How to Calculate Pond Weir Openings

The design of ornamental ponds and waterfalls includes careful engineering to ensure that all elements in the water (re)circulation system are adequately dimensioned. In particular, the designer must calculate the amount of water flowing through weirs -- the weirs may be located at the top of a waterfall, or at outflow points of a pond. You can compute estimates for water flows through weir openings using Francis' formula.

Instructions

- 1
  Measure the width of the weir in feet (W), and the depth of the water flow as it passes through the weir in feet (H). You can perform those measurements on blueprints or on the actual pond or waterfall.
- 2
  Compute the first-level Francis flow estimate, by evaluating this formula
  
  First_flow = 3.33 x ( ( W - 0.2 x H ) x H ^ 1.5 )
  
  First_flow will be expressed in cubic feet per second. If the weir is 3 foot wide or wider (i.e., if W>=3), First_flow is an accurate estimate of water flow.
- 3
  Correct for narrow weirs by computing the second-level Francis flow estimate, by evaluating this formula:
  
  Second_flow = 0.8 x First_flow
  
  Second_flow, expressed in cubic feet per second, is an accurate estimate of water flow for the narrower weirs.

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