Sedimentation Pond Performance from Three-Dimensional Modelling

Stormwater Conference

In two separate studies, three-dimensional numerical modelling has been used to investigate the performance of stormwater and sediment control ponds. A mud transport module was used to model sedimentation and re-entrainment of the sediment the ponds are intended to trap. The wide range of fall velocities found in stormwater sediment results in quite different sedimentation patterns, and therefore required that several fractions of suspended sediment be modelled.

The first study applied either a constant flow of sediment-laden stormwater or a single runoff event, to various pond designs. This was to determine the effect of various parameter on pond efficiency (expressed as the fraction of incoming suspended sediment that is trapped). The variables investigated included pond size and geometry, inlet geometry, and baffles that redirect flow.

The second study investigated the long-term efficiency of a standard stormwater treatment pond design, by modelling the inflow derived from a one-year gauged rainfall record. This approach aimed to determine the pond efficiency in absolute terms, which can be done as long as inflow time series of inflow and sediment concentration are known as well as the range of sediment fall velocities.

The models provided not only the pond efficiency data but also visual and quantitative information on details of the sedimentation. For example:

• Even in symmetric ponds, flows are often asymmetric, reducing trapping efficiency somewhat;

• A circular pond with inlet and outlet at its edge can be inefficient if forced vortex flow results, because the pond’s centre remains free of sediment. Directing the inflow more radially inwards than tangentially prevents this.

Three-dimensional modelling including mud transport thus can increase our understanding of what is needed for an effective sedimentation pond.

Conference Papers Resource - Conference Papers Stormwater

1. Graham Macky.pdf

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21 Jun 2016