Natural Channel Design
Natural channel design uses the principles of fluvial geomorphology (the processes and pressures that shape river systems) to evaluate the potential for a degraded stream to be restored to its historic stable form. The end result is a self-sustaining stream system that provides valuable hydraulic (water transport), geomorphic (sediment erosion and transport) and ecological (habitat, water quality) functions.
In a healthy system, a stream will overflow its base flow channel every 1.5 years (on average). This allows pollutants to be processed on the floodplain, riparian plants to flourish, the flowing stream to be flushed of organic materials, and most stream functions of value to humans to be performed. In a degraded system, the channel has often eroded down so deeply that the stream is no longer connected to its floodplain, except during the most extreme flood events. This limits stream functions and exacerbates bank erosion problems.
Urban streams are commonly channelized and armored with rock or concrete to protect infrastructure built in the floodplain. While this is an effective local flood control method, it does not allow the stream to function as a natural system, and often increases problems downstream. Natural channel design techniques will vary depending on whether there is space adjacent to the channel to allow for increased water on the floodplain without damaging nearby infrastructure.
Bioengineering integrates natural materials
Streambank bioengineering is a restoration method that integrates living plants in bank stabilization, as opposed to using strictly inert materials (logs, rocks, etc). With their flexible stems and dense roots, the native willows and dogwoods that grow right at the water’s edge are invaluable to help stablize stream banks. The roots hold the soil together while above ground vegetation protects against flood damage by dissipating energy and acting as a buffer against the abrasive effects of sand, silt and rocks carried in the stream. These riparian shrubs thrive on the periodic “pruning” caused by seasonal high flows and browsing and will resprout vigorously. Branches that are broken off and carried downstream can take root anywhere along the stem, a feature that makes willows, dogwoods and cottonwoods an important component of streambank bioengineering projects. Cuttings can be harvested in large quantities and then installed in a variety bioengineering applications, per site-specific needs.
Cuttings from dormant riparian shrubs are planted deep enough in streambank soils to reach the dry season water table. These “live stakes” are used in a variety of bioengineering techniques. Stem nodes underground will take root; nodes above ground will leaf out.
Rock is used in conjunction with plants to stabilize banks where velocities are high, long term durability is needed, immediate stabilization is needed, or there is significant threat to life or property. Rock is often used to protect the toe of the bank (where the stream bed meets the stream bank), which is the area of highest stress in the stream channel. Instream rock structures are used to deflect flow away from eroding streambanks, concentrate flow in the center of the channel, create pool habitat for fish, and encourage sediment deposition and scour.
Restoration becomes part of the landscape over time
When designed correctly, natural channel design and bioengineering projects require less maintenance, blend into and become part of the natural landscape over time, and ultimately reduce overall costs.
Bare and eroding banks are reshaped and revegetated in the stream restoration project (image below). This project utilizes natural channel design techniques.
Mouse over the image to see the project after restoration
Photos: San Antonio River Authority