Surface Skimmer and Baffle Sediment Basins, Modeling the Benefits

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1 Surface Skimmer and Baffle Sediment Basins, Modeling the Benefits J.P. Johns, PE Woolpert Ray Vaughan Stormwater Manager SCDOT Brandon Wagner -Woolpert

2 Background o SCDOT sediment basin design required a removal efficiency of 80% TSS from the 10-year 24-hour design storm, or provide 3,600ft 3 of sediment storage volume per disturbed acre. o Prior SCDOT Sediment Basins used perforated risers or staged riser with a low flow orifice for dewatering. o New DOTCGP requires surface withdrawal and baffles and does not allow the use of perforated risers.

3 Goals o Perform Hydrologic and Sedimentology Modeling to determine trapping efficiencies as a result of implementing new DOTCGP requirements. o Use Modeling results to create SCDOT Standard Specifications and Detail for Surface Outlet and Baffle Sediment Basins.

4 Defining Surface Withdrawal o Use only Floating Skimmer to drain basin? Peak Flowrate generated by the 10-yr storm for a 10 acre watershed 25 cfs Flow capacity of standard 3-in skimmer 0.1 cfs 100+ skimmers required to prevent basin from overtopping 10-year storm. o Use Earthen Weir as Primary Spillway?

5 Surface Outlet o Design Primary Riser and Outlet Barrel to operate in weir flow control and transition to pipe/barrel flow control. Orifice flow not allowed for 10-year 24-hour storm event. o Design 10-year, 24-hour storm event peak stage at an elevation of approximately 6 inches above top elevation of Primary Riser. 140 Q Weir Control Orifice Control Pipe Control Head

6 Surface Outlet o Requiring surface withdrawal will complicate the basin design when considering water quantity. o In order to maintain surface withdrawal with limited storage above the riser, the riser size will be significantly larger and the basin surface area will increase compared to traditional designs, but required volume may decrease.

7 Primary Riser o Solid concrete riser, no low flow orifice or staged orifice/weir discharges. o Runoff only enters Primary Riser structure by overtopping and through Floating Skimmer. o New design requires dewatering from water surface and implements 3 spillways: 1. Primary Riser Spillway 2. Floating Skimmer 3. Stabilized Emergency Spillway (pass the 100-year 24-hr storm event)

8 Surface Outlet and Baffle Basin Modeling o Comprehensive sediment modeling effort consisting of 3 primary scenarios; 1. Basins using surface withdrawal and Porous Baffles. 2. Basins using surface withdrawal without Porous Baffles. 3. Basin using traditional default sizing of 3,600 ft 3 per disturbed acre.

9 Surface Outlet and Baffle Basin Modeling o SEDCAD4 Incorporate Baffles by input of 0% dead space. Incorporate Skimmer discharge by selecting User Input Discharge as the spillway type. (constant rate) Calculate required drawdown volume below primary riser crest. Determine design flow rate required for drawdown in hours. Select specific skimmer(s) required to obtain design drawdown rate.

10 Surface Outlet and Baffle Basin Modeling o SEDIMOT II FORTRAN program SEDIMOT Originally created by Dr. Barfield, batch version later created by Richard Warner Datasheets or text files for model inputs 85 manual inputs Operates in MS-DOS

11 Surface Outlet and Baffle Basin Modeling o Model of over 1,200 scenarios using various input parameters to consider construction projects throughout the State of SC. Contrasting inputs included: Soil Type Watershed Area Runoff Storage Volume Watershed Slope 3 Pond Design Scenarios o 7 soil types with varying erodible particle size distributions. o 4 soil grouped as Upper State soils o 3 soil grouped as Lower State soils.

12 Surface Outlet and Baffle Basin Modeling o Soils o Upper State Soils Cecil HSG B Pacolet HSG B Brevard HSG D Blanton HSG A o Lower State Soils Lakeland HSG A Rains HSG D Norfolk HSG B

13 Surface Outlet and Baffle Basin Modeling o Watershed Inputs Watershed sizes varied from 5 acres to 25 acres. Watershed slopes ranging from 5-20%. TOCs calculated based on watershed slopes. Curve Numbers ranged from depending on HSG of the Soil. o Basin Inputs Model scenarios incorporated storage volumes consisting of 3,600ft3/acre, 3,000ft3/acre, 2,400ft3/acre, 1,800ft3/acre, 1,400 ft3/acre, and 1,000ft3/acre. All basins 7 feet deep, with 4 foot high riser. Freeboard Height of 3-ft Barrel Slope of 1.5% if feasible ESW depth of 1.5 ft

14 Surface Outlet and Baffle Basin Modeling o Modeling Porous Baffles & Surface Withdrawal In SEDIMOT II, the user has three options for the outflow withdrawal option: Uniform Basin drained from all depths. Surface Basin drained from the top fall layer. Bed Basin drained from the bottom fall layer. In other modeling software, the user cannot change the outflow withdrawal option.

