Dredging

Sediment removal can provide a variety of long-term benefits, and thus can be viewed as cost-effective despite its considerable expense up front. First, the increase in water volume provided by dredging increases hydraulic retention time, and also provides a larger reservoir of water to buffer against night-time or seasonal declines in dissolved oxygen. Dredging can thus reduce the possibility of fish kills.

Second, highly organic, nutrient-rich sediments are removed from the system. The release of nutrients from such sediments when oxygen becomes depleted near the bottom during summer can cause the return of phosphorus to the water column, defeating attempts to control external inputs of phosphorus from the riparian zone or larger watershed. (By contrast, removal of inorganic sediments resulting from bank erosion or stream transport, while providing benefits of pond deepening, does less to counteract internal nutrient recycling.)

Third, shallow ponds experience frequent resuspension of sediments during storms. The suspended material not only adds nutrients to the water column but also greatly reduces light penetration, often suppressing the growth of aquatic plants. Deeper sediments are less likely to be resuspended.

As sediments accumulate, the pond becomes progressively shallower, and the bottom is increasingly exposed to greater light intensity. This often allows aquatic plants to grow much more rapidly, and pond owners often take notice when submersed plants such as pondweeds start to become obvious at the surface. As stated in Section IV, moderate abundances of aquatic vegetation are a very healthy contribution to pond ecosystems. If plants become dominant throughout the pond, however, their presence can produce large fluctuations in

There can also be unexpected negative impacts of dredging. For example, deepening a pond changes the availability of light for primary producers. A good way to evaluate the probable effect of pond deepening on light reaching the bottom is to compare the present and proposed future depths of the pond with the "compensation depth", the depth to which 1% of incident light penetrates and below which few plants can grow. If the pond is deepened uniformly to a depth exceeding the compensation depth, few rooted aquatic plants will grow, releasing phytoplankton and making the pond appear greener than before. One recommendation is to dredge portions of the pond while leaving shallower areas near shore to sustain healthy plant communities.

There are a wide variety of techniques for dredging, involving either barges positioned on the pond, or more commonly equipment operated from shore. Prior drawdown of the pond is often a cost-effective first step to allow equipment into the pond basin.

Both state and federal permits are required before starting a dredging project (some excavation companies will handle the permitting process for the client). The principal permitting concern relates to disposal of the dredged material. The permits require sediment testing for toxic substances, which, if found, restrict the options for sludge disposal. On-site disposal is less expensive, but requires a natural, non-wetland depression. Off-site disposal involves shipment of the material elsewhere by dump truck. In either case, dewatering is a necessary first step in treatment of the dredged material.

Because the procedure is expensive, cost considerations should be carefully researched; dredging companies are well advertised on the internet, and most environmental firms subcontract the actual sediment removal to firms that specialize in dredging.

 Pond strati graphysm72

Dredging following drawdown during winter at a development in Willistown Township (photograph courtesy of R. Stephanou).

Both state and federal permits are required before starting a dredging project (some excavation companies will handle the permitting process for the client). The principal permitting concern relates to disposal of the dredged material. The permits require sediment testing for toxic substances, which, if found, restrict the options for sludge disposal. On-site disposal is less expensive, but requires a natural, non-wetland depression. Off-site disposal involves shipment of the material elsewhere by dump truck. In either case, dewatering is a necessary first step in treatment of the dredged material.

Because the procedure is expensive, cost considerations should be carefully researched; dredging companies are well advertised on the internet, and most environmental firms subcontract the actual sediment removal to firms that specialize in dredging.