Runoff: Meaning, Factors and Its Determination | Water Engineering

In this article we will discuss about:- 1. Meaning of Runoff 2. Factors Affecting Runoff 3. Methods of Determination.

Meaning of Runoff:

Runoff is that part of precipitation as well as any other flow contribution, which is transmitted through natural surface channels or streams or rivers.

In the general sense runoff includes:

(i) Surface runoff or overland flow received in the stream immediately after a heavy rain;

(ii) Inter-flow which is a portion of soil moisture that flows laterally through the upper soil layers and joins the stream before joining the ground water;

(iii) Delayed runoff or ground water flow that enters the stream after passing through deeper portions of the earth; and

(iv) Other delayed runoff that has been temporarily detained as snow cover or stored in natural lakes and swamps.

Thus runoff is the total quantity of water received by a stream from its drainage basin or catchment area. The runoff is generally classified as direct runoff and base flow (or base runoff). The direct runoff comprises the overland flow and the inter-flow which are usually grouped together, while the ground water flow that enters a stream is termed as base flow (or base runoff).

The runoff is generally considered in terms of the total flow carried by a stream during a month, season or year and accordingly it is termed as monthly, seasonal or annual runoff. The runoff is thus expressed in cubic metres or hectare- metres of water carried by a stream in a certain duration.

However, the runoff is more commonly expressed in centimetres or metres of water depth on the entire drainage basin, or in hectare-metre or cubic metre per unit area of the drainage basin. Further the terms stream flow, discharge of a stream and rate of runoff are generally used to mean one and the same thing, in which case it is expressed in cumec.

Factors Affecting Runoff:

The runoff from the drainage basin or catchment area of a natural stream depends on several factors as noted below:

1. Characteristics of Precipitation:

The pertinent characteristics of precipitation which may affect the runoff are the type of precipitation (such as rain or snow), its intensity, areal extent and duration, and direction of storm movement.

2. Characteristics of Drainage Basin:

The runoff is considerably affected by the characteristics of the drainage basin such as size, shape, surface, orientation, altitude, topography and geology of the drainage basin.

3. Meteorological Characteristics:

The runoff is significantly affected by the meteorological characteristics such as temperature, humidity, wind velocity, pressure variation, etc.

4. Storage Characteristics:

The storage characteristics of a drainage basin also have significant effect on the runoff. If a drainage basin has a large number of natural depressions, pools, lakes, etc., and a number of artificial reservoirs or tanks, which will store a part of the precipitation, then the runoff at the outflow point of the basin will be reduced. However, in this case the drainage basin will have large storage capacity.

Methods of Determination of Runoff:

The runoff from catchments of rivers may be determined by the following methods:

(1) By direct measurement of discharge through rivers.

(2) Estimation of runoff by indirect methods.

Methods of Measurement of Discharge through Rivers:

The measurement of discharge through rivers may be carried out by the following methods:

(i) Area-Velocity method

(ii) Stage-Discharge rating curve method.

(iii) Slope-Area method.

(iv) By the use of structures such as weirs, anicuts, etc.

Each of these methods is briefly described below:

(i) Area-Velocity Method:

In this method the cross-section of the river is divided into a number of segments of equal or unequal widths. The mean depths of flow of the segments are measured along the centre lines by soundings. The mean velocities of flow through segments are measured with the help-of a current meter by inserting it along the centre line of each of the segments to a depth equal to 0.6 times the mean depth below the free water surface. The total discharge through the river is obtained by adding the discharge through each of the segments.

(ii) Stage-Discharge Rating Curve Method:

The elevation of the water surface at any section of the river measured above and arbitrary datum is known as stage. In this method a stage-discharge rating curve is prepared for the river. For this the discharges are measured and corresponding to the measured discharge of the river the stage is also recorded with the help of a gage provided at the section of the river where the discharge is measured.

By plotting the stage against the corresponding discharge a stage-discharge rating curve is obtained for the river which may be used for determining the discharge corresponding to a known stage.

