FLOOD04325

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Rules and Regulations for Dam Safety and Dam Construction
• Date: 9/30/1988
• Prepared For: State of Colorado
• Prepared By: State of Colorado DNR
• Floodplain - Doc Type: Educational/Technical/Reference Information

STATE OF COLORADO

DIVISION OF WATER RESOURCES

OFFICE OF STATE ENGINEER
SPECIFICATIONS TO GOVERN THE CONSTRUCTION OF A EROSION CONTROL DAM

Preparation of Foundation for Dam-All vegetable matter of every description, including roots to a depth of two
feet, shall be removed from the entire area upon which the dam will rest, together with boggy or unstable materials
and deposited outside the toes of the dam. The banks of the stream channel shall be dressed to a slope of about 1 %:1.
A bonding trench, with sloping sides and a bottom width of not less than 5 feet and depth of 4 feet, shall then be
excavated beneath the center line of the dam the full length thereof, which trench shall be refilled with the most imper-
vious materials available. The fou!ldation of the dam shall then be lightly plowed lengthwise of the dam, to provide
proper contact between the foundatlOn and the dam embankment.

Placing of Dam Embankment.-The materials shall be placed in the bonding trench and in the embankment of the
dam in layers not exceeding 6 inches in thickness, after which each layer shall be thoroughly compacted by a heavily
loaded disc cultivator, a corrugated or sheep's foot roller, the treads of a caterpillar or trucks, or by livestock used in the
construction. During the construction period, the top of the embankment shall be maintained as a horizontal plane the
full width and length thereof, and no side dumping of materials shall be permitted. The materials shall at all times
contain sufficient moisture to provide proper compaction. Puddling of material with water shall not be permitted.
No frozen material or large clods or stones shall be incorporated in the dam. The upstream face of the dam shall be
constructed with a slope not steeper than 2%:1, and the downstream face on a slope not steeper than 2:1. The crest or
top of the finished dam shall be not less than 8 feet in width.

The upstream two-thirds of the dam shall be constructed of the most impervious materials, such as clay loam, or a
mixture of clay and sand, and the downstream one-third of more pervious material, such as sand or gravel. The up-
stream face of the dam shall be adequately protected against wave action by stone riprap, or other suitable materials
when required.

Outlet-There shall be located beneath the dam an un gated outlet pipe not less than 12 inches in diameter and
large enough to drain within thir~.-six hours any impoundment in excess of two acre-feet. Such outlet Ripe shall
be provided with cutoff collars, The pipe shall be placed in a trench bottomed in stable formation, and shall be
completely surrounded with well compacted impervious l!laterials.

Spillway-For the protection of the dam, an adequate spillway shall be constructed around one or both ends of the
dam, of sufficient width to provide a capacity to carry the entire discharge from the drainage basin above the dam
during periods of unusual runoff. The spillway shall be located in stable formations not easily eroded, and shall extend
to a point well downstream from the dam. ,The following table shall be used to determine the necessar.y width of spill-
way to meet the above requirements. The top of the dam at all points shall be not less than 4 feet above the bottom of
the spillway.

The fallowing table shows the widths of spillways for corresponding drainage areas with an allowance of a mini-
mum freeboard between the maximum high water line and top of dam, of 2.3 feet, and maximum velocities of 3.5 feet
per second of time.

AREAS OF LOW RAINFALL INTENSITY AREAS OF mGH RAINFALL INTENSITY
AREA OF REQUIRED WIDTH OF AREA OF REQUIRED WIDTH Of AREA Of REQUIRED WIDTH OF
DRAINAGE BASIN SPILLWAY "W" AT DRAINAGE BASIN SPILLWAY "W" AT DRAINAGE BASIN 5PILLWAY"W" AT
ABOVE DAM NARROWEST POINT ABOVE DAM NARROWEST POINT ABOVE DAM NARROWEST POINT
IN ACRES IN fEET IN ACRES IN fEET IN ACRES IN FEET
20 8 20 8 400 76
;0 9 40 9 450 84
60 11 60 11 SOO 90
80 14 80 18 550 98
100 16 100 22 600 105
11.0 21 120 26 700 117
180 25 140 30 , 800 129
220 29 160 34 900 140
260 33 180 37 1000 150
300 35 200 43 1100 160
350 38 220 46 1200 169
400 42 240 50 1300 178
450 44 260 " 1400 187
500 46 280 57 1500 196
600 51 300 60 1600 208
700 55 320 63 1700 212
800 59 340 66 1800 219
900 62 360 70 1900 225
1000 66 380 73 2000 233

ALL AREAS EAST OF THE CONTINENTAL DIVIDE BELOW AN ELEVATION OF 7000 FEET, ARE CONSID-
ERED AS BEING IN THE HIGH RAINFALL INTENSITY ZONE.

The above spillway widths may be reduced at a point 50 feet below intake, by 25 per cent, where the spillway is
located the full length thereof in hard clay or shale, and by 50 per cent when located in hard rock formations, if the
slope or grade of the bottom is increased accordingly. The grade for clay and shale formations should be 0.30 foot per
100 feet and for rock formations 0.90 foot per 100 feet. The width of the entrance to the spillway must in all cases be
one-third wider than shown in the table, and the bottom should slope from the lower end of the funnel section, toward
the reservoir 1.0 foot in the distance of 50 feet, and the slope downstream should-be 0.25 foot in a distance of 100 feet.

Borrow Pits-Pits from which materials are taken to build the dam, shall be cleared of all vegetable matter, and
no material shall be b~rrowed within a distance of 50 feet of any part of the dam. Materials excavated from the spill-

way, when suitable, may be used in building the dam.