An Introduction to Stormwater Ponds in South Carolina

This article is intended for stormwater pond owners, property management professionals, pond management professionals, and county and municipal staff to learn about the design, function, and management of stormwater ponds.

Introduction

Altering land use or land cover across a watershed affects the hydrology of the system. As South Carolina becomes increasingly developed, urban and suburban landscapes generate larger volumes of polluted stormwater runoff. Stormwater runoff is generated when precipitation falls on impervious surfaces, such as roads and parking lots, and cannot infiltrate into the soil. An increase in impervious cover increases runoff volume and peak flows during storm events, which can cause flooding and accelerate erosion. Stormwater ponds are constructed to intercept runoff from the stormwater conveyance systems. Temporary water storage in ponds reduces flood pulse, improves water quality, and minimizes adverse effects to downstream waterways.

While wetlands are a common natural landscape feature across South Carolina, most ponds were constructed to serve a purpose, such as irrigation, recreational fishing, or flood control. Stormwater ponds fall under that latter purpose, flood control, and are increasingly being used to manage water quality in addition to quantity. The proliferation of stormwater ponds across South Carolina is a direct result of regulations placed on new developments beginning in the 1990s. As such, most new housing developments include one or multiple stormwater ponds to manage additional runoff generated due to increased impervious surfaces. The coastal counties alone have over 9,200 stormwater ponds, and the number continues to rise.¹ The large number of ponds present across South Carolina lead to various management challenges and have implications for broader watershed health.

Stormwater ponds are designed to act as flow-through systems. Stormwater is routed to a pond, which acts as a temporary storage site and settling basin, and then water eventually exits the system and flows into downstream water bodies. This connectivity between ponds and natural waterways is important because although ponds help reduce and filter pollutants from stormwater, a poorly functioning pond can act as a pollutant source. Even a well-maintained pond that reduces pollutants, such as excess nutrients, can still export harmful algae and introduce warmer water with lower dissolved oxygen levels downstream.² In some highly urbanized areas, much of the runoff in a watershed may end up passing through stormwater ponds before flowing downstream. As a result, stormwater ponds are an integral part of larger watershed management efforts.

In addition to flood control and pollutant reduction and removal, stormwater ponds provide a number of other ecosystem services, including carbon sequestration, habitat creation, macroinvertebrate biodiversity, and recreational opportunities.^3,4,5

Pond Design Elements

Stormwater ponds are the primary best management practice used across South Carolina to comply with stormwater control requirements.⁶ Generally, there are three types of stormwater ponds designed to capture and detain stormwater runoff for the design conditions:

1. Dry detention ponds – designed to store stormwater temporarily.
2. Wet detention ponds – have a permanent pool of water for water quality benefits and temporarily store water before being released.
3. Retention ponds – have a permanent pool of water and reduce volume through infiltration, evapotranspiration, or a combination of the two.

Wet detention ponds are the predominant stormwater pond type across coastal South Carolina and in areas with a high water table. Dry ponds are likely to be found across the upstate region of South Carolina, although data are lacking on the total number of ponds statewide and the presence of wet vs. dry ponds. In this article, further discussion and references to “stormwater ponds” will focus on wet detention ponds and their design and management.

Various state and local regulations influence the design of stormwater ponds. Stormwater ponds are generally built to comply with construction regulations intended to reduce runoff from newly developed sites. Pond design is site-specific, based on both the local rainfall and watershed characteristics. Ponds may be designed to capture design storms (i.e., a hypothetical storm event of a certain magnitude), reduce peak flows, empty within seventy-two hours of a storm, or store runoff for twenty-four hours to provide additional water quality benefits. Design components of a pond may include a forebay, permanent pool, temporary pool, inlet and outlet structures, low-flow orifice, emergency spillway, geometry, and underlying soils (figure 1).

Figure 1. Design elements common to most stormwater ponds. Image credit: SC Department of Health and Environmental Control.

