River Training Works

River training works are structures built on or along a river to guide and confine the flow, control its erosion and deposition, and keep it in a stable course so that bridges, headworks, towns and farmland are protected. Rivers of the Nepal Terai (Koshi, Narayani, Bagmati, Karnali) are steep, braided and carry heavy bed load from the Churia hills, so they need training works.

Objectives

• Prevent floods and spilling over the banks.
• Protect banks from erosion and the river from shifting its course (flanking a bridge or barrage).
• Give a steady, axial approach flow to a structure.
• Control silt entry and maintain a navigable or regular channel.

Methods normally adopted in the Terai

1. Guide bunds (guide banks) are embankments built on both sides of the river upstream (and a short length downstream) of a bridge or barrage, to guide flow centrally through the opening.

 flow --->      ____ guide bund (curved head)
 ______________/    \______
                   waterway W      = bridge/barrage
 ______________    ______/
               \____/

2. Spurs (groynes) project from the bank into the river to push the current away from the bank, create slack water and cause silt to settle between them. They protect concave banks.

  bank ======+==+==+==+====
             |  |  |  |      spurs (L)
             v  v  v  v      current deflected away

3. Marginal (flood) embankments are earth levees along the river, set back from the bank, which confine the flood within the river corridor.

4. Bank protection (revetment / pitching) covers the bank with stone pitching, concrete blocks or gabion mattresses over a filter, with a launching apron at the toe.

5. Cut-offs are artificial channels across the neck of a meander that straighten the river and lower flood levels.

6. Check dams / bed sills and gabion crate walls control bed degradation in steep tributaries.

7. Bio-engineering (vegetative spurs, bamboo piles, planting of grass and trees) is used as a cheap supplement along banks.

8. Pitched islands and training walls are used where the river must be held in one channel.

Selection depends on river slope, bed material, flood discharge and cost; in practice a combination (embankment, spurs and gabion revetment) is used.

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.

Flood control aims to reduce the damage from floods by storing, diverting, confining or reducing flood flow. The structural methods are as follows.

1. Storage reservoirs

A dam stores flood water and releases it gradually, cutting the peak flow downstream. It is the most effective method but costly. Used on the Koshi, Karnali and Sapta Gandaki river systems of Nepal in the planning stage.

2. Detention basins and retarding basins

Low-lying areas or small ponds hold flood water temporarily.

3. Embankments (levees or dykes)

Earth banks built along the river confine the flood and prevent spilling. The flood level rises inside the embankments, so they must be raised periodically and have free-board.

   ____ levee               levee ____
  /    \_______ river _______/    \
 /  flood plain protected          \

4. Channel improvement

Straightening, widening, deepening, desilting or removal of obstacles raise the carrying capacity and lower flood levels.

5. Cut-offs

A shortcut across a meander neck steepens the slope and lowers flood levels upstream.

6. Diversion and flood-ways

Excess flood is diverted to a side channel, lake or another basin through a spillway or gated weir.

7. Bank protection and river training

Spurs, revetments and guide bunds protect banks and hold the river in its course.

8. Watershed management

Afforestation, check dams and terracing in the catchment cut run-off and sediment.

9. Flood proofing and warning

Raised platforms, flood warning systems and zoning of the flood plain reduce damage to people and property (non-structural, but used with the above).

A river changes character along its course from the hills to the sea. Three stages are recognised.

Stages of a river

In the Terai, rivers leaving the Churia hills spill into the plain where slope falls suddenly, so their load is dropped and they braid and shift.

Meandering process

A meander is a bend that develops in an alluvial channel in a flat reach. It forms by this sequence:

1. Small irregularities cause the flow to attack one bank, creating a bend.
2. In the bend, a transverse (helical) flow develops. The surface water moves toward the outer (concave) bank and bottom water toward the inner (convex) bank.
3. The outer bank erodes and the inner bank gets silt deposits, so the bend grows in amplitude.
4. The meander migrates downstream. Pools form on the outer side and riffles (shoals) in the crossings.
5. Eventually the neck narrows; in flood the river cuts across it (natural cut-off), leaving an oxbow lake.

      outer bank erodes
   ______      ______
  /      \____/      \___   -->
         inner bank deposits

Meandering is controlled by guide bunds, spurs and bank protection on the concave side.

