How the two differ in kind
Both devices do the same physical thing — hold water still enough, and for long enough, that suspended particles fall out of it — and they are designed for completely different flow conditions. Using one where the other belongs is the commonest and most consequential error on a site.
A SILT FENCE is a perimeter control. It is designed to intercept SHEET flow: a thin, slow, distributed film of runoff moving across a surface. The fabric is a filter but the mechanism is mostly impoundment — water ponds behind it, slows down, and drops the coarser particles before the water passes through or around. That works for a limited contributing area and a limited slope length behind it, and those limits are the design.
A SEDIMENT BASIN is a treatment device at the bottom of a catchment. It takes CONCENTRATED flow — a channel, a swale, a pipe, the collected runoff of a large disturbed area — holds a designed volume of it, and gives the particles time to settle before releasing the clarified water through a controlled outlet. It is a piece of temporary infrastructure with a designed volume, a designed outlet and a designed emergency spillway.
The failure mode when they are swapped is not gradual. A silt fence installed across a drainage path receives concentrated flow it was never sized for: it either overtops, or the flow undermines it, and the pond it had accumulated discharges at once. The result is a slug of sediment leaving the site in a single event, which is worse than if nothing had been installed — and it is the reason regulators treat a fence across a channel as a defect rather than an inadequacy.
The factors that actually differ
Show
| Silt fence | Sediment basin | |
|---|---|---|
| Flow it is designed for | Sheet flow — thin, distributed, slow. Never concentrated flow. | Concentrated flow — a channel, a swale, a pipe, or the collected runoff of a catchment. |
| Contributing area | Small, and limited by both area per unit length of fence and the slope length behind it. | Large. This is what basins exist for. |
| Position on site | At the perimeter, along the contour, downslope of disturbed ground. | At the low point, at the outfall, where the catchment collects. |
| How it is designed | By length, contributing area and slope, with a defined maximum flow path behind it. | By storage volume, surface area, detention time and outlet capacity, with an emergency spillway. |
| Installation detail that decides it | The bottom must be keyed into a trench and backfilled. A fence stapled to the ground surface is undermined by the first storm. | The outlet and the spillway. A basin that discharges through an uncontrolled breach delivers everything it held at once. |
| Maintenance | Accumulated sediment removed before it reaches a fraction of the fence height, and inspection after every significant rain. | Sediment removed before it consumes the design storage, and the outlet kept clear. Both are formal duties, not optional tidying. |
| What happens at end of life | Removed once the area is stabilised, and the accumulated sediment removed with it — not simply spread about. | Dewatered, sediment removed, and the basin decommissioned and restored. |
| Effectiveness on fines | Poor. Sand settles; silt is partial; clay largely passes through or stays in suspension. | Better, because detention time is much longer — but clay still largely defeats it without flocculation. |
| Cost and footprint | Low cost, minimal footprint, installed in a day. | Substantial: land take, excavation, outlet structure, and the space has to be available at the low point. |
| What neither replaces | Erosion control upstream. Soil that never moves does not have to be captured. | The same. A basin is the last line, not the strategy. |
Which one, and when
Choose silt fence when…
- The contributing area is small and the flow across it is genuinely sheet flow.
- It is a perimeter control on the downslope boundary of a disturbed area.
- The slope length behind the fence is within the limit the design allows.
- It is one element of a set — with upstream erosion control, inlet protection and stabilised entrances — rather than the entire plan.
Choose sediment basin when…
- The catchment is large, or the flow arriving is concentrated in a channel or a pipe.
- The site discharges to a sensitive receiving water where a measurable standard applies.
- There is space at the low point and the site will be disturbed long enough to justify the infrastructure.
- The regulator requires one — which on larger disturbed areas is usually the case rather than a judgement.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does a silt fence fail across a channel?
- Because it was designed for a thin film of water and it is receiving a concentrated stream. Sheet flow arrives slowly and distributed along the whole fence, which ponds shallowly and lets particles settle. Concentrated flow arrives fast, at one point, with far more volume and energy than the fence's length can dissipate. Two things then happen, and often both: the water rises above the fabric and overtops it, and the velocity at the base scours under the toe and undermines it. Either way, the impoundment that had accumulated behind the fence is released in a single surge — carrying with it all the sediment the fence had been collecting. That is why a fence across a drainage path is worse than no fence, and why concentrated flow gets a check dam, a channel liner, or a basin instead.
- How is a silt fence supposed to be installed?
