Land and Water Conservation Engineering Part 2

Soil Erosion

is the process by which soil particles are dislodged by the action of wind, water, gravity, or mechanical forces

natural or geologic erosion

includes soil forming as well as soil eroding processes that maintain the soil in a favorable balance, suitable for the growth of most plants

man-man or accelerated erosion

includes breakdown of soil aggregates and accelerated removal of organic and mineral particles resulting from improper tillage and removal of natural vegetation

natural or geological erosion

man-man or accelerated erosion

TYPES OF SOIL EROSION

detachment of soil particles from the soil surface

transport of detached particles

two main interactive processes

particle detachment

transport

and deposition

process of erosion

initiation of movement

transportation

deposition

process of wind erosion

initiation of movement

soil movement as a result of turbulence and velocity of wind

transportation

quantity of soil moved is a function of particle size, wind velocity, and distance across the eroding area

deposition

this occurs when the gravitational force is greater than forces holding the particles in the air.

generally occurs when there is a decrease in velocity

wind erosion

soil movement caused by moving air responsible for the formation of desert or sand dune

gravity erosion

erosion caused by force of gravity like landslide, sudden and instantaneous soil mass movement and slow creeping of soil along the slope

mechanical erosion

erosion caused by moving machines or animals

ex. road construction

water erosion

erosion caused by action of runoff and rainfall

randrop/ splash erosion

is soil splash resulting from the impact of water drops directly on soil particles.

sheet erosion

is the uniform removal of soil in thin layers from sloping land resulting from sheet of overland flow occurring in thin layers

rill erosion

is the removal of soil by water from small but well-defined channels or streamlets when there is a concentration of overland flow

advanced stage of sheet erosion

gully erosion

erosion in channels larger than rills.

these channels carry water during and immediately after rains, and, as distinguished from rills

advanced stage of rill erosion

stream channel erosion

consists of soil removal from stream banks or soil movement in the channel

suspension

saltation

bed load

types of sediment transportation in a stream

suspension

sediments that remain in suspension in flowing water for a considerable period of time without contact with the stream bed

saltation

occurs where the particles skip or bounce along the stream bed

bed load

sediments that moves in continuous contact with the stream bed being rolled or pushed along the bottom by the force of the water

climate

soil properties

soil cover

topography

factors affecting water erosion

rainfall

affects the runoff and volume.

it causes the breakdown of aggregates and the dislodgement and dispersal of soil particles, thereby sealing the surface and increasing the amount of runoff

temperature and wind

effect evaporation and transpiration

humidity and solar radiation

less directly involved in the erosion process, but they are associated with temperature

rainfall

temperature and wind

humidity and solar radiation

climatic factors that affect water erosion

soil texture

refers to the size and proportion of the particles comprising the soil

soil structures

refers to the arrangement of soil particles into aggregates.

it influences the water movement and water holding capacity of soil

soil texture

soil structure

soil properties that affect water erosion

soil cover

non vegetative covering involves the application of some surface treatments on the ground to protect the soil from erosive actions associated with rainfall, runoff, wind and temperature

degree of slope

affects the flow velocity and the energy of flow

length of slope

affects both the quantity and the depth of flow

orientation of slope

determined the effectiveness of climatic forces to influence soil erosion

degree, length, and orientation of slope

topographic factors influencing water erosion

Universal Soil Loss Equation (USLE)

it estimates sheet erosion as the product of a series of terms for rainfall, soil, slope, gradient, slope length, crop and cover management and conservation factors

Modified Universal Soil Loss Equation

MUSLE

Rainfall Erosivity Index, R

quantitative measure of erosion potential of rain

is a numerical evaluation of a rainstorm or a rainfall pattern which describes its capacity to erode soil from an unprotected field

Soil Erodibility, K

is a complex interaction of the soil physical and chemical properties

Slope Factor and Slope Length, LS

all the erosion sub-processes are influenced by slope

increasing slope generally increases their rates

Conservation Practice Factor

is the ratio of soil loss from a given area with conservation practice to that from an identical area that is periodically tilled up and down the slope

