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EM 1110-2-1601 1 Jut 91 (1) Method A. For riprap revetments 12 in. thick or less. the normal riprap layer should be extended to areas where velocities will not erode the narurnl channel banks. (2) Method B. For riprap revetments exceeding 12 in. in thiclrness. one or more reductions in riprap thick- ness and stone size may be adopted for a distance a (plate 41) in which velocities decre:lSe to a noneroding narurnl channel veiocity. (3) Method C. For all riprap revetments that do not terminate in noneroding narurnl channel velocities. the ends of the revetment should be enlarged, as shown in Plate 41. The dimensions a and b should be 3 and 2 times the layer thickness. respectively. The decision to terminate the revetment in erosive velocities should be made with caution since severe erosion can cause the revetment to fail by progressive flanldng. c. Length. Riprap revetment is frequently c:uried too far upslre:lm and not far enough downstre:lm of a channel bend. In a lrapezoidal channel. the maximum velocities along the outer bank: are often located in the straight reach immediately downStre:lm of the bend for relatively large distances downsa-eam. In a natural channel, the limit of protection on the downsa-eam end should depend on where the now crosses to the opposite bank:. and should consider future bar building on the opposite banIc. resulting in channel constriction and incre:lSCd velocities. Guidance is generally laclcing in this area. but review of aerial photographs of the subject location can provide some insight on where the crossover flow occurs. Model tests in a sand bed and banIc flume (USACE 1981) were conducted to determine the limits of protection required to prevent scour that would lead to destruction of the revet. ment. These tests were conducted in a llO-deg bend having a constant discharge. The downslream end of the revetment had to be 1..5 channel widths downstream of the end of the bend. Geomorphic studies to determine revet- ment ends should be considered. Section IV Revetment Toe Scour Estimation and Protection. 3-9. General Toe scour is probably the most frequent cause of failure of riprap revetments. This is lrUe not only for riprap. but also for a wide variety of protection techniques. Toe scour is the result of several factors. including these three: 3.8 a. Meandering channels. cirmlge in cross section that occurs after a banlc is protected. In meandering channels the thalweg often moves toward the outer bank: after the bank is protected. The amount of change in cross section that occurs after protection is added is related to the erodibility of the natural channel bed and original banIc material. Channels with highly erodible bed and banks can experience significant scour along the toe of the new revetment. b. Meandering channels. scour at high flows. Bed profile measurements have shown that the bed observed at low flows is nO! the same bed that exists at high nows. At high flows the bed scours in channel bends and builds up in the crossings between bends. On the recession side of the flood. the process is reversed. Sediment is eroded from the crossings and deposited in the bends, thus obscuring the maximum scour that had occurred. c. Braided channels. Scour in braided channels can reach a maximum at intermediate discharges where now in the channel braids attacks banks at sharp angles. Note that local scour is the mechanism being addressed herein. When general bed degradation or headcutting is expected. it must be added to the local scour. When scour mechanisms are not considered in the design of protection works. undermining and failure may result. Plate 42 may be used for gross depth of scour estimates. Neill (1973) provides additional information on scour depth estimation. 3-10. Revetment Toe ProtectIon Methods Toe protection may be provided by two methods: a. Extend to maximum scour depth. Place the lower eXlremity beiow the expected scour depth or found it on nonerodibJe material. These are the preferred methods. but they can be difficult and expensive when underwater excavation is required. b. Piau launchable stone. Place sufficient Jaunch- able SlOne to stabilize erosion. Launchable SlOne is defined as SlOne that is placed along expected erosion areas at an elevation above the zone of attack. As the attack and resulting erosion occur below the SlOne. the SlOne is undermined and rolls/slides down the slope, SIO~ ping the erosion. This method has been widely used on sand bed Slreams. Successful applications include: