2011 LID Symposium. Estimating Annual Runoff Based on the NRCS Runoff Curve Number. Loews Philadelphia Hotel Philadelphia, PA September 25-28, 2011

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1 Estimating Annual Runoff Based on the NRCS Runoff Curve Number Presented at: 2011 LID Symposium Loews Philadelphia Hotel Philadelphia, PA September 25-28, 2011 Randy Greer, PE Steve Wright, PE Delaware DNREC

2 Delaware Urban Runoff Management Model (DURMM) A Little History

3 Green Technology Issues: Stds. & Specs Modeling

4 NRCS Runoff Equation Q = (P - 0.2S) 2 / (P + 0.8S)

5 Limitations of NRCS Methodology Source: Pitt & James (1998)

6 Limitations of NRCS Methodology Source: USDA-ARS/NRCS Curve Number Work Group (2002)

7 Pitt s Accumulative Runoff Curve 80% 2 Source: Pitt & Voorhees (2004)

9 WinSLAMM Predicted vs. Observed Runoff

10 DURMM REL 1.1 Based on NRCS methodology Adjusted to Pitt s WinSLAMM equations

11 WinSLAMM Runoff Equations Pervious Impervious E (Source: DURMM Technical Manual, 2004)

12 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

13 So why DURMM v.2????

14 Resource Protection Event BMPs Runoff Retention Practices (i.e., storage) Runoff Reduction Practices (i.e., surface recharge)

15 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

16 The Problem: Event-Based Methodology Predicting Annual Runoff

17 One Solution:

18 A Better Solution: Find the link between: Event-based methodologies for modeling retention practices Annual rainfall/runoff relationships for modeling runoff reduction practices

19 Retention: Event-Based Modeling

20 Retention: Event-Based Modeling

21 Retention: Event-Based Modeling

22 Retention: Event-Based Modeling

23 Retention: Event-Based Modeling

24 Retention: Event-Based Modeling

25 DURMM v.2 Event RO Equation 1-YR Event Runoff = (RCN) (RCN)

26 Limitations of the Event-Based Methodology to Estimate Annual Runoff Hi CN Lo CN Hi Rain Lo Rain

27 Reduction: Annual Runoff Modeling Correlation Approach Source: Pitt & Voorhees (2004)

28 Reduction: Annual Runoff Modeling Correlation Approach Source: Table 2-2a, USDA-NRCS TR-55

29 Reduction: Annual Runoff Modeling

30 DURMM v.2 Annual RO Equation Annual Runoff = (RCN) 3.5

31 DURMM v.2 Layout User input Pre-set or output Result Workflow

32 DURMM v.2 Runoff 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.

33 DURMM v.2 Annual RO Equation General Form General equation for estimating annual runoff*: Where: Annual Runoff (in.) = C Ra (RCN) 3.5 (Eq. 1) C Ra = annual runoff coefficient RCN = NRCS runoff curve number *Results must be verified for local conditions

34 DURMM v.2 Annual RO Equation General Form Derivation of the annual runoff coefficient (C Ra ): Where: C Ra = [(P)(R v )] / RCN Rv^3.5 (Eq. 2) P = annual precipitation (in.) R v = percent annual precipitation converted to runoff RCN Rv = conjugate NRCS runoff curve number at Rv Based on analysis of Pitt s results, Rv = 0.85 at RCN = 98. Substituting: C Ra = [(P)(0.85)] / (Eq. 3)

35 Additional Information

36 QUESTIONS???

Stormwater Management Impacts Resulting from the Volumetric Abstraction of Runoff from Frequent Storms per PADEP CG-1. Geoffrey A. Cerrelli 1, P.E.

Stormwater Management Impacts Resulting from the Volumetric Abstraction of Runoff from Frequent Storms per PADEP CG-1 Geoffrey A. Cerrelli 1, P.E. 1 Hydraulic Engineer USDA/NRCS, One Credit Union Place,