Dumped Rock Riprap (Rule 57)

01. Placement of Riprap. Riprap shall be placed on a granular bedding material or a compact and stable embankment. (3-18-22) 02. Sideslopes of Riprap. Sideslopes of riprap shall not be steeper than 2:1 (2’ horizontal to 1’ vertical) except at ends of culverts and at bridge approaches where a 1 1/2:1 sideslope is standard. (3-18-22) 03. Minimum Thickness of Riprap. The minimum thickness of the riprap layer shall equal the dimension of the largest size riprap rock used or be eighteen (18) inches, whichever is greater. When riprap will be placed below high water level, the thickness of the layer shall be fifty percent (50%) greater than specified below. (3-18-22) 04. Riprap Protection. Riprap protection must extend at least one (1) foot above the anticipated high water surface elevation in the stream. (3-18-22) 05. Rock Used for Riprap. Rock for riprap shall consist of sound, dense, durable, angular rock fragments, resistant to weathering and free from large quantities of soil, shale, and organic matter. The length of a rock shall not be more than three (3) times its width or thickness. Rounded cobbles, boulders, and streambed gravels are not acceptable as dumped riprap. (3-18-22) 06. Size and Gradation of Riprap. Riprap size and gradation are commonly determined in terms of the weight of riprap rock. The average size of riprap rock shall be at least as large as the maximum size rock that the stream is capable of moving. The maximum size of riprap rock used shall be two (2) to five (5) times larger than the average size. (3-18-22) 07. Methods Used for Determining Gradation of Riprap. There are many methods used for determining the gradation of riprap rock. One of these many acceptable methods is shown in Table 1 below. Another acceptable method is the Far West States (FWS) method shown in APPENDIX A - Table 1A. Table 1 – GRADATION OF RIPRAP IN POUNDS Max. Weight of Stone required (lbs) Min. and Max. Range in weight of Stones (lbs) Weight Range 75 percent of Stones (lbs) 150 25 - 150 50 - 150 200 25 - 200 50 - 200 250 25 - 250 50 - 250 400 25 - 400 100 - 400 600 25 - 600 150 - 600 800 25 - 800 200 - 800 1000 50 - 1000 250 - 1000 1300 50 - 1300 325 - 1300 1600 50 - 1600 400 - 1600 2000 75 - 2000 600 - 2000 2700 100 - 2700 800 - 2700 IDAHO ADMINISTRATIVE CODE IDAPA 37.03.07 Department of Water Resources Stream Channel Alteration Rules Section 057 Page 9 (3-18-22) 08. Use of Filter Material. A blanket of granular filter material or filter fabric shall be placed between the riprap layer and the bank in all cases where the bank is composed of erodible material that may be washed out from between the riprap rock. Filter material shall consist of a layer of well-graded gravel and coarse sand at least six (6) inches thick. (3-18-22) 09. Toe Protection. Some suitable form of toe protection shall be provided for riprap located on erodible streambed material. (3-18-22) a. Various acceptable methods of providing toe protection are shown in APPENDIX B, Figure 2 at the end of this chapter. (3-18-22) b. In addition to the approved methods of providing toe protection as shown in APPENDIX B, any other reasonable method will be considered by the Director during review of a proposed project. (3-18-22) 10. Extension of Riprap Area. Riprap shall extend far enough upstream and downstream to reach stable areas, unless the riprap is protected against undermining at its ends by the method shown in APPENDIX C, Figure 3 at the end of this chapter. On extremely long riprap sections, it is recommended that similar cutoff sections be used at several intermediate points to reduce the hazard that would be created if failure of the riprap occurred at any one (1) location. (3-18-22) 11. Finished Surface. Placement shall result in a smooth, even finished surface. Compaction is not necessary. (3-18-22) 12. Placement of Riprap. The full course thickness of the riprap shall be placed in one (1) operation. Dumping riprap long distances down the bank or pushing it over the top of the bank with a dozer shall be avoided if possible. Material should be placed with a backhoe, loader, or dragline. Dumping material near its final position on the slope or dumping rock at the toe and bulldozing it up the slope is a very satisfactory method of placement, if approval is obtained for the use of equipment in the channel. (3-18-22) 13. Design Procedure. Design procedure using the Far West States (FWS) method. (3-18-22) a. The FWS method uses a single equation to deal with variables for riprap. (3-18-22) D75 = 3.5/CK WDS for Channel Banks where: D75 = Size of the rock at seventy five percent (75%) is finer in gradation, in inches. (3-18-22) b. The coefficient, C, is based on the ratio of the radius of curvature of the stream, (CR), to the water surface width, (WSW), so it is necessary for the user to make field determination of these values. The coefficient varies from 0.6 for a curve ratio of 4 to 6, up to 1.0 for a straight channel. If the computed ratio for a particular project is less than 4, the designer should consider some modification less than 4. W = Specific weight of water, usually 62.4 lbs./cu.ft. D = Depth of flow in stream, in feet in flood stage S = Channel slope or gradient, in ft/ft. C = A coefficient relating to curvature in the stream K = A coefficient relating to steepness of bank slopes IDAHO ADMINISTRATIVE CODE IDAPA 37.03.07 Department of Water Resources Stream Channel Alteration Rules Section 058 Page 10 (3-18-22) c. The coefficient, K, ranges from 0.5 for a 1.5:1 sideslope to 0.87 for 3:1 sideslope. No values are given for steeper or flatter slopes. Slopes steeper than 1.5:1 are not recommended. If slopes flatter than 3:1 are desired, it would be conservative to use the K-value for 3:1 slopes. (3-18-22)