47 5. WEIRS 5.1. GENERAL 5.1. Introduction A weir or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. They are also used to control the flow of water for outlets of lakes, ponds, and reservoirs. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. Dams are structures that cover the entire valley other than its bed and affect the regime of the stream. Weirs are structures that generally only cover the bed of the stream, raise the water level to a certain level rather than accumulate water, turn the water in a certain direction and provide the desired amount of water from the desired level. Weirs are also called regulators or in a broader sense translation structures. Weirs are structures that swell the water in the streambed and turn it into a transmission structure. Therefore, it includes both the elements necessary for taking water and the elements required to transfer the excess water harmlessly to the downstream when the water is excessive. Although it swells the water to a certain amount, unlike dams, there is no important accumulation. Weirs contain control elements such as grids and covers include elements such as a settling basin, the structure in which the sediment is settled. The sediment accumulated in the sedimentation pool is cleaned intermittently by opening the cover of the washing channel. Weirs may be made with a fixed concrete mass called fixed weirs (Figure 5.1a) or with lids placed on a lower called movable (gated) weirs (Figure 5.1b). Sometimes, both kinds of weirs may be used together (combined weirs). Significant changes may occur in the flow conditions in the downstream region of the stream after the construction of the weirs. As the waters passing downstream through the weir leave most of the sediment they carry, bed and bank erosion occurs in the downstream region. Since the construction of the weirs cause elevation difference between the upstream and downstream water levels, some special structures should be made to ensure the access between upstream and downstream. 5.1.2. Construction Aims Weirs may be constructed one of or a number of the following purposes: To increase the upstream water to a certain level, ensuring that it receives water at the desired level, To reduce the water level changes in front of the water intake, To obtain drop height by swelling the water, To reduce the flow velocity in order to prevent shore and bottom erosion in the streambed and to protect the related structures against scouring, To provide the required water depth in transport streams, especially at minimum discharges, To keep the bed and suspension sediment back, even to a limited extent, To regulate flows, because there is limited storage in the swelling of water.
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5. WEIRS
5.1. GENERAL
5.1. Introduction
A weir or low head dam is a barrier across the width of a river that alters the flow characteristics of water
and usually results in a change in the height of the river level. They are also used to control the flow of water
for outlets of lakes, ponds, and reservoirs. There are many weir designs, but commonly water flows freely
over the top of the weir crest before cascading down to a lower level.
Dams are structures that cover the entire valley other than its bed and affect the regime of the stream. Weirs
are structures that generally only cover the bed of the stream, raise the water level to a certain level rather
than accumulate water, turn the water in a certain direction and provide the desired amount of water from the
desired level. Weirs are also called regulators or in a broader sense translation structures.
Weirs are structures that swell the water in the streambed and turn it into a transmission structure. Therefore,
it includes both the elements necessary for taking water and the elements required to transfer the excess
water harmlessly to the downstream when the water is excessive. Although it swells the water to a certain
amount, unlike dams, there is no important accumulation. Weirs contain control elements such as grids and
covers include elements such as a settling basin, the structure in which the sediment is settled. The sediment
accumulated in the sedimentation pool is cleaned intermittently by opening the cover of the washing
channel. Weirs may be made with a fixed concrete mass called fixed weirs (Figure 5.1a) or with lids placed
on a lower called movable (gated) weirs (Figure 5.1b). Sometimes, both kinds of weirs may be used together
(combined weirs).
Significant changes may occur in the flow conditions in the downstream region of the stream after the
construction of the weirs. As the waters passing downstream through the weir leave most of the sediment
they carry, bed and bank erosion occurs in the downstream region. Since the construction of the weirs cause
elevation difference between the upstream and downstream water levels, some special structures should be
made to ensure the access between upstream and downstream.
5.1.2. Construction Aims
Weirs may be constructed one of or a number of the following purposes:
To increase the upstream water to a certain level, ensuring that it receives water at the desired level,
To reduce the water level changes in front of the water intake,
To obtain drop height by swelling the water,
To reduce the flow velocity in order to prevent shore and bottom erosion in the streambed and to
protect the related structures against scouring,
To provide the required water depth in transport streams, especially at minimum discharges,
To keep the bed and suspension sediment back, even to a limited extent,
To regulate flows, because there is limited storage in the swelling of water.
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a. Fixed Weir
b. Movable (Gated) Weir
Figure 5.1. Types of Weirs.
