Current through August 31, 2023
01.
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: (3-18-22)
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. (3-18-22)
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). (3-18-22)
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. (3-18-22)
d. All drop structures shall be constructed
to facilitate fish passage and centralized scour pool development.
(3-18-22)
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).
(3-18-22)
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. (3-18-22)
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.
(3-18-22)
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. (3-18-22)
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.
(3-18-22)
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). (3-18-22)
