10.26.2021-SBN-Standards-Manufacturer-Additions-Minor-Revisions-final

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SECTION 3 – STORMWATER 22 storage facility is dry within 48 hours of a 100-year storm event. All wet storage facilities (retention basins) shall be designed to resume the designed water elevation within 48 hours of a 100 -year storm event. The storage volume provided by a storage facility is a function of the water elevation. A stage (elevation) storage table should be calculated using the average area method. The following is a formula for the average area method. The interval between intermediate elevations should not exceed one foot. V= storage volume 𝑉=∑∆𝐻(𝐴1+𝐴2) 2 ∆𝐻=Elevation2-Elevation1 A= surface area at a given elevation The cumulative storage should be calculated by summing the storage volume at each elevation. For wet storage facilities, only the volume provided above the ordinary water level is considered in the storage volume calculations. A stage storage table showing all storage above the ordinary water elevation should be shown and the total volume of that storage should be equal to or greater than the calculated storage volume required. 3-1.4 CALCULATION OF REQUIRED STORAGE VOLUME: STANDARD METHOD Due to uncertainties of future development patterns and the associated proposed stormwater quantity and quality management systems within a watershed, it is the policy of the City of South Bend to discourage release of runoff from a new development or redevelopment other than by infiltration up to the 100-year storm event. However, the City Engineer may approve a controlled release from a storage facility when the engineer can show that the downstream receiving system has adequate capacity to accept the calculated discharge from the storage facility. At no time shall an increased rate of discharge be directed to a combined sewer system or storm sewers that drain to combined sewers. 3-1.4 (a) CONTROL RELEASE CALCULATIONS 1.) The same hydrologic method shall be used for runoff peak determination for both pre - developed and post-developed condition calculations. 2.) Recommended hydrologic methods i.The Rational Method is acceptable for calculating peak flows when the total tributary drainage area is 5 acres or less and the tributary drainage area has no existing depressional storage. Runoff coefficients for analysis are shown in Table 2 and are adopted from the INDOT Design Manual. Table 2: Runoff Coefficients for Rational Method Land Use Runoff Coefficient “C” Park, Lawn, Grassy Area 0.2 Residential Single Family 0.4 Apartment or Multi-Unit 0.7 Business, Commercial or Industrial 0.8 Pavement 0.9