Information and Advisory Note Number 21 Back to menu
11 The aim of this Information and Advisory Note is to describe the major causes of flooding and to identify the impacts of flooding and river management on bank erosion and channel change. Two different case studies are presented, together with a list of relevant legislation and sources of management guidance This Information and Advisory Note should be read in association with 'Rivers and their Catchments: an overview' (Information and Advisory Note No. 18).
2.1 Floods occur in all rivers, and are most simply considered as events where the
flow of water exceeds the capacity of a river channel. Floods have greatest
visual impact where floodplains are inundated - these are flat areas, sometimes
hundreds of metres wide, through which a river flows, and are formed by the
repeated deposition of sediment by flood waters over thousands of years Through
time, floodplains are built up by fluvial deposition but they also evolve
through river bank erosion and changes in river courses during floods.
2.2 The flood characteristics of a river -the magnitude of floods and the frequency with which they occur - govern all aspects of its channel. By virtue of their energy, flood flows are responsible for erosion, the transport of coarse sediments and, ultimately, for the capacity of the channel. Most river channels are able to accommodate the flow of the average annual peak.
2.3 An inverse relationship always exists between flood magnitude and frequency, the largest floods are the rarest. Records made at flow gauging stations, or obtained from historical sources, allow the relationship between magnitude and frequency to be examined and, where required, an experienced hydrologist can assess the likely return period of a flood of a given magnitude. However, return periods can only indicate the average interval between exceedances of a given magnitude and, m reality, unusually large floods can occur within relatively short periods of time.
2 4 Floods in Scotland are generated by a variety of mechanisms:
2.5 On many rivers, the largest known floods have all been produced by the same type of meteorological and antecedent conditions
2.6 Most bank erosion occurs during the few largest floods each year. In these events, banks are exposed to the greatest height of floodwater, and the erosive stresses resulting from current velocities and water depth are also at their maximum. As water levels recede, the transported material is deposited in areas of low velocity (principally the insides of bends, thus balancing the erosion taking place immediately opposite). Deposition also occurs whenever the floodplain is inundated, as velocities outside the channel are always relatively low.
Development of a meander bend Erosion occurs on the outside of the bend, and a
helical flow of water then deposits eroded material from this bank on the inside
bank of the bend
2 7 The type of channel change which occurs during floods is largely a function of the channel type in steep, upland streams, much of the erosive energy of a flood is directed in the vertical plane, whereas, in fiat areas of floodplain, channel evolution is mostly in a horizontal sense as meander bends migrate across the floodplain. Braided channels often typify the intermediate situation, where floods frequently cause channel switching. Straight channels are generally found in near-coastal areas and evolve only slowly. Most of the material being eroded and reworked by the major Scottish rivers is derived from glacial deposition.
2 8 The processes of flooding, bank erosion and channel change are in the first instance natural, and are responsible for providing a range of fluvial features and natural habitats. However, human intervention in this system, normally aimed at halting erosion or preventing inundation, is never without impact elsewhere and can often be thwarted by these powerful forces of nature.
2.9 From a conservation perspective, there are several fundamental considerations in developing management responses to flooding. These are:
3 1 Channel Effects
3 2 Floodplain effects:
3.5 Channel capacity below reservoirs is reduced by deposition, the flood flows experienced are generally smaller.
3 6 Flood embankments
3.7 Bank protection and river engineering:
3 8 Wildlife.
4 1 The middle River Feshie- an example of a
highly active natural river system
411 The River Feshie is one of the principal rivers draining the western side of the Cairngorm Mountains. Its upper tributaries are steep mountain torrents but, after flowing through a narrow gorge, the river enters a floodplain area of more moderate (10m/km) gradient Here the river consists of a complex network of braided channels. Below this middle reach, the river again descends more rapidly until joining the River Spey just below Loch Insh.
4.1.2 The middle reach of the River Feshie is recognised as one of the most active river reaches in the UK It is one of four on the Feshie which qualify for SSSI designation for the value of their fluvial features. The network of braided channels here reflects a strong tendency for the river to change its course. The floodplain consists of coarse glacial material, and is reworked within and adjacent to the main channels in high flows. Switching of the main flow from one channel to another occurs as the result of sometimes subtle changes in channel cross-section where channels part The high energy of the system (the combined result of moderately steep gradient and high flows) ensures that channel change, both by bank erosion and channel switching, is frequent Research undertaken over the last 20 years has demonstrated that detectable changes often occur several times per year, and has illustrated the sustained nature of change over the past 200 years.
