Janet Hooke

Erosion, flooding and channel management in Mediterranean environments of southern Europe

Soil erosion by water is one of the most important land degradation processes in Mediterranean environments. This process is strongly linked to problems of flooding and channel management. This article reviews existing knowledge on these topics and defines research gaps. In the framework of environmental change studies it is important to consider soil erosion at various spatial and temporal scales. Most field measurements and modelling efforts have hitherto concentrated on water erosion processes operating at the runoff plot scale. Soil erosion processes operating at other spatial scales have received much less attention in the literature. Yet, there are indications that gully and channel erosion are probably the dominant sediment sources in a variety of Mediterranean environments. Beside water erosion, other erosion processes operating within catchments, such as tillage erosion, land reshaping for land preparation (e.g., terracing) or soil quarrying can have significant impacts on soil profile truncation. Land use changes strongly affect the intensity of these processes. The conditions, position and connectivity of the runoff and sediment generating areas within catchments have a profound effect on flood characteristics within the main channels but the dynamics are not well understood. Some research has taken place into meteorological conditions producing catastrophic flooding and into development of hydrological models using catchment variables. Much less is known of the properties and effects of flood waves within channels, partly because of lack of records of these infrequent events. It is not only water but also sediment which causes destruction in floods, yet sediment is frequently ignored in channel management. The extreme conditions associated with floods in the region, the variability of flows and of flood zones, the mobility of the channels and the high sediment loads create particular challenges for channel management. Trends in land use and channel management are tending to exacerbate these problems. From this review it can be concluded that there is still an important need for process-based understanding and modelling of key soil erosion processes operating at a range of scales: i.e., from plots over hillslopes, catchments to regions. In particular, more research is needed on the linkages between upland areas which produce large volumes of runoff and sediment and channels on the other hand. Such linkages are through gullies and sedimentation zones. Monitoring and experi mental data on key soil erosion and channel processes operating within Mediterranean landscapes are crucial for the improvement of soil erosion and channel models for a range of scales. In particular, long-term monitoring of soil erosion processes and stream channel changes seems to be essential to observe the effects of infrequent torrential rain events on severe erosion, flooding and stream channel changes as well as on the transient response of Mediterranean landscapes to changes in land use and climate. Systematic collation of historical evidence of changes would be valuable. Implications of land and water use need to be examined in detail. A wide range of alternative strategies and techniques of channel and basin management must be explored and modelled. A holistic approach to management of the fluvial system is recommended.

Coarse sediment connectivity in river channel systems: a conceptual framework and methodology

Identification of coarse sediment connectivity down river-channel systems is important for understanding the linkages between river reaches, the influence of sediment sources on channel morphology and the mechanisms and liability to propagation of morphological change. A conceptual framework is developed in which several types and degrees of connectivity are proposed. It is hypothesised that sediment connectivity varies with sediment sources and with stream competence to transport the coarse fractions. A method of identifying the status of reaches is developed, based on field mapping of morphology and sedimentology. The analysis is exemplified by two channels: one an ephemeral channel in SE Spain; the other a perennial channel in NW England. Questions arise over whether reaches that lack channel stores transfer coarse sediment. Three scenarios are suggested: (1) lack of flux due to lack of competence of the reach; (2) flushing through of sediment due to high competence; and (3) potential transport of coarse sediment but exhaustion or lack of availability. Changes in connectivity will take place over different time scales depending on the nature of exchange and time scales of storage. Short-term variations in connectivity can be identified by repeated mapping.

An analysis of the processes of river bank erosion

Abstract Measurements of erosion have been made on sections of river banks in Devon, England over a period of 2.5 yr. Erosion pins were used and found to be satisfactory. Data of the hydrological and meteorological conditions during each erosion period were collected; these were incorporated into a stepwise multiple regression analysis of the conditions controlling the amount and distribution of river bank erosion. Field observations are shown to aid considerably in understanding the processes of removal of material. Two main methods of bank erosion are identified, corrasion and slumping, and these appear to be associated with the influence of river flow levels and antecedent precipitation conditions, respectively. Antecedent precipitation index emerges as the variable providing highest explanation of the erosion characteristics but the diversity of results from different sites illustrates the complex combinations of conditions related to erosion events. A tentative grouping of the sites according to the types and amounts of erosion and the variables explaining their distribution is suggested.

