Idaho Administrative Code
Title IDAPA 37 - Water Resources, Department of
Rule 37.03.07 - STREAM CHANNEL ALTERATION RULES
Section 37.03.07.057 - DUMPED ROCK RIPRAP

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.

(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)

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)