Sediment basins Part 1 – Introduction, Use, Design, Construction and Maintenance.

What are sediment basins and why are they used?

Sediment basins are temporary pond structures that are used to detain sediment-laden runoff long enough to allow majority of the sediment to settle out. Sediment accumulates at the bottom and desilting is required to maintain the capacity of the sediment basin from subsequent sediment additions. 

This part one of a series of two sediment basin articles introduces sediment basins, what they are, how they work and then focus on sediment basin design, construction and maintenance. 

How do sediment basins work?

• Slow water velocity.
• Moderate peak flows.
• Allow for sediment to accumulate in the bottom.
• Use release structures to control the discharge.
• Is an emergency spillway to release flow from larger storms.
• Serve as permanent stormwater management facilities.
• Facilitate water collection and reuse.

Where are sediment basins typically used?

The typical uses of sediment basins is spread across a variety of industries. Sediment because of water erosion is common in every exposed area and typically sediment basins are used in these areas:

• Construction sites.
• Mines.
• Farms.
• Catchment areas.
• Residential sites.
• Highway projects.
• Downstream treatment sites.

What are the key considerations for a sediment basin design?

The key elements in the design considerations for a sediment basin include the designed outlet inlet and outlet structures, settling pond, and high flow, overflow structures and storage volume. The key design considerations are as follows:

1. Sedimentation basin key roles in the treatment train.
2. Sizing of the sediment basin to match the settling velocity of target sediment size.
3. Provision of adequate storage for settled sediment.
4. Outlet design based on design flow entering the basin.
5. Landscape design and location.
6. Vegetation specification for scour and erosion control to the basin batters.
7. Maintenance and sediment removal happens relative to target basin depth.
8. Access pad for excavator access for desilting.
9. Batter slope between 1:3 – 1:5.
10. Importance of flow path lengths within the system.
11. Impact of flocculants/coagulants.
12. Effectiveness during flood flow conditions.

Sediment basin design example

Our sediment basin design example is adopted from the Gold Coast Planning Scheme Policies document. The proposal involved constructing a wetland system to treat runoff from a freeway in Pimpama on the Gold Coast. The sediment basin formed the ‘inlet zone’ of the wetland system. The key design elements to ensure effective operation of the constructed wetland and sediment basin in this example include:

• notional permanent pool depth of sedimentation basin of 2m
• permanent pool 0.3 m (3.8 AHD)
• wetland macrophyte zone extended detention depth of 0.5 m (permanent water level of 3.5 m AHD)
• sedimentation basin permanent pool level (control outlet pit level) 0.3 m above the permanent pool level of the wetland (3.8 m AHD)
• ‘spillway’ outlet weir set at the top of extended detention for the wetland and 0.3 m above the sediment basin permanent pool level (4.1 m AHD).

What is a sediment basin design construction inspection checklist?

A sediment basin design construction inspection checklist is documentation that monitors functional installation, earthworks, erosion and sediment control and operational establishment during sediment basin construction. Some of the key components of the checklist include:

Preliminary works: 

• Erosion and sediment control plan adopted.
• Locations same as plans.
• Site protection from existing flows.

Earthworks:

• Integrity of banks.

• Compaction process as designed.
• Check for groundwater intrusion.

Structural components:

• Location and levels of outlet as designed.

• Inlets appropriately installed.

• No seepage through banks.
• Provision of maintenance drain.

Vegetation:

• Stabilization immediately following earthworks.

• Planting as designed (species and densities).
• Vegetation layout and densities as designed.

Erosion and sediment control:

• Sediment basins to be used during construction.
• Stabilization immediately following earthworks.
• Silt fences and traffic control in place.

Operational establishment:

• Sediment basin desilted.

What are the key questions in a sediment basin maintenance checklist?

This checklist seeks to address the pertinent maintenance questions and compliance regulations for a sedimentation basin. These are the key questions to be answered in the checklist:

1. Litter accumulation?
2. Sediment accumulation at inflow points?
3. Sediment requires removal (record depth, remove if >50%)?
4. All structures in satisfactory condition?
5. Evidence of dumping (building waste, oils etc.)?
6. Littoral vegetation condition satisfactory (density, weeds etc.)?
7. Replanting required?
8. Weeds require removal from within sediment basin?
9. Settling or erosion of bunds/batters present?
10. Damage/vandalism to structures present?
11. Outlet structure free of debris?
12. Maintenance drain operational (check)?
13. Resetting of system required?

In conclusion – watch out for part two.

Are you looking to design, construct, maintain, rehabilitate or decommission your sediment basin? Speak to Daniel Grundy and hear how GRT can assist you in compliance with the stipulated regulations of your erosion and sediment control plan. Find out more in part two of our sediment control article series in which we further discuss all things sediment basins. 

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