Sediment Transport

is the movement of organic and inorganic particles by water

bed load

moves along the channel bed in almost constant contact with the bed

suspended load

is carried in the fluid away from the bed

saltation

transport of sediments by hopping and bouncing along the channel bed

bed load

suspended load

saltation

types of sediment transport by a stream

wash load

the fine fraction which is mostly in suspension

bed material load

the coarse fraction normally found in the bed

wash load

bed material load

sediment load of a stream can be divided into two parts

sediment discharge

the transport rate of sediments in a stream, thus the terms bed load discharge, suspended load discharge, bed material discharge, etc.

sediment yield

total annual sediment discharge at a reference point across a stream system

trap efficiency

percentage of incoming sediments retained in the reservoir

earth embankments

are important structures to control and regulate excess water flows that may cause excessive erosion and flooding in downstream areas

simple embankment

embankment constructed of relatively homogenous soil material and is either keyed into an impervious foundation stratum or is constructed with an upstream blanket of impervious material

core or zoned type

this design utilizes within the dam a central section of highly impermeable materials extending from above the water line to an impermeable stratum in the foundation

diaphragm type

uses a thin wall of plastic, butyl, concrete, steel, or wood, to form a barrier against seepage through the fill

full diaphrahm

cutoff extend from above the waterline down to and sealed into an impervious foundation stratum

partial diaphragm

cutoff does not extend through this full range and is sometimes referred to as a cutoff wall

farm reservoir

is a conservation structure use to store water for irrigation livestock, fish production, recreation or any combination of these uses

dugout ponds

this is constructed by excavating or digging out a depression

on stream ponds

constructing a dam in a natural ravine

off stream ponds, by-pass ponds

is constructed adjacent to a continuously flowing stream, and an intake through either a pipe or open channel, diverts water from the stream into the pond

small farm reservoir (SFR)

is a water impounding earthen structure that has an open water area of less than 1 ha and a maximum embankment height of 3 meters

can be built in flat areas or on slope of up to 15%

maximum of 3 meters

have a water depth depth of 1.5 meters or higher

reservoir capacity

this is the volume of water stored at maximum water level

product of the open water area at maximum water level and the average depth of the reservoir

4 m or more

dugout ponds may have a water depth of ___

0.5 to 1.1 per 1000 cubic meters

SFR

a catchment area of ____ per ___ of reservoir capacity is sufficient

0.3 per 1000 cu. m

SFR

a suitable service area is approximately ____

VEGETATIVE

ENGINEERING

VENGINERING

TERRACING

CONTROL SOIL EROSION STRATEGIES

Vegetation

absorbs the energy of falling water and reduces the runoff volume and erosive velocity. The effectiveness of vegetation in controlling erosion varies according to density, composition, and structure.

Cover cropping

is the growing of crops to cover the soil and protect it from the impact of raindrops which causes soil detachment and dispersion.

PLANTS

• act as protective shields to the soil

• less impact of rainfall, wind, excessive watering and ice melt

• help stabilize the soil

• prevent from becoming prone to soil erosion.

CROP ROTATION

practice of growing different crops in succession on the same land to preserve the productive capacity of the soil

is the systematic planting of different crops in succession on the same piece of land

STRIP CROPPING

A method of farming which involves cultivating a field partitioned into long, narrow strips which are alternated in a crop rotation system

It is used when a slope is too steep or when there is no alternative method of preventing soil erosion.

s the growing alternate strips of different crops in the same field. For controlling water erosion, the strips are always on the contour. T

Contour Strip Cropping

Field Strip Cropping

Buffer Strip Cropping

The three general types of strip cropping

Contour Strip Cropping

The growing of row crops and soil-conserving crops planted alternately on strips laid on the contour or on the level across the slope. One strip is planted to a crop with a closegrowing habit and the next strip is planted to a row crop.

Field Strip Cropping

the strips of uniform width are placed across the general slope. This practice may also be used for wind erosion contro

Buffer Strip Cropping

designed to work on rolling topography with irregular slope gradients. It has strips of grass or legume crop between contour strips crops. Buffers may be irregular in width and placed on critical slope areas of the field.

