Hydrographs
Hydrographs are the plots between the river discharge in me’ls and the time. Hydrograph may be plotted on daily, weekly, monthly or yearly basis.
It can predict the availability of power from the stream at different time of a day during the year . Hydrograph also help in assessment of flood flow of rivers and its effect on storage. Therefore, the following information may be obtained from hydrographs
- Rate of flow at any time during the year.
- Volume flow rate of water.
- Average stream flow per month.
- Maximum and minimum stream flow per month.
- The rate of stream flow, duration and frequency of floods.
- Availability of head.
Figure A shows the typical hydrograph of a steady flow river on yearly basis and Figure B shows the flow duration curve which is the rearrangement of all the run-off of a hydrograph in descending order. The flow duration curve indicates the number of days the run off in m3/s is available.
The flow duration curve from hydrograph can be drawn as follows :
- Cut the hydrograph in number of horizontal strips.
- Point A represents the maximum flow rate of water for zero hours.
- Now take various values like a, b, c, d and e at various values of run-off. Measure days a, b, c, d, e having the intercepts at run-off rate corresponding to A, A2, A3 and A4.
- Plot at run-off A, the no. of days (a), A2 having days (b), A3 having (c + d) days and so on as shown in Figure B.
- The locus of all the points A, A1, A2 —etc represents the flow duration curve.
The flow duration curve shows the period of low volume flow rates & the period of minimum water flow rates which helps in designing of power capacity of power plants. It also helps in requirement and design of drainage system and in flood control studies.