Laserfiche WebLink
Table 4.0 indicates that large velocities and Froude numbers are associated with channels <br />DS4, DS5 and DS7r. In general, velocities in excess of 6 ft/s and Froude numbers greater <br />than 0.8 would indicate the potential for excessive erosion of the channel. This is <br />primarily due to the steep slope of the channels in these areas. Note that the geometries <br />shown in Table 4.0 are approximate and provide a preliminary working design for <br />construction. Modifications may be made in the field to accommodate space limitations <br />including bedrock issues. All channels constructed in unconsolidated materials of <br />moderate grade will be compacted with excavation equipment after final grading and <br />seeded to establish vegetation. The following paragraphs describe construction <br />restrictions and rationale for each drainage structure design. <br />DS1: <br />Atriangular-shaped or a narrow (small-based) trapezoidal channel is anticipated for <br />drainage structure DS 1, on the Upper Bench area because of limited space. The <br />estimated geometry is a 1-ft base, a 1 ft depth, and 2H:1 V side slopes. The channel will <br />be constructed at the toe of the recontoured slope below the former Daylight Portal. The <br />west flank of the channel will be bounded by a small bench feature that will be amended <br />with topsoil and seeded. The channel is expected to consist mostly of gravel to cobble- <br />sized shale with smaller amounts offinely-divided silt and sand-sized material. <br />DS2n, DS2s, and DS3: <br />Triangular-shaped or relatively nattow (small-based) trapezoidal channels are anticipated <br />for drainage structures DS2n, DS2s, and DS3 on the Upper Bench area because of limited <br />space or shallow depth to bedrock. The estimated geometry of channels DS2n and DS2s <br />is a 1-ft base, a 1 to 1.2 ft depth, and 2H:IV side slopes. The estimated geometry of <br />channel DS3 is a 1.5-ft base, a 1.2 ft depth, and 2H:1 V side slopes. The channels will be <br />constructed near the base of highwall of the bench area. The channel is expected to <br />consist mostly of gravel to cobble-sized shale with smaller amounts offinely-divided silt <br />and sand-sized material. <br />DS4: <br />Because the area of DS4 is characterized by exposed bedrock or shallow fill or <br />colluvium, erosion of the bed of the channel is expected to be minimal. To accommodate <br />the peak discharge anticipated in the 10-yeaz storm, and to minimize erosion of channel <br />side slopes, the preliminazy design of this channel involves a broad shallow channel on <br />one side that is in fact, the road grade itself. The estimated geometry of channel DS4 is a <br />1-ft base, a 1 ft depth, and 5H:1 V and 1H:1 V side slopes. This is the practical design to <br />accommodate the 10-year flows without conducting an expensive excavation in bedrock. <br />Although the calculations indicate a potential high velocity in this channel, bedrock is <br />exposed in places along this alignment and is expected to be shallow in other azeas, and <br />therefore control potential erosion associated with velocities. Where possible, vegetation <br />will be established in the channel to increase channel stabilization. Note again that <br />diverting runoff off the road onto the outslopes is not the preferred method of storm <br />control as the development of vegetation in the outslope areas is of primary importance. <br />If conditions allow, deeper, more triangular-shaped channels will be constructed in these <br />areas. <br />Western Water S Land Inc. 12 <br />