Safety factors for design of stormwater ponds?
Author: Bill Noell
Source: Pattle Delamore Partners Ltd
Stormwater attenuation ponds are widely used to reduce the downstream water quality and flooding effects of stormwater discharges in urban areas. The design of attenuation storage is dependent on a number of factors including downstream flow requirements, inflows, outlet size and volume stored. The estimation of these variables can create considerable uncertainties with the sizing of such attenuation devices.
The use of safety factors in hydrological design is often not as commonly visible compared with other engineering disciplines. While “freeboard” is commonly included in hydraulic design of spillways and downstream flow paths to account for design uncertainties, limited attention is often applied to the performance of the basin below spill level.
Significant uncertainties in the pond performance can occur through the following:
• Temporal and spatial variability of rainfall and runoff
• Estimation of outlet flow requirements
• Estimation of outflow performance with transitional hydraulic conditions
• Construction tolerances
• Operational losses in volume (sediment, vegetation)
This paper examines provisions for these factors in commonly used design standards and the effect these uncertainties can have on the performance of attenuation structures. It also considers requirements to manage these design uncertainties including the provision buffer storage depth below the spill level and other options.
With respect to freeboard, there are significant differences between minimum freeboard allowances used in design of stormwater ponds and those specified in the Dam Safety Guidelines (2015). This paper considers where the Dam Safety Guidelines may be applied for urban stormwater ponds.
In summary, there is a strong case for both positive (i.e. above spillway level) and negative freeboard (below spillway level) to provide appropriate design safety factors for stormwater attenuation ponds with a consequence based setting of freeboard(s) on a case by case basis.