Examining The Advances In Bank Erosion Studies Environmental Sciences
This paper reviews the advances in bank erosion studies. Bank erosion has been an area of interest by researchers in various disciplines such as geological, geotechnical, hydraulic, hydrology and river engineering. In conjunction with global issues namely climate change, bank erosion studies have been one of the challenges issues in sustainable environment. The evolution in the theoretical and laboratory findings have led to the advances in bank erosion established that contribute to new knowledge in the same field. Review will give a summary of the findings by previous investigations that include measurement approach of bank erosion, behaviour and prediction of bank erosion. Detail on modelling bank erosion in term of laboratory approach and numerical approach will also be discussed.
Knowledge and progress of bank erosion area today have been broadly explored by researcher. It can be seen on the management of the river today especially on the developed country. However, increasing population, industrial activities and technology have been creating a new modern problem which one of it is climate change. As one of the element in river system that contributes to the river erosion, damage on the flood plain land, and also change in channel width adjustment, bank erosion is one of the challenges issues in sustainable environment. This paper review a finding by previous investigation in bank erosion at specific area where need to take consideration in order to face sustainable environment issues. The specific areas are measurement approach of bank erosion, behaviour and prediction of bank erosion. Besides that, modelling bank erosion in laboratory approach and numerical approach also need be highlighted.MEASUREMENT APPROACH OF BANK EROSION
Actual data in understanding bank erosion need to have a proper technique measurement. Technique measurement is important to have high accuracy, persistence and less error. Earlier review on field measurement has been done by  by summarize several method measurement and monitoring bank erosion namely mapping, cross section surveyed, erosion pin, spray printing, reprofiling, side-scan sonar and remote sensing. His review have been update by , by summarize the field investigation approach into categorize base on timescale which is long timescale (10 000 ââ‚¬" 20 000 years), intermediate timescale (1-30 years), and short timescale (few month to a few years). Table 1 shows available measurement technique for bank erosion investigation.
From time to time, researcher was upgrading some of the technique to reduce the limitation of the technique and increase the ability of the technique. One of the rapid enhancement methods is Photo-Electronic Erosion Pin (PEEP) system. Photo-Electronic Erosion Pin has been testing for the suitability of the series erosional and depositional situation, one of the series is done by . In their study shows that, PEEP was suitable to use in dynamic upland headwater site. Consequence to that, this method was enhanced by  with new PEEP system to quasi-continuous monitoring river bank erosion and deposition event and their temporal variability. Then,  was come out with PEEP-3T as a response to the Challenges in geomorphological method and technique issues.
Besides upgrading the technique, new techniques of measurement bank erosion have been developed. Recent technique was developed namely LIDAR remote sensing  and Diffuses Reflection Infrared Fourier Transform (DRIFT) . LIDAR remote sensing was capable to estimate the contribution of bank erosion in suspended sediment, where DRIFT was capable to identify and quantify the sources of sediment.
One of the issues of the measurement technique is the resolution of the technique. The resolution of measurement technique was important in order to know detail change in the measurement. To have high quality of accuracy data, high resolution technique is needed. This is what has been highlighted by  in their improvement of Photo-Electric Erosion Pin (PEEP). Figure 1 shows graph relationship between timescale and resolution of several field measurement techniques done by . The ability of the measurement technique to give high resolution and at same time suitable with large timescale is where need to be focus now. The ability PEEP-3T as show in the figure where the arrow on the figure shows the PEEP-3T can increase the resolution and timescale because of the measurement was control by data logger scan frequency. PEEP-3T is the most flexible measurement technique in term of resolution and timescale recently.BEHAVIOUR OF BANK EROSION
Behaviour of bank erosion basically is the action of bank due to erosion either mass failure or hydraulic erosion. This behaviour also describe as mechanism by researcher. The mechanism of bank erosion explore by researcher was already summarize by  as shown in table 2. In group of gravitational we can see the mechanism is wedge failure, popout failure and cantilever failure. While the tractive force contain mechanism such as undercutting, bed degradation and basal cleanout. Besides that, one type of mechanism can be occurring in both condition gravitational and also tractive force which is preferential flow-induced failure.
