RESERVOIR PLANNING BY S N NANAWARE Visit for

RESERVOIR PLANNING BY S. N. NANAWARE Visit for more Learning Resources

Reservoir Planning For the best and efficient design of Irrigation project the investigation is must so it is carried out in three stages Surveys for Irrigation project : 1. Reconnaissance Survey 2. Preliminary Survey 3. Detailed Survey / Construction Survey

Reconnaissance Survey : In reconnaissance survey the find out number of project sites where storage of water can be done. In this survey inspection of region is done and data collected physically without using precise survey instrument. In reconnaissance survey following information is collected. The size of catchment area, vegetation in the catchment area, soil type, slopes, existing ponds etc

Preliminary Survey : This survey is carried out by experienced supervision with modern instruments it may take few month to one or two year depending on size of project. It is carried out in dry season with careful planning. Preliminary survey divided into parts 1. Engineering survey 2. Hydrological survey 3. Geological survey

1. Engineering Survey : In this type of survey various types of surveys are carried out such as Plane table survey, Traverse survey, aerial and photographic survey etc. the aim of this survey is to prepare a contoured map of the site. Contour map gives the following information. 1. Water spread. 2. Capacity of the reservoir. 3. Suitable dam site. 4. Site for waste weir and outlets.

2. Hydrological Survey : Whenever any storage work is constructed, it is very essential to ascertain that required amount of water will be available at the site for storage. Therefore hydrological survey is of catchment area is done to collect the data for determining availability of water for the reservoir. It is necessary for 1. To study runoff pattern and to estimate yield 2. To determine maximum discharge. 3. Determination of hydrograph of worst flood to determine spillway capacity and its capacity.

3. Geological/Exploration Survey : It is done in two stages i) Survey inspection and surface mapping. ii) Sub surface exploration gives information 1. Water tightness of the basin 2. Stability of foundation for the dam. 3. Faulty structure in the basin. 4. Type and depth of overburden. 5. Ground water condition of region. 6. Location of quarry sites for construction materials.

Detailed Survey : After the preliminary survey only a few sites are selected for further consideration. In this type of survey office studies and estimates for each of the selected site is prepared. Detailed designs of various storage structures are prepared in the drawing office and the cost is worked out. So the relative merits and demerits of two or more sites are take into consideration so as to adopt one site finally.

Points to be considered for the site selection of reservoir : 1. Stable foundation for the dam should be available. 2. At the selected site, the river valley should be narrow so that the length of the dam may be short. 3. The proposed reservoir basin should be water tight and free from cracks, fissures so that there is loss of water due to percolation. 4. The reservoir basin should not submerged valuable land properties.

Points to be considered for the site selection of reservoir : 5. The tributaries which carry high content of sediments should be avoided. 6. The size should be such that the optimum storage capacity is available for the desired purpose. 7. The size should be easily accessible by road and railway. 8. The construction materials should be available in nearby area.

Data to be collected for irrigation project : The preliminary investigation consists of 1. Approximate quantity of water. 2. Period of deficient water supply. 3. Technical feasibility of project such as i) command area, ii) nature of soil, iii) types of crops, iv) demand for water at present and in future. 4. Approximate cost of all proposed works.

Data to be collected for irrigation project : The detailed investigation consists of 1. Soil survey 2. Field work for availability of water 3. Gauging of river 4. Fixing dam site 5. Availability of materials of construction 6. Sedimentation problem 7. Contour survey of reservoir basin 8. Types of dams 9. Alignment of main canals

Area Elevation and Storage Elevation Curve ( Area – Capacity Curve ): -

Area elevation curve: From the contour plan of reservoir site the area of successive contours A 1, A 2, A 3………An are determined with the help of planimeter. The area A 1 is minimum at the bottom of the basin and area An is maximum at the top of the basin. This curves gives the area submerged at different elevation.

Storage - elevation or volume elevation curve: The volume of the storage of water between successive contours is calculated by prismoidal formula. The volume of storage between successive contours, goes on increasing from bottom of the basin towards its top.

Silting of reservoirs : Silting of reservoirs means the deposition of silt and clay. This mainly occurs when runoff occurs Runoff contain smaller soil particles such as silt clay, fine sand which may deposited at the bed of reservoir which reduces the storage capacity of reservoir. When these particles reaches into reservoir tries to settle down due to action of gravity, coarser one settle down while fine remain suspension therefore it is very essential to check silting of dam and water stored in reservoir.

Method to reduces the entry of silt: 1. Selection of proper dam site such that catchment area has less erodable portion. 2. Constructing the dam in stages. 3. Providing vegetation screens. 4. Proper reservoir design causing silt water to escape. 5. Dredging of silt.

Factor affecting Silting : 1. Catchment Area : - In case of fan shaped catchment the amount of deposition of finer soil fractions will be more as compared to fern shaped catchment. 2. Slope of country : - If the slope is steep more particles are deposited through runoff 3. Climatic condition : - climatic condition also affect the production of silting like dryness, rain, heat helps in production of silting. 4. Nature of surface soil : - If soil is loose, silting is more along with runoff.

Control Level in Dam : -

Control Level in Dam : 1. Top Bund Level (TBL) : - This is RL of dam which depends on capacity, storage flood lift & free board. The height of dam at any section is difference between TBL & GL. 2. High Flood Level (HFL) : - It is the maximum RL upto which water can rise in the reservoir. 3. Full Tank Level (FTL) : - Water in reservoir is stored upto a level known as FTL or FRL. Usually the crest of spillway is kept at this level. 4. Lowest Water Level (LWL) : - It is the lowest RL of water surface in the reservoir. If water goes below LWL then water can not be drawn out of the reservoir.

1. Dead storage: - The storage below lowest sill level (LSL) can not be used and hence it is called as dead storage. LSL is a upstream sill level is kept 50 cm to 100 cm below LWL. This is generally equal to volume of silt. 2. Live storage : - the storage between LWL and FTL is called as live storage. Live storage = irrigation requirement + Tank losses 3. Gross storage : - It is the storage at FTL in the reservoir. Gross storage = Dead storage + Live storage 4. Free Board : - It is the vertical distance between TBL and HFL. It is clear margin kept as a measure of safety so that wave form should not cross dam. For more detail contact us