53:078 Principles of Hydrology
Unit Hydrographs Practice (Solutions)

  1. 26 cfs-hours.

  2. Rainfall excess hyetograph associated with this 2-hour unit hydrograph event:

    Time (hours)
    0-1
    1-2
    Pe (in)
    0.5
    0.5

  3. S-hydrograph for the watershed is:

    Time (hours)
    0
    1
    2
    3
    4
    5
    6
    7
    S-Hyd (cfs)
    0
    4
    14
    20
    24
    26
    26
    ..

    The value at t = 10 hours is 26 cfs. In fact, the value for all times greater than 5 hours is 26 cfs.

  4. A 2-hour UH is needed. The 2-hour UH (given) is used to predict the direct runoff hydrograph.

    Time (hours)
    0
    1
    2
    3
    4
    5
    6
    7
    8
    9
    DRH (cfs)
    0
    2
    7
    12
    19
    19
    11
    6
    2
    0

    The volume of direct runoff is 78 cfs-hours = 3(26 cfs-hours). OK

  5. Need first to compute the 4-hour UH. This is used to predict the direct runoff hydrograph.

    Time (hours)
    0
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    DRH (cfs)
    0
    1
    3.5
    5
    6
    7.5
    10
    11.5
    12.5
    11
    6
    3
    1
    0

    The volume of direct runoff is 78 cfs-hours = 3(26 cfs-hours). OK

  6. Need first to compute the 1-hour UH. This is used to predict the direct runoff hydrograph.

    Time (hours)
    0
    1
    2
    3
    4
    5
    6
    7
    8
    DRH (cfs)
    0
    4
    18
    30
    26
    16
    8
    2
    0

    The volume of direct runoff is 104 cfs-hours = 4(26 cfs-hours). OK

  7. Need first to compute the 3-hour UH. This is used to predict the direct runoff hydrograph.

    Time (hours)
    0
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    DRH (cfs)
    0
    1.33
    4.67
    6.67
    9.33
    13.33
    15.33
    15.33
    12.67
    10.67
    8
    4
    2
    0.67
    0

    The volume of direct runoff is 104 cfs-hours = 4(26 cfs-hours). OK

  8. The 4-hour unit hydrograph is:

    Time (hours)
    0
    1
    2
    3
    4
    5
    6
    7
    8
    9
    DRH (cfs)
    0
    1
    3.5
    5
    6
    5.5
    3
    1.5
    0.5
    0

    The volume of direct runoff is 26 cfs-hours (required for a UH for this watershed).

Last changed on 11/13/13 by aab.