Stream Restoration: Bank Stabilization Practices
Bank stabilization practices, often referred
to as bioengineering, are a nonstructural means of stabilizing streambanks
from further accelerated erosion. Bank stabilization practices that rely on
vegetation to protect streambanks are much more sensitive to the effects of
urbanization than more structural practices. While the effects of increasing
imperviousness are less noticeable with structural practices, bank stabilization
practices in highly impervious watersheds tended to be less successful. This
is the primary reason bank stabilization (e.g., nonstructural) practices are
utilized less frequently or used in combination with bank protection practices.
While these practices have been found to be very effective on rural and agricultural
stream channels (i.e., low impervious cover), they are less able to withstand
the elevated storm flows, high stream velocities, and rapid water level fluctuations
that occur in urban streams.
Coir Fiber Rolls
Coir fiber rolls are commercially made erosion control products. They consist of tightly bound cylinders of coconut fiber (coir fiber) held together by a coir fiber netting. They are generally available in 10 to 20 foot lengths and are 10 to 12 inches in diameter. They are excellent at providing toe protection where scour is not severe. Once installed, the coir fiber log becomes saturated with water and vegetation can be planted directly in them. Coir fiber rolls provide a natural, unobtrusive appearance and decompose over a three to six-year period leaving the roots of colonizing vegetation to secure the toe of the streambank. They are relatively lightweight (10' length = 75 lbs) and can be installed with a minimum of site disturbance. The only limitations to coir fiber rolls are that in areas of severe scour they are not appropriate and there must be sufficient sunlight available for colonizing plant growth.
Coir fiber rolls are installed by excavating a shallow (3 to 4 inches deep) trench along the toe of the stream bank. The coir fiber log is placed in the trench so that the bottom and back are in tact with the stream substrate and the streambank. Stakes are then driven down along its sides. Coir or nylon twine is woven between and around the stakes and the stakes are driven in firmly, securing the coir fiber log to the streambed. The streambank above the coir fiber log is stabilized using other bank stabilization techniques (Figures 33 and 34).
Figure 33: Section View of Coir Fiber Log Installation
Figure 34: Profile of Coir Fiber Log Installation
Live fascines are tightly bound bundles of live but dormant willow, alder, or dogwood branch cuttings. Each fascine is approximately eight to 10 feet long and eight to 10 inches in diameter. The bundles are bound with either wire or twine. The fascines can be used as toe protection in areas where toe scour is not severe, or combined with bank protection measures and placed higher on the bank where scour is a greater threat. The typical installation is to place the fascines in a shallow trench along the streambank parallel to the stream. Once installed the dormant cuttings will root and grow, adding structural stability to the streambank and preventing down slope erosion and rill formation. On taller streambanks, multiple rows of fascines can be installed for stabilization. Live fascines are intended to take root and grow, but should this not happen, the woody cuttings will still provide several years of physical stabilization to the streambank. As live fascines utilize dormant cuttings, they must be installed during the non-growing season; generally early spring is best (Figures 35 and 36).
When used along the toe of the streambank, live fascines have significant potential to enhance stream habitat by promoting the creation of undercut banks and overhanging bank cover. When used higher on the streambank they can provide overhanging bank cover and a source of organic material to the stream.
Figure 35: Section View of Live Fascine Installation
Figure 36: Profile of Live Fascine Installation
A brush mattress is another technique that utilizes dormant branch cuttings. Rather than a tight bundle, a brush mattress is a thick mat of dormant cuttings placed on the bank and held down with stakes. The intention of a brush mattress is to create structural streambank protection that in time will root and provide vegetative stabilization.
The brush mattress is installed by first grading the streambank to the desired stable angle. Brush mattresses are most successful on slopes not exceeding 2:1. A shallow trench is then cut behind the toe protection (coir fiber log, boulder revetment, etc.) and the cut ends of the branch cutting placed in the trench. This trench is to ensure good soil contact and water for the branches to root. The branches are laid down perpendicular to the stream flow until the bank is barely visible through the branches. Stakes are then driven partially into the brush mat on two-foot centers. Wire or strong twine is then woven between and around the stakes. In order to insure good soil contact, as much loose dirt as possible is then agitated into the brush mat. Once the dirt has been added, the stakes are driven in fully to tightly press the brush mattress against the streambank. It is important for the growth of the brush mattress that as much brush mat/bank soil contact is made as possible. As brush mattresses utilize dormant cuttings, they must be installed during the non-growing season; early spring is best (Figures 37 and 38).
Figure 37: Section View of Brush Mattress
Figure 38: Profile of Brush Mattress
Erosion Control Matting
Erosion control matting is a geotextile fabric made of either natural or man-made material with the purpose of providing temporary soil stabilization while vegetative stabilization germinates or roots.
Erosion control matting is manufactured in many forms. A commonly used product in stream restoration is matting made from coir (coconut) fiber. The advantage of coir fiber is that it is long lasting but biodegradable. Similar matting is also made out of wood fiber (curlex). However, many of the wood fiber products are not fully biodegradable, as they utilize a nylon mesh to hold the fibers in place. There are also several types of non biodegradable erosion control matting, generally made of plastic. These mats are utilized in the same way.