Question

A gravelly sand channel slopes away from a reservoir, through the reservoir rim, to an adjacent drainage area. The channel slopes at a gradient of 23 m (75 ft) vertically to 30.5 m (100 ft) horizontally. In cross section the channel is 1.8 m (6 ft) thick and 4.6 m (15 ft) wide. The channel intersects the adjacent drainage area at a horizontal distance of 152.4 m (500 ft) from the reservoir.

a. At minimum pool elevation in the reservoir the water is 1.5 m (5 ft) above the sandy gravel channel. What is the total pressure head in the gravelly sand seam at the upstream end (or at the reservoir)? What is the total pressure head at the downstream end of the channel where it daylights into the adjacent drainage area 152.4 m (500 ft) away? What is the hydraulic gradient? What is the average velocity of flow if k = 10–2 cm/sec? What is the flow loss in m3/sec (ft3/sec) and in gal/min for the channel (use Darcy’s law, v = ki relationship, see also Chapter 13, Groundwater)? How long does it take for the water to flow from the reservoir to the adjacent drainage area?

b. At maximum pool the water stands at 15 m (50 ft) higher than minimum pool. What is the flow loss in m3/sec (ft3/sec) and gal/min for this case? Does a quick condition occur when the pool reaches this maximum level? If not, what is required to cause a quick condition? Explain.

Answer 1