Objectives of RT works and spur spacing

River training works (1 mark)

River training works are structures built on or along a river to guide the flow, control the river course, and protect banks and structures from erosion and floods.

Objectives (2 marks)

1. Prevent the river from outflanking a bridge, barrage or town, and hold it in a stable course.
2. Protect banks from erosion and the land from flooding.
3. Provide an axial, uniform approach flow to structures, and control silt entry.
4. Maintain a good channel for navigation or water supply.

Principles behind spur spacing (3 marks)

The spacing must be such that the eroding current cannot reach the bank between two spurs.

1. The flow leaving the head of a spur spreads out at a small angle (about 10° to 15° with the spur line). The next spur must lie inside the protected zone, so spacing is limited by this expansion angle.
2. Spacing is related to spur length $L$: about $2$ to $2.5L$ on a concave bank, $3$ to $4L$ on a straight reach and larger on a convex bank.
3. Spacing becomes smaller for: sharper curvature, higher velocity, weaker bank soil, and greater deflection.
4. The area between spurs should silt up quickly. A very large spacing leaves the bank unprotected; a very small one wastes money.
5. The river width and spur length should not choke the channel: $L \le 0.2W_{river}$.

Longitudinal and cross-section of a spur (2 marks)

 Longitudinal section (along spur axis)
  top level HFL+1 m ________ nose
  bank  ---------/        \
      root       slope 1:1 ... 3:1 at nose
  bed ---------------------------- launching apron

 Cross-section
        3-6 m
      _________
     /         \  side slopes 2:1 (3:1 at head)
 ---/ pitching   \---
 bed__ filter ____ apron

Top width is 3 to 6 m, top level HFL $+$ 1 m (sloping down toward the head), and the head is pitched with stone and an apron.