2008-04-07_REVISION - M1980149 (3)

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Ms. Erica Crosby RE: Weld County. Hokestra Pit April 3, 2008 Page 2 of 4 4. Weld County is constructing a French drain around the site to mitigate any groundwater impacts from the project, Weld County however feels that it is important to monitor the effectiveness of the French drain to avoid impacts to adjacent water rights. Monitoring wells have been installed to monitor any' impacts that the project may have on local groundwater levels. • Trigger Points - In addition to the installation of the perimeter drain, Weld County will monitor groundwater elevations monthly in the established monitoring wells. Trigger points will depend upon the location of the well, but will be set at two feet above or below the historic high or low elevations in each well. Proposed trigger points are shown in the following table. Wells 1 through 3 will indicate ifmounding occurs and wells 4 and 5 will indicate if shadowing occurs. Well 1 2 3 4: 5' .. Depth to Depth to Depth to Depth to Depth to Trigger Point Water (ft) Water (ft) Water (ft) Water (ft) Water (ft) 11.8 6.8 5.9 13.8 13 To evaluate the effectiveness of the perimeter drain, and to protect against adverse effects on existing structures, the trigger elevation for each monitoring well will be two feet above or below the elevation of the historic high or low water level at each well for three consecutive months. Three consecutive months is necessary to account for seasonal fluctuations in groundwater flow in the area, the relatively slow movement of groundwater to the perimeter drain, and to allow the perimeter drain sufficient time to effectively drain the groundwater. Measured groundwater elevations in excess of two feet above or below the depths shown in the above table for three consecutive months will indicate that the perimeter drain is not functioning properly, or that groundwater mounding is in excess of the drain's capacity. Weld County will provide the DBMS with monthly monitoring well data on a quarterly basis, except in the event that the monitoring wells show three consecutive months of exceedence of the two-foot trigger in which case Weld County will notify DBMS within 48 hours. If the three-month, two-foot trigger is reached, -Weld County will work with DBMS to evaluate the cause of issue, .and identify potential solutions or additional mitigation measures at that time. Weld County will identify the additional mitigation measures or potential solutions within 1 S working days after notifying DBMS of the exceedence. Weld County will commit to monitoring groundwater levels monthly for a period of 12 months, after which Weld County will evaluate the need for additional monitoring. If after I2 or more months of monitoring Weld County believes there is sufficient -data to demonstrate that the groundwater monitoring is no longer needed at the site, Weld County will submit a request to DBMS to discontinue the groundwater monitoring. 5. There seem to be several different requests contained within this comment. I have addressed each as a separate bullet point below. a. Attached is a copy of the model and results illustrating the flooding characteristics of the St. Vrain and the resulting design parameters. The reservoirs were modeled by using information from the existing HEC-RAS model and several other sources. The effectiveness of a spillway for these reservoirs is directly tied to the rate of rise of the river hydrograph. The faster the river rises, \\Hercules\projects\06125 Hokestra lining\DMG permit\Submittal\?.dequacy review\Reply2.docx