BFC 21103 Hydrology

Chapter 4. Surface Run-off

Zarina Md AliBased on BFC 32002 Hydrology Module

Email: zarinaali75@gmail.comPhone Nu: 07456 / 0197722315

BFC32002_Ch4/ZARINA'S 1

Learning Outcomes

At the end of this chapter, students should be able to:

• explain catchment area and its characteristics

• quantify runoff and river flow based on various

methods.

• develop Intensity-Duration-Frequency (IDF) curves

of rainfall event by applying Rational Method.

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4.1 IntroductionSurface runoff (also

known as overland flow)

is the flow of water that

occurs when excess

stormwater, meltwater, or

other sources that

travels down hill on a

catchment area over the

time

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The water in small channels flows to larger channels, and then

connects to a larger stream, and so on till the flow reaches the

catchment outlet at the downstream. Runoff is extremely

important in that not only does it keep rivers and lakes full of

water, but it also changes the landscape by the action of

erosion.

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4.2 Catchment AreaThe watershed or known catchment area, drainage area and

drainage basin is basic hydrologic unit in the analysis of

runoff phenomenon. Since large catchment areas are made

up from many smaller catchment areas, in general,

catchment area consists of all land area that sheds water to

the outlet during a rainstorm.

Catchment area is a system which is complex and

heterogeneous consists of collection of some sub systems.

Each sub system is considered homogeneous, and every sub

system is determined by its physical character, where it can

be grouped as follows:

(a) the characters of its surface (land use and topographic),

(b) the characters of top soil layer, and

(c) the characters of sub soil layer.

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4.2 Catchment CharacteristicsEvery catchment area is

different because factors

that affect catchment area

vary with location. The

catchment area has different

shapes and sizes, for

example, as shown in figure

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4.2 Catchment CharacteristicsEvery catchment area is different because factors that affect

catchment area vary with location. Size and shape of

catchment will have direct effect on surface runoff generation.

Long, narrow drainage basins will generally display the most

dramatic effects of surface runoff. Stormwaters reach the main

channels far more rapidly in long narrow basins than in other

types of basins.

Physical characteristics: area, shape, slope and drainage

channel pattern - major characteristics that affect the

volume of surface runoff and the shape of runoff

hydrograph from a catchment.

Linear measurements: channel length, the drainage

pattern, channel roughness and cross-sectional properties,

time of flow parameters, and the land cover.

4.3 Run-off Determination

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Runoff normally applies to water that flows over a surface to

the downstream, and the term streamflow is used to describe

the amount of water flowing in a river. Rain falling on a

catchment in quantities that exceeding the soil or vegetation

uptake and becomes surface runoff. Water infiltrating the soil

may eventually return to a stream and combine with surface

runoff in forming the total drainage from the basin.

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. 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.

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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. If there is no impeding layer in the subsoil the

infiltrating water percolates into the ground as deep seepage

and builds up the groundwater table. The groundwater may

also contribute to the stream flow if it 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.

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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 (from rainfall).

Direct surface flow can be analysed 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 groundwater contribution,

which takes days or months to reach the stream, in all probability, has

no direct relation with the immediately preceding storm. The

groundwater flow into the stream would have continued even if there

had been no storm immediately proceeding. For this reason, it is

termed as base flow in hydrograph analysis.

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When the overland flow starts (due to a storm) some flowing water is

held in puddles, pits and small ponds; this water stored is called

depression storage. The volume of water in transit in the overland

flow which has not yet reached the stream channel is called surface

detention or detention storage. The portion of runoff in a rising flood

in a stream which is absorbed by the permeable boundaries of the

stream above the normal phreatic surface is called bank storage.

These various types of runoff and its path can be illustrated as in

Figures 4.3 and 4.4.

Overall, it can be concluded that runoff will occur when rainfall

exceeds the infiltration rate at the surface; excess water begins to

accumulate as surface storage in small depressions. As depression

storage begins to fill, overland flow will occur as surface runoff. Main

components of runoff are overland flow/surface runoff, subsurface

flow/interflow and baseflow/groundwater flow

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4.3.1 Factors Affecting Surface-OffSeveral factors can affect surface runoff can be characterisedinto meteorological and physical factors, some of them are:1. Meteorological Factors:

(a) Type of precipitation (rain, snow, sleet, etc.)(b) Rainfall intensity, amount and duration(c) Distribution of rainfall over the drainage basin(d) Direction of storm movement(e) Precipitation that occurred earlier and resulting soil

moisture(f) Other meteorological and climatic conditions that affect

evapotranspiration, such as temperature, wind, relativehumidity, and season

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2. Physical Characteristics (a) Land use (b) Vegetation (c) Soil type (d) Drainage area and shape (e) Elevation and topography, especially the slope of the land (f) Drainage network patterns (g) Ponds, lakes, reservoir or anything that accumulates in a

basin which prevent runoff from continuing downstream

The extent of runoff is a function (ƒ) of geology, slope, climate,precipitation, saturation, soil type, vegetation, and time.Geology includes rock and soil types and characteristics, aswell as degree of weathering.

