Mass Curve

• A mass curve is a plot between the cumulative volume of water that can be stored from stream flow and the time in day, weeks or in months. 
• Such a plot helps in designing the size of the storage required for hydro-electric power plants. Also, with the help of mass curves it is possible to predict the maximum possible uniform flow for a given storage capacity and the periods of low stream flows. 
• The mass curve can be expressed mathematically as :

Where, V = Volume of run off Discharge in m/s. A typical mass curve is shown in Figure A. It gives following information :

• The slope of curve at any point indicates the rate of flow of water at any point of time. If the curve is steep, it shows high flow rate of water, if horizontal then flow rate of water is zero. If the curve is concave it shows that period of dry days i.e. in flow of water is less than evaporation.

• The rain water is only available for certain period of time. Due to this run-off is deficient during the large part of the year. 
• Thus, to meet the certain power demand, an artificial storage of water is necessary in the form of pondage and storage to meet and regulate the requirement of water during deficient days. It helps in meeting the power demands. 
• A pondage is defined as small pond or reservoir built near the power house to regulate the small quantity of water. 
• It meets the water requirement for meeting the hourly changes in power demand. 
• While, the storage is built in the form of dam across the river, whose function is to store very large amount of water during excess run-off period (rainy days) and supply this water during the deficient period of run-off.

• Thus the storage increases the plant capacity to generate more power. The storage capacity of dam can be determined with the help of hydrographs or mass duration curves. 
• The power house with pondage is shown in figure B. The pondage regulates the variable water flow to meet the power demand. 
• It is required to meet the short term fluctuation of water requirement to cater the requirements of : 

(a) Sudden small changes in load on power plant. 
(b) Sudden changes in inflow of water. 

• The pondage is located at suitable site near the power house as shown in Figure B. A weir is provided across the river and water from the river is directed through the water channel to pondage and to the power house through small penstock (water feeding pipes). 
• Figure C shows the location of storage reservoir. It is built across the dam to store huge quantity of water. 
• It increase the capacity of river from 6 months to 2 years depending on its size while the pondage increase the capacity of river for a very short time.