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