5 CHAPTER: Erosion
5.1 Introduction
Erosion is the number one problem in Australia’s creeks today.
Erosion can lead to:
- A loss of fertile land
- A loss of biodiversity
- A loss of infrastructure such as fences and bridges
- The smothering of down stream properties
- The contamination of in stream water
- Damage to the surrounding water table
- The excessive sedimentation of water storage areas such as reservoirs and dams
While there is a range of different forms of creek erosion the root cause is nearly always the same, flowing water cutting into the creek bed or under cutting the creek bank. Once the creek bank or bed has become unstable then the way is open for a range of other forms of erosion to take place, see below.
It is estimated that in any given year approximately xxxx kms of usable farmland is lost to erosion.
5.2 A typical erosion story: From beginning to end
100 years ago a farmer buys a rural of 100 acres. The property is nestled in a small range of hills and contains an alluvial plane of rich soil. From the farmers perspective the major downside of the property is that the alluvial plane is a virtual marsh for much of the year which both excludes his stock from the area while at the same time denying him permanent access to the richest soils on his property.
The farmer determines to drain the marsh by running a single plough line trough it in summer and thus encouraging the water in the marsh to drain away. The problem is that the soil in the marsh is rich largely because it is sedimentary soil that has accumulated over thousands of years and as a result has very little structural integrity besides that provided by the surrounding vegetation. As soon as the water starts running along the plough line it begins a process of scour, removing soil from the sides and bottom of the furrow created by the plough.
Within just a couple of years the single furrow line has eroded to become a small incision in the landscape. The deeper the incision becomes due to scour the more water from the adjacent land starts to find its way into the incision. As more and more water flows down the incision increasing both the volume and speed of the flow the faster and faster the erosion occurs.
Soon any remaining vegetation that may have been left along the length of this new water channel has been eaten by stock, trampled by stock or been undercut by the water flow and washed away. With the removal of adjacent vegetation the last significant impediment to a full-scale process of erosion has been removed.
While the incision, which has now has eroded into a recognisable channel, is relatively stable in time of low flow, However once there is a flood the erosion process truly takes hold.
In a flood event all the surrounding water in the local landscape is now being funnelled by the lie of the land into the emergent creek. The water becomes a raging torrent ripping away at the sides of the creek and cutting into the creek bed.
Within 20 or so years of the original plough line being cut into the marsh the creek has become so deep that even when the creek is not in full flood, the flowing water is under cutting the creek banks causing every increasing segment of adjacent land to fall into the creek only to be quickly washed away by the moving water. In addition where concentrations of water is entering the creek along its length smaller side channels or gullies are forming and beginning to become major erosion problems in their own right.
Of course this on going and expanding erosion problem is not confined simply to the original property. As the creek channel deepens, a series of “head cuts” begin to work their way up steam spreading the erosion problem onto neighbouring properties. While at the same time all this eroded material is being washed down stream burying wildlife and crops in a deep layer of sludge.
Worse still the depth of the creek has now reached the water table and water from well beyond the creek is being syphoned out of the landscape and into what has become more like an eroded drain than a natural waterway.
When will the erosion problems associated with this creek end? If the creek erodes to a depth were a firm base if found then this will limit the creek from getting any deeper and this will slow down the rate of erosion. However having an solid base in the creek but it will not stop the creek from continuing to undermine the creek banks making the banks unstable and causing them to continue to fall into the flowing water. In addition to an ongoing bank collapse erosion will continue in side gullies long after the main channel may have hit a firm base.
100 years after the initial draining of the marsh significant erosion is continuing and is unlikely to cease until at least basic rehabilitation works, such as the exclusion of stock and revegetation, are undertaken.
5.3 Types of Erosion
There are a specific range of terms that are used in technical literature to describe the erosion process that occur waterways and they are included below with the source of the definition in brackets attributed next to them. However there are also some specific forms of erosion which occur which are not yet widely discussed in the technical liturature. These additional forms of erosion and their description are also attached to the list below.
Scour:
Scour is a term that relates to the removal of soil, clay gravel etc, as a result of friction generated by a moving current of water.
Scour is the most destructive form of erosion in creeks, not only in its own right, but because it leads either directly or indirectly to virtually all other types of significant creek erosion.
Scour erosion is most obvious where water can be seen undercutting banks or at the point of step erosion in the creek bed.
Calving
Calving describes a situation where a part of the verge peals away from the rest of the verge and falls into the creek channel. The area of verge involved in calving event is usually relatively small and tends to be a strip shaped section of verge. Calving occurs when the channel bank becomes too steep to support a particular area of the verge. The catalyst for the calving event may either be continued undercutting of the bank or saturation of the channel bank increases to a point where the calving event occurs. While not directly caused by scouring, if scouring had not occurred lower down the bank making the bank unstable, the verge would not have calved.
Slumping
Slumping is similar to calving but is typically used to describe and area of verge which slides, or slumps in to the creek channel. The other major difference to a calving event is that in a slumping event the amount of verge that ends up in the channel can be considerable. It is not uncommon of a serious slumping event to even block the low flow channel until scouring washes a new path through the blockage. The causes for slumping are similar to the causes of calving, namely under cutting of the bank and or excessive subsoil moisture
Step erosion
Step erosion occurs when there is a rapid change in creek bed gradient coupled with a soft creek bed. What happens is that a step occurs in the bed of the creek looking like a mini waterfall? As water runs down the face of the step it causes both a mini plunge pool at the base of the step while eroding the face of the step. This duel dynamic can cause the step to progress rapidly up stream in even modest water flow events.
