Problem Solving in Paradise Pond: Usage of Wetlands to Reduce Sediment Accumulation

12 May

Over the last few weeks of this semester, students in our Ecohydrology class at Smith College have taken on challenging hydrologic questions that are relevant to our community. In this post, Brittany Bennett and Nicole DeChello address the question:   What is the potential for riparian wetlands along the Mill River – either constructed or natural – to reduce sediment accumulation in Paradise Pond?

Few landmarks are as cherished as Paradise Pond at Smith College, but maintaining the idyllic space is not without a price. Sediment from the Mill River builds up at the bottom of the pond over time. The college must dredge the pond on a regular basis, about once every 8 years, in order to prevent the formation of an additional island. In 1990, 27,000 cubic feet of sediment were removed from the pond (Hartwell, 2014). For comparison, that is almost a third of an Olympic sized pool of sediment. In addition to the high costs associated with this process, dredging disturbs the local environment and is highly disruptive to all who use the pond. The installation of a riparian wetland upstream could reduce sediment in Paradise Pond after specific rain events, but in the long term will not solve the problem of sediment accumulation.

ParadiseParadise Pond, Smith College

A wetland is an area of soil persistently covered with slow moving water. Riparian wetlands, shown in the image below, exist alongside rivers and are distinguished by distinct vegetation, such as cattails and pondweed, and have a variable water level that is affected by the river flow (pulse-fed wetland). The hydroperiod of a wetland is the seasonal pattern of the water level, and it defines the type of wetland and affects nutrient cycles.

wetland
Riparian Wetland Location (Mitsch, 2000).

Wetlands are complicated habitats because the three main characteristics -hydrology, physiochemical environment and biota- are all interconnected. As a result, hydrologists have not been able to use traditional equations, such as Penman or Thornthwaite, to accurately quantify the wetland water balance (water into and out of the wetland), seen in the image below.

balance
Water balance for a wetland (Mitsch, 2000)

Water comes into the wetland from the stream (S), from groundwater (G), which exists under the soil, and from precipitation (P). The water can leave back through the stream, into the groundwater or through interception and evapotranspiration, which involve turning liquid water into vapor that enters the atmosphere. The inflows to a wetland are largely related to season. During rainy periods, the inputs fluctuate according to rain events, such that the water level of the wetland goes up when it has rained because of the precipitation input and the increased water in the stream.

Rain events also correspond to increased sediment in the river because rainfall washes away dirt when running over land and can cause erosion along the riverbanks. As the sediment travels with the river, some will encounter wetlands. As the water enters the wetland, it slows down due to a wider space to travel through, which causes the flow to lose energy and its ability to continue to carry the sediment load. The deposition of sediment in turn creates niches for diverse habitat development. Some of the sediment that enters the wetland will decompose, but sediment can also start to build up in a wetland and prevent water from flowing through. Wetlands do not keep the sediment forever; water also flushes the wetland of waste products, particularly from soil and root metabolism (Mitsch, 2000). Furthermore, fish, wind, and/or other factors can cause sediment to become resuspended in the water, especially during times of shallow water depth (Dieter, 1990). The life cycle of wetland sediment is summarized in the image below.

flows
Sediment flows in a wetland (Kadlec, 1996)

The sediment from the river, as seen above, can accumulate in the wetland and some will decompose, but some sediment will also become resuspended in the water and the aquatic life can create sediment that travels downstream. The sediment that is traveling down the Mill River is mostly coarse sand and organic debris. The image below shows the banks of the Mill River.

erosion
Erosion along the Mill River (photo: Alex Julius)

During rain events, precipitation will run over the top of the soil and, at areas similar to the one shown above, continue to erode the banks carrying soil and organic debris into the river. The sediment travels downstream and ends up in Paradise Pond. When the Mill River joins the pond, the area widens and allows the stream to slow down. The reduction in velocity leads to sediment accumulating at the bottom of the pond, similar to the way sediment accumulates in a wetland. The main area of deposition is at the junction of the river and the pond, adjacent to the island, as seen in the picture below.

mr map
Map of Mill River through Paradise Pond (Google Maps)

As mentioned previously, about 27,000 cubic feet of sediment were removed from the pond in Smith’s largest dredging project. To assess whether a riparian wetland could eliminate the need to the dredge the pond, the maximum volume of sediment a wetland could store was determined. A study of constructed wetlands in Illinois measured accumulation rates in wetlands to be 0.5 to 1.0 centimeters per year, which were admittedly overestimations (Fennessy). For a hypothetical one-acre one-foot-deep wetland, at the maximum observed rate it would take 18.9 years for 27,000 cubic feet of sediment to accumulate. An almost 19 year timespan exceeds the average 8 years between dredging projects. This simple calculation does not take into account the decomposition of sediment over time or the resuspension of sediment. Several acres of wetland could be added upstream to reduce the timespan, but considering the issues of private property and number of acres necessary this does not seem reasonable. At the simplest level, an upstream wetland does not seem to be a feasible alternative to dredging.

Citations

Brooks, Kenneth N. Hydrology and the Management of Watersheds. Ames, IA: Iowa State UP, 1997. Print.

Dieter, C.D. 1990. The importance of emergent vegetation in reducing sediment resuspension in wetlands. J. Freshw. Ecol. 5:467-473

Fennessy, M. Siobhan, Christopher C. Brueske, and William J. Mitsch. “Sediment Deposition Patterns in Restored Freshwater Wetlands Using Sediment Traps.” Ecological Engineering 3.4 (1994): 409-28. Print.

Kadlec, Robert H., and Robert L. Knight. Treatment Wetlands. Boca Raton: Lewis, 1996. Print.

Mitsch, William J., and James G. Gosselink. “The Value of Wetlands: Importance of Scale and Landscape Setting.” Ecological Economics 35.1 (2000): 25-33. Print.

One Response to “Problem Solving in Paradise Pond: Usage of Wetlands to Reduce Sediment Accumulation”

  1. google.com August 28, 2014 at 9:37 am #

    Thanks for finally talking about >Problem Solving in Paradise Pond: Usage of
    Wetlands to Reduce Sediment Accumulation | (CEEDS) <Loved it!

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