Drainage Design Manual - the City of San Luis Obispo

• Live Rock Wall. Rocks can be end dumped on a stream bank, or they can be carefullyhand crafted into aesthetic walls that can support vegetation (Figure 6-10). While suchwalls are very structural, they also have a role in restoration. In constricted situations, ahand-laid rock wall can be built to provide a stable bank. Concrete rubble pieces cansometimes be used in place of rock and still look similar to a rock wall, as in the MissionPlaza area. The spaces between the rocks or rubble pieces can be planted with cuttings asthe wall is constructed. This sort of wall is referred to as rock work, as opposed to rip-rap.This approach preserves bank top land allowing for nearly vertical banks. Typically thisis used when bank top encroachment is severe and valuable property or structures arethreatened.Post and Wire Fencing is a continuous single or double row of pilings with a facing ofwoven wire (Figure 6-11). The pilings (either wood or metal posts) are typically placedsix to eight feet apart, with maximum heights of 1.8 meters (6 feet). The posts areinserted in pre-drilled holes or are driven in to a depth of at least half their height, refusal,and/or below anticipated scour depth. Live brush cuttings, inter-layered with rock andsoil can be laid behind the post-and-wire fencing. Vertical slopes to six feet in height canbe constructed using this technique. The structure can have a significant affect onchannel flow resistance, which needs to be carefully considered in flood prone areas.They are also susceptible to damage from woody debris carried in high flows. Thistechnique has limited applicability in the SLO area.• Planted Gabion Baskets consist of rectangular containers prefabricated from twistedheavy gauge galvanized steel wire (Figure 6-12). The empty baskets are placed inposition on a prepared bed, wired to adjacent baskets, and filled with rock. The basketscan be planted by placing live branches on a layer of soil between successive baskets. Itis very important to toe in the bottom basket at least the depth of anticipated scour,usually at least one basket in depth. Gabions could be used on the project site but therecost is typically more then some of the other options and are usually reserved whenproperty and structures must be protected. This technique is often difficult to permit andis relatively expensive. Unplanted gabion baskets normally will not be considered in theSLO watershed, except for at very small and constrained sites.• Reinforced Concrete. Reinforced concrete may be used selectively to armor seriouslyfailing sections of streambanks. Structural elements include walls, bulkheads, and banklinings. Positive drainage behind these structures is required to relieve uplift pressures.These systems are inflexible and prove to fail under lateral forces, or when undercut bychannel bed erosion. Generally the use of reinforced concrete channel lining should beavoided or used in short segments where other methods cannot be considered because oflack of space or other reasons.• Modular Pre-cast Units. Interlocking modular pre-cast units of different sizes, shapes,heights and depths have been developed for a wide variety of applications. These servein the same manner as gabions. Units with void areas can allow the establishment ofvegetation. They provide verticality in tight areas as well as durability. Many types areSLO Creek Drainage Design Manual 74 February 2003