3 Water Module

Weir Width Requirements (m)

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)

     Private Bag 3038   Waikato Mail Centre   Hamilton 3240   Fax (07) 859 0998     Freephone 0800 800 401