Data and design basis

• Silt factor given: $f = 1.10$
• Flood depth: $h = 5.00$ m
• Design discharge for scour/apron: $Q = 4000$ m³/s

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{4000} = 300.42$ m. Adopt $W \approx 300$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 300 = 375\ \text{m} \\ L_{ds} &= 0.25W = 75\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 150$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 75$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 150\times 2.094 = 314$ m, so total length on the upstream side $\approx 689$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 4000^{1/3} = 0.06\times 15.87 = 0.95\ \text{m}$$

Adopt $t \approx 1.0$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{4000}{1.10}\right)^{1/3} = 7.23\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 0.95$ m, so the launched apron thickness is $1.5t = 1.5\times0.95 = 1.43$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.13$ m (about $2.25t$).

Summary

Sketches

Plan of guide bunds:

 U/S                                          D/S
                       bridge / weir axis
   Shank (straight)                |
 ________________________________  |  ___
/  1.0-1.5 W upstream            \ | /   \  tail
\________________________________/=|=\___/  0.25 W
 curved head                       |
 R, sweep 120 deg    <-- waterway W -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/

Section at the shank (straight portion):

           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)

Data and design basis

• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{1.0} = 1.76$
• Flood depth above bed: $h = 225.0 - 222.0 = 3.00$ m
• Design discharge for scour/apron: $Q = 5000$ m³/s

1. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{5000}{1.76}\right)^{1/3} = 6.66\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 1.03$ m, so the launched apron thickness is $1.5t = 1.5\times1.03 = 1.54$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.29$ m (about $2.25t$).

Answer (straight portion): length of launching apron $=1.5D' = 10.48$ m, thickness $=1.5t = 1.54$ m launched (about 2.29 m as laid).

Data and design basis

• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{1.0} = 1.76$
• Flood depth above bed: $h = 114.0 - 111.0 = 3.00$ m
• Design discharge for scour/apron: $Q = 7000$ m³/s

1. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{7000}{1.76}\right)^{1/3} = 7.45\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 1.15$ m, so the launched apron thickness is $1.5t = 1.5\times1.15 = 1.72$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.57$ m (about $2.25t$).

Answer (straight portion): length of launching apron $=1.5D' = 12.25$ m, thickness $=1.5t = 1.72$ m launched (about 2.57 m as laid).

        pitching
   ____/\__ 
  /\ apron laid: width 1.5D', T = 2.24 t
 bed ___________ river bed
   after launching: 2:1 slope, 1.5 t thick

Data and design basis

• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{1.0} = 1.76$
• Flood depth above bed: $h = 128.5 - 125.0 = 3.50$ m
• Design discharge for scour/apron: $Q = 8000$ m³/s

1. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{8000}{1.76}\right)^{1/3} = 7.79\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 1.20$ m, so the launched apron thickness is $1.5t = 1.5\times1.20 = 1.80$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.68$ m (about $2.25t$).

Answer (straight portion): length of launching apron $=1.5D' = 12.27$ m, thickness $=1.5t = 1.80$ m launched (about 2.68 m as laid).

A guide bund is a pair of embankments, built along both banks upstream (and a short length downstream) of a bridge or barrage, which guide the river through the opening without allowing it to outflank the structure.

Purposes

1. Confine and guide the river axially through the waterway.
2. Prevent the river from attacking the approach embankments and abutments (flanking).
3. Prevent the formation of eddies and cross-currents near the structure.
4. Keep the scour holes and deposits away from the structure.
5. Maintain a smooth, symmetrical entry of flow, so the afflux is minimum.

Design criteria (Lacey and Garg)

Plan

• Waterway $W = 4.75\sqrt{Q}$.
• Length of upstream shank: $1.0$ to $1.5\,W$; downstream tail: $0.2$ to $0.5\,W$.
• Upstream curved head: radius about $0.5W$ and sweep $120^\circ$ to $145^\circ$; downstream curved head: smaller radius, sweep about $45^\circ$.

