Strawberry Creek has been degrading or downcutting into its streambed for some time due to natural equilibrium processes. This has caused considerable undercutting of streambanks. Bank erosion has been accelerated by the high peak storm flows in the creek. In addition, channelization and channel confinement have resulted in further downstream erosion and scouring of the streambed, contributing to loss of aquatic habitat Streambanks in narrow confined areas should be stepped back if space permits and the conf'ming walls replaced with "biotechnical" bank stabilization techniques. All areas of "hardened" concrete streambed should be removed. Central campus creek sites in need of rehabilitation or stabilization are shown on Figure 16 and described in Table 24.
The use of biotechnical solutions for bank erosion are emphasized in this report because of their cost effectiveness, durability, and environmental compatibility compared to strictly structural methods. The engineering and aesthetic aspects of these methods improve over time in contrast to strictly structural methods which tend to deteriorate over time. Biotechnical methods allow plants and structures to function together in an integrated and complimentary manner. Biotechnical or "bioengineering" systems generally combine live native vegetation with indigenous natural structural materials such as wood, stone, or brush. The following methods are recommended for future use in Strawberry Creek.
1. Revegetation: This is one of the simplest methods and has many advantages. The flexibility and resilience of vegetation increases surface roughness and reduces water velocity. This reduces streambank erosion and provides an opportunity for sediment filtration and deposition. The binding network of plant roots increases the shear strength of the soil which further reduces erosion. Vegetation provides a canopy which shades the creek and provides cover which enhances both wildlife and aquatic habitat The high regenerative capacity of many riparian species allows vegetative stabilization techniques to be self-healing and self-suRportive in contrast to traditional structural methods. Vegetation also provides an aesthetically pleasing appearance over a long time period. In addition, vegetative cover intercepts precipitation, reducing the impact of direct rainfall on soil and thereby reducing compaction and erosion. When selecting vegetation for streambank: stabilization or erosion control, a range of riparian vegetation should be considered. Pioneer or invasive species are particularly suitable for disturbed sites. Willows, alders, and other native riparian species have been used successfully and inexpensively in urban stream restoration projects. Another factor to consider in vegetation selection is the length of time needed for the plants to become established. Species selection should be based on soil compatibility, moisture holding capacity of the soil, and streambank slope. In scoured areas, woody plants established at the toe of the streambank in conjunction with grass planted above the toe have proven to provide good bank protection. The list of native riparian vegetation given in Appendix C should be used as a guide for revegetation and restoration of riparian plant communities. These are most effective for streambank: stabilization if planted during the autumn dormant period. Vegetation can be combined with a variety of other biotechnical stabilization techniques.
2. Live fascines (wattles): This is a flexible simple method that requires little soil disruption and grows into a natural aesthetic installation. Fascines are bundles of live plant cuttings which are wired together and secured into a streambank with live or dead stakes. These bundles are often placed on slopes parallel to the contour and can be used in combination with other vegetative stabilization techniques. Fascines are ideal for protecting streambanks from undercutting and seepage. They are particularly effective at the toe of banks subjected to scour and in situations where water level fluctuation occurs. At the water's edge at the toe of banks this method is durable even before the live cuttings have rooted. Willows are ideal plants to use in fascines.
3. Live cuttings/Brush matting or layering: This method involves planting rows of live riparian species cuttings to protect eroding streambanks or to modify a stream meander (Figure 16). This technique also includes "Palmiter" methods of staking dead trees or limbs (brush matting) to enxiing banks in order to slow water velocities on outside bends to allow for volunteer plant growth to establish itself. Live cuttings can also be planted in the brush matting. Matting may also employ live plant materials stacked on the streambanks. Brush layering techniques use live branches of riparian shrub or tree species inserted perpendicularly into the banks so that rooting occurs back into the slope of the bank rather than parallel to the slope as in brush matting. The established vegetation then stabilizes the bank through the root system which resists shear failures or slippage.
4. Gabions: These are rock filled wire mesh baskets which are wired together and filled in place with rock and soil to form a continuous bank stabilization structure (Figure 17). This methcxi may be used in narrow channels or in situaHons where the banks are too steep for other types of protection. However, use of gabions is recommended only if it supports revegetation of the channel. Gabions are flexible and porous, therefore they eliminate hydrostatic pressure buildup by allowing streambank seepage. Soil may be mixed with the rocks during filling of the gabions and subsequently planted with cuttings of native riparian species. If necessary, filter cloth can be used to help retain the soil in the gabions. As the vegetation matures, the durability and permanency of the installation will increase.
5. Wooden crib walls: These structures are built sloping back against steep streambank:s in situations where a retaining wall is needed for stability. Crib walls are particularly effective in areas with steep banks that experience high velocity flows. A rectangular framework of timbers or logs is assembled in log cabin fashion (Figure 19). The void spaces between the wooden members are filled with soil and rocks to provide strength and weight. Vegetation can be incorporated into crib walls by planting through the slats in the cribwork. This will result in an installation that achieves slope stabilization while retaining a natural appearance. By the time the wooden members rot out, the plant materials will have taken over the structural function of bank: stabilization.
6. Concrete crib walls: These crib walls are constructed of reinforced concrete structural members. Most crib walls in use today are built in this manner. Concrete crib walls are generally more durable and can be built to greater heights than wooden timber cribs. The basic construction and vegetation planting principles are identical to those employed in wooden cribs. Several concrete crib wall systems or designs are available. These systems differ in the shape of the structural members, wall configuration, structural connections, and erection procedures. Two examples of the range of designs available are the Hilfiker Concrib wall and the Humes Pincrib or Minicrib walls.
7. Welded wire walls: These are composite wire and granular fill structures built by placing welded steel wire mats between successive layers of fill material. The L-shaped form of the wire mats is designed to both reinforce the fill material and contain the face of the structure. The wire mats are lightweight and may be easily prefabricated. In addition to their low cost, flexibility, and ease of construction, welded wire walls can be planted with vegetation that will grow through the wire mesh. Plant roots help bind the structure together as well as providing aesthetic amenity.
8. Dry stone walls/Rock walls/Wooden plank: walls: In particularly difficult areas characterized by steep or vertical banks, confined working space, and high velocity flows these techniques may be useful. Extensive hand labor and skill is required to build a mortarless dry stone wall, but they are quite effective and durable. The rocks can be interplanted with native vegetation that serves both an aesthetic as well as an important structural function.
9. Riprap (stone dikes): Riprap consists of carefully placed layered stones or boulders generally placed parallel to the toe of the bank to deflect flows and provide toe protection. It is one of the most common and effective methods of bank protection and is applicable under most conditions where bank: erosion occurs. Riprap may be placed along the bank: slope's natural angle of repose to help control the erosion and induce sedimentation behind the riprap. It is flexible enough to settle and conform to the final streambed contour if scour is a problem. Riprap may also protrude from the bank: as a "hard point" which provides some degree of flow deflection, allows sedimentation, and acts as a tieback to riprap placed along the bank: to prevent flanking. Areas not covered by riprap may be protected by vegetation. Limitations to the use of riprap include the availability of suitable rocks as well as the difficulty and expense of rock placement.
10. Chemical stabilization: A number of chemical substances such as acetates, resins, latexes, and lime can be used to help prevent erosion of streambank: soils by increasing the cohesiveness of the soil. Researchers found that when lime was mixed with clay, complex calcium and aluminum silicates formed as reaction products which imparted hydrophobic or impervious properties to the soil (U.S. Army Corps, 1981). A thin layer of topsoil can then be placed over the soil-lime mixture to permit the growth of vegetation.