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Anchorage Canal Drainage Area Stormwater Retrofit

Project Status: Complete

The Anchorage Canal Drainage Area Stormwater Retrofit Project

A Collaborative Community Demonstration Project to Clean Up Stormwater Runoff to the Little Assawoman Bay

Ditch along South Pennsylvania converted to a wet detention swale

The coastal corridor from Dewey Beach to Fenwick Island is very developed with a high percentage of the land covered in parking lots and roofs–impervious surfaces that prevent storm water from soaking into the ground. These communities were built prior to the establishment of current storm water management regulations and, because of their close proximity to the Inland Bays, these areas are major sources of runoff to the Bays.

 

 

The Anchorage canal is the northernmost canal in South Bethany and it connects to Little Assawoman Bay. Compared to other canals in South Bethany, it has a large drainage area of 120 acres, about half of which is impervious surface. High levels of nitrogen, hydrocarbons, and sediment enter the canal from the watershed which produces runoff even during light rains. Much of the runoff is collected through stormwater drains along Route 1 and is piped untreated to the canal.

Read details about the impact of stormwater runoff on the Anchorage Canal here.

The canals and the Little Assawoman Bay are in poor to fair condition and experience unhealthy levels of dissolved oxygen and bacteria as described in the 2011 State of the Delaware Inland Bays report. Residents have long supported the cleanup of the Bay.

In 2008, the Center for the Inland Bays partnered with the Town of South Bethany, the Town of Bethany Beach, Middlesex Beach, Sea Colony, the Delaware Department of Transportation, and the University of Delaware to develop a strategy to address runoff to the Anchorage Canal. Read the Anchorage Canal runoff strategy here (11mb). The strategy identified and prioritized 25 storm water retrofit concepts according to their pollution reduction efficiency. Storm water retrofits are management practices designed for locations where storm water controls did not previously exist or were ineffective. The goal of the strategy is voluntarily to reduce nitrogen and phosphorus loads to the Canal by 40% in accordance with a Total Maximum Daily Load regulation from the State of Delaware.

Stormwater infiltration pit under construction

Implementation began in 2011 with a plan to treat storm water from the Sea Colony high rise complex through the creation of wet-swale bio-retention areas and a series of infiltration pits along South Pennsylvania Avenue. The project was funded by the DNREC Community Water Quality Grants program, the CIB, and Sea Colony; and was designed and constructed by JMT, an engineering firm. The project treats 35 acres to remove 24 lbs. of nitrogen and 3.4 lbs. of phosphorus per year, and is planted in native vegetation. The planting scheme was designed by the one of our partners at the University of Delaware.

In the second phase of the project, sixteen bio-retention areas (engineered rain gardens) were created in the medians of the Coastal Highway to slow down and filter storm water off the road. Each area is estimated to trap and filter 0.15 pounds of phosphorus and 1.5 pounds of nitrogen every year. The project was funded by the DNREC Non-point Source Program, the CIB, the Town of South Bethany, Middlesex Beach, and the Delaware Department of Forestry.

A partially finished highway median bioretention area

 

The project team inspects the bioretention area

In the third phase of the project completed in 2014, seventeen bioretention areas were constructed within the highway right of ways.  On average, each bioretention area treated 0.7 acres of impervious surface.  Some of the areas were within the Anchorage Canal Drainage Area and some in drainage areas of other South Bethany Canals.  The areas were created by excavating the first 6 to 12 inches of soil from minimum 2 foot wide areas surrounding storm drains in a similar fashion to the highway median bioretention areas.  They were planted with different vegetation designs as desired by the participating communities of South Bethany and Middlesex Beach.  Biochar, a product of pyrolysis of waste biomass, was also added as an experiement to the areas to increase water retention and treatment.  In total the bioretention areas were modeled to treat 0.91 lbs/yr of phosphorus and 6.7 lbs/yr of nitrogen.

Excavation of a highway one right of way bioretention area.

 

Mulched bioretention area planted with native coastal grasses and shrubs.

Additional projects in the works include an innovative storm water wetland and wetpond, and more bioretention areas and infiltration areas around stormdrains in the Sandpiper Pines subdivision of the Town of South Bethany. Together these projects are anticipated to nearly meet the overall pollution remediation goals of the strategy.


Progress towards pollution reduction goals in the Anchorage Canal drainage area