Runoff Volume and Depth NRCS Curve Number

7/27/2019 Runoff Volume and Depth NRCS Curve Number

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RUNOFF VOLUMEUNOFF VOLUMEAND DEPTHND DEPTH

NRCS METHODSRCS METHODSAGSM 335GSM 335

READING ASSIGNMENTEADING ASSIGNMENTCHAPTER 5 OF TEXTHAPTER 5 OF TEXT

ANDNDChapter 5 of handouts (web)hapter 5 of handouts (web)

SURFACE RUNOFF USING THEURFACE RUNOFF USING THENRCS CURVE NUMBER TECHNIQUESRCS CURVE NUMBER TECHNIQUES

THE NRCS METHOD IS THE MOSTHE NRCS METHOD IS THE MOSTCOMMONLY USED METHOD FOROMMONLY USED METHOD FORESTIMATING RUNOFFSTIMATING RUNOFF1.. The effects of soil and surfacehe effects of soil and surfacet ti ll tti ll tegetation, as well as management,egetation, as well as management,are estimated with the CURVEre estimated with the CURVENUMBER or CN.UMBER or CN.CN IS THE APPROXIMATE PERCENTAGEN IS THE APPROXIMATE PERCENTAGE

OF INCIDENT RAINFALL WHICHF INCIDENT RAINFALL WHICHBECOMES RUNOFF.ECOMES RUNOFF.

NRCS CURVE NUMBERRCS CURVE NUMBER2.. The Curve Number is between 0 andhe Curve Number is between 0 and100 which is related to the amount of00 which is related to the amount ofrunoff generated from a watershedunoff generated from a watershedunoff generated from a watershed.unoff generated from a watershed.3.. Larger CN values are associated witharger CN values are associated withgreater runoff volumes.reater runoff volumes.

NRCS CURVE NUMBERRCS CURVE NUMBER4.. Function of antecedent moistureunction of antecedent moistureconditions, surface cover, landonditions, surface cover, land

management and soil type.anagement and soil type.5.. For multiple land uses and soil types,or multiple land uses and soil types,use a weighted average (see Examplese a weighted average (see Example5.2 in the text). Weight by area within.2 in the text). Weight by area withina watershed.watershed. WE WILL HAVEE WILL HAVEPROLEMS AND LABS with examples.ROLEMS AND LABS with examples.


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NRCS CURVE NUMBERNRCS CURVE NUMBER

Determine the total volume ofetermine the total volume ofrunoff generated by a storm event.unoff generated by a storm event.

)8.0(

)2.0( 2

SP

SPQ

)2.0( 2SP

WHERE:HERE: Q is the runoff volume (inches)is the runoff volume (inches)P is the precipitation (inches)is the precipitation (inches)S is the precipitation surface storageis the precipitation surface storagebefore onset of runoff (inches)efore onset of runoff (inches)

)8.0( SP


Surface storage is estimated byurface storage is estimated bythe curve number.he curve number.)1000(

WHERE: CN is the NRCS Curve Number.HERE: CN is the NRCS Curve Number.CN

SUMMARYSUMMARY

1.1. Given rainfall duration, return periodiven rainfall duration, return periodand location, determine depth ofnd location, determine depth ofprecipitation (P).recipitation (P).2 D t i i ht d l f tht i i ht d l f th.. Determine weighted value of theetermine weighted value of theCURVE NUMBER.URVE NUMBER.3.. Adjust of ANTECEDENT SOILdjust of ANTECEDENT SOILMOISTURE as necessary.OISTURE as necessary.4.. Calculate S and Q.alculate S and Q.

The 0 2S Factorhe 0 2S FactorThe precipitation must exceed 0.2She precipitation must exceed 0.2Sbefore runoff occursefore runoff occurs0.2S = Initial abstraction (I.2S = Initial abstraction (Ia) that includes:that includes:

Surface depressionsurface depressionsurface depressionsurface depressions Vegetation interceptionegetation interception Evaporationvaporation Infiltrationnfiltration

S = Potential maximum retention after= Potential maximum retention aftersurface runoff beginsurface runoff begins


Determine the total volume ofetermine the total volume ofrunoff generated by a storm event.unoff generated by a storm event.)2.0( 2SP

WHERE:HERE: Q is the runoff volume (inches)is the runoff volume (inches)P is the precipitation (inches)is the precipitation (inches)S is the precipitation surface storageis the precipitation surface storagebefore onset of runoff (inches)efore onset of runoff (inches)

