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HomeMy Public PortalAboutSection 4 Design Requirements for Storm Drainage FacilitiesRevised 10/15/12 42 4. DESIGN REQUIREMENTS FOR STORM DRAINAGE FACILITIES 4.010 General Stormwater sewers or channels provide the facility for removing and transporting surface runoff produced from rainfall. Design requirements differ from those for either sanitary or combined sewers. This section gives the minimum technical design requirements of the District storm drainage facilities. In general, the formulae presented herein for hydraulic design represent "acceptable" procedures not necessarily to the exclusion of other sound and technically supportive formulae. Any departure from these design requirements should be discussed before submission of plans for approval and should be justified. All construction details pertaining to storm sewer improvements shall be prepared in accordance with the District Standard Construction Specifications unless otherwise noted. 4.020 General Requirements of Storm Sewer Construction All storm sewers shall meet the following general requirements: 4.020.01 Size and Shape The minimum diameters of pipe for stormwater or combined sewers shall be twelve (12) inches. Sewers shall not decrease in size in the direction of the flow unless approved by the District. Circular pipe sewers are preferred for stormwater sewers, although rectangular or elliptical conduits may be used with special permission. 4.020.02 Materials All materials shall conform to the District Standard Construction Specifications. Reinforced concrete pipe joints shall be Type "A" or better, as required. 4.020.03 Bedding The Project Plans and Specifications shall indicate the specific type or types of bedding, cradling, or encasement required in the various parts of the storm sewer construction if different than current the District Standard Construction Specifications. Special provisions shall be made for pipes laid within fills or embankments and/or in shallow or partial trenches, either by specifying extra strength pipe for the additional loads due to differential settlement, or by special construction methods, including ninety percent (90%) modified proctor compaction of fill to prevent or to minimize such additional loads. Compacted granular backfill shall be required in all trench excavation within public (or private) streets rights-of-way or areas where street rights-of-way are anticipated to be dedicated for public use. Under areas to be paved, the compacted granular backfill shall be placed to the subgrade of the pavement. Under unpaved areas, the compacted granular backfill shall be placed to within two (2) feet of the finished surface, and generally not more than two (2) feet beyond street pavement or curb lines. Local street jurisdiction shall govern where more stringent. Pipes having a cover of less than three (3) feet shall be encased in concrete, unless otherwise directed by the District. The Storm and sanitary sewers are parallel and in the same trench, the upper pipe shall be placed on a shelf and the lower pipe shall be bedded in compacted granular fill to the flow line of the upper pipe. 4.020.04 Concrete Pipe or Conduit Strengths Reinforced Concrete pipe shall be Class II, minimum. Any concrete pipe, conduit or culvert beneath a street right-of-way, or with reasonable probability of being so located, shall be a minimum of Class III, but Revised 10/15/12 43 also shall account for all vertical loads, including the live load required by the highway authority having jurisdiction. In no case shall the design provide for less than HS-20 loading of the AASHTO. For other locations, the minimum design live load shall be the HS-10 loading. 4.020.05 Monolithic Structures Monolithic reinforced concrete structures shall be designed structurally as continuous rigid units. Generally these are poured in place units. Wall thickness shall be 8” minimum with one row of reinforcement, horizontal and vertical. Wall thickness 10” and greater shall require 2 rows of reinforcement, horizontal and vertical. (Where approved District precast structures are allowed, less steel & thickness may be accepted). 4.020.06 Alignment Sewer alignments are normally limited by the available easements, which in turn should reflect proper alignment requirements. Since changes in alignment affect certain hydraulic losses, care in selecting possible alignments can minimize such losses and use available head to the best advantage. Sewers shall be aligned: 1. To be in a straight line between structures, such as manholes, inlets, inlet manholes and junction chambers, for all pipe sewers thirty (30) inches in diameter and smaller. 2. To be parallel with or perpendicular to the centerlines of straight streets unless otherwise unavoidable. Deviations may be made only with approval of the District. 3. To avoid meandering, off-setting and unnecessary angular changes. 4. To make angular changes in alignment for sewers thirty (30) inches in diameter or smaller in a manhole located at the angle point, and for sewers thirty- six (36) inches in diameter or larger, by a uniform curve between two tangents. Curves shall have a minimum radius of ten times the pipe diameter. 5. To avoid angular changes in direction greater than necessary and any exceeding ninety (90) degrees. 4.020.07 Location Storm sewer locations are determined primarily by the requirements of service and purpose. It is also necessary to consider accessibility for construction and maintenance, site availability and competing uses, and effects of easements on private property. Storm sewers shall be located: 1. To serve all property conveniently and to best advantage. 2. In public streets, roads, alleys, rights-of-way, or in sewer easements dedicated to the District. 3. On private property along property lines or immediately adjacent to public streets, avoiding diagonal crossings through the central areas of the property. 4. At a sufficient distance from existing and proposed buildings including footings, and underground utilities or other sewers to avoid encroachments and reduce construction hazards. 5. To avoid interference between other stormwater sewers and house connections to foulwater or sanitary sewers. Revised 10/15/12 44 6. In unpaved or unimproved areas whenever possible. 7. To avoid, whenever possible, any locations known to be or probably to be beneath curbs, paving or other improvements particularly when laid parallel to centerlines. 8. To avoid sinkhole areas if possible. However, if sinkhole areas cannot be avoided, see sub-section 4.020.08 for requirements. 9. Crossing perpendicular to street, unless otherwise unavoidable. 4.020.08 Sinkhole Areas 1. Sinkhole Report Where improvements are proposed in any area identified as sinkhole areas, a sinkhole report will be required. This report is to be prepared by a Professional Engineer, registered in the State of Missouri, with demonstrated expertise in geotechnical engineering, and shall bear his or her seal. The sinkhole report shall verify the adaptability of grading and improvements with the soil and geologic conditions available in the sinkhole areas. Sinkhole(s) shall be inspected to determine its functional capabilities with regard to handling drainage. The report shall contain provisions for the sinkholes to be utilized as follows: a. All sinkhole crevices shall be located on the plan. Functioning sinkholes may be utilized as a point of drainage discharge by a standard drainage structure with a properly sized outfall pipe provided to an adequate natural discharge point, such as a ditch, creek, river, etc. b. Non-functioning sinkholes and sinkholes under a proposed building may be capped. c. If development affects sinkholes, they may be left in their natural state; however they will still require a properly sized outfall pipe to an adequate natural discharge point. d. An overland flow path shall be required for all sinkholes assuming the outfall pipe and sinkhole become blocked. Where the topography will not allow for an overland flow path: 1. The storm sewer shall be designed for the 100-year, 20-minute storm, and 2. If this storm pipe is smaller than thirty six (36) inches in diameter, a designated ponding area shall be identified, assuming the pipe is blocked, and 3. The ponding area shall be based on the 100-year, 24-hour storm, and 4. The low sill of all structures adjacent to the ponding area shall be a minimum of two (2) feet above the 100-year highwater elevation. 5. Special siltation measures shall be installed during the excavation of sinkholes and during the grading operations to prevent siltation of the sinkhole crevice. Revised 10/15/12 45 2. Procedure for Utilization of Sinkholes a. Excavation. Prior to filling operations in the vicinity of a sinkhole, the earth in the bottom of the depression will be excavated to expose the fissure(s) in the bedrock. The length of fissure exposed will vary, but must include all unfilled voids or fissure widths greater than one-half (1/2) inch maximum dimensions which are not filled with plastic clay. b. Closing Fissures. The fissure or void will be exposed until bedrock in its natural attitude is encountered. The rock will be cleaned of loose material and the fissures will be hand-packed with quarry-run rock of sufficient size to prevent entry of this rock into the fissures, and all the voids between this hand-packed quarry-run rock filled with smaller rock so as to prevent the overlying material's entry into the fissures. For a large opening, a structural (concrete) dome will be constructed with vents to permit the flow of groundwater. c. Placing Filter Material. Material of various gradations, as approved, will be placed on top of the hand-packed rock with careful attention paid to the minimum thicknesses. The filter material must permit either upward or downward flow without loss of the overlying material. The fill placed over the granular filter may include granular material consisting of clean (no screenings) crushed limestone with ten (10) inch maximum size and one (1) inch minimum size or an earth fill compacted to a minimum density of ninety percent (90%) modified Proctor as determined by ASTM D-1557. d. Supervision. Periodic supervision of the cleaning of the rock fissures must be furnished by the Engineer who prepared the Soil Report. Closing of the rock fissures will not begin until the cleaning has been inspected and approved by that Engineer. During the placement and compaction of earth fill over the filter, supervision by the Engineer shall be continuous. Earth fill densities will be determined during the placement and compaction of the fill in sufficient number to insure compliance with the specification. The Engineer is responsible for the quality of the work and to verify that the specifications are met. 4.020.09 Flowline The flowline of storm sewers shall meet the following requirements: 1. The flowline shall be straight or without gradient change between the inner walls of connected structures; that is, from manhole to manhole, manhole to junction chamber, inlet to manhole, or inlet to inlet. 2. Gradient changes in successive reaches normally shall be consistent and regular. Gradient designations less than the nearest 0.001 foot per foot, except under special circumstances and for larger sewers, shall be avoided. Revised 10/15/12 46 3. Sewer depths shall be determined primarily by the requirements of pipe or conduit size, utility obstructions, required connections, future extensions and adequate cover. 4. Stormwater pipes discharging into lakes shall have the discharge flowline a minimum of three (3) feet above the lake bottom at the discharge point or no higher than the normal water line. 5. A concrete cradle is required when the grade of a sewer is twenty (20) percent or greater. A special design and specification is required for grades exceeding fifty percent (50%). 6. For sewers with a design grade less than one percent (1%), field verification of the sewer grade will be required for each installed reach of sewer, prior to any surface restoration or installation of any surface improvements. 7. The District may require the submittal of revised hydraulic calculations for any sewer reach having an as-built grade flatter than the design grade by more than 0.1%. Based on a review of this hydraulic information, the District may require the removal and replacement of any portion of the sewer required to ensure sufficient hydraulic capacity of the system. 