Spacing of Berms for Erosion Control along Pipeline Rights-of-way

Diverter berms installed on slope sections on the Western Route Export Pipeline near Tbilisi, Georgia, to control runoff and erosion and support biorestoration work have not prevented extensive rill and gully erosion from occurring. The berm spacings in the field are compared with those obtained from empirical equations currently used by pipeline engineers, the Universal Soil Loss Equation (USLE) and analytical methods based on the maximum non-eroding velocity of runoff and a maximum acceptable wash-out depth. Only the USLE and the maximum acceptable wash-out depth consistently yield spacings which are closer than those adopted on the slopes. Empirical methods cannot be used with safety. For a storm with 10-year return period, the USLE yields spacings on very steep slopes that are so close together as to be impracticable. For the conditions in which berms would be most commonly used, methods based on maximum non-eroding velocities give very similar results but spacings are still too wide at some of the study sites. Using a maximum non-eroding velocity which is 80% of the published values allows for scour by sediment-laden flow and produces closer spacings. If wider spacings are used than those required to achieve a specified level of performance, there is a risk that seeds planted for biorestoration will be washed out and that erosion will expose the pipe. A procedure is presented for determining the time period over which exposure may occur for a given level of risk.