2003-10-14_REVISION - M1992069 (2)

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Ms.Kate Pickfard Division OjMinerals and Geology October 8, 2003 Page 1 of 4 3. The North Eagleview Regional Detention Pond Design Report estimates that, during the 100- year stonnwater event, the groundwater lake elevation will rise approximately 2.5 feet. The design report also estimates that the lake elevation will return to the normal surface elevation within several days following the event. Events of this magnitude aze also very infrequent. There is only a one percent chance of a 100-year storm event occurring each year. Therefore, flooding of the wetlands would not be common. With this level, duration, and frequency of flooding, impacts to the wetlands located to the north of the lake are not anticipated. 4. Data as to how the 150 cubic feet per second (cfs) outflow figure was calculated are presented in the North Eagleview Regional Detention Pond Concept Phase Report and North Eagleview Regional Detention Pond Design Report. The reports also include data used to predict potential stormwater flows to the future groundwater lake. It is difficult to predict what the lake outflow would be following the 100-year stormwater event if the site were to be reclaimed under the current plan. The pit and future groundwater lake are located in the historic drainage path for the watershed being addressed by the North Eagleview stormwater drainage project. Therefore, it is probable that, under the current reclamation plan and without the stormwater channel construction, flood flows to the groundwater lake would be similaz to those predicted in the concept phase report and design report. During the 100-year stonnwater event, the future groundwater lake would likely flood and drain in an uncontrolled scenario. Flooding and drainage under this scenario would produce increased risk of localized erosion and failure of lake banks and side slopes. Design specifications for the proposed channel and box culvert under F Street are included in the attached materials. The culvert connecting the western and eastern lakes is part of the original approved reclamation plan and is located at an invert elevation of 4675 feet. The technical revision does not propose changing the design or location of this culvert. It is not anticipated that the eastern groundwater lake surface elevation will reach the culvert invert elevation during the 100-year stormwater event. Sentember 23, 2003 Letter ]. Dimensions of the stonnwater channel are provided in the attached materials. 2. An analysis of the groundwater lake outlet and channel under the railroad bridge is provided in the North Eagleview Regional Detention Pond Concept Phase Report. It is estimated that the railroad bridge could safely pass stormwater flows of nearly 500 cfs. Wells located within 600 feet of the groundwater lake'and outlet channel are illustrated on the attached Figure 1 and well ownership and use infonnation is provided in the attached Table 1. The information provided was obtained from records on file with the Colorado Office of the State Engineer.