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HomeMy Public PortalAbout460848Revised 10/15/12 91 5. FLOOD PLAIN REQUIREMENTS 5.010 Flood Plain Study/100 – Year Hydraulic Study 5.010.01 When Required 1. A flood plain study will be required for all tracts or parcels of ground located in the 100-year flood plain shown on the current Federal Emergency Management Agency's (FEMA) Flood Boundary and Floodway Map. In incorporated areas, a letter will be required from the municipality stating that the project is in compliance with the municipality’s flood plain ordinance. 2. A 100 – year hydraulic study will be required for any watercourse, not identified by FEMA, for which the 15-year, 20-minute design storm exceeds the capacity of a (60) inch diameter sewer. 3. A flood plain study/100 – year hydraulic study will be required when development of any type, i.e., new construction, additions to existing facilities, rehabilitation of existing facilities, or site grading is planned within an area subject to 100-year flooding. 5.010.02 Criteria For Preparation 1. The flood plain study /100 – year hydraulic study shall be submitted by a Professional Engineer, registered in the State of Missouri. Each page of the study shall be signed and sealed. a. All assumptions shall be clearly stated in the report. The modeling process, review of existing data, necessary background, purpose, flooding potential of the existing land, existing or previous studies, existing computer model input and output, proposed conditions computer model input and output, 100-year and 15-year water surface elevations, cross-sections and profiles should all be included in the report. An electronic copy of the models shall be submitted on a disk. b. Sufficient hydraulic computations shall be submitted for review to verify the 100-year and 15-year water surface elevations with the proposed development in place. The computations should also verify that the conveyance characteristics of the water course are not restricted by the proposed development when compared with the pre-developed conditions. c. Flood plain elevations and design of bridges and culverts will be reviewed by the District using the flow and elevation numbers that provide the most protection to the public. The FEMA flow/elevation will be used for those instances where these values will provide the most protection for the residents and public since that is the official guide. However, since the FEMA numbers were developed based on an empirical equation representing all the St. Louis area and upon existing conditions at the time of the FEMA modeling (late 70’s and early 80’s generally) they may not accurately reflect the current the current situation and knowledge. The Stormwater System Master Improvement Plans (SSMIP) contain more current information based upon SWMM modeling having been performed during the 1995-1999 time period and are based upon actual gauged or measured rainfall/runoff to calibrate the model. Consequently, the SWMM flow/elevation numbers in several instances provide a better estimate of actual current conditions within the watersheds. Therefore, the District will use the more conservative flow/elevation values for each location (either FEMA or SWMM depending on which provides the most conservative value) on all review of future projects. Revised 10/15/12 92 2. A Metropolitan St. Louis Sewer District bench mark and U.S.G.S. datum shall be referenced and shown on the study with reference made to the on-site temporary bench mark. 3. The study shall determine the 100-year high water elevation through the site using the elevations obtained from the most recently published Federal Emergency Management Agency Flood Insurance Study or, if more conservative, from the District’s SSMIP watershed studies. 4. The method used to determine the water surface elevation shall be dependent on the stream characteristics. a. FEMA Streams For streams for which a detailed Flood Insurance Study has been performed by FEMA, the water surface elevation shall be computed by the same method as used in the FEMA Flood Insurance Study. b. Non-FEMA Streams The method used to determine the water surface elevation for streams for which a hydraulic model has not been developed by FEMA, shall depend on the conveyance characteristics of the stream. For generally prismatic streams with a mild, constant slope, the water surface elevation shall be computed using the direct-step method. For non-prismatic streams with varying channel shape or slopes, the standard- step method shall be used. Both the direct-step and standard-step methods are described in detail in Open Channel Hydraulics by Ven Te Chow, a McGraw-Hill work published in 1959. 5. The determination of a starting water surface elevation used in the hydraulic computations must be stated in the study. The high water elevations shall be checked against data showing downstream influences, such as culvert restrictions and/or flood elevations of larger streams or rivers. The downstream and upstream study limits shall be measured from the most downstream and upstream edges of the development. The hydraulic computations shall extend a minimum of one hundred (100) feet beyond the cross section where the post and pre-development water surface elevations are coincident. There may be situations where the post and pre-development water surface elevations are not coincident. In those situations, the post development water surface elevation must be lower than the pre-development water surface elevation. Also, the hydraulic slope and energy grades should not indicate a possible rise in water surface elevation beyond the post development water surface elevation within the one hundred (100) feet study limits. A sufficient number of cross sections should be utilized within the study limits to accurately determine the water surface elevation. The District reserves the right to extend the study limits. 