 U/S                                 D/S
   ___________________________  |  ____
  /  shank 1.0-1.5 W           \ | /    \ tail
  \____________________________/=|=\____/ 0.2-0.5 W
  curved head                     axis
        <------ waterway W ------>

Cross-section at the shank

• Top width 6 m (3 to 6 m) and top level $=$ HFL $+$ 1.5 m free-board.
• Side slopes 2:1 on the river side (pitched) and 2:1 on the country side.
• Pitching thickness $t = 0.06\,Q^{1/3}$ on a filter 0.15 to 0.30 m thick.

Launching apron

• Scour depth $R = 0.47(Q/f)^{1/3}$; maximum scour below HFL is $2R$ at the nose and $1.5R$ at the shank.
• Depth of apron below bed $D' =$ (scour below HFL) $-$ (flood depth).
• Width $= 1.5D'$ and thickness $= 1.5t$ when launched on a 2:1 slope (laid thickness about $2.25t$).

 Section at shank                 Section at nose
      ____6 m___                      ___6 m___
     /          \ HFL+1.5            /         \
 HFL/____________\                 HFL/___________\
   / pitching t   \                  /             \
 _/_______apron____\_             _/____apron 2R_____\_

At the nose the scour is deeper (2R), so the pitching continues around the head and the apron is wider and thicker; sections are flatter (3:1) at the head.

The design of a guide bund (guide bank) follows Lacey's regime theory and Garg's recommendations. The steps are listed first and then worked for an example.

Design steps

1. Find the design discharge $Q$, HFL, bed level, bed material size $d$.
2. Silt factor $f = 1.76\sqrt{d_{mm}}$.
3. Waterway $W = 4.75\sqrt{Q}$.
4. Length: upstream shank $= 1.0$ to $1.5W$, downstream $= 0.2$ to $0.5W$.
5. Curved heads: radius and sweep angle.
6. Top level $=$ HFL $+ 1.5$ m (free-board); top width 6 m; side slopes 2:1.
7. Pitching thickness $t = 0.06\,Q^{1/3}$.
8. Scour depth $R = 0.47(Q/f)^{1/3}$; maximum scour $2R$ (nose), $1.5R$ (shank) below HFL.
9. Apron: $D' =$ scour $-$ flood depth; width $1.5D'$; thickness $1.5t$ (launched).
10. Draw plan and sections.

Example

$Q = 5000$ m³/s, bed material $d = 1$ mm, HFL $= 100.0$ m, bed $= 96.0$ m.

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{5000} = 335.88$ m. Adopt $W \approx 336$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 336 = 420\ \text{m} \\ L_{ds} &= 0.25W = 84\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 168$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 84$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 168\times 2.094 = 352$ m, so total length on the upstream side $\approx 772$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 5000^{1/3} = 0.06\times 17.10 = 1.03\ \text{m}$$

Adopt $t \approx 1.1$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{5000}{1.76}\right)^{1/3} = 6.66\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 1.03$ m, so the launched apron thickness is $1.5t = 1.5\times1.03 = 1.54$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.29$ m (about $2.25t$).

Result: $W \approx 336$ m, length of upstream bund $= 420$ m, $t = 1.03$ m, shank apron width $= 8.98$ m and nose apron width $= 13.97$ m, launched thickness $= 1.54$ m.

 Plan:  ________ shank ________   bridge
       (  curved head           |=|
        \____________________   |=|
 Section: top 6 m, HFL+1.5, slope 2:1, pitching t, apron

Data and design basis

• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.1} = 0.56$
• Flood depth above bed: $h = 154 - 150 = 4.00$ m
• Design discharge for scour/apron: $Q = 4500$ m³/s

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{4500} = 318.64$ m. Adopt $W \approx 319$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 319 = 399\ \text{m} \\ L_{ds} &= 0.25W = 80\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 160$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 80$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 160\times 2.094 = 334$ m, so total length on the upstream side $\approx 733$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 4500^{1/3} = 0.06\times 16.51 = 0.99\ \text{m}$$

Adopt $t \approx 1.0$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{4500}{0.56}\right)^{1/3} = 9.43\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 0.99$ m, so the launched apron thickness is $1.5t = 1.5\times0.99 = 1.49$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.21$ m (about $2.25t$).

