5.1 Hazard Identification - ccma.vic.gov.au · Soil Erosion 2008-201273 SOIL EROSION - HAZARD...

Soil Erosion 2008-2012 73

SOIL EROSION - HAZARD IDENTIFICATION

The following sections provide insight into theidentification of soils susceptible to erosion. Theydescribe the different erosion processes and knownexamples within the Corangamite CatchmentManagement Authority (CCMA) region.

Information has been assembled from various sourcesfrom around Australia; these are recognised andacknowledged at the start of each section.

5.1.1 What is Soil Erosion?

Source: F.M. Garden & P.J Feehan. SCA, 1978, Soil ConservationTheory; Snowy Mountains Hydro-electric Authority 1969 Manual ofSoil Conservation Practice; Soil Conservation Handbook for NewSouth Wales “Soils – Their Properties and Management.”

Introduction

Soil erosion is the detachment, transportation anddeposition of soil by water or wind. Eroded soil in transportis ‘sediment’; sedimentation occurs when sediment isdeposited. The rate of erosion depends on the climate,soil, topography, plant cover and land use.

Water erosion processes

Water erosion is usually classified into stagescorresponding with progressive concentration of surfacerun-off. Water erosion generally starts with sheet erosion(the more or less normal removal of surface soil), then asthe water concentrates into small rivulets in the field sheeterosion progresses into rill erosion, and then, as the erodedchannels become larger, gully erosion.

The first and most important stage in the erosion process isthe impact of the raindrop. Research has shown that rainfalling at an intensity of 50mm/hr for 30 minutes generatessufficient energy to raise the top 10cm of soil to a height of45cm. Sheet erosion conjures up a picture of soil beingremoved uniformly by the even flow of thin sheets of water.This rarely happens. Laminar flow of water only scours atvelocities much higher than usually occurs on arable landor overgrazed pastures, and run-off is seldom in flatsheets. The term “Sheet erosion” will therefore be replacedby “splash erosion”.

The erosion process can also be thought of as three parts– the detachment of the soil particle from the soil mass,their transportation and their deposition.

(a) Splash erosion - the splattering of small soil particlescaused by the impact of raindrops. The loosened orspattered particles may or may not be subsequentlyremoved by surface run-off. While splash erosion on steepslopes can aid the movement of soil downhill, it does nottransport it away from the base of the slope to any greatextent.

(b) Sheet Erosion - the more or less even removal ofsurface soil. Flowing water is usually the major transportingagent when erosion is caused by rain storms. The flowingwater may move as:

• a shallow sheet across broad surfaces that have novisible channels – sheet flow; or

• rills, gullies or valley channels.

Running water gains the energy to perform work by gainingmass in its movement downhill or by gaining velocity as itfalls over a rapid change in slope. Erosion by surface flowis most apparent near the base of the slope, where thegreatest amounts of surface flow can concentrate.

A shallow-flowing sheet of water outside rills or gulliesusually only has a velocity of about 0.1m/sec. It contributesto the transport of soil particles only in a small way due tothe low velocity.

With the removal of raindrop impact from the flowing water,both the detachment and transporting capacities rapidlydecrease. If the surface is fully protected against raindropimpact, very little soil will be transported except that whichis detached and moved by channel flow.

5.1 Hazard Identification

Fig. 5.1: Research has shown that rain falling at an intensity of50mm/hr for 30 minutes generates sufficient energy to raise the top10cm of soil to a height of 45cm.

(c) Rill erosion - the formation of gutters or miniaturegullies by the action of run-off water concentrating intosmall rivulets.

As free water concentrates to form channelised flow itacquires both rotational and translational energy. Therotational energy detaches soil by scouring and thetranslational energy enables the flowing water to transportthe detached material downhill.

It is at this stage when flows of water start to concentrate,that scouring, or scour erosion starts.

Rills are usually considered to be eroded channels whichare small enough to be eliminated by normal culturalmethods.

