07-22-1985 Council Packet - Laserfiche WebLink

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RICE CREEK WATERSHED DISTRICT WETLAND PRESERVATION GUIDELINE Marshes (wetlands) are important to the environment of the Rice Step 1: Calculate the area of each proposed land -use in the Creek Watershed District. basin which drains to the wetla.td. Wetland vegetation hzs the natural capacity to assimilate Step 2: Calculate the nutrient load (lb./year) generated by the nutrients (nitrogen and phosphorus) washed from the the watershed that drains to the wetland according surrounding watershed by storrtrwater runoff. to the'ultimate land -use. See Table 1. Wetlands provide a natural means of temporarily storing stormwater runoff to minimize the potential for flooding downstream. The Rice Creek Watershed District Board of Managers has set the general policy that requires all property improvements and land ' developments that involve wetland alteration to preserve enough wetland to: 1. At its ultimate development, assimilate the nutrient load from the total area that drains to the wetland. 2. Provide adequate storage for stormwater such that the runoff rate from ?he watershed does not increase due to development of the land, based on a 100 year- 24 hour duration rainfall event. Step 3: Calculate the nutrient assimilative capacity of the ' + wetland. See Table 2. Step 4:' Balance the lost assimilative capacity per acre of fill plus the additional loading due to the use of that area with the surplus nutrient assimilative capacity of the wetland. This can be written in the following equation: Total Assimilative Capacity — Total Loading from Fillable of Existing Wetland ultimate Runoff Acres Assimilative Capacity of f Ultimate Loading Due . Wetland (per acres to Use (per acre) + B. stormwater Storage Determination The acreage of the wetland that meets these requirements is called the Minimum Wetland Preservation Area. This pamphlet describes s " ; In Order •to calculate the minimum area needed to store how the Minimum Wetland Preservation Area is determined using runoff and to minimize potential downstream flooding, a set of generalized parameters for nutrient generation and assimi- the following procedure has been developed. lation, soils, slope and land use. However, there may be occasions ,when the preservation of a unique natural resource or a unique Runoff Determination development may require an analysis of an additional set of para- R = CPA meters to determine the Minimum Wetland Preservatioo Area. R = Runoff in acre-feet MINIMUM WETLAND PRESERVATION AREA CALCULATION • • There are two considerations when the Minimum Welland Preser- vation Area is determined: the area necessary to assimilate nutri- ent loads and the area necessary to minimize the potential of downstream flooding. The larger of the two areas determin�,l considered the Minimum Wetland Preservation Area. A. Nutrient Assimilation Capacity and Possible Fill Determine tion P = Total Rainfall for 100-year return ftequencyi24-hour duration rainfall r •, . event in feet (normally 0.5 feet) C r Runoff Coefficient from Table 4 s• • A = Drainage Area in acres Step 1: Calculate the runoff for the entire watershed pr.or to all land development. Step 2: Calculate the runoff for the entire watershed considering the ultimate land use Step 3. Calculate the, storage required in the d,flerence To calculate the mr- a needed to a<s,m,ldte nutr, between the ultrmdte runwf and the ur.developed ents washed off lh+ a and to deter+r•j•,e possible runoff plus the rauif I'll or, the wetland, w.ihuut fill the following pi, s been devriuped causing flood Jamdge ,-(-W)d tl,r , +rid