PERMFILE131144

• Permit No: C1981022
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: 404 PERMIT FOR WATER SUPPLY INTAKE
• Section_Exhibit Name: Exhibit 2.03-E4 Part 9a
• Media Type: D
• Archive: No

3. DROP STRUCTURES. A drop structure is a group of rocks,
boulders and%or logs placed in a stream to act as a low level dam.
• Placement.vf a drop structure will raise the streambed elevation,
decrease the stream gradient immediately upstream, cause
deposition upstream of the drop, dissipate stream energy,
decrease current velocity, and reduce streambank erosion while
promoting streambed stability.
The placement of drop structures is authorized provided:
(a) Drop structures shall be constructed so that the
maximum change (increase) in the ordinary low water surface
elevation upstream of the drop does not exceed 2 feet above the
ordinary low water surface elevation immediately downstream of
the drop. The majority of drop structures employed in the State
of Colorado would not need to exceed 2 feet in height above the
pre-construction stream bottom elevation to promote stream bank
and stream bed stability;
(b) Materials acceptable for drop structure
construction include large boulders, large angular stone or rock-
filled gabion baskets and logs or a combination with any of the
preceding [Cabions and logs can be more suspectible to damage
from high flows and may need frequent maintenanceJ;
(c) Material size shall be large enough to withstand
expected high flow velocities and associated turbulence;
• (d) Drop structures shall be constructed straight
across a stream channel perpendicular, or at an angle, to the
current or more preferably, in an inverted ~V" shaped
configuration which will direct overtopping flows to the middle
of the river and not. against adjacent stream banks. The point of
the "V" should be directed upstream. Note that flows overtopping
a drop structure will 6e directed perpendicular to the downstream
face.
(e) All drop structures shall be tied or keyed into
the existing banks and protected by large rock anchored several
feet into the bank. Protecting the flanks of the structure is
critical to prevent extensive damage from high flows;
(f) Construction equipment should access the stream
only at a location immediately upstream of the proposed
structure;
(g) Construction or placement of rock for a drop
structure shall start at both banks and proceed toward the middle
of the stream. A backhoe or front-end loader, preferably with an
opposing "thumb" on the bucket to manipulate the rocks, has been
found most, useful constructing drop structures; and
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