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

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1 10 100

Disc

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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

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1 10 100

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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

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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