Sediment management keeps silt from filling a channel by trapping it in settling basins at the intake and every 500-1000 m, and by cleaning both basins and channel on a schedule. A basin 1 m × 1 m on a 5 L/s flow catches sand coarser than about 0.09 mm and lets finer silt through, so the channel still needs cleaning; fit a drain pipe at the basin bottom so it can be flushed.
Part of Aqueducts and Channels
Sediment management is the combination of traps, flushing and cleanout that stops soil carried by the water from settling in a channel and reducing its capacity. A channel without it silts up from the slow stretches inward.
Silt Settles Wherever The Water Slows Down
Water carries sediment only while it moves fast enough. In a wide, flat or shallow stretch it slows and drops the load on the bed, which then raises the bed, narrows the section and slows the water further. A basin is a deliberate slow stretch, put where the deposit can be removed easily.
The aqueducts page places a basin at the channel intake and every 500-1000 m along the route, a wider, deeper section of 1 m × 1 m × 0.5 m deep, with a drain pipe at the bottom for periodic flushing. Settling Basins covers design in more depth.
What A Small Basin Does And Does Not Catch
A particle settles in a basin if it sinks faster than the surface overflow rate, the flow divided by the basin’s surface area.
For a 5 L/s channel: 5 L/s = 0.005 m³/s = 18 m³/h. Basin surface area 1 m × 1 m = 1 m². Overflow rate = 18 ÷ 1 = 18 m/h = 5 mm/s.
Settling speeds from the Settling Basins page: coarse sand (0.2 mm) sinks at 25 mm/s; fine sand (0.02 mm) at 0.3 mm/s; silt (0.002 mm) at 0.003 mm/s. A particle is caught only if its settling speed exceeds 5 mm/s. Sinking speed rises with the square of the diameter, so scaling the 0.2 mm row (25 mm/s) down to 5 mm/s gives a diameter of 0.2 × √(5 ÷ 25) = 0.2 × 0.447 = 0.09 mm, a rough figure. Coarse sand is caught; fine sand is not. To catch fine sand (0.02 mm, 0.3 mm/s = 1.08 m/h) would need 18 ÷ 1.08 = 17 m² of surface.
So the small basins are grit traps. Fine sand and silt go on down the channel, and settle where the water slows. The cleanout plan has to cover it.
What You Need
- Basins at the intake and at intervals (Open Channels)
- Drain pipes and plugs or valves at the bottom of each basin
- Shovels, buckets, rakes and a way to carry spoil away
- A schedule: before and after each rainy season, and after any storm that makes the water muddy
- A place for spoil away from the channel bank, where rain cannot wash it back
Method: Trap, Flush, Clean, Check
Step 1: Build the basin with the drain at the lowest point.
Step 2: Clean the basin before it fills. Open the drain and flush; shovel out what remains. A full basin no longer slows the water and releases its sediment again in a high flow.
Step 3: Walk the channel after storms and look for banks of silt, standing pools and reduced flow.
Step 4: Clean the channel bed from the downstream end upwards, so cleaned water does not run through a dirty section.
Step 5: Dispose of the spoil well away from the bank.
Step 6: On lined channels, use wooden or blunt tools. A steel shovel gouges concrete lining and tears plastic.
Worked Example
A channel is losing capacity. Before cleaning, measure the flow at the intake and at the end (Flow Comparison) to separate seepage from blockage. A silted section raises water levels upstream of it and lowers the flow, while seepage loses water without a change in level.
Common Mistakes
- Never cleaning the basin. It fills and stops working.
- Spoil on the bank. It washes straight back.
- Cleaning from upstream. Dirty water refills the cleaned stretch.
- Using steel tools on the lining. It damages it.
- Treating the basin as a fine-silt filter. It is not.
Safety
Covered sections and deep basins can hold bad air, and the rescuer dies second
Rotting leaves and silt in a roofed channel, an access chamber or a deep basin can use up the oxygen and give off hydrogen sulphide or carbon dioxide, which is heavier than air and pools low down. A person who steps in can collapse in seconds. Clean from the surface with long-handled tools. If someone must go in, ventilate at two openings, use a harness tied to a line held by an attendant, and keep a second person outside who pulls the line and never follows; the commonest fatality pattern is a second person going in after the first. Do not test the air with a flame.
What’s Next
- Learn next: Vegetation Control — the other routine job
- Combine with: Optimal Gradient — a steeper grade carries silt through