2022 08 Drainage Report

Downstream (Off-Site) Stability Analysis Infiltration may be used to meet peak flow reduction requirements (outlined below) for the purposes of documenting stability of the downstream receiving channel, provided that the complete loss of infiltration function does not result in an increase in peak flow values above the predevelopment levels. No infiltration is proposed as part of the proposed development to reduce flow rates. Point of Discharge - Methods for Achieving Stability 1. No well-defined waterway below the point of discharge: There is no well-defined waterway below the point of discharge; therefore, this method will be used. Stability can be achieved by one of the following alternatives: A. Retain pre-existing runoff characteristics. Do no increase the amount and rate of runoff for the development and do not concentrate flows, Due to the increase in impervious surfaces for the proposed development, this alternative is not feasible; therefore, alternative “b” will be utilized as described below. B, Where there is no well-defined channel, no sandy condition, no trees or brush to substantially concentrate the flows and it can be reasonably assumed that the flow will disperse over a broad area. The combinations of slopes and soils in table 21-1 and the following criteria are considered stable for flows of 10 cfs or less for a 25 year, 24 hr design storm, SHI Builders August 2022 DEC# 4266-99-001 -8-