Chapter 3 (Dis 2013)

CHAPTER 3 : SURFACE RUNOFFBy : PN HALINA BINTI HAMID

LEARNING OUTCOME :

Upon completion of this course students will be able to :•Explain clearly the component of runoff•Define the definition of catchments area and the catchments characteristics•Calculate the Stream flow measurement using velocity – area method, mean section and mid section method•Calculate the infiltration using Phi Index- Φ method

What that means ofSURFACE RUNOFF ???

SURFACE RUNOFF :

Surface runoff is water, from rain, snowmelt, or other sources, that flows over the land surface, and is a major component of the water cycle

COMPONENT OF RUNOFF

1. Direct Runoff– It is that part of the runoff which enters

the stream immediately after the rainfall.– Include : surface runoff, prompt interflow

and rainfall on the surface of the stream.– Direct Runoff (DRO) is the total of

surface runoff and interflow

COMPONENT OF RUNOFF

2. Interflow– the water that travels laterally or

horizontally through the zone of aeration (vadose zone) without reaching the water table during or immediately after a precipitation event and discharges directly into a stream or other body of water

3. Baseflow– The delayed flow that reaches a stream

essentially as groundwater flow called base flow.

– In the annual hydrograph of a perennial stream the base flow is easily recognized as the slowly decreasing flow of the stream in rainless periods.

DEFINITION OF CATCHMENT AREA

• The area of land draining into a stream or a water course at given location is known as Catchment Area .

• It is also called as drainage area or drainage basin.

• A catchment area is separated from it neighbouring areas by a ridge called divide in USA and watershed in UK

Fig : Schematic Sketch of Catchment of River

Watershed(divide)

River Flow

CHARACTERISTICS OF THE CATCHMENT AREA

• The number of streams• The length of streams• Stream density• Drainage density

• Catchment Characteristics Affecting Runoff :– Land Use– Vegetation– Soil type– Drainage area– Basin shape– Elevation– Topography, especially the slope of the land– Drainage network patterns

• Rainfall Characteristics Affecting Runoff :– Type of precipitation (rain, snow, sleet, etc)

– Rainfall intensity

– Rainfall amount

– Rainfall duration

– Distribution of rainfall over the drainage basin

– Direction of storm movement

– Precipitation that occurred earlier and resulting soil moisture.

– Other meteorology and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity and season

STREAM FLOW MEASUREMENT

• Stream flow gauging using :– Velocity – area (V-A) method– Mean Section– Mid Section

STREAM GAGING USING VELOCITY AREA METHOD

• Stream gaging is a technique used to measure the discharge, or the volume of water moving through a channel per unit time, of a stream.

• The height of water in the stream channel, known as a stage or gage height, can be used to determine the discharge in a stream.

• The area-velocity methods as above using the current meter is often called as the standard current meter method

Procedure to Measurement Using Current Meter :

STEP 1 :

•String a measuring tape across the stream at right angles to the flow. Tie the tape off at both sides of the stream. Make it taut enough so that it doesn’t sage near the middle. Measure the stream width. Leave the tape in place.

STEP 2 :

•First, determine the width intervals that will measure. The official method requires that at least 20 points of measurement be made across the width of the stream. To do this, divide the total stream width by 20 to calculate the distance between points.

Layout of channel cross-section for obtaining discharge data by the velocity-area procedure.

STEP 3 :•At each measuring point, read and record the total depth, multiply the total depth by 0.6 to determine the depth of average velocity, set the propeller at the new depth, read and record the velocity.

STEP 4 :•The total amount of water moving through your section is a function of the size of the stream (cross-sectional area) and the velocity. Used the velocity measurements and the depth and distance measurements you recorded to calculate the total volume of water flowing through the section (total discharge).

Using Weathering the River Method

Using Cabel Method

Example :

The data pertaining to a stream-gauging operation at a gauging site are given below.

The rating equation of the current meter is

v = 0.51 Ns + 0.03m/s where Ns = Revolutions per second. Calculate the discharge in the stream.

