Drainage Design Manual - the City of San Luis Obispo

• A-Jacks are a potentially cost-effective alternative to rock rip-rap or other hard toeprotection techniques. They consist of a three dimensional star shaped structure (similarin shape to children’s jacks) constructed from concrete (Figure 6-9). Rows of A-Jacksare assembled to interlock in horizontal and vertical directions. Typically they have apyramidal structure, with a base of three to four, or more units, decreasing in numberwith height. The voids in the matrix are back filled with soil and planted to nativeriparian trees and shrubs. Commonly 609-mm (24-inches) A-Jacks are used inmoderately sized streams, are toed in 0.3 to 6 meters (1 to 2 ft.), and rise in height from1.2 to 2 meters (4 to 7 feet). The 609- mm (24-inches) A-Jacks weigh 35 kilograms (78pounds) and can be constructed using hand labor or light equipment. Willow staking,willow wattling, erosion blankets, or other upper slope biotechnical approaches are wellsuited for use with A-Jacks. A-jacks do have aesthetic limitations that need to be takeninto consideration for use at some highly visible sites.• Traditional placed rock rip-rap consists simply of placement of rock on existingslopes. Often, this technique has been employed with little or no slope preparation, butgenerally a toe trench is constructed and key-ways are excavated into the bank at eitherend of the revetment. When carefully constructed, rocks can be placed around obstaclessuch as trees. This technique requires careful selection and placement of rock material.This technique is not recommended for widespread usage in the SLO creek watershedbecause of concern over biological and geomorphic impacts.• Loose rock rip-rap consists simply of placement of rock on existing slopes grades withlittle or no slope preparation. Generally a toe structure is not constructed. The advantageof this method is that rocks can be placed around obstacles such as trees. Thedisadvantage is that the finished embankment is not as stable as a properly engineeredand constructed slope, because of the possible lack of toe support and the fact thatfrequently the slopes are steep, to 1H:1V. Sometimes pipes are driven into the toe of thechannel to provide support for the loser rack. This technique requires careful selectionand placement of rock material. As with planted rock rip-rap, live willow stakes can beinserted in voids between rocks. Loose rock can often be used as a transition structureupstream and downstream from the main bank protection element.• Concrete block walls can be constructed in a variety of ways, including reinforcedmortared cinder blocks, stacked slab concrete, concrete or cinder blocks with open rearfaces that are set on top of each other at slight incline and filled with soil (i.e., Keystoneblock walls), or larger concrete bins that are backfilled with soil and have their front edgeplanted with vines. These walls provide good shear resistance and slope support, and canbe designed to be somewhat flexible, however they deflect flows more than most of theother techniques listed here, and are very difficult to completely vegetate and disguise.This technique has very limited applicability in the SLO Creek watershed forenvironmental reasons, but could be considered where top of bank land constraints andchannel capacity limitations require near vertical walls be used.SLO Creek Drainage Design Manual 73 February 2003