Control structures

Control structures are common elements in flumes and are used for the following purposes:

  • raising the water level, for example creating a suffi cient navigable depth for ships, use of hydropower, erosion protection due to lower fl ow velocity

  • controlling the discharge

  • measuring the discharge

Typical control structures are weirs or gates. The difference between the two is whether the water flows over (weir) or under the structure (gate).

What is a weir? Watch the video below.

There are fixed or movable control structures. Gates are usually movable; they can regulate the water level and discharge. Possible movements are: lifting, retracting, rotating, tilting, rolling or combinations of these. Weirs can be constructed as a fixed or movable weir. Fixed weirs cannot regulate the water level, offering the advantage that they do not contain any moving parts prone to failure and requiring intensive maintenance. A special form of the fixed weir is the siphon weir (see below). There is a flow transition from subcritical to supercritical discharge in the area around the control structure. Real control structures consist of the following components:

  • damming body (generates increase of water level); can be fixed, movable or a combination of both

  • stilling basin: energy dissipation of the discharge

  • bed pitching in the upstream and downstream water, structural connection (weir sidewalls)

  • structures for ecological consistency

Overfall condition at the weir

There may be two overfall conditions present at a weir. In the case of free overfall, the upstream water remains unaffected by the downstream water. There is critical discharge at the weir crest. The weir crest is above the downstream water level. The weir is called a free overfall weir.

In submerged overfall the upstream water is affected by the downstream water. The weir acts like a submerged weir and in many cases is completely under water.

In the case of free overfall, weirs remove any connection between the water level in the upstream water and the water level in the downstream water. As soon as the downstream water has accumulated to the weir crest to the extent that the critical depth over the crest is exceeded, there is submerged overfall.

1 free overfall, 2 submerged overfall; W height of weir, ho weir head, hc critical depth, Q discharge, hd downstream water discharge depth, hw discharge depth at weir crest

Types of weir

We can essentially distinguish between three different types of weir:

  • sharp-crested

  • ogee-crested/rounded (free-overfall weir)

  • broad-crested

Sharp-crested weirs are preferred for measuring weirs. Ogee-crested weirs are often found being used as a retaining weir and fl ood overfl ow. Broad-crested weirs are often used as a sill and overfl owed structure. These three weir types are all considered in the GUNT experimental flumes.

Control structures: flow over fixed weirs

Simplified control structure: ogee-crested weir with stilling basin

1 weir crest, 2 weir body, 3 rounded weir outlet, 4 stilling basin; ZH highest top water level, ho weir head, E specific energy; red basic triangle of the weir as an aid to design

Fixed weirs are often used to retain a river. The weir itself consists of a massive damming body. The applied moment of the water pressure is compensated by the weight of the dam wall. For this reason, weirs are normally constructed so that the outer contours roughly correspond to a triangle. The weir downstream sides can be designed to improve flow, in order to achieve the largest possible discharge Q. A hydraulically good discharge profi le is the WES profile, which was developed at the Waterways Experimental Station in Vicksburg, Massachusetts, USA, by the US Army. The WES profile design does not assume a design discharge. Usually discharges smaller than the design discharge flow over the weir. The weir is therefore optimised for a slightly smaller discharge. For discharges that are smaller than or equal to the “chosen design discharge”, the discharge profile remains stable and nappe separations can be avoided. With the design discharge, small negative pressures occur at the downstream side of the weir, but these do not represent a danger to the weir.

Control structures: types of overfall at the weir

There are two types of overfall: sharp-crested overfall and hydrodynamic overfall. In both types of overfall, the overfall condition can be free or submerged.

In the case of sharp-crested overfall, it is important that the nappe is aerated so that it falls freely. Lack of aeration may result in disturbances and thus to reduced discharge.

In hydrodynamic overfall at a fixed weir, it is important that nappe separations (reduced discharge) and excessive negative pressures (risk of cavitation) are avoided.

Control structures: calculation of discharge at the weir

Calculating the discharge plays a key role in fl ow over control structures. To calculate the discharge we use the Poleni equation. For a weir with free overfall:

μ is a factor that takes into account the weir geometry (see table below), b is the weir’s crest width, ho the weir head. In submerged overfall the equation is supplemented with a reducing factor that is taken from appropriate diagrams. From the Bernoulli equation we can see that the specific energy E can be calculated from the kinetic energy (velocity of approaching flow vu) and the discharge depth hu in the upstream water. In many cases vu is relatively small and is ignored.

In the GUNT experimental flumes, the models studied are approached normally, i.e. perpendicular to the flow direction. The weirs considered all belong to the group of fixed weirs.

In practice there are also lateral weirs, which are used as flood spillways. Lateral weirs are installed parallel to the flow direction. Lateral weirs are also fixed weirs.

Discharge coefficient μ for weirs with different shaped crests

Control structures: ogee-crested weirs

Fixed ogee-crested weirs are the preferred weir to be used as a retaining weir and flood overfl ow. They usually have a spillway for optimum flow, such as the WES profile.

