Exercise 7-Working rivers Part 3 RIVER DISCHARGE AND FLOODING ream\'s discharge

Question

Exercise 7-Working rivers Part 3 RIVER DISCHARGE AND FLOODING ream's discharge (volume) is Q-Ax V, where Q is discharge per second; A is the cross sectional area of the stream The equation for calculating a st measured in cubic feet (or cubic meters) determined by multiplying the a measured in either feet per second (or meters per second). If a stream h feet, is 32 feet wide, flows at a rate of 4 feet per second, what is its discharge? 1- verage depth by the width of the stream; and V is velocity as an average depth of 7 2. Floods occur when the discharge of a river exceeds the volume of the river channel and water flows over the channel edge onto the floodplain. Reconsider the equation for discharge. If the river is now flowing over a much wider area and the depth remains almost constant, what effect does that increase in A have on the equation, if Q remains the same? A river's capacity refers to the volume of sediment it can carry. Capacity is related to discharge in that a larger river can transport more sediment than a smaller river. However, another measure of 3. a stream's ability to move sediment is competence, which refers to the largest sized particle a river can carry. Competence is controlled primarily by the river's velocity. A fast moving stream of any size can carry larger particles than a slower flowing stream. Consider what happens to river's capacity when it floods onto its floodplains. a. Does its discharge decrease? b. Does its capacity decrease, remain the same, or increase? Now consider what happens to its competence on the floodplains. c. Referring to your answer for question 1, does competence decrease, increase, or remain the same? d. Based on all your answers, what happens on the floodplain during flooding in terms of deposition and/or erosion'?

Explanation / Answer

a. When river overflows its channel it deposit its material on the side of channel and this filling up in the side is known as floodplain (as it is formed when river floods and because of its planar surface) and since its transfering its material from channel to the side hence its dischare decreases.

b. While forming this flood plain a lot of river's material is deposited, which is why formation of flood plain takes place, hence now it can hold more material therefore its capacity increases.

c. After the flood plains the gradient of river decreases because flood plains occur during the mature phase of river lifetime where its velocity is relatively low when compared to initial valley and hence its capability to carry large size sediment also decreases and hence its competence also decreases.

d. Deposition of sediment takes place on flood plain because till flood plain the river have high capacity and competence and when this river over flows its channel it deposit this material in the flood plain.

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