Let s Go to the Beach Evaluation and Modeling of a Coastal Community s Storm Drain System and Outfalls Jeff Riling GHD November 2016
Acknowledgments City of Rehoboth Beach Mayor Sam Cooper Sharon Lynn Bob Stenger Bill Woods GHD Jeff Sturdevant Maureen Wingfield Ross Fryar Ivan Botev
Project location City of Rehoboth Beach, DE Beach community with summer tourism Ocean water quality important to sustaining economic prosperity of beach tourism Historically, very high water quality with low pollutant concentrations
Project background and driver DNREC published TMDL for Rehoboth Bay all point sources into Bay must be eliminated RoD issued by DNREC - Stormwater evaluation required as a condition for effluent outfall construction 1998 2010 to 2013 Jan 2015 2015 to 2016 EIS developed for Rehoboth Beach WWTP Effluent Disposial Ocean Outfall Recommended Alternative Stormwater evaluation and preliminary modeling developed
Project goal and scope Project goal: To help the City prioritize spending of limited capital funds to maximize enterococcus reduction within the surf zone. Project scope: To perform a stormwater evaluation to identify methods to reduce enterococcus within runoff and protect swimming area water quality. Evaluation included: Investigation of additional onshore devices / practices Investigation of alternative outfall configurations and/or extensions Hydraulic and hydrodynamic modeling
Enterococcus as the Fecal Indicator Bacteria Fecal indicator bacteria (FIB) Predictive of the potential for human infectious disease Enterococcus: FIB for marine waters Found in guts of warm-blooded animals Widely distributed in a variety of environmental habitats Water Quality Criteria Currently 104 cfu/100 ml BAV of 60 cfu/100 ml to be adopted in 2017 Photo Courtesy of Bacteria in Photos
City-wide stormwater management evaluation
Existing storm drain system GIS geodatabase of the City s stormwater collection system developed from field surveying and record drawings 36,000 LF of storm drain pipe varying from 6 to 36 Storm water discharges into surf zone through five (5) ocean outfalls
Existing stormwater management City currently employs: Good management practices Structural best management practices Lake management plans
Additional best management practices evaluation Opportunities for additional stormwater management were investigated: Non-structural practices Structural practices Photo Courtesy of Filterra
Historical rainfall and water quality evaluation
Hydraulic and hydrodynamic modeling
Modeling methodology Design storm hyetograph Pipe network Hydraulic model Ocean data - Currents & wave height - Conductivity / temperature / density Drainage areas - Size - Curve number - Slope Outfall hydrographs Hydrodynamic model Land use Enterococcus conc. by land use Enterococcus conc. at outfalls Enterococcus plumes
Hydraulic model Uncalibrated model developed in CivilStorm utilizing the EPA SWMM calculation engine Model includes inlets, manholes, pipes, and drainage areas Rainfall distribution for a theoretical 10-year 24-hour storm applied Model exported hydrographs at each ocean outfall
Enterococcus concentration No data on enterococcus concentration within the collection system available Literature review conducted to approximate enterococcus concentration Land use Assumed enterococcus concentration (cfu / 100 ml) Commercial area 8,500 Residential area 6,800
Hydrodynamic and transport model 233-mile by 65-mile hydrodynamic and transport model developed in 2010 for the EIS Model refined to simulate stormwater discharge in the City Flow rates based on EPA SWMM hydrographs Concentrations based on land use and literature review
Outfall reconfiguration alternatives Alternative 1: Existing Outfalls Alternative 2: Rehoboth Avenue Extension Alternative 3: Combined Outfall Extension
Hydrodynamic and transport model results Alternative 1: Existing Outfalls Alternative 2: Rehoboth Avenue Extension Alternative 3: Combined Outfall Extension
Hydrodynamic and transport model results Alternative 1: Existing Outfalls Alternative 2: Rehoboth Ave. Extension Alternative 3: Combined Extension Day 1 0:00 to 6:00
Hydrodynamic and transport model results Alternative 1: Existing Outfalls Alternative 2: Rehoboth Ave. Extension Alternative 3: Combined Extension Day 2 0:00 to 6:00
Hydrodynamic and transport model results Alternative 1: Existing Outfalls Alternative 2: Rehoboth Ave. Extension Alternative 3: Combined Extension Day 3 0:00 to 6:00
Hydrodynamic and transport model conclusions Model Conclusions Concentrations greatly reduced by beginning of second day No observable benefit of single outfall extension Substantial reduction in predicted peak concentrations at the beach with a combined outfall extension Current Model Limitations: Hydraulic model uncalibrated Extremely conservative - Concentrations obtained from literature and assumed to be constant
Next steps Detailed stormwater data collection, including: Flow monitoring Enterococcus sampling Hydraulic Model Calibration Identification of priority drainage areas and optimum BMP locations Analysis of ocean enterococcus concentration during theoretical return period storms Photo Courtesy of Teledyne ISCO
Questions? jeff.riling@ghd.com