Cress Help - Scour - Scour behind stills and groynes - B28.6

Local scour in constrictions due to groynes

The local erosion caused be a flow constricting construction can be determined by using:

     equation 1

with

    equation 2

and

    equation 3

in which:

Note that in this equation neither the geometry nor the constricting construction is neglected.

In the case of upstream transport of sediment the following equation can be used:

    equation 4

In which:

  • hb = dune height [m]
  • KB = shape factor [-]

The shape factor KB can be determined from the following table :

 
type code b/L a KB 
Circular pier CP     1.5
semi-circular pier SCP     1.5
  SCE1 < 3   1.5
semi-circular end SCE2 3 .. 5   2.25
  SCE3 > 5   3.0
vertical wall abutment VSG     3.0
wing-wall abutment (streamlined) WWS1 0.2 45ْ 0.75
  WWS2 0.3 35 - 45 1.25
wing-wall abutment (blunt) WWB1 0.5 -1.5 30 1.5
  WWB2 1.5 - 2.5 30 2.0
spill-through abutment (1.5H:1V) ST1 0.2   0.75
                                 (1H:1V) ST2 0.2   1.0
  ST3 0.5 - 1.5   1.5
Triangular shaped TS   45 1.0

In which L the length of the construction in the direction of flow (see also the figures below).

The codes mentioned in the table can be determined with the aid of the following figures:

    figure 1