South Wilmington Wetland Park

RK&K worked with the City of Wilmington, the Southbridge community, and many regulatory and environmental groups to envision, design, and construct the South Wilmington Wetland Park. This park transforms a Phragmites-infested, heavily contaminated 17-acre public property into a vibrant wetland park that offers stormwater storage capacity to address chronic flooding while creating a public amenity with trails and state-rare wetland habitat and encouraging economic development opportunities. The design required creative engineering solutions to tackle numerous constraints including preserving access to a Delmarva Electric transmission line crossing the site; heavily developed areas between the park and its source of hydrology, the Christina River; and narrow ecological parameters to support a robust wetland ecosystem. The resulting wetland park design is a highly engineered and constrained solution, with relatively little flexibility regarding design elements. An innovative pinch valve-based connection to the Christina River provides an opportunity to create a tidal marsh, enhanced wildlife habitat, and an outfall for the redirected community stormwater and will provide the versatility to manage the site in response to storms and sea level rise. A boardwalk across the site provides public access to the wetland and connects the Southbridge community to economic opportunities while improving utility maintenance access.

In July 2020 Delaware Senator Tom Carper visited the South Wilmington Wetlands Project and learned about the project’s community benefits, innovative environmental design, and ongoing progress of construction and discussed the importance of green initiatives and partnering to make projects like this possible.

In 2021, the City of Wilmington’s South Wilmington Wetland Park project took home a Government Award from the Water Resources Association (WRA) of the Delaware River Basin during the WRA’s 38th Annual Recognition Dinner. During their acceptance speech, the City of Wilmington recognized RK&K, noting our team’s dedication and commitment that helped make this complicated project a reality.

Wetland Creation and Enhancement Design

The South Wilmington Wetland Park team assessed multiple wetland hydrology options including groundwater, stormwater, and tidally inundated habitats, selecting tidally inundated hydrology as both the most ecologically valuable and technically feasible option. They reviewed avenues for creating a hydrologic connection to the Christina River and identified a flap gate-regulated outfall to the river from an on-site drainage ditch. They coordinated with the client and nearby landowners to select an innovative pinch valve system to replace the flap gate, providing adjustable, regular tidal inundation to the wetland with minimal spatial or visual disruption to the existing riverfront businesses. Proposed site grades were analyzed using 3D site modeling (InRoads) and tidal exchange modeling (TIDEROUT2). Model runs were used to set the wetland restoration elevations and ensure adequate wetland hydrology. RK&K targeted inundation periods that minimize the opportunity for invasive species colonization while supporting a mosaic of wetland habitats including tidal channels, tidal marsh flats, deciduous swamps, and coniferous swamps, as well as open water and upland islands. The site design required close coordination with Brightfields, Inc. to comply with the site’s Final Plan of Remedial Action for contamination. The wetland design incorporated hazardous material cleanup elements such as water treatment, contaminant hotspot excavation, contaminated soil disposal, PCB remediation with SediMite^TM, and capping with clean fill.

Hydrologic/Hydraulic Analysis/Flooding/Sea Level Rise

RK&K conducted an extensive hydraulic analysis of existing and proposed conditions for several alternatives for this tidally influenced system in the Christina River coastal plain floodplain using a standard 2D hydraulic model, XPSWMM. Long-term surface water level monitoring stations and rainfall data were used to calibrate the hydraulic model. The adjacent Southbridge area was analyzed to determine the stormwater volume/area that could be separated from the combined sewer and directed into the wetland park. Model results were used to size the forebay treatment system in the wetland park and optimize the grading, to reduce cost, and achieve reduction in flooding and allow for varied planting zones, creating a variety of native Delaware habitat types, while minimizing the opportunity for invasive species colonization. Model runs were used to set the wetland restoration elevations and ensure adequate wetland hydrology. The hydraulic analysis assisted in sizing the tidal channels and confirming a depositional, non-erosive tidal system. Tidal influxes for proposed conditions were analyzed to provide the optimal volume and timing of inundation required to maximize biological diversity in the system and minimize the opportunity for the establishment of invasive vegetation.

Wetland Evaluation and Delineation

RK&K conducted a wetland and waters delineation for the City of Wilmington. One wetland and five waters were identified as potentially jurisdictional. A wetland delineation report was developed, and a jurisdictional determination was conducted with the USACE. To permit the South Wilmington Wetland Park project within existing wetlands, the team conducted a functional assessment of the current wetlands and of the proposed enhanced wetland to document the ecosystem function uplift, which will result from the proposed project. The existing wetland serves well for shoreline bank erosion and sediment control. Functions that will be improved are water quality, wildlife habitat, fish habitat, education/scientific value, and uniqueness. An increase in vegetative diversity and structure and the creation of open-water habitat would improve the existing marsh, as would greater connectivity to adjacent surface waters, including tidal flushing and the removal of contaminated debris. The trail network could include educational signage and will provide opportunities for wildlife study.

Permitting

The South Wilmington Wetland Park project required environmental permits from USACE, DNREC Subaqueous Lands, DNREC Coastal Zone, and DNREC Site Investigation and Remediation Section. RK&K developed permit application packages and coordinated with the regulatory agencies to secure authorizations for this project. The team developed a USACE individual permit application, a DRNEC Wetlands and Subaqueous Lands joint permit application, and a DNREC Coastal Zone Consistency Certification. The application packages included permit plates accurately documented regulated impacts, enhancement areas, and creation areas. The applications included a project description, project purpose, project need, alternatives analysis, impact avoidance and minimization, mitigation, functional uplift, and a monitoring plan. A field review with the regulatory agencies and inclusion of the regulatory agencies as stakeholders ensured a smooth permitting process.

Construction Management

RK&K provided on-site construction management services to ensure a well-constructed final park. In addition to typical construction management activities, on-site engineers responded quickly and creatively to unforeseeable issues as they arose. When hundreds of boulders were uncovered unexpectedly during construction, RK&K engineers identified tasteful reuse opportunities within the site. They responded to multiple instances of conflicts with undocumented underground utilities with workable and cost-effective solutions. RK&K countered pandemic-related cost increases with innovative solutions, such as swapping out timber for stamped concrete at the site entrances and reducing costs by $90,000.