In this article we will discuss about:- 1. Units of Hydrograph 2. Area of Hydrograph 3. Complex Storm 4. Isolated Storm.
A hydrograph may be defined as a graphical representation of time versus discharge. Fig 2.10.
Units of Hydrograph:
The unit for time on the x-axis may be:
1. Hours.
2. Days.
3. Months.
This unit is selected depending upon the purpose and the nature of the study. If the study is floods or flood routing, then the scale may be in hours. If the study is for estimation of runoff for a year, then it may be in days or in months.
On the y-axis, the unit for discharge is m3/sec. Sometimes, the discharge is expressed as the depth of water per unit area of the catchment per second expressed in cm/s/m2.
Area of Hydrograph:
The area under the hydrograph is the runoff, i.e., the volume of water.
Complex Storm:
When subsequent storm occurs before the runoff of the previous storm ceases, such combined storms are called complex storm (Fig. 2.12).
Isolated Storm:
When subsequent storm does not occur before the runoff of the previous storm ceases, that storm is called isolated storm (Fig. 2.11).
Hydrograph of an Isolated Storm:
The hydrograph of an isolated storm is represented as follows:
This is the normal way of presenting a hydrograph.
The hydrograph or the total excess rainfall uniformly distributed over the entire catchment area is shown with an axis parallel to the x-axis of the hydrograph, but the y-axis from a distance and in the reverse direction (Fig. 2.11 and Fig. 2.13).
The discharge Qb is the discharge in the river before the storm. This does not include the storm runoff. This discharge is known as base flow or basic flow or sustained flow or fair weather runoff. Thus, a hydrograph covers the base flow as well as storm runoff.
Analysis of a Hydrograph due to an Isolated Storm:
The shape of the storm hydrograph (Fig. 2.13) can be analysed as follows:
1. The rising curve AC also known as concentration limb starts at A. From this point, the flood hydrograph starts. This is also the beginning of excess rainfall. This curve is normally steep and is concave upwards from A to B, and from B to C it is convex upwards. Point B is, thus, called the point of contraflexure or point of inflection. The shape of the rising curve depends upon the catchment characteristics as well as the storm precipitation.
2. BD is the crest segment with the two parts BC and CD having curvature convex upwards.
3. The peak discharge is at C.
4. The falling curve starts from the point C up to E, i.e., from the point of peak discharge to E. The curve is convex upwards from C to D and then concave upwards from D to E. Point D is also called point of contraflexure or point of inflection.
The nature of curve changes at the points of contraflexure, viz. B and D. Point D is the point up to which the surface runoff from the most distant point on the ridge of the catchment area reaches the gauging site. This time, i.e., from A to D, is called the concentration time or the time of concentration.
From point D onwards, there is no addition to the surface runoff. However, when the river flows, there is some water stored in the flowing river channel. The discharge at the gauging site is due to draining of this water stored in the river channel, i.e., draining of surface retention, and hence the nature of the recession curve is always the same for a specific catchment. It depends only on the basin characteristics of the catchment area and is, therefore, the independent of the precipitation.
The curve from D to E is called recession curve.
An equation in the following form is derived for the recession curve:
Qt+1 = QtK(t – t0)
where, Qt = Discharge in m3/s at time t
Qt+1 = Discharge in m3/s at time t0
K = A constant known as recession constant.
By observing recession curves of a number of storm runoffs of a specific catchment, a master recession curve may be prepared with the help of a tracing paper. This is also known as composite recession curve, master depletion curve or type curve. Point E denotes the end of surface runoff due to the storm precipitation. The discharge in the stream after E is again the base flow. It is rather difficult to locate the point E. (Theoretically, the recession curve is tangential to the base flow at E.).
Thus, the storm hydrograph is from A to E and the time interval from A to E is known as base period. It may be noted that the period for the rising curve is much less than that of the falling curve.