Distance from left water edge (m)

0 1.0 3.0 5.0 7.0 9.0 11.0 12.0

Depth (m) 0 1.1 2.0 2.5 2.0 1.7 1.0 0

Revolution of a current meter kept at 0.6 depth

0 39 58 112 90 45 30 0

Duration of observation (s)

0 100 100 150 150 100 100 0

Solution :

Distance from initial points

(m)

Revised Depth, m

Depth of Observation,

mRevolutions

Time, s

Ns (rev/s)

VELOCITY

Area of section

(m2)

Mean Depth of Section

(m)

Width of Section

(m)

Discharge(m3/s)

RemarksVelocity at point,

V

Mean Velocity

in vertical

Mean Velocity

in section

0 0 0 0 0 0 0.030 0.030 0.129 0 0 0 0.000

1.0 1.1 0.66 39 100 0.390 0.229 0.229 0.277 0.55 0.55 1.0 0.152

3.0 2.0 1.2 58 100 0.580 0.326 0.326 0.368 3.10 1.55 2.0 1.141

5.0 2.5 1.5 112 150 0.747 0.411 0.411 0.373 4.50 2.25 2.0 1.679

7.0 2.0 1.2 90 150 0.600 0.336 0.336 0.298 4.50 2.25 2.0 1.341

9.0 1.7 1.02 45 100 0.450 0.260 0.260 0.221 3.70 1.85 2.0 0.818

11.0 1.0 0.6 30 100 0.300 0.183 0.183 0.107 2.70 1.35 2.0 0.289

12.0 0 0 0 0 0 0.030 0.030 0.015 0.50 0.50 1.0 0.008

TOTAL 5.428

0.6 x Depth ( Based on observation

depth – 0.6D, 0.8D and 0.2D)

Ns = Time / Rev

V = 0.51Ns + 0.03 m/s ( Based on calibration of current meter )

Area = Mean Depth of section x Width of section

Discharge = Area x Mean Velocity in section

(0.03+0.229)/2

(0 + 1.1)/2

(1.1 + 2.0)/2

0 - 0

1.0 - 0

3.0 – 1.0

5.0 – 3.0

(0.229+0.326)/2

Problem Based 1 :

Taking the rates of current meter as V = 0.05 + 0.8Ns, where V is m/s and Ns is in rev/s, calculate the stream flow according to the following observations in Table below.

Distance from river bank, m

Depth, m Current meter depth, m

Revolutions Time, s

0 0 0 0 00.6 1.0 0.6D 15 501.2 4.0 0.2D 30 55

0.8D 48 532.0 5.5 0.2D 40 46

0.8D 60 543.0 6.5 0.2D 45 48

0.8D 67 523.8 4.5 0.2D 33 54

0.8D 51 504.5 2.5 0.2D 26 48

0.8D 44 555.0 1.0 0.6D 20 47

ESTIMATION OF INFILTRATION

• The rate at which water infiltrates into a ground is called the infiltration capacity.

• When a soil is dry, the infiltration rate is usually high compared to when the soil is moist.

• For an initially dry soil subjected to rain, the infiltration capacity curve shows an exponentially decaying trend as shown in Figure below.

:

INFILTRATION INDICES

• The two commonly used infiltration indices are the following:

φ – index

W – index

THE Φ - INDEX

• The rate of infiltration - the rainfall volume equals runoff volume.

PROCEDURE FOR CALCULATION OF φ – index

Example :

A storm with 10 cm of precipitation produced a direct runoff of 5.8 cm. The duration of the rainfall was 16 hours and its time distribution is given below.

Time from start (h)

2 4 6 8 10 12 14 16

Intensity of rain (cm/h)

0.20 0.45 0.75 1.15 0.90 0.80 0.50 0.25

Estimate the φ-index of the storm.

Solution :

Here duration of rainfall D = 16h, Δt = 2h and N = 8

Trail 1 :

Assume M = 8, Δt = 2h and hence te = M, Δt = 16h

Since M = N, all the pulses are included

Runoff, Rd = 5.8 cm =

Hyetograph and Rainfall Excess of Storm

Mc = number of pulses having that is

Trial 2 :

Trial 3 :

Problem Based 2 :

The following rainfall distribution was measured during a 12 hour storm :

Time (hour) 0-2 2-4 4-6 6-8 8-10 10-12

Rainfall Intensity (cm/h)

1.0 2.0 4.0 3.0 0.5 1.5

Runoff depth was 16 cm. Calculate the Φ-index for this storm.