(d) Gully erosion - the formation of steep sided channelsby concentrated run-off. Gullies often have more or lessvertical heads and are cut into sub-soil or overlying rock.They may range in depth from 0.3 m to as deep as 30m.

Most gullies have formed in natural drainage lines. Themajor cause of gully erosion is a change in the naturalecosystem by clearing, cultivating and grazing. Diminishedground cover during summer leads to faster run-off,reduced time available for infiltration and a reduction in soilpermeability (surface sealing) due to compaction byhoofed-stock and raindrop action.

Gully erosion can also be caused by sub-surface sapping.This occurs when sub-surface flow from up-slope washesout loose grains or a dissolved clay layer in a gully head,leaving a potential for collapse of the soil above. Anunderstanding of the mechanisms of gully erosion in eachparticular case is therefore important to select the correctcontrol method.

(e) Tunnel erosion - the formation of undergroundchannels or pipes through the removal of (dispersible) sub-soil by seepage water.

Tunnel erosion is a most insidious form of erosion becauseof the amount of deterioration which occurs before there isany real visible sign of damage. The earliest stages oftunnel erosion are marked by small patches of yellow claywhich have oozed through small cracks or “ant-holes” tothe surface. At a later stage, there may be conspicuous"fans” of yellow clay material which has been washed downslope from small holes. At an even later stage, there maybe a line of holes upslope from the point where the clay isfirst washed out. This is due to collapse of the tunnel. Thefinal stage is total collapse of the tunnel to form a gully.

Wind Erosion

Wind erosion is not normally as serious as water erosionbut there are occurrences where it is more severe than theworst water erosion. Severe and damaging wind erosion isusually confined to arid and semi-arid climates but can alsooccur in humid areas.

There are three main factors affecting the incidence of winderosion:

(a) Soil condition - The physical nature of the soil willaffect the ease with which particles are dislodged; but byfar more important than this is the fact that only dry soilblows. Any soil will be unmoved by wind while its surface ismoist.

(b) Rainfall - Wind erosion is more frequent where themean rainfall is low. As a rough guide, areas most affectedare those with less than 300mm, and those with higherrainfall but long dry periods.

(c) Vegetation - Soils is least vulnerable to wind erosionwhen well vegetated. The vegetation physically preventsthe wind coming into direct contact with the soil. In thesame way, a good cover of vegetation prevents splasherosion by water.

In the Corangamite catchment the soils most vulnerable towind erosion are those on high hill tops that have beenovergrazed or used as stock camps and are devoid ofprotective vegetation. Other soils are light sandy soils alongthe coast.

5.1.2 Soil Erosion in the Field

i). Sheet erosion:

74 Corangamite Catchment Management Authority Training Manual 2008-2012


Fig. 5.2: Sheet erosion on steep hills removes the topsoil and plantnutrients and develops a hard impermeable surface which increasesthe rate of runoff creating further erosion problems down slope.



ii). Rill Erosion:

iii). Gully Erosion:

iv). Tunnel erosion:

v). Wind erosion:

Fig. 5.3: Sheet and rill erosion can be a major contributor ofsediment to public infrastructure such as roads and streams. Fig. 5.6: Tunnel erosion can cause havoc on building blocks. There

was no sign of this problem prior to its collapse.

Fig. 5.4: Rill erosion in a paddock situation can create problemswith paddock trafficability and concentrates water flow whichincreases velocity and leads to formation of gullies if unattended.

Fig. 5.7: The first sign of tunnel erosion is yellow clay emanatingfrom pop-holes or formation of silt fans. The final stage is thecollapse of the tunnel and formation of a gully.

Fig. 5.8: Wind erosion destroys the soil structure and fertility byblowing away the fine soil particles which also contain soil nutrients.Wind blown soil can impact on paddock structures such as fencingand water supplies, as well as public amenities, roads, houses androad safety during a wind storm.

Fig. 5.5: Gully erosion in a paddock situation can cause disruptionto cultivation, stock movement and general access across thepaddock. Sediment from the gully can impact on public utilitiessuch as roads and bridges.



