Estimating Annual RunoffBased on the

NRCS Runoff Curve Number

Presented at:

2011 LID SymposiumLoews Philadelphia Hotel

Philadelphia, PASeptember 25-28, 2011

Randy Greer, PESteve Wright, PEDelaware DNREC

Delaware Urban Runoff Management Model

(DURMM)

“A Little History”

“Green Technology”

Issues:• Stds. & Specs• Modeling

NRCS Runoff Equation

Q = (P - 0.2S)2/ (P + 0.8S)

Limitations of NRCS Methodology

Source: Pitt & James (1998)

Source: USDA-ARS/NRCS Curve Number Work Group (2002)

Limitations of NRCS Methodology

Pitt’s Accumulative Runoff Curve

80%

2”

Source: Pitt & Voorhees (2004)

WinSLAMMPredicted vs. Observed Runoff

• Based on NRCS methodology

• Adjusted to Pitt’s WinSLAMM equations

DURMM REL 1.1

WinSLAMM Runoff Equations

(Source: DURMM Technical Manual, 2004)

E

Pervious Impervious

DURMM REL 1.1, 2004

• Event-based methodology• Design storm: 2”, 24-hr event

(~6-month return frequency)• Runoff calculated separately

for pervious & impervious areas

• Limited ability to link GTBMPs in a “treatment train”

So why DURMM v.2????

Resource Protection Event BMPs

•Runoff Retention Practices (i.e., storage)•Runoff Reduction Practices (i.e., surface recharge)

Modeling Runoff Reduction Practices• Storage Practices

– Ex.: infiltration trench, bioretention, etc.– Runoff reduction a function of storage volume (constant)– Typically modeled using event-based methodology– Recommendation: MDE’s CN* method

• Non-Storage Practices– Ex: filter strip, bioswale, soil amendment, etc.– Runoff reduction a function of rainfall (variable)– Typically modeled using deterministic methods (ex.,

SWMM) or empirical methods based on regression equations (ex., Simple Method) using annual precipitation

– Needed: simple computational method based on RCN

The Problem:

Event-BasedMethodology

Predicting AnnualRunoff

One Solution:

A Better Solution:

• Find the link between:– Event-based

methodologies for modeling retention practices

– Annual rainfall/runoff relationships for modeling runoff reduction practices

Retention: Event-Based Modeling

Retention: Event-Based Modeling

Retention: Event-Based Modeling

Retention: Event-Based Modeling

Retention: Event-Based Modeling

Retention: Event-Based Modeling

DURMM v.2 – Event RO Equation

1-YR Event Runoff = 0.000466(RCN)2 – 0.023230(RCN) + 0.263672

Limitations of the Event-Based Methodologyto Estimate Annual Runoff

Hi‐CN Lo‐CN

Hi‐Rain Lo‐Rain

Source: Pitt & Voorhees (2004)

Reduction: Annual Runoff ModelingCorrelation Approach

Reduction: Annual Runoff ModelingCorrelation Approach

Source: Table 2-2a, USDA-NRCS TR-55

Reduction: Annual Runoff Modeling

DURMM v.2 – Annual RO Equation

Annual Runoff = 0.000004(RCN)3.5

DURMM v.2 Layout

User input

Pre-set or output

Result

Workflow

DURMM v.2Runoff Reduction Methodology Caveats

• The methodology was developed as an empirical compliance tool. It should not be considered as a replacement for physically-based hydrologic modeling tools.

• Under actual rainfall conditions, low magnitude events would be expected to be fully captured by the runoff reduction practices. However, as magnitude increases, the percentage of runoff volume captured decreases. Therefore, the runoff reduction calculated for GTBMPs using this methodology should be viewed as an average value based on the annual rainfall distribution.

DURMM v.2 – Annual RO EquationGeneral Form

General equation for estimating annual runoff*:

Annual Runoff (in.) = CRa(RCN)3.5 (Eq. 1)

Where:

CRa = annual runoff coefficientRCN = NRCS runoff curve number

*Results must be verified for local conditions

DURMM v.2 – Annual RO EquationGeneral Form

Derivation of the annual runoff coefficient (CRa):

CRa = [(P)(Rv)] / RCNRv^3.5 (Eq. 2)

Where:

P = annual precipitation (in.)Rv = percent annual precipitation converted to runoffRCNRv = conjugate NRCS runoff curve number at Rv

Based on analysis of Pitt’s results, Rv = 0.85 at RCN = 98.Substituting:

CRa = [(P)(0.85)] / 983.5 (Eq. 3)

Additional Information

http://www.dnrec.delaware.gov/swc/Drainage/Pages/Technical_document.aspx

QUESTIONS???

http://www.dnrec.delaware.gov/swc/Drainage/Pages/Technical_document.aspx