6. If a new channel or channel realignment is proposed which would require a FEMA map amendment, the municipality must submit to FEMA all information required by the current FEMA "Conditions and Criteria for Floodway Revisions." Those projects, which require map revisions, will not be approved until a conditional letter of map revision (CLOMR) is provided by FEMA to the municipality. Revised 10/15/12 93 7. If the development is in an area where no discharge data exists, the Engineer shall use the following guidelines for establishing the discharge for the study: a. Determine the 100-year discharge value from the last known discharge, at most upstream portion of the receiving stream, as shown in the Flood Insurance Study (FIS). b. Determine the 100-year discharge by using the Rational Method as outlined in the Storm Drainage section of this manual. c. The 100-year Rational Method discharge shall be used until that value equals the FIS discharge determined above d. If the 100-year discharge determined by the District’s SSMIP watershed study is more conservative than the above value, then the SSMIP discharge shall be used. 8. For flood plain areas not covered by FEMA regulations, the District will not allow offsite rise in the 100-year water surface elevation due to an encroachment in the channel or overbanks. 5.010.03 Filling in Flood Plain Areas 1. If a tract of land, located in the flood plain, has been the subject of unauthorized fill, the flood plain study shall be based on ground conditions prior to placement of unauthorized fill. 2. Placement of fill shall be in accordance with FEMA guidelines as allowed by the governing municipality's flood plain ordinance and consistent with any other governmental restrictions. In Unincorporated St. Louis County, the floodway fringe may be filled without providing equivalent storage in the flood plains of the Missouri River, the Mississippi River and the Meramec River. However, there may be the need for wetland delineation first, since flood plain areas have high probability for wetlands. 3. Placement of fill, roadway embankments or placement of buildings may occur within areas designated as a floodway on the Flood Insurance Floodway maps, but only as allowed by municipal Floodplain Ordinance and supported approved calculations. 5.010.04 Detention Basins in Flood Plains Detention basins will be allowed in flood plain areas if all other flood plain requirements are satisfied. 5.010.05 Bridges in Flood Plains 1. Bridges, culverts, roadway embankments or any encroachment planned in the floodway shall not increase the 100-year water surface elevation adjacent to or beyond the project site. 2. Bridges in flood plain areas shall be designed on a 100-year storm frequency and shall provide a minimum of one (1) foot of freeboard between the bottom of the superstructure and the 100-year water surface elevation. 5.010.06 Parking Lots in Flood Plains 1. No parking lots will be allowed within the 15-year flood plain. Revised 10/15/12 94 5.010.07 Site Plan Requirements 1. The site plan shall show existing and proposed contours, existing and proposed 100-year floodway limits, and existing and proposed 100-year high water elevations as a minimum. The contour interval shall be two (2) feet or less. The site plan shall also show locations of the proposed encroachment and structure(s), together with existing topography. 2. The site plan shall show the entire flood plain (not just the project site area) a minimum of two hundred (200) lineal feet downstream and upstream of the proposed project. 3. Cross section locations and alignment shall be shown on the site plan. The sections should be located perpendicular to the flow in both the channel and overbanks. The sections shall be taken and plotted left to right looking downstream. The flood profile shall be plotted for inclusion in the study. 4. Cross sections shall show the following items: a. 100-year water surface elevation. b. Manning's "n" values c. Top of bank locations d. Horizontal stationing, vertical elevations e. Existing and proposed ground lines f. Existing and proposed sewers g. Cross section identification as it relates to the site plan and hydraulic study h. Existing or proposed structures i. Floodway limits 5. Crossroad culverts in the flood plain shall be designed on a 100-year storm frequency and provide a minimum of two (2) feet of freeboard at the road shoulder line. 6. Basement elevations, with the exception of walkouts, must be one (1) foot above the 100-year flood elevation. Basements outside the flood plain limits may be below the 100-year flood elevation if a sanitary pump is installed in the basement with the outgoing plumbing located above the 100-year elevation. 7. The lowest finished floor elevation of all new or redevelopment structures, including slab on grade, shall be minimum of one (1) foot above the 100-year flood elevation. . The low sill of all structures in or adjacent to the floodplain shall be a minimum of two (2) feet above the 100-year flood elevation. 8. Channel and overbank (flood plain) velocities should generally not increase by more than fifty percent (50%) over pre-developed conditions. Velocities in excess of five (5) feet per second (fps) in channels shall require erosion protection.