2016-09-01_REVISION - M2004031 - Laserfiche WebLink

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existing 3:1 (horizontal to vertical)reclamation slopes. The depth of the channel will be 2 feet excavated into the reclamation slope with channel side slopes of 3:1 horizontal to vertical. The channel bottom will be 3 feet wide. The rundown channel will be reinforced with grouted, Type M riprap bedded on 12 inches of Type 2 riprap bedding, The rundown channel will terminate in a grouted, Type M riprap dissipation channel at the base of the mine. The Type M riprap will be bedded on 12 inches of Type 2 riprap bedding. The dissipation channel will be 10 feet long by 3 feet wide by 2 feet deep. The proposed permanent drain system headwall and orifice has a flowline elevation of 5036.50. The maximum capacity of the drain is estimated to be 12.8 cfs, based on an orifice equation calculation. The capacity assumes that approximately 6.5 feet of surcharge (up to elevation 5043.0) may be applied to the orifice during high water conditions with a water surface elevation less than 5036 in the reservoir. Elevation 5043.0 is the approximate elevation of the highest adjacent grade in the vicinity of the structure. As mentioned above, the rundown channel into the reservoir has a trapezoidal cross-section. The proposed channel will have a 3-foot bottom width and 3H:1V side slopes. The longitudinal slope is approximately 3H:1V, matching the finished reclamation slopes. The velocity of the flow in the channel is approximately 8.3 fps. Using the UDFCD riprap sizing equation (Eq. MD-13), type VH un-grouted riprap is allowable for use in the rundown channel. However, due to supply constraints, it was elected to use grouted Type M riprap. Calculations for the channel velocity are attached. A series of monitoring points will be used to monitor both ground and surface water levels and their responses to the various stop log levels. These monitoring points will consist of a series of staff gauges and monitoring wells. These points will be monitored by Aggregate Industries. The data collected will be forwarded to the DRMS. Orr Drain System Limitations The permanent drain is not intended to convey all the surface water flows through the Bull Seep that are influenced by surrounding land and water uses which are not associated with Aggregate Industries' operations. Consequently, there may be some instances when flooding will occur at the Orr Property. This includes, but is not limited to, turnout flows from the Fulton Ditch, runoff from rainfall events, or flooding from the South Platte River. FEMA Flood Insurance Rate Maps (FIRM)show that the Orr Property may become partially inundated during some flood events. The drainage system will hydraulically connect the Orr Property with the future reservoir that is planned for the reclaimed mine. During flooding events on the South Platte River, some floods are expected to enter the future reservoir. As a result, some flooding flows are expected to spill into the Orr Property. Reservoir Outlet Pipes A pipe system is proposed on the north side of the reservoir to convey Orr Drain flows and release some flood waters when the reservoir is operating at the normal high water surface elevation. These drains were added at the request of the City of Thornton. No performance criteria for the pipes were defined by the City of Thornton, with the exception of the pipe invert elevations, which were set at elevation 5036.0. The outlet of the pipes are located on Property owned by Denver Water. An easement will need to be obtained to allow construction and release of drainage onto their Property. Three 24-inch reinforced concrete pipes were selected. This selection as made because the pipe outlet structure was the largest that could be constructed but still be less than the width of the outlet channel (Bull Seep). In addition, 24 inches was the maximum pipe size that could be constructed with a minimum of 1 foot of cover in the