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• River  training  implies  certain  measures  to  be adopted  on  a  river  to  stabilize  the  river channel  along  a  certain  alignment  with  a certain cross section.

• The river training works are required to achieve the following; 

  •       Prevent out flanking of the structure
  •       It protects the river bank
  •       Prevent flooding by the river lands upstream 
  •       It contracts a river channel to improve its depth
  •       It trains the flow along a certain course

MORPHOLOGY OF A RIVER

• River/Stream Morphology describes the shape of river channels and how they change in shape & direction with respect to time. 

• THALWEG of a river - a thalweg or talweg is the line of lowest elevation within a valley or watercourse.

CLASSIFICATION OF RIVER TRAINING WORKS

 High  Water  Training  –  It  is  undertaken with  the  purpose  of  providing  the efficient  disposal  of  the  maximum floods  &  thereby  protecting  against damage due to floods.

• It is also called as training for discharge.

Low Water Training is undertaken with the purpose of providing sufficient depth for navigation during the low water season.

• It is usually concerned with contraction of width

• It is also known as training for depth.

Mean Water Training  –  It  is undertaken  to  provide  efficient disposal  of  bed  &  suspended sediments and thereby protecting the river channel in designed shape.

• It is also known as training for sediments.

RIVER TRAINING WORKS (Groynes/Spurs)

GROYNES/SPURS

• Groynes are structures constructed transverse to the river flow and extend from the bank into river up-to a limit. 

• This form of river training works perform one  or  more functions such as: 

  • training the river along the desired course to reduce the concentration of flow at the point of attack, 
  • creating a low flow for silting up the area in the vicinity, and
  • protecting the bank by keeping the flow away from it.

TYPES OF GROYNES

Groynes are classified on the basis of function

  1. Repelling/Reflecting Spur,
  2. Deflecting Spur, 
  3. Attracting Spur
  4. T-Shaped (Denehey), Hockey (Or Burma) Type, Kinked Type, E t c.   

1.      REPELLING/REFLECTING GROYNE

• It  points  towards  the  U/S  at  an  angle  of 10-30  degrees  to  the  line  normal  to  the bank, 

• The head of this spur causes the flow to be deflected in a direction nearly perpendicular to itself

• A still water pocket is formed U/S of it and suspended load gets deposited in the pocket

2.      DEFLECTING GROYNE

• It has much shorter length than repelling groynes and it is built perpendicular to the bank 

• It only deflects the flow, locally

3.      ATTRACTING GROYNE

• The Attracting Groyne points D/S of the direction of normal flow. 

• It causes formation of scour holes closer to the banks than the repelling groynes.

• Therefore, they tend to maintain deep current close to the bank.


Meandering of Rivers

• A meandering type of river flows in consecutive curves of reverse order connected with short strait called as crossing.

Causes of Meandering as per Ingus’ Theory

• When  there  is  heavy  load  of  bed  material  in movement  during  floods,  excess  turbulent energy  is  developed  due  to  unevenness  of  the bed and the banks.

• In  such  a  condition,  it  is  observed  that symmetrical axial flow is not maintained & the flow  tends  to  concentrate  towards  one  of  the banks.

• Due  to  this  concentration  of  flow,  further erosion  takes  place  &  finally  giving  the meandering shape to the river.

Sinuosity or Tortuosity

It is the ratio of the Actual length (along the curve) to the Meander Length (along straight line) between the end points of the curve of a meandering river.

Effect of Meandering

The  meandering  action  increases  the length  of  the  stream  or  river  and  tends  to reduce the slope. 

DIVERSION HEADWORKS

GUIDE BUNDS OR BANKS

• Guide Bunds/Banks are made for guiding the river near a structure so as to confine it in a reasonable width of river.

• The  Guide  Banks  usually  consists  of  a heavily  built  embankment  in  the  shape  of Bell  Mouth  on  both  sides  of  constricted channel.

• It is necessary at many instances to narrow down  and  restrict  the  course  of  the  river through  the  barrage  and  it  is  achieved  by the use of the guide bunds.

LEVEES OR MARGINAL EMBANKMENTS

• Embankments  &  dykes  also  known  as levees are earthen banks constructed parallel to the course of river to confine it to  a  fixed  course  and  limited  cross- sectional width. 

• The  heights  of  levees  will  be  higher  than the  design  flood  level  with  sufficient  free board.  The  confinement  of  the  river  to  a fixed  path  frees  large  tracts  of  land  from inundation and consequent damage. 

DIVIDE WALL

• The wall which divides the waterway into two parts, one is the weir portion and the other from which canal takes off is called the divide wall.

FISH PASS OR FISH LADDER

• Some barrages require providing special structures to allow migratory fishes to flow up and down the river through structures called Fish Passes or Fish Ladders

SILT EXCLUDER & SILT EXTRACTORS

• Silt Excluders are those works which are constructed on the bed of the river upstream of the head regulator. The silted water enters the silt excluders and clear water enters the head regulators.

• In  this  type  of  works,  the  silt  is  therefore, removed  from  the  water  before  it  enters  the canal.

• Silt  Extractors  or  Silt  Ejectors  remove  the remaining  silt  in  the  off-take  canal  and  are constructed some distance downstream of the head regulator at the bed of the canal.

UNDERSLUICES

• They are gate controlled openings in the weir with crest at low level.

• They are located on the same side as off take canal

• They lower the highest flood level

• They preserve a clear and defined river channel approaching the canal regulator

RIVERBANK PROTECTION WORKS

Cutoffs

• Cut-offs are barriers provided below the floor of the barrage both at the upstream and the downstream ends. They may be in the form of concrete or steel sheet-piles. The  cut-offs  extend  from  one  end  of  the barrage  or  dam  up  to  the  other  end  (on the other bank). The purpose of providing cutoff is two-folds as explained further below:

1.       During  low-flow  periods  in  rivers,  when  most  of  the gates are closed in order to maintain a pond level, the differential  pressure  head  between  upstream  and downstream may cause uplift of river bed particles. A cutoff increases the flow path and reduces the uplift pressure, ensuring stability to the structure

2.       During flood flows or some unnatural flow condition, when  there  is  substantial  scour  of  the  downstream riverbed,  the  cutoffs  or  sheet  piles  protect  the undermining of the structures foundation

LACEY’S SCOUR DEPTH

• Lacey’s scour depth equation helps to find the  depth  of  sheet  piles  by  finding  the level of deepest scour below HFL.

• The sheet pile must be taken at least up to the level of possible deepest scour below the bed of the river.

• The design depth of scour below HFL may be taken as 1.5 times Lacey’s Scour Depth.

Launching Apron as River Protection Work

• Whenever  a  sloping  face  is  protected  by stone  pitching  against  scour,  the  pitching  is extended  beyond  the  toe  on  the  bed  in  the form of packed stones, this stone dumping is known as launching apron

• If no such protection is provided, scour will occur at the toe with consequent undermining.

• The  launching  apron  is  generally  laid  in  a width  equal  to  1.5  times  the  depth  of  the scour D below the original bed.

• The  total  scour  below  HFL  is  taken  as  XRs where  Rs  is  Lacey’s  normal  regime  scour depth. The value of X depends upon the type of river training work and its location 

• In order to remove such a danger to the slope, the  pitching  is  extended  on  the  horizontal river  bed  portion,  which  falls  down  into  the scoured  portion  as  soon  as  the  scouring occurs.