4.020.10 Manholes Manholes provide access to sewers for purposes of inspection, maintenance and repair. They also serve as junction structures for lines and as entry points for flow. Requirements of sewer maintenance determine the main characteristics of manholes. Cast in place or precast manhole structures are generally allowable, though the former requires approved shop detail drawings. 1. For sewers thirty (30) inches in diameter or smaller, manholes shall be located at changes in direction; changes in size of pipe; changes in flowline gradient of pipes, and at junction points with sewers and inlet lines. For sewers thirty-three (33) inches in diameter and larger, manholes shall be located on special structures at junction points with other sewers and at changes of size, alignment change and gradient. A manhole shall be located at one end of a short curve and at each end of a long curve. 2. Spacing of manholes shall not exceed four hundred (400) feet for pipe sewers thirty-six (36) inches in diameter and smaller; five hundred (500) feet for pipe sewers forty-two (42) inches in diameter and larger, except under special approved conditions. Spacing shall be approximately equal, whenever possible. 3. When large volumes of stormwater are permitted to drop into a manhole from lines twenty-one (21) inches or larger, the manhole bottom and walls below the top of such lines shall be of reinforced concrete. Special structural design may be required for large pipes and/or large drops. 4. Manholes shall be avoided in driveways or sidewalks. 5. Connections to existing structures may require rehabilitation or reconstruction of the structure being utilized. This work will be considered part of the project being proposed. 6. When a project requires a manhole to be adjusted to grade a maximum of twelve (12) inches of rise is allowed if not previously adjusted. When adjustments to raise or lower a manhole is required, the method of adjustment must be stated on the project plans and approved by the District. Revised 10/15/12 47 4.020.11 Overflow/Design System The "design" components of the drainage system include the inlets, pipe, storm sewers, and improved and unimproved channels that function during typical rainfall events. The "overflow" system comprises the major overflow routes such as swales, streets, floodplains, detention basins, and natural overflow and ponding areas. The purpose of the overflow system is to provide a drainage path to safely pass flows, which cannot be accommodated by the design system without causing flooding of adjacent structures. The criteria for the design of the overflow and design systems shall be as follows: 1. The "design" system shall be designed in accordance with Section 4.030. 2. The "overflow" system shall be designed for the 100-year, 20-minute event, assuming the "design" system is completely blocked. The capacity of the "overflow" system shall be verified with hydraulic calculations at critical cross-sections. The "overflow" system shall be directed to the detention facility, or as approved by the District. 3. The low sill of all structures adjacent to the "overflow" system swales shall be above the 100-year highwater elevation. 4. Where the topography will not allow for an overland flow path: a. The storm sewer shall be designed for the 100-year, 20-minute storm, and b. If this storm pipe is smaller than thirty-six (36) inches in diameter, a designated ponding area shall be identified, assuming the pipe is blocked, and c. The ponding area shall be based on the 100-year, 24-hour storm, and d. The low sill of all structures adjacent to the ponding area shall be above the 100-year highwater elevation. 5. The "overflow" system shall be designated on the drainage area map and on the grading plan. 6. All overflow systems will be considered on a site specific basis. 7. The stormwater design for projects within designated levee districts such as Monarch- Chesterfield, Earth City and Riverport will be based on the Stormwater Master Plan for these districts. 4.030 Stormwater Design Criteria 4.030.01 Flow Quantities Flow quantities are to be calculated by the "Rational Method" in which: Q = API where: Q = runoff in cubic feet per second A = tributary area in acres I = Average intensity of rainfall (inches per hour) for a given period and a given frequency P = runoff factor based on runoff from pervious and impervious surfaces Revised 10/15/12 48 P (Runoff Factors) for various impervious conditions are shown in Table 4-1. P.I. values for various impervious conditions are shown in Table 4-2. 1. Rainfall Frequency A twenty (20) year rainfall frequency is to be used in the City of St. Louis and areas of St. Louis County where combined sewers are used. A fifteen (15) year rainfall frequency is to be used in areas of St. Louis County where storm sewers are separated from sanitary sewers. In the design of local storm sewer systems, a twenty (20) minute time of concentration shall be used. Figure 4-1 gives rainfall curves for 2, 5, 10, 15, 20 and 100 year frequencies. 2. Impervious Percentages and Land Use Minimum impervious percentages to be used are as follows: a. For manufacturing and industrial areas, 100%* b. For business and commercial areas, 100%* c. For residential areas, including all areas for roofs of dwellings and garages; for driveways, streets, and paved areas; for public and private sidewalks; with adequate allowance in area for expected or contingent increases in imperviousness: In apartment, condominium and multiple dwelling areas: 75%* In single family areas: 1/4 Acre or less 50% 1/4 Acre to 1/2 Acre 40% 1/2 Acre to 1 Acre 35% One acre or larger Calculate impervious percentage* Playgrounds (Non-Paved) 20-35%* d. For small, non-perpetual charter cemeteries, allow 30% For parks and large perpetual charter cemeteries 5% *NOTE: Drainage areas may be broken into component areas, with the appropriate run-off factor applied to each component, i.e. a proposed development may show one hundred percent (100%) impervious for paved areas and five percent (5%) impervious for grassed areas. Use of actual component areas may be required, however, where minimum impervious percentages are deemed misleading, or too approximate. The design engineer shall provide adequate detailed computations for any proposed, expected or contingent increases in imperviousness and shall make adequate allowances for changes in zoning use. If consideration is to be given to any other value than the above for such development, the request must be made at the beginning of the project, must be reasonable, fully supported, and adequately presented, and must be approved in writing before its use is permitted. Although areas generally will be developed in accordance with current zoning requirements, recognition must be given to the fact that zoning ordinances can be Revised 10/15/12 49 amended to change the currently proposed types of development, and any existing use. Under these circumstances the possibility and the probability of residential areas having lot sizes changed or re-zoned to business, commercial, or light manufacturing uses should be given careful consideration. e. Average 20-minute values of P.I. (cfs per acre) to be used are as follows: Percent 20 Minute Duration Imperviousnes 15 Year 20 Year 5 1.70 1.78 10 1.79 1.87 20 2.00 2.09 30 2.19 2.28 40 2.39 2.50 50 2.58 2.69 90 3.36 .. 3.50 100 3.54 3.70 *Roofs (City of St. Louis) 6.0 *Roofs 4.2 (Other than City of St. Louis) *For Direct Connection to Sewer 3. Reduction in P.I. with Time and Area Reduction in P.I. values for the total time of concentration exceeding twenty (20) minutes, and for tributary areas exceeding three hundred (300) acres will be allowed only in trunk sewers and main channels. The reduced average P.I. value for the tributary area shall not be less than the value determined as follows on the basis of: a. Time. As the time of concentration increases beyond twenty (20) minutes, select the appropriate P.I. value from Table 4-1. The travel time through a drainage channel should be based on an improved concrete section. These reduced values shall be used unless a further reduction is allowed for area. b. Area. As the total tributary area at any given location in a channel increases in excess of three hundred (300) acres, the P.I. value may be further reduced by multiplying it by an area coefficient "Ka". (The area coefficient is obtained from data in a special study of a major storm in the St. Louis area by the U.S. Corps of Engineers.) The average rainfall rate, for a given storm, for a given period for the tributary area, is less than the corresponding point value as determined from recording rainfall gauges. The curve data is as follows: P.I. Coefficients Ka Area (Abscissas) "Ka" (Ordinates) 300 to 449 Acres 1.00 450 to 549 Acres .99 550 to 749 Acres .98 750 to 999 Acres .97 1000 to 1280 Acres .96 1281 to 1600 Acres .95 1601 to 1920 Acres .92 1921 to 2240 Acres .91 Revised 10/15/12 50 4.030.02 Hydraulic Grade Line for Closed Conduits 1. Computation Methods The hydraulic grade line is a line coinciding with (a) the level of flowing water at any given point along an open channel, or (b) the level to which water would rise in a vertical tube connected to any point along a pipe or closed conduit flowing under pressure. The beginning point for hydraulic grade line computations for storm sewers shall be at least the higher of the elevations listed in 3.030.07.1. Field-verification shall also apply. The hydraulic grade line shall be computed to show its elevation at all structures and junction points of flow in pipes, conduits and open channels, and shall provide for the losses and the differences in elevations as required below. Since it is based on design flow in a given size of pipe or conduit or channel, it is of importance in determining minimum sizes of pipes within narrow limits. Sizes larger than the required minimum generally provide extra capacity, however consideration must still be given to the respective pipe system losses. There are several methods of calculating "losses" in storm sewer design. The following procedures are presented for the engineer's information and consideration. It is expected that the design will recognize the reality of such "losses" occurring and make such allowances, as good engineering judgment requires. a. Friction Loss The hydraulic grade line is affected by friction loss and by velocity head transformations and losses. Friction loss is the head required to maintain the required flow in a straight alignment against frictional resistance because of pipe or channel roughness. It is determined by the equation: hf =L x S h Where: hf = difference in water surface elevation, or head in feet in length L L = length in feet of pipe or channel S h = hydraulic slope required for a pipe of given diameter or channel of given cross- section and for a given roughness "n", expressed as feet of slope per foot of length From Manning's formula: S h = [V n /(1.486 R0.667)]2 Where: R = hydraulic radius of pipe, conduit or channel (feet) (Ratio of flow area/wetted perimeter) V = velocity of flow in feet per second (fps) n = Manning's value for coefficient of roughness Use: n = .013 for pipes of concrete, vitrified clay, and PVC pipe n = .012 for formed monolithic concrete, i.e., vertical wall channels, box culverts and for R.C.P. over 48" in diameter n = .015 for concrete lining in ditch or channel Revised 10/15/12 51 inverts and trapezoidal channels n = .020 for grouted riprap lining on ditch or channel side slopes n = .033 for gabion walled channels Note: "n" will have a weighted value for composite lined channels. "n" values for unlined channels to be determined on an individual basis. b. Curve Loss Curve loss in pipe flow is the additional head required to maintain the required flow because of curved alignment, and is in addition to the friction loss of an equal length of straight alignment. It should be determined from Figure 4-2, which includes an example. c. Entrance Loss at Terminal Inlets Entrance loss is the additional head required to maintain the required flow because of resistance at the entrance. The entrance loss at a terminal inlet is calculated by the formula: H ti = (V2/2g) Where: V = Velocity in flow of outgoing pipe g = Acceleration of gravity (32.2 Ft/Sec/Sec) d. Turn Loss Head losses in structures due to change in direction of flow (turns) in a structure, will be determined in accordance with the following: Multiplier of Change in Direction Velocity Head of of Flow (A) Water Being Turned (K) 90 Deg. 0.7 60 Deg. 0.55 45 Deg. 0.47 30 Deg. 0.35 15 Deg. 0.18 0 Deg. 0.0 Other Angles By Interpolation Revised 10/15/12 52 DIAGRAM: Formula: H L = K(V L )2/2g Where: H L = Feet of head lost in manhole due to change in