Mechanisms for Pollutant Treatment and Removal

Pollutants generally enter a stormwater pond via stormwater runoff and reflect upland sources of pollution that flow into ponds during storm events. Stormwater reaches the pond through two key pathways: (1) through pipe discharge at the inlet, and (2) through overland flow across the pond banks. Stormwater pollutants entering the pond via pipe discharge reflect the surrounding land use. In a residential neighborhood, common pollutants sources include excess nutrients from fertilizer use, bacteria from pet waste and wildlife, and sediment from unvegetated/bare ground and eroding banks. The warmer temperatures of runoff from impervious surfaces can be problematic for ponds, as can freshwater inflows to brackish ponds.

Traditionally, stormwater ponds were designed to manage localized flooding. However, because ponds can be designed to mimic many of the ecosystem services provided by natural wetlands, they are generally effective at improving water quality via (1) sedimentation, (2) pollutant uptake (via assimilation in vegetative biomass), and (3) biogeochemical transformations, such as denitrification, in the water column and underlying soils.

Sedimentation

Sedimentation is a settling process in which suspended particles and sediment-associated pollutants in the water column drop out of solution and settle on the pond bottom, where they eventually become buried by additional layers of sediment accumulation.⁷ Sedimentation is one of the primary processes of pollutant removal in stormwater ponds. In general, there are four zones of sedimentation in ponds: an inlet zone, a sedimentation zone, a sludge zone, and an outlet zone.⁸ Designing stormwater ponds with long, circuitous flow paths can enhance the hydraulic retention time, ultimately increasing sedimentation of suspended solids and sediment-associated pollutants from the water column.⁹ Pollutant storage in the sediments can serve as a temporary or a long-term sink but is not considered a permanent solution. Disturbance of the pond bottom can resuspend sediments and associated pollutants within the water column.¹⁰ Inclusion of a forebay at the inlet can increase the removal of sediment and associated pollutants through the sedimentation process prior to entering the main storage reservoir of the pond.¹¹ Increased sedimentation over time eventually decreases the storage capacity of the stormwater pond, and ponds may require dredging to restore capacity.³ Proper maintenance and management of the pond are necessary to reduce erosion, both along pond banks and in upland areas draining to the pond, to decrease the total amount of sediment entering the pond.

Plant Uptake

Plants both in and around the pond can enhance sedimentation and sediment-associated pollutant removal in several ways.¹² They provide resistance to flow, thereby reducing the water velocity and increasing the hydraulic retention time. Slower water velocities enhance the settling of sediment and sediment-associated pollutants from the water column to the bottom of the pond. Plants can also take up nutrients and pollutants via bioaccumulation, which results in the storage of pollutants in plant tissues. Plants also help in other processes such as filtration, infiltration, and adsorption. Filtration occurs when plants remove the pollutants from the water column, protecting surface water and groundwater quality. Infiltration helps in the reduction of dissolved pollutants from the water column. Adsorption refers to the process in which pollutants bind to the surface of organic matter from the plants or soils associated with their root systems, helping reduce the concentration of pollutants in the stormwater. While increased stormwater flows in the ponds could resuspend sediment and sediment-associated pollutants within the water column, the roots of the plants/vegetation help to hold the sediment and sediment-associated pollutants in place and prevent erosion.¹³ Vegetation management is a critical component of pond management if pollutant mitigation and management are design goals.

Biogeochemical Processes

Biogeochemical processes refer to ways by which different elements or pollutants are transferred between the organisms and the environment. Biogeochemical processes in ponds occur when microorganisms mediate chemical transformations, transforming one form of a pollutant to less harmful versions or removing them entirely.¹⁴ For example, organic material that accumulates on the pond bottom can host a variety of beneficial microbes that help to assimilate and transform nitrogen pollution. Nitrification and denitrification are important reactions that transform and remove inorganic nitrogen from the water column.¹⁵ Denitrification reduces nitrate (NO₃^–) into N₂ gas via a microbially-mediated pathway, which is considered a permanent removal of nitrogen from the system. Microbial update can also help to remove soluble reactive phosphorus from stormwater.¹⁶

Implications for Management

Mechanisms for pollutant mitigation in stormwater ponds systems have implications for management enhancements to improve target pollutant removal efficiencies or deter pollutant introduction into the pond system. Popular strategies considered include vegetated buffers, floating treatment wetlands, aeration, and incorporation of upland stormwater control measures (figure 2).

