§ 2.2.1. Design Assumptions For Storm Runoff Analysis

A.

When analyzing an area for channel or storm drain design purposes, urbanization of the full watershed without stormwater detention facilities shall be assumed (except as noted in (D.) below). Zoning maps, future land use maps, and master plans should be used as aids in establishing the anticipated surface character of the ultimate development. The selection of design runoff coefficients and/or percent impervious cover factors are explained in the following discussions of runoff calculation.

B.

An exception to (A.) above may be granted if the channel is immediately downstream of a City maintained regional detention facility and written approval is obtained from the Director of the Watershed Protection Department.

C.

In designing a storm drain system, full development of adjoining and interior tracts without detention shall be assumed.

D.

In the event the engineer desires to incorporate the flow reduction benefits of existing upstream detention facilities, the following field investigations and hydrologic analysis will be required: (Please note that under no circumstances will the previously approved construction plans of the upstream detention facilities suffice as an adequate analysis. While the responsibility of the individual site or subdivision plans rests with the engineer of record, any subsequent engineering analysis must ensure that all the incorporated detention facilities work collectively.)

1.

A field survey of the existing physical characteristics of both the outlet structure and ponding volume. Any departure from the original engineer's design must be accounted for. If a dual use for the detention facility exists, (e.g., storage of equipment) then this too should be accounted for.

2.

A comprehensive hydrologic analysis which simulates the flow attenuation produced by the existing detention facility in the upstream contributing area. This should not be limited to a linear additive analysis but rather a network of hydrographs which considers incremental timing of discharge and potential coincidence of outlet peaks.