The main purpose of buffer is to give protection from erosion or allow for areas of deposition.

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MULCHING

is the covering of the soil with crop residues such as straw, corn stalks, and standing stubbles to protect both crop and soil from damage by water erosion.

Assist the soil to soak in water slowly

lessen the impact of rainfall as it penetrates through to the soil

stabilize the affect soil by regaining its pH levels to be healthy and neutralized

Multiple cropping

the growing of two or more crops a year in sequence, or intercropping, the growing of two or more crops on the same piece of land at the same time.

This practice aims to increase productivity while providing protection of the soil from erosion.

H E D G E R O W S

is the planting of perennial crops close together, usually along the contour of the sloping land, to form a living wall.

This wall will slow down surface runoff and deposit or trap the eroded soils at the base of the hedgerows

made up of two or three staggered lines of grasses or shrubs also operate as very effective permeable microdams

WAT T L I N G

consists of interwoven fences of brushwood in shallow trenches where pegs/stakes are driven between in which sprouting branches of species like wild sunflower and lantana are woven.

is done across the slope of the field. It stabilizes the slope of the field and the soil is kept right in place by its retaining stability.

Reforestation

is the planting of forest trees to bring back vegetation on denuded areas.

Afforestation

is the establishment of a forest or stand of trees in an area where there was no recent tree cover.

MATTING

• Placed on the soils surface and prevent erosion from occurring

• Allow plants, crops and trees to grow through it and the soil will be healthy and stabilized

Wood fibers

environmentally friendly and biodegradable

TERRACE

is a broad channel constructed across the slope of rolling land to control runoff in areas of high rainfall and for conservation of water in low rainfall areas.

are used where simple and less expensive measures will not provide adequate control of erosion

Runoff water is collected in the channel and conveyed from the field in such a way that erosion is kept under control.

VEGETATED WATERWAY

• A diversion ditch is a channel is designed to intercept runoff water from upper areas and carry it away at non-erosive velocity so that it will not cause damage to lower lying areas.

• Diversion channels are usually constructed at the top or the foot of steep slopes or on property line to protect productive soil from erosion or inundations.

Drop structures

are small dams used to stabilize steep waterways. They are placed at interval along the channel to stabilize it by changing its profile from continuous steep gradient to series or more gently sloping reaches which will slow down the velocity of flowing water.

chute structure

is designed to carry flow down a steep slope through a concrete-lined channel rather than by dropping the water in a free overfall. It is used for the control of elevation changes up to 6 meters.

CHECKDAMS

are structures constructed across the natural waterway to temporarily store runoff water and raise the waterhead.

These structures are used when vegetation in the waterway cannot be established and runoff rates are very high.

FARM PONDS

are designed to store runoff water and minimize the potential runoff that may cause soil erosion.

R I P R A P

are stones/rocks fitted or filed on top of each other to from retaining walls.

are constructed on necessary points of hillsides in order to reduce the slope incline, support collapsible slopes, prevent washout of grading soil, and preserve drainage work.

CONTOUR ROCKS/ ROCK WALLS

• Stones/rocks are filed along the contour of the sloping areas to form a wall.

• This is used in areas with abundant rocks and where terracing is not appropriate.

• A one meter wide area along the contour is leveled to provide a good base for the wall. The height of the wall is from 30-50 cm, depending upon the slope gradient

JETTY

is a structure built to direct the current of a stream/river away from the bank.

are set at various angles with the current, ranging from as small as 10 or 15 degrees to a right angle

a dike-like structure extending from the stream bank out into the streambed

STONE WALLS

• These walls are carefully built by piling up flat stones, wedged with small rock chips.

• found in sandstone hills

LOW STONE BUNDS

Two or three layers of stones are so arranged along the contour line as to reinforce one another.

FIBER ROLL

is a temporary erosion control and sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment erosion.

It is made of straw, coconut fiber or similar material formed into a tubular roll.

GABION WALL

is a retaining wall made of stacked stone-filled gabions tied together with wire.

are usually battered (angled back towards the slope), or stepped back with the slope, rather than stacked vertically.