While the step erosion itself is problematic through the damage it causes to the bed of the creek, it is the rapid deepening of the entire creek channel step erosion leaves in its wake that leads to even more damaging secondary erosion events such as slumping calving and increased gully erosion. Left un-checked step cut erosion will quickly work its way continuously all the way upstream until it his a creek base such as rock that it can not erode.
Another common feature of step erosion is that once the conditions are suitable for one step erosion event then is quite common for there to be multiple step erosion events working their way up a creek at any one time.
The final difficulty with step erosion is that halting a typically fast moving step erosion event is rarely possible with vegetation alone. This means that rock works involving machinery and significant cost is often the only viable way of stopping step erosion.
Gully erosion
Gully erosion relates to what is often in essence a high-speed scour event that occurs where concentrations of water from the surrounding landscape enter the creek channel proper. The reason behind these accelerated scour events is that gully channels nearly always have a bed gradient which is significantly steeper gradient than the main creek channel and the increased gradient of the gully leads to greater water velocity and hence erosion.
In addition gullies can be particularly vulnerable to erosion if stock begin to use the gullies along the creek side as access points to the bed of the creek proper.
Tunnel erosion
Tunnel erosion refers to an event where surface water finds a path to the creek where it is easier to flow under ground than on the ground. Common situations where this occurs where there may be an below ground weakness, not uncommonly initiated by an animal burrow such as a rabbit or wombat burrow or where the surface layer of soil is reinforced by plant roots and or where there are highly dispersive clays beneath the top soil.
5.4 Major factors effecting erosion
Geology
In this context we are using the term geology to describe the various soils, clays, rocks etc that the creek runs though and which are located in the surrounding area and their impact rates of erosion.
The single biggest effect geology has on creek erosion is quite simply the level of resistance that that geology offers to erosive forces. In creeks that have a hard rock bed and rocky banks even significant changes in the local catchment which lead to much larger water flows entering the creek, are unlikely to lead to significantly greater levels of erosion.
On the flip side if the geology that a creek runs through is highly dispersive the incidence of erosion of various types is likely to be correspondingly high.
Other major ways that creek geology can affect rates of erosion include:
- The more fertile the soils the greater the chance of vegetation establishing within the creek channel
- The likelihood of different geologies to slump and calve
- The speed and quantity of ground water delivery to the creek
- The type and quantity of surrounding vegetation that the geology encourages
Hydrology
When discussing hydrology in context of erosion we referring to both surface and subsurface water as it affects the creek and its surrounds.
Different quantities and quantities combined with frequency will have different effect in different situation however there are likely to be substantial impacts on:
- Rates of scour
- Frequency of slumping and calving
- In stream and surrounding vegetation growth
Vegetation
In situations where other conditions such as gradient and water flow are not major inhibitors a balanced cover of native vegetation can have a significant impact in slowing or even repairing various types of erosion.
Vegetation can make a positive impact on vegetation in the following ways:
- Large tree roots can help calving and slumping
- Grassy plants such as rushes, reeds and sedges when located correctly can bend over with the water flow armouring parts of the creek from scour
- Suitable plant species can quickly re-colonise bare or damaged areas of the creek extending the beneficial areas of plant cover at no cost
- Plants can filter sediments out of the water helping build up damaged areas of creek structure while providing additional nutrients to assist
Gradient
The steeper the gradient in a creek bed the higher the velocity of the water in the creek. This increase in water velocity will create a greater the likelihood of erosion occurring.
The key aim of any creek rehabilitation project, particularly a rehabilitation project in a soft creek, is to lower the gradient to a level where normal flows are not causing significant erosion. In soft creeks this can be shown to be achieved when a chain of ponds has been re-established and in a hard creek where ponds and riffles are established.
The optimum creek gradient will differ from situation to situation depending on key factors, such as those listed else where in this chapter, but a good an indicative gradient would be 1 in xxxx for a soft creek.
Angle of repose
The likelihood of calving or slumping in a creek is highly dependent on the steepness of the creek toe and bank, the steeper the bank the higher the likelihood of erosion events occurring.
To minimize erosion events the angle of the bank needs to “lie back” until it reaches an angle of repose. An angle of repose is the point where the bank is self-supporting and will permit the establishment of an appropriate vegetation cover.
The angle of repose for the banks of different creeks and different reaches in the creek will vary depending on a range of factors such as geology, hydrology and vegetation. However as a general rule of thumb banks that have an angle of xxxx to the vertical are likely to be close to an angle of repose
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Land management practices
Land management practices in and surrounding the creek will have significant impacts on rates of erosion in a creek.
Allowing stock unrestricted access to the creek will usually lead to greatly increased rates of erosion. Stock breaks down the soil profile, increase channelization both in the creek and along the bank, break down vegetation where they tread and eat vegetation that may be assisting with controlling erosion.
Activities which encourage the concentration and speed of runoff from land adjacent to the creek will lead to increased water velocities and a likely increase in the rates of erosion.