Maintenance is an inevitable part of any trail system. The Wissahickon Valley Park has miles of trails. Many of these trails are located on steep slopes and stream sides. This makes for interesting walking, but it also makes more frequent trail repair work.
Unpaved footpaths are, by their very nature, prone to more erosion. As a trail is used, the top of the trail is worn away. When it rains, stormwater runs down the trail and picks up material from the trail. As a trail ages, the center of the trail tends to become gullied. Water is directed to a narrower and usually steeper area. In some places, trails have been eroded several feet below their original path.
As a trail degrades, users tend to widen the trail by walking or cycling around damaged areas. These 'rogue' trails increase the disturbance to the surrounding natural area and increase the amount of maintenance work that will have to be done in the future.
An important part of trail repair is directing water off and away from trails. Many of the techniques used in trail repair are focused primarily on keeping stormwater off of the trails.
Below are descriptions of some of the techniques used to repair trails.
These are diagonal humps that run across the trail in order to direct water into a storm drain, or to simply divert the water away from the trail. They are usually obvious mounds of material.
This is similar to a water bar, except that rolling dips are usually longer and more subtle than the water bars. The primary goal is to divert water away from the trail. They work with the natural slopes and grades in the trails.
Once a trail has been severely degraded to the point that deep gullies have formed in the trail, work must be done to restore the tread of the trail. Generally, the gullies are filled with layers of rock and soil. The material becomes finer towards the surface of the trail. Ideally the top material would include a mix of clay and fine rock to prevent erosion due to use.
Swales, or stormwater drains, are often built along the edge of trails to keep water away from the middle of the trail and to drain water to areas that are more capable of handling the water.
Stormwater runoff is a serious issue in this area due to the large amounts of impervious surfaces found here. Impervious surfaces are any surfaces for which water cannot penetrate, such as roads and parking lots. When stormwater moves across impervious surfaces, it picks up sediment and other pollutants. In a natural environment, some water would be absorbed by the ground and be released slowly over time. On impervious surfaces, water moves quickly and gets to receiving waters quickly (bringing with it whatever pollutants it picks up along the way). This results in more water moving faster into nearby streams. When stormwater reaches the stream, it scours the streambanks and the stream bed. This causes the streambanks to become steeper and less stable. Stream channels then become wider. The combined result is a loss in habitat and a deterioration of water quality. These conditions also cause higher flows during storm events and drier streambeds during periods of drought.
In addition to the stormwater that flows across surfaces into streams, stormwater is also diverted into storm drains, which then discharge water into a stream directly from a pipe. The receiving stream is further impaired at the point where the storm outfalls discharge water.
The Wissahickon Watershed is characterized, especially in the areas in Philadelphia, by large amounts of impervious surfaces and the resulting large amounts of stormwater. Stormwater management is an important priority for the overall health of the Wissahickon Creek and the stability of the park. By restoring riparian areas (areas near a stream), we are helping to reduce the impact of stormwater on the streams of the Wissahickon Watershed. Planting trees and shrubs is an important element to stormwater management.