Progress in Physical Geography

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Erosion, flooding and channel management in Mediterranean environments of southern Europe J.W.A. Poesen and J.M. Hooke Progress in Physical Geography 1997; 21; 157 DOI: 10.1177/030913339702100201 The online version of this article can be found at: http://ppg.sagepub.com/cgi/content/abstract/21/2/157

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157

Erosion, flooding and channel management in Mediterranean environments of southern

Europe

J.W.A. Poesena and J. M. Hookeb Fund for Scientific Research — a

Geomorphology,

Department b University

of

of

Flanders, Laboratory for Experimental

K.U. Leuven,

Redingenstraat 16, B-3000 Leuven, Belgium Geography, Buckingham Building, Lion Terrace,

Portsmouth, Portsmouth

PO1 2HE, UK

Abstract: 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


158

other hand. Such linkages are through gullies and sedimentation zones. Monitoring and experimental 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.

Key words: Mediterranean, Europe, soil erosion, soil loss, gully erosion, inter-rill and rill erosion, tillage erosion, mass movements,badlands, extreme rainfall, sediment yield, trampling erosion, flooding, flood routing, flood frequency, flood forecasting, flood impacts, channel management, structural flood control measures, nonstructural flood control measures, policy recommendations, research gaps.

I

-

Introduction

Over the last decade, there has been a growing awareness of the gravity of problems related to erosion, flooding and channel management in Mediterranean Europe because of two major reasons. First, it is believed that the Mediterranean could be strongly affected by desertification: i.e., land degradation (reduction or loss of the biological or economic productivity and complexity of cropland or range, pasture, forests and woodlands) in arid, semi-arid and dry subhumid areas resulting from various factors, including climatic variations and human activities (UNEP, 1994). Secondly, southern Europe has suffered regularly over the last years from catastrophic floods. For instance, 11 catastrophic floods occurred during the last 50 years in Catalonia and six of them were recorded in the 1980s (Figure 1; Llasat and Puigcerver, 1994). The most recent and dramatic ones occurred in Italy (Tuscany, 19 June 1996) and in Spain (Biescas, Huesca, 7 August 1996, Garcia Ruiz et al., 1996) with several tens of casualties. Soil erosion is considered to be a key desertification process (UNEP, 1994). In a strict sense, soil erosion is defined as detachment and transport of soil particles by a moving fluid and the corresponding processes are described as soil erosion by wind and soil erosion by water. In a larger sense, soil erosion also includes mass movements which cover various kinds of processes leading to a downslope movement of soil or rock due to the pull of gravity (e.g., creep, earth and mudflow, landslides, rockfalls, debris avalanche, subsidence, etc.). More recently it has become clear that soil movement by tillage operations (i.e., tillage erosion) or slope reshaping for land preparation may cause locally considerable soil losses. The rate of soil erosion is usually expressed as soil loss per soil surface unit and per time interval: e.g., ton/ha/y. Erosion rates are often compared to a soil loss tolerance level (T-factor) in order to decide whether soil loss rates are excessive and whether soil conservation measures should be taken. T-factor values are defined based on on-site and/or on off-site considerations. For example, a T-factor of 13 ton/ha/ year (= 1 mm/year) has been accepted for a deep medium-textured soil under a temperate humid climate since this soil loss rate keeps pace with the corresponding weathering rate. But T-values may be much less for many soils in the Mediterranean belt because these soils are often shallow (hard unweathered rock is at limited depth in the


159

1 Main catastrophic rainfall events causing flooding between 1939 and 1989 in the Spanish Mediterranean basin (after Llasat and Puigcerver, 1994). Continuous line: maximum rainfall recorded in 24 hours. Dashed line: maximum cumulative rainfall recorded at any point during the event. The month, year and number of casualties (between brackets) are also shown

Figure

soil profile) and because of lower weathering rates. In addition, if the off-site consequences of soil erosion are considered, e.g., the silting up of reservoirs, flooding or channel management problems, T-values may be still lower. From the previous discussion, it becomes clear that erosion of soils in catchments, flooding and channel management are environmental problems which are strongly linked to each other. Flooding is of natural occurrence in the Mediterranean region but magnitudes and frequency change with land use and climate. Most of the tendencies and prediction of change under these scenarios will exacerbate the flood problem rather than reduce it. In addition, there are increasing pressures from settlement and development of floodplains leading to increasing channel management but major questions arise in relation to the long-term sustainability of certain policies and strategies. The objectives of this article are 1) to present a state of the art on soil erosion, flooding and channel management in Mediterranean environments; and 2) to indicate important research gaps.