5.1.3 Known extent of soil erosion sites in Corangamite CMA region

Sheet and Rill Erosion

CORANGAMITE

GLENELG HOPKINS

PORT PHILLIP & WESTERNPORT

NORTH CENTRAL

Colac

Terang

Cressy

Lismore

CobdenTorquay

Skipton

Macedon

GEELONG

Ballarat

Werribee

Mortlake

Beaufort

Anglesea

Lorne

Blackwood

Lake Bolac

Inverleigh

Camperdown

Apollo Bay

Cape Otway

Princetown

Glenaire

Queenscliff

Diggers Rest

Port Campbell

Riddells Creek

0 10 20 30 405

Kilometres

Tunnel Erosion sitePublic Land

Fig. 5.9: Known tunnel erosion sites in the Corangamite Catchment Management Authority region

Fig. 5.10: Gully erosion susceptibility in the Corangamite CMA region



Fig. 5.11: Gully erosion susceptibility in the Ballarat region



Fig. 5.12: Gully erosion susceptibility in the Colac Otway Shire region



Fig. 5.13: Gully erosion susceptibility in the Corangamite Shire region



Fig. 5.14: Gully erosion susceptibility in the Golden Plains Shire region



Fig. 5.15: Gully erosion susceptibility in the Moorabool Shire region



Fig. 5.16: Gully erosion susceptibility in the Surf Coast Shire region



Sheet and Rill Erosion

Fig. 5.17: Sheet and rill erosion susceptibility in the Corangamite CMA region



Fig. 5.18: Sheet and rill erosion susceptibility in the Ballarat region



Fig. 5.19: Sheet and rill erosion susceptibility in the Colac Otway Shire region



Fig. 5.120: Sheet and rill erosion susceptibility in the Corangamite Shire region



Fig. 5.21: Sheet and rill erosion susceptibility in the Golden Plains Shire region



Fig. 5.22: Sheet and rill erosion susceptibility in the Moorabool Shire region



Fig. 5.23: Sheet and rill erosion susceptibility in the Surf Coast Shire region

5.1.4 Field Recognition

Source: Shelley McGuinness (1991) Soil Structure Assessment Kit,Department of Conservation and Environment, Victoria; TroyClarkson “South West Victorian SoilSmart series: Dispersive soils”Agriculture Notes, DPI 2003.

Soil Stability Test:

This test is a modification of the laboratory-based EmersonDispersion Test and is used to determine:

• if the soil is dispersive

• whether the soil will slake?

Requirements:

• two to four clean, shallow, flat-bottomed glass jars

• about a cup (250 ml) of rainwater or distilled water, (donot to use dam or bore water because dissolved saltswill impact on the results).

Note: It is best to use dry soil samples from the site you willbe working on.

Procedure:

• select three aggregates from each sample area, eachapproximately the size of a pea

• put enough water in each jar to cover the aggregates(6 – 10 mm deep) and then place the aggregatescarefully into each jar.

Note: Use one jar for the top soil and the other jars forsubsoil samples

• watch the aggregates carefully for the first few minutes.

Results:

Slaking (Figure 5.24) will occur almost immediately if it isgoing to happen at all. You will see small bits of soil fall offthe side of the aggregate, small bubbles of air escapingfrom the aggregate and eventually the entire aggregatemay ‘fall to bits’.

If a soil is dispersive (Figures 5.25 and 5.26) you will seethe water become cloudy but the soil aggregate will notbreak up as it did in the slaking example. This may occurwithin the first hour, but may also take some longer time,e.g. overnight.



Fig. 5.24: Unlike dispersion, slaking is a mechanical process andoccurs when the soil structure is weak. When a dry soil is wetrapidly water moves into pores within the aggregate and forces airout. The force of the escaping air causes the aggregate to burst.The soil mictoaggregates are washed into the soil and block soilpores and form a crust on the soil surface. In the field soil crustingcan be caused by slaking or dispersive soils. Organic matter isimportant as a treatment for holding soil particles together andpreventing them from slaking.