Summary

Data and design basis

• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.1} = 0.56$
• Flood depth above bed: $h = 245 - 240 = 5.00$ m
• Design discharge for scour/apron: $Q = 3000$ m³/s

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{3000} = 260.17$ m. Adopt $W \approx 260$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 260 = 325\ \text{m} \\ L_{ds} &= 0.25W = 65\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 130$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 65$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 130\times 2.094 = 272$ m, so total length on the upstream side $\approx 597$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 3000^{1/3} = 0.87\ \text{m}$$

Gabion slope protection: use gabion boxes (1.0 m thick) in one layer, so adopt $t_g = 1.0$ m (next standard size above 0.87 m) over a 0.15 m gravel filter.

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{3000}{0.56}\right)^{1/3} = 8.24\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 0.87$ m, so the launched apron thickness is $1.5t = 1.5\times0.87 = 1.30$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 1.93$ m (about $2.25t$).

Summary

Gabion details

• Slope pitching: gabion boxes $2\times1\times1$ m laid in one layer (1.0 m thick) on a 0.15 m geotextile or gravel filter, wire mesh 8x10 cm, galvanised and PVC coated.
• Launching apron: launched thickness $1.5t = 1.30$ m; laid thickness $\sqrt5\,t = 1.93$ m, so adopt 2.0 m (two layers of 1 m gabion boxes or four layers of 0.5 m mattress) over a width $1.5D'$ = 11.04 m (shank) and 17.22 m (nose).
• Gabion boxes are flexible, so they follow the scour hole and launch without breaking.

Methods of river training (3 marks)

River training works guide a river and protect its banks. The main methods are:

1. Guide bunds for the approach to a bridge or barrage.
2. Spurs (groynes) projecting from the bank.
3. Marginal (flood) embankments along the banks.
4. Revetment (bank pitching) with stone, concrete or gabion and a launching apron.
5. Cut-offs across meander necks.
6. Bed sills / check dams to arrest bed degradation in steep reaches.
7. Bio-engineering and pitched islands.

Types of spurs used (5 marks)

Spurs are classified by orientation, material and shape.

By orientation to the flow

 Repelling (up)    Deflecting (90)    Attracting (down)
  \  /              |   |               /   /
   \/ <-flow        |   |              /   /
 -----------       -----------        -----------
  bank              bank                bank

• Repelling spur points upstream (angle 100 to 120° to the bank). It pushes the current strongly away from the bank; the scour at its nose is large. Used on concave bank bends.
• Deflecting (neutral) spur at right angles. It gives moderate effect and is the most common type.
• Attracting spur points downstream (60 to 70°). It draws the current toward the bank and has less scour, but protects less.

By material and permeability

• Impermeable (earth core with stone pitching, stone, gabion): deflect flow completely.
• Permeable (pile and brushwood, wire-crate or bamboo): reduce velocity and induce silting.

By shape

• Straight spur with a round or tapered head.
• T-head spur and hockey (curved) spur: the head protects against scour and gives better protection downstream.

By height

• Submerged spurs (overtopped at high flood) and non-submerged spurs (above HFL).

Spurs are laid in a series with spacing of about 2 to 2.5 times the spur length on a concave bank.

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.

Definition (2 marks)

River training works are structures constructed on, along or across a river so as to guide the flow, control the river's behaviour and protect the banks and the structures from erosion, floods and shifting.

Three major objectives (3 marks)

1. To prevent flooding and spilling over the banks by confining the river within a defined course.
2. To protect banks and structures (bridges, barrages, embankments, towns, farmland) from erosion, scour and outflanking.
3. To ensure axial flow to a structure and control silt, so that a stable and uniform channel is maintained for headworks, navigation or water supply.

Two basic purposes of spurs (2 marks)

1. Training the river: to deflect the current away from an eroding bank and hold it near the centre of the channel.
2. Creating slack water for silting: to reduce velocity near the bank and promote deposition between the spurs, forming a new, stable bank (land reclamation).

River training works serve five main objectives.

