UNDERGROUND BANDHARAS
Underground bandharas of various types have been constructed at Yavatmal, Amaravati and Satara districts. Large rivers such as Krishna, Wardha, Painganga , Pus, Bembla, have been crossed successfully by RCC, masonry bandharas. Similarly, nallas small rivers have been crossed by masonry, soil bandharas. Overall total more than 50 bandharas have been constructed using the site conditions. One Arch type bandhara on Arunavati river has been successfully constructed. Everywhere the experience was different. However, every bandhara was useful to the water supply system. While constructing new concepts were added. Confidence level for all the staff was high. Funding was mostly done by the Collectorate from the scarcity funds. Problems tried by construction of bandharas were completely solved and they did not crop up again. In other districts MJP did not contribute in construction of bandharas to a larger extent. But this experience was shared to many people and canvassed for the benefits. A large size river crossing within a very short time of about three weeks is a challenging job. It is a scheduled work and time bound activities are required to be planned. Dewatering problem was not a problem. It was better to plan for diversion than pumping. Hence, it was economical and it is worth noting that none of bandhara was costing more than five lakhs in the years 1992 to 2000. That is why it could be accommodated in the scarcity funds.
Underground bandharas are constructed to arrest the underground water currents and also create a barrier to hold water upstream in the subsurface. It is provided as a conservation measure. They are economical structures. They are not expected to flood the upstream ground surface. They create a potential difference in subsurface water levels. Underground filtration is also expected. Ground water level is also improved. Scarcity may or may not be required; it creates a positive interaction of source improvements.
Combination of underground bandhara and above ground barrier can be done. Both these structures can be in one line or having horizontal distance in between. Above ground structures are built across the natural drain and try to arrest the surface water. It has a chance to percolate water downstream in the subsurface. When the underground water is pumped out of the system from the wells, recharging takes place and water currents are established.
Apart from increase in water quantity in soil, quality is also improved. Further soil movement along with current is restricted.
SOIL BANDHARA
Underground bandharas can be constructed in soil, cement concrete and/or in masonry. In case of soil bandhara, however, excavation across the stream has to be done up to hard strata. Bottom width has to be designed for depth of excavation up to hard strata. This depth is normally found to be 3 to 4 m. Accordingly, base width for soil bandhara comes to minimum 3 to 4 m. Section at the top can be minimum 1 m, which is about 0.3 m below the bed level in natural drain.
For better and stable construction soil filled in plastic bags of cement can be used and constructed in fashion of brick masonry, is recommended. For protection from sides and top two layers of brick on edges shall be provided. Approximate cost of this type of construction works out to Rs. 5000/- per meter length of underground bandhara.
Soil bandhara can be constructed across a nalla or small size river. Time available for construction is about 30 days in the summer, when water level is very low in the natural drain and dewatering involved can be minimum. Similarly, dewatering during construction can be minimized by diverting the flow, rather than pumping out water. Practically, no skilled labour is involved for construction. Local labour can do the job satisfactorily.
Barrier constructed across the natural drain causes resistance to natural full flow of water in the rainy season. Flow carries silt and it will try to accumulate along upstream side of underground bandhara. It will exert pressure on the section of bandhara and this will be maximum at the center. The decided section should withstand the pressures. Further, on downstream side there can be a void, created due to barrier. In this void on rear side a scouring is expected. But after one season the conditions get stabilized. Effects are seen immediately in same season. Water retention upstream becomes effective and becomes useful during summer period.
If the soil bandhara is constructed without packing black cotton soil in plastic bags, then there are chances that soil can get displaced and effects of bandhara are not seen in the consecutive years.
Black cotton soil in wet conditions swells and it exerts pressures on the edges and it tries to come out of the confined position. It is therefore necessary to use plastic empty cement bags for the black cotton soil. Similarly the trapezoidal section of the soil bag masonry has to be retained in shape by providing brick on edge packing in two layers.
MASONRY BANDHARA
In case of masonry bandhara length of bandhara is more important in design. At every 2 m to 3 m distance along length a pier like structure should be provided to break the continuity. Size of pier should be distinctly more than the wall thickness. For a depth of 4 m, masonry thickness should be minimum 2 m at the base and then it can be tapered up to the top. Minimum top width should be 60 cm and should be walk-able. Normally, masonry bandharas are taken to 1 m to 1.5 m above the bed surface. This ensures impounding of some surface water. For occasional draining of water flanged pipe pieces are required to be provided. On the flanged ends sluice valves shall be fitted or blank flanges can be fitted. These can used to remove the accumulated silt near the upstream base of bandhara.
