Weirs and Barrages MCQ Quiz in বাংলা - Objective Question with Answer for Weirs and Barrages - বিনামূল্যে ডাউনলোড করুন [PDF]

Concepts:

Barrage is an artificial barrier across the river to prevent flooding, aid irrigation or navigation. In other words, barrages are the structures which are only meant  to stop the river flow and here, storage is secondary purpose  of barrage.

Advantages of Barrage

1. Barrage has number of gates. During floods the gates are regulated as per magnitude of floods.
2. There is good control over silt entry into the canal.
3. A roadway can be conveniently provided over the structure at a little additional cost.

4. Facilities for inspection and repair of various structures.

Disadvantages of Barrage

1. The initial cost of the barrage is quite high.

2. Frequent gate regulation during rainy season.

3. Water storage will be less as such their main purpose is to divert the flow of river.

Concept:

Minimum thickness of the floor is given by,

\(t = \dfrac{h}{{G - 1}}\)

Where h = seepage head and G = specific gravity

Calculation:

Given,

h = 4.8, G = 3.4

∵ \(t = \dfrac{h}{{G - 1}}\)

⇒ \(t = \dfrac{{4.8}}{{3.4 - 1}} = 2\ m\)

Explanation:

IRC Code of practice for the design of substructure and foundations of the bridge gives clear guidelines regarding the scour depth for the condition of flow at noses of piers, and that is equal to 2.0d [where d = depth of scouring according to lacey's theory]

d = \(0.475{\left( {\frac{Q}{f}} \right)^{\frac{1}{3}}}\)

Additional Information

Additional scour depths are given from the IRC Code of practice for the design of substructure and foundations of the bridge 

Weirs:

Weirs are sharp-crested, overflow structures that are built across open canals. They are easy to construct and can measure the discharge accurately when correctly installed. It is important that the water level downstream is always below the weir crest, otherwise the discharge reading will be incorrect.

According to the discharge conditions the weirs are classified as follows:

Freely discharging weir:

If the water level on the downstream of a weir is well below the weir crest , then the nappe emerges freely in the atmosphere and thus it is called as freely discharging weir.

Submerged weir:

If the water level downstream of a weir is above the crest level of the weir, then the weir is known as submerged weir. 

Important point:

Weirs can also be classified according to the shape of opening, shape of crest, and effect of sides on the issuing nappe as follows,

1) According to the shape of opening:

Weirs may be classified as rectangular, triangular and trapezoidal weirs.

A particular type of trapezoidal weir having sides slopes as one horizontal to four vertical is known as cipoletti weir.

2) According to the shape of crest:

Sharp crested weir:

If the jet of liquid falls free as it leaves the upstream face of a weir, the weir is known as sharp crested weir.

Ogee shaped weir:

If the water flowing over the crest is made to glide smoothly over the curved profile of spillway then it is called ogee shaped weir.

Based on the relation between B ( width of weir) and H (maximum head of water under which weir has to operate) weirs are classified as follows:

If B < 0.625H → Thin shaped weir

If B ≥ 0.625 H and B < 2.5H → Narrow crested weir

If B ≥ 2.5H and B ≤ 10H → Broad crested weir

3) According to the effect of sides on the issuing nappe:

Suppressed weir:

When the crest length of a weir is the same as that of the width of the channel, the weir is said to be suppressed weir.

Contracted weir:

When the crest length of the weir is less than the width of the channel, there will be lateral contraction of the nappe, such type is called contracted weir.

Sharp crested weir: 

The crest shape of the weir is very sharp. It is the most widely used weir.

• A sharp crested weir is a type of flow control structure used in open channels to regulate the flow of water.
• The weir consists of a horizontal crest or sill over which the water flows. The thickness of the weir refers to the dimension perpendicular to the direction of flow.
• To ensure the proper functioning of the weir, the thickness should be kept less than one-half of the height of water on the sill.
• This is done to minimize the effect of submergence and ensure that the flow over the weir remains sharp-crested.
• If the thickness exceeds one-half of the height of water on the sill, the flow characteristics may change, resulting in a broader and less well-defined flow pattern over the weir.
• This can affect the accuracy of flow measurement and the overall performance of the weir. Therefore, it is important to adhere to the recommended thickness guidelines for a sharp crested weir.

Concept:

Weir is a physical structure of masonry constructed across the channel width to calculate the discharge of the channel section.

