MODULE 2. RUNOFF
Engr Deodatus G. Sagadal
OVERVIEW
Module 1 introduced you to the fundamentals of soil and water conservation. The
scope of soil and water conservation engineering is so broad and it is essential for
students to learn first the basic terminologies and concepts of this course. You were
taught about the importance of soil & water conservation in agricultural development;
and the issues, problems, and the present state of soil and water conservation in the
Philippines and abroad along with the soil and water conservation laws and
regulations. In this Module, you will learn about runoff, runoff processes, and factors
affecting runoff. You will also learn how to estimate the design runoff rate.
LEARNING OUTCOMES
At the end of this module, the students should be able to:
1. Describe runoff, runoff process and factors affecting runoff
2. Estimate the design runoff rate
3. Estimate runoff volume and water yield
4. Derive the runoff hydrograph.
5. Estimate rainfall intensity
MODULE REQUIREMENTS
The whole module-based teaching/learning activities will help you obtain the learning
outcomes independently. You will need laptop/computer/cellphone, internet
connection facility, and internet data to attend online meetings, access this module
and other teaching-learning materials from the UEP Learning Management Systems
Portal, group chat in Facebook, Google Classroom and Google Meet. At the end of
each lesson or topic, you are required to comply the assessment activities provided
in this module, and submit your outputs through private message to a designated
account online.
LEARNING PLAN
This module introduces you to runoff, runoff processes, and the factors affecting
runoff. This lesson is a preliminary to control soil and water erosion and land
degradation. You need to understand how runoff happens and what the factors that
affect runoff are. This can be obtained through online discussions, and
independently reading of lectures provided in this module and the reference books
suggested. The module will be presented in logical manner with a number of
examples and figures so that you may properly understand the topic. You will be
required to research the internet for your homework. We will use Google Meet for
online meetings and discussions, and Google Classroom for the access of teachinglearning materials, while a Group Chat in FB Messenger will be our tool for
notifications. Assessment will be done through online platforms as scheduled in the
syllabus.
2.0 RUNOFF
Runoff is the portion of precipitation that makes its way towards the stream
channels, lakes or oceans as surface or subsurface flows. Runoff occurs when
precipitation rate exceeds infiltration rate, and is the most destructive component of
rainfall. Water that flows across the earth’s surface is the same as the water that does
not soak into the ground or evaporate.
2.1 THE RUNOFF PROCESS
The paths water can take in moving to a stream are illustrated in Figure 1.
Precipitation may be in the form of rain or snow. Vegetation may intercept some
fraction of precipitation. Precipitation that penetrates the vegetation and may consist of
both precipitations that does not contact the vegetation, or that drops or drains off the
vegetation after being intercepted is referred to as throughfall. A large fraction of
intercepted water is commonly evaporated back to the atmosphere. There is also flux
of water to the atmosphere through transpiration of the vegetation and evaporation
from soil and water bodies. The surface water input available for the generation of
runoff consists of throughfall and snowmelt. This surface water input may accumulate
on the surface in depression storage, or flow overland towards the streams as
overland flow, or infiltrate into the soil, where it may flow laterally towards the stream
contributing to interflow. Infiltrated water may also percolate through deeper soil and
rock layers into the groundwater. The water table is the surface below which the soil
and rock is saturated and at pressure greater than atmospheric. This serves as the
boundary between the saturated zone containing groundwater and unsaturated zone.
Water added to the groundwater is referred to as groundwater recharge.
Immediately above the water table is a region of soil that is close to saturation,
due to water being held by capillary forces, this is referred to as the capillary fringe.
Lateral drainage of the groundwater into streams is referred to as baseflow, because
it sustains streamflow during rainless periods. Subsurface water, either from interflow
or from groundwater may flow back across the land surface to add to overland flow.
This is referred to as return flow. Overland flow and shallower interflow are processes
that transport water to the stream within the time scale of approximately a day or so
are classified as runoff. Water that percolates to the groundwater moves at much
lower velocities and reaches the stream over longer periods of time such as weeks,
months or even years. The terms quick flow and delayed flow are also used to
describe and distinguish between runoff and base flow. Runoff includes surface runoff
(overland flow) and subsurface runoff or subsurface stormflow (interflow).
2.2 FACTORS AFFECTING RUNOFF
These include both catchment factors and rainfall factors.