- Along the contour, with the bottom of the fabric keyed into a trench and backfilled, and the posts on the downstream side with the fabric on the upstream face. The trench is the part most often skipped and it is the part that decides whether the fence works: water takes the easiest path, and fabric laid on the ground surface simply lets the flow pass underneath, at which point the fence is a fence. Following the contour matters for the same reason — a fence running downhill concentrates flow at its low end, producing exactly the channel-flow failure it was never designed for, and the usual remedy is to turn the ends of the run upslope so water cannot run around them. Joints between rolls are wrapped rather than butted, since a butt joint is a slot.
- How big does a sediment basin need to be?
- It is sized on storage volume relative to the disturbed catchment and on the detention time needed for the particles you are trying to remove, with an outlet sized to release the design flow slowly and an emergency spillway sized for the event that exceeds it. Your regulator's figures govern, and they differ. Three things matter beyond the headline volume. Surface area matters as much as depth, because settling is a function of how far a particle must fall and how long it has. The outlet should draw from the surface rather than the bottom, so it releases the clearest water rather than stirring the settled material. And storage has to be maintained: a basin half full of accumulated sediment has half the design volume and a fraction of the detention time, so removal is scheduled rather than left until it is obviously full.
- Why does neither remove clay?
- Because settling velocity falls steeply as particle size falls, and clay particles are small enough that they stay suspended almost indefinitely in still water — far beyond any detention time a temporary basin can offer. Sand drops out in minutes, silt takes considerably longer, and clay is the reason a basin discharges water that is still visibly turbid. Two responses exist. Flocculation adds a treatment that binds fine particles into larger aggregates that will settle, and it is effective where the site's soils and the receiving water justify the added complexity of dosing and monitoring. The better response is upstream: keep the clay in the ground. Stabilising surfaces, limiting the area disturbed at any time, mulching, matting and seeding all prevent the particles entering the water at all, which is the only intervention that genuinely works on fines.
- How often do they need inspecting?
- On a routine interval and after every significant rainfall, because a control that worked before a storm may not have survived it — and the storm is exactly when it mattered. The routine checks are specific: for a fence, accumulated sediment against the fabric, undermining at the toe, tears, sagging between posts, and flow finding its way around the ends; for a basin, remaining storage volume, the condition of the outlet and any screening on it, the spillway, and the embankment. Sediment removal is triggered at a fraction of the design capacity rather than at full, because performance degrades long before the control is visibly full. These inspections are usually a formal requirement with a record, and the record is what demonstrates the site was managed rather than merely equipped.
- Can I use both?
- Yes, and on most sites of any size that is the expected arrangement rather than an alternative. They occupy different positions in the same treatment train: erosion control on the surfaces to stop soil moving at all, inlet protection at drainage structures, silt fence at the perimeter where sheet flow leaves disturbed ground, check dams in channels to slow concentrated flow, and a basin at the low point to treat what the catchment collects. Each element handles a flow condition the others do not, which is why a plan consisting of one measure repeated everywhere performs badly whatever that measure is. The sequencing matters too — perimeter controls and the basin are installed BEFORE the ground is disturbed, not once earthworks are under way.
- What replaces a silt fence for concentrated flow?
- Something designed for velocity rather than impoundment. In a channel, check dams — rock, wattles, or manufactured products — placed at intervals that back water up to the toe of the one above, slowing flow and trapping coarse sediment without creating a single large impoundment. Where the channel itself is eroding, a liner: rock, turf reinforcement, or an erosion control blanket, so the flow no longer picks up material at all. At a drainage inlet, purpose-made inlet protection rather than fabric wrapped around the grate, which blinds and causes flooding. And where the flow represents a whole catchment, a basin. The common thread is that concentrated flow is managed by reducing velocity along its path, not by trying to stop it at a line.
- What happens to all this at the end of the job?
- It is removed, and the removal is part of the work rather than an afterthought — with a condition attached: controls stay in place until the areas draining to them are permanently stabilised. Taking a fence out while the ground behind it is still bare simply releases the next storm's load. Once stabilisation is established, the fence is pulled, the accumulated sediment is collected and disposed of or incorporated properly rather than raked flat, and the trench line is reinstated. A basin is dewatered — the water pumped out is itself sediment-laden and needs treatment or discharge to a suitable place — the accumulated material excavated, the outlet structure removed, and the area regraded and stabilised. The disturbed footprint of the basin is then itself bare ground, so it is one of the last areas to be seeded.