River channel adjustment to meander cutoffs on the River Bollin and River Dane, northwest England

Morphological adjustment of a channel after cutoff should be expected, but rarely has the mode and timescale of adjustment in natural channels been measured. Four natural cutoffs which have occurred since 1980 on two rivers in northwest England have been monitored from pre-cutoff stage. Two of the cutoffs were neck cutoffs of large loops; two were chute cutoffs in which significant bend curvature remained. All the cutoffs were the terminal result of progressive bank erosion and/or scour of the floodplain rather than overflow and all the cutoffs took place in peak flow events, though two of these were of relatively low magnitude-high frequency. Rapid widening and sedimentation took place within the straight channels produced by neck cutoffs, immediately after breaching. Adjustment was mainly by formation of multiple riffles and bars, producing a variable morphology in the first 2–4 years. Subsequently, the morphology became more regular and in one case, had stabilised within about eight years. Progressive steepening and acceleration of bank erosion appears to have been propagated upstream in one case, but in the other cases erosion was very localised. Rates of vertical accretion at the entrances to the old channel were much higher than most quoted in the literature. Rates of change in all morphological, sedimentary and biotic components exhibit an exponential decline with time but with slightly differing timescales. On these active, gravel-bed streams major adjustment within the new channels may be completed within 6–12 years though lakes persist in the abandoned channels for much longer.

Geomorphological impacts of a flood event on ephemeral channels in SE Spain

A flood event occurred on 30 September 1997, in three catchments, Torrealvilla, Salada and Nogalte, in SE Spain which were being monitored for morphological change. Peak discharges were high in the Torrealvilla catchment, moderate in the Salada and low in the Nogalte. Detailed topographic surveys were made before and after the flood and from these DEMs of difference were constructed. The two sites with highest flow both underwent significant incision of the channel. At other sites, both scour and aggradation of the channel took place, with scour holes of up to 1 m depth eroded. Elsewhere, within the channels major erosion took place downstream of tracks and structures such as check dams that impeded sediment movement. The pattern and amounts of change show little relationship to independent factors, other than a scale effect of peak discharge. Behaviour of channels is closely related to local sediment supply. Sediment flux calculated from changes in volumes at the site is comparable with the other figures produced for ephemeral channels and much higher than for perennially flowing streams.

River meander behaviour and instability: a framework for analysis

Recent empirical and theoretical work on river meanders suggests that instability is inherent. Within this context, an occurrence of multiple cut-offs could be interpreted as a clustering associated with self-organized criticality. Types of meander behaviour ranging from stable to chaotic are examined as trajectories or attractors within the phase space of rate of meander movement and bend curvature and change to the system is explored as shift from one attractor to another. It is suggested that this new approach provides insights into meander dynamics and provides a basis to identify the conditions, limits and constraints under which different behaviour occurs. Basic controls of energy and resistance underlie planform behaviour, but need to be refined in relation to the morphology and stability of channel courses.