Figure 2: Clockwise from top left: (1) vegetated buffer management encourages the use of an alternating mowing schedule to permit the growth of beneficial aquatic perennials and grasses. Image credit: Guinn Wallover, Clemson University. (2) bottom diffusers are a recommended aeration method to reduce the risk of fish kills related to turnover in deeper pond systems – the pond surface looks like it is “bubbling” when diffusers are used. Image credit: The Lake Doctors. (3) floating wetlands enhance sediment settling and nutrient uptake when appropriate surface coverage of the stormwater pond is achieved. Image credit: Charleston Aquatic and Environmental, Inc.

The benefits of vegetated buffers in reducing pollutant transport or minimizing shoreline erosion along waterbodies are well-documented.^17,18 Buffers have been adapted for use along with stormwater ponds and are a common recommended best practice to help reduce shoreline erosion and associated sedimentation and water quality concerns. Where communities can achieve wider buffer widths, increased water quality benefits may be achieved through enhanced management of runoff from properties adjacent to the pond.

The incorporation of native wetland vegetation for enhanced pond performance also includes floating treatment wetland practices. Floating treatment wetland systems provide opportunities for multiple pollutant removal mechanisms to work simultaneously, including wave attenuation resulting in sedimentation and reduced bank erosion, the formation of biofilm along the dense root systems that support microbial decomposition of organic material and denitrification, and plant uptake of nutrients and pollutants by vegetation. Barriers to the use of floating treatment wetlands include installation costs and required pond surface area coverage. Studies suggest a marked increase in pollution removal efficiency when floating treatment wetlands achieve coverage of 20% of pond surface area, suggesting their use for pollution removal enhancement is more feasible for smaller pond systems.¹⁹ Field studies that examine the relationship between pollutant removal efficiencies and floating wetland area versus the area of the pond show that a range exists in the recommended coverage area, which may also be influenced by additional variables.^19,20,21

Aeration systems in ponds are traditionally used to help maintain dissolved oxygen levels in deeper pond systems. Aerator use is recommended to help prevent stratification of the water column and pond turnover. Pond turnover can result in rapid mixing and low dissolved oxygen conditions following seasonal weather shifts. Creating or maintaining aerobic conditions in the pond can also affect nutrient levels by decreasing phosphorus release from the sediments, although other studies have found minimal effects on phosphorus dynamics due to mixing.^22,23 Notably, effective aeration and mixing of the water column in lakes can shift the phytoplankton community composition from cyanobacteria (which can release toxins into the water upon cell death) to green algae and diatoms; however, this effect may be limited in shallow pond systems.²¹ A preferred approach to cyanobacteria bloom prevention is to limit the runoff of nutrients into the pond.

While stormwater ponds can effectively treat water quality for a variety of pollutants, the best way to ensure their functionality over the long term is to reduce the load of pollutants that reach the pond in the first place. Upland best management practices focused on infiltration can reduce total runoff reaching the pond, thereby reducing pollutant inputs. A treatment train approach focused on using best management practices in series can iteratively reduce runoff volume and treat pollutants nearer to the source as stormwater travels across the landscape. Changing behavior is another important approach to reducing or eliminating pollutants at the source. Research on coastal South Carolina stormwater ponds suggests that terrestrial sources of organic material (e.g., grass clippings, leaf litter, etc.) are significant contributing sources to organic material accumulation in pond bottoms, as opposed to aquatic algae and plant material.²⁴ Homeowner adoption of best practices in the landscape can help to reduce pollutants in ponds and reduce the volume of stormwater runoff generated. Practices like removing and properly disposing of pet waste, keeping yard waste out of storm drains, limiting fertilizer use, avoiding excessive irrigation, and converting impervious surfaces to permeable areas all help safeguard and protect stormwater ponds and the ecosystems they were designed to protect.

