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Pond Dredging – Why and When to Dredge My Pond?

Is your pond getting shallow? Accelerated sedimentation is one of the most common problems in storm-water ponds. In fact, all urban water bodies have a tendency to fill in with time and become dry land. Therefore, reserve funds should always be set aside for pond renovation, including pond dredging.

Disturbed soil is carried by storm-water runoff across surfaces such as rooftops, streets, and parking lots. Most of this material ultimately ends up in the nearest pond or canal. The accumulation of organic “muck” also contributes. Organic sediment is created by the annual growth and decay cycle of algae, aquatic weeds, fallen leaves, and grass clippings. This process called _eutrophication_ is accelerated by landscaping practices and over-fertilization within the watershed.

How pond dredging can lower costs long term

If done correctly, pond dredging will remove most of the excess nutrients (phosphorus and nitrogen) accumulated in the pond bottom. The end result of renovation is reduced management costs, better water quality, and a more aesthetically pleasing resource.

Sedimentation can be detrimental in several ways. It creates optimum conditions for the growth of algae and aquatic weeds. A general rule of thumb is that ponds should reach 3-4 feet deep as quickly as possible from the shoreline. Maximum depths should be a minimum of 8-10 feet. This reduces sunlight penetration to the bottom where algae and aquatic weeds begin their growth. Heavy sedimentation also increases the likelihood of fountains and submersible pumps getting clogged; increasing long term maintenance and repair costs. In extreme cases, sedimentation elevates the risk of urban flooding by reducing the storm-water holding capacity of the pond. And lastly, mosquito larvae are more likely to survive in shallow, stagnant areas with little or no water movement. This is important in this day and age of West Nile and Zika viruses.

Sediment surveys from Magnolia Fisheries

Sediment surveys are essential for establishing a baseline for future monitoring and obtaining dredge cost estimates. The cost of dredge projects are driven by the amount of accumulated material that needs to be removed. A general rule of thumb is that storm-water ponds should be renovated every 15-20 years. However, urban development and soil disturbance within the watershed may expedite the need for attention.

Sediment surveys include several steps. The first being onsite data collection using sediment probes and survey grade GPS equipment. The next step involves using high tech software to convert the data into detailed contour maps that show the location and depth of the accumulated material. The same software is also used to create a bathymetric map of the pond. And finally, the information is summarized into an easy to understand report. Most importantly, the report will include a calculation of the total volume of sediment in the pond. This number is critical when obtaining dredge estimates.