Suitability of Surface Water for Use | Surface Water | Water Engineering

In this article we will discuss about the suitability of surface water with regard to quantity and quality.

Rainfall directly affects the quantity of surface water. As the rainfall is not uniform throughout the year, the quantity of surface water also has large variations. The discharge in rivers and streams remains maximum in rainy season and minimum in summer.

If the quantity of water in summer is not sufficient to meet the demand, it should be stored in imponded reservoirs. The site for the dam or imponded reservoir should be very carefully selected. In hilly areas having large lakes, the construction of artificial reservoirs is not necessary.

Surface water mostly contains large amount of impurities in both suspended and dissolved form. Surface water is contaminated by the impurities while travelling on the ground. The suspended impurities contain disease-producing bacteria, therefore, surface water should not be used before treatment.

In lakes and reservoirs, the suspended impurities settle down in the bottom, but in their beds algae, weeds, vegetable and organic growth take place, which produce bad smell, taste and colour in water. Therefore, this water should also be used after purification.

The sewage of towns and cities situated near the rivers is also discharged in the rivers, which pollute the river water up to certain length. Therefore while taking water for water supply purposes; intakes should always be installed in up-streamside, which is free from contamination due to sewage. When water is stored for long time in reservoir it should be aerated and chlorinated to kill the microscopic organisms which are born in water.

Reservoir Storage Capacity:

The capacity of the storage reservoirs depends on the rates of inflow, losses and demand or outflow. For determining the capacity of the storage reservoir to be constructed, these are to be calculated first.

The inflow and outflow values are to be determined for various months of the years. The deficits and surpluses of water are calculated, and the storage capacity is made equal to the total deficit. For remaining on the safer side the dry year in which the inflow was minimum and the outflow was maximum is generally chosen for this purpose.

Practically, it has been noted that sometimes the drought conditions continue for longer periods, therefore, the storage capacity must be sufficient to meet the requirements in the worst possible conditions. For the design or calculation of reservoir storage capacity the records of at least previous three consecutive years should be collected.

Following example clearly illustrates the method of determination of storage capacity:

Example:

Table 3.1 gives the yield from various sources in the reservoir, losses due to evaporation, percolation etc. and the estimated consumption of water by the town. All the figures are in millions of litres per month. Determine the storage capacity of the proposed reservoir.

After thoroughly examining the Table 3.1, following points will be observed:

Table 3.1. Method of determining the storage capacity of the proposed estimated consumption of water are given. (All quantities of water reservoir when yield from various sources in the reservoir, all losses and are in millions litres/month)

Table 3.1

1. In the month of June of first year total estimated consumption of 0.0863 millions litres/month is more than the total yield of this month (0.0385 millions litres/ month). The deficit shall be met from the surplus of the storage reservoir.

2. From July to June of the first year the total surplus is 1.3837 million litres and the total deficit is 0.4961 millions litres, which can be stored in the storage reservoirs.

3. From the Table 3.1 it is clear that the second year is drought year and the surplus is 0.2997 million litres, whereas the deficit is 0.7221 million litres. This situation can be saved due to carry over of 0.4961 million litres saving of the first year. Therefore, the minimum storage capacity of the reservoir should be 0.3224 say 0.33 million litres to meet the dry weather demand.

4. Third year is also the wet year in which the total yield is more than the demand.

Table 3.1. also shows the months when the filling, emptying or overflow of reservoir will take place.

Effect of Valley Storage:

In case of large capacity reservoir for conditions of normal flow, no serious error is introduced if it is assumed that the hydrograph of inflow is the same as the hydrograph at the dam site. The effect of valley storage must be taken into account while considering the floor-control reservoirs, where the valley storage during flood is of significant proportions with the reservoir storage volume.

The volume of water stored by a stream within its banks may be small in relation to the volume of water during flood, when the stream is out of banks. The effect of change in timing after the construction of the reservoir is very important, and it may effect a coincidence of peak reservoir outflow with peak downstream tributary inflows. Such critical condition may require the provision of additional storage for avoiding such possibility.

While applying valley storage correction, its volume is determined under various conditions. The reservoir is divided into various sections and the volume of each section is determined for various water surfaces for various water discharges. In flood control reservoirs the effect of valley storage is very important.

As it is uneconomical to provide storage for the entire flood volume, because it is necessary to discharge some flow during filling period. This flow is fixed usually by channel conditions below.

Now when the rate of discharge has been fixed, the flow hydrograph, modified by discharge, determines the size of the reservoir. Generally when the effect of valley storage is taken into account of size of the reservoir is about 30% more, than its size, if the effect of valley storage is not taken into account.

Reservoir Sedimentation:

Deposition of all the materials is the bed of the reservoir, transported by flowing water is known as the sedimentation. The soil particles eroded by the water from the ground surface, are generally carried by the water to the reservoir where they settle in its bed.

The problem of sedimentation is very serious in arid regions where the ground is not covered or supported by good vegetable. The results is excessive erosion by heavy rains. The vegetable cover in humid climates prevents excessive erosion.

The percentage of sedimentation carried by the streams varies from stream to stream and on the characteristics of the catchment area. The sediment particles try to settle in the bottom of the river due to gravitational force.

Some particles remain in suspension position due to turbulent flow and these particles reach in the reservoir and settle down in the reservoir bed. Very fine sedimentation particles, which could not settle, may escape from the dam through turbines, sluiceways, spillways etc. The deposition of the sediments is commonly known as silting.

As the quantity of silting increases, the capacity of the reservoirs reduces. If the silt is not removed from the reservoirs, its quantity will continuously increase and will make the reservoir useless in longer course of time.

During the design of reservoir sufficient allowance is made for silting. When the quantity of the silt reaches the designed volume, it should be removed. The allowance for silt storage may be as high as 1/4th the total capacity of the reservoir.

Control of Sedimentation:

It is very costly to remove the sediment silt by means of mechanical methods. It is common practice to remove the silt by sluices. But removal of silt by sluices is not very effective. If large gates are provided near the bed of the dams and flooded water is allowed to pass through it, a large portion of the silt can be removed.

The most effective method to control sedimentation is by watershed control. In this method the ground surface and the river beds are controlled, that minimum possible suspended particles should enter the reservoir.

To control the entry of suspended particles in the reservoir, special check dams and debris barriers may be constructed at suitable places. The catchment area surface should be covered with grass, vegetable and other special variety of trees which would prevent scouring and erosion of ground particles.

Following are the various methods, which can be adopted in reducing the quantity of silt:

(i) Dam site should be selected at such a place, that its catchment area should be less erodible.

(ii) The construction of the dam should be done in stages. When the storage capacity will be less in the beginning, large quantity of silt will be washed along with the surplus overflowing water. Thus will reduce the silting rate.

(iii) Under-sluices should be constructed in the Dam, to remove the silt of the reservoir.

(iv) The vegetation screens should be provided at suitable places to check the entry of suspended particles in the reservoir.

(v) As the sediment content increases just after the flood, as far as possible this water should not be collected in the reservoir.

(vi) If possible the sediment should be kept in suspended particles will flow out through the sluices.

(vii) Attempts should be made to grow vegetables, trees, grass etc. which will bind the soil particles of the ground, and will prevent their erosion.