Hydrodynamic overfall on the ogee-crested weir, pressure distribution on the weir crest at different discharge

1 nappe lying on the crest, 2 weir downstream side roughly corresponds to the contour of the free nappe, 3 nappe lifts off where appropriate; Q discharge, QB design discharge

Control structures: sharp-crested weirs

There is also free and submerged overfall in the case of a sharpcrested weir. For the optimal discharge at a sharp-crested weir, it is important that the nappe is aerated. Ambient pressure prevails at the top and bottom of the aerated nappe. Typical variables include the height of weir W, the weir head ho above the weir crest in the upstream water and the discharge depth hd in the downstream water. Together with the width of the weir b these variables are entered into the Poleni equation to calculate the discharge. Some variables are included indirectly in coefficients or reducing factors.

Aerated free overfall at a sharp-crested weir

1 weir, 2 nappe, 3 draw down;

vu velocity in the upstream water, v1 velocity in the nappe, hd downstream water discharge depth, ho weir head, hu upstream water discharge depth, W height of weir

Submerged overfall

1 at a partially submerged sharp- crested weir,

2 at a fully submerged sharp-crested weir (undulating discharge)

Control structures: broad-crested weirs

Broad-crested weirs are overflowed structures that are used in rivers where there is little variation in the discharge and only a rather small top water level is desired. They can also be the foundation for a movable control structure. Broad-crested weirs are characterised by a short section of almost uniform discharge with critical depth occurs on the weir crest (see illustration below). In this section, there is a hydrostatic pressure distribution. The streamlines extend almost horizontally. These conditions apply as long as the ratio of weir head to weir length ho/L is between 0,08 and 0,5. Broad-crested weirs with these dimensions can also be used as a measuring weir.

Once ho/L is <0,08, friction losses can no longer be ignored and the weir body is too long to be used as a measuring weir. At ho/L > 0.5, i.e. short weir bodies, the streamlines do not run horizontally and the pressure distribution is not hydrostatic, so that we cannot use the calculation approaches presented here. For ecological reasons, a broad-crested weir is rarely used as a sill in rivers. Instead, a ramp is built so that fish and other aquatic creatures can swim upstream. GUNT experimental flumes facilitate the investigation of various broad-crested weirs and the their respective discharges Q.

Broad-crested weir

vu upstream water flow velocity, hu upstream water discharge depth, W height of weir, hc critical depth, L length of weir; arrows indicate streamlines

Control structures: siphon weir

The siphon weir is a fixed weir. The illustrations below show the hydraulic principle of the syphone when used as a flood overfl ow. When the water level of the storage lake rises just above the weir crest of the damming body, the siphon comes into play, soon resulting in free overfall. If there is a slight increase in water level, i.e. a slight increase in discharge, the nappe deflector directs the water jet to the siphon hood. This leads to an evacuation in the siphon duct, resulting in the discharge pressure in the pipe with full flow. This discharge pressure has a high discharge capacity, which only increases a little with rising water level.

If the water level of the storage lake falls again so that it is below the edge of the inlet lip, air is sucked into the siphon and the siphon vented. This abruptly stops the flow of water. The discharge can be interrupted at any time by an additional device for venting. Siphon weirs can only be adjusted to a limited extent and cannot be overloaded. In the past they were often incorporated as spillways in dams on the basis of their high specific discharge capacity.

Principle of a siphon weir

1 air vent, 2 weir body, 3 nappe defl ector, 4 siphon duct, 5 siphon hood; ZS top water level, ZH highest water level

Control structures: flow under gates

Discharge under a sluice gate

1 free discharge, 2 submerged discharge; hu upstream water discharge depth, a gate opening, hd downstream water discharge depth, h1 minimum discharge depth, L position of the minimum discharge depth, E specific energy, ΔE loss of specific energy

Gates may be subject to either free or submerged discharge, in a similar way to flow over weirs. Discharge leads to jet contraction, also called “vena contracta” (minimum discharge depth h1). Free discharge prevails as long as the discharge passes under the gate without disturbance and the downstream water does not form a backwater to the gate. In free discharge, there is supercritical discharge directly downstream of the gate. In a similar way to the flow over weirs, the free discharge Q is calculated from Bernoulli’s equation, the momentum equation and the continuity equation giving

where μ = discharge coefficient, b = gate width, a = gate opening.

Gates are movable control structures, i.e. the gate opening a and thus the discharge Q is altered and adjusted to actual needs. In practice, there are therefore characteristic diagrams which show the discharge Q (upstream and downstream water discharge depth hu and hd and gate opening a are given). One type of gate commonly used in practice is the circular radial gate used to control discharge. It can be rotated about a bearing point. The radial gate is often placed on the weir crest of a control structure. Flow does not just go under the radial gate, but can also go over into a flume (radial weir).

Discharge under a radial gate

hu upstream water discharge depth, a gate opening, hd downstream water discharge depth