Laserfiche WebLink
-. safety=1.03). The very large size is a result of the excessive shear stress generated on <br />the steep slope (0.3571). There are several alternatives for armoring these channels. <br />\• <br />'~~ <br />1. Rip-rap -Although an option, rip-rap is probably not economically feasible. This <br />alternative would require a prepared bed (trapezoidal, depth=1.5 ft, bottom width=2, <br />side slopes of 2:1) lined with a filter cloth, then a layer of well graded gravel of sizes <br />3/4" to 3", 0.5 ft thick. The rock rip-rap would then be placed on the gravel bedding. <br />Since the total depth is only 1.5 ft and the tots{ width 8 ft, it wll be difficult to use large <br />stones and maintain the channel geometry. The channel would need to be over <br />excavated to make room for the blanket and rip-rap. <br />2. Gabion blanket -Amore feasible alternative would be to use Gabions to line the <br />channel. This option would require a prepared bed (trapezoidal, depth=1.5 ft, bottom <br />width=2, side slopes of 2:1) lined with a filter cloth to preven# erosion of the <br />subgrade. Gabion baskets constructed to fit the channel (one basket across the <br />entire width of the channel) would be 8.7 ft wide (channel wid#h) , 5 ft tong and 8 <br />inches thick. The rock filled Gabions would then be placed side by side extending up <br />the channel in 5 ft sections to form the channel. <br />3. Concrete channel - A reinforced concrete channel. Detailed concrete specifications <br />will be required if this option is selected. <br />4. Grouted rip-rap - An alternative to option one would be to use a smaller rip-rap size <br />(D~=12 inch) an then cover the rock with a sprayed on grouting layer. Detailed <br />grouting specifications will be required if this option is selected. <br />6. Sediment Ponds <br />6.1 Hydrology <br />Regulations impose several constraints on the hydrologic design of sedimentation <br />ponds. First, the pond must be capable of storing the 10yr-24hr event volume. in <br />addition to this volume it must also be constructed and maintained to account for <br />watershed sediment yield. !n time; sediment inflow to the pond will decrease the <br />effective storage capacity. Estimation of sediment yield is discussed in section 6.2. <br />The next hydrologic requirement stipulates that the pond must be capable of either <br />storing or safely passing the 25yr--24hr event immediately following the 10yr-24hr event. <br />One foot of freeboard from the maximum water surface elevation to the; top of the pond <br />embankment must be maintained at all times. These criteria are satisfied through the <br />combined effect of storage volume, a !ow level outlet and a primary spillway. The two <br />outlet works must be separate structures. The final hydrologic regulation specifies that <br />the entire 10yr--24hr volume, stored in the pond, must drain through the low level outlet <br />in 48 hrs. <br />Flowrates and volumes were computed using HEC-1, as previously discussed. They <br />were routed through the ponds using the Modified Puls method assuming level pool <br />10 <br />January 1998. <br />i <br />