direction of lateral flow V L = Velocity of flow in lateral in Ft/Sec g = Acceleration of gravity, (32.2 Ft/Sec/Sec) K = Multiplier of Velocity Head of water being turned e. Junction Chamber Loss A sewer junction occurs for large pipes or conduits too large to be brought together in the usual forty two (42) inch diameter manhole or inlet where one or more branch sewers enter a main sewer. Allowances should be made for head loss due to curvature of the paths and due to impact at the converging streams. Losses in a junction chamber for combining large flows shall be minimized by setting flowline elevations so that pipe centerlines (springlines), will be approximately in the same planes. At junction points for combining large storm flows, a manhole with a slotted cover shall be provided. A computation method for determining junction chamber loses is presented below: H j = y + V h1 - V h2 Where: H j = junction chamber loss (ft.) V h1 = upstream velocity head V h2 = downstream velocity head y = change in hydraulic grade line through the junction in feet Where: Revised 10/15/12 53 y = [(Q 2 V 2 )-((Q 1 V 1 ) + {(Q 3 V 3 Cos θ 3 ) + (Q n V n CosӨ n )})] 0.5(A1 +A 2 )g Where: Q 2 = Discharge in cubic feet per second (cfs) at the exiting conduit V 2 = Velocity in feet per second (fps) at the exiting conduit A 2 = Cross sectional area of flow in sq. ft. for the exiting conduit Q 1 = Discharge in cfs for the incoming pipe (main flow) V 1 = Velocity in fps for the incoming pipe (main flow) A 1 = Cross sectional area of flow in Sq. Ft for the incoming pipe (main flow) Q 3 ,Q n = Discharge(s) in cfs for the branch lateral(s) V3,V n = Velocity(ies) in fps for the branch lateral(s) Ө 3 , Ө n = The angle between the axes of the exiting pipe and the branch laterals(s) g = Acceleration of gravity (32.2 ft/sec/sec) Where: Ө = is the angle between the axes of the outfall and the incoming laterals f. Losses at Junctions of Several Flows in Manholes and/or Inlets The computation of losses in a manhole, inlet or inlet manhole with several flows entering the structure should utilize the principle of the conservation of energy. This involves both the elevation of water surface and momentum (mass times the velocity head). Thus, at a structure (manhole, inlet or inlet manhole) with laterals, the sum of the energy content for inflows is equal to the sum of the energy content of the outflows plus the additional energy required by the turbulence of the flows passing through the structure. DIAGRAM: Revised 10/15/12 54 The upstream hydraulic grade line may be calculated as follows: H u = [V D 2/2g]-[((Q U /Q D )(l-K)(V U 2/2g))+((Q L1 /Q D )(l-K)(V L1 2/2g)) +((Q LN /Q D )(l-K)(V LN 2/2g))] + H D Where: H U = Upstream hydraulic grade line in feet Q U = Upstream main line discharge in cubic feet per second Q D = Downstream main line discharge in cubic feet per second Q L1 -Q LN = Lateral discharges in cubic feet per second V U = Upstream main line velocity in feet per second V D = Downstream main line velocity in feet per second V L1 -V LN = Lateral velocities in feet per second H D = Downstream hydraulic grade line in feet K = Multiplier of Velocity of Water being turned g = Acceleration of gravity, 32.2 ft/sec/sec The above equation does not apply when two (2) almost equal and opposing flows, each perpendicular to the downstream pipe, meet and no other flows exist in the structure. In this case the head loss is considered as the total velocity head of the downstream discharge. g. Transition Loss The relative importance of the transition loss is dependent on the velocity head of the flow. If the velocity and velocity head of the flow are quite low, the transition losses cannot be very great. However, even small losses may be significant in flat terrain. The sewer design shall provide for the consideration of the necessary transitions and resulting energy losses. The possibility of objectionable deposits is to be considered in the design of transitions. For design purposes it shall be assumed that the energy loss and changes in depth, velocity and invert elevation, if any, occur at the center of the transition. These changes shall be distributed throughout the length of the transition in actual detailing. The designer shall carry the energy head, piezometric head (depth in an open channel), and invert as elevations, and work from the energy grade line. Because of inherent differences in the flow, transitions for closed conduits will be considered separately from those for open channels. (1) Closed Conduits Transitions in small sewers may be confined within a manhole. Special structures may be required for larger sewers. If a sewer is flowing surcharged, the form and friction losses are independent of the invert slope; therefore, the transition may vary at the slopes of the adjacent conduits. The energy loss in a transition shall be expressed as a coefficient multiplied by the change in velocity head (V2/2g) in which V is the change in velocity before and after the transition. The coefficient may vary from zero to one, depending on the design of the transition. Revised 10/15/12 55 If the areas before and after a transition are known, it is often convenient to express the transition loss in terms of the area ratios and either the velocity upstream or downstream. For an expansion: H L = K(V 1 -V 2 )2/2g≈[K(V 1 )2/2g][1-(A 1 /A 2 )]2 in which H L is the energy loss; K is a coefficient equal to 1.0 for a sudden expansion and approximately 0.2 for a well-designed transition and the subscripts 1 and 2 denote the upstream and downstream sections, respectively, i.e., A 1 = Area Before Transition and A2 = Area After Transition. For a contraction: H L = [K(V 2 )2/2g][(1/C c )-1]2≈[K(V 2 )2/2g][1-(A 2 /A 1 )]2 in which K is a coefficient equal to 0.5 for a well-designed transition, C c is a coefficient of contraction, and the other terms and subscripts are similar to the previous equation. Losses in closed conduits of constant area are expressed in terms of (V2/2g). The above equations may be applied to approximate the energy loss through a manhole for a circular pipe flowing full. If the invert is fully developed, that is, semi-circular on the bottom and vertical on the sides from one-half depth up to the top of the pipe, for the expansion (A1 /A 2 ) = 0.88, and for the contraction (A2 /A 1 ) = 0.88. The expansion is sudden; therefore, K = 1. The contraction may be rounded if the downstream pipe has a bell or socket. In this case, K may be assumed to be 0.2. The expansion energy loss is 0.0l4 [(V 1 )2/2g] and the contraction energy loss is 0.010 [(V 2 )2/2g]. If the invert is fully developed, the manhole loss is small, but if the invert is only developed for one-half of the depth, or not at all, the losses will be of considerable magnitude. (2) Open Channel Transitions The hydraulics of open channel transitions are further complicated by possible changes in depth. As a first approximation to the energy loss, unless a jump occurs, the equations given above may be used with a trial-and-error solution for the unknown area and velocity. The K value for a well-designed expansion should probably be increased to 0.3 or 0.4. Whether the properties of the upstream or downstream section will be known will depend on the characteristics of the flow and the channel, but can be determined by a profile analysis. In transitions for supercritical flow, additional factors shall be considered. Standing waves of considerable magnitude will be produced in transitions. The height of these waves must be estimated to provide a proper channel depth. In addition, in long transitions, air entrainment will cause bulking of the flow with resultant greater depths of the air-water mixture. 4.030.03 Hydraulic Grade Line Limits The hydraulic grade line shall not rise above the following limits as determined by flow quantities calculated per Section 4.030.01. Stormwater conveyance systems are usually designed for 15 year or 20 year, 20 minute rainfall frequencies per 4.030.01.1. Revised 10/15/12 56 1. The hydraulic grade line at any inlet or storm manhole shall not be higher than two (2) feet below the inlet sill or top of manhole. 2. Storm sewers shall not flow with greater than three (3) feet of head. 3. The hydraulic grade line for combined sewers shall not rise above the pipe intrados. 4. The beginning point for the hydraulic grade line computations shall be the higher (i.e. more conservative) elevation as determined below: a. For connection to existing pipe system: (1) Top of pipe intrados of at least two reaches downstream of the connection point of the existing system; or (2) The hydraulic grade line computed for the existing system. b. For connection to channels or ditches: (1) Top of pipe intrados of the proposed pipe, or (2) The hydraulic grade line computed for the channel or ditch as approved by the District. c. For upstream system pipe connection to dry and wet detention basins: (1) The starting hydraulic grade line for all incoming pipes shall be the 100 year-24 hour blocked low flow water surface elevation, where County maintained streets are located adjacent to or upstream of the basins. (2) The starting HGL for all other situations may be the 100 year – 24 hour unblocked low flow water surface elevation, unless the local road authority requires something higher. 5. When storm sewers are designed to convey 100 year flows, effusion at low lying inlets is not allowed, unless 100 year ponding easements are so delineated, granted, and recorded. Those associated temporary “ponding” easements however, should not be confused with 100 year overland flow paths, for which no conveyance area easements are presently required. Also, such intentional effusive designs may be prohibited for St. Louis County maintained streets or highways. 4.030.04 Inlets Inlets function entirely as entry points for stormwater flow. They also may be constructed to serve as a manhole on separate stormwater sewers, and are then termed inlet-manholes. Steep gradients may give such low inlet capacities that additional inlets should be located at more favorable grade locations or special inlets designed for steep gradients must be used. Provision must be made to control by-pass flow and to provide additional capacity in the inlet and line affected by such increased flow. Six (6) inch open throat inlets should be used at all times. The open throat should not be obstructed or otherwise restricted by bars, wires or screens. Grated inlets, without an open throat or other provision for overflow shall be avoided except under exceptional conditions, and are prohibited in grade pockets. Any exceptions shall be used only with District approval. Curb inlets shall be placed at street intersections or driveways such that no part of the inlet structure or sump is within the curb rounding. Revised 10/15/12 57 1. Inlets are shown in the Standard Details of Sewer Construction. The minimum depth of a terminal inlet is four (4) feet from the top of the inlet to the flowline of the outlet pipe. Greater depth shall be used for intermediate inlets if necessary for the required depth of the hydraulic grade line. Trapped inlets shall have the depth shown in the Standard Details of Sewer Construction. 2. Inlet capacity should not be less than the quantity flow tributary to the inlet and by- pass flow shall be avoided whenever possible. “Multiple type” inlets, used in the past, had one integral chamber shallower than the other chamber, and tops of different size stones, as well. Use of “multiple type” inlets is prohibited for new construction, as are bars, screens, or wires across inlet openings. Use “double inlets” instead, if necessary for capacity. Bypass, if unavoidable, must be identified, including amount and spread; local road jurisdiction approval must also be provided. Inlets at low points or grade pockets should have extra capacity to compensate for possible flow by-pass of upstream inlets. Figure 4-3 shows inlet capacity/maximum gutter capacity with a given gutter line grade and flow. Inlets angled in opposition to direction of vehicular travel may be dangerous and are to be avoided. 3. Connections to existing structures may require rehabilitation or reconstruction of the structure being utilized. This work will be considered part of the project being proposed. 4. Grated trough drains will not be accepted for dedication to the District. Where appended to the District curb inlet structures, design shall provide clear and workable separation for purposes of maintenance responsibility by others for their part of the drain. 