Supercritical flow often also has a high flow energy, which is composed of the kinetic energy necessary for further flow and excess energy. The excess energy can lead to erosion of the bottom, amongst other things. Therefore it is important to dissipate this excess energy. This can be realised in the hydraulic jump mentioned above (naturally occurring or intentional in a stilling basin) or in specially designed overfalls (stepped, ski jump style). A spillway fitted with a ski jump results in a free jet that sprays into the air and that has dissipated its energy after hitting the bottom (see photo on the right).
Excess energy can be found at the following locations:
at cross-sectional constrictions, e.g. weirs, gates
in spillways chutes/steep slopes
upon change in the discharge depth due to obstacles
HM 162 with ogee-crested weir HM 162.32 and sills from HM 162.35
Supercritical flow at the overflow weir with subsequent energy dissipation in the stilling basin
ho weir head, vu upstream water flow velocity, W height of weir, E specific energy, Q discharge, h1 smallest discharge depth, h2 discharge depth after hydraulic jump, hd downstream water discharge depth, L1 length of weir body, L2 length of stilling basin, ΔE dissipated energy (specific energy loss); dashed line energy line
Stilling basins have the following functions:
stabilisation of the hydraulic jump at a defined location (depending on discharge depth h and/or backwater conditions in the downstream water, the position of the hydraulic jump may vary)
in addition to the hydraulic jump, further energy dissipation through structural elements such as baffle blocks, sills
protection of the flume bottom against erosion and scour formation (funnel or kettle-shaped deepening in the flume bottom)
conversion of the water’s excess energy (kinetic and potential) into thermal and sound energy; good energy conversion occurs at Froude numbers from 4 to 8.
It is important that the hydraulic jump does not migrate out of the stilling basin into the downstream water, where it may cause scour. A slight backwater is recommended to avoid this from happening. The ratio of the actual discharge depth h to the theoretically required discharge depth req. h can be used as a measure of the backwater in the stilling basin.
The stilling basin can be made more efficient through various design measures. It is possible to widen the flow cross-section or to use what are known as chute blocks.
In GUNT experimental flumes, chute blocks and sills can be installed on the bottom of the stilling basin. These energy dissipation elements support the energy conversion and dissipate excess energy more quickly.
Stilling basin designs
1 basin with end sill, 2 trough-shaped, 3 flat;
a positive step, Q discharge, L length of the stilling basin, h1 discharge depth at the beginning of the stilling basin, h2 sequent depth in the hydraulic jump, hd discharge depth in downstream water, req. h2 theoretically required discharge depth