Anchorage Canal Drainage Area Stormwater Retrofit


The Anchorage Canal Drainage Area Stormwater Retrofit Project

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

The coastal corridor from Dewey Beach to Fenwick Island is very developed with a high percentage of the land covered in roads, parking lots, and roofs—impervious surfaces that prevent stormwater from soaking into the ground. These communities were built prior to the establishment of current stormwater 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 125 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 until recently has been 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.


Development of the Pollution and Stormwater Control Strategy

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, or master plan, to address polluted runoff flowing to the Anchorage Canal. Read the Anchorage Canal runoff strategy here (11mb). The strategy identified and prioritized 25 stormwater retrofit concepts according to their pollution reduction efficiency. Stormwater retrofits are management practices designed for locations where stormwater controls did not previously exist or were ineffective. The goal of the strategy was to reduce total nitrogen and phosphorus loads to the Canal by 40%, in accordance with a Total Maximum Daily Load regulation from the State of Delaware.

Strategy Implementation

Phase 1 (Completed)
An infiltration pit under construction.

Implementation of the Pollution and Stormwater Control Strategy began in 2011 with a plan to treat stormwater from the Sea Colony high rise complex through the creation of wet swales and a series of infiltration pits along South Pennsylvania Avenue. The project was funded by the DNREC Community Water Quality Grants program, the Center, and Sea Colony and was designed and constructed by JMT, an engineering firm. The project treats 35 acres to remove 23.7 pounds of nitrogen and 3.4 pounds of phosphorus per year and is planted in native vegetation. The planting scheme was designed by one of our partners at the University of Delaware.

In the second phase of the project, 16 bioretention areas (engineered rain gardens) were created in the medians of the Coastal Highway to slow down and filter stormwater off the road. These bioretention areas are expected to trap and filter 27.4 pounds of nitrogen and 3.3 pounds of phosphorus every year. The project was funded by the DNREC Nonpoint Source Program, the Center, the Town of South Bethany, Middlesex Beach, and the Delaware Urban and Community Forestry Program.

Bioretention projects in the median of Coastal Highway will help address stormwater pollution concerns.
The project team inspects the bioretention area

The third phase of the project was completed in 2014, where 17 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 experiment to the areas to increase water retention and treatment. In total the bioretention areas were modeled to treat 6.7 pounds of nitrogen and 0.91 pounds of phosphorus every year.

Excavation of a highway one right of way bioretention area.
Mulched bioretention area planted with native coastal grasses and shrubs.

In June 2016, the fourth phase of the project was completed. Eight stormwater green infrastructure practices were designed and installed in the Sandpiper Pines neighborhood of the Town of South Bethany. Three infiltration trenches and five combined bioretention/infiltration facilities were designed and constructed within the Sandpiper Pines subdivision of South Bethany. Biochar, a type of charcoal, was added to the soil to increase nitrogen removal and improve infiltration rates. Together these green infrastructure practices treat stormwater runoff from 10 acres of residential development. This project reduces inputs of nitrogen and phosphorus to Anchorage Canal and Little Assawoman Bay by an estimated 15 pounds of nitrogen and 2 pounds of phosphorus annually. The project was funded by DNREC Community Water Quality Improvement Grant, the Center, Town of South Bethany, and the US Environmental Protection Agency.

Progress toward pollution reduction goals in the Anchorage Canal drainage area.

The fifth and final phase of the project, located at the intersection of Coastal Highway and South Pennsylvania Avenue near the Sea Colony high-rise condominiums in Bethany Beach, began construction in March 2020. This innovative stormwater treatment facility includes a pond and wetland with native vegetation to remove pollutants and beautify the area. The South Pennsylvania Avenue northbound slip ramp off of Coastal Highway has been closed to vehicles and converted into a safe and attractive walking and bike path.

“This is a significant achievement for clean water,” says Dr. Marianne Walch, the Center’s Science & Restoration Coordinator. “The Anchorage Canal stormwater initiative has been a great demonstration of how communities, government agencies, and nonprofit organizations like the Center can work together to achieve important water quality goals.”

Eighty percent of the design and construction of the project was funded by DelDOT’s Transportation Alternatives Program, with 20% match from the Sea Colony Recreation Association. The Center also contributed $10,000 toward design. Total cost for the project, including design and construction of the pond and roadway/intersection improvements, was $846,160.

This portion of the initiative will stop about 95 pounds of nitrogen and 21 pounds of phosphorus from reaching nearby waterways every year.

The total annual pollutant load reduction predicted once all projects are constructed is 167.8 pounds of total nitrogen and 30.6 pounds of total phosphorus every year.