Agenda - Council Work Session - 02/23/2021

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may not be counted toward infiltration volume requirement because evaporation does not yield water quality benefits, achieve groundwater recharge goals, and because it is highly variable based upon climatological conditions. Infiltration Facilitv Desi )n and Construction The chapters on Infiltration in the Minnesota Stormwater Manual should be used as a reference in the design and construction of infiltration basins for the stormwater management. This includes, but is not limited to, soil borings, period of inundation, pretreatment, and protection during construction. Robust pretreatment to remove suspended solids is especially important to long term functioning of the practice. Infiltration rates used in design of infiltration practices shall be either those in the MN Stormwater Manual (see Table 1 below) or those measured on -site by a double -ring infiltrometer. A maximum infiltration rate of 3 inlhr is allowed. Rates used for design higher than those listed in Table 1 below shall be verified by post -construction testing. TABLE 1. Infiltration Rates for Hydrologic Soil Groups Hydrologic Soil Group Soil Textures Gravel, sand, sandy gravel, silty gravel, loamy sand, sandy loam oam, silt loam Sandy clay loam Corresponding Unified Soil Classification GW — Well -graded gravel or well - graded gravel with sand GP — Poorly graded gravel or poorly graded gravel with sand GM — Silty gravel or silty gravel with sand SW — Will -graded sand or well -graded sand with gravel SP — Poorly graded sand or poorly graded sand with gravel ylylyl-www www www SM — Silty sand or silty sand with gravel MI— Silt OL — Organic silt or organic silt with sand or gravel or gravelly organic silt nfiltration Rate (in/hr) 1.6 GC — Clayey gravel or clayey gravel with sand SC — Clayey sand or clayey sand with gravel Comprehensive Watershed Management Plan Lower Rum River Watershed Management Organization October 2011 IF 11 11 01 0.2 E-6