Subdivision Stormwater Management Report Service

As part of the subdivision professional engineering services RMA prepares a Stormwater Management Report that is typically submitted to the local jurisdictional authority.

The study examines the pre-developed and post-developed site hydrology for 1, 2, 10, 25, 50 and 100-year 24-hour rainfall events, and shows that using detention pond, the post-development peak dis-charge rates will not increase above the pre-developed rates for each study point evaluated in the hydrological study area.

For determining overall flows and detention pond requirements, the Soil Conservation Service (SCS) Method (TR-55) is used to develop hydrographs for estimating stormwater runoff from the site for 1, 2, 10, 25, 50, and 100-year storm frequencies. The “Georgia Stormwater Management Manual” is used as a guide in evaluating and assigning Curve Numbers, “C” values, and Manning’s “n” values. The SCS TR- 55 method is used to evaluate Times of Concentration (Tc). AutoDesk “Hydraflow Hydrographs” is used to compute pre-developed and post-developed hydrographs for the different drainage areas within the site and the total basin. “AutoDesk Hydraflow Storm” is used to evaluate minor watersheds draining to proposed catch basins and the storm piping system. The “Rational Method” is used only to evaluate flows in these minor basins. Finally, “AutoDesk Hydraflow Hydrographs” is used as an aid in evaluating detention pond size and type of outlet control device. All evaluated storm events are routed through a pond to determine adequacy of the pond and structures, and to ensure that post-developed flows were at and below pre-developed flows. The USGS mapping is used in evaluating 10% drainage basin. On site analysis is based on a ground run topographical survey while offsite inflow areas are delineated per the local jurisdictional authority.

Pre-Developed Site Conditions — For the purpose of the hydrology study, the site is subdivided into multiple basins. A description of each pre-development drainage basin and outfall is determined. A pre-developed drainage map is also developed.

Post-Developed Site Conditions — The proposed site conditions consists of a fully development subdivision with paved access streets and driveways. Peak discharge rates are calculated at the Study Point for each basin. A description of the post-development of each drainage basin and outfall is determined. A post-developed drainage map is also developed.

Extended Dry Detention Pond — A proposed detention pond is typically required so that the post-development peak discharge rates will not increase above the pre-developed rates for each study point evaluated in the hydrological study area. Based on the pre-developed and post-developed site conditions the location and volume of the detention pond can be calculated. An outlet control structure de-signed to restrict peak flow rates and an emergency overflow weir to facilitate additional flows are designed as part of the detention pond facility.

Downstream Summary — A downstream analysis is necessary to insure that after development the storm water leaves the site in the same way prior to the development. This is performed by evaluating the calculated run-off values prior to the development and downstream of the development. As part of this analysis a downstream study is performed summarizing the per and post development peak flows for the for 1, 2, 10, 25, 50 and 100-year 24-hour rainfall events.

It is the requirement that the post-development flows are below the pre-construction levels for each study point and for the whole development at the downstream point, therefore there is no anticipated detri-mental impact to any surrounding property or downstream drainage systems, natural or manmade. The stream receiving channel will be undisturbed, therefore all discharge velocities will not increase from pre-development level.