5.1.3. Factors that Affect the Type of Weirs
a. Topography of the Weir Place: In the case of fixed weir in flat and plain streams, since swelling will be
high in the passage of flood waters, agricultural land, access roads and basements of residential areas may be
submerged. In such places, the swelling level can be kept constant to some extent by choosing the movable
weir. The choice of fixed or mobile weir in mountainous rivers and valleys with steep slopes is decided by
looking at other factors. Generally, fixed weirs are more economical in such cases. Planning a fixed weir
wide valleys and movable weirs in narrow valleys is appropriate in terms of the passage of the flood flow.
b. Sediment Discharge: In streams carrying a large amount of sediment, the upstream side of the fixed weir
fills in a short time and a large amount of bed material enters from the water intake. In movable weirs, it is
possible to wash the solid materials accumulated on the upstream side by opening the lids, especially during
flood, and eliminating the aforementioned drawbacks for fixed weirs. For this reason, the choice of mobile
weir is appropriate in rivers with high solid material transport.
c. Minimum and Maximum Discharges: If the difference between the maximum and minimum discharge of
the stream is too large, the choice of movable weir is appropriate, since the swelling level will increase
during the flood in fixed weirs.
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d. Operation and Maintenance Costs: Since additional energy is needed to open the gates in movable weirs,
the operation and maintenance costs of these types are higher than the fixed weirs.
e. Comparison of Costs: The points mentioned above are examined one by one and compared with each
other. The advantages and disadvantages of each type are determined; The most appropriate weir type to be
built is decided by calculating the initial investment costs and estimated maintenance and operating costs.
5.1.4. Determination of Backwater Elevation
When a weir body is constructed (for fixed weir) or weir gates (for gated weir) are constructed upstream of
the structure water level will swell; the water profile at upstream of a weir is called backwater (Figure 5.2).
As going from weir to upstream, the water level will descend and the swelling will be zero enough far from
weir. When the water level difference between before and after swelling is 2 cm, it is supposed that the
effect of the weir will end (no swelling upstream this point). This point is called swelling (backwater)
boundary and the distance between swelling boundary and weir is named as backwater length.
Figure 5.2. Backwater Profile
Backwater elevation is determined according to: The flooding condition of the backwater (swelling) zone,
the permissible seasonal peak values of the ground and surface water levels in the upstream region,
foundation conditions, the costs of the energy breakers (dispersion structures). In particular, the selection
should be made by considering the lands with high agricultural value, the settlements in the swelling zone,
the drainage status of the region, the water supply and wastewater networks and the existing water rights in
the region and by considering the goals that serve everyone.
The swelling elevation, which determines water level upstream of the weir, is calculated by adding friction
and local energy (head) losses at the transmission channel and local losses at the water intake. Low weirs are
not suitable as they will lose their effect in a short time as a result of sediment accumulation. High
agricultural value lands can be protected by constructing berms. It should be noted that the swelling
elevation will increase over time as a result of sediment accumulation in the swelling zone.
4.1.5. Weir Foundations
Foundation soils at the weir place are classified into 3 groups; a. Rock, b. Sand-Gravel and c. Clay-Mud.
Rock is the most suitable and inexpensive foundation type. However, the weirs were mostly built on
sand-gravel ground. If the sand-gravel layer is of sufficient thickness, there is no problem in terms of
transferring the loads to the ground and the stability of the structure. If the permeability is large, serious
problems are encountered as a result of bottom leakage. In this case, special measures such as cutoff wall
and sheet pile are required to increase the infiltration length. The foundations planned on clay-mud are the
most expensive and difficult. Clay soils can be used as a weir foundation, taking into account the swelling,
shrinkage properties and pore water pressure and reinforcing it. If the soil is mud, it should be removed and
the ground with high bearing capacity should be filled instead.
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5.2. FIXED WEIRS
5.2.1. Definitions
The fixed weir is essentially a wall that blocks the front of the water, is resistant to the pressure of the water
that will swell behind it and the pressure of the accumulated sediment material; and has a hydraulic suitable
cross section. The swell level and the passage of waters from upstream to downstream are provided by a full
body that covers the stream bed. The top of the full body is fixed. If discharge changes, the water level in the
upstream region also changes greatly. For this reason, fixed weirs are planned in places where the harmful
effects of swelling level changes are not observed. The solid body also acts as a spillway to discharge flood
waters downstream. Since the accumulation of sediments in front of the water intake will cause clogging
over time, it is necessary to leave a covered section near the connecting body. This section is called the
undersluice (gravel passage). After high discharges, the covers of the undersluice is opened and cleaned.
Some terms related to fixed weirs are given below (Figure 5.2):
Water Nappe: A layer of water that passes over the weir and has a lower and an upper surface.
Weir Load: Height difference between the weir crest elevation and the water level passing over the weir.
Weir Crest Elevation: It is found by subtracting the minimum sluice load from the swell level.
Swelling Level: Minimum swelling level of the water that is desired to be swelled by weir.
Maximum Swelling Level: It is found by adding the maximum weir load to the weir crest elevation.
Weir Height: It is found by subtracting the stream base elevation from the weir crest elevation.
