Session 10: Post-Processing with GWLF

1. Session 10: Post-Processing with GWLF Virginia Tech Biological Systems Engineering Department DEQ TMDL Modeling Workshop; Richmond November 19-20, 2013

2. To describe the various post-processing steps in TMDL development based on GWLF output Objective

3. Process downstream sub-watersheds

4. GWLF and Sub-Watersheds Modeling without sub-watersheds provides no spatial distribution of watershed characteristics, except as they relate to different land uses

5. Modeling Sub-watersheds with GWLF TMB3 2,000 ha SDR = 0.1720 TMB2 1,000 ha SDR = 0.1845 Sub - Area TMB1 Total Watershed (TMB) 2,000 ha SDR = 0.1408

6. Distributing Modeled GWLF Sub-watershed Loads TMB3 TMB1 TMB2 Total Watershed (TMB)

7. Incorporate BMPs

8. Representing BMPs in Models • Land use changes • Riparian pasture to buffer • Hi-till to Lo-till cropland • Source reduction • Livestock exclusion • Reduced PS discharges • Streambank stabilization • Application of removal efficiencies • DCR Ag Cost-share tracking • Chesapeake Bay Model accounting • Reductions from upslope land uses • Grass and forest buffers

9. The load reduction fraction from a given landuse for any individual BMP can be thought of as: area of BMP / landuse area* reduction efficiency (acres) (acres) (fraction) The aggregate reduction fraction from all non-overlapping BMPs applied on a given landuse in a given watershed are then summed as a total reduction fraction. The remaining load fraction is referred to as the passthrufactor: passthru = 1 – total reduction fraction Since we simulate loads (not reductions) with the model, we apply the passthru factor to simulated loads to represent these BMPs. Incorporation of current BMPs Passthru factor = 1 – 0.09 = 0.91

10. Incorporate point source loads

11. NPDES Loads – Existing Conditions • NPDES Loads – Future Conditions • NPS Permitted Loads • Stormwater runoff permits • MS4 accounting • Sediment detention accounting Post-Processed Load Calculations

12. VPDES Loads – Existing Conditions

13. VPDES Loads – Future Conditions

14. Stormwater load = area * runoff depth * concentration • Schueler’sSimple Method Runoff = Rainfall • Rv • where Rvis the runoff coefficient, defined as: Rv = 0.050 + 0.009 * (% impervious area) • Simulate runoff from the relevant land use categories • Barren • Developed NPS Loads – Runoff Estimation

15. NPS Loads – Construction Site Runoff

16. NPS Loads – Industrial Stormwater Runoff

17. Area-weighted partitioning (post-processed) • Urban impervious • City – 36% • County – 64% • Different land uses (modeled) • Urban impervious (city) • Urban impervious (county) Alternative Simulation of MS4 Loads

18. Develop Allocation Scenarios

19. Aggregate Source Categories? Consider Reduction Exemptions? Determine Basis for Reductions Allocation Options

20. Alternate Aggregation Options • More flexibility for implementation • Less specific targeting ability

21. Permitted loads Source loads < 1-2% of Total Load Sources with very small UALs Area Load UAL HYSadj = reference TMB = impaired Possible Reduction Exemptions

22. Consider exemptions Equal % Reductions Largest Source Loads Largest UALs Basis for Reductions

23. TMDL Allocation Scenarios TMDL = WLA + LA + MOS Modeling Endpoint = TMDL - MOS

24. Concentration-based Allocation Scenario Modeling Endpoint = Overall average TP concentration = 0.070 mg/L BMP Scenario 1 = Future Load + all Big Otter River IP BMPs. BMP Scenario 2 = BMP Scenario 1 + all Little Otter River sediment TMDL reductions. Allocation Scenario = BMP Scenario 2 + WWTP at current flow and 0.5 mg/L TP effluent limits.

25. GWLF Allocation Exercise

26. 26 Questions?