More information on stormwater pond maintenance can be found in the Clemson HGIC factsheet 1881, Stormwater Ponds: Inspection and Maintenance Considerations.

References Cited

1. Smith E, Sanger D, Tweel A, Koch E. Chapter 1: A stormwater pond inventory for the eight coastal counties of South Carolina. In: Cotti-Rausch BE, Majidzadeh J, Devoe MR. [eds.] Stormwater ponds in coastal South Carolina: 2019 state of the knowledge full report. Charleston (SC): SC Sea Grant Consortium; 2019. p. 1–14.
2. DeLorenzo ME, Fulton MH. Water quality and harmful algae in southeastern coastal stormwater ponds. NOAA Technical Memorandum; 2009. NOS NCCOS 93. p. 27.
3. Schroer WF, Benitez-Nelson CR, Smith EM, Ziolkowski LA. Drivers of sediment accumulation and nutrient burial in coastal stormwater detention ponds, South Carolina, USA. Ecosystems. 2018;21(6):1118–1138.
4. Greenfield DI, Smith EM, Tweel AW, Sitta K, Sanger DM. Chapter 4: The ecological function of South Carolina stormwater ponds within the coastal landscape. In: Cotti-Rausch BE, Majidzadeh, J and MR Devoe [eds.] Stormwater ponds in Coastal South Carolina: 2019 State of the knowledge full report. Charleston (SC): SC Sea Grant Consortium; 2019. p. 81–101.
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7. Haan CT, Barfield BJ, Hayes JC. Design hydrology and sedimentology for small catchments. San Diego (CA): Academic Press; 1994.
8. StormOps IDEAL User Manual [Computer software]. Columbia (SC): StormOps; 2007. https://stormopssoftware.com/.
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16. Frost PC, Prater C, Scott AB, Song K, Xenopoulos MA. Mobility and bioavailability of sediment phosphorus in urban stormwater ponds. Water Resources Research. 2019;55(5):3680–3688.
17. Lovell ST, Sullivan WC. Environmental benefits of conservation buffers in the United States: Evidence, promise, and open questions, Agriculture, Ecosystems & Environment. 2006;112(4):249–260.
18. Bentrup G. Conservation Buffers – Design guidelines for buffers, corridors, and greenways. Gen. Tech. Rep. SRS–109. Asheville (NC): U.S. Department of Agriculture, Forest Service, Southern Research Station; 2008. p. 1–110.
19. Winston R, Hunt W, Kennedy S, Merriman L, Chandler J, Brown D. Evaluation of floating treatment wetlands as retrofits to existing stormwater retention ponds. Ecological Engineering. 2013;54:254–265.
20. McAndrew B, Changwoo A, Spooner J. Nitrogen and sediment capture of a floating treatment wetland on an urban stormwater retention pond – the case of the rain project. Sustainability. 2016;(8)10:972. doi:10.3390/su8100972.
21. Nichols P, Lucke T, Drapper D, Walker, C. Performance evaluation of a floating treatment wetland in an urban catchment. Water. 2016;8(6):244. doi:10.3390/w8060244.
22. Beutel MW. Inhibition of ammonia release from anoxic profundal sediments in lakes using hypolimnetic oxygenation. Ecological Engineering. 2006;28(3):27–279.
23. Visser PM, Ibelings BW, Bormans M, Huisman J. Artificial mixing to control cyanobacterial blooms: a review. Aquatic Ecology. 2016;50:423–441.
24. Hehman L. The effects of aeration on phytoplankton community composition and primary production in stormwater detention ponds near Myrtle Beach, SC. Columbia (SC): University of South Carolina; 2014. p. 1–69. https://scholarcommons.sc.edu/etd/2591.