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!I I I I I I I I I I I It is also interesting to note that the village is at an elevation that is about the same level as the banks of the Conejos River. There have been no detailed floodplain studies done in the area, so it is uncertain where the limit of the lOO-year floodplain is relative to the village. IV. GROUNDWATER INVESTIGATION On November 8, 1995 under the direction of Colorado Water Conservation Board staff, a groundwater investigation was undertaken to determine soil types and infiltration rates for the Guadalupe area. Rather than installing expensive monitoring wells to determine groundwater flow rates, it was more feasible to perform a visual test through an open trench analyis (test holes). The analysis consisted of excavating two 4' wide by 8' long trenches using a backhoe tractor. The test holes were excavated in one foot increments of depth. At each increment, the soil type and groundwater infiltration rate was recorded. Soil types were estimated by visual inspection only, and the infiltration rate was estimated by measuring the rate of rise of the water within each test hole. A summary of the groundwater investigation is presented in figure 3, and the locations of the test holes are shown on plate 1. V. ALTERNATIVE PROJECT PLANS Two different methods of lowering the water table and reducing sub-surface flooding problems were investigated. They are as follows: 1. A subdrain system using a lift station (pump) II , I I II I II I II 2. A subdrain system using gravity flow The two methods were evaluated based on the criteria of engineering constraints, construction methods and costs, and operation and maintenance costs. Overall, a subdrain system with a lift station would be more expensive to construct, operate, and maintain than a gravity system. A subdrain system using gravity flow would serve the purpose of lowering the groundwater levels, and would require less maintenance and would have essentially no operating cost once constructed. Based on the above considerations, a subdrain system using gravity flow was selected as the recommended plan of improvement. The outfall to the subdrain system would be on the left bank of the Conejos River, and would be located to the southeast of the village. The elevation of the outlet pipe would allow free draining of the subdrain system during times of nonnal flows in the Conejos River. A flap gate would be installed on the outlet pipe to prevent water from backing up in the subdrain during high stage events in the Conejos River. 5