Hydraulics & drainage · Flooding & scour

Stream & debris force on a pier calculator

Drag force from flow on the pier and on a debris mat, with the resultant height for overturning.

Standard: AS 5100.2 Cl 15 (drag form, simplified)

Inputs in the worked example

The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.

Result	Value	Unit
Pier drag 0.5 Cd ρ V² b h	15.12	kN
Debris drag 0.5 Cd ρ V² A	54.00	kN
Total	69.12	kN
Moment at bed (pier force at 0.6h... debris at h)	252.3	kNm

Diagram for the worked example

Pier drag 0.5 Cd ρ V² b h = 0.5*0.7*1000*3^2*1200/1000*4/1000 = 15.12 kN
Debris drag 0.5 Cd ρ V² A = 0.5*1*1000*3^2*12/1000 = 54 kN
Total = 15.12+54 = 69.12 kN
Moment at bed (pier force at 0.6h... debris at h) = (15.12*0.6*4+54*4) = 252.29 kNm

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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.