1. Flood protection: confine the river within a fixed channel and prevent it from spilling over the banks and flooding adjacent land and settlements.
2. Bank protection and prevention of avulsion: stop erosion of the banks and prevent the river from shifting its course or outflanking bridges, barrages, towns and roads.
3. Axial approach to structures: guide the flow centrally and smoothly to the opening of a bridge, barrage or headwork, avoiding eddies and cross-currents.
4. Control of silt and sediment: induce silting where land is to be reclaimed and prevent deposits or scour holes at critical locations (such as in front of a canal head regulator).
5. Maintenance of a stable channel for use: keep a definite, stable channel and depth for navigation, water supply or irrigation intake and protect the river against excessive meandering.

Common works used are guide bunds, spurs, marginal embankments, revetments and cut-offs, chosen to suit the river and its objectives.

A groyne (or spur) is a structure built out from the river bank, roughly at right angles or inclined to the current, to deflect the flow away from the bank, reduce velocity near the bank and induce silting. The root is anchored in the bank and the head projects into the stream.

Types of groynes

1. According to the effect on the current (inclination)

• Repelling groyne: inclined upstream (100 to 120° with the bank). It pushes the current toward the middle, but the scour at the head is heavy.
• Attracting groyne: inclined downstream (60 to 70°). It attracts the current, and has little scour.
• Deflecting or neutral groyne: at right angles to the bank. The commonest type.

 Repelling         Deflecting        Attracting
  \                 |                  /
   \ flow->          |                 /
 ---------        ---------        ---------

2. According to permeability

• Impermeable groynes are of earth, stone, concrete or gabion. They block the flow.
• Permeable groynes are of piles, bamboo or brushwood, or wire crates with stone. They let a slow flow pass, causing silt deposits, and have little scour; they suit sandy, low-velocity rivers.

3. According to shape of the head

• Straight groyne, T-head groyne and hockey (curved) head groyne. A T-head gives better protection and reduces scour at the nose.

4. According to the height

• Submerged groynes are overtopped in floods.
• Non-submerged groynes stand above HFL.

5. According to the material

• Earth with stone pitching, boulder or stone masonry, concrete block, gabion and bio-engineering types.

Design features

• Length limited to about 20 % of river width; spacing about 2 to 2.5 times the length on concave banks.
• Top width 3 to 6 m and top level at HFL plus a margin.
• Heads protected with pitching and a launching apron.

Definition (1 mark)

River training works are structures built on or along a river to guide the flow, stabilise its course, protect its banks and safeguard structures like bridges and barrages from floods and erosion.

Methods (2 marks)

1. Guide bunds (guide banks).
2. Spurs (groynes).
3. Marginal embankments (levees).
4. Bank pitching (revetment) with a launching apron.
5. Cut-offs.
6. Bed sills, check dams and bio-engineering.

Design of the components of a guide bank (5 marks)

Waterway: Lacey's regime width $W = 4.75\sqrt{Q}$ (Q in m³/s). Silt factor $f = 1.76\sqrt{d_{mm}}$.

 Section at shank
      _6 m_
     /     \  HFL+1.5 m
 HFL/_______\ 2:1
   / pitching \
 _/___filter___\_ apron (width 1.5D', launches at 2:1)

The apron is laid at the toe of the pitching and, as the river scours the bed, the stones slide down the 2:1 slope and protect the bund against undermining.

A Bell's bund is a guide bund (guide bank) on the pattern recommended by Bell, in which a pair of embankments with curved heads on the upstream and a short tail on the downstream, are laid symmetrically about the axis of a bridge or barrage so that the river approaches the waterway axially. The waterway is based on Lacey's regime width.

Plan and sections (3 marks)

 U/S                                       D/S
   ______________________________   |   ____
  /   upstream shank 1.0-1.5 W   \  |  /    \ tail
 (  curved head, R = 0.5W,        )=|=(      ) 0.25 W
  \ sweep 120 deg                 /  |  \____/
   \_____________________________/   |
         waterway W (Lacey)        axis of bridge

 Section at shank              Section at nose
     __6 m__                      __6 m__
    /       \  HFL+1.5           /        \
 HFL/_________\ 2:1          HFL/__________\ 3:1
  / pitching t \                / pitching t \
 /___ apron ____\ (1.5R)    /____ apron (2R) ___\

