AEG-101-Class-3 : Definition and agents of soil erosion

Introductory Soil and Water Conservation Engineering

II Semester 3^rd Feb to 30^th June 2020, 2019-20

Teacher Information

Professor	Email	Phone
Dr. K. C. Shashidhar	shashidhar.kumbar@gmail.com	9448103268

Class-3 Reference Material

Definition and agents of soil erosion

Runoff

• Runoff is the Portion of total precipitation from given area that appears in natural or artificial surface streams.
• Runoff is that portion of the rainfall or irrigation water applied which leaves a field either as surface or as subsurface flow.

Run-off is that portion of the precipitation, which is:

1. Not absorbed by the deep strata
2. Not utilized by vegetation
3. Not held in detention over the surface
4. Not lost by evaporation

and which finds its way into streams as surface or sub-surface flow.

Before runoff occurs, the rainfall must satisfy.

1. Evaporation
2. Interception
3. Infiltration
4. Surface storage
5. Surface detention
6. Channel detention

The runoff process may be described as below.

I Phase: Rain less period just prior to beginning of rainfall dry soil and low ground water high E.T.

II Phase:

• Initial period of rain. As rain starts its amount is divided among channel precipitation, interception by vegetation, infiltration in to the soil, temporary retention in surface depressions.
• The infiltrated water results in a gradual increase of water in the zone of aeration after the natural storage
• Slight evaporation and transpiration
• There is low overhand flow except on impervious surface

III phase: Continuation of rain at variable intensity.

• As rain continues the capacity of vegetation interaction and retention or surface depressions are reached and the excess rain becomes a source of runoff and detention storage on hand surface and in channels
• Over land flow occurs when the net rate of rain exceeds the infiltration rate, but it may not depending on retention and detention capacities of the land surface over which it travels
• If rain continues the water table will raise and the ground water contribution to stream flows will increase.
• As the zone of aeration is saturated sub surface runoff may contribute also to the stream flow.

IV phase:

The rainfall continuous until, all-natural storage has been satisfied. The infiltration rate will approach the rate of water transmission through the zone of aeration to both ground water table and sub surface runoff – Peak runoff.

V phase:

This is the period between the termination of rain and time when the first stage is to be reached.

Some definitions

1. Runoff: Precipitation as well as any other flow contribution, which appears in surface streams of either perennial or intermittent form. This appears at the out let of the basin.
2. Virgin flow: Stream flow unaffected by artificial diversions.
3. Over land flow: The part of surface runoff that flows over the land surface towards the stream channels
4. Stream flow: Over land flow plus other components of flow to foam total runoff in the stream.
5. Subsurface flow: It is the runoff due to that part of precipitation, which infiltrates the surface soil and moves laterally through the upper soil horizons towards the stream as ephemeral, shallow perched ground water above the main ground water level.
6. Ground water flow: Deep percolation of infiltrated water.
7. Direct runoff: Runoff portion, which enters the stream promptly after the rainfall or snow melting.
8. Channel precipitation: Precipitation, which falls directly on the water surfaces of lakes and streams.
9. Base flow: Sustained or fair-weather runoff.
10. Precipitation excess: that part of precipitation that contributes directly to surface runoff i.e. total rainfall mines abstractions (which eventually do not become surface runoff such as interception evaporation depression storage and infiltration).
11. Effective precipitation: The part of precipitation that contributes entirely to the direct runoff i.e. precipitation excess plus that part of precipitation which becomes prompt surface runoff.
12. Depth of runoff: The total runoff from a drainage basin divided by the area.

Factors affecting runoff

I. Climate

i) Precipitation characteristics:

a) Duration ii) Temperature

b) Intensity iii) Humidity

c) Distribution iv) Wind velocity

5. Direction

II) Watershed characteristics

1. Geological shape of the catchments
2. Size and shape of the catchments
3. Topography
4. Drainage pattern

III. Vegetation

Climate

The major climatic factors influencing runoff are rainfall temperature, humidity and wind; of these rainfall is the most important.