2.2.1 Catchment factors
How do catchment factors affect runoff?
Runoff is influenced by catchment factors such as topography, vegetation,
infiltration rates, soil storage capacity and drainage pattern. In addition, the size of the
catchment, its shape, orientation, geology and surface culture also affect runoff. The
larger a catchment, the more runoff it will generate.
Slope. The steepness or slope of the land also determines runoff. Water moves
quickly down steep slopes so it has little time to soak into the ground. Water moves
more slowly down gentle slopes and across flat area, allowing time to soak into the
ground.
Vegetation. Vegetation, such as grass and trees, also affects the amount of runoff.
Runoff slows down when it flows around plants. By slowing down runoff, plants and
their roots prevent soil from being carried away.
Gravity. Gravity is the attracting force that all objects have for one another. The
greater the mass of an object, the greater the forces of gravity. Water runs downhill
because of gravity. When water runs down a slope it picks up speed. As speed
increases so does its energy. Fast moving water carries more soil than slow moving
water does.
Slope steepness is particularly important as soil erosion is more prone on steeper
slopes. Surface culture includes the soil tilth, whether there is vegetative cover or not,
and other land management activities, e.g. cultivation that would increase erosion.
2.2.2 Rainfall factors
How do rainfall factors affect runoff?
The amount of rain and length of time it falls are two factors that determine
whether water will soak into the ground or run off. Light rain falling over several hours
will have time to soak into the ground. Heavy rain falling in less than an hour will run
off because it can’t soak into the ground fast enough.
Rainfall factors associated with surface runoff and erosion include; rainfall
amounts, storm duration, intensity and distribution, as well as seasonal patterns,
e.g. dry areas are more prone to erosion than wet areas because prolonged dry spells
destroy vegetation cover, and rain storms tend to be high intensity and thus erosive.
The most significant component of rainfall is its intensity, which is a function of the
energy the raindrops impact on the soil. The intensity-duration relationship of rainfall
gives an indication of expected runoff.
For example:
I = a/ (t+b)
Where: I = Rainfall intensity
t = Duration of rainfall (min)
a & b are constants
For any given duration, the graph or equation will indicate the highest average
intensity which is probable for a storm of that duration. This is calculated as:
I = kTx/tn
Where,
T = is the return period in years
t = is the duration in minutes
k, x, and n are all constants
Calculations involving rainfall probability must relate to a chosen return period,
e.g. for conservation works on small farms, about 10 years.
2.2.2.0 Rainfall Intensity
Rainfall intensity is one of the most important rainfall characteristics and
defined as the rate at which rainfall occurs, expressed in depth per unit time, usually
mm per hour. The fundamental unit of rainfall is depth measured by rain gauges. The
average intensity is calculated by dividing a rainfall depth by the duration, the time
over which the rainfall accumulates. Rainfall intensity is classified into light, moderate
and heavy intensity as under:
Type of Intensity
Range (mm/hr)
Light intensity
< 2.5
Moderate intensity
2.5 - 7.5
Heavy intensity
> 7.5
Very intense storms are not necessarily more frequent in areas having a high
annual rainfall. Storms of high intensity generally last for short periods and cover
small areas. The infrequent combination of relatively high intensity and long duration
produce large amounts of rainfall causing erosion and devastating floods. The rainfall
intensity is used in the preparation of hydrographs of maximum probable and
maximum possible flood flows. The peak intensity produces the largest runoff rate. If
rainfall was constant throughout a storm, any duration less than the storm duration
would produce the same intensity. However, rainfall is rarely constant for the storm
duration and intensity varies.
Rainfall intensity-duration- (ID) relationship comprises the estimates of rainfall
intensities for different durations and is commonly required for designing the water
resource projects. The depth of rainfall recorded on the graph sheet of a recording
type rain gauge is read hourly or half hourly to determine the variation in rainfall
intensity for different durations of the storms.
Intensities are calculated using the rainfall depth and storm time in the first two
rows as given in the following table. Each of the duration rows show intensities
calculated based on different durations. For example, i5 is the intensity calculated
over a period of 5 minutes starting at t = 0 and ending at t = 5 minutes, or starting at t
= 5 and ending at t = 10 minutes, etc.
