Both armour a bank with stone. Riprap relies on the weight of each boulder; a gabion mattress ties small stone into one flexible blanket — so it holds the same water with far smaller stone.
Loose riprap must be heavy enough, stone by stone, not to be moved. A common estimate is the Isbash equation, d50 = V² ÷ (2g C² (Ss − 1)), with C = 0.86 for turbulent flow and stone of specific gravity 2.65:
| Water velocity | Loose riprap, median stone (approx.) | Gabion mattress that holds it (IRC:SP:116) |
|---|---|---|
| 3.5 m/s | 0.51 m | 0.15–0.17 m mattress, 70–100 mm stone |
| 4.5 m/s | 0.85 m | 0.23–0.25 m mattress, 70–150 mm stone |
| 5.0 m/s | 1.04 m | 0.30 m mattress, 100–150 mm stone |
So where only small or broken stone is available — most of the plains and the delta — a mattress does the job riprap cannot. See revet mattress thickness.
| Gabion mattress | Loose riprap | |
|---|---|---|
| Stone | Small, 70–150 mm, often local | Large boulders, sized to the velocity |
| Layer | 0.17–0.30 m thick | Usually 1.5–2 × the median boulder |
| Behaviour | One flexible blanket; follows scour and settles | Individual stones; displaced stones leave gaps |
| Steep banks | Holds on steeper slopes | Needs flatter slopes to stay in place |
| Under water | Mattresses or sack gabions placed by crane | Dumped; hard to control |
| Weak point | Wire coating where bedload abrades it — use PVC | Stone size and supply |
Where large, hard boulders are cheap and close, the bank slope is flat and the flow carries heavy bedload that would abrade wire, riprap is simple and durable. Many schemes use both: riprap or boulders in the launching apron, mattresses on the slope.
Where large stone is scarce or the bank is steep, yes: a 0.23–0.30 m mattress of 70–150 mm stone holds 4.5–5 m/s, which loose riprap needs stone of about 0.8 m or more to resist.
By the Isbash equation, about 0.51 m median stone for 3.5 m/s and 0.85 m for 4.5 m/s in turbulent flow.
Tell us your dimensions, wire specification and quantity — we'll come back with a quotation and lead time.