4.030.05 Open Channels *NOTE: This section contains some excerpts relating to design and are attributed to Open Channel Hydraulics by Ven Te Chow, a McGraw-Hill work published in 1959. All open channels shall meet the following requirements: 1. Size and Shape Open channels shall not decrease in size in the direction of flow. Open channels shall be vertical walled except in special cases where other approved materials are being considered. 2. Materials Channels may be constructed with reinforced concrete or other approved material. Gabions, articulated mattresses or other systems may be approved. However, the District shall have the right to approve or disapprove any channel material and shall select the appropriate channel material if a proposed material is rejected. Swales shall be sodded unless velocities are excessive (greater than 5 fps or where velocities are less than 2 fps causing deposition of soil particles, then concrete swales may be used. Swales used as BMP’s shall be appropriately vegetated and maintained, or otherwise stabilized in an approved manner. Revised 10/15/12 58 3. Bedding Special provisions shall be made for channels or paved swales laid over fill on non- supportive soils to support the channel on paved swales. Pipes extended to the channel in a fill area shall have compacted crushed limestone bedding for support. 4. Structural Considerations Provision must be made for all loads on the channel. 5. Alignment Open channel alignments may be limited by available easements, physical topography, existing utilities, buildings, residential development, maintenance access and roadways. 6. Locations Storm channel locations are determined primarily by natural drainage conditions. It is also necessary to consider accessibility for construction and maintenance, site availability and competing uses, and evaluating effects of easements on private property. Storm channels shall be located: a. To serve all adjacent property conveniently and to best advantage. b. In easements or rights-of-way dedicated to the District. c. In easements on common ground when feasible. d. On private property along property lines or immediately adjacent to public streets, avoiding crossings through the property. e. At a sufficient distance from existing and proposed buildings and underground utilities or sewers to avoid future problems of flooding or erosion. f. To avoid interference between stormwater sewers and house connections to foulwater or sanitary sewers. g. In unpaved or unimproved areas whenever possible. h. Crossing perpendicular to streets, unless unavoidable. 7. Flowline The flowline of open channels shall meet the following requirements: a. Gradient changes shall be kept to a minimum and be consistent and regular. b. Gradient designations less than the nearest 0.001 foot per foot shall be avoided. c. Channel and swale depths shall be determined primarily by the requirements of the channel size, utility obstructions and any required connections. 8. Other Open Channel Considerations and Requirements a. All natural channels and ditches shall be improved unless otherwise authorized by the District. Revised 10/15/12 59 b. Drainage within private property should be controlled to prevent damage to the property crossed. Swales, or broad shallow grass lined ditches with non-erosive slopes, are generally located at or near rear lots and along common property lines. If a paved gutter is utilized, then appropriate erosion protection shall be used at both ends. c. Drainage channels and water courses draining through a subdivision shall be enclosed if the required pipe size does not exceed sixty (60) inches unless a BMP plan is approved which incorporates the channel specifically. Drainage channels originating within a project site are encouraged to be incorporated into the development’s stormwater management plan. When it is undesirable or impractical to enclose a channel with a pipe across a road or street, a suitable bridge or culvert shall be required. d. For flows greater than 4 cfs, area inlets or inlet manholes are required to intercept the gutter or swale flow unless part of a workable, recognized and approved BMP. e. All improved concrete channels shall have a forty eight (48) inch chain link fence on each side of the channel, or other protective measures as directed by the District. f. Channels and water courses draining large areas shall be located in rights-of-way or easements previously approved by the District as a part of an adequate overall plan for drainage. 9. Design Limitations a. The flow quantity shall be calculated by the method presented in Section 4.030.01 of this manual. b. If the channel is within an area designated in a community's flood insurance study, then the channel shall also meet all District and the community's floodplain requirements. c. Other agencies of jurisdiction, for example FEMA or MoDNR, may have requirements which must be met. A U.S. Army Corps of Engineers permit may be required for any construction affecting a watercourse. 10. Hydraulic Grade Line a. Computation Methods In open channels the water surface is identical with the hydraulic grade line. The hydraulic grade line shall be computed throughout the channel reach to show its elevation at junctions with incoming pipes or channels and at the ends of the channel reach under consideration. It shall also provide for the losses and differences in elevations as required below. Since it is based on design flow in a given channel, it is of importance in determining minimum sizes within narrow limits. The depth at which the actual flows will occur is controlled by the two end conditions of the reach considered, and by the relationship between the energy available and by the energy required to overcome the losses that are encountered along the channel. There are several methods of calculating "losses" in channel design. The following procedures are presented for the engineers information and consideration. Revised 10/15/12 60 It is required that the design recognize the reality of such "losses" occurring and make such allowances as good engineering judgment indicates. (1) Control Sections The engineer should locate all possible control sections for the reach in question. A control section refers to any section at which the depth of flow is known or can be controlled to a required stage. At the control section, flow must pass through a control depth which may be the critical depth, the normal depth or any other known depth. Three types of control sections include (a) Upstream Control Section; (b) Downstream Control Section; (c) Artificial Control Section, which occurs at a control structure, such as a weir, dam, sluice gate, roadway embankment, culvert, bridges or at the confluence with a major river or stream. (2) Friction Loss The friction loss may be calculated by the same procedure as is presented in (3) Flow in Curved Channels The centrifugal force caused by flow around a curve produces a rise in the water surface on the outside wall and a lowering of the inner wall. This phenomenon is called superelevation. The flows tend to behave differently according to the state of flow. In subcritical flow, friction effects are of importance, whereby in supercritical flow, the formation of cross-waves is of major concern. (a) Curve Losses Curve losses may be estimated from Figure 4-2 by replacing D, diameter, with b, width of channel. (b) Superelevations In addition to curve losses, an evaluation of superelevations should be considered and, if required, an allowance made in the top elevation of outside wall. Equations are presented below which may be used to determine the superelevation at channel bends. 1) Trapezoidal Channels Subcritical Flow: ΔH w = 1.15(V2/2gr c )[b+D(ZL+ZR)] Supercritical Flow: ΔH w = 2.6(V2/2gr c )[b+D(ZL+ZR)] 2) Rectangular Channels Subcritical Flow: ΔH w = (V2b/2gr c ) Supercritical Flow: ΔH w = (V2b/gr c ) Where: ΔH w = Change in water height above the Revised 10/15/12 61 centerline water surface elevation. V = Average velocity of design flow in Fps g = Acceleration of gravity (32.2 Ft/Sec/Sec) r c = Radius of curve on horizontal alignment in feet b = Base width of channel in feet D = Depth of flow in straight channel ZL = Left side slope (ft/ft) ZR = Right side slope (ft/ft) (4) Transitions Transitions should be designed to accomplish the required change in cross section with as little flow disturbance as possible. The following features are to be considered in design of transition structures. (a) Proportioning For a well designed transition, the following rules should be used: 1) The optimum maximum angle between the channel axis and a line connecting the channel sides between the entrance and exit sections is 12.5o. 2) Sharp angles in the structure should be avoided. (b) Losses The energy loss in a transition consists of the friction loss and the conversion loss. The friction loss may be estimated by the Manning Formula. The conversion loss is generally expressed in terms of the change in velocity head between the entrance and exit sections of the structure. H t = K t ΔV H Where: H t = Conversion loss K t = Coefficient of head loss in transition ΔV H = Absolute change in velocity head Average design values for K t are presented in the table below: Contracting Expanding Type of Transition Section Section Warped 0.10 0.20 Wedge 0.20 0.50 Cylinder-quardrant 0.15 0.25 Revised 10/15/12 62 Straight Line 0.30 0.50 Square End 0.40 0.75 See Figure 4-4 for sketches of each type of transition. (c) Freeboard A transition shall have a minimum of one (1) foot of freeboard above the hydraulic grade line. (d) Hydraulic Jump The existence of a hydraulic jump in a transition may become objectionable, and the design of the transition should be checked for such. (e) Sudden Enlargement and Contraction A sudden enlargement results when an intense shearing action occurs between incoming high-velocity jet and the surrounding water. As a result, much of the Kinetic energy of the jet is dissipated by eddy action. The head loss at a sudden enlargement, H Le , is: H Le = K e ( ΔV2/2g) Where: K e = Coefficient of head loss for enlargements = 1 ΔV = Change in velocities between incoming and outgoing sections g = Acceleration of gravity (32.2 Ft/Sec/Sec) The flow in a sudden contraction is first contracted and then expanded resulting in high losses as compared to a sudden enlargement. Thus the head loss at a sudden contraction, H Lc , is: H Lc = K c ( ΔV2/2g) Where: K c = Coefficient of head loss for contractions = 0.5 ΔV = Change in velocities between incoming and outgoing sections g = Acceleration of gravity, Revised 10/15/12 63 (32.2 Ft/Sec/Sec) (5) Constrictions A constriction results in a sudden reduction in channel cross section. The effect of the constriction on the flow depends mainly on the boundary geometry, the discharge and the state of flow. When the flow is subcritical, the constriction will induce a backwater effect that extends a long distance upstream. If the flow is supercritical, the disturbance is usually local and will only affect the water adjacent to the upstream side of the constriction. A control section may or may not exist at a constriction. The control section, when it exists, may be at either side of the constriction (upstream or downstream), depending on whether the slope of the constricted channel is steep or mild. The entrance and outlet of the constriction then acts as a contraction and an expansion, respectfully. (6) Obstructions An obstruction in open-channel flow creates at least two paths of flow in the channel. Typical obstructions include bridge piers, pile trestles, and trash racks. The flow through an obstruction may be subcritical or supercritical. b. Hydraulic Grade Line Limits (1) The hydraulic grade line at any point along a channel shall not be higher than one (1) foot below the top of the channel wall. (2) The hydraulic grade line at any point along a channel shall not cause the hydraulic grade limits of the storm sewer system to be exceeded as stated in Section 4.030.03 of this manual. 11. Hydraulic Jump When flow changes from the supercritical to subcritical state, a hydraulic jump may occur. A study should be made on the height and location of the jump, and for discharges less than the design discharge, to ensure adequate wall heights extend over the full ranges of discharge. 