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<sup>a</sup>This rate is consistent with the infiltration rate provided for the lower end of the Hydrologic Soil Group A soils in the Wisconsin Department of Natural Resources Conservation Practice Standard: Site Evaluation for Stormwater Infiltration.</font size>
 
<sup>a</sup>This rate is consistent with the infiltration rate provided for the lower end of the Hydrologic Soil Group A soils in the Wisconsin Department of Natural Resources Conservation Practice Standard: Site Evaluation for Stormwater Infiltration.</font size>
  
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Revision as of 21:10, 5 May 2014

Caution: The original Manual had two different infiltration rates for B soils (0.3 and 0.6 inches per hour). In spring of 2013 the infiltration rate for all B soils was changed to 0.3 inches per hour. Following a meeting of technical experts on December 16, 2013, a rate of 0.45 inches per hour was established for SM soils.

Design infiltration rates, in inches per hour, for A, B, C, and D soil groups. Corresponding USDA soil classification and Unified soil Classifications are included. Note that A soils have two infiltration rates that are a function of soil texture.*
Link to this table

Hydrologic soil group Infiltration rate (inches/hour) Soil textures Corresponding Unified Soil Classification
A
1.63a

gravel
sandy gravel

silty gravels

GW - well-graded gravels, sandy gravels
GP - gap-graded or uniform gravels, sandy gravels
GM - silty gravels, silty sandy gravels
SW - well-graded gravelly sands

0.8

sand
loamy sand
sandy loam

SP - gap-graded or uniform sands, gravelly sands

B
0.45 SM - silty sands, silty gravelly sands
0.3 loam, silt loam MH - micaceous silts, diatomaceous silts, volcanic ash
C
0.2 Sandy clay loam ML - silts, very fine sands, silty or clayey fine sands
D
0.06

clay loam
silty clay loam
sandy clay
silty clay
clay

GC - clayey gravels, clayey sandy gravels
SC - clayey sands, clayey gravelly sands
CL - low plasticity clays, sandy or silty clays
OL - organic silts and clays of low plasticity
CH - highly plastic clays and sandy clays
OH - organic silts and clays of high plasticity

*NOTE that this table has been updated from Version 2.X of the Minnesota Stormwater Manual. There are no longer two different infiltration rates for B soils and a value of 0.06 is used for D soils (instead of < 0.2 in/hr).
Source: Thirty guidance manuals and many other stormwater references were reviewed to compile recommended infiltration rates. All of these sources use the following studies as the basis for their recommended infiltration rates: (1) Rawls, Brakensiek and Saxton (1982); (2) Rawls, Gimenez and Grossman (1998); (3) Bouwer and Rice (1984); and (4) Urban Hydrology for Small Watersheds (NRCS). SWWD, 2005, provides field documented data that supports the proposed infiltration rates. (view reference list)
aThis rate is consistent with the infiltration rate provided for the lower end of the Hydrologic Soil Group A soils in the Wisconsin Department of Natural Resources Conservation Practice Standard: Site Evaluation for Stormwater Infiltration.