How to size litter boom skirt depth for your river

Most skirt-depth questions start the same way: a supplier hands you a spec sheet with three stock depths, and you're supposed to pick one before you've even seen the site at low flow. That's backward. Skirt depth is a site decision: get it wrong and debris slides under the boom, or the whole unit drags under at the first rise in flow.

What skirt depth does

The skirt is the part of the boom that hangs below the waterline. Its job is to stop debris from ducking underneath the floating section. It isn't there to catch everything in the water column.

Litter booms are built for surface debris: bottles, bags, foam, branches, the stuff that rides the top 30 to 60 cm depending on turbulence and flow velocity. They are not submerged-debris collectors. A boom with a 1.2 m skirt in a 1.5 m channel catches no more debris than one with a 0.4 m skirt. It just sits closer to the bed, where it fouls on submerged logs and shopping carts, the single most common cause of torn skirts and failed anchor points.

The working rule most crews use: skirt depth should reach roughly 1.5 to 2 times the depth of the surface debris layer you're trying to intercept, with enough clearance above the channel bed to avoid snagging on bedload and submerged obstructions. In a slow, shallow tributary that might mean a 0.3 m skirt. In a tidal reach with heavier surface loading after storm flow, crews often run 0.6 to 0.9 m. Past that, you're fighting drag and current loading for very little debris-capture gain, because surface debris behaves exactly as the name says: it stays on the surface.

Freeboard matters as much as skirt depth

Freeboard is the part of the boom riding above the waterline, and it's the half of the sizing question that gets skipped because skirt depth gets all the attention. If freeboard is too low, wake from wind or a passing wash overtops the boom and debris goes straight over the top. No skirt depth fixes that. As a rough field check: freeboard should sit at roughly half to two-thirds of your skirt depth in calm reaches, and closer to equal in reaches with regular wake, wind fetch, or flashy storm response. If your reach floods fast after rain, undersized freeboard is usually the reason a boom that worked fine in September fails in a December storm.

Boom draft depth versus channel depth

Draft depth, how far the whole assembled unit sits below the surface including skirt and ballast, has to leave clearance above the bed and above any known submerged hazards: sunken debris piles, low bridge footings, gravel bars that shift after high flow. A boom sized for a 2 m pool at summer base flow can be sitting on the bed by October if that reach drops with seasonal withdrawal. This is the part that trips up a lot of first-time siting decisions: the bathymetry at the site visit isn't the bathymetry three months later.

Surface debris vs submerged debris: a quick gut check

If your field notes or photos show mostly floating bottles, bags, and foam at the surface with no visible mass below the waterline, a shallow skirt sized to that surface layer is the right call. If you're seeing submerged mats of debris, snagged branches breaking the surface, or debris banking up against something unseen below, that reach probably needs a trash rack or a different interception method entirely. A litter boom isn't built to intercept a submerged load, and trying to size the skirt deep enough to catch it usually just gets the boom fouled or breached.

Where accumulation sits in the reach

None of this sizing math matters if the boom goes in at the wrong bend. A skirt built for the right depth, anchored at a point where debris doesn't collect, still catches nothing. A boom sited off last year's cleanup location reflects where the crew happened to be last time, which may have nothing to do with where debris is piling up this season. Plastic Accumulation reads monthly multispectral satellite imagery to flag where floating plastic is accumulating along a reach, so the depth and freeboard math above gets applied at the bend where it counts.

If you're about to size a skirt for a new stretch of river, it's worth checking where the debris is collecting before you cut the fabric.