Gated Weir

The Gated Weir models flow through a gated weir where the crest elevation can vary with time.

Data

Theory and Guidance

The Gated Weir models flow through a gated weir where the crest elevation can vary with time. It can be used with all the types of control available for the multi-gated Radial Sluice Gate and Vertical Sluice Gate, namely with gate opening set according to upstream water level, downstream water level, level at some remote node or according to a set of logical rules.

Three flow regimes can be observed for flow over a gated weir:

• when the gate is lowered and the weir operates as a flume or broad crested weir with critical depth in the body of the structure.
• when the gate is raised and critical depth occurs at the gate crest.
• when the direction of flow is from the free crest towards the hinge when performance is similar to a weir with a triangular profile.

These are shown on the diagram below.

When in logical' mode the weir will use logical rules that are contained within a specific RULES' data block to control the movements of the weir crest; this data block must appear immediately after the main part of the gated weir datafile entry. For full details of how to use this option, please refer to the Rules topic.

When operating in logical mode there are two sub-modes of operation; automatic and manual control. Gated weir unit can switch freely between these two modes of operation depending on the instructions the user has put into the datafile entry.

When in automatic mode the crest height will be controlled by instructions from connected control units (in controller mode) or by currently valid rules within the RULES data block (in logical mode). These instructions will be updated when the polling time interval has elapsed. Instructions are interpreted as a command to move to a target position - between fully closed and the maximum opening value - and the gate will be moved to this target position at the maximum movement rate over the subsequent polling time interval.

When operating in manual control mode the target gate positions are obtained from the appropriate line of the time switch data in the datafile entry for the controlled sluice unit. When the model time reaches or exceeds the time value in the switch data, the crest height will be set according to the gate opening value, moving at the maximum rate possible.
Note that when in manual mode the gate will attempt to move instantaneously from one setting to another (i.e. intermediate positions are not obtained via linear interpolation, as they are in Time control mode).

Equations

Gated Weir Parameters

m = Modular Limit

b = Breadth of sluice at control section (normal to the flow)

C_tc = Discharge coefficient for throat control

C_gt = Discharge coefficient for gate control

C_rev = Discharge coefficient for reverse gate control

Mode 0 - Dry sill

Mode 1 - Dry gate crest

Modes 2 - 5

Mode 2: Free weir flow - throat control

Mode 3: Drowned weir flow - throat control

Mode 4: Free weir flow - gate control

Mode 5: Drowned weir flow - gate control

Determination of throat/gate and free/drowned flow combinations:

Mode 2 - Free weir flow - throat control

Mode 3 - Drowned weir flow - throat control

Mode 4 - Free weir flow - gate control

Mode 5 - Drowned weir flow - gate control

Mode 6 - Free weir flow - reverse gate control

Mode 7 - Drowned weir flow - reverse gate control

Modes 8-13 - Reverse flow modes

Mode 8 - Free weir flow - reverse gate, throat control, reverse flow

Mode 9 - Drowned weir flow - reverse gate, throat control, reverse flow

Mode 10 - Free weir flow - reverse gate control, reverse flow

Mode 11 - Drowned weir flow - reverse gate control, reverse flow

Mode 12 - Free weir flow - forward gate control, reverse flow

Mode 13 - Drowned weir flow - forward gate control, reverse flow

The equations and conditions are the same as for modes 2-7, but with reverse flow (Q<0) . Note that the orientation of the gate specified above for modes 8-13 is relative to the sill, not the flow direction, e.g. for Mode 8, the position of the gate is not physically oriented the same as for Mode 2. For Modes 2 and 12, the gates are in the same position, relative to the sill.

General

The equations used for the Gated Weir were derived and tested on a physical model. The work is described in HR Wallingford EX1296.

The unit will model reverse flow (i.e. flow from label2 towards label1) and considers the FORWARD gate orientation as reverse gate control in this case. Conversely REVERSE gate orientation corresponds to either throat or gate control for reverse flow.

Gate orientation cannot change from FORWARD to REVERSE during a run.

A check is applied to verify that a particular crest height is physically possible since the crest height cannot exceed the sum of the gate depth and the sill height.

When using 'controller' or 'logical' modes, if the datafile is set up so that for a particular run a gated weir starts up in 'AUTO' mode, the crest height will initially be set to the value given in the unit state (ustate) field in the initial conditions. If this is outside the possible range, the corresponding datafile entry in the switch data set must have a gate setting value, which will be used. Other 'AUTO' entries in the switch data set need not have a corresponding value as the weir will receive its target gate positions from the control system or from the logical rules sub-block. If gate settings are given in this instance they will be ignored by the weir unit.

When in automatic mode the sluice gates will be driven from instructions from connected control units (in controller mode) or by currently valid rules within the RULES data block (in logical mode). These instructions will be updated when the polling time interval has elapsed. Instructions are interpreted as a command to move to a target gate position - between fully closed and the maximum opening value - and the sluice gates will be moved to this target position at the maximum movement rate over the subsequent polling time interval.

When starting in 'AUTO' mode for 'controller' operation, it may be necessary to ensure that the initial gate settings in the switch data set are compatible with the initial output value from the corresponding SETSLUICE unit. Otherwise the signal from the control system may result in a large initial change in gate setting.

For reasons of stability it is often desirable to run this unit in MANUAL mode for a short time even if an automatic run is required in `controller' mode. This is to ensure that control unit variables (such as outputs, errors, etc.) can stabilise without any interfering feedback effects.

As indicated above, this unit can be used in conjunction with logical control rules (however the direct steady method has not yet been modified to allow for this feature). The direct steady method will apply the equations for gate control unless the gate crest is below the sill height.

Users must convince themselves that sensible control rules have been specified. In 'water' operating modes it is possible to specify rules such that there are regions where no equations apply. It is recommended that logical control rules are used in preference to water control modes.

The unit state for this unit is the gate opening in metres above sill level. Note: the opening (when set in time or logical mode) is gate crest above sill level, i.e. 0 opening is flat on the sill and max opening equates to fully upright position.

Datafile Format

Line 1 - keyword `GATED WEIR' [comment]

Line 2 - Label1, Label2, [Label3]

Line 3 - C_tc, C_gt, C_rev, m, gtdir

Line 4 - b, z_c, h_g, BIAS

Line 4a - 1, tm, rptflg

Line 5 - O_mode[, oprate, opemax, opemin, Clabel]

Line 6 - keyword `GATE'

Line 7 - n₁

If O_mode = 'time':

Line 8 to Line 7+n₁ - t_i, hO_i

If O_mode = 'water1' or 'water2' or 'water3':

Line 8 to Line 7+n₁ - y_i, hO_i

If O_mode = 'controller' or `logical':

Line 8 to Line 7+n₁ - t_i, opmod, hO_i

Note: The RULES sub-block should appear immediately after this unit if `logical' operating mode is to be used - see the Rules section.