There is a fundamental difference between sediment retention ponds and smaller structures like sediment traps. The philosophy behind the location and design of the structures, discussed in this chapter so far, has been to reduce sedimentation by dispersing water as close to source as possible. Sediment retention ponds rely more on a ‘concentrate and treat’ approach. As their name suggests, these are ponds designed to retain sediment, so they can be large structures that can hold high volumes of sediment laden water. They allow coarse to moderately fine particles to settle out of water before it is discharged. Sediment retention ponds are not effective where colloidal clay particles are in suspension, or where there is concentrated flow. Decanting earth ponds are a retention pond variant that uses an inverted syphon pipe in the pond centre as an outlet. These are common in large earthwork construction, such as new residential housing estates, where water flow paths are part of the earthwork design.
Sediment retention ponds in forestry are typically only used in situations over very large earthworks to minimise the discharge of sediment laden stormwater into highly sensitive receiving environments. They are used when other methods are inadequate for the site.
It is best to seek specialist advice to determine sizing and design flow capacity. Unless they are designed and constructed to specifications, there are a potential safety hazards to personnel and machinery working in the vicinity, and off-site risks if the structure fails. The consequences of failure can be significant.
Design the structure to fit the terrain. Do not try to use a retention pond if the site is not large enough to size it properly. Never construct them in fill material. Large volumes of water create high static pressure that could cause fill to saturate and fail. Also ensure that they are not installed within the active flood plain of rivers.
When constructing sediment retention ponds, excavate in original ground. If the inflow or outflow must pass through fill, then flume the water into or out of the structure. The outlet is generally constructed at the opposite end to the inlet to maximise settling time. The length to width ratio should be at least 3:1 to extend the flow path from the inlet to the outlet, and provide time for sediment to settle out from the stormwater. Keep the slope of the inlet/outlet reasonably flat and broad to avoid erosion, or if not possible, form a rock armoured spillway or flume if necessary. Ensure the outflow is on stable ground. Slash or long grass can assist with sediment retention.