)8.0( SP


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Hydrologic Soil Groups (HSG)ydrologic Soil Groups (HSG)4 hydrologic soil groups => A,B,C,Dhydrologic soil groups => A,B,C,D

A = sand, loamy sand or sandy loam= sand, loamy sand or sandy loam B = silt loam or loam= silt loam or loam C = sandy clay loam= sandy clay loam D = clay loam, silty clay loam, sandy= clay loam, silty clay loam, sandyclay, silty clay or claylay, silty clay or clay

based on surface soil textureased on surface soil texturewhen choosing a group must consider:hen choosing a group must consider:compaction by heavy equipment,ompaction by heavy equipment,exposure of subsoil, etc.xposure of subsoil, etc.

From TEXT

NOTE ERRORSNOTE ERRORS

0 1515 -0 3030.15.15 -0.30.30

HSGs based on SaturatedHSGs based on Saturated

Hydraulic Conductivity (KHydraulic Conductivity (Ksatsat))

A soil (high infiltration rates)soil (high infiltration rates)Ksatat > 0.30 in./hr0.30 in./hrsatat 0.30 in./hr0.30 in./hrB soil (moderate infiltration rates)soil (moderate infiltration rates)0.15 in./hr < K.15 in./hr < Ksatat < 0.30 in./hr0.30 in./hr

HSGs based on SaturatedHSGs based on Saturated

Hydraulic Conductivity (KHydraulic Conductivity (Ksatsat))

C soil (slow infiltration rates)soil (slow infiltration rates)0.04 in./hr < K.04 in./hr < Ksatat < 0.15 in./hr0.15 in./hrD soil (very slow infiltration rates)soil (very slow infiltration rates)Ksatat < 0.04 in./hr0.04 in./hrSee Appendix D (HANDOUT NOTES) for aee Appendix D (HANDOUT NOTES) for alist of the most common soils in eachist of the most common soils in eachstate with HSG type (A, B, C or D)tate with HSG type (A, B, C or D)

APPENDIX D from HANDOUT NOTES

Other hydraulic conductivityvalues are provided in Soilsurveys which are available onurveys, which are available ona county basis.


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Curve Numbers (CN) andCurve Numbers (CN) and

Antecedent Moisture ConditionsAntecedent Moisture Conditions

CN = SRO (SURFACE RUNOFF)N = SRO (SURFACE RUNOFF)potential of areaotential of area Table 5.3 (very limited)able 5.3 (very limited) TEXTEXT

OTHER references for moreTHER references for moreextensive listingxtensive listing CN's developed for land use / landN's developed for land use / landtreatment combinationsreatment combinations

H d NO Sd NO Sandout NOTESandout NOTES

Curve Numbers (CN) andurve Numbers (CN) andAntecedent Moisture Conditionsntecedent Moisture Conditions(ASM)ASM)Antecedent Soil Moisture Conditionsntecedent Soil Moisture Conditions(ASM) indicates the moistureASM) indicates the moistureASM) indicates the moistureASM) indicates the moisturecontent of the soil prior to the stormontent of the soil prior to the stormto be considered.o be considered.



Antecedent moisture conditions I, II, IIIntecedent moisture conditions I, II, III condition I => initially dry soilondition I => initially dry soilyy condition II => average soilondition II => average soilmoistureoisture

condition II => Table 5.3ondition II => Table 5.3 condition III=> initially wet soilondition III=> initially wet soil


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CN for conditions I (driest) and III (wettest)N for conditions I (driest) and III (wettest)can be adjusted using condition II CN'san be adjusted using condition II CN's(average)average) See Table following.ee Table following. Adjustment based on 5djustment based on 5-day antecedentay antecedentrainain

5-DAY ANTECEDENTAY ANTECEDENTRAINFALL (inches)AINFALL (inches)AMC CLASSMC CLASS CONDITIONONDITION DORMANTORMANTSEASONEASON GROWINGROWINGSEASONEASON

I DRIESTRIEST 2.12.1

TABLE 5.2 BELOW IS TAKEN FROMHANDOUT NOTES, CHAPTER 5

Multiply CNultiply CNIII by these factorsy these factors

ANTECEDENT MOISTURENTECEDENT MOISTURECONDITIONS (ASM)ONDITIONS (ASM)II

I CN

CNCN

058.010

2.4

II

II

II

III

CN

CNCN

13.010

0.23



CN develop from experimental plotsN develop from experimental plotslots of measured rainfall / runoff dataots of measured rainfall / runoff dataots of measured rainfall / runoff dataots of measured rainfall / runoff data throughout the U.S.hroughout the U.S. CN's correlated w/ land useN's correlated w/ land use

