Saturated Hydraulic Conductivity Rates
A professional geotechnical engineer, geologist, or soil scientist licensed in Utah must identify basin location and perform site-specific geotechnical investigation in accordance ASTM D5856 to determine the infiltration rate for the selected site. This applies to native material below the pond and any topsoil or landscaping material that will be placed to obtain a true post-construction infiltration rate.
Retention is a feasible option when the soil where the retention is planned has "saturated infiltration rate" such that the volume of the 1%, 24-hour chance of occurrence storm can be drained in 48-hours or less. For a retention basin the following condition application must be met.
Infiltration Test Results and Recommended Rates
Planned retention basins must have the native soil below the proposed basin identified according to (USCS) (ASTM D4318). Use the table below for the design infiltration rate. Provide hydraulic conductivity testing of in situ soils according to ASTM D5856 if applicant believes in situ soils differ from this table. The results must be accepted by the city engineer. A Safety Factor of 2.5 is required for the infiltration rates listed in this table.
NRCS Hydrologic Soil Group Typical Soil Texture Saturated Infiltration Rate (mm/h)* Saturated Infiltration Rate (in/h)* Porosity Field Capacity A Sand 200 8.0 0.437 0.062
A Loamy Sand 50 2.0 0.437 0.105 B Sandy Loam 25 1.0 0.453 0.190 B Loam 12.7* 0.5 0.463 0.232 C Silt Loam 6.3* 0.25 0.501 0.284 C Sandy Clay Loam 3.8* 0.15 0.398 0.244 D Clay Loam & Silty Clay Loam <2.3 <0.09 0.465 0.325 D Clay <1.3 <0.05 0.475 0.378
Soils must meet drain time requirements of the City Storm Water Design Standards