Design steps (5 marks)

1. Design discharge $Q$, HFL, bed level and median grain size $d$.
2. Silt factor $f = 1.76\sqrt{d_{mm}}$, and waterway $W = 4.75\sqrt{Q}$.
3. Length of upstream shank $1.0$ to $1.5W$, downstream tail $0.2$ to $0.5W$.
4. Radius of upstream curved head about $0.5W$ with sweep $120$ to $145^\circ$; tail with a smaller radius, sweep about $45^\circ$.
5. Top level $=$ HFL $+ 1.5$ m, top width 6 m, side slopes 2:1.
6. Pitching thickness $t = 0.06Q^{1/3}$.
7. Scour depth $R = 0.47(Q/f)^{1/3}$; max scour below HFL: $2R$ at nose, $1.5R$ at shank.
8. Launching apron: depth below bed $D' =$ scour depth $-$ flood depth, width $1.5D'$, launched thickness $1.5t$.
9. Draw plan and sections.

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 $2.25t$).

Desired waterway of the barrage

Lacey's regime waterway (wetted perimeter) is

$$W = 4.75\sqrt{Q} = 4.75\sqrt{7500} = 411.4\ \text{m}$$

For a barrage the waterway is generally kept equal to this regime width (1.0 to 1.2 times); adopt $W = 411$ m between abutments. Flood depth $h = 238.0 - 232.0 = 6.0$ m.

Data and design basis (Bell's guide bund)

• Bell's guide bund is designed on Lacey's regime width; bed material is gravel (5.07 mm).
• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{5.07} = 3.96$
• Flood depth above bed: $h = 238.0 - 232.0 = 6.00$ m
• Design discharge for scour/apron: $Q = 7500$ m³/s

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{7500} = 411.36$ m. Adopt $W \approx 411$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 411 = 514\ \text{m} \\ L_{ds} &= 0.25W = 103\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 206$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 103$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 206\times 2.094 = 430$ m, so total length on the upstream side $\approx 944$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 7500^{1/3} = 0.06\times 19.57 = 1.17\ \text{m}$$

Adopt $t \approx 1.2$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{7500}{3.96}\right)^{1/3} = 5.81\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 1.17$ m, so the launched apron thickness is $1.5t = 1.5\times1.17 = 1.76$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.63$ m (about $2.25t$).

Summary

Sketches

Plan of guide bunds:

 U/S                                          D/S
                       bridge / weir axis
   Shank (straight)                |
 ________________________________  |  ___
/  1.0-1.5 W upstream            \ | /   \  tail
\________________________________/=|=\___/  0.25 W
 curved head                       |
 R, sweep 120 deg    <-- waterway W -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/

Section at the shank (straight portion):

           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)

Data and design basis

• Total waterway $W = 658.88$ m (given). The flood depth is not given, so Lacey's regime depth for the intensity $q = Q/W = 13100/658.88 = 19.88$ m²/s is used: $R_L = 1.35(q^2/f)^{1/3} = 10.03$ m. $f = 1.76\sqrt{0.3} = 0.964$.
• Discharge is increased by 20 % for the apron design: $Q_a = 1.2\times13100 = 15720$ m³/s.
• Silt factor given: $f = 0.96$
• Flood depth: $h = 10.03$ m
• Design discharge for scour/apron: $Q = 15720$ m³/s

1. Waterway and length of guide bund

Waterway between abutments $W = 658.88$ m (given). Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 659 = 824\ \text{m} \\ L_{ds} &= 0.25W = 165\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 330$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 165$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 330\times 2.094 = 690$ m, so total length on the upstream side $\approx 1514$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 13100^{1/3} = 0.06\times 23.57 = 1.41\ \text{m}$$

Adopt $t \approx 1.5$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{15720}{0.96}\right)^{1/3} = 11.92\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 1.41$ m, so the launched apron thickness is $1.5t = 1.5\times1.41 = 2.12$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 3.16$ m (about $2.25t$).

