Hydrology and Water Resources Engineering

HYDROLOGY AND

WATER RESOURCES ENGINEERING

• Reservoir Types, Investigations, Site selection, Zones of storage, Safe yield, Reservoir capacity, Reservoir sedimentation and control.

Syllabus

Reservoir

• A Reservoir is a artificial lake or impoundment from a dam which is used to store water.

• Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.

Reservoir

Purpose of Reservoir

• The Storage reservoir is formed for the following purpose:

• Flood Control• Irrigation• Water Supply• Hydroelectric Power Generation• Development of fishery• Navigation• Soil Conservation

Flood Control

Irrigation

Water Supply

Hydro Electric Power Generation

Fisheries

Navigation

Soil Conservation

Reservoir may be designated by.

• Single Purpose Reservoir: This type of reservoir is formed mainly to serve a single purpose, such as irrigation, flood control, water supply, etc.

• Multipurpose Reservoir: This type is formed to serve many purpose such as,

• Irrigation and water supply.• Irrigation water supply and flood control.• Irrigation, water supply, flood control,

hydroelectric power generation, fishery, etc.

Reservoir

Classification of Reservoir

Storage Reservoir• Storage Reservoir The storage reservoir is formed

by constructing a dam across a river valley. The idea of constructing such a reservoir is to store the excess water which flows through the river during the high floods or rainy season. This stored water is then utilized for various purposes, such as irrigation, water supply, fishery, hydroelectric power generation, etc. Again the storage reservoir may be named as single purpose reservoir or multipurpose reservoir according to its utility.

Storage Reservoir


• Flood Control Reservoir: Flood control reservoir are for are formed by constructing dams at suitable places in the catchment area or river valley to arrest the flood water temporarily so that the downstream area may not get damaged by sudden high flood discharge. The arrested water is then allowed to flow or released gradually without causing any harm to the low lying area on the downstream side. This type of reservoir is designated as single purpose reservoir. The flood control reservoir can be of two type:

Flood Control Reservoir

Classification of ReservoirDetention Reservoir • In this type of reservoir, the spill ways with adjustable gates

provided with the dam so that the flood water may be detained for sometime and then released according to the situation of the downstream area by operating the gates of the spillways.

Retarding Reservoir • In this type of reservoir spillways are provided with the dam

at such a level an capacity so that the flood discharge is retarded and it takes long time for the flood water to flow completely towards the downstream area. The discharge stops when the water level falls below the crest of the spillways.

Detention Reservoir


Distribution Reservoir: • The distribution reservoir is not formed by

constructing a dam across a river or valley. It is constructed by masonry work or concrete work in the form of a rectangular or circular tank at suitable places near the town or city. The water from the river or lake is pumped into this reservoir and stored there for supplying to the consumers of the town or city. The water may be supplied to consumers by pumping system or gravity system.

Distribution Reservoir

Site Selection for Reservoir• The following points should be remembered while

selecting a site for a reservoir.• Stable foundation for the dam should be available

where the reservoir basin is proposed to be formed.• At the selected site the river valley should be narrow

and well defined so that the length of the dam may be short.

• The proposed reservoir basin should be watertight and free from cracks, fissures, etc. so that there is no loss of water due to percolation.

• The reservoir water should not submerge valuable land or property.

Site Selection for Reservoir• The tributaries which carry high content of sediment

should be avoided.• The site should be such that a deep reservoir may be

formed with minimum surface area to control the loss of water due to evaporation.

• The site should be easily accessible by road or railway.• The construction material for the dam and other allied

works should be available in the vicinity of the site.• Suitable area should be available for labour colonies,

staff quarters, godowns, stack yards etc.

Site Selection for Reservoir

Investigation works for the Reservoir

• The following investigation should be carried out for planning the storage reservoir.

• Engineering Survey • The area of the dam site and reservoir basin should be

surveyed thoroughly to prepare topographical map and contour map. From the contour map the storage capacity of the reservoir should be fixed by ascertaining the height of the dam. Thus, the valuable land and properties which may get submerged in the reservoir can be found out. So, the amount of compensation to be paid to the owner can be estimated. A detailed statement may be prepared for the land acquisition.