Fig. 5.25: Complete dispersion. A cloud of dispersed clay covers thebottom of the dish and the aggregate has almost disappeared.

Fig. 5.26: Incomplete dispersion. The dispersed clay spreads in thinstreaks and crescents on the bottom of the container.



5.1.5 Soil erosion Case Studies

Case Study 1: Road Construction andErosion Prevention

Source: F.M. Garden, Gravel Road Erosion Prevention Guidelines,Department of Conservation, Forests and Lands, Vic

Gravel Road Erosion Prevention Guidelines

Roads are a major source of erosion and sediment. Gravelroads have an inherent potential for causing erosion. Theyhave a surface, batters and table drains of subsoil materialwhich is frequently erodible and exposed to splatteringaction of raindrops.

They have an impermeable surface that generates highrun-off which in turn causes rill erosion along the tabledrain or where the run-off is redirected from the road to thesurrounding area. Roads interrupt the natural drainagepattern, concentrating flows and converting overland flowinto concentrated flow. The combination of these factorsoften leads to off-site erosion.

Careful location of a road can significantly reduce itsconstruction, drainage and maintenance costs. Forexample, roads on ridges and spur tops require a minimumof earthworks because they receive only that rain whichfalls on them and are easy to drain. (Figure 5.28). Roadsthat run across slopes require an increasing amount ofearth-moving as the steepness of the cross-slopeincreases. The gentlest cross-slopes available should beselected and use made of natural benches whereverpossible. (Figure 5.29 a, b, c). As these roads also receiverun-off from slopes above them, they require more drainsat closer intervals.

Where the velocity of water is doubled, its capacity toerode and transport sediment increases 64 times. It istherefore important to provide more drains and use gentlegradients wherever possible.

Design of roads

Gradient

The gentler the gradient, the less the erosion hazard andthe further apart cross-drains may be placed.

On major roads, try to keep the gradients below 1 in 12 orabout 8%, with only short gradients of above this level.

On minor roads including farm tracks, gradients of up to 1in 10 (10%) are common, with occasional short gradients of1 in 5 (20%).

Surface

Roads should be designed to have a crowned surface thatis 150 – 300mm higher than the table drain on one or bothsides. This surface will drain readily and is less likely toerode or become boggy.

Batters

Road batter slopes should be designed to suit the materialin which they are constructed. They should range from 1 in4 in sand or other free moving material, to steeper than 1 in1 in rocky or more stable material.

Fig. 5.27: Roads have a surface, batters and table drains of subsoilmaterial which is frequently erodible and continually exposed toerosion by rain and surface run-off. Note that this road has noapparent opportunity to divert run-off away from the road.

Fig. 5.28: Roads on ridges and spur tops require a minimum ofearthworks, because they receive only that rain that falls on themand are easy to drain.

Fig. 5.29 a, b, c: Roads that run across the slope require anincreasing amount of earth works as the steepness of the cross-slope increases. The gentlest cross-slope available should beselected and use made of natural benches.

Drain capacity

Culvert pipes should have sufficient capacity to allow atleast 1 in 10 year flood flows. In forested areas, normalroad drainage culverts should have a minimum diameter of450mm to minimise the risk of blockage by leaves andtwigs.

The required size of culverts can be calculated using theformula contained in the Road Design Manual published byVicRoads. For small farm tracks, culvert size can becalculated using the old empirical rule of thumb: Squareroot of the catchment (in acres) divided by 4 = diameter ofthe culvert required in feet.

Example:Catchment Area is 100 acresSquare root of 100 = 10; Culvert diameter is 10/4 = culvert pipe of 2.5 feet diameter.

Construction and drainage of roads

The problems to be minimised in road construction anddrainage are deterioration of the road surface and tabledrains by erosion. (Figure 5.31).

Reducing flow velocity and subsequent erosion in the tabledrains may be achieved by constructing a wider drainbase, lining it with graded rock or establishing protectivevegetation cover.

