Stormwater Management at Thompson Park Zoo

What is Stormwater Runoff?

Stormwater runoff is all of the rainfall or snowmelt water that is not absorbed into the ground and instead flows over land. When not managed properly, stormwater runoff causes issues like pollution in our waterways, flooding, and erosion. Stormwater runoff has been cited in multiple studies as a leading cause of water quality impairment to our local lakes and rivers. And, with increasing levels of rainfall from climate change impacts, stormwater management is an especially critical issue for communities all across the U.S.  

What is Stormwater Management?

Stormwater management focuses on reducing runoff and improving water quality through a variety of techniques. 

Traditional stormwater management methods include things like storm drains, retention ponds, and culverts. Green stormwater infrastructure uses vegetation, soil, and other natural components to manage stormwater. Green stormwater infrastructure systems mimic natural hydrology to take advantage of interception, evapotranspiration, and infiltration of stormwater runoff at its source. Examples include rain gardens, constructed wetlands, vegetated bioswales, and living shorelines.  Many stormwater systems include a combination of grey and green infrastructure management practices. 

Stormwater management treatment “trains” combine multiple stormwater management processes in order to prevent pollution and decrease stormwater flow volumes that negatively affect the receiving waterbody.

Let’s Take a Look at a Stormwater Treatment Train in Action

The Thompson Park Zoo in New Jersey

Thompson Park is a 675-acre recreation area – the largest developed park in the Middlesex County park system – with numerous attractions including playgrounds, ballfields, hiking trails, and a zoo. The zoo is an animal haven that houses over 50 geese and fowl, goats, and approximately 90 deer in a fenced enclosure. The park also features Lake Manalapan. 

Within the zoo is a 0.25-acre pond that impounds stormwater runoff from adjacent uplands and two stormwater-fed tributaries to Lake Manalapan and Manalapan Brook. There are three tributaries to the pond  with varying levels of erosion. The western tributary contains a headcut that is approximately four feet high. A headcut is created by a sudden down-cutting of the stream bottom. Similar to a miniature waterfall, a headcut slowly migrates upstream and becomes deeper as it progresses. The headcut in the Zoo tributary had destabilized the stream by eroding and incising its channel and banks. Additionally, foraging by Zoo inhabitants had removed most ground cover around the pond and associated tributaries, which also caused erosion. 

The bare soil conditions, headcut, and manure from the Zoo animals were contributing sediment, nutrient, and pathogen loading to the Zoo pond and subsequently Lake Manalapan. The Zoo pond drains to an outlet structure, a 24-inch reinforced concrete pipe (RCP), and subsequently to a vegetated swale via a stormwater outlet. A second outlet pipe drains stormwater runoff from an asphalt parking lot which discharges to the vegetated swale. 

The shoreline of Lake Manalapan where the vegetated swale drains into the lake was the subject of a previous restoration project during which a diverse suite of native plants was installed; however, the swale was not included in this project and a maintained lawn, which does not adequately filter stormwater runoff or provide any ecosystem services. The swale also had little access to its floodplain where vegetation can help filter non-point source (NPS) pollutants from the Zoo pond and adjacent uplands.

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Implementing a Stormwater Management Treatment Train

In order to increase channel stability, decrease erosion, improve water quality and ecological function, and reduce the NPS pollutants originating from the Zoo, a stormwater management treatment train was designed and constructed. 

Middlesex County Office of Parks and Recreation and Office of Planning, the New Jersey Department of Environmental Protection (NJDEP), South Jersey Resource Conservation and Development Council (SJRC&D), Middlesex County Mosquito Extermination Commission, Freehold Soil Conservation District, Rutgers Cooperative Extension, Enviroscapes and Princeton Hydro worked together to fund,  design, permit,  and construct the following stormwater management measures: 

• stabilizing the western tributary to Lake Manalapan and its existing headcut by constructing a rock step-pool sequence; 
• installing BioChar filter bags within the Zoo pond to remove excess nutrients from the water column and bed sediments; 
• daylighting a portion of the existing 24-inch RCP in order to widen the stream channel and allow for more stormwater absorption 
• grading the vegetated swale to provide positive drainage and reduce mosquito breeding habitat; 
• grading a floodplain bench adjacent to the swale to allow for increased water storage and absorption times and thus greater nutrient removal; 
• installing outlet protection measures to reduce stormwater velocity and prevent scour within the swale; and 
• replacing the manicured grass with native vegetation within transition areas to reduce erosion potential and increase biodiversity.