12. Open Channel Junctions a. General (1) Consideration shall be given in the design of open channel junctions to the geometry of the confluence of flows in order to minimize undesirable hydraulic effects due to supercritical velocities. b. Confluence Design Criteria (1) The momentum equation can be applied to the confluence design if the below stated criteria is used. (2) The design water-surface elevations in the two joining channels should be approximately equal at the upstream end of the confluence. Revised 10/15/12 64 (3) The angle of the junction intersection can vary from 0-12 degrees. (4) The width of the main channel shall be expanded below the junction to maintain approximate flow depths throughout the junction. (5) Flow depths should not exceed ninety percent (90%) of the critical depth. 13. Erosion Protection Properly designed rock blankets, minimum one (1) foot thick, shall be required at each end of the improved channel. The minimum length of the rock blanket shall be twenty five (25) feet. A rock toe wall, minimum two (2) foot deep, shall be constructed at the free end of each blanket. Alternative erosion protection products may be considered, such as articulated block mattresses, etcetera. 14. Sanitary Sewer Crossings The characteristics of any sanitary sewer crossing shall be given consideration in the design of the channel floor. 4.030.06 Culverts The design of culverts shall include consideration of many factors relating to requirements of hydrology, hydraulics, physical environment, imposed exterior loads, construction and maintenance. With the design discharge and general layout requirements determined, the design requires detailed consideration of such hydraulic factors as shape and slope of approach and exit channels, allowable head at entrance (and ponding capacity, if appreciable), tailwater levels, hydraulic and energy gradelines, and erosion potential. 1. Hydraulic Design The hydraulic design of a culvert for a specified design discharge involves (1) selection of a type and size, (2) determination of the position of hydraulic control, and (3) hydraulic computations to determine whether acceptable headwater depths and outfall conditions will result. Hydraulic computations will be carried out by standard methods based on pressure, energy, momentum and loss considerations. 2. Entrances and Headwalls - Outlets and Endwalls Where an existing culvert is to be extended, the possibility for maintaining or improving existing capacity should be investigated. Marked improvement may be obtainable by proper entrance design. All culverts shall be designed for possible extension unless there are extenuating circumstances. 4.040 Bridges Bridges shall be designed to meet the current criteria of the governing agencies. 4.040.01 Waterway Capacity and Backwater Effects Sufficient capacity will be provided to pass the runoff from the design storm determined in accordance with principles given elsewhere in this manual. Revised 10/15/12 65 4.040.02 Clearance The lowest point of the bridge superstructure shall have a (freeboard) clearance of two (2) feet above design water surface elevation for the 15-year frequency in St. Louis County (20-year frequency in the City of St. Louis) and one (1) foot for the 100-year frequency. 4.040.03 Waterway Alignment The bridged waterway will be aligned to result in the least obstruction to stream flow, except that for natural streams consideration will be given to future realignment and improvement of the channel. 4.040.04 Erosion Protection To preclude failure by scouring, abutment and pier footings will usually be placed either to a depth of not less than five (5) feet below the anticipated depth of scour, or on firm rock if such is encountered at a higher elevation. Large multispan structures crossing alluvial streams may require extensive pile foundations. To protect the channel, revetment on channel sides and/or bottom, consisting of concrete or grouted rock blanket should be placed as required. The governing authority should be contacted regarding their design requirements. 4.050 Outlet Erosion Protection If outlet velocities exceed 5 fps, an appropriate erosion protection must be provided. Erosion protection may be required at outlets where velocities are less than 5 fps if soil conditions warrant. For paved channels a cutoff wall will be required at the termini with appropriate protection. The cutoff wall shall extend a minimum depth of two (2) feet into the existing ground line. 4.060 Limitations on Areas Draining Across Sidewalks or Driveways [Deleted by Amendment 4] In the City of St. Louis, per Ordinance No. 60664, up to three thousand (3,000) square feet of parking area may discharge via a driveway to the public or private street. An additional three thousand (3,000) square feet may discharge into a public alley. Areas larger than this must have any excess area discharge into interceptor basins as set forth in the (City of St. Louis) Plumbing Code. In unincorporated areas of St. Louis County, area inlets shall be required to intercept overland flows greater than 1 cfs to prevent that flow from crossing sidewalks or curbs. 4.070 Impervious Areas - In City of St. Louis [Deleted by Amendment 4] Any area which is to be paved, repaved, expanded or otherwise improved, that is over three thousand (3,000) square feet in area, whether presently paved or not, shall at such time as it is to be paved, repaved, expanded or be otherwise improved, be provided with storm water drainage facilities constructed in accordance with plans and specifications submitted to and approved by the District. Such approval is a pre-requisite of the City of St. Louis for acceptance of an application for the paving or any other construction necessary to bring an impervious area into compliance with City of St. Louis Ordinance 52552 as amended. Any impervious area, less than three thousand (3,000) square feet in area, shall not require on-site storm water drainage facilities. The area draining toward the rear alley is counted separately from the area draining toward the street, in actual practice, if both are receiving flow. 4.080 General Performance Criteria for Stormwater Management 4.080.01 When Required 1. The requirements of stormwater quantity and quality management shall be evaluated for all projects submitted to the District for review and approval. Stormwater management facilities shall be provided and designed in accordance with the requirements of this section. If another Revised 10/15/12 66 local jurisdiction requires more stringent design standards, then they shall govern in that locale. A Stormwater Management Facilities (BMP) Operation and Maintenance Design Report and Plan, including specific continuing resources, procedures and schedules to be used, shall be submitted for approval. If required and approved, the Plan shall be included in a recorded Maintenance Agreement by reference. 2. Stormwater quantity and quality management requirements shall be evaluated for all projects, and specifically, will be required for projects including: a. All new development and redevelopment projects that disturb greater than or equal to one acre, including projects less than 1 acre that are part of a larger common parcel or project that is greater than one acre. However, existence of downstream stormwater problems may require quantity detention on the proposed site, where less than 2 cfs differential is proposed. b. Projects which have a differential runoff of 2 cfs or greater for the 15-year, 20-minute event (in St. Louis County).The differential runoff is calculated by the Rational Method using PI factors. Subsequent development or redevelopment of sites without prior stormwater detention shall provide detention or retention, when cumulative differential increase, since January 15, 2000, equals 2 cfs or greater. Projects with prior detention shall provide additional detention or retention for increasing runoff irrespective of the 2 cfs threshold. The degree of commonality between subsequent or concurrent projects, sites or parcels within same watershed shall be as determined by the District for purposes of this section. 3. When existing stormwater management facilities are going to be used to accommodate additional runoff from building or parking lot expansions or subdivision additions, the facilities shall be retrofitted to meet the current stormwater management requirements for the drainage area, served by the facility. Projects, which cannot meet this requirement due to physical constraints, will be evaluated for alternatives on a case by case basis. 4. The stormwater design projects within designated levee districts such as Monarch-Chesterfield, Earth City, Howard Bend, and Riverport will be based on the Stormwater Master Plan for these districts. If the Stormwater Master Plan does not address water quality, the requirements of this manual shall apply. 5. [Added by Amendment 3. See Section 16] Controls shall be designed and implemented to prevent or minimize water quality impacts by reasonably mimicking pre-construction runoff conditions on all “new development” projects to the maximum extent practicable. (This is a Small MS4 NPDES Stormwater Discharge Permit requirement. It necessitates controls and practices that reduce runoff volume through infiltration, evapotranspiration, and/or rainwater harvesting.) On “redevelopment” sites, controls shall be designed and implemented to prevent or minimize water quality impacts by effectively utilizing water quality strategies and technologies, including those that reduce runoff volume, to the maximum extent practicable. When micro-detention is required in the combined sewer area to address sewer capacity problems, these controls should also apply runoff reducing strategies and technologies. Sites with 20 percent or less existing imperviousness are considered new development for determining if the “mimicking pre-construction runoff conditions” requirement is applicable. Any subsequent or additional development or expansion projects on those sites will also be considered new development. Subdividing does not affect that requirement. Revised 10/15/12 67 4.080.02 Unified Stormwater Sizing Criteria 1. General This section presents a unified approach for sizing stormwater Best Managements Practices (BMP’s) to meet pollutant removal goals, reduce channel erosion and prevent flooding and pass extreme floods. A very brief summary is listed below. SUMMARY OF THE KEY COMPONENTS AND STORMWATER CRITERIA Water Quality Volume (WQv ) (acre-feet) WQv = [(P)(Rv)(A)]/12 P = rainfall depth in inches and is equal to 1.14 Rv = volumetric runoff coefficient, and A = area in acres Channel Protection Storage Volume (Cpv) Cpv = 24 hour extended detention of post-developed one-year, 24 hour storm event Flood Protection Volume (Qp2 & Qp100) The post-developed routed peak flow from the site may not exceed the existing routed peak flow for the 2-year and 100-year, 24-hour events, or the allowable release rate for differential runoff greater than 5 cfs. The following sub-sections provide more expanded information, directories, explanations and resource references. 2. Water Quality Volume (WQ v ) [See Amendment 4] a. The Water Quality Volume (denoted as the WQ v ) is the storage needed to capture and treat the runoff from 90% of the recorded daily rainfall events. In numerical terms, it is equivalent to 1.14 inches of rainfall multiplied by the volumetric runoff coefficient (R v ) and site area. The WQ v is directly related to the amount of impervious cover created at a site. A minimum WQ v of 0.2 inches per acre shall be met at all sites where WQ v is required. b. As a basis for determining water quality treatment volume the following assumptions may be made: (1) The water quality volume WQ v for offsite areas is not required. The following equations are used to determine the storage volume, WQv (in acre- feet of storage): WQ v = [(P)(Rv)(A)]/12 P = 1.14 inches of rainfall Where: WQ v =water quality volume (in acre-feet) R v =0.05 + 0.009 (I) where I is percent impervious cover A =drainage area to BMP in acres (2) Measuring Impervious Cover: The measured area of a site plan that does not have vegetative or permeable cover shall be considered total impervious cover. (3) Multiple Drainage Areas: When a project contains or is divided by multiple drainage areas, the WQ v volume shall be addressed for each drainage area. (4) Offsite Drainage Areas: The WQ v shall be based on the impervious cover of the proposed site. Offsite existing impervious areas may be excluded from the calculation of the water quality volume requirements. Revised 10/15/12 68 (5)(4) BMP Treatment: The final WQ v shall be treated by an acceptable BMP(s) from the list presented in Section 4.080.05, or as approved by the District. (6)(5) Subtraction for Non-structural Practices: When non-structural practices are employed in the site design, the WQ v volume can be reduced in accordance with the conditions outlined in Section 4.080.06. (7)(6) Extended Detention for Water Quality Volume: The water quality requirements can be met by providing a 24-hour draw down of a portion of the water quality volume (WQ v ) in conjunction with a stormwater pond or wetland system. Referred to as ED, this is different than providing the extended detention of the one-year storm for the channel protection volume (Cp v ). The ED portion of the WQ v may be included when routing the Cp v . 