Recent research has shown that the surface of earthen orgravel roads deteriorates rapidly when rills are allowed toerode deeper than 25mm.

The distance that water flows down a road surface beforeeroding rills deeper than 25mm determines the spacingsthat are required between cross drains installed to reducethe volume of flow. A “rule of thumb” calculation for culvertspacing is 300 divided by the % grade of the road.

Example:Gradient of road 10%, Culverts would need to be spaced at 300/10 = 30mintervals.

Once flow is collected by the culvert and diverted off theroad, it needs to be spread over as wide a path aspossible to reduce the velocity.

Cut-off Drains

On all-weather roads, drainage is usually provided bycrowned surface and table drains, with cut-off drainsdispersing water to either side as required on ridge-toproads, (Figure 5.31) and culverts carrying it across the roadon side-cut roads, (Figure 5.32).

Out-sloping

Another means of obtaining cross drainage on gravel roadsis by out-sloping; that is grading the road surface so that itslopes downward across the road from the cut batter to theshoulder. This method is not suitable on roads that canbecome slippery when wet. (Figure 5.33).



Fig. 5.30: An eroded road surface and table drain

Fig. 5.31: Run-off from a ridge- or spur-top road is dispersed to bothsides as required by means of mitre or cut-off drains.

Fig. 5.32: On side-cut roads, culverts carry run-off water from thetable drain across the road to a safe diversion point.



Protection of road batters

Batter Slopes

Batters should be constructed with slopes that suit thestability of the soils encountered. On a cut batter, batterstability may be enhanced by construction of a tapered toeon the batter, with the table drain offset on the incurves. Thisform of construction reduces the effects of undercuttingduring grading operations. The toe of the batter should beslashed following revegetation (Figure 5.34).

Road Maintenance

A considerable amount of careful planning and work goesinto construction of a stable road free of acceleratederosion, but this continued state is often destroyed by poormaintenance.

The four most common problems are:

1) Failing to re-open mitre or cut-off drains blocked by roadsurface grading (Figure 5.35)

2) Leaving an unnecessary ridge or bank of graded earthor gravel along the outside edge of the road so that thewater cannot flow freely off the road but concentratesinstead to scouring volumes and velocities (Figure 5.36)

3) Removing the toe of the cut-off batters whilst clearingout table drains with a grader (Figure 5.37). This oftenupsets the whole stability of the cut batter leading tofurther undercutting by water in the table drains causingeither slumps or the onset of rill erosion.

4) Dumping unwanted sediment from drains, or material fromcut batter slumps, where the table drain and other drainsdischarge at the end of the cut section (Figure 5.38).

Fig. 5.36: The windrow of earth on each side of this road preventsdrainage of run-off water from the road.

Fig. 5.33: An outsloped road. Care should be taken where soils becomeslippery when wet.

Fig. 5.34: Tapered toe to cut batters on incurves, with off settabledrain

Fig. 5.35: A cut off drain blocked by road grading must be re-opened for effective road drainage.

Case Study 2: Erosion Control and Prevention

Source: Trevor Cox “Soil Conservation Structures” 1993,Department of Conservation and Natural Resources; “Guidelines forMinimising Soil erosion and Sedimentation from Construction SitesTC-13” Soil Conservation Authority

Severe erosion periodically occurs in the CorangamiteCatchment. Heavy rain falling on bare soil provides thetrigger for large scale erosion.

There are many techniques that can be undertaken tominimise the effects of soil erosion. Broad-scale catchmentimprovement can be used to protect the soil, improve soilstructure, increase infiltration and reduce run-off. Onceerosion has occurred, soil conservation structures may benecessary to reduce the erosive energy of water,particularly when it becomes concentrated.

The previous Case Study focused on proper design andlocation of minor roads to prevent and minimise erosion.This section will provide basic detail on soil conservationtechniques for existing gully erosion.

Soil Conservation Structures

Soil Conservation structures are often needed to supportbroad catchment improvement activities. The reasons whythese structures are required can vary with each situation.There could be concentration of considerable surfaceflows, a geomorphic imbalance that provides a trip-point,or there is a need to protect a damaged area.

