Drainage Design Manual - the City of San Luis Obispo

cylindrical structures (Figure 6-1). They are placed in shallow trenches on contour. Theyare principally used to prevent slope erosion and very shallow slumping and rilling. Sincethese techniques rely on rapid plant growth for stabilization, they are best used on sunnysites, or in combination with erosion control blankets at most bank repair sites.• Erosion Control Fabric Planted w. Rooted Trees/Shrubs. This technique involvespreparing a smooth surface along the upper bank, seeding with grass and appropriatefertilizer, and covering this surface with an appropriate biodegradable erosion controlfabric (Figure 6-2). The erosion control fabric provides temporary protection againstsurface rilling and gullying while the grass becomes established and, when properlyinstalled and stapled to the ground, can provide protection against scour from high streamflow. Rooted native trees and shrubs selected based on the sun and water availability onthe bank are planted through the erosion control fabric at relatively close spacings. Theseplantings are then irrigated periodically for several years until they become permanentlyestablished and can be counted on for permanent scour protection for the upper bank.This is one of the most labor-intensive upper bank protection techniques and is thepreferred alternative for projects where toe erosion or stream velocities are not high orwhere equipment access is poor. At some locations use of rock toe protection is neededwith this technique.• Brush Layering. This technique is also called branch packing. It consists ofconstructing a slope fill section by alternating layers of live sprouting branches andcompacted earth backfill (Figure 6-3). The front of the live branches should protrudeslightly into the channel, while it is important that the butt ends contact native bank soilsat the rear of the fill. Brush layering can be used to repair small, localized slumps andholes in streambanks. The upper limit of the fill section should be less than 2.7 to 3meters (9 or 10 feet) in thickness, while depth can extend back 2.4 to 3 meters (8 to 10feet). Fill sections are designed in consideration of soil conditions, slope, and watervelocities. Typical vertical section spacing ranges from .9 to 1.5 meters (3 to5 feet), andslopes as steep as 1.5H: 1V can be constructed. This technique has limitations on dry orshady sites.• Brush Mattress. A brush mattress is a combination of live stakes (usually willow), livefascines or willow wattles, and branch cuttings installed to cover and stabilize erodingstreambanks (Figure 6-4). Usually twine or wire constructed and held in place usingstout stakes as a sort of cross-laced grid system is used to hold the brush down until someof it begins to sprout. This method can be used to form a live armor against fast flowingwater with abrasive power, but is less effective combating toe scour or slumping. Thebrush layer is usually cross-tied to the slope using stakes and rope or wire. Adequatechannel flow capacity must also exist. This technique is best used in rural areas wherethere is a supply of brush near the job site, where inexpensive labor is available, andwhere banks are not particularly steep or high.• Fiber rolls/ fiber rock rolls/ coir erosion blankets. Fiber rolls are cylindrical structuresfilled with coconut husk fibers bound together with twine woven from coconut(Figure 6-5) Typically they are available in 304 mm to 457 mm (12 or 18 inches)SLO Creek Drainage Design Manual 71 February 2003