(iii) Slope-Area Method:

In this method at the gaging station the cross- sectional area of the river is obtained by taking soundings below the water level at equal interval and plotting the profile of the cross-section and drawing the water surface level. From the plotted profile of the cross-section of the river, wetted perimeter is obtained and the hydraulic radius is computed. The water surface slope is determined by means of gages placed at the ends of the reach (say 1 km upstream and 1 km downstream of the gaging station).

The slope may also be determined by means of flood marks on either side and their subsequent levelling. Further depending on the nature of the bed and banks of the river the values of the Manning’s n or Chezy’s C may be found, and by using either Manning’s or Chezy’s formula the mean velocity of flow in the river may be computed.

The mean velocity of flow multiplied by the cross-sectional area of the river gives the discharge through the river. The slope-area method is often used to estimate the peak or maximum flood discharge through rivers.

(iv) By the Use of Weirs or Anicuts:

If a weir or anicut is constructed across a river, then as the water flows over it, by measuring the head over it the discharge through the river may be determined by using the formula for discharge over a weir or anicut. However, this method is suitable only for determining discharge through small streams or rivers.

From the discharge measurements of a river recorded by any of these methods the total daily flow of the river may be computed and by adding the total daily flow of the river the total weekly, monthly and yearly (or annual) flow or yield of the river may be obtained.

Estimation of Runoff by Indirect Methods:

The runoff may be indirectly estimated by the following methods:

(a) Using empirical formulae, curves and tables relating rainfall and runoff.

(b) Infiltration method

(c) Rational method

(d) Unit hydrograph method

Empirical Formulae, Curves and Tables Relating Rainfall and Runoff:

Several empirical formulae, curves and tables relating the rainfall and runoff have been developed which may be used for the estimation of runoff.

Some of these are as indicated below:

(i) Binnie’s Percentage:

In India the earliest efforts in estimating runoff from the rainfall were those of Alexander Binnie.

He made observation on two rivers in Madhya Pradesh and established certain percentages of runoff from rainfall which are given below:

(ii) Barlow’s Tables:

T.G. Barlow carried out studies of catchments mostly of area under 130 square kilometres in Uttar Pradesh, and correlated runoff R and rainfall P as-

R = KP …(3.4)

where K is runoff coefficient, the value of which depends on the type of catchment and the nature of rainfall season. He divided catchments into five classes and assigned values of runoff coefficient K for each class as given below.

These values of the runoff coefficient K are for average rainfall season and are to be multiplied by the following coefficients according to the nature of the rainfall season.

(iii) Stragne’s Tables and Curves:

W.L. Strange was the next to evolve ratios between rainfall arid runoff. Based on the rainfall and runoff data of the catchments in Maharashtra he obtained the values of the runoff coefficient K as a function of the catchment character. For the purpose of calculating the runoff yield from the total monsoon rainfall, the catchments were characterized as ‘good’, ‘average’ and ‘bad’. The values of K given by Strange for these three types of catchments are as indicated below.

Strange also gave the values of the runoff coefficient for calculating the daily runoff from daily rainfall. In this the runoff coefficient depends not only on the catchment character and the amount of rainfall but also on the condition of the ground surface prior to rain. Strange considered three original (or prior to rain) ground surface conditions viz., dry, damp and wet. and gave the values of the runoff coefficient for these conditions of ground surface in an average catchment as indicated below.

Strange has also given two sets of curves:

(1) Daily runoff v/s daily rainfall for the three conditions of ground surface prior to rain; and

(2) Annual runoff v/s annual rainfall for the three types of catchments, from which runoff corresponding to known rainfall may be directly determined.

(iv) Inglis and De Souza’s Formula:

On the basis of the data collected from 37 catchments in ghats and plains of Maharashtra, C.C. Inglis and De Souza gave the following two different formulae for ghat areas and for plains.

where R is average annual runoff in centimetres and P is average annual rainfall in centimetres over the entire drainage basin.