Good for higher infiltration ratesood for higher infiltration rates Not so good for lower infiltrationot so good for lower infiltrationratesates

Curve Numbers for ImperviousCurve Numbers for Impervious

SurfacesSurfaces

Impervious areasmpervious areas Impervious areas ==> CN = 98 to 100mpervious areas ==> CN = 98 to 100 expressed as of total areaxpressed as of total area directly connected => flow directlyirectly connected => flow directlyto a drainage system (gutters,o a drainage system (gutters,sewers, etc)ewers, etc)


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Area Weighted Curve NumberArea Weighted Curve Number

Mixed land uses and HSG'sixed land uses and HSG's Calculate an areaalculate an area-weighted CNeighted CN

Get a single CN for the entireet a single CN for the entireet a single CN for the entireet a single CN for the entirearearea CN = (AN = (Ai CNNi) / ( A/ ( Ai) CNNi = CN for the part ofCN for the part ofcatchment having area Aatchment having area Ai

Example ProblemExample Problem

Given:iven: Precipitation (P) = 4.0 in.recipitation (P) = 4.0 in. A watershed that has:watershed that has:

35 cultivated with a D soil group5 cultivated with a D soil group5 cultivated with a D soil group5 cultivated with a D soil group 30 meadow with a B soil group0 meadow with a B soil group 35 thin forest with a C soil group5 thin forest with a C soil group

Required:equired: Calculate the surface runoff (excessalculate the surface runoff (excessrainfall)ainfall)

35 Cultivated (Row Crop 30 Meadow

Watershed with Land UseAnd HSGs Listed

35 Cultivated (Row CropHSG = D (POOR) HSG = B

35 Woodlands (POOR)HSG = C


Firstirst ----- find the curve numbersind the curve numbersUsese HSGSG CN*N*Cultivatedultivated D 355 911Meadoweadow B 300 588Thin Foresthin Forest C 355 777

*Table 5.3 text (reference is important)Table 5.3 text (reference is important)


Secondecond ----- calculate a weighted CNalculate a weighted CNWeights based on areaeights based on areaCNNavgvg = 0.35(91) + 0.30(58) + 0.35(77)0.35(91) + 0.30(58) + 0.35(77)CNN av gvg = 76 2 = 7676 2 = 76


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Thirdhird ----- calculate the S termalculate the S termS = 1000 / CN= 1000 / CN 10 = (1000 / 76)0 = (1000 / 76) 100S = 3.16 in.= 3.16 in.


Fourthourth ----- check to see if P > 0.2Sheck to see if P > 0.2S0.2S = 0.2(3.16) = 0.63 in..2S = 0.2(3.16) = 0.63 in.

P > 0.2S (OK)> 0.2S (OK)


Fifthifth ----- calculate surface runoff (Q)alculate surface runoff (Q)Q = [(P= [(P - 0.2S)^2] / (P + 0.8S).2S)^2] / (P + 0.8S)Q = [(4.0= [(4.0 0.2(3.16)].2(3.16)]2 / [4.0 + ((0.8)3.16)][4.0 + ((0.8)3.16)](( ( )])] [ (( ) )](( ) )]Q = 1.74 in.= 1.74 in.

For a rainfall event = 4.0 in. on theor a rainfall event = 4.0 in. on thegiven watershed with average soiliven watershed with average soilmoisture conditionsoisture conditions

Runoff = 1.74 inRunoff = 1.74 in

For a rainfall event = 4.0 in. onFor a rainfall event = 4.0 in. onthe given watershed withthe given watershed withavera e soil moistureavera e soil moistureconditionsconditions

SUMMARYUMMARY1.1. Given rainfall duration, return periodiven rainfall duration, return periodand location, determine depth ofnd location, determine depth ofprecipitation (P).recipitation (P).2 Determine weighted value of theetermine weighted value of the.. Determine weighted value of theetermine weighted value of theCURVE NUMBER.URVE NUMBER.3.. Adjust of ANTECEDENT SOILdjust of ANTECEDENT SOILMOISTURE as necessary.OISTURE as necessary.4.. Calculate S and Q.alculate S and Q.

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Runoff Volume and Depth NRCS Curve Number