Information and Advisory Note Number 21 Back to menu

Rivers and their Catchments: Flooding, Bank Erosion and Channel Change

1. Introduction

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. Processes and Concepts

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:

Convective storms in the summer months produce short duration, high intensity, localised rainfall of greatest importance in generating floods on small, upland streams
Frontal rainfall is more prolonged, occurs over wider areas and produces floods on larger rivers by integrating the flows from their many smaller tributaries
Snowmelt often magnifies the impact of winter rainfall, the combined effect being known to produce many of the largest floods on Scotland's main rivers such as the Tay, Dee, Spey, Ness, Tweed and Clyde Very occasionally, snowmelt alone may produce floods without significant accompanying rainfall.

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:

river management should always be based on an understanding of the fluvial system, which requires recognition of the dynamic nature of Scotland's rivers and the links between rivers and other components of catchment hydrology;
river management should be planned in a catchment framework to ensure that upstream and downstream effects of particular developments are fully evaluated,
possible impacts of river management on other aspects of the catchment, such as
flora, fauna and habitats should be fully appraised,
design of flood alleviation measures should be appropriate for the particular circumstances concerned and should include the use of alternative low technology/low impact approaches where possible

3. Flood Impact and Management Practices

3 1 Channel Effects

erosion on the outsides of bends by undercutting, often cutting back into fields;
deposition on the insides of bends (point bars) - sediment accumulation provides new areas for colonisation by vegetation; bars may also form in the centres of channels,
destabilisation of channel gravels, many years may elapse after a major flood before new, stable channel forms are produced;
high slopes (e g. in moraines or outwash terraces) may remain unstable for years after erosion by undercutting during floods.

3 2 Floodplain effects:

inundation, which may be prolonged if drainage is impeded by embankments;
deposition of fine sediments, adding to the soil and causing temporary blocking of soil pores,
nutrient input important in sustaining wetland ecosystems
attenuation of downstream flood peak magnitudes

3.3 Agriculture:

bare and thinly vegetated soils are heavily
eroded by floodwaters,
short-term loss of fertility due to pore blockage, but balanced by nutrient gains;
failure of crop due to loss of seeds and fertiliser,
grazing animals weaken river banks, making them more susceptible to erosion, unless fencing used to keep stock back from banks

34 Reservoirs.

reservoirs cause a reduction in the number of small and moderate floods experienced in downstream areas but have little impact on the magnitude of large floods (typically above a 10-year return period),
channel features (e.g. gravel bars, riffles) may become less stable,
fine sediment deposition predominates because of the reduced frequency of moderate flows;
sediment starvation occurs immediately below reservoirs due to deposition in the reservoir of all material earned from upstream areas, locally increased rates of erosion result

3.5 Channel capacity below reservoirs is reduced by deposition, the flood flows experienced are generally smaller.

3 6 Flood embankments

embankments protect agricultural or built-up floodplain areas from inundation, but divert all the flow downstream, thus causing flood peaks further down the river to be higher than expected;
in large floods, these flood embankments are inevitably overtopped and breached,
resulting in locally severe erosion (scour holes) and therefore also deposition

3.7 Bank protection and river engineering:

gabions are most often used to protect river banks where erosion problems are experienced and, while sometimes effective, they frequently deflect erosive energy to areas immediately downstream;
such efforts generally cannot change the direction in which a river is attempting to alter its course and, wherever possible, it is best to work with the river rather than against it;
in the long term, this approach is likely to cause the least loss of land to the river -many attempts at river engineering have been known to cause further unexpected erosion problems,
downstream of completely channelised reaches, high erosive stresses may arise during floods because of the lack of material available for transport immediately upstream.

3 8 Wildlife.

floods provide the energy to maintain river channels as dynamic environments and, accordingly, provide the basis of most aquatic habitats,
at the same time, bedload movement in major floods causes substantial short-term disturbance to in-stream habitats (e g to fish spawning grounds),
spring floods are responsible for mortality in many species of aquatic fauna (e g loss of
eggs)

4. Case Studies

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. Legislation

5 1 The following aspects of legislation relate to some of the issues described in this note

Scottish local authorities have discretionary powers for urban flood protection under the Flood Prevention (Scotland) Act 1961;
riparian owners have the right to erect flood defences in rural areas, and partial grant aid is payable by the Scottish Office Agriculture and Fisheries Department under the Farm Conservation Grant Scheme or the Land Drainage (Scotland) Act 1958,
planning decisions are made under the Town and Country Planning (Scotland) Act 1972, and local authorities may refuse planning permission on the basis of known flood risk. Policies are in place across Highland Region and in Perth following major floods in the late 1980s and early 1990s;
the Secretary of State authorises abstractions and diversions for hydropower schemes under the Electricity Act 1989,
there are no formal guidelines on the planning of flood embankments.

6. Sources of Guidance on River Management

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:

Alternative Methods of River Management for Scottish Rivers (Hoey et al, 1995) emphasizes that 'in many cases the required level of protection can be achieved using natural materials, by deflecting the flow away from the bank or by using vegetation to stabilise the bank. It may also be possible to reduce erosion by reducing artificial pressures on the bank (from livestock, people, vehicles). Methods which involve
vegetation are encouraged as, used carefully, they can promote habitat diversity and have considerable conservation benefits for the river corridor as a whole'. The report describes and critically evaluates a number of alternative approaches applicable to the management of gravel and boulder-bed streams.
The New Rivers and Wildlife Handbook (RSPB, NRA & RSNC, 1994) is a practical guide to alternative techniques of management for lower energy, lowland rivers The approaches recommended are set within a framework of catchment planning and incorporate both flood alleviation and conservation interests in an integrated approach.