2008-04-07_REVISION - M1980149 (3)

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Ms. Erica Crosby RE: Weld County: Hokestra Pit April 3, 2008 Page 3 of 4 The rate of rise for this analysis was assumed to be 4000 cfs per hour. This number was determined after evaluating several nearby hydrographs on similar drainages. Once this was determined a model of the reservoir was constructed to evaluate different spillway widths. According to Urban Drainage's recommendations the spillway size for reservoirs II and N would be 19 ft and 11 ft respectively, however they also recommend a minimum width of 100 ft. The analysis for reservoir II was performed for 4 following scenarios: • A 20 ft spillway, and a full reservoir at the start of the flood • A 20 ft spillway, and a half full reservoir • A 100 ft spillway, and a full reservoir • A 100 ft spillway, and a half full reservoir A review of the existing HEC-RAS model revealed that the river berm would overtop. uniformly as the river rose. Therefore water coming into the pit over the river berm was modeled as a broad crested weir 1300 ft wide with a.weir coefficient of 2.8. The results of this modeling are attached. These results show that if the reservoir was to be half full at the start of the flood the maximum difference in water surface elevation across the river berm fora 100 ft wide spillway would not be significantly different than a 20 ft wide spillway. Furthermore the amount of water overtopping the river berm would require 9" D50 riprap on the pit side slope in both cases. Therefore, it was determined that it is more economical to build a small, 20 ft wide spillway and riprap the entire river berm to 3 ft below the invert of the river, than build a 100 ft wide spillway that would also require armoring of the river berm. The small spillway will serve to release any flood waters entering the reservoir during a flood event as well as prevent the reservoir from exceeding the maximum normal water surface elevation during filling. The spillway will be . located in the northeast corner of the reservoir and spill into the adjacent slough. The riprap for the river berm was sized using several sources which are also attached. The proposed armoring for the river berm is 9" D50 riprap placed 18" thick on top of a 9" thick layer of type II bedding. A similar analysis was performed for Reservoir IV but only for the half full reservoir scenario since that is the more conservative approach. The reservoir was evaluated for 11 ft and 100 ft wide spillways as well as a 24 inch pipe spillway. The pipe size was selected based on the maximum anticipated flowrate (15 cfs) entering the reservoir via an interconnect pipe from reservoir II. The pipe spillway would pass the incoming flow without allowing the reservoir to overtop and would also serve to release,any accumulated flood storage after a flood event. The river berm was assumed to be 800 ft long: The results of the analysis revealed that by installing a 100 ft spillway the rock diameter protecting the river berm could be decreased to 6" D50. However due to the presence of the gas line a pipe spillway and slightly larger riprap, 9" D50 or engineer approved equal, was the most economical solution. Exhibit F has been updated to show the approximate spillway locations. b. The DRMS expressed a concern regarding the potential for erosion of the berm between reservoirs II and N during an overtopping event. The proposed berm between reservoirs II and N is consistent with the adjacent approved reclaimed mined areas. Reviewing the flooding characteristics modeled by the HEC-RAS model and the models discussed above, the only scenario where flood water overtopping this berm would have a significant rundown into reservoir N would be if reservoir N was significantly lower that reservoir II at the start of the flood. The pump station which will serve the surrounding reservoirs is located at the northwest corner of reservoir IV thereby making reservoir N the last reservoir in the chain before making \\Hercules\projects\06125 Hokestra lining\DMG permit\SubmittalWdequacy review\Reply2.docx