As masonry bandhara is designed to impound water, it will be replenished continuously by the surface flow. If surface flow is in excess of any drawal from the system then water will overflow from the artificial barrier created. Downstream structure has to be designed for the overflow. A small height bucket has to be constructed along the downstream edge so that its energy gets dissipated. This arrangement protects the foundation. Also, it smoothens the downstream flow.
Impounded water percolates underground and recharges the supply wells. This action ensures the availability of water in the summer period. Natural underground water currents are established. Thus by construction of bandhara, localized raising of surface water is ensured and all its advantages are deployed.
When there is a natural ponding in the stream, some impervious barriers exist downstream. Some of rocky outcrops also rise above surface across the stream bed. The geography has to be studied carefully and the advantage can be taken to join these rock outcrops by constructing masonry to create a bandhara just above the existing natural underground bandhara. This is the most economical proposal of constructing masonry bandhara. The joints have to be carefully constructed. For having substantial anchorage in the harder strata, bores can be drilled and steel anchor bars can be provided and grouted with cement mortar.
Constructing masonry and soil bandhara is choice as per the available site conditions. Masonry bandharas are slightly costlier as far as per meter cost is concerned. But these are semi-permanent structures and can be relied upon. When soft rock is available at nearer depths masonry bandhara is appropriate choice and in case of soft strata for deeper depths is available then, soil bandhara is a better choice. Similarly when banks are not sound soil bandhara is a better proposal.
R.C.C. BANDHARA
Wherever masonry is proposed RCC bandhara can be proposed. RCC bandhara becomes economical due to its design. The sections are less as compared to masonry sections. Designed as couterforts, the structures are really sturdy. Fixtures, such as pipes and valves, are easily adoptable. Further construction of downstream water bucket is also easy. As far as economics is concerned per meter cost of RCC is slightly less than that of masonry bandhara. Composite RCC and masonry structures in case of joining of out-crops, can be thought of because they are further economical.
PROTECTION OF BANKS
It has been observed that the floodwater has a tendency to damage the banks. If these banks are of soil, they are likely to be cut by the floodwater. These banks need to be protected. Masonry or RCC bandhara structure has to be taken into the banks for getting sufficient anchorage. Any loose portion of the bank has to be removed. Provide a stone pitching for sufficient area upstream as well as downstream. Very high soil banks shall be avoided. Further, try to provide full overflow section for the bandhara. The top should have a central depression, so that, initially the flow should start from the central portion and as the flow increases it should spread all over the section. During floods all the valves at the bottom shall have to be opened. It will carry silt and large quantity of water, which will flow downstream.
This is the part, which has to be carefully attended to during maintenance. Removal of silt and protection of banks has to be carefully attended to. It will increase the life of bandhara.
EXPECTED STORAGE
When the bandhara top is above the bed surface (1.5m), then some water has to be stored in the nalla/river bed. If the slope of bed is known then the length of stored water can be known e.g.
Assume, 1 = length of bed
b = width of bed
h = height of bandhara, then,
storage = 0.5*b*i*h
Assume, b = 150 m, 1 = 1500 m, and h = 1.5 m, then,
Storage = 0.5*150*1500*1.5 = 168750 cum = 168.75 ML
There are number of other factors which affect the storage quantity. Cross-section of bed width, depression in the bed and outcrops in the, bed strata, etc., has influence on the possible stored quantity.
Thus, when bandhara is full and overflowing assume about 150 ML of water is stored. When overflow stops depletion starts. Depletion may be due to pumping, evaporation and by seepage. Still, the available water can be used for pumping the storage builds up confidence in the maintenance of water supply system.
For urban water supply scheme this storage can be calculated in terms of days of city water supply. For population of 50000 souls, at the rate of 70 lpcd, daily demand is 3.5 ML. Storage in the above example will be adequate for 45 days minimum.
When bandhara is full, the surface area is 1500*150 = 225000 sq. m, and storage acts as large settling tank. Instead of increasing the height of bandhara, in the natural drain, it is always better to construct a small bandhara upstream beyond the possible maximum spread of the existing bandhara. Such series of storages upstream, will be giving natural background to the river course. If the water is flowing in the river, then it will create series of storages in the bed. In case of letting out water in the river course through a dam, then also water utilization through bandharas downstream is justified.
Some of the water is utilized for agricultural irrigation purposes. These are the system losses. However, the green area development is an asset. Water stored has multiple uses and benefits. These benefits are interactive. All concerned indirectly contribute to continue the flowing conditions of natural stream. In short, the environment is changed by construction of bandhara.
