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HomeMy WebLinkAboutWATERFIELD P.U.D., 2ND FILING - FINAL - 7-95D - SUBMITTAL DOCUMENTS - ROUND 1 - STORMWATER-RELATED DOCUMENTSI Mr. Rob Wilkinson Page 3 November 17, 1997 If you have any questions regarding these preliminary results, please call Terry Geiselman at 970- 484-7573. Sincerely, Riverside Technology, inc. Terry Geiselman Project Manager cc: P. Plumley (RTi) B. Curtiss (Northern Engineering) oealwnkin.w.dac 11117197 ,� Mr. Rob Wilkinson Page 2 November 17, 1991 layers with the boundary between the top layer and bottom layer being flat at an elevation of 4,945 feet above mean sea level. The model has 140 cells in the east -west direction and 134 cells in the north -south direction. Each cell is 20 feet wide and 20 feet long. The cell thicknesses in the top layer vary with topography. Cell thicknesses for the bottom layer are all 45 feet. The top of the model mimics the planned graded elevations and the bottom of the model is at 4,900 feet amsl. The model incorporated evaporation and evapotranspiration from the wetland area. Using the water levels recorded in the monitoring wells and at the pond staff gage as calibration points, the model was run and hydrogeologic parameters were adjusted until the ground water elevation predicted by the model matched (within I foot) the measured ground water elevations at the observation points. The process of adjusting hydrogeologic parameters to calibrate the model required approximately 40 steady-state simulations. With the model properly calibrated to show current ground water conditions, the model was used to estimate the flow from the proposed drain system. The drain system design from Northern Engineering Inc. was incorporated into the ground water model using MODFLOW's drain subroutine. All of the basic hydrogeologic model parameters determined from the model calibration were retained when simulating the drain system. The preliminary results of the modeling are as follows: • Estimated Drain Discharge: The estimated discharge from the subdrain required to lower the ground water levels approximately 15 feet in the vicinity of the first filing is on the order of 5,000 ft'/day. Field data collected during this investigation suggest that an existing tile drain may exist in the vicinity of the proposed housing. The suspected tide drain system apparently discharges to a ditch located along the east side of the pond/wetlands. The estimated discharge from the pipe is unknown but was estimated in the field at around 1 to 2 felsec. This existing drain discharge appears to be a major component of flow to maintain water levels within the pond and associated wetland. It is likely that the excavation of the subdrain system will also intercept the flow from the existing tile drain. Based on our current understanding, the combined flow from the subdrain system (flow from lowering the heads plus the flow captured from the existing tile drain system) is estimated at approximately 180,000 ft'/day. It should be noted that this probably reflects a peak flow rate that would only be realized for a few months each year. However, this flow rate of 180,000 ft'/day is well within the capacity of the proposed 8-inch diameter pipe. • Estimated impacts to the wetland system: We understand that all of the water captured by the subdrain system would be discharged into the pond wetland system. Furthermore, we understand that the pond would be installed with a control structure to allow for the pond elevation to be maintained at approximately 4,950 feet amsl. The subdrain outlet would be located at or above an elevation of 4,950. Therefore, installation of the subdrain system should not adversely impact the pond and wetland system. n441/wilkinxadoc 11/17N7 Riverside Technology, inc. Water Resources Engineering and Consulting November 17, 1997 Mr. Rob Wilkinson City of Fort Collins Natural Resources Department 281 N. College Ave PO Box 580 Fort Collins, CO 80522-0580 Re: Preliminary Ground Water Modeling Results Dear Rob: The purpose of this letter is to provide a brief summary of the results of the ground water modeling performed for the First Filing of the Waterfield P.U.D. We assume that for the purposes of this evaluation the general layout of the subdrain system is identical or was not modified for the Second Filing. The modeling was performed to estimate the flow rate that could be produced from the subdrain system designed to lower the water table in the vicinity of the First Filing. A complete description of the existing baseline data, model setup, model calibration, and model simulations will be provided in a technical report for the project. We understand that the objectives of the project include (1) lowering the water table in the vicinity of the housing, and (2) maintaining the existing pond and wetland system. Prior to modeling, the project site was investigated by drilling and installing eight monitoring wells around the site to measure water levels. The wells were also used to conduct field tests to estimate the hydraulic conductivity of the hydrostratigraphic units. A staff gage was also installed as part of this investigation to monitor the elevation of the water surface in the pond area. The results of the site investigation and monitoring were used to develop a conceptual model of the ground water flow system. The conceptual model defines the general hydrostratigraphic conditions and flow system across the site. the major components of the ground water system were simulated using Visual MODFLOW, a PC -based ground water modeling program developed by Waterloo Hydrogeologic Software. Visual MODFLOW is a graphics -based implementation of the MODFLOW program (McDonald and Harbaugh 1988) developed by the U.S. Geological Survey. MODFLOW is commonly used to analyze changes in ground water levels and flow cause by pumping or drains. It is also very useful in applications involving the interaction of ground water with surface water. The ground water model covers the Waterfield P.U.D. from the Larimer-Weld Canal southward to approximately 500 feet south of the wetland area. The western model boundary is County Road 11 and the eastern boundary is Summitview Drive. The model consists of two hydrostratigraphic n44//wi(kinw.dnc 11117197 2290 E. Prospect Road, Suite 1 Fort Collins, Colorado 80525 (970) 484-7573 FAX: (970) 484-7593