Section 37.03.07.058 - DROP STRUCTURES, SILLS AND BARBS01.Drop Structures. A drop structure shall be constructed of rocks, boulders and/or logs placed within a stream channel to act as a low level dam. Placement of a drop structure perpendicular to stream flow will decrease the stream gradient, dissipate stream energy and decrease stream velocity through an increase in water surface elevation immediately above the structure. Drop structures shall comply with the following criteria: a. Maximum water surface differential across (upstream water surface elevation minus downstream water surface elevation) a drop structure shall not exceed two (2) feet. The department shall approve the final elevation of any structure.b. Rock drop structures shall be constructed of clean, sound, dense, durable, angular rock fragments, and/or boulders of size and gradation, such that the stream is incapable of moving the material during peak flows. Rocks shall be keyed into the stream banks to minimize the likelihood of bank erosion, (See APPENDIX D located at the end of this chapter).c. Log drop structures are acceptable in four (4) designs including the single log dam, the stacked log dam, the three (3) log dam, and the pyramid log dam. Log ends shall be keyed into both banks at least one-third (1/3) of the channel width or a distance sufficient to prevent end erosion. To prevent undercutting, the bottom log shall be embedded in the stream bed or hardware cloth, cobbles or boulders shall be placed along the upper edge. Minimum log size for a single log structure shall be determined by on-site conditions and shall be placed to maintain flow over the entire log to prevent decay. Each log drop structure must be accompanied by downstream scour protection, such as a rock apron (See APPENDIX E located at the end of this chapter.d. All drop structures shall be constructed to facilitate fish passage and centralized scour pool development.02.Sills. A sill shall be constructed of the same material and in the same manner as a drop structure. The top of the sill may not exceed the elevation of the bottom of the channel. The purpose of a sill is to halt the upstream movement of a headcut, thus precluding the widening or deepening of the existing channel. (See APPENDIX F located at the end of this chapter).03.Barb or Partial Drop Structure. A barb or partial drop structure shall be constructed in the same manner and of the same material as a drop structure and placed into the stream channel to act as a low level dam and grade control structure. The barb will decrease stream gradient, dissipate stream energy and redirect stream flow.a. Barbs shall be constructed of clean, sound, dense, angular rock fragments, of size and gradation such that the stream is incapable of moving the material during peak flows.b. Barbs shall be constructed with a downstream angle of no less than one hundred (100) degrees and no greater than one hundred thirty-five (135) degrees unless otherwise specified.c. Barbs shall "extend" into the channel a distance of not more than twenty percent (20%) of the width of the channel unless otherwise specified by the Director.d. Barbs shall be keyed into the bank a distance equal to or greater than the width of the structure and down to bed level. Whenever moisture is encountered in the construction of the keyways, willow cuttings or clumps shall be placed before and during rock placement in such a manner that the base of the cutting is in permanent moisture and the top extends a minimum of six (6) inches above grade (see APPENDIX G located at the end of this chapter).Idaho Admin. Code r. 37.03.07.058