Underground bandharas of various types have been constructed at Yavatmal, Amaravati and Satara districts. Large rivers such as Krishna, Wardha, Painganga , Pus, Bembla, have been crossed successfully by RCC, masonry bandharas. Similarly, nallas small rivers have been crossed by masonry, soil bandharas. Overall total more than 50 bandharas have been constructed using the site conditions. One Arch type bandhara on Arunavati river has been successfully constructed. Everywhere the experience was different. However, every bandhara was useful to the water supply system. While constructing new concepts were added. Confidence level for all the staff was high. Funding was mostly done by the Collectorate from the scarcity funds. Problems tried by construction of bandharas were completely solved and they did not crop up again. In other districts MJP did not contribute in construction of bandharas to a larger extent. But this experience was shared to many people and canvassed for the benefits. A large size river crossing within a very short time of about three weeks is a challenging job. It is a scheduled work and time bound activities are required to be planned. Dewatering problem was not a problem. It was better to plan for diversion than pumping. Hence, it was economical and it is worth noting that none of bandhara was costing more than five lakhs in the years 1992 to 2000. That is why it could be accommodated in the scarcity funds.
Underground bandharas are constructed to arrest the underground water currents and also create a barrier to hold water upstream in the subsurface. It is provided as a conservation measure. They are economical structures. They are not expected to flood the upstream ground surface. They create a potential difference in subsurface water levels. Underground filtration is also expected. Ground water level is also improved. Scarcity may or may not be required; it creates a positive interaction of source improvements.
Combination of underground bandhara and above ground barrier can be done. Both these structures can be in one line or having horizontal distance in between. Above ground structures are built across the natural drain and try to arrest the surface water. It has a chance to percolate water downstream in the subsurface. When the underground water is pumped out of the system from the wells, recharging takes place and water currents are established.
Apart from increase in water quantity in soil, quality is also improved. Further soil movement along with current is restricted.
SOIL BANDHARA
Underground bandharas can be constructed in soil, cement concrete and/or in masonry. In case of soil bandhara, however, excavation across the stream has to be done up to hard strata. Bottom width has to be designed for depth of excavation up to hard strata. This depth is normally found to be 3 to 4 m. Accordingly, base width for soil bandhara comes to minimum 3 to 4 m. Section at the top can be minimum 1 m, which is about 0.3 m below the bed level in natural drain.
For better and stable construction soil filled in plastic bags of cement can be used and constructed in fashion of brick masonry, is recommended. For protection from sides and top two layers of brick on edges shall be provided. Approximate cost of this type of construction works out to Rs. 5000/- per meter length of underground bandhara.
Soil bandhara can be constructed across a nalla or small size river. Time available for construction is about 30 days in the summer, when water level is very low in the natural drain and dewatering involved can be minimum. Similarly, dewatering during construction can be minimized by diverting the flow, rather than pumping out water. Practically, no skilled labour is involved for construction. Local labour can do the job satisfactorily.
Barrier constructed across the natural drain causes resistance to natural full flow of water in the rainy season. Flow carries silt and it will try to accumulate along upstream side of underground bandhara. It will exert pressure on the section of bandhara and this will be maximum at the center. The decided section should withstand the pressures. Further, on downstream side there can be a void, created due to barrier. In this void on rear side a scouring is expected. But after one season the conditions get stabilized. Effects are seen immediately in same season. Water retention upstream becomes effective and becomes useful during summer period.
If the soil bandhara is constructed without packing black cotton soil in plastic bags, then there are chances that soil can get displaced and effects of bandhara are not seen in the consecutive years.
Black cotton soil in wet conditions swells and it exerts pressures on the edges and it tries to come out of the confined position. It is therefore necessary to use plastic empty cement bags for the black cotton soil. Similarly the trapezoidal section of the soil bag masonry has to be retained in shape by providing brick on edge packing in two layers.
MASONRY BANDHARA
In case of masonry bandhara length of bandhara is more important in design. At every 2 m to 3 m distance along length a pier like structure should be provided to break the continuity. Size of pier should be distinctly more than the wall thickness. For a depth of 4 m, masonry thickness should be minimum 2 m at the base and then it can be tapered up to the top. Minimum top width should be 60 cm and should be walk-able. Normally, masonry bandharas are taken to 1 m to 1.5 m above the bed surface. This ensures impounding of some surface water. For occasional draining of water flanged pipe pieces are required to be provided. On the flanged ends sluice valves shall be fitted or blank flanges can be fitted. These can used to remove the accumulated silt near the upstream base of bandhara.