3. Channel Protection Storage Volume Requirements (Cp v ) [See Amendment 1, Chapter 16] a. General To protect channels from erosion, a 24-hour extended detention of the one-year, 24-hour storm event shall be provided. The rationale for this criterion is that runoff will be stored and released in such a gradual manner that critical erosive velocities during bankfull and near-bankfull events will seldom be exceeded or prolonged in downstream channels. A detention pond or underground vault is normally needed to meet the CP v requirement (and subsequent flood protection criteria Q p2 and Q p100 ). b. As a Basis for determining Channel Protection Storage Volume the following assumptions may be made: (1) The model TR-55 (or approved equivalent) shall be used for determining peak discharge rates. (See 4.080.02 Paragraph 4.b.(1) for additional TR-55 information. (2) The rainfall depth for the one-year, 24-hour storm event is 2.50 inches. Use Type II rainfall distribution. (3) The length of overland flow used in time of concentration (t c ) calculations is limited to no more than 100 feet for post project conditions. (4) The 24-hour extended detention is defined as providing a 24-hour detention lag time (T) for the one-year storm. The lag time is defined as the interval between the center of mass of the inflow hydrograph and the center of mass of the outflow hydrograph. (5) A Cp v orifice diameter of less than 3.0” will require a special internal control for orifice protection. For Cp v orifice between 3” and 1 1/2” diameter, an internally controlled orifice shall be used with slot width less than or equal to 1/3 of orifice diameter. Less than 1 ½” orifice will not be allowed. (6) Cp v shall be addressed for the entire site. If a site consists of multiple drainage areas, Cp v may be distributed proportionately to each drainage area. (7) Extended detention storage provided for the Cp v does not fully meet the WQ v requirement (that is Cp v and WQ v should be treated separately). Revised 10/15/12 69 (8) The stormwater storage needed for Cp v may be provided above the WQ v storage in stormwater ponds and wetlands; thereby meeting all storage criteria in a single facility with appropriate hydraulic control structures for each storage requirement. (9) Infiltration is not recommended for Cp v control because of large storage requirements. If proven effective, appropriate and desirable however, in some rare situations it may be permissible. c. Exemptions To protect channels from erosion, a 24-hour extended detention of the one-year, 24-hour storm event shall be required at all sites that do require Flood Volume Detention (Qp). Also, the existence of downstream stormwater problems may require treatment for channel protection (CP v ), regardless of any other possible exemptions. At some sites however, the provision of traditional extended detention (Cp v ) may not be effective or may be best achieved by other means. Specifically, the following sites may be exempted from the channel protection storage requirement: A.) For sites 5 acres or larger, exempt if: 1.) No detention for Flood Protection Volume (Qp) is required, and 2.) The project is a redevelopment site (at least 20% of the existing site was impervious coverage as of January 15, 2000) and 3.) The watershed is a less sensitive, i.e. “zero increase” watershed (per Table 4-5) And either 4.) or 5.) below 4.) Surface BMP’s are utilized to treat the required WQ v and more pervious area is created on the site and within the tributary area. OR 5.) Underground devices are utilized to treat the required WQ v but the proposed site will be at least 20% more pervious than the existing site. B.) All sites less than 5 acres will normally be exempt. C.) For all sites, regardless of size, exempt if any of the following apply: 1.) The project site will be 100% pervious, and does not increase annual or more frequent discharge from site (for a 1-year event per TR-55). 2.) The project is a redevelopment site (at least 20% of the existing site was impervious coverage as of January 15, 2000) and the project reduces the site impervious area to less than 10% of the total site area. 3.) The project is located within levee district boundaries or within the very flat portions of the FEMA defined flood plain of the Mississippi, Missouri, or Meramec Rivers. 4.) The project is located upstream of permanent lakes, concrete lined channels, or enclosed pipe systems. However, the presence of any intervening reach or downstream, natural channel which does need extended detention channel protection may then nullify this exemption. Revised 10/15/12 70 a. To protect channels from erosion, a 24-hour extended detention of the one-year, 24-hour storm event shall be provided. The rationale for this criterion is that runoff will be stored and released in such a gradual manner that critical erosive velocities during bankfull and near-bankfull events will seldom be exceeded in downstream channels. A detention pond or underground vault is normally needed to meet the CP v requirement (and subsequent flood protection criteria Q p2 and Q p100 ). As a Basis for determining Channel Protection Storage Volume the following assumptions may be made: (2) The model TR-55 (or approved equivalent) shall be used for determining peak discharge rates. (See 4.080.02 Paragraph 4.b.(1) for additional TR-55 information. (2) The rainfall depth for the one-year, 24-hour storm event is 2.50 inches. Use Type II rainfall distribution. (3) The length of overland flow used in time of concentration (t c ) calculations is limited to no more than 100 feet for post project conditions. (10) The 24-hour extended detention is defined as providing a 24-hour detention lag time (T) for the one-year storm. The lag time is defined as the interval between the center of mass of the inflow hydrograph and the center of mass of the outflow hydrograph. (11) Cp v is not required at sites where the one-year post development peak discharge is less than or equal to 2.0 cfs. A Cp v orifice diameter of less than 3.0” is not allowed. (12) Cp v shall be addressed for the entire site. If a site consists of multiple drainage areas, Cp v may be distributed proportionately to each drainage area. (13) Extended detention storage provided for the Cp v does not fully meet the WQ v requirement (that is Cp v and WQ v should be treated separately). (14) The stormwater storage needed for Cp v may be provided above the WQ v storage in stormwater ponds and wetlands; thereby meeting all storage criteria in a single facility with appropriate hydraulic control structures for each storage requirement. (15) Infiltration is not recommended for Cp v control because of large storage requirements. If proven effective, appropriate and desirable however, in some rare situations it may be permissible. 4. Flood Protection Volume Requirement (Q p2 & Q p100 ) a. To protect downstream areas from flooding stormwater shall be detained on site or offsite as approved and released at a rate not to exceed the allowable release rates for the 2-year and 100-year 24-hour events, as determined by the District for the watershed in question. The allowable release rates have been determined by watershed modeling (see Table 4- 5). The engineer has the option to calculate a site specific release rate based on procedures provided by the District's Engineering Department (Volumetric Method). Note that stormwater pipes, downstream from the control structure, shall be sized to carry the runoff from the 15-year 20-minute design storm for the total tributary upstream watershed. No reduction in outfall pipe size shall be permitted because of detention. Revised 10/15/12 71 b. As a Basis for Determining the Flood Protection Volume the following assumptions may be made. (1) The 2-year and 100-year, 24-hour inflow hydrographs shall be determined by using Technical Release 55 (TR-55), "Urban Hydrology for Small Watersheds" from the Natural Resources Conservation Service, formerly Soil Conservation Service (SCS). The inflow hydrograph shall be developed based on the actual flow and timing characteristics upstream of the detention facility. The rainfall distribution shall be Type II. The rainfall quantities to be used are from the Illinois State Water Survey Bulletin 71, and shall be as follows: 3.1” for the 2- year, 24-hour storm and 7.2” for the 100-year, 24-hour storm. (2) The volume of detention may be provided through permanent detention facilities such as dry basins or ponds, permanent ponds or lakes, underground storage facilities or in parking lots. The engineer shall make every effort to locate the detention facility at or near the lowest point of the project such that all of the on- site runoff will be directed into the detention facility. Multiple use of detention basins is encouraged. Multiple use may include parking lots, ball fields, tennis courts, play grounds and picnic areas. This is subject to the approval of the District. Flows from offsite, upstream areas should be bypassed around the detention facility to ensure that the proposed detention facility will function as designed and will provide effective control of downstream flows with development in place. If offsite flows are directed into a detention facility, the allowable release rates shall not be modified without District approval. Modifying the release rate to accommodate offsite flows may reduce or eliminate the effectiveness of the detention facility, because it will no longer control the increased volume of runoff during the critical time period of the watershed. As stated in Item 4.a above, the engineer has the option to calculate a site specific release rate based on procedures provided by the District's Engineering Department. The engineer shall provide detailed modeling to prove that the increase in runoff volume has been limited to existing conditions during the critical time period of the watershed (For Volumetric Method see Appendix – forthcoming). (3) Detention basin volume will be based on routing the post-developed 2-year and 100 year, 24-hour inflow hydrographs through the detention facility while satisfying the appropriate allowable release rate. The routing computations shall be based on an application of the continuity principle, (i.e., level pool routing). Municipalities or St. Louis County may by ordinances, have other volume requirements. The more stringent volume requirements will govern. 4.080.03 Limits of Maximum Ponding in Stormwater Ponds 1. The maximum ponding elevation shall be calculated based on a routing of the design storm (100-year, 24-hour event) assuming the low flow outlet is blocked with water ponded to the overflow structure’s sill. 2. The limits of maximum ponding in dry basins or ponds and permanent lakes or ponds shall not be closer than thirty (30) feet horizontally to any building, and not less than two (2) feet vertically below the lowest sill elevation of any building. Revised 10/15/12 72 3. The limits of maximum ponding in parking lots shall not be closer than ten (10) feet horizontally from any building and not less than (1) foot vertically below the lowest sill elevation of any building. 4. A minimum of one (1) foot of freeboard shall be provided from the top of the basin to the maximum ponding elevation. 5. Micro-Detention will be looked at case by case but in general the following apply: a. 12” Freeboard Requirement Exceptions allowed: (Freeboard 6” or less) (1) Upper terraces of multiple in-line detention basin(s). (2) If the basin’s outfall is directed onto a natural water way with no downstream property to be impacted. This could be obtained by providing swales and/or using existing swales that are around the downstream perimeter of the site and which direct flows to natural waterways or property with no future build out possibilities (6” freeboard still preferred). b. Other Exceptions: 30’ Requirement for Horizontal Distance From HW Elevation (1) If the basin is very shallow (2) If the basin’s H.W. elevation is 2’ below the low sill of building. It is recommended that additional construction and grading notes be put on the plans, as well as a landscape schedule of plantings. Grading tolerances should be kept at + 0.1 and as-builts provided. Infiltration rates should be accurate for basins without outfall structure. Copy of maintenance and operations notes should be included on the plans. 