Citation:

Detenbeck, N., V. Zoltay, A. Morrison, T. Garrigan, J. Leclair, R. Abele, AND M. Tenbrink. WMOST v2 Case Study: Monponsett Ponds. Monponsett Ponds Watershed Workgroup, Halifax, MA, August 13, 2015.

Impact/Purpose:

This is a presentation made for a group of stakeholders at the monthly meeting of the Monponsett Ponds Watershed Workgroup. It provides an overview of results from a Region 1 RARE project case study application of version 2 of EPA's Watershed Management Optimization Support Tool. At their request, this case study overview will be posted on the town's web site for Monponsett Ponds Watershed Association and other citizens to review. The full case study description is being published as an appendix to an EPA report.

Description:

This webinar presents an overview of the preliminary results of a case study application of EPA's Watershed Management Optimization Support Tool v2 (WMOST) for stakeholders in the Monponsett Ponds Watershed Workgroup. Monponsett Ponds is a large water system consisting of two basins, East and West. The two ponds are connected by a small culvert at their southern end. East Monponsett Pond serves as water supply to the City of Brockton through periodic diversions of water to Silver Lake (located in the adjacent Jones River watershed). West Monponsett Pond drains into Stump Brook, a tributary of the Satucket River. There are estimates that Stump Brook only receives about one third of its potential annual discharge due to water diversions to Silver Lake (Princeton Hydro, 2013 ). Monponsett Ponds’ two basins have differing water quality, with higher pollutant loadings (notably phosphorus) in West Monponsett Pond resulting in the listing of the waterbody as impaired or threatened for one or more uses and requiring a TMDL. The WMOSTv2 model was successfully validated for the Monponsett Ponds watershed based on a 2002-2006 period of record covering both dry and one very wet year using inputs from a Taunton River watershed HSPF model, estimated operating rules for the dam outlet, and comparing modeled pond volumes with estimated volumes based on measured stage and pond bathymetry. Validation required the assumption that the dam operating regime shifted suddenly in October 2006 during Tropical Storm Tammy, which dumped up to 20 inches of rain across New England. Multiple potential management scenarios were evaluated, including variations in water demand by Halifax, repair of leaks in water distribution systems, variations in timing and magnitude of pond diversions by Brockton, different operating rules for minimum and maximum pond stage, increased target flows for the Stump Brook outlet to more closely approximate historic flows, and application of green infrastructure stormwater BMPs, interbasin transfers, and aquifer storage and recharge. Management scenarios with a uniform distribution of withdrawals yielded more volume deficits than did the current seasonal withdrawal pattern. No scenarios successfully met a target of the 75th percentile of unimpacted flows. Three scenarios were identified which met target flows of the 25th percentile of unimpacted flows for pond ranges between 52 ft and 53.5 to 54.5 feet above sea level. Implementation of green infrastructure stormwater best management practices (BMPs) were a common feature in successful scenarios or those with minimum volume deficits. The analysis also revealed a tradeoff between storage volume available for water storage and potential flooding damage to buildings surrounding the ponds. Next steps will include 1) a regional analysis to evaluate the tradeoff between GI BMP costs and reduction of flooding costs in the upper Taunton River watershed, which could also enhance groundwater supplies outside of the Monponsett Ponds basin and reduce the need for water diversions, and 2) development and application of a water quality module in WMOST.

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