2022 08 Drainage Report

wv. Conduit outlet protection shall be provided in accordance with that Standard and may include: flat aprons, preformed scour holes, impact basins, stilling wells, plunge pools, etc. Level spreaders are not an acceptable design. The velocities existing the proposed flared end sections are within the allowable velocity in Table 12-1 of 5.0 fps as detailed below. The highest proposed velocity rate is 4.80 fps. = WW, VI E ¥ SOIL TEXTURE ALLOWABLE VELOCITY (fi/sec.) Sand 18 Sandy loam 25 Silt loam (also high lime clay), loam 3.0 Sandy clay loam 35 Ta 4.0 (Clay. fine gravel, loam to gravel 5.0 obbles: 3.3 Shale (non-weathered) 60 —— ——— v Topography shows broad uniform outlet area where flows will not concentrate. The topography downstream of the point of discharge is broad and generally uniform and flows will not concentrate. vi, Discharge locations shall contain perennial, erosion resistant vegetation. The discharge location contains perennial, erosion resistant vegetation. vii, Peak discharge velocities (in the last pipe section) do not exceed 2 fps for the 10-year storm. The maximum peak 10-year flow from any of the four (4) proposed basins is 0.20 CFS. As shown in the enclosed stormwater conveyance calculations, the velocity in the last pipe section is 0.13 fps. vil. The maximum length of slope below the outlet(s) is 100 feet. The proposed basin discharge is tributary to the downstream area and therefore this criterion is satisfied. Downstream of the Point of Dischar; ESite Stability Analysis, In addition to ensuring erosion does not occur at the point of discharge, areas downstream and beyond the immediate area of site development may be damaged due to erosive forces resulting from extended duration of hydrograph peak flows. An unintended consequence of the practice of detaining and slowly releasing stormwater is the ability for peak flows to be sustained Sor longer periods of time, offering an opportunity for upstream discharges to coincide with project site discharges. The resulting combined discharge may be equal to or even exceed that of the pre-development condition. To limit the potential for such an occurrence the designer may choose either of two approaches for downstream stability protection: 1. Examine hydraulic characteristics of the receiving stream channel considering upstream discharge in combination with site discharge to

ther of two approaches for downstream stability protection: 1. Examine hydraulic characteristics of the receiving stream channel considering upstream discharge in combination with site discharge to assess channel stability. The scope and scale of the analysis shall be appropriate to the scale of SHI Builders August 2022 DEC# 4266-99-001 -10-