15 Surface Outlet and Baffle Basin Modeling o Modeling Porous Baffles & Surface Withdrawal Turbulence Factor For the turbulence factor, the user can select a value within the range of (0.1 2). For the traditional basin design scenarios the turbulence factor was 1.0. The turbulence factor was reduced to 0.5 for basins with porous baffles. In other modeling software, the user cannot change the Turbulence Factor

16 Surface Outlet and Baffle Basin Modeling o Modeling Porous Baffles & Surface Withdrawal Pond dead space For the traditional basin design scenarios the dead space was 30%. The dead space was reduced to 10% for basins with porous baffles.

17 Surface Outlet and Baffle Basin Modeling Results Trapping Efficiency All Modeling Results % Trapping New Design with Baffles New Design without Baffles Traditional Design All Soils Silt-Clay Soils Sandy Soils

18 Surface Outlet and Baffle Basin Modeling Results Trapping Efficiency All Modeling Results 100 All Soils Upper State Soils Lower State Soils 90 % Trapping Traditional Design 1800ft3/ac SW w/o Baffles 1800ft3/ac SW with Baffles 1800ft3/ac Traditional Design 2400ft3/ac SW w/o Baffles 2400ft3/ac SW with Baffles 2400ft3/ac Traditional Design 3000ft3/ac SW w/o Baffles 3000ft3/ac SW with Baffles 3000ft3/ac

19 Surface Outlet and Baffle Basin Modeling Results Trapping Efficiency for Upper State Soils % Trapping SW with Baffles 3000ft3/ac SW w/o Baffles 3000ft3/ac Traditional Design 3000ft3/ac SW with Baffles 2400ft3/ac SW w/o Baffles 2400ft3/ac Traditional Design 2400ft3/ac SW with Baffles 1800ft3/ac SW w/o Baffles 1800ft3/ac Traditional Design 1800ft3/ac

20 Surface Outlet and Baffle Basin Modeling Results Trapping Efficiency for Lower State Soils % Trapping SW with Baffles 3000ft3/ac SW w/o Baffles 3000ft3/ac Traditional Design 3000ft3/ac SW with Baffles 2400ft3/ac SW w/o Baffles 2400ft3/ac Traditional Design 2400ft3/ac SW with Baffles 1800ft3/ac SW w/o Baffles 1800ft3/ac Traditional Design 1800ft3/ac SW with Baffles 1400ft3/ac SW w/o Baffles 1400ft3/ac Traditional Design 1400ft3/ac SW with Baffles 1000ft3/ac SW w/o Baffles 1000ft3/ac Traditional Design 1000ft3/ac

21 Surface Outlet and Baffle Basin Modeling Sediment Basin Modeling Results o Required runoff storage with surface withdrawal (SW) & Porous Baffles 2,400ft 3 /acre for Upper State soils 1,000ft 3 /acre for Lower State soils o Required sediment storage (3-month cleaning schedule) 415 ft 3 /acre for Upper State soils 450 ft 3 /acre for Lower State soils o Sediment basins conservatively over designed for sediment trapping efficiency requirement of 80% TSS for 10yr 24hr storm event.

22 Surface Outlet and Baffle Sediment Basins New Design Aids

23 Surface Outlet and Baffle Sediment Basins New Design Aids MAX AREA DRAINING TO BASIN SEDIMENT CONTROL STRUCTURE SIZE (RISER DIA) PIPE BARREL DIAMETER 5 48" 24" RECESSED RISER TOP OF RISER TO BASIN BOTTOM FREEBOARD HEIGHT EMERGENCY SPILLWAY DEPTH EMERGENCY SPILLWAY BOTTOM WIDTH 10 48" 30" 18' 15 60" 36" YES 4' - 0" 3' - 0" 1' - 6" 22' 20 72" 42" 22' 25 72" 42" 28' 10'

24 Required Basin Volume (ft 3 ) Surface Outlet and Baffle Sediment Basins New Design Aids GRAPH 1 - REQUIRED STORAGE VOLUME (RUNOFF + SEDIMENT ) Uppper State Lower State Watershed Area (ac)

25 Required Surface Area (ft 2 ) Surface Outlet and Baffle Sediment Basins New Design Aids GRAPH 3 - REQUIRED SURFACE AREA AT TOP OF DAM Uppper State Lower State Watershed Area (ac)

26 Basin Volume (ft 3 ) Surface Outlet and Baffle Sediment Basins New Design Aids Graph 2-48 HR FLOATING SKIMMER DE-WATRING RATES Flowrate (gpm)

27 Surface Outlet and Baffle Sediment Basins

28 Surface Outlet and Baffle Sediment Basins Sediment Control Structure

29 Surface Outlet and Baffle Sediment Basins Sediment Control Structure

30 Floating Skimmers Excavate Skimmer Pit to account for sediment that accumulates on Basin bottom. Simmer Pit allows Floating Skimmer to completely drain the basin. o Min dimensions of 4ft x 4ft, depth of 2 ft. o Fill with Class A or Class B Riprap to the top elevation of Pit. o Ensure top elevation of Pit is lower than invert of the outlet barrel from the riser.

31 New Specifications DOTCGP contains new design requirements that had to be addressed. The most significant new requirements included: o Porous Baffles o Floating Skimmers o Surface Outlet and Baffle Sediment Basins

32 Questions 100% Trapping for Vehicles