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Porous material (sand, gravel, and soluble rock) absorbs waterfar more readily than does fine-grained, dense clay or un-fractured rock. Well-drained soil material (porous) has a lowerrunoff potential therefore has a lower drainage density.Poorly-drained soil material (non-porous) has a higher runoffpotential, resulting in greater drainage density. Drainagedensity is a measure of the length of channel per unit area.Many channels per unit area mean that more water is movingoff of the surface, rather than soaking into the soil.

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4.4 River Flow DeterminationDischarge or streamflow or total surface and subsurface runoff), Q is

the volume of water flows down in a stream/ river/ channel per unit of

time, commonly expressed in cubic feet per second, cfs (ft3 /s) or

cubic meter per second (m3 /s). The discharge can be calculated

using the following equations:

There are numerous methods and types of equipment to measure

stream or river or channel water level, cross-sectional area and

velocity, hence discharge can be computed.

4.4.1 Measurement of Stage

The stage or water level of a river is defined as its water-surface

elevation measured above a datum. This datum can be the mean-sea

level (MSL) or any arbitrary datum connected independently to the

MSL. Three common methods of water level measurement: staff

gauge, wire gauge and automatic stage recorders.

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• The simplest method is made by noting the water elevation which

contact with a fixed graduated staff.

• The staff is made of a durable material with a low coefficient of

expansion with respect to both temperature and moisture.

• It is fixed rigidly to a structure, such as an abutment, pier, wall, etc.

• The markings are distinctive, easy to read from a distance and are

similar to those on a surveying staff.

Staff Gauge

Sometimes, it may not be possible to read the entire range of water

surface elevations of a stream by a single gauge and in such cases

the gauge is built in sections at different locations. When installing

sectional gauges, care must be taken to provide an overlap between

various gauges and to refer all the sections to the same common

datum.

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Wire Gauge

It is a gauge used to measure the water-surface elevation from

above the surface such as from a bridge or similar structure. By

using this apparatus, a weight is lowered by a reel to touch the water

surface. A mechanical counter measures the rotation of the wheel

which is proportional to the length of the wire. The maximum length

of wire is about 25 m.

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Automatic Stage Recorders

Bubble gauge Recorders

For this gauge, compressed air

or gas is made to bleed out at a

very small rate through an outlet

placed at the bottom of the river

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Float-gauge Recorders

Automatic Stage Recorders

The float-operated stage recorder is

the most common type of automatic

stage recorder being used. This float

gauge operates in a stilling well,

which is balanced by a counterweight

over the pulley of a recorder.

4.4.2 Stage Data

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4.4.3 Measurement of Velocity

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The measurement of velocity

is an important aspect of many

direct stream flow

measurement techniques.

The upper figure is a cross-

sectional view with contours

indicating how velocity varies

from top to bottom and across

the stream channel.

Figure below shows an example of

a velocity profile changes with

increasing depth and reach Surface

Runoff 93 the average velocity at

approximately 0.6 of the total depth

(or 0.4 of the depth from the bottom)

4.4.3 Measurement of Velocity

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Stream velocity is typically faster at the surface and toward the

middle of the channel, and slower along the sides and bottom of

the channel due to differences in friction. The velocity profile

shows the average velocity is usually at 0.6 times the total depth

from the water surface, or 0.4 times the total depth from the bottom

of the channel. A mechanical device, called current meter and float

method are commonly used in measuring velocity.

1. Current meters

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Typical values of a and b for a standard size 12.5 cm diameter Price

current meters (cup-type) is 0.65 and 0.03, respectively. While,

smaller meters of 5 cm diameter cup assembly called pigmy meters,

which rotate faster and are useful in measuring small velocities. The

values of the meter constants are a = 0.30 and b = 0.003.

1a. Vertical-Axis Meters

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These instruments consist of a series of conical cups mounted around

a vertical axis (Figure 4.9). The cups rotate in a horizontal plane and a

camera attached to the vertical axial spindle to record and generated

signals proportional to the revolutions of the cup assembly. The Price

current meter and Gurley current meter are typical instruments under

this category.

The normal range of elocities is from 0.15 to 4.0 m/s. The accuracy of

these instruments is about 1.5% at the threshold value and improves

to about 0.30% at speeds in excess of 1.0 m/s.