Storm Time
(minute)
0
5
10
15 20 25 30 35 40 45
Cumulative
precipitation (mm)
0
10
40
60 75 90 100 100 100 100 100 100 100
i5
-
120 360 240 180 180 120 00 00 00
i10
Rainfall intensity
i30
(mm/hr)
-
-
-
-
-
-
-
- 200 180 120 80
i60
-
-
-
-
-
-
240 300 210 180 150 00 00 00
-
-
-
-
50 55
60
00 00
00
00 00
00
50 20
00
-
-
100
CALCULATIONS
Requirements
•
•
Recording type rain gauge, and
Calculator.
Procedure
'
• Take out graph sheet from the recording type rain gauge.
• Read rainfall depth for every 30-minute duration.
• Compute rainfall intensity.
EXAMPLE
In a particular rainfall storm of 4-hour duration, the rainfall depth recorded is as
follows:
Time, hours
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Cumulative rainfall, mm
0
15
27
36
43
49
54
57
59
Solution
The rainfall depth at 30-minute interval is computed by subtracting rainfall depth
corresponding to a preceding duration from cumulative rainfall depth. Rainfall intensity
is computed by dividing the rainfall depth with duration.
Time, hr
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Cumulative rainfall, mm
0
15
27
36
43
49
54
57
59
Rainfall depth, mm
0
15
12
9
7
6
5
3
2
Rainfall intensity, mm/hr
0
30
24
18
14
12
10
06
04
2.2.2.1 Time of Concentration (Tc)
The storm duration which corresponds with the maximum rate of runoff is
known as the time of concentration (Tc). It is assumed that during the time of
concentration, all parts of the watershed are contributing simultaneously to the
discharge at the outlet. Tc is also described as the longest time for water to travel by
overland flow from any point in the catchment to the outlet. It is equivalent to the time it
takes water to flow from the furthest corner of the catchment to the outlet. The time of
concentration (Tc) is the time by which water from most distant parts of the catchment
has reached the outlet.
The formula to estimate Tc is;
Tc = 0.0195 L0.77 S-0.385
Where:
Tc = is in minutes
(equation 2.2)
L = the maximum length of the catchment in m, and
S = slope of the catchment in m m-1 over the total length L
2.2.2.2 Design Storm
A design storm is a storm of known return period. It is used as a basis for
designing structures. For example, a 10-year, 1-hour rainfall is the maximum rainfall
amount expected in a 1-hour period with a 10-year return period.
2.3 DESIGN RUNOFF RATE
The capacity to be provided in a structure that must carry runoff may be termed as the
design runoff rate. Structures and channels are designed to carry runoff that occurs
within a specified return period (TR). E.g. 10 years for vegetative waterways, and 100
years for permanent channels.
2.3.1 Estimation of surface runoff
It is important to know the quantities of water to be handled. The importance of
estimating runoff or design of conservation or conveyance structures is to avoid failure
due to overtopping. The main objectives are to impound water (e.g. dams, peak
volumes are used) and to convey water (e.g. channels/waterways, peak runoff rates are
used). Estimating the rates of surface runoff depend on two processes: 1) estimating
the rate of rainfall, and 2) estimating how much of the rainfall runoff becomes
2.4.
METHODS OR EQUATIONS USED IN ESTIMATING RUNOFF RATE
2.4.1. The Runoff Coefficient
The simplest method is to use a single coefficient which represents the ratio of
rainfall loss. If half of the rainfall is “lost” by infiltration, the other half appears as runoff,
and then the coefficient, C is 0.5.
Examples of runoff coefficients:
Woodland on flat sandy loam, C=0.10 (means only 10% of rainfall is runoff)
Woodland, flat tight clay C = 0.40
Cultivated, hilly clay soil, C = 0.60
Urban, rolling, 50% built up, C = 0.65.
Example: Estimate the runoff in an urban rolling land with runoff coefficient of 0.65 for a
rainfall of 135 mm/sec.
Solution: Runoff Estimate = 0.65 x 135mm/sec = 88mm/sec Ans
Runoff Coefficient C for permanent pasture
The value of C is the ratio of the peak runoff rate to the rainfall intensity and is
dimensionless. It represents the proportion of rainfall that becomes runoff and is determined,
to a large extent by catchment conditions. Examples of these C values are given in Tables 2.
