Spurs: functions, types and layout

A spur (groyne) is an embankment-type structure built out from the bank into the river, at an angle to the flow, to protect the bank and train the river in a bend.

How spurs assist river control

• The current is turned away from the concave (eroding) bank toward the centre of the river.
• Behind the spur, velocity is low and silt settles, building a new bank (land reclamation).
• The river is narrowed to a desired width and the thalweg is held in the channel centre.

Functions

1. Protect the bank from erosion and prevent flanking of embankments.
2. Contract the river and improve the navigable or approach channel.
3. Induce silting (sedimenting) and create a new bank line.
4. Guide the river toward the entrance of a structure.

Types

Layout in a bend

 convex bank (silting)
        .  .  .
 -----------------------------
    flow ->     (centre line)
 -----------------------------
 | | | | |   concave bank: spurs at spacing S
 v v v v v   spur length L, pointing slightly upstream

• Spurs are placed on the concave bank, starting upstream of the eroding reach and ending where the bank is stable.
• Spacing is about 2 to 2.5 times the spur length ($S \approx 2$ to $2.5L$) on a concave bank, more on straight reaches.
• Length is limited to about 20 % of the river width so the channel is not choked.
• Heads are well protected with stone pitching and a launching apron; the root is anchored into the bank.