Discuss how geomorphologists can make a contribution to river channel rehabilitation
Over the last few decades the application of geomorphology to river engineering has become an important aid to the effective management of river channels and systems. In particular river restoration and channel rehabilitation is now a major theme in river engineering, and one in which major developments in the contribution of fluvial geomorphology have taken place over the past decade (Douglas, 2000).
Radical changes in policies and approach to river management as a result of increasing public scrutiny given failures to prevent flood hazards and economic and environmental concerns now highlight the need for fluvial geomorphology to be an integral part in the planning, implementation and post-project appraisal stages of engineering projects.
Some of the main contributions that geomorphology can make to river and floodplain management include: the recognition of connectivity in the fluvial system at all scales and the inter-relationships that occur within catchments; to stress the importance of understanding fluvial history and chronology in order to fully comprehend the behaviour of fluvial systems; to highlight the sensitivity of fluvial systems to disturbances and change; and to demonstrate the importance of promoting ecologically acceptable engineering through a trans-disciplinary integrated approach to river management (Gilvear, 1997).
In the past, river management has been a contentious environmental issue concerned with conflicts such as those between flood mitigation or irrigation development and habitat preservation. Attitude towards physical management of fluvial systems and of hazards such as flooding tended to be one of dominating and controlling nature (Hooke, 1999), compounded by an attitude that all economic assets needed to be protected. River engineering involved the use of straight channels, impoundments, embankments and a range of training structures to control rivers and their flow (Gilvear, 1997).
Thus until around the early 1990s management policies centred on an accelerating rate of control and protection of both coasts and rivers in England and Wales. There have, however, been great changes in attitudes and practice in recent decades. This was as a result of an increasing realisation that many engineering schemes had longer-term detrimental and unanticipated consequences, often in areas adjacent to original works. For example everywhere there are concerns over the degradation of rivers downstream of dams (Douglas, 2000).
The change also arose from a general desire to minimise flood damage and reduce environmental degradation as a result of river engineering schemes. It had been shown that a wider perspective was needed that considered consequences of management schemes and did not use only short-term, localised data. By the 1980s these realisations had produced a momentum for changes in engineering practices there were inherently based on a geomorpholgical approach to river management (Douglas, 2000).
If the processes and changes in fluvial systems were to be understood and the detrimental consequences of schemes were to be avoided, then more research was needed into the processes of rivers, and particularly into the interconnections between areas. The contribution of geomorphologists to such an analysis was exemplified by the mid-1980s, as the need for understanding river valley planning and management became firmly ensconced in planning, river management and flood mitigation literature and policies (Brookes and Shields, 1996, cited by Douglas, 2000).
In terms of controlling bank erosion in the UK, for example, two major perceptions and practices of river managers occurred as a result of geomorpholgical analysis. Firstly, bank erosion began to be considered in the context of the total sediment dynamics of a river system, thus examining both upstream and downstream effects of bank protection work. Secondly, river managers began to prescribe softer, more natural materials for bank protection works, such as traditional vegetation like willow and ash (Douglas, 2000).
This symbolised the recognition by national government in 1993 of a change in policy which the Minister for Agriculture, Fisheries and Food called ‘working with nature’, thus providing the platform for the understanding of river processes and landforms – that is geomorphology – as being an integral and fundamental part of river engineering and management (Hooke, 1999). The application of geomorpholgical skills can therefore provide important contributions to river management.
The need for wide spatial perspectives and the importance of understanding systems as a whole is one such skill. Geomorphologists are traditionally concerned with the ways in which rivers vary from source to mouth, a unidirectional approach that results in an important understanding of the longitudinal connectivity in the river system, with upstream impacts having downstream consequences (Gilvear, 1997). In the past, engineers have failed to appreciate such connectivity and have at best concentrated on short-term and local impacts. Thus as Thorne at al. 1997) state, traditional river engineering works have ‘created major instability and environmental problems, because they impose an unnatural condition on the river by modifying bankfull dimensions and/or discharge and sediment transport regimes with little consideration of longer term implications on the catchment as a whole’. An appreciation of the geomorphic significance of trapping sediment load by engineering structures is now resulting in the use of substrate replenishment to prevent bed degradation and changes in character downstream (Gilvear, 1997).
A lack of geomorphic appreciation results in problems and changes which have to be constantly revised. For example, the Kelang River in Kuala Lumpur was channelised following the great flood of 1926, with a double trapezoidal cross-section created through the city centre. After continued annual flooding and another great flood in 1971, further chanelisation was undertaken. The problem was that new training works merely served to pass water and much of the silt further downstreamwhile floods may be alleviated in one reach, they are often aggravated elsewhere.
Constant additions to such works are required. Clearly, then, any success at the basin scale of river engineering projects depends on the integrated management of component parts: the ‘total catchment concept’ has to be applied (Douglas, 2000), thus bringing the different elements and applications of fluvial geomorphology to the fore. Catchment management plans are now produced in the fluvial field where problems are arising, and the technique of fluvial auditing is being applied, where processes and landforms are identified in a river reach through a method of detailed geomorphological mapping.
Consultations have taken place since 1999 on ways of developing an integrated catchment plan for the Kelang River. Another critical component of what Amoros et al. (1996, cited by Gilvear, 1997) term ‘the fluvial hydrosystem concept’, which is increasingly being used as a framework for river management investigations, is the temporal dimension of fluvial systems. River managers need to understand river systems as they change through time in order to appreciate flux as a result of anthropogenic changes and channel hydraulics.
Such studies have been the focus of fluvial geomorphologists but have been sadly lacking in past engineering schemes because often ‘historical methodologies are not conducive to the application of quantitative analysis’ (James, 1997). The geographical environments in which engineering structure are placed are highly irregular in ways that are difficult to anticipate or model, nonetheless the inability of most numerical approaches to adequately incorporate full representation of processes over extended time periods leaves most methods susceptible to extreme errors of judgement.