4.1.3 The confluence of the River Feshie with the Spey is also designated on account of the large alluvial fan which has formed by the deposition of sediment from the Feshie where it enters Strathspey River engineering has been attempted here at a variety of scales for many years as a means of trying to avoid flooding of adjacent agricultural land All the evidence to date, however, suggests that the course of this
powerful river will continue to evolve in response to natural processes
4.2 The Perthshire River Garry: an example of severe regulation and its effects
4 21 The River Garry is one of the least environmentally acceptable examples of river regulation in Scotland. The cause is hydro-power development whereby Loch Garry is dammed to provide runoff for Loch Ericht, and a further intake downstream on the River Garry (adjacent to the A9 and Perth-Inverness railway) similarly supplies runoff to Loch Errochty. Because of its early development preceding establishment of the principle of supplying compensation water for fisheries interests, no residual flow is supplied to the river downstream of the latter site and, as a consequence, the river is devoid of running water for most of the time
4 2 2 Significant flows in this part of the river are therefore only experienced as a result of heavy rainfall, and subject to the effects of the reservoir and intake upstream: the river switches from no flow to high flow conditions, and quickly back again Consequently sand blanketing can be expected as a result of infrequent moderate flows
4 2 3 Compensation arrangements have been mandatory for a long time in reservoir schemes, so that the River Garry represents an extreme
case The benefits of such arrangements are also accepted as being of a much wider nature than simply catering for fisheries interests.
However, hydro-power reservoirs are found across much of the Highlands, and supply reservoirs occur on many smaller rivers and streams in the Central Belt, the effects of flow regulation on channel stability are therefore widespread, along with the implications for aquatic habitats
5 1 The following aspects of legislation relate to some of the issues described in this note
6.1 A range of approaches has been applied to the management of Scottish rivers, mostly at a local scale. Increasingly, the application of traditional engineering solutions which frequently involve a high impact on conservation interests (flora, fauna, habitats, landforms and physical processes) is being questioned, particularly because there is now available a range of environmentally sensitive methods which are based on an understanding of flu vial geomorphology. River mangers are therefore encouraged to consider such options and to incorporate conservation interests in the planning and design of particular schemes. Two important sources of information on environmentally sensitive river management are:
Gilvear, D J. 1994. River flow regulation. In Maitland, P.S., Boon, P.J. and
McLusky, D S (eds) The Fresh Waters of Scotland: a National Resource of
International Significance. Chichester, John Wiley, 463-487.
Hoey T.B., Smart D.W.J., Pender G. and Metcalfe N. 1995 Alternate Methods of River Management for Scottish Rivers Scottish Natural Heritage Review No. 33.
Leopold, LB. 1994 A View of the River. London, Harvard University Press.
Richards, K. 1982 Rivers- Form and Process in Alluvial Channels London, Methuen
RSPB, NRA & RSNC 1994 The New Rivers and Wildlife Handbook' Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire, SG19 2DL
Werritty A 1995. Integrated Catchment Management a Review and Evaluation. Scottish Natural Heritage Review No 58.
Werritty, A and Brazier, V 1991. The Geomorphology, Conservation and Management of the River Feshie SSSI Report to the Nature Conservancy Council, Peterborough
Werritty, A., Brazier, V., Gordon, J.E. and McManus, J 1994 Geomorphology. in Maitland, P.S, Boon, P J. and McLusky, D S. (eds) The Fresh Waters of Scotland a National Resource of International Significance. Chichester, John Wiley, 65-88.
Werritty, A and McEwen, LJ. (In press) The fluvial geomorphology of Scotland's rivers. In K.J. Gregory (ed) The Fluvial Geomorphology of Britain. London, Chapman and Hall.
Authors: Dr. Andrew Black, Institute of Hydrology, Stirling (now Department of Geography, University of Dundee). Jonathan Clark, Earth Sciences Branch, Scottish Natural Heritage, Edinburgh
Back to menu