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XII.3. Downstream Reach A small swale located at the toe of the excavation does not intercept much surface runoff; its primary purpose is to keep water away from the toe of the excavation. The swale begins at the southwest corner of the property and travels for about 400 feet along the toe of the excavation. From there, it veers away from the toe of the slope and travels along the southern and eastern limits of the interior of the property at a distance of 50 to 250 feet from the toe of the slope. Along the downstream end of its length in the upstream reach, this interior ditch begins traveling west, where it begins to intercept some interior drainage. None of the runoff will pond behind the Levee or otherwise pose any problem for the Levee. A 1.9 -acre pond located near the middle of the site intercepts a small amount of overland flow. It is the opinion of the author of this report that the interior drainage requirements of 44 C.F.R. 65.10(6) are satisfied for the upstream reach of Levee. See Figure 34. XII.2. Middle Reach Report in Support of the Recertification of the University of Colorado 's Flood Control Levee Page 20 All of the surface drainage from the area behind the middle reach of Levee is generated from rain falling on the immediate /adjacent area and from surface runoff from some of the area behind the upstream reach of Levee. The ditch from the upstream reach widens into a channel which flows first west, then north along the base of the excavation. The outflow from the 1.9 -acre pond flows into the channel, along with some of the surface runoff from the upstream reach. The remaining runoff from the area behind the middle reach of Levee flows as overland flow to the north. See Figure 34. Except for overtopping of the Levee in the Downstream Reach during the 100 -year flood, all of the surface drainage from the area behind the downstream reach of Levee is generated from rain falling on the immediate /adjacent area and from surface runoff from portions of the area behind the upstream and middle reaches of Levee. The majority of this runoff occurs as overland flow offsite to the north, without being intercepted by any drainage facility. The channel from the middle reach of the Levee continues at the toe of the excavation and flows through an existing 2.3 -acre pond. A channel conveys the overflow from the pond off the site. See Figure 34. XII.4. Normal Outfall for Interior Drainage After the interior channel exits from the north end of the mining area, it travels another 500 feet where it discharges into the Veile Gulch channel (see Figure 34). The remaining portion of the overland flow from the site goes directly into the Veile Gulch channel. Veile Gulch channel is an improved drainageway located north of the gravel pit. The design flow for the Veile Gulch channel is for its 100 -year flow of 750 cfs. It traverses from southwest to northeast, where it goes under the southbound - Foothills- Parkway -to- east - bound - Highway -36 on -ramp and then under the highway itself in triple 60 -inch concrete culverts. Eventually, it finds its way to South Boulder Creek (see Figure 34). The Veile Gulch channel and culverts are large enough to handle normal storm flows from the interior of the mined site in addition to the normal Veile -Gulch flows. Under normal conditions, the site - drainage and Veile -Gulch peak flows occur at different times. © Leonard Rice Engineers, Inc. June 2009 — 207FIP05