Underestimates of the magnitude of natural phenomena over time have led to erroneous judgements about flood control and channel conditions on the Sacramento Rover in California (James, 1997). Here, the effects of human-induced alluvial deposits from mining activities have continued much longer than anticipated, and rates of transport of historical alluvium out of the Sacramento Valley continue to be overestimated.
The ever-growing need for flood control has compelled engineers to concentrate on hydraulic aspects of rivers, dams and levees without a full understanding of long-term, morphologic adjustments of the fluvial system. Practical solutions to immediate problems take precedence over a search for causality in nature where a long term perspective toward channel change over a geological time based on field evidence is surely needed (all taken from James, 1997). The contribution of geomorphology to such a need is now becoming better documented.
Modes of meander development and mechanisms of bank erosion on meandering rivers are gradually being incorporated into engineering designs. Careful reading of the landscape in which the dynamics of past and present processes are continually manifested is the realm of the geomorphologist. Geomorphology wil interpret fluvial landforms as indicators of stability and instability and document past channel change using cartographic and sedimentological evidence in order to predict future change (Hooke, 1999).
Coates (1990) describes how the use of an ‘ergodic’ model, involving the selection of landforms that can be arranged in a sequence showing changes through time, is a geomorphic principle that can be used in solving problems dealing with stream adjustments and channelisation. One model by Harvey and Watson (1986, cited by Coates, 1990) describes the evolution of an incised channel from a state of disequilibrium to a state of new dynamic equilibrium.
By application of the model, such factors as bank stability, effective discharge and hydraulic energy could be calculated – all factors that create morphological channel adjustments. On this basis grade-control structures were implemented that were designed to reduce slope and induce upstream deposition of sediment in the bed, thus enabling supply and transport capacity of the system to be balanced. The new LTS approach (‘location for time substitution’) has been successful in preventing further erosion of the channel, which was threatening a number of public facilities (Coates, 1990).
Clearly, then, geomorphologists have an important role and contribution in predicting channel adjustment over historic time scales in order to aid the implementation of new engineering designs. The question of sensitivity of fluvial systems, or the ability of a river to resist changes in its morphological variables resulting from external stress such as engineering, is an important field in which geomorphologists can contribute to river channel rehabilitation. Effective management strategies have to be built on sound science and an understanding of river dynamics.
Knowledge of how rivers adjust their morphology is an important facet of sensitivity and is critical to river engineering. Channels will often exhibit different degrees of sensitivity to change, and in particular they will be sensitive if they lie near a threshold (Gilvear, 1997). Analysis can be used, for example, to assess the extent to which bed forms need to be engineered in river restoration projects: in some cases they will develop naturally and rapidly, in other cases they will not.
In many cases channel restoration, involving river flow augmentation of slight alterations in river’s sediment budget, can lead to renewed channel adjustments, depending on the sensitivity of the system. For example flows were augmented along the Arkansas River, Colorado, and this resulted in a shift from meandering patterns to less sinuous braided ones and an increase in bankfull channel depth and width (Douglas, 2000). Sound design must therefore consider these possibilities and ensure that improvements in one place do not serve to merely push the problem further downstream.
Geomorphologists are increasing being called upon to identify zones at risk or of differing behaviours in order to apply sensitivity concepts into land-use planning. Geomorphologists can also make an important contribution to river channel rehabilitation by addressing particular objectives in river management. With the acceptance of the idea that river design has to be addressed in a more holistic manner, river engineers are seeking to integrate engineering concerns such as flood mitigation with the preservation and enhancement of habitats.
In this the importance of geomorpholgical control on habitat type and diversity needs to be more fully appreciated. Conservation of species can often be achieved through the preservation of geomorphologically defined habitats such as pools, riffles, undercut banks and backwaters to riverine species. For example, on the River Blackwater in Ireland, a programme of habitat reinstatement that introduced pools and spawning areas to reaches has led to localised increased in channel roughness and increased the biodiversity of the river (Douglas, 2000).
The influence of velocity upon all major groups of organisms in running waters must also be considered. Geomorphologists can, in the future, be influential in designing flow management strategies that are designed to maintain intsream velocities within ecologically accepted values (Gilvear, 1997). These developments will only be possible, however, with close monitoring of key, intensively used rivers. A further key geomorphological contribution that is gaining momentum is post-project appraisal (Thorne et al, 1997).
The need to evaluate the outcome and impact of schemes has been sorely lacking in past engineering projects. The provision of information on timescales and patterns of responses will greatly benefit river rehabilitation research, especially in the field of conservation work. It seems clear, therefore, that the work of geomorphologists on river management projects can make a number of key contributions to processes of river channel rehabilitation.
By providing research methods and evidence that demonstrates the significance of connectivity within the fluvial system; the importance of historical legacy of past river changes; land sensitivity considerations; and the interplay between engineering, geomorphology and ecology, geomorphologists give a wider perspective on river systems which greatly furthers previous work done by river engineers at specific site and design levels.
Geomorphologists contributed directly to the profound move away from conventional engineering solutions towards more environmentally sustainable solutions. Having set this agenda, therefore, future engineering activity will simultaneously be required to achieve traditional objectives such as maintenance of bank stability or reduction of flood levels, but also maintenance of instream and floodplain habitats and wildlife.
This, together with river restoration and rehabilitation, represents an enormous challenge to engineers in geomorphological approaches and input will need to be the major component. Perhaps the greatest contribution that geomorphology can make to future river engineering and rehabilitation, therefore, is for geomorphologists to continue to strive for improved understanding of geomorphic systems, so that this challenge may be more easily overcome.