Precipitation characteristics

Rainfall intensity: It is obvious that surface runoff will increase rapidly with an increase in rainfall intensity as soon as infiltration capacity is exceeded.

When the rate of rainfall exceeds the infiltration rate of the soil, water that is not absorbed where it falls move across the land as surface flow.

Duration of rainfall: If the duration of the rainfall is more quantity of runoff will be more when 
rainfall intensity is more than the infiltration rate.

• Though duration is more if the intensity remains constant than the rate of runoff will remain constant (Catchments).

Distribution on small drainage basins the highest peak flow will be produced by intense thunderstorms covering small area. Generally, such storms will not produce the maximum peak flow on larger catchments / drainage basis.

The distribution co-efficient is the ratio between the mean on the basin for any particular storm and the maximum rainfall at any point.

If the distribution co-efficient is one we can except peak runoff from that storm but getting one is a remote chance in case of larger water sheets.

Direction: Both the magnitude of the peak flow and the time of duration of surface runoff are influenced by the direction of the storm with respect to the direction of the flow of the drainage system. Most striking differences occur in the case of the stream flow and storm directions are in the opposite direction.

Temperature humidity wind: These are most evident through their effects on evaporation and transpiration.

Watershed characteristics:

The major watershed factors influencing runoff and erosion are:

1. Geologic features of the catchments
2. Size and shape of the catchments
3. Topography
4. Drainage pattern

Geological features of the catchments

1. Types of the soil and sub-soil and their permeability
2. Location of the outlet or point of discharge of the previous deposit

Soil: Soil property which influences the runoff is the infiltration rate, when the rainfall intensity is less than infiltration capacity of the soil all the water “soaks in and there is no runoff.

The more important factors affecting infiltration capacity are:

• Permeability of soil profile
• Condition of the soil surface
• Soil moisture content

• The infiltration rate decline with the increase in soil moisture content
• Compact surface – low infiltration
• The size of soil particles generally determines the size of the openings between soil particles, which influence infiltration rate.

Sandy soils – High rate of infiltration

Clay soils – Pores are small – infiltration slow

Size and shape of the catchments

Size: - Both runoff volume and rates increase as watershed size increases. However, both rate and volume per unit area of watershed / catchment decreases as the area increases. This is due to the effect of time of travelling for the water to reach a given stations to the effect of the distribution of the rain storm over the area.

Shape: Long and narrow watersheds are likely to have longer time of concentration resulting in lower runoff rates than more compact watershed in size.

Farm shaped catchments give greater runoff because tributaries are nearly of same size and hence time of concentration of runoff is nearly same on the contrary discharge over fern leaf arrangements of tributaries are distributed over long period.

Topography: The inclination of the surface and the degree of inclination will have significant influence on runoff.

This may be very important factor which is related to infiltration capacity, time of over land flow and concentration of rainfall in stream channel.

• Flat land erosion in usually not a problem
• Velocity of the runoff water is influence mainly by the degree of slope. If the land slope is increased four times the velocity of water flowing over it is approximately doubled.

Vegetation: A good vegetative cover such as a thick growth of grass of a dense forest may negate completely the effects of climate, topography and soil on runoff.

Effective of vegetation

1. Interception of rainfall
2. Decreasing runoff velocity
3. Root effects
4. Biological influences
5. Transpiration effects

1. intercept, absorb the impact of raindrop
2. hindrance to runoff water slows down the rate at which travels down the slope
3. knitting and binding effect aggregates the soil into granules the soil into granules

• die & decay increase pore space and water holding capacity
• one cubic meter of soil has several kilometers of root fiber

4. More vegetative cover most active soil fauna, channels of earth worm beetles and other life
5. Vegetation increases the storage capacity of the soil for rainfall by the transpiration of large quantities of moistures from the soil.

