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3 Water Module

3.6 Damming and Diverting

3.6.7 Spillway Requirements

Weir Width Requirements (m)

    Catchment Size km2
0.2 0.4 0.6 0.8 1
Depth (m) 0.3 - - - - -
0.5 7 - - - -
1.0 3 4 4 6 6
1.5 2 2 3 3 4
5.0 - 2 2 2 3


Spillway requirements.

3.6.7.1 Calculation Assumptions

There have been several assumptions made to generate the above guidelines for spillway requirements.

  • The Rational Method was used to estimate flows.
  • The rainfall event was assumed to be a 50 year event multiplied by 1.6 which was then used to derive the maximum required spillway flow.
  • An average for the Region was used.
  • The dam is assumed to be full when the rainfall event occurs.
  • Rainfall time of concentration has been assumed to be dependent on the catchment size by the following table:

    Time of concentration relative to catchment size

    Catchment Size km2 0.2 0.4 0.6 0.8 1.0
    Assumed Time of Concentration (m) 10 20 30 30 30
  • The run-off coefficients were also assumed to be dependent on the catchment size:

    Run-off coefficient relative to catchment size

    Catchment Size (km2) 0.2 0.4 0.6 0.8 1.0
    Run-off Coefficient 0.8 0.8 0.7 0.7 0.6
  • The weir flow calculations were derived using the following formula:

    q=2/3*H*(2/3*g*H)1/2

    where:

    q = flow (m3)/meter width (m)
    H = depth of water about the weir crest (m)
    G = gravity (9.81m2/s)

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