GrahamC001
Civil/Environmental
I have a common problem relating to sizing of rock armour downstream of a number of existing low head (~2m) gated hydraulic strucutres with a level invert/apron (ie no stilling basin).
In the absence of good downstream scour protection these structures typically dig a hole in the river bed downstream. During low/medium discharge the tailwater remains relatively low and the high velocity jet emerging from the gates tends to skim across near to the surface. This sets up recirculatory movement in the water below, which may cause scour. We need to design scour protection to prevent the depth of the scour hole increasing and threatening the stability of the strucure (most likely rock armour as it is flexible and relatively easy to install in deep water).
All the references I have seen do not consider the existence of a drop and use the velocity of the flow on the apron or the depth averaged velocity to determine rock sizing - however in this scenario these don't seem appropriate (and you can end up with a requirement for some very large rock!).
I am aware that the form of the jet downstream will vary with tailwater level - and that under some circumstances this will dive into the hole. However I need something to allow a reasonable and economic design to be developed where this is not the case.
Has anyone come across any papers or information that would allow an assessment of the likely velocities of the reciculatory flow beneath the jet? I would like to use this as the basis of rock design rather than the other values suggested.
Many thanks in advance
In the absence of good downstream scour protection these structures typically dig a hole in the river bed downstream. During low/medium discharge the tailwater remains relatively low and the high velocity jet emerging from the gates tends to skim across near to the surface. This sets up recirculatory movement in the water below, which may cause scour. We need to design scour protection to prevent the depth of the scour hole increasing and threatening the stability of the strucure (most likely rock armour as it is flexible and relatively easy to install in deep water).
All the references I have seen do not consider the existence of a drop and use the velocity of the flow on the apron or the depth averaged velocity to determine rock sizing - however in this scenario these don't seem appropriate (and you can end up with a requirement for some very large rock!).
I am aware that the form of the jet downstream will vary with tailwater level - and that under some circumstances this will dive into the hole. However I need something to allow a reasonable and economic design to be developed where this is not the case.
Has anyone come across any papers or information that would allow an assessment of the likely velocities of the reciculatory flow beneath the jet? I would like to use this as the basis of rock design rather than the other values suggested.
Many thanks in advance