These structures need to be carefully designed and built tosafely deliver run-off water to a relatively stable and safedisposal area within the landscape.

Protection of these structures against damage by watercan be achieved by using either vegetative or non-vegetative mediums such as masonry, fibreglass, metal, orreinforced concrete.

Vegetated Structures

These are generally cheaper than non-vegetated structuresand therefore should be used wherever possible. Thechoice between vegetated or non-vegetated structuresdepends on a number of factors.• availability of space to build a suitable vegetated

structure• required capacity• amount of erosion in the catchment• slope of site• cost of maintenance• suitability of soil for building structure.

Figure 5.39 can be used as a general guide to decidingwhether vegetated or non-vegetated should be used.

Grassed Chutes

These structures are designed to safely convey run-off fromone level to a lower level by means of sloping surfacewhich has been prepared and planted with vegetationselected for its resistance to flowing water.

Grassed Chutes are generally installed in gully heads ordam spillways with a longitudinal slope of between 5% and25%. Generally, they are limited to gullies up to two metresin depth.



Fig. 5.37: Removal of the toe of the cut batter during grading upsetsthe stability of the batter leading to slumping or rilling and blockingof table drains.

Fig. 5.38: Spoil from the slumped cut batters or silted table drainsshould not be dumped near culvert or table drain outlets.

Fig. 5.39: Selection Guide for Vegetated Structures.



Non-Vegetated Structures

For non-vegetated structures, the choice between thevarious types of structures that are available depends on:

• required capacity

• drop to be controlled

• suitability of soils for structure.

Figure 5.41 can be used as a general guide for decidingwhich type of non-vegetated structure is most likely to bethe most suitable at a given site.

Non–vegetative structures can be in the form of loose rock,rock chutes, rock in wire mesh, reinforced concrete dropstructures made of concrete inlet box structures, flexiblepipe structures. The type of structure used will depend onthe depth of gully to be controlled and the volume of waterto be handled. The non-vegetative structures usually haveto deal with deeper gullies and larger volumes of water.

Essential reading: “Guidelines for Minimising Soilerosion and Sedimentation from Construction SitesTC-13” Soil Conservation Authority provides all of thedetail for designing these structures and is a valuablereference publication.

Tunnel erosion Rehabilitation

Tunnel erosion is such an insidious type of erosion; itusually appears after the damage has been done. It isimportant to know and be able to recognise the first stagesof tunnel erosion. These include small yellow clay moundsor fans emanating from holes in the ground. This material isdispersible clay that has been washed out of the subsoil.As more material is washed out, the tunnel undergroundincreases in size until the roof of the tunnel becomes tooheavy to sustain its own weight and collapse to form a gully.

As tunnel erosion occurs out of site, the full extent of thetunnel and its exact location is unknown.

Treatment

To be able to control tunnel erosion it is essential tocompletely destroy the tunnel. This is usually done with adeep ripper behind a bulldozer. It is important that thetunnel is ripped to its full depth and the tunnel cavitydestroyed. Experience suggests that this takes time todetermine the full location and complete the ripping task.

Because tunnel erosion occurs in dispersible soils, therehabilitation process requires the addition of gypsum as asoil ameliorant to prevent or minimise further dispersion.Gypsum should be applied to the ripped area at a rate ofat least 2t/ha prior to sowing the ripped area down todeep-rooted perennial species.

Fig. 5.40: Grassed Chutes are limited to gullies up to 2 m in depth.

Fig. 5.42: The pipe structure is one of the effective ways of deliveringflow to a gully floor safely. A trench should be prepared for laying thepipe no steeper than 2.5h:1v. The trench must be tightly backfilledafter laying the pipe and have anti- seepage collars installed.

Fig. 5.41: Selection Guide for Non-Vegetated Structure

5.1 Hazard Identification - ccma.vic.gov.au · Soil Erosion 2008-201273 SOIL EROSION - HAZARD...

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