Drainage patterns of the area

Ill drained – more runoff.

Estimation of runoff

Runoff is estimated by various methods. These can be classified under the following headings.

1. Empirical formulae and tables
2. Estimating losses
3. Infiltration method
4. Rational method
5. Unit hydrograph method
6. Synthetic unit graph method

Classroom Videos

Video bt Dr. Shashidhar K. C.

Estimation of Runoff

 

Rational method of estimating runoff

The rational method correctly used by many designers is given below.

Q = 0.0278 CIA

Q = Design peak runoff rate (cubic meters per second)

C = Runoff coefficient

I = Maximum average rate of rainfall centimeters per hour over the entire area which may occur during the time of concentration (cm/hr).

A = Watershed area (hectares)

This formula was proposed by CE Ramser of the USA. He proposed the formula as a result of measurements of rates of runoff and rainfall from small Agricultural Watershed.

Originally six different values for C were suggested to suit certain conditions of slope and vegetation but later other research workers have established a number of values of C to sit more specifically type of soil and vegetation cover these values are summerised and tabulated.

Values of C in the rational formula

Vegetative cover & slope	Soil texture
	Sandy loam	Clay & silt loam	Stiff clay
I wood land
0-5% slope	0.10	0.30	0.40
5-10% slope	0.25	0.35	0.60
10-30% slope	0.30	0.50	0.60
II pasture land
0-5% slope	0.10	0.30	0.40
5-10% slope	0.16	0.36	0.55
10-30% slope	0.22	0.42	0.60
III cultivated land
0-5% slope	0.30	0.50	0.60
5-10% slope	0.40	0.60	0.70
10-30% slope	0.52	0.72	0.82

The rational method is applicable to watersheds of area less than 1300 hectares.

This method is based on two assumptions.

1. Rainfall occurs at uniform intensity for duration at least equal to the time of concentration of watershed.
2. Rainfall occurs at a uniform intensity over the entire area of the watershed

Since there is hardly a rainfall satisfying both these conditions exactly, the estimates of runoff based on this method is rather approximate.

However, the method is considered sufficiently accurate for runoff estimation in the design of relatively in expensive structures where the conseques of failures are limited.

Time of concentration vary greatly with the nature and extent of the vegetation in a given watershed. However, a reasonable estimate of the time of concentration can be obtained by using the following empirical formula.

T_c = 0.0195 L ^0.77S^-0.385

In which

T_c = Time of concentration in minutes

L = The maximum length of the flow meters

S = Average slope of the area meters meter

Use graph for converting the 1-hour rainfall intensities to intensities at other duration.

Determine the design peak runoff rate for 25 years occurrence interval from an area in clay loam containing 20 ha of cultivated land on 1% slope 35 ha of cultivated land on 7% slope and 30 ha wood land on 12% slope. The most remote point in the watershed is 3750 m away from the out let. The maximum intensity of 1 hr rainfall expected during the reoccurrence interval is 7.5 cm.

Solution

20 ha cultivated land on 1% slope C = 0.5

35 ha of pasture land on 7% slope C = 0.36

30 ha of wood land on 12% slope C = 0.5

Total area = 20 + 35 + 30 = 85 ha

Weighed value of C=(0.5x20+0.36x35+0.5x30)/85=0.443

T_c = 0.195 L^0.77S ^–0.385

Weighed slope = (0.01 x 20+0.07 x 35+0.12 x 30)/85

S = 0.74

T_c = 0.0195 x 3750^0.77 x 0.074^-0.385

Tc=30

From Fig. the rainfall intensity is 11 cm / hr when the 1 hr intensity is 7.5 cm/hr and Tc is 30min

Q = 0.0278 CIA

= 0.0278 x 0.443 x 11 x 85

= 11.51 cum/sec

Classroom Videos

Video bt Dr. Shashidhar K. C.

Rational Formulae