For rectangular weir:

The discharge through weir is given by: \({\rm{Q}} = \frac{2}{3}{{\rm{C}}_{\rm{d}}}\sqrt {2{\rm{g}}} {\rm{\;L}}{{\rm{H}}^{3/2}}\)

∴ Q ∝ H^1.5

where H = still water head, and L = Length of the weir

Flow over a triangular weir (V-weir):

\(Q = \frac{8}{{15}}{C_d}\sqrt {2g} \;{H^{\frac{5}{2}}}\tan \left( {\frac{\theta }{2}} \right)\)

θ: Included angle of Notch.

Flow over a trapezoidal weir (or) Notch:

\(Q = \frac{2}{3}{C_{{d_1}}}\sqrt {2g} \;LH_1^{\frac{3}{2}} + \frac{8}{{15}}{C_{{d_2}}}\sqrt {2g} .\tan \frac{\theta }{2}.{H^{\frac{5}{2}}}\)

Where

\(\left( {\frac{\theta }{2}} \right)\) : weir angle of inclination with the vertical.

\({C_{{d_1}}}\) = Coefficient of discharge for the rectangular portion.

\({C_{{d_2}}}\) = Coefficient of discharge for the triangular portion.

Freeboard:

A) → Free board

B) → Surcharge storage

C) → Active Storage capacity

D) → Inactive storage capacity

E) → Live storage capacity

F) → Buffer storage

NWL → Maximum water level or also called highest flood level (HFL)

FSL → Full supply level

MDDL → Minimum Draw down level

DSL → Dead storage level (No outlet available to drain the water in reservoir by gravity)

Freeboard is the vertical distance between the top of the dam and the reservoir water surface.

Free board can be defined in different terms such as:

Normal freeboard:

Normal freeboard is defined as the difference in elevation between the top of the dam and the normal reservoir water level as fixed by design requirements.

Minimum freeboard:

Concept:

Bligh’s Creep Theory:

• Bligh assumed that the percolating water follows the outline of the base of the structure which is in contact with the subsoil.
• The length of the path traversed by the percolating water is called the length of creep or creep length.
• He further assumed that the head loss per unit length of creep (i.e. H/L) which is called hydraulic gradient is constant throughout the percolating passage i.e. Loss of head is proportional to length of the creep.

\({\rm{Hydraulic\;Gradient\;}}\left( {\rm{i}} \right) = \frac{{{\rm{Head\;Loss}}}}{{{\rm{length\;of\;creep}}}} = \frac{{\rm{H}}}{{\rm{L}}}\)

• The reciprocal of hydraulic gradient (i.e. L/H) is known as Bligh’s Coefficient of creep.

\({\rm{Bligh's\;Coefficient\;of\;creep\;}}\left( {\rm{C}} \right) = \frac{1}{{\rm{i}}} = \frac{{\rm{L}}}{{\rm{H}}}\)

Where, C lies between 5 and 15 and depends on the type of soil.

To ensure safety against piping failure, \({{\bf{L}}_{{\bf{req}}}} \ge {\bf{CH}}\)

Note:-

Percolation Coefficient:

It is defined as the ratio of coefficient of permeability to porosity of soil.

\({{\bf{K}}_{\bf{p}}} = \frac{{\bf{K}}}{{\bf{\eta }}}\)

Rockfill dam is the structure constructed by loose rocks and boulders piled in the river bed and concrete structure on the upstream side.

A spillway is a structure which is constructed near the dam for collecting the surplus water above the freeboard level. It ensures the safety of the dam and avoids flooding condition.

The reservoir is the water storage structure constructed across the river.

Gravity dam is the structure which its own weight resist the hydraulic force exerted by water. 

Trap efficiency of a reservoir is the ratio of total sediments retained by reservoir to the total sediments entering into reservoir (inflow sediments).

Trap efficiency of reservoir is the function of ratio of reservoir capacity and total inflow.

The reservoir capacity is inversely related to the amount of sediments deposited. As the time passes, rate of silting or rate of deposition of sediments reduces but amount of sediments deposited will be increased which in turn reduces the capacity of reservoir. Hence, trap efficiency also gets reduced.

Other factors affecting trap efficiency:

1. Size of reservoir and stream: A larger reservoir on small stream has less capacity inflow ratio and hence having higher trap efficiency.