To see the project elements taking shape and being completed, watch our video:

The project is funded by a Water Quality Restoration 319(h) grant awarded to SJRC&D by the NJDEP for continued implementation of watershed-based measures to reduce NPS pollutant loading and compliance with a total phosphorus (TP) Total Maximum Daily Load (TMDL) established by the NJDEP for Lake Manalapan. The TMDL is a regulatory term in the U.S. Clean Water Act, that identifies the maximum amount of a pollutant (in this case phosphorus) that a waterbody can receive while still meeting water quality standards. 

“The South Jersey Resource Conservation and Development Council was pleased to participate in this project. Partnering with these various governmental agencies and private entities to implement on the ground conservation and water quality improvements aligns perfectly with our mission.  We are thrilled with the great work done at Thompson Park and look forward to continuing this partnership.”

Craig McGee, South Jersey Resource Conservation and Development Council District Manager

Construction of the stormwater treatment train components began in early August 2021 and was completed by the end of September 2021. 

The first step of the stormwater treatment train was to stabilize the tributary to Lake Manalapan and its associated headcut. Streambank stabilization measures included grade modifications to create a gradual stream slope and dynamically stable form with improved habitat features, including riffles and pools, with gravel and cobble substrate. On August 17, grading of the floodplain bench began, the RCP was exposed, and the team started  excavation for the lower three steps in the step-pool sequence.

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On August 20, the rock grade and step-pool sequence were completed. And, fabric was installed along both sides of the rock-lined channel to increase stream-bank stability. Rock was placed within the pools to cover the edge of the fabric. We are very pleased to report that the newly restored channel held up to two large storm events during the construction process.

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Bags of BioChar, a pure carbon charcoal-like substance made from organic material, were installed across the Zoo pond using an anchor and line system. The BioChar bags help to remove TP and other nutrients from the water column and bed sediments of the Zoo pond and subsequently Manalapan Brook Watershed. The team also built, planted and installed a floating wetland island, an effective green infrastructure solution that improves water quality by assimilating and removing excess nutrients that could fuel algae growth.

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After conclusion of pipe lighting, excavation of the floodplain bench and installation of scour protection, native perennial vegetation was planted within the floodplain and swale in order to provide sediment deposition and nutrient uptake functions, as well as aquatic food web services and water temperature moderation before flows are discharged to Lake Manalapan. The plantings also enhance and create suitable avian and pollinator species habitat, and greater flora and fauna diversity.

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This stormwater treatment train project improves the habitat and water quality of the Manalapan Brook Watershed by addressing NPS pollutants that originate from Thompson Park Zoo. The completed work also supports the Watershed Protection and Restoration Plan for the Manalapan Brook Watershed by reducing TSS and TP loads in compliance with the TMDL. Additionally, the project improves the overall ecosystem by stabilizing eroded streambanks, installing native and biodiverse vegetation, and reducing the quantity of pollutants entering Lake Manalapan. 

“Thompson Park Zoo is an excellent model for showcasing a successful and comprehensive approach to stormwater management and watershed restoration through a dynamic multi-stakeholder partnership. We are so proud to be a part of this project and continue to support the Manalapan Brook Watershed Protection Plan through a variety of restoration activities.”

Amy McNamara, E.I.T, Princeton Hydro Project Manager and Water Resource Engineer

At Princeton Hydro, we are experts in stormwater management; we recognize the numerous benefits of green infrastructure; and we’ve been incorporating green infrastructure into our engineering designs since before the term was regularly used in the stormwater lexicon. Click here to learn more about our stormwater management services.

• Engineering
• Flood Mitigation
• Green Infrastructure
• Natural Resource Management
• Stormwater Management