Summary

Sketches

Plan of guide bunds:

 U/S                                          D/S
                       bridge / weir axis
   Shank (straight)                |
 ________________________________  |  ___
/  1.0-1.5 W upstream            \ | /   \  tail
\________________________________/=|=\___/  0.25 W
 curved head                       |
 R, sweep 120 deg    <-- waterway W -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/

Section at the shank (straight portion):

           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)

Data and design basis

• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.4} = 1.11$
• Flood depth above bed: $h = 205 - 202 = 3.00$ m
• Design discharge for scour/apron: $Q = 4000$ m³/s

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{4000} = 300.42$ m. Adopt $W \approx 300$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 300 = 375\ \text{m} \\ L_{ds} &= 0.25W = 75\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 150$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 75$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 150\times 2.094 = 314$ m, so total length on the upstream side $\approx 689$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 4000^{1/3} = 0.06\times 15.87 = 0.95\ \text{m}$$

Adopt $t \approx 1.0$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{4000}{1.11}\right)^{1/3} = 7.20\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 0.95$ m, so the launched apron thickness is $1.5t = 1.5\times0.95 = 1.43$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.13$ m (about $2.25t$).

Summary

Sketches

Plan of guide bunds:

 U/S                                          D/S
                       bridge / weir axis
   Shank (straight)                |
 ________________________________  |  ___
/  1.0-1.5 W upstream            \ | /   \  tail
\________________________________/=|=\___/  0.25 W
 curved head                       |
 R, sweep 120 deg    <-- waterway W -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/

Section at the shank (straight portion):

           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)

Data and design basis

• Bed level 105.00 m, flood depth 5 m, so HFL $=110.00$ m; top level of bank $=111.50$ m.
• Silt factor given: $f = 1.00$
• Flood depth: $h = 5.00$ m
• Design discharge for scour/apron: $Q = 6500$ m³/s

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{6500} = 382.96$ m. Adopt $W \approx 383$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 383 = 479\ \text{m} \\ L_{ds} &= 0.25W = 96\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 192$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 96$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 192\times 2.094 = 401$ m, so total length on the upstream side $\approx 880$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 6500^{1/3} = 0.06\times 18.66 = 1.12\ \text{m}$$

Adopt $t \approx 1.2$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{6500}{1.00}\right)^{1/3} = 8.77\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 1.12$ m, so the launched apron thickness is $1.5t = 1.5\times1.12 = 1.68$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.50$ m (about $2.25t$).

Summary

Sketches

Plan of guide bunds:

 U/S                                          D/S
                       bridge / weir axis
   Shank (straight)                |
 ________________________________  |  ___
/  1.0-1.5 W upstream            \ | /   \  tail
\________________________________/=|=\___/  0.25 W
 curved head                       |
 R, sweep 120 deg    <-- waterway W -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/

Section at the shank (straight portion):

           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)

Data and design basis

• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.1} = 0.56$
• Flood depth above bed: $h = 78.8 - 75.5 = 3.30$ m
• Design discharge for scour/apron: $Q = 2500$ m³/s

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{2500} = 237.50$ m. Adopt $W \approx 238$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 238 = 298\ \text{m} \\ L_{ds} &= 0.25W = 60\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 119$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 60$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 119\times 2.094 = 249$ m, so total length on the upstream side $\approx 547$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 2500^{1/3} = 0.06\times 13.57 = 0.81\ \text{m}$$

Adopt $t \approx 0.9$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{2500}{0.56}\right)^{1/3} = 7.75\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 0.81$ m, so the launched apron thickness is $1.5t = 1.5\times0.81 = 1.22$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 1.82$ m (about $2.25t$).

Summary

Sketches

Plan:

 U/S                                          D/S
                       bridge / weir axis
   Shank (straight)                |
 ________________________________  |  ___
/  1.0-1.5 W upstream            \ | /   \  tail
\________________________________/=|=\___/  0.25 W
 curved head                       |
 R, sweep 120 deg    <-- waterway W -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/

Section at the straight portion (shank):

           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)

Section at the nose (upstream curved head):

 Section at u/s curved head (nose)
            ______  <- HFL + 1.5 m
           /      \   top 6 m, side 3:1 (flatter)
 HFL ----/---------\----
        / pitching t \
 bed ___/_____________\__
         \_ apron: 2.0R scour, 1.5D' wide _/