As shown in table 2, all the mechanism was depend on their factor influence to be occurring. Although all the mechanism was already being investigated and find, the analysis of factor influence still need to be deeply study.PREDICTION OF BANK EROSION
Prediction of bank erosion is important area need to be highlighted especially in term of river managing issues. The prediction of bank erosion is important in order to know change of river channel and amount of sediment input from bank erosion source with respect to the time. It is also important in engineering application to design the river or structure deal with river and also sustained it in long term of period. Prediction of bank erosion in previous study was come out in term of equation and combination of the equation was creating the simulation of the prediction.
Some of the prediction was the dominance domain process of bank erosion along river channel. Base on conceptual model and previous report, upstream is dominance by sub-aerial preparation process, middle stream is dominance by fluid entrainment, and low reaches is on mass failure . On the other hand, Bank erosion rate can be prediction by treating bank failure as a probilistic phenomenon as done by . Besides that, the best predictor of bank erosion in term of hydraulic erosion was flow peak intensities .  and , predict and quantify temporal progression of an incised stream through the different stages of channel evolution, change in thalweg elevation, and change in channel top width by using CONCEPT model.MODELLING OF BANK EROSION
Same as other area of study, modelling is one of the approach can be use to investigate the process, in this paper was bank erosion. Two type of modelling can be usually namely laboratory modelling and numerical modelling. Here is summary on previous investigation on laboratory modelling and also numerical modelling.Laboratory Approach
Most of the model is use to verified or validate the numerical modelling. Table 3 shows the reviewed laboratory models of bank erosion have been develop.
Laboratory model of non-cohesive material can be seen start by . A lot of laboratory study was base on their model as  studies. There was use laboratory experiment similar to the  to validate the three-dimensional computational fluid dynamics to model free-forming meander evolution. Besides , model experiment have been upgrade by , in term of meander formed and relationship of meander formation to water discharge, sediment load, bank composition and valley slope to validate their numerical simulation of the inception of channel meandering.
Other than that, , was developed model to determine the adequacy of a channelization scheme for Salt River near Sky Harbor International Airport, Phoenix, Arizona. There develop model consist of two type of bank and bed. , develop laboratory model to validate their numerical simulation relating wide, shallow channels with erodible banks.
On the other hand,  also develop non-cohesive laboratory model. The model was use widely to validate the numerical experiment.  use the experiment model to verify applicability of their numerical model to simulate the widening processes of a sine-generated channel.  as use laboratory model similar to the  to verified the ability of their three-dimensional modeling of bank erosion and morphological change. In term of cohesive laboratory model, only one model has been found done by . The model use to determine the erosional strength of the cohesive soil.
For stratified bank material, a study done by  was using this type of material. The experiment was use to validate two-dimensional bank erosion model by add slump block phenomena on two layer of bank material laboratory model. This experiment was study deeply the slump block phenomenon without know effect of the variation of structure and composition of stratified soil.
Recent laboratory model have been develop by considering stratified material (double layer) have been develop by , cohesion on the upper layer and non-cohesion on the bottom layer of the bank. The model was develop in small scale and focusing on evolution of bank erosion as main study is slump block phenomena, it will make difficult to study the specific profile of bank erosion at bend. Besides that, other laboratory model considering stratified material only focusing on influence of seepage such as lysimeter experiment made by . Their experiment uses three type of material which is silty loam in the upper layer, loamy sand in the middle and clay loam in the bottom layer. These models have less considered actual river flow, but on seepage flow on stratified bank. The limitation on laboratory model will make possible to make a new design of laboratory model to study the profile of bank erosion.Numerical Approach
Applications of numerical approach use to simulate and predict the bank erosion phenomenon become rapidly improved. This approach is more effective in term of cost to simulate the morpholodynamic of river channel . It can classify in term of one-dimensional, two-dimensional and also three dimensional. One-dimensional study by , produce the derivation of one-dimensional erosion law. This derivation is only suitable for laminar flume on cohesion less material.
