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13: Streams and Floods (Exercises)

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    Q13.1 How Long Does Water Stay in the Atmosphere?

    The volume of the oceans is 1,338,000,000 km3 and the flux rate is approximately the same (1,580 km3/day).

    What is the average residence time of a water molecule in the ocean?

    1,338,000,000/1,580 = 846,835 days average residence time for water in the ocean (or 2320 years)

    Q13.2 The Effect of a Dam on Base Level

    How does the formation of a reservoir affect the stream where it enters the reservoir, and what happens to the sediment it was carrying?

    The velocity of the streams slows to zero and most of the sediment is deposited quickly.

    The water leaving the dam has no sediment in it. How does this affect the stream below the dam?

    With nothing to deposit, the water below the dam can only erode, so there will be enhanced erosion below the dam.

    Q13.3 Understanding the Hjulström-Sundborg Diagram



    1. A fine sand grain (0.1 mm) is resting on the bottom of a stream bed.

    (a) What stream velocity will it take to get that sand grain into suspension? ~20 cm/s

    (b) Once the particle is in suspension, the velocity starts to drop. At what velocity will it finally come back to rest on the stream bed? ~1 cm/s

    1. A stream is flowing at 10 cm/s (which means it takes 10 s to go 1 m, and that’s pretty slow).

    (a) What size of particles can be eroded at 10 cm/s? No particles, of any size, will be eroded at 10 cm/s, although particles smaller than 1 mm that are already in suspension will stay in suspension.

    (b) What is the largest particle that, once already in suspension, will remain in suspension at c0 cm/s? A 1 mm diameter particle should remain in suspension at 10 cm/s.

    Q13.4 Determining Stream Gradients


    Gradients of Priest Creek (in red) [SE]

    The length of the creek between 1,600 m and 1,300 m elevation is 2.4 km, so the gradient is 300/2.4 = 125 m/km.

    1. Use the scale bar to estimate the distance between 1,300 m and 600 m and then calculate that gradient. 5.2 km, with a gradient of 700/5.2 = 134 m km
    2. Estimate the gradient between 600 and 400 m. 3.6 km, with a gradient of 200/3.6 = 56m /km
    3. Estimate the gradient between 400 m on Priest Creek and the point where Mission Creek enters Okanagan Lake. 4 km, with a gradient of 60/4.0 = 15 m/km

    Q13.5 Flood Probability on the Bow River

    1. Calculate the recurrence interval for the second largest flood (1932, 1,520 m3/s). Ri = 96/2 = 48 years
    2. What is the probability that a flood of 1,520 m3/s will happen next year? 1/48 = 0.02 or 2%
    3. Examine the 100-year trend for floods on the Bow River. If you ignore the major floods (the labelled ones), what is the general trend of peak discharges over that time? In general the peak discharges are getting lower (from an average of around 400 m3/s in 1915 to an average of about 300 m3/s in 2015)

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