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14. APPENDIX – VOLUMETRIC METHOD
Introduction
The volumetric design procedure is to be used in cases where detention cannot be designed using the simple
maximum release rate procedure and involves detailed modeling of the local tributary drainage system. The
volumetric approach would be necessary to do the following:
• Determine the release rate when there are off-site upstream flows that cannot be bypassed around the
detention facility.
• Determine whether a detention rate less than the prescribed watershed release rate might meet the
planning criteria.
• Evaluate “interim” conditions when a regional detention facility is planned.
• Determine the detention basin release needed to compensate for development areas that cannot be
drained to the detention pond.
The volumetric procedure has been demonstrated to lead to detention basin designs that are consistent with the
objectives and criteria established for the Stormwater System Master Improvement Plan (SSMIP).
Procedures
Regulatory Requirements and Approach
Prior to conducting the analysis, the regulatory requirements for the watershed should be determined. This is
best accomplished by meeting with MSD staff. At the meeting, the following information can be obtained:
• Planning watershed available information.
• MSD regulatory release rate.
• MSD special requirements due to special watershed problem or capacity limitations.
• Availability of MSD SWMM model at the project location.
The meeting will assist the engineer in determining whether the volumetric approach is required and which
computer model strategy is appropriate. An appropriate model such as SWMM, HEC-1, or TR-55 plus a
routing package must be used.
Analysis
The following steps outline the basic modeling procedure to develop or verify a volumetric-based release rate for a
detention facility. Some of this information may not be needed for simple cases:
1. Delineate sub-basins
a. Proposed development area draining to pond.
b. Proposed development area not draining to pond (should be minimized).
c. Off-site areas draining to proposal pond.
d. Off-site areas draining to proposed bypass.
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2. Define hydrologic parameters, such as area, % impervious, curve number, sub-basin width and time of
concentration. Requirements depend on whether SWMM, HEC-1, or TR-55 is being used. Determine
parameters for:
a. Existing land use.
b. Post development land use.
3. Determine design rainfall distributions for the 2-year and 100-year 24-hour storm events using the
SCS TYPE II distribution.
4. Run Model for 100-year rainfall and obtain the following runoff hydrographs:
a. Existing land use hydrograph for area to be developed plus off-site upstream area draining to
detention basin.
b. Future land use hydrograph for development plus off-site areas (using existing conditions)
draining to detention basin.
c. Future land use hydrograph for area to be developed but not draining to detention basin.
d. Future conditions flow to be bypassed.
5. Determine Critical Time Period
The critical time period will be based on SWMM model results developed during the MSD watershed
planning. The critical time period will be determined by MSD using the following definitions:
a. Start of Critical Time Period is the time of peak runoff from the proposed development under
future land use conditions
b. End of the critical time period is the peak of the existing conditions hydrograph at the
downstream outlet of the watershed or as determined by previous study.
If the SWMM watershed model is not yet available then an approximate critical time period will be defined as
starting at the time of peak rainfall intensity with a duration equal to the time of concentration for the planning
watershed.
6. Determine Critical Time Period outflow volume.
This is the existing conditions runoff volume from the area to be developed plus any runoff from
offsite area that will be routed through the detention basin. The volume is computed based on flow
during the Critical Time Period from the hydrograph in step 4a. above. The runoff volume during the
critical time period must be limited to this volume using detention.
7. Obtain first estimate of the release rate.
The Critical Time Period outflow volume divided by the length of the critical time period in seconds is
the average release rate allowed during the critical time period.
8. Obtain first estimate of required storage.
The required storage is the difference between average release rate in 7. above and overall future
hydrograph. (The overall future hydrograph may the sum of hydrographs 4b and 4c).
9. Conduct analysis of future detention.
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The hydrograph from step 4b. is routed through the proposed detention basin. The detention basin
outflow is then combined with hydrograph from step 4c. if there is one. This is the proposed site
outflow hydrograph. The site outflow volume during the critical time period must be less than or equal
to the existing conditions volume for the same drainage area. Adjust outlet structure and storage
volume until site outflow volume during critical period is equal to existing conditions target volume.
10. Repeat step 4 for the 2-year, 24-hour storm hydrograph. Adjust the outlet design as needed to limit
2-year flows to the prescribed maximum release.
11. Plot performance results
Plot hydrographs of existing conditions flow (4a.) and proposed site outflow (routed pond outflow
plus additional site runoff). The plot should show the critical time period and the target volume
expressed as an average outflow.