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NRCS Hydrologic Soil Classification, How Good Is It?
Rohin Saleh, MS. PE. Supervising Civil Engineer, ACFCD
Frank Codd, PE. Associate Civil Engineer, ACFCD
Alameda County Flood Control District
Established in 1949
Agenda
• Impacts of NRCS Hydrologic Soil Classification on runoffs peak, volume & discharge frequencies
• Frequency of NRCS Soil Classification revisions (the old and the new ways as observed)
• Field investigation and tests of Hydrologic Soil Classification
• Concluding Remarks
Impacts of NRCS Hydrologic Soil Classification on Runoff Peak
SAN RAMON
M.A.P=26"
STORM Runoff Peak of D,C,B Soils Relative to A Soil
FREQ SOIL D SOIL C SOIL B SOIL A
100 42% 31% 6% 0
25 57% 42% 8% 0
10 79% 55% 9% 0
5 120% 83% 13% 0
Impacts of NRCS Hydrologic Soil Classification on Runoff Volume
SAN RAMON
M.A.P. = 26 "STORM Runoff Volume of D,C,B Soils Relative to A SoilFREQ SOIL D SOIL C SOIL B SOIL A100 92% 67% 15% 050 101% 72% 15% 010 158% 102% 21% 05 1383% 554% 68% 0
ARROYO MOCHO
M.A.P. = 16"
STORM Runoff Volume of D,C,B Soils Relative to A SoilFREQ SOIL D SOIL C SOIL B SOIL A100 292% 136% 28% 050 306% 139% 32% 010 540% 192% 42% 05 1542% 537% 108% 0
100
1000
10000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
San Ramon Creek_Soil D
95% Low Bull 17B 95% Up Model Log. (Model)
Impacts of NRCS Hydrologic Soil Classification on Discharge Frequency Curves
100
1000
10000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
San Ramon_Soil B
95% Low Bull 17B 95% Up Model Log. (Model)
Impacts of NRCS Hydrologic Soil Classification on Discharge Frequency Curves
100
1000
10000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
San Ramon Final
95% Low Bull 17B 95% Up Model
Impacts of NRCS Hydrologic Soil Classification on Discharge Frequency Curves
History of Hydrologic Soil GroupClassification
History of HSG Early mention of a system like HSG is in the 1955 Yearbook of Agriculture in an article by George W. Musgrave called “How Much of the Rain Enters the Soil”
This article introduces four infiltration classes:
A – Highest Group 0.30 to 0.50 inch/hrB – Above Average Group 0.15 to 0.30 inch/hrC – Below Average Group 0.05 to 0.15 inch/hrD – Lowest Group 0 to 0.05 inch/hr
Based on data for about 115 soil types
It is important to recognize that the ranges of infiltration rates published by Musgrave were developed for generally homogeneous hydrologic soils classes that he had data for and not mixes. However the NRCS surveys available today contains large quantity of mix hydrologic soils under one class. Therefore it is important to study the NRCS GIS soils polygons before assigning infiltration ranges.
NRCS Old Soil Survey publications used to remain effective for decades
NRCS Hard Copy Publications
When hydrologic modeling results do not match with historic observations of runoffs and you are confident of your rainfall parameters
Morrison Canyon and Mission Creek Watershed
Focus on two type of soils and slopes:
1-Los Osos 7-30% & 30-45%2-Millshom 30-45% & 45-75%
Land Use
MethodologyThe NRCS uses a mixture qualitative and quantitative criteria for assigning a Hydrologic Soil Group. The quantitative infiltrometer method can be used to more definitively define the soil group as described in tables 7-1 and 7-2 from the NRCS.
Infiltration Rate Assessments Soils in Mission and Morrison Creek Watersheds
Site No. SOIL TYPE Soil Sample DescriptionTested Preliminary
Perm., in/hr Soil Group3 Millsholm Silty Clay w/gravel, brown, Lw/Md Plasticity, rootlets 2.0 B4 Millsholm Silty clay w/sand, Lw Plasticity, rootlets, organics 1.0 C5 Millsholm Gravelly silt, brown, some clay, Lw Plasticity, rootlets 4.0 B
Average 2.3 BMedian 2.0 B
Site No. SOIL TYPE Soil Sample Description Tested PreliminaryPerm., in/hr Soil Group
6 Los Osos Not available 2.5 B7 Los Osos Silt, brown, some clay, Lw Plasticity, rootlets 9.0 A8 Los Osos Silt, brown, some clay,Lw Plasticity, rootlets 6.8 B
Average 6.3 A/BMedian 6.8 A/B
Floodplains
• Accurate representation of floodplain boundaries
• Impact to the community
Active rain gauges & flow monitoring stations
60 Stream or flow stations
50 Recording rain gauge stations
150 Daily rainfall observers
Conclusions:
• Impacts of HSG classification on runoff peaks and volume are substantial
• NRCS HSG classification are not homogeneous and contains large amount of mixes and fuzziness
• Must watch for frequent revisions and have data to verify
• Hydrologic calibrations and field tests should be used to supplement and refine NRCS HSG classifications