Sediment traps and soak holes are small excavated structures. They capture sediment-laden water, allow sediment to settle, then water to either discharge or drain. They reduce the volume of sediment that can enter sensitive sites, such as waterbodies. They should be located close to roads and landings to enable easy access for maintenance.
To be effective, sediment traps need to be large enough to slow flow, and allow some of the larger sediment particle sizes to settle, before the water is discharged. Soak holes are constructed in porous soils such as sand and pumice, allowing sediment-laden water to soak into the soil. Although sediment traps and soak holes are constructed similarly, they work differently. Some sites not suitable for either type of sediment control. These include a fill batter, where they increase the risk of bank collapse, or where the site is located within land that carries flood flows of rivers.
Construct sediment traps near culvert inlets and outlets, and immediately after water is directed, near a road, track, or landing as necessary. A recommended effective sediment trap size is 1 m deep x 1.5 m long. A good length to width ratio is 3:1, but this is not always practical at culvert inlets due to topographical constraints and safety concerns. For example, on steep terrain, adequately sized cut-outs are difficult to construct near culvert mouths as they may encroach into the roadway, creating a health and safety hazard. Multiple small traps of at least the bucket width may be an alternative option. Do not construct sediment traps or soak holes in fill where they increase the risk of bank collapse or within the floodplain of rivers.
Soak holes are constructed similarly to sediment traps, but are constructed in porous soils such as sand and pumice, allowing sediment-laden water to soak into the ground with no discharge.
Soak hole spacing guide
Excavate the trap to well below the culvert inlet level, to ensure maximum sediment retention capacity for the trap. Install on hard ground rather than fill or disturbed soil. Use a rock bucket to excavate. Make them large enough so that an excavator bucket can be used during maintenance to remove the retained sediment. Keep the slope of the inlet into the soak hole reasonably flat, to avoid erosion. If the inflow or outflow is through fill, then flume into and/or out of the sediment trap. Consider sediment retention from the outflow. Additional sediment controls like slash or grassing or the use of polymers may assist, if needed.
