(A) Peak discharge. Site runoff control for large storms, up to the 100-year event, is essential to protect against immediate downstream erosion and flooding.
(1) Post-development discharge rates for new developments and redevelopments shall not exceed the existing condition discharge rates.
(2) Any concentrated stormwater discharges leaving the site must be conveyed into an existing channel, storm sewer, or overland flow path with adequate downstream stormwater capacity and will not result in increased erosion, flood damage, or other drainage hazards. In addition to discharge rate, demonstration of this shall include a comparison of existing and post-development velocity, volume, and hydrograph at the location where stormwater leaves the site. Post-development velocity and runoff volume shall be less than 110% of the existing conditions for the 100-year critical duration storm.
(3) The design of stormwater management systems shall not result in any transfer of water between watersheds unless no reasonable alternative exists as determined by the City Engineer.
(B) Calculation methodology.
(1) Unless a continuous simulation approach to drainage system hydrology is used, all design rainfall events shall be based on the Illinois State Water Survey's Bulletin 70.
(a) The first quartile point rainfall distribution shall be used for the design and analysis of conveyance systems with critical durations less than or equal to six hours.
(b) The second quartile distribution shall be used for storms with durations greater than six hours and less than or equal to 12 hours.
(c) The third quartile point rainfall distribution shall be used for the design and analysis of detention basins and conveyance system with critical durations greater than 12 and less than or equal to 24 hours.
(d) The fourth quartile distribution shall be used in the design and analysis of systems with durations greater than 24 hours.
(e) The NRCS Type II distribution may be used as an alternate to the Huff distributions.
(f) The total rainfall value for the design storm shall be adjusted for the “St. Louis Urban Effect” as given in Table 4, Illinois State Water Survey Circular 172.
(2) The rational method may be used to calculate discharges for drainage areas of 20 acres or less and the storm water conveyance (i.e. storm sewer) design. The rational method shall not be used to determine detention storage requirements.
(a) TR-55 may be used to calculate discharges for areas that drain less than 100 acres. TR-55 with a pond-routing program such as TR-20 or HEC-1 may be used to confirm stormwater storage requirements.
(b) TR-20, WinTR-20, HEC-1, HEC-HMS, or a city approved hydrograph producing a hydrotogic model shall be used to determine the peak runoff rates for areas with a drainage area greater than 100 acres and to confirm the stormwater storage requirements for facilities that have a drainage area of ten acres or more.
(c) Runoff calculations for all off-site tributary land shall be based on either the anticipated future land use conditions or existing land use conditions as approved by the City Engineer.
(C) Antecedent moisture. Average antecedent moisture conditions shall be assumed when calculating runoff curve numbers for use in the NRCS (SCS) TR-55 method.
(Ord. 2017-03, passed 3-6-2017)