RUNOFF

1

• When a storm occurs, a portion of rainfall infiltrates into the

ground and some portion may evaporate. The rest flows as a thin

sheet of water over the land surface which is termed as overland

flow.

• Runoff is the drainage of precipitation from a catchment, which

flows out through its natural drainage system. The plot of the

stream discharge against the time, gives the flow hydrograph.

• The term direct runoff is used to include the overland flow and the

interflow. If the snow melt contributes to the stream flow it can be

included with the direct runoff.

• Direct surface flow can be analyzed for – relatively large drainage areas by the unit hydrograph method and for

– Smaller areas by overland flow analysis.

• The direct runoff results from the occurrence of an immediately

preceding storm while the ground water contribution has no direct

relation with the immediately preceding storm.

2

RUNOFF Runoff consists of water from three sources:

1. Surface flow

It is the product of effective rainfall i.e., total rainfall minus hydrologic

abstractions. Surface flow is also called direct runoff. Direct runoff has

the capability to produce large flow concentrations in a relatively short

period of time. Therefor, direct runoff is largely responsible for flood

flows.

2. Interflow

A part of infiltrated rainwater moves parallel to the land surface as

subsurface flow is called interflow.

If there is a relatively impermeable stratum in the subsoil, the infiltrating

water moves laterally in the surface soil and joins the stream flow, which

is termed as underflow (subsurface flow) or interflow

3. Groundwater flow

The other part of infiltrated water percolates downwards to groundwater

and moves laterally to emerge in depressions and rivers and joins the

surface flow. This type of flow is called groundwater flow. It is also

known as base flow.

• If there is no impeding layer in the subsoil the infiltrating water

percolates into the ground as deep seepage and builds up the ground

water table.

• The ground water may also contribute to the stream flow, if the GWT is higher

than the water surface level of the stream, creating a hydraulic gradient towards

the stream.

• Low soil permeability favors overland flow.

• While all the three types of flow contribute to the stream flow,

– it is the overland flow, which reaches first the stream channel,

– the interflow being slower reaches after a few hours and

– the ground water flow being the slowest reaches the stream channel after some days.

3

Runoff

4

Infiltration - Runoff Processes for Constant Rainfall

5

Time

Saturated

Overland Flow

Interflow

Base flow

Rising limb

Falling limb

Separation of sources of streamflow on an idealized hydrograph

Separation line

Peak

Hyetograph

Components of Runoff Hydrograph

6

Superposition of Runoff Hydrograph

Time

Runoff Hydrograph

Surface Runoff

Interflow

Groundwater flow

The superposition of these hydrographs results in runoff

hydrograph exhibiting a long tail (positive skew).

7

Input Rainfall

System Watershed

Output Runoff

hydrograph

The objective of many hydrologic design and analysis

problems is to determine the surface runoff (volume and peak

discharge) from a watershed due to a particular storm. This

process is commonly referred to as rainfall-runoff analysis

or modeling.

Rainfall-Runoff Modeling

8

Runoff Requirement

Two components of a storm runoff are generally required for

engineering works to be estimated, which are runoff volume

and peak runoff rate.

There are many methods available for their estimation.

The most commonly adopted methods are the curve number

method of USSCS for estimation of both excess runoff volume

and peak flow rate (by using the dimensionless unit

hydrograph) and the Rational method for the peak flow rate.

Many empirical relationships are also widely used for

estimation of flow rates.

9

Runoff Estimation

A runoff hydrograph is a continuous record of streamflow over time. A

complete runoff hydrograph contains information on runoff volume as

the area under the hydrograph and peak runoff rates as the maximum

flow or peak of the runoff hydrograph.

Actual Runoff Hydrograph

Conceptual Runoff Hydrograph

•The Santa Barbara Urban Hydrograph Method

•A hydrodynamic Model

The Unit Hydrograph

Synthetic Unit Hydrographs

Dimensionless Unit Hydrographs

10

Factors Affecting Runoff

1. Meteorological Factors

Precipitation Characteristics

Type of Precipitation

Rainfall Intensity

Duration of Rainfall

Rainfall Distribution

Direction of Storm

Antecedent (past) ppt

Other Factors

Temp, Wind

2. Physio-graphic Factors

Characteristics of the Drainage Basin

Soil Type

Land Use

Shape and Size of Catchment

Topography of area

Natural Drainage Net

11

12

Effects of storm shape, size and

movement on surface runoff

13

Time

Time

Effect of Time variation of rainfall

intensity on the surface runoff

14

Effect of storm size on surface runoff Storm A

Storm B

Time

Storm A

Storm B

15

Effect of storm movement on surface runoff

A

B

Storm Movement

B A

Time

A B

16

Effects of basin characteristics

on the discharge hydrograph

17

Fan Shaped

Catchment

Fern Leaf

Catchment

• Fan shaped catchments give greater runoff because

tributaries are nearly of the same size and therefore

time of flow is nearly the same and is smaller.

• Whereas in fern leaf catchments, the time of

concentration is more since the discharge is

distributed over a long period.

Types of Catchments

18

Relationship of slope to peak discharge

t

Q

Gentler

t

Q

Steeper

19

How to find the slope of catchment

• Horton has suggested a method of determining the slope of

large drainage areas, i.e.,

• The area is subdivided into a number of square grids of equal

size. The number of contours crossed by each subdividing line

is counted and the lengths of the grid lines are scaled.

• Then the slope of the basin is given by

• S =1.5 (CI) Nc/(∑L)

• where S = slope of the basin, CI = contour interval

• Nc = number of contours crossed by all the subdividing lines

• Σ L = total length of the subdividing lines

20

21

Relationship of hydraulic roughness to runoff

t

Q

Less rough

t

Q

More rough

22

Relationship of storage to runoff

t

Q

Little storage

t

Q

More storage

23

The characteristics of the drainage net may be

physically described

(i) the number of streams (ii) the length of streams

(iii) stream density (iv) drainage density

The stream density of a drainage basin is expressed as the number

of streams per square kilometer.

stream density, Ds =N/A

where Ns = number of streams

A = area of the basin

24

Relationship of Channel length to runoff

t

Q

Shorter length

t

Q

Longer length