Engineering Survey


• Geological Survey: It should be carried out to determine the following informations:

• (i) Dam foundation: The sub-surface exploration at the dam site should be carried out to locate the cracks, fissures, etc, which are responsible for the percolation loss. Necessary measures can then be recommended for percolation zones to control the losses.

Investigation works for the Reservoir• (ii) Characteristics of reservoir basin: The geological

survey of the basin should be carried out to locate the cracks, fissures etc. which are responsible for the percolation loss. Necessary measures can then be recommended for percolation zones to control the losses.

• (iii) Availability of Construction Materials: Huge quantity of materials like fine aggregate and coarse aggregate are required for the construction of dam and allied structures The place where these materials are available should be located and they should preferably be near the site. If the quarries and sand pits are situated in the vicinity of the project, it reduces the cost of the project.

Geological Survey


• Hydrological Survey: This survey includes the collection of following data:

• River gauging: River gauging should be carried out for the main river and its tributaries in the catchment area and on the upstream side of the site. This is to estimate the probable discharge of the river throughout the year.


• Rainfall records: The rainfall records from the rain gauge stations in the catchment area for the past few decades are collected and the runoff is estimated. Then the hydrographs are prepared from the high flood discharge for the previous decades. The probable flood discharge for the future decades are ascertained from the graphs.

• Determination of discharge from empirical formulae: The peak flood discharge may also be determined by empirical formulae like Ryve’s formulae, Dickens's formulae, etc. The number and capacity of spillway are determined according to the available highest flood discharge from different observations.

Hydrological Survey

Yield of Reservoir

• Yield The yield of reservoir is defined as the amount of water which is available in the reservoir at a specified interval of time.

• Safe yield: It is the maximum quantity of water available in the reservoir during critical dry period.

Yield of Reservoir

Capacity of Reservoir

• Capacity The capacity of a reservoir is defined as the total volume of water that can be stored in it. The capacity of a reservoir may be determined by the following methods.

• By Mass Curve• By Actual Discharge• By Empirical Formulae

Capacity of Reservoir

Zones of Storage Reservoir• The storage capacity of a reservoir is designated by several

zones which are demarcated by certain pools.• Normal Pool Level (N.P.L) It is the maximum elevation

of the water surface which is to be stored in the reservoir during normal working period. The water above this level will flow over the spillway crest. This water level is also known as full reservoir level (F.R.L)

• Maximum Pool Level (M.P.L) It is the maximum elevation to which the water surface is allowed to rise in the reservoir during the period of flood. This level can be controlled by providing spillway gate (M.W.L).

Zones of Storage Reservoir Minimum Pool Level: • It is the lowest elevation to which the water

level is allowed to drop in a reservoir. This level is fixed by providing outlets in the dam. In case of hydroelectric power generation, the minimum head is essential. So, the water level of the reservoir should not drop below the minimum pool level (L.W.L).

The following are the various Storage Zones:

• Dead Storage: The volume of water which is stored below the minimum pool level of a reservoir is known as dead storage. This water cannot be utilized under normal operating Condition.

• Live Storage: The volume of water stored in the reservoir between normal pool level and minimum pool level is known as useful storage or live storage. This water can be used for various purposes, such as irrigation, water supplies, fisheries, hydroelectric power generation.


• Surcharge Storage: The volume of water stored in a reservoir between the normal pool level and maximum pool level is known as surcharge storage, this storage is not meant for any work, but it is mainly for the detention of flood water so that it may not cause any danger on the downstream side.


• Valley Storage: The minimum volume of water which flows through a river before the construction of dam is known as valley storage. This water may always be available from the river even before the formation of reservoir. This water may always be available from the river even before the formation of reservoir.. So, the effective storage capacity is given by.

• Effective Storage= (Live storage + Surcharge Storage- Valley Storage).

Zones of Storage Reservoir

Causes of Sedimentation in Reservoir

• In the catchment area the soil may get eroded and rocks may get disintegrated due to various reasons. The disintegrated rocks and loose soil form the sediment which is carried by the river and gets deposited on the reservoir bed near the base of the dam. The sediment mainly consists of sand and silt. The process of deposition of sand and silt in the reservoir is designated as reservoir sedimentation.

Causes of Sedimentation in Reservoir

The following are the causes of sedimentation

• Characteristics of soil in Catchment area: If the catchment area is composed of loose soil, then it may get easily eroded and get carried away by the river. On the other hand, if the soil of the catchment area is hard and rocky, the river would not be able to carry sediment.

