07-15-1985 Planning Packet - Laserfiche WebLink

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• RICE CREEK WATERSHED DISTRICT WETLAND PRESERVATION GUIDELINE Marshes (wetlands) are important to the environment of the Rice Creek Watershed District. Step 1: Calculate the area of each proposed land-use in the basin which drains to the wetland.II •t Wetland vegetation has the natural capacity to assimilate ‘ the nutrients (nitrogen and phosphorus) washed from the surrounding watershed by stormwater runoff. • % . Step 2: Calculate the nutrient load {lb./year) generated by • the watershed that drains to the wetland according s to the ultimate land-use. See Table 1. • i • • I \ \ . Wetlands provide a natural means of temporarily storing stormwater runoff to minimise 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 '<' • • j. .._______________________. ..•_____________j ___________________...______________________ __________: developments that involve wetland alteration to preserve enough * wetland to: Step 3: Calculate the nutrient assimilative capacity of the . *•* V; / » wetland. See Table 2. . * *. • . • •• « •><. *• • * . , ^ f » , / ^ V • • V .' * • ♦ ». * , . ' • '*.*•*, ' \ • '*.«•*, . • ' ■ • # Step 4:’ Balance the lost assimilative capacity per acre of t * * , . V . fill plus the additional loading due to the use of . ’ v • that area with the surplus nutrient assimilative capadty of the wetland. This can be written in ] • • • the following equation: • ^ \ K. * * * * ' ■ * » w • . */ ‘t \ f I .' * j ‘ apment, assimilate the nutrient r,’:‘ ‘V j1. • At its ultimate development ' load from the total area that drains to the wetland.Total Assimilative Capacity — Total Loading from Finable of Existing Wetland Ultimate Runoff2. , ^ Provide adequate Storage for stormwater such that ‘ ____P±K^L^llDRJ!}L^^tiand____ultimate^Hun^ott i - ‘ • •’ the runoff rate from the watershed does not increase •''* f. Assimilative Capacity of ' ■+ Ultimate Loading Due . ^ • due to development of the land, based on a 100 year-5 ^.'Wetlarnd (per acre) ‘ . to Use (per acre) | • 24 hour duration rainfall event. •' ^ • ■* I ; • . ’i • .. ' B. , Stormwater Storage Determination \ The acre^e of the wetland that meets these requirements is called ?.• -j. i *. ^ . the R4inimum lA/etland Preservation Area. This namrihtet rtesi-fiHi^s i In orde' .K a ..a. ..aw .a ..a..a .. the Minimum Wetland Preservation Area. This pamphlet describes , j ‘ ... ’ how the Minimum Wetland Preservation Area is determined using a set of generalized parameters for nutrient generation and assimi- .*. lation, soils, slope and land use. However, there may be occasions .when the preservation of a unique natural resource or a unique development may require an analysis of an additional set of para- / rder to calculate the minimum area needed to store V» ': runoff and to minimi?e potential downstream flooding, r • • j the following procedure has been developed. . . • . • t I*/• '. t • • • I•. • - * - t t < » « . :• .meters to determine the Minimum Wetland Preservation Area. % f • • MINIMUM WETLAND PRESERVATION AREA CALCULATION ‘ ^ ^ Runoff Determination R = CPA • • •, / R ^ Runoff in acre-feet ‘ P ■ Total Rainfall for 100 year return • • • ) . • \ _ • ■ • . . . ‘'frequency/24-hour duration rainfall .. ( .Ievent in feet (normally 0.5 feet)•. • .V ... • I' V; There are two considerations when the Minimum Wetland Pieser- ^ ^ •, '/• , •' ; * . * ' /*• ■ . vation Area is determined: the area necessary to assimilate nutri- A = Drainage Area in acres is . ’ * / ' T . Vi * * • . r , , . . • • • \ T. ent loads and the area necessary to minimize the potential of « V . j "a . .V * • - " .. VVownstream flooding. Tha larger of the two areas determined is ‘ 1: Calculate the runoff for the entire.watershed prior , • *'considered the Minimum Wetland Preservation Area. ‘ 1o*l| l^nd development. • . , . .. • • ’ *•*.» • . .. ' • ► 9 \ -• • • » •. • •• •.. ; A. * Nutrient Assimilation Capacity and Possible Fill Determine- % • . . tion N . Step 2: Calculate the runoff for the entire watershed ( '*yconsidering the ultimate land use. » I • I ^ • *• « • • . * • •* •To calculate the minimum area needed to assimilate nutri- • ents v^ashed off the land surface and to determine possible fill the following proceduie has been developed: Step 3: Calculate the storage required in the difference between the ultimate runoff and the undevelop>ed , runoff plus the rainfall on the v/etland, without •* cjusing flood damage around the wetland. J