PROJ00382

• Contract/PO #: C153685
• Contractor Name: Monument, Town of
• Contract Type: Loan
• Water District: 10
• County: El Paso
• Bill Number: SB 94-029
• Loan Projects - Doc Type: Feasibility Study

Monument Lake Dam Feasibility Study

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Chapter IV - Formulation of Alternatives

D. Reservoir Dredging Alternative

The Town would like to regain reservoir storage, which has been reduced by sediment for

recreational purposes. Blackhawk Geometrics conducted bathymetry and sub-bottom profiling at

Monument Lake. The purpose of the survey wa~ to map the water depth (bathymetry) and the

sediment thickness (sub-bottom profiling). The fieldwork was performed on Thursday, June I,

2000. The sub-bottom profiling did not obtain usable data due to the composition of the

reservoir sediment. The bathymetry survey was used to determine the amount of sediment that

has accumulated within the reservoir basin.

The Global Positioning System (GPS) equipment and bathymetry equipment were

installed on a 20-foot Boston Whaler boat poweted by a gasoline engine. A GPS base station

was established at a survey marker that was on tl)p of the spillway weir crest.

Figure IV-12 presents a plot showing the track of the boat during the survey. As can be

seen from the figure, the line spacing is about 100 feet. Generally, the outer perimeter boat track

maps the shape of the lakeshore. Generally, this track was completed as close to the shore as

reasonable while maintaining either a few feet of water depth or being within about 20 feet of the

shore.

The bathymetry data is presented in Figure IV-B. This figure shows colored grid with
contours of the depth to the bottom of the lake. The contour interval is 0.5 feet. The plot shows
the depth varying from less than 4 feet to over 13 feet with the depth increasing from north to
south. The deepest part of the lake lies close to the center of its southeastern shore.

The bathymetry results were then compared with the original reservoir topographic

survey that was performed during the construction of the dam. Table IV-11ists the results of the

comparisons. The reservoir storage has been reduced by sediment by approximately 355 AF.

Wetlands have established above elevation 691 S. To avoid extensive environmental permitting

activities, dredging operations will be restricted below reservoir elevation 6915. Therefore, the

maximum amount of sediment that could be renlOved is approximately 200 AF (322,666 CY).

Due to site constraints within the reservoir area approximately 50 percent of the lost storage

could be recovered through dry bed dredging. Dry bed dredging is performed by draining the