The impact of rainstorms on floods in ephemeral channels in southeast Spain

Abstract This paper presents and discusses data on rainfall, stage and estimated discharge for a large flood occurring in two catchments in southeast Spain in September 1997. Rainfall and stage were recorded using automatic logging equipment and discharge was estimated using measurements of channel cross-sections and water depth estimated from trash lines. Total precipitation in the Rambla de Torrealvilla was 50 mm in 2 days with maximum rainfall intensities of 80 mm h −1 . Total rainfall in the Rambla de Nogalte was 195 mm in 3 days, with maximum intensities of 200 mm h −1 . In the Torrealvilla, this rainfall produced three flood peaks with maximum stage approaching 2.5 m. In the Nogalte, there was only one flood peak, which was 0.5 m deep. Estimated discharge varied widely throughout both catchments with maxima of 120 m 3 s −1 in the Torrealvilla and 60 m 3 s −1 in the Nogalte. Maximum discharges occurred at times of high rainfall intensity, but intensity alone did not explain why some tributaries had very small discharges. Variations in discharge in the ephemeral channels were due to combinations of lithology, morphology and land use. The predominantly marl catchment of the Torrealvilla had a lower threshold rainfall intensity than the schists of the Nogalte. Within each catchment sub-basins characterised by steep, gorge like terrain and sub-basins where agriculture had been abandoned both resulted in higher flood discharge. The contributing areas for the September storms were up to two thirds of tributary catchment areas. Comparison of rainfall data records shows that the September flood was the fifth largest on record and had a recurrence interval of 7 years. The largest (1973) flood, which is known to have caused substantial damage and a number of deaths, was only a 30-year event. The floods on the Torrealvilla destroyed at least two check dams and evidence suggests that these had little effect on reducing the impact of the floods.

Use of terrestrial photogrammetry for monitoring and measuring bank erosion

This paper examines the use of terrestrial photogrammetry as a technique for measuring bank erosion in a rapidly changing fluvial environment. It has been recognized that there are a number of advantages when applying photogrammetric techniques to geomorphological situations. In this study the enhancement of spatial sampling combined with the ability to capture additional information, such as soil moisture, on film, is of particular importance in enabling the identification of specific processes involved in bank erosion as well as detailed volumetric analysis of losses. Metric terrestrial photography was taken of the river bank on several dates, and data were abstracted by the use of analytical photogrammetry. This enabled the generation of digital terrain models from which morphological and volumetric changes could be assessed.

Vegetation in dryland river channels and its interaction with fluvial processes

Vegetation forms a major component of the channel environment of dryland rivers in Mediterranean Europe yet the interactions between vegetation and fluvial processes in dryland river channels have received relatively little attention. Characteristic of dryland channels is a degree of complexity and irregularity in morphology coupled with abrupt changes in patterns of riparian vegetation along their course. This is in contrast to more temperate and humid channels, which show strong downstream trends in morphology and a regularity in the distribution of vegetation across the valley floor. The general effect of vegetation in these channels appears to be to enhance the processes of sedimentation and increase resistance to erosion. However, at high stress levels vegetation may be removed. The limited work that has been undertaken indicates that there is considerable variability in the effect that vegetation has on channel processes, depending on both the magnitude of the flow and characteristics of the vegetation, such as their distribution and positioning within the channel and factors such as plant flexibility, cross-sectional area (blockage ratio), number of stems. Studies of sediment connectivity are beginning to yield new insights into the dynamics of dryland rivers, and in particular the role of vegetation in reducing the connectivity of sediment transfers downstream. Detailed surveying at monitored sites such as is being carried out within the EU-funded project RECONDES is required to study the interactions between vegetation, sediment and flow in dryland rivers of the Mediterranean region.

Modelling vegetation interactions with channel flow in river valleys of the Mediterranean region

Flooding and erosion are major hazards in semi-arid areas. As a result of climate and/or land use changes these hazards could increase. Vulnerable areas include the Mediterranean region of southern Europe. As part of the European Union (EU)-funded MEDALUS (Mediterranean Desertification and Land Use) research programme on impacts of desertification a model to simulate channel changes is being developed. This is designed to simulate morphological changes in the channels and floodplains of ephemerally flowing streams over time scales of a year to decades. It incorporates morphological, sedimentological and vegetation characteristics and simulates the effects of flood events and of climate and human activities for reaches of valley floor. In this paper the selection of characteristics and processes in the vegetation component of the model is discussed. The effects of vegetation upon flow, erosion and deposition processes and the feedback of flow, sediment and moisture on vegetation are identified. The conceptualisation of the model is outlined then the problems and strategies for parameterisation are indicated.