1b. Horizontal-Axis Meters

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The meters consist of a propeller mounted at the end of horizontal

shaft (Figure 4.10). The instrument has a wide variety of size with

propeller diameters from 6 to 12 cm, and can register velocities in the

range of 0.15 to 4.0 m/s. The meters are fairly rugged and are not

affected by oblique flows of as much as 15°. The accuracy of the

instrument is about 1% at the threshold value and is about 0.25% at a

velocity of 0.3 m/s and above.

2. Float

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A floating object on the surface of a stream when timed can yield the

surface velocity by the relation

t

Svs where S = distance travelled in time t.

This simple method used to determine velocity in special

circumstances, such as:

(a) a small floodedstream,

(b) a small stream with a rapidly changing water surface, and

(c) preliminary or exploratory surveys.

Any floating object can be used in this method, but it needs to be

leakproof and easily identifiable floats (Figure 4.11). A simple float

moving on stream surface is called surface/loot, however, surface

floats are affected by surface winds.

2. Float

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Special floats, i.e float canister under water can be used to

measure the average velocity in the vertical axis. Besides, rod float

in which a cylindrical rod is weighed so that it can float vertically.

Example 4.1

A floating object method has been used in determining velocity of

small flooded stream. If the length of buoyance object flows is 3

km in 30 minutes between 2 cross-sections, find velocity of

stream.

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30

s

SV

t

30001.67m/s

30 60 sV

x

4.4.4 Determination of Stream-flow

Direct determination Methods or Stream Gauging:

(a) Area-velocity methods, 1. mid-section, 2. mean-section

(b) Tracer-dilution techniques,

(d) Electromagnetic method, and

(e) Ultrasonic method.

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Indirect determination of stream flow:

(a) Hydraulic structures, such as weirs, flumes, orifices and

gated structures, and

(b) Slope-area method.

Stream flow is measured in units of discharge (m3/s) occurring at a

specified time and constitutes historical data.

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a) Area-Velocity Method

Mid-section method

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According to figure below, total area to be divided into N-1 segments and

velocity averaged over the vertical at each section is known.

The total discharge is calculated by the method of mid-sections as follows:

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Example 4.2The data pertaining to a stream-gauging operation at a gauging site are

given in the table below. The rating equation of the current meter is v = 0.51

Ns + 0.03 m/s. Calculate the discharge in the stream using mid section

method.

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Distant from left

of water edge(m)0 1 3 5 7 9 11 12

Depth(m) 0 1.1 2 2.5 2 1.7 1 0

Revolutions of a

current meter

kept at 0.6 depth

0 39 58 112 90 45 30 0

Duration of

observation (s)0 100 100 150 100 100 100 0

Example 4.2

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Distance from

left water edge

(m)

Average

width W (m)

Depth

y

(m)

Velocity

Vave

(m/s)

Segmental

discharge

ΔQi (m3/s)

0 0 0 — —

1 2.00 1.10 0.229 0.504

3 2.00 2.00 0.326 1.304

5 2.00 2.50 0.411 2.055

7 2.00 2.00 0.336 1.344

9 2.00 1.70 0.260 0.884

11 2.00 1.00 0.183 0.336

12 0 0 - -

ΔQi 6.457

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12

2

21

2

W

2 2

2 2W

v = 0.51 Ns + 0.03

v = 0.51 (39/100) + 0.03

Q = v x y x W

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Mean-section methodAt each vertical location along the cross section as in figure, the average

velocity of flow for the area between two verticals is considered to be equal to

the average of the mean velocities for each of the bordering verticals.

The discharge between two verticals is thus the average velocity for the

section multiplied by the area of the section. The individual discharges are then

summed to provide an estimated total flow for the channel at that location.

Note that it is important to have enough measurements of cross section.

Example 4.3

Table below provides the field measurements of width, depth,

and velocity. Calculate the discharge at the river cross section

using mean section method.

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Vertical

Section

No.

Section

width (m)

Depth

(m)

Average

velocity

(m/s)

0 0 0 0

1 4.2 4 2.1

2 3.3 5 2.3

3 4.8 7.2 2.7

4 5.2 7.4 2.8

5 3.7 7.1 2.5

6 5.1 4.7 2.2

7 5.9 0 0

Sub-

area

Cross-

sectional

area (m2)

Average

velocity

(m/s)

Discharge

Q (m3/s)

0 - 1 8.40 1.05 8.82

1 - 2 14.85 2.20 32.67

2 - 3 29.28 2.50 73.20

3 - 4 37.96 2.75 104.39

4 - 5 26.83 2.65 71.10

5 - 6 30.09 2.35 70.71

6 - 7 13.87 1.1 15.26

Total 161.28 376.15

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A = (4.2 )x (0+4)x (1/2)

V = (2.5 + 2.2)/2

Q = VA

A = (5.1 )x (7.1+4.7)x (1/2)

Tracer-Dilution Method

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Tracer-Dilution Method

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qCC

CCQ

02

21

c0 = background conc. already present

c1 = the known conc. of tracer added at a constant rate q

c2 = sustained final conc. of the chemical in the well mixed flow

This method has the major advantage that the discharge is estimated directly.

