Protecting the Shoreline

The shoreline is the interface between terrestrial inputs and in-pond processes, and its protection is a major component of pond protection. This report focuses on three general shoreline considerations.

First, soil erosion of the shoreline (e.g., slumping banks) can be a major source of suspended particles in the water column (reducing light penetration, inhibiting the growth of primary producers, and discoloring the water), and nutrient input (especially phosphorus, large quantities of which are loosely attached to soil particles).

turbid pond

Turbid, brown water is usually an indication of excessive sediment runoff, and can rapidly fill in a pond if not checked. Sediments are also particularly rich in P, and can thus stimulate algal growth.

Shoreline erosion may be exacerbated by fluctuations in the pond water level (leaving exposed soil) and by livestock (which may be a substantial contributor of both suspended sediments and nutrients).

Engineered solutions for protecting shorelines from erosion have traditionally included "hard armor" such as "riprap" (large, loose stones placed atop screening) along banks to control sediment erosion during storm events. More recently, bioengineering approaches have used the roots and stems of natural vegetation to stabilize shorelines. Biodegradable organic materials such as coconut fiber are used initially to prevent erosion while new plantings are becoming established.

selective riparian buffer

Selective removal of riparian vegetation at key access locations can enhance the view while maintaining pond water quality.

Eventually, the plants take over the task of bank stabilization as the organic materials slowly decompose. Bioengineered shorelines provide additional aesthetic benefits, serve as wildlife habitat and, once established, require little further maintenance, whereas traditional "hard armor" structures may weaken over time. Many ponds in Chester County use riprap or concrete "sea-walls", which are really more appropriate to environments receiving heavy wave stress than to the banks of small ponds.

cows in pond

(Left) Cows are frequent visitors to ponds when given the chance, eroding banks and adding nutrients. (Right) A mud bank produced by sediment runoff is exposed during dry times of the year (photo courtesy of B. Lathrop).

Second, establishing riparian buffer strips of vegetation along the shoreline to replace mowed lawn (currently the predominant riparian land use in Chester County) may likewise improve pond water quality. Turf grass has little root penetration and proportionally little capacity for sediment and nutrient retention, and the application of lawn fertilizers can add to the pond's nutrient load. Just how wide a riparian buffer should be is subject to debate, but minimum widths of 7.5 m (25 feet) are often recommended; any buffer is better than none. It is often useful to plan both an upland zone and aquatic zone within the buffer. Suggestions for planting and landscape design are provided under "Pondscaping" (Section VI).

riparian buffer

Installation of a riparian buffer of mixed prairie wildflowers produced this scene the following summer.

Establishing a riparian buffer zone of natural meadow, shrubs or trees can actually be financially logical as well. The property manager at a farm in East Bradford Township recently replaced 2 acres of mowed grass with wildflowers. The initial estimated cost of seeding was recouped within approximately 1 year by reduced mowing expenses, and the new buffer resulted in a profusion of wildflowers.

Trees can be a hindrance to recreational uses, and may interfere with the view of the pond. One way to have a riparian buffer while at the same time retaining an attractive "viewscape" is to trim lower branches and reduce the height of the herb and shrub layers The creation of adequate access points for fishing and landscaping specific locations where the pond can be seen, while keeping much of the remaining shoreline protected, can likewise facilitate enjoyment of the pond while maintaining its water quality.

Third, protection of an inflowing stream can be just as important as protecting the shoreline of the pond itself. Bathymetric (depth contour) maps of most ponds show that the shallowest areas occur near the inflow, typically because of sediments carried in by the stream. If land is scheduled to be developed upstream, properly installed sediment fences and well designed erosion control measures are absolutely necessary for pond protection. Where sediment inflows cannot be reduced by streamside buffers, a portion of the pond near the inflow can sometimes be engineered with a berm built just below the surface, forming a sediment trap in which the suspended material settles before entering the main portion of the pond. Trapped sediments must be removed regularly, however, for the design to be effective. Alternatively, the inlet area of the pond can be graded to encourage the growth of emergent aquatic plants, forming a wetland that will also function to trap sediments as they enter the pond.

bathymetric map

Bathymetric map of a pond in Westtown Township, showing the location of an underwater berm designed to trap sediments entering the pond.