Laserfiche WebLink
1 <br /> CONDITION DURING MINING <br />' <br /> During mining of the proposed quarry, measures must be taken to minimize <br /> potential damage to the operating area and downstream property owners <br />1 and water users. The proposed mining development will have a relatively <br />minor effect on the overall runoff characteristics of the various <br /> gulches due to their naturally high runoff rates. Consequently, the <br /> quarry expansion will not have a significant impact on the peak runoff <br /> rates from the basins during rare storm events. Due to the potential <br /> for ponding within the excavated quarry there will be a lower peak <br /> runoff rate. <br />' <br /> As mining in the quarry progresses, its configuration will be constantly <br /> changing. In the course of the operations, interim storm drainage plans <br /> will have to be developed to convey large amounts of storm runoff either <br />' around or through the area with a minimum of damage to site facilities <br /> and keeping erosion of the open excavations to a minimum. The following <br />' sections contain three basic alternative measures which may be taken to <br />control storm runoff, protect the operations area and minimize impacts <br /> of mining operations downstream. <br /> Alternative I <br /> To the extent possible and practicable, runoff from above the quarry <br /> site should be directed around the excavation to the channel along the <br />' south side of the quarry. Provisions gust be made to convey the flows <br /> in this gulch around the edge of the quarry. The channel should either <br />' be relocated south of the mining area or the mining area should not <br />disturb the gulch. <br /> <br /> A contour diversion channel may be feasible to collect and convey water <br /> from gulches which would otherwise drain into the open mine area. <br /> (Construction of this channel may not be possible due to the shallowness <br /> of the soil on top of the rock.) Figure 2 illustrates the possible <br /> location of such a diversion channel. The channel would intercept <br />' runoff from portions of Gulches A and B above the disturbed area and <br /> carry it around the pit and into the natural channel of Unnamed Gulch B. <br />' Based on a comparison of the tributary area above this channel with the <br />historic area of Unnamed Gulch B, the proposed diversion channel would <br /> require a capacity of approximately 155 cfs to carry the 6-hour, 100- <br />' year peak flow. This flow rate is approximately 20 percent higher than <br />the historic peak flow for Unnamed Gulch B due to the addition of runoff <br /> carried by the diversion channel from Unnamed Gulch A. <br />t To mitigate potential adverse impacts downstream as a result of an <br />increase in the 100-year peak flow rate, the diversion channel described <br />above should be designed for a bank-full capacity of 100 cfs. The <br />' natural channel of Unnamed Gulch B should also be left intact with <br />sufficient capacity to convey 100 cfs. `dith such a design, runoff <br />intercepted by the channel from all major storms with a peak flow of 100 <br />' cfs or less will be carried around the pit and into the natural channel <br />below. Should a rare storm occur which results in a peak flow in excess <br />of 100 cfs, the excess flow would spill out of the diversion channel and <br />1 <br /> 3 <br /> <br />