It is particularly suitable for small turbulent streams, such as those in

mountainous areas. It can be used as an occasional method to check the

calibration, stage-discharge curves, etc, obtained by other methods.

Example 4.4

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A 25 gm/l solution of a fluorescent tracer was discharged into a stream at a

constant rate of 10 cm3/s. The background concentration of the dye in the

stream water was found to be zero. At a sufficiently distance downstream

section, the dye was found to reach an equilibrium concentration of 5 parts

per billion. Estimate the stream discharge.

Electromagnetic Method

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• This method is based on the Faraday's principle that an electro magnetic

field (EMF) is induced in the conductor (water in the present case) when it

cuts a normal magnetic field.

• Large coils buried at the bottom of the channel carry a current (I) to produce

a controlled vertical magnetic field.

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• Electrodes provided at the sides of the channel section measure the small

voltage produced due to flow of water in the channel. It has been found

that the signal output E will be of the order of millivolts and is related to the

discharge Q

• The method involves sophisticated and expensive instrumentation and

has been successfully tried in a number of installations.

• Total discharge was determined once it has been calibrated and really

suited for field situations where the cross-sectional properties can change

with time due to weed growth, sedimentation, et. Another specific

application is in tidal channels where the flow undergoes rapid changes

both in magnitude as well as in direction.

Ultrasonic Method

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This method is measured using ultrasonic signals. Consider a channel carrying

a flow with two transducers A and B fixed at the same level h above the bed

and on either side of the channel

The transducers can receive as well as send ultrasonic signals. Let A send an

ultrasonic signal to be received at B after an elapse time t1. Similarly, let B

send Hydrology 106 a signal to be received at A after an elapse time t2. t1 and

t2 can be calculated as,

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Ultrasonic method was carried out to determine velocity of river. Data

measured are: L = 5 km, t1 = 20 s, t2 = 25 s and θ = 30o. By using

Equation 4-14, calculate velocity of river.

Example 4.5

1 2

1 1 5000 1 128.87m/s

2cos 2cos30 20 25

LV

t t

4.4.4.2 Determination of Stream-flow

Direct determination Methods or Stream Gauging:

(a) Area-velocity methods, 1. mid-section, 2. mean-section

(b) Tracer-dilution techniques,

(d) Electromagnetic method, and

(e) Ultrasonic method.

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Indirect determination of stream flow:

(a) Hydraulic structures, such as weirs, flumes, orifices and gated

structures, and

(b) Slope-area method.

Stream flow is measured in units of discharge (m3/s) occurring at a

specified time and constitutes historical data.

4.5 IDF Curve

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• An intensity-duration-frequency (IDF) curve is a mathematical

function that relates the rainfall intensity with its duration and

frequency of occurrence.

• The data are normally presented as curves displaying two of the

variables, such as intensity and duration, for a range of

frequencies.

• These curves are commonly used in hydrology for flood

forecasting and civil engineering for urban drainage design.

• The IDF can be developed from the historical rainfall data and

available for most geographical areas in Malaysia.

• The IDF curve development will be explained briefly in this

module and refer to MSMA (2012) for detailed.

4.5.1 ARI

• Rainfall and subsequent discharge estimation is based on the

selected value of frequency or return period

• termed as the Average Recurrence Interval (ARI).

• ARI is the average length of time between rain events that

exceeds the same magnitude, volume or duration and is

expressed as:

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1100rT x

P

where;

Tr = Average Recurrence Interval, ARI (year) and

P = Annual Exceedance Probability, AEP (%).

Example 4.6

What is the AEP (P) of a peak flow occur in 50 years (ARI)?

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1 1100 100 0.02%

50r

P x xT

4.5.2 Empirical IDF Curve

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Based on Table 4.1 and Equation 4-16, calculate rainfall intensity at

station 1 if d = 2 hours and T = 5 years.

Example 4.7

4.6 Peak Flow Estimation

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Time of Concentration

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Runoff Coefficient for Mixed Development

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Rainfall Intensity

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Rational Method Estimation

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Example 4.8

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Calculate a 20 years ARI peak discharge for drain AB using Rational

Method. Given data: AAB = 31.83 ha, Cave = 0.526, tc = 7.5 min, and i =

300.36 mm/hr