Flow Duration Curve

A graphical representation of runoff is called a flow duration curve. It is denoted by FDC. It relates flow rate with duration.

At first, data like discharge and time are calculated. Then the Flow Duration Curve is plotted which is used to learn or study floods, evaluating many dependable stream data, designing of drainage works, etc.  In the FDC:

  • Discharge is plotted on the y-axis(ordinate).
  • Percentage of time duration is plotted on the x-axis(abscissa).

 

flow duration curve

Steps for drawing Flow Duration Curve

  1. Calculate the total number of data, say N.
  2. Give rank to the data for the data 1,2,3,…., N ; say n.
  3. Compute frequency(f)
    f= total number of data(N) / Rank(n)
  4. Compute probability of ecceedance(p)
    p= 1/f
    p= n/N
  5. Now place the discharge in descending order.
  6. Draw the probability of exceedance or % of the time versus discharge.
    This curve is the FDC.

Uses of FDC

  1. FDC helps to evaluate low-level flows.  Flow relating to any % of the time can be obtained by using FDC.
  2. Planning and Designing of the hydropower project. In hydro power project; firm power is calculated using flow duration curves. In the case of projects without storage  (ROR) flow available in 90% to 95% of the time.
  3. The total amount of sediment transported by the river can be found out if FDC along with sediment rating curve is available.
  4. FDC also provides runoff variability of the stream.
    – If the curve has a steep slope throughout: This indicates that the stream has highly variable discharge. This is the typical conditions where the flow is mostly surface runoff.
  5. It is used for designing drainage systems.
  6. It is in flood control studies.
  7. It is used in the calculation of the sediment load and dissolved solids load of a stream.
  8. It is used for comparing streamflow data.
  9. It is used for evaluating many dependable flows in water engineering projects.
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