In this article we will discuss about:- 1. Procedure to Separate Base Flow 2. Methods to Separate Base Flow 3. Base Flow Separation of a Complex Storm.
The flood hydrograph observed during the storm is a combination of the flood runoff and the base flow. It is necessary to study the excess rainfall and its corresponding runoff from a catchment. Hence, it is necessary to separate the base flow from the combined by hydrograph to arrive at flood runoff.
When the flood occurs, the water level in the river rises substantially. The groundwater inflow, if any, depends mainly on the difference between the level of groundwater table and water surface level in the channel. It is a major portion of the base flow. When the water level in the channel rises, this inflow may reduce during the floods and may be less than what it was at the beginning.
A typical hydrograph with base flow is shown in Fig. 2.14.
Procedure to Separate Base Flow:
All the methods for the separation of base flow follow a step-by-step procedure as indicated below:
1. Locate the point A.
2. Locate the point E.
3. Join A and E by a suitable curve.
The discharge below the line AE is the base flow and above it is the flood runoff.
Normally, locating A is not difficult because there is a sudden rise in the hydrograph from this point.
Methods to Separate Base Flow:
The different methods to separate base flow are as follows:
a. Simple Judgement:
Make a guess. Locate A and E by judgement and join these two points by a straight line (Fig. 2.15).
The discharge below the line AE is the base flow and above it is the flood runoff:
b. Equation of the Recession Curve:
Locate A- Then locate E from the following equation- t0 = 0.84 A0.2
where, t0 = Time in hours from the peak discharge, i.e. the point C to E
A = Catchment area in km2
Knowing the value of t0, E can be located. Join A and E by a straight line (Fig. 2.16).
The discharge below AE is the base flow and above AE is the flood runoff.
c. Master Depletion Curve:
Trace the master depletion curve of the catchment area on a paper and try to match it with the hydrograph in question and then locate E. Join A and E by a straight line (Fig. 2.17).
The discharge below AE is the base flow and above AE is the storm runoff.
d. Semi-Log Analysis:
Plot the recession curve including some part of the base flow on a semi-log paper (Fig. 2.18).
Since the recession curve is plotted on a semi-log paper, it will be represented by two straight lines in the region close to E. There will be a sudden change in slope at a point. This will be E since the recession curve and the base flow curve have different equations and hence will have different slopes.
Join AE by a straight line.
The flow below AE is the base flow and above AE it is the flood runoff.
e. Recession Discharge Analysis:
Plot a graph of Qn versus Qn+1 and the graph will change its nature from E. Here Qn is the discharge at any time on the recession curve and Qn+1 is the discharge immediately after Qn after a time lag (Fig. 2.19).
Locate E, Join AE by a straight line.
Flow below AE is the base flow and above AE it is the surface runoff.
f. Extension of Base Flow Graph:
Extend forward the base flow graph, prior to storm, from A up to the peak discharge by a straight line. Similarly, extend backward the base flow up to the peak point by a straight line. Join these two straight liens smoothly by making a good judgment (Fig. 2.20).
The flow below this line AE is the base flow and above it is flood discharge.
Review of the Methods:
Firstly, all these methods are arbitrary. Not all may be applicable for a specific hydrograph. Proper judgment will have to be made before applying any method.
Secondly, if there is some error is assessing the base flow, the result will be as follows:
1. The base flow time T will slightly change.
2. The runoff volume also will slightly change.
However, it will not make much difference in the storm runoff.
Base Flow Separation of a Complex Storm:
Separating the base flow from flood hydrograph resulting from a complex storm is a difficult job. The basic principles are the same and it is a matter of experience. Fig. 2.21 shows the base flow separation for a complex storm.
