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e e e TRMs permit soil filling and/or retention and the development of vegetation within the matrix. Both rigid and flexible lining systems can be used in waterways to protect channel beds from scouring and to stabilize side slopes. Vegetative cover, rip rap, and RECP's are all considered "flexible linings" when employed as channel protection. Rigid linings, like concrete or asphalt, generally withstand higher discharges, flow velocities, and shear stresses. However, they may fail entirely if a portion of the lining is damaged, specifically scour due to over topping flow events. Other drawbacks include: I) susceptible to subsidence and frost heave; 2) prohibit infiltration; 3) cannot capture sediment; 4) increase runoff velocities and discharges; 5) do not look natural; 6) provide no wildlife habitat, and 7) costly to construct. According to the Georgia Department of Transportation (GADOT), where hydraulic conditions permit, the use of flexible lining materials will provide a more economic alternative to rigid lining materials. Furthermore, fI,exible linings have a greater ability than rigid linings to adjust to dynamic foundation conditions such as freeze-thaw, swelling and soil pore water pressures.5 Flexible linings can conform to changes in channel shape and are capable of rehealing themselves when slightly damaged. Flexible linings are also less expensive, allow infiltration and exfiltration, and provide temporary and permanent erosion protection while allowing vegetation to reestablish itself, and they possess a natural and aesthetically pleasing appearance6 The GADOT began using RECPs in the early 1980's. RECP prices then averaged $5.40 per square yard compared to c;oncrete ditch paving at $ I3, 64 per square yard. (These prices are roughly the same today; in addition, typical costs of rock rip rap range from about $20 to $45 per square yard.) Through Dc:cember of 1987, the use ofRECPs instead of concrete ditch paving had saved the GADOT over $5.4 million in contract lettings7 The EPA supports vegetation as the most effective way to control erosion.s Vegetative covers reduce erosion by shielding the soil surface from faI1ing rain; dispersing and decreasing the velocity of surface flow, which allows sediments to drop out or deposit; physically restraining soil movement by holding the soil in place with plant roots; increasing infiltration rates by improving the soil's structure and porosity through the incorporation of roots and plant residues; and -4-