(v) Lacey’s Formula:

G. Lacey gave the following formula:

where R is average annual runoff in centimetres; P is average annual rainfall in centimetres over the entire drainage basin. F is a monsoon duration factor and S is a catchment (or drainage basin) factor.

Lacey gave the following values for the catchment factors S corresponding to the Barlow’s five classes of catchments.

It may, however, be observed that the value of the monsoon duration factor F varies form 0.5 to 1.5.

(vi) Khosla’s Formula:

A.N. Khosla considered mean annual temperature as a measure of the various factors affecting losses by evaporation, transpiration, sunshine and wind velocity and hence gave the following formula-

where R is average annual runoff in centimetres; P is average annual rainfall in centimetres over the entire drainage basin; and T is mean annual temperature in °C for the entire drainage basin.

(vii) Formula for Some of the Drainage Basins in India:

For some of the drainage basins in India empirical formulae have been developed which are as follows:

where R is average annual runoff in centimetres and P is average annual rainfall in centimetres over the entire drainage basin.

Infiltration Method:

In this method the runoff may be estimated either by the use of the infiltration capacity curve, or by the use of the infiltration indices. In the first Method the runoff is estimated by subtracting the infiltration loss from the rainfall. The area under the infiltration capacity curve represents the total infiltration loss.

As such as shown in Fig. 3.10 the infiltration capacity curve for a given soil and moisture conditions is subtracted from the curve of rainfall pattern to derive the excess rainfall which represents the runoff. This method would give good results if there is high degree of areal uniformity in rainfall pattern and in infiltration capacity, and if reasonably accurate infiltration curves are available.

In the second method by subtracting the predetermined values of the infiltration indices from the rainfall intensity the excess rainfall representing the runoff may be obtained. This method is largely empirical and the derived values of the indices are applicable only when the rainfall characteristics and initial soil moisture conditions are identical to those for which these are derived.

Rational Method:

In the rational method the basic equation which correlates runoff and rainfall is as follows:

Q = CIA … (3.15)

where Q is runoff in hectare-metre per hour (or cubic metre per hour); I is intensity of rainfall in metres per hour; A is area of the drainage basin in hectare (or square metre); and C is a runoff coefficient.

The value of the runoff coefficient C depends on the characteristics of the drainage basin such as soil type, vegetation, geological features etc.

For different types of drainage basins the values of C are as given below:

In the rational method the entire area of the drainage basin lying on the upstream of the point where the runoff is estimated, is divided into a number of sub-areas such that the time taken for the flow to reach the reference point-

(i) From the extreme dividing line of the nearest sub-area is one hour;

(ii) From the extreme dividing line of the next sub area is two hours and so on.

Then knowing the intensity and duration of the rainfall and the values of the runoff coefficient for each sub-area at different times, and using the rational formula the runoff contributed by each sub-area at different times can be obtained. By adding the runoff contributed by each of the sub-areas during every hour, the direct runoff obtained from the entire drainage basin during every hour at the reference point is determined.

Unit Hydrograph Method:

A hydrograph is a graphical plot of discharge of a stream or river versus time. A unit hydrograph is defined as a hydrograph of direct surface runoff resulting from one centimetre of effective rainfall falling uniformly over the drainage basin or catchment in space as well as in time for a specified or unit duration. The effective rainfall also called rainfall excess is that part of the total rainfall that enters the stream as a direct runoff. The specified or unit duration is the period within which the effective rainfall is considered to be uniformly distributed.

In this method a unit hydrograph of suitable unit duration is first derived from an observed hydrograph of a drainage basin.

From the unit hydrograph so derived the hydrograph that would result from a rainfall of known intensity and its duration same as the unit duration of the unit hydrograph may be obtained as follows:

(i) Deduct the infiltration losses from the rainfall intensity to obtain the intensity of effective rainfall.

(ii) Multiply the ordinates of the unit hydrograph by the intensity of effective rainfall to obtain the ordinates of the direct runoff hydrograph.

(iii) Add the corresponding base flow ordinates to the direct runoff ordinates to obtain the ordinates of the anticipated hydrograph which may be plotted.