Scour, the digging out of the stream bed around foundations during floods, is the main way bridges fail, and riprap only works if each stone is too heavy for the flow to move: by Isbash’s formula a stone about 0.38 m across holds in a 3 m/s turbulent flood, so the usual 30-60 cm riprap suits floods of roughly 2.7-5.3 m/s. Place the stone around piers and abutments, set foundations below a scour hole that can reach 2.4 pier widths, and inspect after every major flood.
Part of Bridge Types & Construction
Protecting bridge piers and abutments from moving water that digs out the bed beneath them, using riprap, wing walls and paved beds.
Floods Dig Where The Bridge Speeds And Disturbs The Flow
Water running past a pier or an abutment speeds up and swirls round it. The extra speed lifts bed material, and the pit it leaves is the scour hole. A bridge that narrows the channel, or whose deck acts as a dam and backs water up behind it, makes it worse; Flood Planning notes that floods destroy more bridges than any other cause, usually by scour.
The parent article lists three measures. Riprap is large stone, 30-60 cm, placed around the base of piers and abutments so the water cannot move it and the finer material beneath is protected. Wing walls are stone walls angling upstream from each abutment, directing flow smoothly through the opening. Streambed paving is flat stones set in the bed under and around the bridge. The parent adds: inspect after every major flood, and repair immediately, because once the foundation begins to wash away the bridge is on borrowed time.
What You Need
- Stones of the right size — angular rock that interlocks. Rounded river cobbles roll.
- A flood estimate — high-water marks on trees and banks, and an estimate of the speed of the current.
- Levers and rollers — for stones of 30-300 kg.
- A pole — to probe the depth of the bed around the foundation.
Method: Size And Place Riprap
Step 1: Find the flood speed Float a stick over a measured 10 m in a flood and time it. Speed = distance / time. Surface float speed is higher than the average, so use the highest speed you can reasonably expect, and add margin.
Step 2: Size the stone Use the worked example, or take the stone from the 30-60 cm rule if the speed is within its range.
Step 3: Place the stone Extend riprap around the base and 1-2 m upstream and downstream, as Abutment Building recommends. Pack it tightly and put the largest stones against the foundation.
Step 4: Build the apron below bed level Dig the riprap into the bed so it can slump into a scour hole instead of being undermined.
Step 5: Inspect after each flood Probe the bed around each foundation. Replace stones that moved.
Worked Example: Stone Size Against Flood Speed
Isbash’s formula for the speed at which a loose stone starts to move is V = C √(2 g (S − 1) d), where d is the stone’s diameter in metres, S is the stone’s density relative to water (2.65 for typical rock), g = 9.81 m/s², and C is 0.86 for stones in high-turbulence flow and 1.20 in low turbulence. These are the values from US Federal Highway Administration guidance (HEC-23); I am recalling them and have not rechecked the source here. Rearranged: d = V² / (C² × 2 g (S − 1)).
- For V = 3 m/s and C = 0.86: d = 3² / (0.86² × 2 × 9.81 × 1.65) = 9 / (0.7396 × 32.37) = 9 / 23.9 = 0.38 m.
- For V = 3 m/s and C = 1.20: d = 9 / (1.44 × 32.37) = 9 / 46.6 = 0.19 m.
- Speed that holds a 0.30 m stone: V = 0.86 × √(19.62 × 1.65 × 0.30) = 0.86 × 3.12 = 2.7 m/s (turbulent); with C = 1.20, 3.7 m/s.
- Speed that holds a 0.60 m stone: 0.86 × √(19.42) = 0.86 × 4.41 = 3.8 m/s; with C = 1.20, 5.3 m/s.
So the standard 30-60 cm riprap suits floods of about 2.7-5.3 m/s, depending on turbulence. A stone 0.38 m across weighs about 2,650 × (π / 6) × 0.38³ = 76 kg; a stone 0.60 m across weighs 300 kg.
How deep the hole can be. For piers, HEC-18 (also recalled, not rechecked) gives a maximum local scour depth of about 2.4 times the pier width for slow flows and 3 times for fast. A 1 m wide pier can have a hole 2.4 m deep, so the foundation must go below the bed by more than that plus any general lowering of the stream bed.
How the bridge speeds the flow. A stream 8 m wide and 1 m deep at 1.5 m/s carries 12 m³/s. If the bridge narrows the opening to 5 m at the same depth, the speed is 12 / 5 = 2.4 m/s. The stone that was stable before may not be after.
Common Mistakes
- Rounded cobbles — they roll. Use angular rock.
- Stones too small — a stone just under the threshold is carried off.
- Riprap sitting on the bed surface — scour undermines the edge. Dig it in.
- Narrowing the opening — less width, more speed.
- Checking once — the parent says inspect after every major flood.
Safety
Floods and falling stones
Never work in or beside a stream in spate. Rocks of 100-300 kg crush limbs; move them with levers and rollers, not by hand, and keep clear of the downhill side. Do not stand on a bridge in flood, since scour can drop a foundation without warning.
What’s Next
- Learn next: Maintenance — what to inspect and when
- Combine with: Abutment Construction — the structure being protected