Energy Dissipation Downstream of a Weir

measuring the water level downstream of a rectangular weir The usual method of dissipating the energy of a weir's nappe is by a hydraulic jump downstream of the weir. In an earthen channel, if the is no provision for this dissipation of energy, there will be a tendency to sour the downstream channel. This scour and erosion of the channel may become excessive resulting in the undermining of the weir and potentially eroding the downstream channel banks.

A reinforced apron downstream of the weir is one approach to controlling the dissipating the energy. In general, this reinforcement should extend at least 4 H_max (maximum anticipated head) or 5-feet [1.5 m], whichever is greater.

When rip rap is used to armor the downstream channel, generally, the larger the rip rap the greater the local velocity / energies that can be controlled.

Where armoring is not possible another solution is to provide a depression in the downstream channel floor so as to create a stilling basin. Properly designed, a stilling basin forces the hydraulic jump in a defined area.

Now naturally not all sites will require energy dissipation – particularly those bedded in rock or secured in concrete, however were the weir is installed in a natural, earthen channel where neither of these are present, energy dissipation should be considered when selecting a site.