4.080.04 General Stormwater Basin Design Requirements 1. Underground Basins’ Special Requirements: a. Adequate access for basin maintenance and inspection shall be provided. A means of visual inspection from the ground surface of the low flow device, overflow weir, and outlet structure is necessary. Access shall also be provided to allow for cleaning of the low flow device from the ground surface. b. The basin should have sufficient volume and spillway capacity to pass/contain the 100-year 24-hour event with the low flow outlet blocked. In some situations it may be desirable to have control structures with at least 2 outlet openings, one above the other. c. Underground basins shall be acceptable for non-residential projects only, except by special municipal request, cooperation and a lack of other technical options. (See also sub-section 4.080.05.1.c also). d. Acceptable materials for underground basins are poured in place reinforced concrete, RCP and aluminized CMP. The CMP gauge shall be approved by the District prior to installation. Restricted use of other systems may be allowed in the future. Revised 10/15/12 73 e. Provide immediate manhole access from ground surface for both sides of the low flow device. Also provide a manhole at upstream end of underground basins, for access, inspection, to facilitate maintenance and air release. f. Adequate flowline spot elevations, sections and profiles including pipe length and slope shall be labeled to define basin and pipe geometry. 2. General Detention Report Requirements For detention facilities in general, the Engineer must submit 2 copies of a report sealed by a Missouri Registered Professional Engineer containing the following for review of the detention facility: (see 4.100 also) a. Elevation vs Discharge tables or curves for all frequencies. b. Elevation vs Storage tables or curves for all frequencies. c. Inflow calculations and data for all frequencies. d. Hydraulic gradeline computations for pipes entering and leaving the basin for all frequencies. e. If the embankment contains fill material a geotechnical report may be required. f. Site plan showing appropriate design information. g. Structural calculations for the outlet control structures (if required). h. Cross sections defining the size, shape and depth of the detention basin shall be required. At a minimum, three sections, one at each end and one in the middle of the basin will be required. 3. All ends of pipes discharging into a dry basin or pond shall be connected with the low flow pipe or control structure, by means of a permeable swale. The swale shall a minimum 4:1 lateral (25%) slope to the center and a minimum 2.0% longitudinal slope to ensure positive drainage. Swales shall be a minimum of six (6) inches deep and four (4) feet wide or 1.3 times the diameter of the pipe entering the basin, whichever is greater. . The bottom of the basin shall be sloped a minimum of two percent (2%). towards the edge of the swale. No concrete swales will be allowed unless approved by the District. Permanent erosion control protection must be provided at the ends of discharging pipes. Lengths of the swale in which the velocity exceeds 2 feet per second analyzed by the 15 year –20 minute storm event, shall have appropriate permanent erosion control protection. Aggregate, porous concrete blocks or appropriate vegetation are required, unless otherwise approved. 4. Railroad tie walls cannot be used where water will be in contact with the railroad tie wall. 5. Permanent detention ponds or lakes are to be designed to minimize fluctuating lake levels. Maximum fluctuation from the permanent pool elevation to the maximum ponding elevation shall be six (6) feet. 6. The maximum side slopes for dry basins or ponds, and the fluctuating area of permanent ponds or lakes shall be 3:1 (three feet horizontal, one foot vertical) without fencing. 7. Dry basins or ponds and the fluctuating areas of permanent ponds or lakes are to be appropriately vegetated to the maximum high water elevation. . Areas above that elevation Revised 10/15/12 74 shall be appropriately stabilized and vegetated. Sod and mowing above that elevation may be approved and is required for dam embankment slopes and downstream toe areas for wet basins where riprap is not appropriate. 8. Control structures and overflow structures are to be reinforced concrete, including precast. Only for underground proprietary or aluminized CMP basins may other approved materials be allowed. 9. The outflow pipe shall be sized for the developed flow rate. 10. In basins with concrete walls or rip rap covered slopes, provisions should be made for mowing equipment to reach the bottom (ramps, etc.). 11. Maximum Depths: a. The maximum depth of water in a dry detention basin or pond shall not exceed eight (8) feet. Projects which need a deeper basin to attain the required detention volume due to physical constraints may be evaluated on a case by case basis. The design and construction of dams greater than eight (8) feet or as directed by the District must be sealed and certified by a Professional Engineer registered in the State of Missouri with demonstrated expertise in geotechnical engineering. b. Parking lots used for automobiles shall have a maximum depth of eight (8) inches of water. c. Parking lots used for trucks or truck trailers shall have a maximum depth of water of twelve (12) inches. 12. Detention Basin Fencing A four (4) foot (minimum height) approved fence shall be provided around the perimeter of any basin where the side slopes exceed 3:1 (three (3) feet horizontal, one (1) foot vertical). Fencing such as post and rail, or fencing which prevents easy observation of required detention basin maintenance, such as tall privacy fencing, should not be used. 13. Detention Basin Elevation If the detention basin discharges to a piped sewer system, the low elevation of the detention basin shall be above the 15-year, 20-minute hydraulic elevation of the receiving storm system, or the 20 year, 20 minute hydraulic elevation of the receiving combined systems, as applicable. Basin backflow contamination must be prevented, if the downstream combined system is known to surcharge. 14. If the detention basin discharges to an open channel, or to a piped sewer system affected by flood levels in a nearby downstream open channel, then the low elevation of the basin is desirable to be above the 100-year flood elevation in the open channel as established by the FEMA Flood Insurance Study or the District SSMIP, whichever is greater. 15. In all cases mentioned above, if the low elevation of the basin is below the receiving system hydraulic grade or channel flood elevation, then the basin shall be sized to store the entire design storm volume, unless directed otherwise by the District. 4.080.05 Acceptable Urban BMP Options [See Amendment 4] 1. This section sets forth five acceptable groups of BMPs that can be used to meet the Water Quality volume criteria (WQ v ). The design and selection of these BMPs shall comply with the Maryland Stormwater Design Manual, Volumes I & II (October 2000; Revised 10/15/12 75 effective date July 1, 2001), as prepared by the Center For Watershed Protection and the State of Maryland Department of the Environment (MDE). Where the District criteria or requirements are more stringent, then they shall govern. Adaption to local Missouri environment and natural conditions should be expected but shall be as approved by the District or a higher authority. The Manual can be purchased through MDE’s website. A simple search for Maryland Stormwater Design Manual will provide a direct link. a. The acceptable BMP designs are assigned into five general categories for stormwater quality control (WQ v ): BMP Group 1 stormwater ponds BMP Group 2 stormwater wetlands BMP Group 3 infiltration practices BMP Group 4 filtering practices BMP Group 5 open channel practices b. To be considered an effective BMP for stand-alone treatment of WQ v , a design shall be capable of: 1. capturing and treating the required water quality volume (WQ v ) 2. removing 80% of the TSS, and 3. having an acceptable longevity rate in the field c. A combination of BMPs and/or credits is normally required at most development sites to meet all three stormwater sizing criteria. d. NOTE: Groundwater sump pump discharge may be problematic if directed towards storm water BMPs. In general this practice should be avoided unless otherwise allowed by the District. 1. BMP Group 1. Stormwater Ponds a. Practices that have a combination of permanent pool, extended detention or shallow wetland equivalent to the entire WQ vs include: P-1 micropool extended detention pond P-2 wet pond P-3 wet extended detention pond P-4 multiple pond system P-5 pocket pond 2. BMP Group 2. Stormwater Wetlands a. Practices that include significant shallow wetland areas to treat urban stormwater but often may also incorporate small permanent pools and/or extended detention storage to achieve the full WQ v include (Modification of existing wetland areas will require a CWA 404/401 permit: Corps 404 permit): W-1 shallow wetland W-2 ED shallow wetland W-3 pond/wetland system W-4 pocket wetland b. U.S. Army C.O.E. Jurisdictional Wetlands shall not be used for control of water quantity (i.e. the flood protection volume). Wetlands shall not be used for control of water quantity (i.e. the flood protection volume). Revised 10/15/12 76 c. Wetlands shall be designed by a qualified and experienced team. 3. BMP Group 3. Infiltration Practices a. Practices that capture and temporarily store the WQ v before allowing it to infiltrate into the soil over a two day period include: I-1 infiltration trench I-2 infiltration basin b. Infiltration practices will be allowed on sites where it is proven that infiltration will work. This must be supported by a soils report. 4. BMP Group 4. Filtering Practices a. Practices that capture and temporarily store the WQ v and pass it through a filter bed of sand, organic matter, soil or other media are considered to be filtering practices. Filtered runoff may be collected and returned to the conveyance system. Design variants include: F-1 surface sand filter F-2 underground sand filter F-3 perimeter sand filter F-4 organic filter F-5 pocket sand filter F-6 bioretention* F-7 proprietary filtering system *may also be used for infiltration b. Filtering practices may be allowed on commercial projects. They are not allowed on residential projects. c. A maintenance agreement and maintenance schedule shall be required. 5. BMP Group 5. Open Channel Practices a. Vegetated open channels that are explicitly designed to capture and treat the full WQ v within the dry or wet cells formed by checkdams or other means include: 0-1.01 dry swale 0-2 wet swale 0-3 filter strips b. Open channel practices shall be designed with the proper plantings. They are not allowed on single-family residential projects. They may be allowed on condominium or apartment projects if maintenance is provided by a management company. c. Wet swales shall be designed to drain out over time. 4.080.06 Stormwater Credits Non-Structural BMPs are increasingly recognized as a critical feature of stormwater BMP plans, particularly with respect to site design. In most cases, non-structural BMPs shall be combined with structural BMPs to meet all stormwater requirements. The key benefit on non-structural BMP is that they can reduce the generation of stormwater from site; thereby reducing the size and cost of structural BMPs. In addition, they can provide partial removal of many pollutants. The non-structural BMPs have been classified into seven broad categories. To promote greater use of non-structural BMPs, a series of credits and incentives are provided for developments that use these progressive site planning techniques. Further Revised 10/15/12 77 details can be found in Chapter 5 of the Maryland Stormwater Design Manual, Volumes I & II (October 2000). • natural area conservation • disconnection of rooftop runoff • disconnection of non-rooftop impervious area • reserved buffers • open channel use • environmentally sensitive development • impervious cover reduction 4.080.07 Easement Required In subdivisions, the detention basin, BMP’s, access roads or paths, control structures, and outfall pipes are to be located in easements dedicated to the subdivision trustees. Lack of appropriate easement(s) will not relieve trustees of responsibility for required maintenance of the Stormwater Management System BMP’s. 4.080.08 Maintenance Agreement Prior to plan approval the property owner(s) of the Detention Basin site(s) shall execute a District Maintenance Agreement for the urban BMPs and the detention basin or pond to insure the urban BMPs and the detention area will be kept in working order, to the satisfaction of the District. The District will not be responsible for maintenance of detention basins or BMPs. Annual trustee or non-residential Property Owner’s certification and reporting of performance of required maintenance, operation and repairs shall commence upon MSD Construction Approval of detention facilities; final closeout of the subdivision or project SWPPP; or as otherwise specified. The annual report will be required for those projects where the recorded Maintenance Agreement requires the reporting directly or by reference included in the Agreement (see Sub-section 4.080.01 Item 1. regarding need for BMP Maintenance Plan). The annual report shall be submitted to the Engineering Department, Design Division, Development Review at 2350 Market St., St. Louis MO, 63103 (return receipt requested). 