• Topography of the catchment Area: If the catchment area consists of steep slope, then it will develop high velocity of flow which will cause more erosion of the surface soil thereby making the river carry a lot of sediment.

• Intensity of Rainfall in Catchment Area: If the intensity of rainfall in catchment area is high, then it will increase the rate of run-off and the river will carry more sediment.

The following are the causes of Sedimentation

• Cultivation in Catchment Area: The intensive cultivation in the catchment area will make the soil loose and rain water will carry a lot of sediment to the river.

• Vegetation cover in Catchment Area: If the catchment area is covered with vegetation cover like grass, plants, forest area, etc. then the erosion of the soil will be controlled and the river will not carry sediment. If there is no such vegetal apron or cover, the soil may be easily eroded and the river will carry much sediment.

Effect of SedimentationWhen the sediment laden water of the river approaches the zone of reservoir, the velocity of flow reduces gradually and thus the heavier particles are settled down at the head of the reservoir, i.e.. starting zone of reservoir. This zone is termed as delta. Most of the sediments get deposited at this zone. The fine sediments remain in suspension for a considerable time and are carried towards the dam. These sediments are deposited at the foot of the dam. The very fine particles are carried with water as turbid flow and ultimately discharged through the outlets. The floating debris is collected above delta. The clear water circulates the middle portion of the reservoir. Thus, the life of reservoir depends on the trend of the sedimentation.

Effect of Sedimentation

Control of Sedimentation• In order to increase the useful life of a reservoir, it is necessary to control

the phenomenon of sedimentation. The following steps should be taken to control the deposition of sediment:

• Selection of a Reservoir Site The reservoir site should be selected in such a way that so that load of sediment in the flowing water from tributaries is considerably low. If it is found that a tributary carries heavy sediment, the reservoir site should be shifted towards the upstream to avoid the tributary.

• Design of Reservoir At the beginning the reservoir should not be constructed to its full capacity. Initially a reservoir of less capacity is constructed. When the reservoir gets silted, gradually the capacity should be increased stage by stage, by increasing the height of the dam.

• Sufficient outlets should be provided in the dam at different elevations so that the water carrying heavy sediment may be discharged to the downstream by opening the outlets.

Selection of a Reservoir Site

Control of Sedimentation• Removal of Sediments The sediment already deposited in

the reservoir may be removed through the scouring sluices provided at the bottom of the dam by loosening the sediments by mechanical agitator.

• Afforestation New plantations should be planned in the catchment to extend the forest area which serves as a vegetal apron to reduce the soil erosion.

• Control of Deforestation The cutting of Trees, i.e. deforestation in the catchment should be restricted by the government so that the vegetal cover is not destroyed. The deforestation may cause soil erosion and this may cause soil erosion and this may impart sedimentation load to river water.

Removal of Sediments

Control of Sedimentation• Control of Cultivation: The intensive cultivation in the catchment

makes the soil loose which may then get easily carried by the rainwater to the river. So the cultivation should be done in a planned manner and necessary measures should be taken to avoid the loose soil getting carried away by the rain.

• Control of Grazing The grazing of cattle in the catchment should be restricted and they should not be allowed in the area where the soil can get easily eroded by their feet.

• Construction of Check Dams: If the tributaries of a river are found to carry heavy sediment load, then check dams of low height are constructed across the tributaries at different stages to arrest the sediments just in the catchment area. Here, the check dams serve the purpose of detention reservoir where the heavy sediments get settled down.

Construction of Check Dams

Control of Sedimentation• Construction of Contour Bunds: In hilly area contour

bunds are constructed on the slope of the catchment at different elevations so that the heavy sediments are arrested at the base of the bunds and comparatively less turbid water passes over the bunds.

• Stabilization of Slips: Most of the tributaries of a river obtain the sediments from the land slides which are caused due to heavy rainfall in the hilly area. It is generally composed of loose soil or permeable rock formation. These slips should be stabilized by suitable methods so that the debris may not be carried by flowing water.

Construction of Contour Bunds

Reference

• Irrigation Engineering : N.N. Basak• Irrigation Engineering & Hydraulic

Structures: S.K.Garg• Internet Websites

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