Estimation of Surface and Sub-Surface Water Resources

ESTIMATION OF SURFACE AND SUB-SURFACE WATER RESOURCES

1. Natural Ponds and Lakes

2. Streams and Rivers

3. Impounding Reservoirs

 

1. Natural Ponds and Lakes as surface sources of supplies

Natural large-sized depression formed on earth surface, when filled with water is called a pond or lake.

Surface runoff from nearby catchment area drains water into lakes

Sometimes, small springs also drain underground water into ponds and lakes.

 

Quality of water in Lakes:

• Good quality

• There is no need of much purification

• Self purification of water occurs in lakes due to:

* Sedimentation of suspended matter

* Bleaching of colour

* Removal of bacteria etc.

• Larger and older lakes are more purer.

• As lakes are still and standing waters, it contribute to growth of algae and aweeds, imparting bad smell, taste, colour to waters.

Quantity of water

• Small quantity of water

• The quantity depends on  - Catchment area

- Annual rainfall

- Geological formation

• Lakes cannot be used as a principal source of water supply.

• Lakes are useful for small towns and hilly areas only.

• When no other sources are available, lakes may be used for water supply.

• Example - Water supply to Bombay city is from lakes.

 

2. Streams and Rivers as surface sources of supplies

(a) Streams

* As the quantity of water is less, it is not suitable for water supply schemes.

* They are useful for small villages and hilly areas.

* However, large perennial streams may be used as sources of water by providing storage reservoirs.

(b) River 

* Most important source of water for public water supply.

 * Rivers may be perennial or non-perennial.

* In perennial rivers, water is available throughout the year.

* Perennial rivers can be used as a source for water supply.

Quantity - Good source of water supply to cities mostly located nearby the rivers.

Quality - Water quality is not reliable.

* River water has large amount of silt, sand and suspended matter.

* The disposal of sewage into river contaminates the waters.

* Proper treatment is required for river waters before supply to public.

 

3. Storage reservoirs / Impounding reservoirs as surface sources of supplies

The rivers or streams when directly used for water supply schemes fail due to the following problems.

• During low flows (dry season), it cannot meet the demand of the consumers.

• During high flows, it leads to devastating floods. Hence, it is necessary to build a barrier or dam or storage / impounding reservoirs at the upstream of rivers to store the excess water (during high flows) and supply to public (during low flows).

An Impounding Reservoir is an artificial lake created by connstruction of dam across a water course.

The objective is to impound (store) water for water supply schemes.

Parts of an Impounding reservoir

 (i) Dam to hold back water

(ii) Spillway to discharge excess flow

(iii) Gate valves to regulate water flow

Location of impounding reservoir is selected based on the following factors

(i) Existence of suitable dam site - short and narrow dams.

(ii) Quantity of water available should be sufficient to meet the demand throughout the year.

(Rainfall, runoff, catchment area characteristics.)

(iii) Distance and elevation of reservoir with reference to point of distribution.

Longer distance - cost will be high

Higher elevation – cost is low due to gravity flow. Pumping is not required

(iv) Density and distribution of population over the catchment area. If the population is less, the pollution of streams will be less.

(v) Existence of towns, highways, railyards, cultivable areas.

(vi) Geological conditions of storage basin.

Existence of calcareous bed rocks may cause hardness to water.

Existence of fissures may lead to water loss by percolation.

Selection of dam site

The selection of a site for constructing dam is governed by following factors:

(i) Suitable foundation.

(ii) Length of dam should be small with maximum storage volume. The river valley should be narrow with large upstream basin for reservoir.

(iii) Bed level of dam site should be higher than river basin.

(iv) Construction materials should be locally available.

(v) Reservoir basin shuld be water tight

(vi) Land value should be low

(vii) Accessible and well connected by roads, rails to towns and cities.

(viii) Labour easily available.

Storage capacity of reservoir

It is determined from the Contour maps of the area (Topographic survey)

The volume of water that can be stored by the reservoir at a certain water surface elevation can be computed by summing up the increment between two elevations.

In the absence of topographic maps, capacity can be determined using, formulae.

Catchment yield = annual inflow to the reservoir

Reservoir yield = amount of water drawn from the reservoir

Storage capacity = Inflow = Outflow

Quality of water in impounding reservoir - similar to that of natural lakes

Quantity of water in impounding reservoir