3 for the USA (temperate region' 700-1000 mm average annual rainfall) and 2.4 below for
Malawi in central southern Africa (average annual rainfall from < 400 mm to > 1000 mm).
Different hydrological conditions according to soil groups are accounted for.
Table 2.3: Coefficient C values for USA
Table 2.4: Coefficient C values (Malawi)
2.4.2. Catchment Characteristics Cook’s Method
The method consists of summing numbers each of which represents the extent to
which runoff from the catchment will influence a particular characteristic. The effect of
four features is considered in Cook’s method, which are:
a.
b.
c.
d.
The relief
Soil infiltration
Vegetal cover
Soil surface storage
Each of these is considered in turn and the condition of the watershed compared
with four descriptions, i.e. extreme, high, normal, and low. Each description/feature has
a number. For example, an arithmetic total (e.g. 30+10+15+10=65) is the watershed
characteristic and will lie between the extreme values of 100 and 25. The main problem
of this method, which estimates by addition, is that the errors are propagated.
2.4.3. Runoff Curve Numbers
This is an extension of Cook’s method, which allows for variations in the physical
conditions of a catchment and also the land use. Like in Cook’s method, four variables
are considered and in each case, a selection has to be made from a list of options. Ten
categories of land use or cover are offered (row crops, pasture, woods, fallow,
farmstead etc) with a choice of soil conservation practices such as contouring and
terracing. The hydrologic condition of the catchment is graded good, fair or poor and a
subjective assessment of this factor is designated one of four major hydrologic soil
groups described earlier. The method relies on subjective non-measurable assessment.
2.4.4. The Rational Formula
The Rational method predicts runoff through this equation:
Q = 0.0028CIA
Where:
Q = the design peak runoff rate in m3/s
C = Runoff coefficient (a function of catchment vegetation, slope, surface culture)
A = Area of the watershed in hectares
I = Rainfall intensity in mm/hr. for the design return period and for a duration equal
to the time of concentration of the watershed.
The Rational method is developed on the assumption that:
1. Rainfall occurs at uniform intensity for a duration equivalent to the time of
concentration, and
2. Rainfall occurs at a uniform intensity over the entire area of the catchment.
Example problem:
Consider a section of a downtown business area of 35,400 square feet that drains to a
particular storm water inlet. The runoff coefficient for this drainage area has been
estimated to be 0.85. Based on a specified design return period and the time of
concentration of the drainage area, the design storm intensity has been determined to
be 5.1 in/hr. What is the peak runoff rate from this area to be used for design of the
storm water inlet?
Solution: q = CiA = (0.85)(5.1)(35,400/43,560) cfs = 3.52 cfs.
2.5.
RUNOFF HYDROGRAPH
What is a runoff hydrograph?
A hydrograph is a graph showing the rate of flow (discharge) versus time past a
specific point in a river, or other channel or conduit carrying flow. The rate of flow is
typically expressed in cubic meters or cubic feet per second (cu.m/sec or cfs).
What are Hydrographs used for?
This type of hydrograph is known as a storm or flood hydrograph and it is generally
drawn with two vertical axes. One is used to plot a line graph showing the discharge of
a river in cu.m/sec (cubic meters per second) at a given point over a period of time.
What is meant by unit hydrograph?
Unit hydrograph is a direct runoff hydrograph resulting from one unit (one inch or one
cm) of constant intensity uniform rainfall occurring over the entire watershed. The
concept of unit hydrograph is based on linear systems theory and follows the
principles of superposition and proportionality
What is the definition of storm hydrograph?
A storm hydrograph is a way of displaying how the discharge of a river can change
over time in response to a rainfall event. The discharge of a river is just the amount of
water passing a certain point every second, and is calculated by multiplying the cross
sectional area of the river by its velocity.
Why would two Hydrographs be different?
The shape of a hydrograph is altered by a few different things. One factor is the
shape of the drainage basin. Drainage basins come in a wide assortment of shapes.
Basins with steep slopes will have a high peak discharge and a short lag time because
the water can travel faster downhill.
What is the base flow?
Base flow (also called drought flow, groundwater recession flow, low flow, lowwater flow, low-water discharge and sustained or fair-weather runoff) is the portion of
streamflow that comes from "the sum of deep subsurface flow and delayed shallow
subsurface flow".