New Spillway Gates at Maple Grove Reservoir
By Mark R. Hunter, P.E., Manager, Maintenance Program


Maple Grove Reservoir; Dec, 2004 Looking at the downstream side of the new 40-foot long by 10-foot tall steel crest gate as it is being tested for the first time.

Maple Grove Reservoir, Dec, 2004 Looking north toward the upstream side of the new 40-foot long (left) and 30-foot long steel crest gates as they are being tested.

Maple Grove Reservoir, 1979 Looking south at spillway and inflatable fabridams under the roadway bridge. The fabridams had been installed about two years prior. This photo is near the time the 30-foot dam on the left was sliced open and deflated by vandals. In this photo both fabridams are fully inflated.

By mid-December, 2004 the two new steel crest gates located in the spillway of Maple Grove Dam were ready to hold back the water that was re-filling the reservoir. The new gates replaced two inflatable dams (fabridams) that were installed in 1977 and were near the end of their service life. The fabridams were installed to improve the operational control of the reservoir and to reduce spillway outflow to be in line with the flood-carrying capacity of the downstream channel.

Maple Grove Reservoir is situated on Lena Gulch at 27th Avenue near Youngfield Street in Lakewood, Colorado. Consolidated Mutual Water Company owns the reservoir and uses it to store raw water. The water is eventually treated and distributed to Consolidated's customers.

When Maple Grove Reservoir was constructed in 1955 it inadvertently provided substantial downstream flood protection. This benefit to the communities downstream of the reservoir was recognized in the 1975 Lena Gulch Drainage Study.

In 1974, Consolidated was directed by the Colorado State Engineer's office to enlarge the Maple Grove Dam spillway to pass the calculated Standard Project Flood. This is the flood that would result from a storm with a recurrence interval of 250 years in the Lena Gulch drainage basin upstream of the reservoir.

Portions of Wheat Ridge, Jefferson County, and Lakewood benefited from the flood protection provided by the reservoir. Along with the District, these communities cooperated with Consolidated in 1976 to design a unique spillway enlargement that met the needs of both Consolidated and the flood-prone communities downstream of the reservoir.

The coordinated plan called for Consolidated to own the 30-foot long by 6-foot tall dam and the District to own the 40-foot long by 10-foot tall dam, both of which were located in the newly-enlarged spillway. The dams in the spillway allowed discharges through the spillway to be controlled by the mechanically operated gates. The result was that the District and downstream local governments could anticipate acceptable spillway outflows during 100-year storm events while Consolidated could be confident in being able to withstand the Standard Project Flood without overtopping the reservoir.

The fabridams were inflated by a combination of air and water and were difficult to operate and maintain. They showed their vulnerability in March of 1979 when vandals using knives sliced open the 30-foot long dam allowing a relatively small but certainly unexpected flood to occur. The peak flow immediately below the reservoir was about 750 cubic feet per second and caused some residential basement flooding and first floor damage to some commercial buildings.

The replacement system uses hydraulic cylinders to raise and lower the two independent crest gates. They will operate under the same discharge parameters as did the fabridams. The new steel crest gates are more resistant to vandalism and are far simpler to operate and maintain than the fabridams. The new system affords renewed confidence in the integrity of Maple Grove Dam to the District, Consolidated, and the communities downstream of the reservoir.


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