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Posted by: Michael Caswell
« on: January 28, 2020, 11:36:11 am »

The following was extracted from a US Army Corps Of Engineers document date from April 2014

ETL 1110-2-583
c. Vegetation-Free Zones. Vegetation-free zones shall, when dry, be mowed to a height of 3–6 in. at any time the grass reaches a height of 12 in.. Mowing shall be triggered by grass heights of less than 12 in. if important to the health maintenance of the particular grass species.

The maximum height of grasses shall be 12 in.

The minimum vegetation-free zone requirements for specific structures are:
(1) Embankment Dams.

At a minimum, for“dry”reservoirs, the entire dam embankment surface, including upstream impervious blankets and the upstream and downstream areas within 50 ft of the embankment toe, shall be a vegetation-free zone (see Figure A-20).

For “normal pool” conditions, the entire embankment surface from the upstream toe of the dam or from the upstream limit of the impervious blanket, as applicable, to a minimum distance of 50 ft from the downstream toe shall be a vegetation-free zone (see Figure A-21).

(2) Dam Abutments. The dam abutment is defined as the part of a natural valley side-wall against which a dam is constructed. At a minimum, the vegetation-free zone shall extend for a horizontal distance of 15 ft beyond the embankment/abutment contact line (i.e., groin).

(3) Spillways.The safety of embankment dams requires the unobstructed operation of spillways. The minimum vegetation-free zone shall include the spillways and spillway channels, including spillway slopes and approaches.

(4) Outlet-Works Discharge Channels.The minimum vegetation-free zone shall include the entire outlet channel, outlet structure headwalls and wingwalls, and surrounding areas to a distance of 50 ft from the top of the bank of the outlet channel.