RT works for a bridge; launching apron

River training works for a bridge (4 marks)

At a bridge on an alluvial river, the river is contracted to the bridge waterway, so training works are provided to guide the flow axially and prevent flanking: guide bunds on both sides upstream, approach embankments, pitching of slopes and the river banks, launching aprons and sometimes spurs upstream.

 Plan
 U/S                                     D/S
  ____________________      |       ____
 /   guide bund        \ approach |=| /    \
(   curved head         )========|=|=(     )
 \____________________/ embankment|=| \____/
 flow --->  waterway W between abutments

 Section through guide bund
      __6 m__    HFL+1.5
     /       \
 HFL/_________\ 2:1
   / pitching   \
 _/__ filter ____\_ launching apron

Launching apron (2 marks)

A launching apron is a layer of loose stone (or concrete blocks or gabion) laid on the bed at the toe of the pitching. When scour develops at the toe, the stones slide into the scour hole and form a protective layer on the slope, preventing undermining of the bund.

Design of launching apron (2 marks)

1. Scour depth $R = 0.47(Q/f)^{1/3}$ with $f = 1.76\sqrt{d_{mm}}$.
2. Maximum scour below HFL: $2R$ (nose) or $1.5R$ (shank). Depth below bed $D' =$ scour $-$ flood depth.
3. Launching slope 2H:1V, so the stone volume must cover a slope length $\sqrt5 D' = 2.236D'$.
4. Thickness of launched apron $= 1.5t$ where $t = 0.06Q^{1/3}$ is pitching thickness.
5. Width laid $= 1.5D'$ and thickness laid $= \dfrac{1.5t\times 2.236D'}{1.5D'} = 2.24t$ (about $2.25t$).