As masonry bandhara is designed to impound water, it will be replenished continuously by the surface flow. If surface flow is in excess of any drawal from the system then water will overflow from the artificial barrier created. Downstream structure has to be designed for the overflow. A small height bucket has to be constructed along the downstream edge so that its energy gets dissipated. This arrangement protects the foundation. Also, it smoothens the downstream flow.
Impounded water percolates underground and recharges the supply wells. This action ensures the availability of water in the summer period. Natural underground water currents are established. Thus by construction of bandhara, localized raising of surface water is ensured and all its advantages are deployed.
When there is a natural ponding in the stream, some impervious barriers exist downstream. Some of rocky outcrops also rise above surface across the stream bed. The geography has to be studied carefully and the advantage can be taken to join these rock outcrops by constructing masonry to create a bandhara just above the existing natural underground bandhara. This is the most economical proposal of constructing masonry bandhara. The joints have to be carefully constructed. For having substantial anchorage in the harder strata, bores can be drilled and steel anchor bars can be provided and grouted with cement mortar.
Constructing masonry and soil bandhara is choice as per the available site conditions. Masonry bandharas are slightly costlier as far as per meter cost is concerned. But these are semi-permanent structures and can be relied upon. When soft rock is available at nearer depths masonry bandhara is appropriate choice and in case of soft strata for deeper depths is available then, soil bandhara is a better choice. Similarly when banks are not sound soil bandhara is a better proposal.
R.C.C. BANDHARA
Wherever masonry is proposed RCC bandhara can be proposed. RCC bandhara becomes economical due to its design. The sections are less as compared to masonry sections. Designed as couterforts, the structures are really sturdy. Fixtures, such as pipes and valves, are easily adoptable. Further construction of downstream water bucket is also easy. As far as economics is concerned per meter cost of RCC is slightly less than that of masonry bandhara. Composite RCC and masonry structures in case of joining of out-crops, can be thought of because they are further economical.
PROTECTION OF BANKS
It has been observed that the floodwater has a tendency to damage the banks. If these banks are of soil, they are likely to be cut by the floodwater. These banks need to be protected. Masonry or RCC bandhara structure has to be taken into the banks for getting sufficient anchorage. Any loose portion of the bank has to be removed. Provide a stone pitching for sufficient area upstream as well as downstream. Very high soil banks shall be avoided. Further, try to provide full overflow section for the bandhara. The top should have a central depression, so that, initially the flow should start from the central portion and as the flow increases it should spread all over the section. During floods all the valves at the bottom shall have to be opened. It will carry silt and large quantity of water, which will flow downstream.
This is the part, which has to be carefully attended to during maintenance. Removal of silt and protection of banks has to be carefully attended to. It will increase the life of bandhara.
EXPECTED STORAGE
When the bandhara top is above the bed surface (1.5m), then some water has to be stored in the nalla/river bed. If the slope of bed is known then the length of stored water can be known e.g.
Assume, 1 = length of bed
b = width of bed
h = height of bandhara, then,
storage = 0.5*b*i*h
Assume, b = 150 m, 1 = 1500 m, and h = 1.5 m, then,
Storage = 0.5*150*1500*1.5 = 168750 cum = 168.75 ML
There are number of other factors which affect the storage quantity. Cross-section of bed width, depression in the bed and outcrops in the, bed strata, etc., has influence on the possible stored quantity.
Thus, when bandhara is full and overflowing assume about 150 ML of water is stored. When overflow stops depletion starts. Depletion may be due to pumping, evaporation and by seepage. Still, the available water can be used for pumping the storage builds up confidence in the maintenance of water supply system.
For urban water supply scheme this storage can be calculated in terms of days of city water supply. For population of 50000 souls, at the rate of 70 lpcd, daily demand is 3.5 ML. Storage in the above example will be adequate for 45 days minimum.
When bandhara is full, the surface area is 1500*150 = 225000 sq. m, and storage acts as large settling tank. Instead of increasing the height of bandhara, in the natural drain, it is always better to construct a small bandhara upstream beyond the possible maximum spread of the existing bandhara. Such series of storages upstream, will be giving natural background to the river course. If the water is flowing in the river, then it will create series of storages in the bed. In case of letting out water in the river course through a dam, then also water utilization through bandharas downstream is justified.
Some of the water is utilized for agricultural irrigation purposes. These are the system losses. However, the green area development is an asset. Water stored has multiple uses and benefits. These benefits are interactive. All concerned indirectly contribute to continue the flowing conditions of natural stream. In short, the environment is changed by construction of bandhara.
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