4.090 Dam Permit Requirements Dams with a height of thirty five (35) feet or greater will require approval from the Missouri Department of Natural Resources. 4.100 Detention Report 1. A Detention Report shall be submitted. 2. The Detention Report shall contain a complete table of contents and a summary. 3. The Detention Report shall be signed, dated and sealed by the Missouri Professional Engineer who is responsible for its preparation. If the report is prepared by another person, a note on the cover shall state the preparer’s name and that he or she is under the direct supervision of the Missouri Professional Engineer whose seal is shown. 4. The Detention Report shall be in a binder, not loose leaf. 5. The engineer shall submit two copies of the Detention Report. 6. A copy of the current “Checklist for Review of Storm Water Detention” shall be completed by the design engineer and submitted with the reports. The checklist is shown on Exhibit 4-A hereinafter, or on the website if updated in the future. Revised 10/15/12 78 EXHIBIT 4-A Check List for Review of Stormwater Detention The following check list is intended to provide guidance in reviewing detention designs. It is not intended to supersede any criteria stated in the District’s “Rules and Regulations and Engineering Design Requirements for Sanitary Sewage and Stormwater Drainage Facilities”, February 2006. the District P-No Project Name: Location: 1. Check planning maps for downstream problems. 2. Check SSMIP report for downstream problems. 3. Detention required if differential runoff, per 4.080.01.2(b), is greater than 2 cfs, or if required by the District due to flooding problems. 4. If differential runoff is greater than 5 cfs, use watershed release rates if applicable (see release rate table, figure 4-5) _______ 5. For building and parking lot additions, detention is required if there is existing detention. Check existing calculations to see if addition is covered. 6. No reduction in outfall pipe size permitted because of detention. 7. Basin located at or near lowest point of site such that on-site runoff will be directed to basin. 8. Offsite flows bypassed around basin. _______ 9. If site specific release rate is used (volumetric procedure), no increase in runoff volume during the critical time period of the watershed. 10. Underground basin has adequate access for maintenance. 11. Provide means of visual inspection of both sides of low flow device from ground surface for underground basin. 12. Underground basin has volume and spillway capacity to pass/contain 100-year, 24-hour event with low flow blocked. It is further recommended that the control structure have at least 2 openings, one above the other. 13. Check flow capacity of downstream pipe with inlet control nomographs found in Hydraulic Design of Highway Culverts, U.S. Department of Transportation publication. _______ 14. Check inlet control constants as entered in the analysis to match the Hydraulic Design of Highway Culverts per US Department of Transportation. 15. Aluminized corrugated metal pipe (CMP) allowed for commercial projects only, gauge specified. 16. No underground basins allowed for residential projects, except by special municipal request, cooperation and a technical lack of other options. Revised 10/15/12 79 17. TR-55 or similar SCS method used for hydrology; Type II rainfall distribution. 18. 2-year and 100-year, 24-hour rainfall amounts of 3.1” and 7.2”, respectively. _______ 19. Legible Detention Basin Area maps showing flow paths used in time of concentration calculations and CN values for existing and proposed conditions, 24” X 36” exhibits are preferred, (Existing Conditions Map is not applicable to a “Fixed” Release Rate Analysis). 20. Elevations vs. Discharge table, including modeling data. 21. Elevation vs. Storage table. 22. Hydraulic gradeline calculations for incoming and outgoing pipes. 23. Provide a copy of the SCS soils map and label site. 24. Geotechnical report may be required for embankment. 25. Structural calculations may be required for control structure. 26. Details of control structure showing reinforcing. _______ 27. Minimum of 3 cross sections through basin for as-built calculations tied to a baseline or known point or to Property Line. 28. Incoming pipes should discharge at the toe of the slope in dry basins. 29. Earthen Pilot swale provided from incoming pipes to control structure. 30. Earthen Pilot swale is a minimum of 6 inches deep. 31. Details for permanent erosion control for earthen swales with slopes greater than 3%. 32. Minimum longitudinal slope of earthen swale is 2.0%, slope called out on plan. 33. Bottom of basin is sloped a minimum of 2% laterally towards earthen swale and called out on plan. 34. Rock blanket provided along outside of curved swale downstream of incoming pipe to prevent erosion. 35. Concrete headwall provided around protruding low flow pipe. 36. Trash rack provided for low flow openings less than 6” wide. 37. No railroad tie walls within ponding area of basin. 38. Maximum fluctuation above permanent pool is 6’. 39. Maximum side slopes are 3:1 without fencing. _______ 40. Dry basins and the fluctuating areas of lakes are to be appropriately vegetated to the maximum high water elevation, call out on plan. Sod and mowing may be approved above that level and on dam slopes (required, if not riprapped). _______ 41. Control structures are to be reinforced concrete; no brick allowed; wall thickness is at least 8” w/one row of steel or 10” w/two rows of steel. Underground basins, as appropriate. 42. In basins with walls, provide access ramp. Revised 10/15/12 80 43. No wetland mitigation in detention basin. 44. Maximum depth of water in a dry basin is 8’ exceptions on a case-by-case basis. 45. Maximum depth of water in a parking lot is 8”, 12” for truck parking lots. 46. Maximum ponding elevation calculated with low flow blocked and water ponded to sill of overflow structure. _______ 47. Limits of maximum ponding are 30’ horizontally and 2’ vertically from a building; 10’ horizontally and 1’ vertically for parking lot detention. 48. Freeboard from top of berm to maximum ponding elevation is at least 1’. 49. Basin is located in common ground or easement dedicated to subdivision trustees. 50. Owners of the basin execute a District Maintenance Agreement. 51. Four foot high fence required if side slopes are steeper than 3:1. 52. Low elevation of basin is above 15-yr, 20-min hydraulic elevation of receiving system; or detain whole storm and provide a backflow preventer (Tideflex) normally installed inside of the outfall structure. 53. Dams with a height of 35’ or greater require MDNR approval. _______ 54. Hydraulic calculations showing 100-year flow is conveyed to basin; calculations at ditch sections, sills of structures set above 100-yr elevation. 55. Smallest low flow opening is 3” diameter or 4” x 2” slot. However, special trashrack or opening protection may be required. 56. Detention cannot cross watershed boundary. 57. Discharge pipe into wet basin shall be a minimum of 3’ above bottom, or flowline of pipe shall be no higher than the normal pool elevation. 58. The report is sealed by a Missouri Professional Engineer. 59. The report has a Table of Contents, a summary and is bound. _______ 60. The starting hydraulic grade line for all incoming pipes shall be the 100 year – 24 hour blocked low flow water surface elevation, or an elevation approved by the District. _______ 61. For some channel and wetlands work, a 404 and/or 401 permit may be required from the Corps and MoDNR, respectively. Revised 10/15/12 81 Revised 10/15/12 82 Revised 10/15/12 83 Revised 10/15/12 84 Revised 10/15/12 85 TABLE 4-5 [See Amendment 4] WATERSHED RELEASE RATES FOR DESIGN OF ROUTED DETENTION FACILITIES FOR DEVELOPMENTS WITH A POST-DEVELOPMENT DIFFERENTIAL RUNOFF GREATER THAN 5 CFS* Note: *Differential runoff based on the 15-year, 20 minute storm. See “Rules and Regulations and Engineering Design Requirements for Sanitary Sewage and Stormwater Drainage Facilities”, Chapter 4, Section 4.080.02. Differential runoff of 5 cfs or less, use “Zero Increase” for all watersheds and both routing frequencies. **Differential runoff and detention routings should be computed from a completely undeveloped condition for projects in the Harlem & Baden watersheds, Grand & Bates (March 9, 2001) sewershed, and other sewersheds as may be directed by the District, in the City of St. Louis. System capacity has been found overcharged for rainfall events as small as once every two (2) years. Physical limitations of the site may be considered in special cases. **Projects which have any increase in differential runoff in the Harlem & Baden watersheds, Grand & Bates sewershed, and other sewersheds as may be directed by the District will have stormwater management requirements as shown in items A.) through D.) as follows: WATERSHED FINAL ROUTED RELEASE RATE 100-YEAR 24 HOUR STORM FINAL ROUTED RELEASE RATE 2-YEAR 24 HOUR STORM VOLUMETRIC METHOD CRITICAL TIME PERIOD OF WATERSHED Baden ** Zero Increase Zero Increase NA Bonfils (Cowmire) 1.0 cfs/acre 0.4 cfs/acre Hour 12.0-14.0 Bonhomme 1.8 cfs/acre 0.25 cfs/acre Hour 12.0-15.5 Caulks 1.4 cfs/acre 0.2 cfs/acre Hour 12.0-14.0 Coldwater Zero Increase Zero Increase NA Creve Coeur 1.2 cfs/acre 0.13 cfs/acre Hour 12.0-17.2 Deer Zero Increase Zero Increase NA Dunn 1.0 cfs/acre 0.4 cfs/acre Hour 12.0-13.0 Fee Fee 1.3 cfs/acre 0.15 cfs/acre Hour 12.0-14.1 Fenton Zero Increase Zero Increase NA Fishpot 1.5 cfs/acre 0.3 cfs/acre Hour 12.0-14.7 Grand Glaize Zero Increase Zero Increase NA Gravois Zero Increase Zero Increase NA Harlem ** Zero Increase Zero Increase NA Kiefer 2.2 cfs/acre 0.7 cfs/acre Hour 12.0-13.0 Maline Zero Increase Zero Increase NA Martigney Zero Increase Zero Increase NA Mattese Zero Increase Zero Increase NA Mill 1.5 cfs/acre .13 cfs/acre Hour 11.5-15.5 River Des Peres Zero Increase Zero Increase NA Spanish Lake 1.0 cfs/acre 0.37 cfs/acre Hour 11.0-16.0 University City Zero Increase Zero Increase NA Watkins Zero Increase Zero Increase NA Williams 0.7 cfs/acre 0.2 cfs/acre Hour 12.0-15.1 Yarnell 1.3 cfs/acre 0.3 cfs/acre Hour 12.0-14.0 Revised 10/15/12 86 STORMWATER MANAGEMENT REQUIREMENTS FOR HARLEM AND BADEN WATERSHEDS AND OTHER WATERSHEDS AS DIRECTED BY THE DISTRICT A.) For project differential increase 0 to < 2 cfs: Volume reduction type BMPs sized for 75% of the 90th % rainfall 1.14” WQv calculation as shown in 4.080.02.2.b(1). In general, the minimum amount of tributary impervious area to be treated by the BMPs shall be the acreage of added impervious area. The BMPs shall be designed to capture 75% of the WQv from the total drainage area tributary to the BMP. The BMP shall be designed to drain down completely in 2 days. The BMP overflow path shall be designed for the 100-year, 20- minute storm. For an example, see MSD website BMP toolkit. B.) For project differential increase 2 cfs or >: TR55 analysis for 2-year and 100-year events, with level pool routing to meet release rates as specified in Table 4-5. C.) In the case of new directly sewer connected roof area replacing existing pavement area, MSD will evaluate these on a case-by-case basis. D.) For all cases, where localized downstream flooding or localized downstream sewer surcharging occurs, differential runoff calculations and detention routings shall be computed from a completely undeveloped condition as directed by the District. Revised 10/15/12 87 Revised 10/15/12 88 Revised 10/15/12 89 Revised 10/15/12 90