07/25/2001 Env Bd Packet - Laserfiche WebLink

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2/90 There is a need for evaluate a design storm (DS) and a freeboard evaluation storm (FES) to provide for cost effective improvements and adequate flood protection. The DS should give the water surface elevation for an event of a given probability of recurrence. The DS provides a realistic estimate of water surface elevations based on rainfall amounts, durations, distributions, and soil conditions for a given recurrence interval. The DS is then used to design the facility. Using the same criteria, the FES is defined as that storm used to estimate practical maximum water surface elevations for all possible rainfall amounts, durations, distributions and soil conditions for a given recurrence interval. The FES is used to evaluate freeboard. - BACKGROUND A series of questions and concerns led to the development of the DS /FES concept. They are summarized in the following paragraphs. 1. In the Minneapolis -St. Paul Metropolitan Area (Twin Cities) the local SCS has indirectly recommended the use of the Type I distribution. This is contrary to the national recommendation of a Type II. The term "indirectly recommended" is used since they have not officially endorsed the use of the type I, but have published "Hydrology Guide for Minnesota" which is based on the Type I distribution. The Engineer must determine which method is appropriate, adding to the uncertainty of design. 2. Recently, the State of Minnesota passed legislation requiring the formation of Watershed Management Organizations (WMO's), causing the preparation of surface water management plans for each watershed in the Twin City Area. The WMOs were charged with preparing overall management plans and reviewing the detailed plans developed by municipalities. Most of the WMO plans require cities to control the rates and volumes of discharge from the "critical storm" to the rates and volumes occurring from existing or undeveloped conditions. - "Critical storm" is most often defined as that 100 year frequency event causing the largest peak discharge rate and /or water surface elevation in a particular water body. This definition leaves the rainfall distribution question unanswered. No matter what event is used to analyze a watershed, a more critical event could be realized. The term "critical storm" is thus a vague definition of a principal element in design. 3. In the Twin Cities, several WMO's have expressed concern regarding the accuracy of high- intensity storms being modeled with a Type I distribution. The districts have generally recommended the use of a Type II distribution because if contains all rainfall depths form storm durations up to and including 24 hours. In small urban watersheds, short duration -high intensity storms most often produce the highest peaks. The Type II distri- bution is thought to adequately cover these short duration D - 8