In term of two dimensional study,  already start add the bank erosion mechanism and bank erosion product in their model but the result is poor because lack of detail understanding in the mechanism of bank erosion. , by considers the basal erosion which is erosion under water level was producing other bank erosion rate equation in their model. , were updating the model  by add mass wasting from bank failure in their model. To be notice that,  was use to enhance CCHED2D and  was move to next step of improvement CCHE2D by develop mass failure and use mesh stretching technique to dynamically the mesh and also counter the moving boundary or bank movement problem. Besides that, in , was developed new two-dimensional morphodynamic model of bank erosion by take into account the slump block effect. As their consideration the slump block occur in nature river that consist normally cohesive layer on the upper bank and non-cohesive on the lower layer.
In term of computational tool,  was presented CONCEPTS (Conservational Channel Evolution and Pollutant Transport System) model which is a process-based modeling tool to evaluate stream-corridor restoration designs. This model tool was able to simulate open-channel hydraulic, sediment transport and also channel morphology. In bank erosion scope, it is able to predict undercutting of streambanks. Consequence to that,  and  was upgrade the CONCEPT and shows that the CONCEPTS model can analyze planar slip failure and cantilever failure of overhanging bank. It is also considered the stratified bank material in their analyzing. The use of CONCEPTS model was proved in to simulated streams in northern Mississipi, James Creek, and the Yalobusha River .
Three dimensional studies allow to accurately studying on behaviour of bank erosion especially in term of sedimentation and flow . Three dimensional study also allow to simulate many complex river geometry directly rather than using simplification in two dimensional study (; ). To develop three-dimensional model, the complexity and long duration need to simulate is a problem need to be face by researcher. , was counter duration problem by reduce 73% of completion time by using equivalent channel-forming discharge.CONCLUSION
The aim of this study was to review specific area of bank erosion as the current river managing issues needed. The conclusion of the review of the previous finding in bank erosion as below:
Researchers already develop a lot of measurement technique of bank erosion. In order to face with modern river managing issues, the flexibility in term of resolution and timescale measurement technique needs to take into account.
There is less focusing on stratified bank material development either in term of prediction, behaviour or modelling bank erosion. Although it is more complex then homogeneous bank material, and the stratified bank material might be not important as much as homogeneous bank material at certain area, it is also need to be considered.
Understanding on behaviour was greatly discovered by researcher, as being noticed before, all the behaviour was related with their factor influence, further work need to be done for certain factor influence.ACKNOWLEDGEMENT
The author would like to thanks to Flood-Marine Excellent Centre as well as Faculty of Civil Engineering, University Teknologi MARA for their contribution in this review.REFERENCE
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Difficult for overhanging profile
Difficult for overhanging profile
Less suitable for non-cohesive
Less suitable for Nnn-cohesive (fine material)
Printed section/ pebbles
cross section surveyed
Photo-electronic erosion pin (PEEP)
Lawler (1994), Lawler (2008)
LIDAR remote sensing
Diffuse Reflectance Infrared Fourier Transform (DRIFT)
Table 1: Measurement technique of bank erosion (; ; ; ; ; ; )MechanismClassificationTypical flow characteristicSediment characteristicBank moistureDescription
Tension crack formed behind bank. Culmann wedge analysis
Small blocks forced out at base of channel bank due to excessive pore pressure and overburden.
Preferential flow-induced failure
Selective removal of course material due to preferential flow. Removal of support during rapid drop in stage.
Tension crack form near base of cantilever. Linked to undercutting
Shear stress applied to the lower bank. in general, rate increases with discharge.
Relatively erodible bed
Shear stress applied to the channel bed. Banks fail due to gravitational mechanism.
Banks made unstable by removal of material at base. Residual strength of material determines requisite flow.
Table 2: Mechanism and their associated by typical sediment and moisture condition. 
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