5.2.2. Elements of Fixed Weirs
The main elements of a fixed weir are the main weir structure, water intake structure and special structures.
Information on intake structures and special structures will be given later. Here, information is given about
parts of the main weir structure (Figure 5.3 and 5.4).
a. Weir Body: It is the structure that allows the adjustment of the upstream water level by closing the stream
bed from one end to the other. If a service bridge that provides passage from one shore to another is
necessary, the bridge piers are placed on the weir body.
b. Undersluice (Gravel Passage): These are the covered passages planned at the lowest level (thalweg) of the
stream bed in order to prevent the accumulation of bed sediment in front of the water intake. The gravel pass
is separated from the full body by the separation (guide) wall that guides the sediment.
c. Abutments: They are the retaining walls that restricts the weir with the coasts, resists the soil effects here
and acts as a support for the service bridge.
d. Fall Bed (Apron): It is the protective layer that is planned as the continuation of the full body in order to
prevent the water passing over the weir from damaging the connection by breaking the energy and
preventing the river bed from being eroded.
e. Riprap: It is the part protected by stones and rock fragments of certain length, at the downstream side the
fall bed in order to prevent scouring in the stream bed.
f. Sealing Structures: The leakage under weir is controlled by sealing structures; such as cutoff wall, sheet
pile, injection curtain, upstream cover, reverse filter, etc.
g. Other Facilities: Depending on the purpose of weir, structures such as water intake structure, power plant,
fish passage, ship passage etc. are planned.
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Plan
a-a Section
b-b Section
c-c Section
d-d Section
Figure 5.3. Plan and Sections of a Fixed Weir
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Figure 5.4. Elements of a Fixed Weir
5.2.3. Calculation Principles
Calculation principles of fixed weirs are examined in four main sections: Determination of weir height and
width, and profile of the sluice, and design of undersluice. In order for the desired amount of flow to enter
from the water intake, the water level at the upstream of weir should not fall below a certain level. This level
is called swell level and is calculated as follows:
Swell Level = Elevation of water arrival +
Energy losses in transmission facilities + Energy losses in water intake (5.1)
5.2.4. Determination Crest Elevation and Length of Weir
The discharge passing on sluice crest is calculated as:
5.1CbhQ (5.2)
Where, Q is discharge (m3/s), C is sluice coefficient (between 1.7 and 2.2), b is crest length of sluice (m) and
h is the water depth on weir (m). Since the critical situation in receiving the demand flow (Qd) into the
transmission channel occurs at minimum discharge (Qmin), the following steps are taken in determining the
weir crest elevation:
Case 1: 0'Q minmin QQd : In this case, the sluice load (water depth of sluice) that will occur when the Q
discharge passes over the weir is calculated. The weir crest elevation level is found by subtracting the sluice
load from the swell elevation.
Case 2: 0Qmin dQ In this case, since the water will not pass over the weir, the weir crest elevation is
taken as swelling elevation by adding 0.1 m safety margin for wave swelling effect.
The weir body is the most expensive part of the facility, so it is desirable to keep its length as short as
possible. If the weir length is chosen large, the maximum swelling level during the flood is small, but the
weir cost is large. The opposite happens if the weir length is chosen small. For this reason, it is necessary to
examine from which swelling elevation the settlement and agricultural areas in the upstream region will be
damaged and submerged, and to what extent this situation can be prevented by berms; thus, the technically
and economically most suitable weir length should be determined. The fixed weir length can be determined
to be q = 5 m3/s/m flow per unit length as a rough approximation. In principle, the weir length should not be
less than 0.5-0.6 times the width of the streambed to prevent ice accumulation and erosion.
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5.2.5. Undersluice
Undersluices are the gated passages arranged to transfer the gravels to the downstream from time to time in
order to prevent gravel entering the settling basin and blockage of the water intake mouth due to excessive
accumulation in front of the sill. It is appropriate to keep the undersluice width between 3 to 3.5 m. Since the
cost of large gates is high, it is also economically beneficial not to make the gate very wide. In the weirs up
to 2.5 m height, the gate height may be equal to weir height. For high weirs, it is suitable to use a breast wall
(a reinforced concrete curtain extending into the water) together with the gate.
5.2.6. Determination of Hydraulic Profile of Sluice
The fixed weir crest over which the water flows freely is called sluice. The upper and lower nappe values of
the sluice should be determined to avoid excessive stresses on the body. Two different methods are used to
determine the hydraulic profile of sluice: Creager Profile and Ogee Profile.
a. Creager Profile: The lower and upper nappe elevations (y) corresponding to various x horizontal distances
from the starting point of the weir are given in Table 5.1 and Figure 5.5 for the unit sluice load (water depth)
(h0 = 1 m). The calculated coordinates for other sluice loads are multiplied by the sluice load.