II

Soil erosion

Increasingly, the scale-dependent nature of runoff generation and soil erosion is being recognized (Poesen et al.,1996a). For example, the magnitude and even process of erosion


160

very different at the square metre scale from that of

catchment

(e.g., Poesen et al., change on soil erosion and flooding requires a sufficient understanding of the scale-dependent nature of erosion processes and their intensities (Poesen et al., 1996a). In the following sections we investigate the various erosion processes (and their controlling factors) operating at two important scales: i.e., the Mediterranean basin scale and the catchment scale. are

a

1994). Accurate prediction of the impact of environmental

1

Mediterranean basin scale

De Ploey (1989) produced, after compiling information from various sources (i.e., literature and field observations throughout Europe), a soil erosion map of western Europe, indicating the areal extent of various erosion processes and erosion-related phenomena (e.g., flash floods and badlands). Figure 2, which is based on this soil erosion map, identifies the major erosion-prone areas in the European Mediterranean.

Soil erosion

by wind: Soil erosion by wind occurs essentially on sandy soils found (e.g., such as in the Landes, France) or on dunes and deltaic deposits (e.g., Rh6ne and Po deltas). Tillage of dry soils may produce local dust clouds. In addition, dust input from sources at greater distance occurs, but this process has received relatively little attention. However, the area affected by severe wind erosion in southern Europe is fairly limited (Figure 2.). a

in coastal

zones

b Soil erosion by water: Figure 2 indicates areas with arable land where soil erosion rates due to inter-rill, rill and gully erosion may exceed 10 ton/ha/year, a rate that is likely to be higher than most soil regeneration rates in southern Europe. This map shows many scattered areas throughout the Mediterranean region. From a spatial point of view, soil erosion by water is the dominant erosion process in the Mediterranean belt. In a section below the dominant processes and major controlling factors will be discussed. Mass movements:

Mass movements are or morainic materials

quite important where clayey rocks (clayoutcrop. The presence of these rock types triggers various types of mass movements, including large landslides. Mass movements also occur frequently in areas where morphotectonic events (uplift, faulting and earthquakes) occur, the latter being important throughout the Mediterranean basin (e.g., c

stones, marls,

flysch)

Almeida-Teixera et al., 1991). Figure 2 indicates the most important regions affected by mass movements such as the area north of Lisbon in Portugal, the Pyrenees, the Alps, the Apennines and various parts of Greece. Cascale et al. (1994) and Gostelow et al. (1996) give a detailed account of the frequency, the magnitude and the external causal factors of landsliding (human, climatic and tectonic) in various study areas of southern Europe, covering different climatological, geological, geomorphological and land use characteristics. From this study it appears that earthquakes play a subordinate role in the triggering or activation of earth movements by comparison with anthropic and above all climatic causes. The most significant human actions causing mass movements are the abandonment of cultivation and of hay fields, deforestation, the opening up of rural roads, the installation of mechanical ski-lifts, the remodelling of ski-runs in the mountains and the urbanization of, for instance, peninsular

Italy. Mass failures also affect (ephemeral) channel walls and can occur directly or accompanied by fluvial action, depending on the location within the channel. Scour of the bed


161


162

and

slope angle, decreasing its stability with usually follow periods of heavy rain or high respect water flow that increase bank material weight (and hence shear stresses) and decrease its shear strength (La Roca Cervig6n and Calvo-Cases, 1988). and bank toe increases the bank’s to

mass

height

movement. Mass failures

d Badlands: Badlands result from both water erosion and mass movement processes. These processes interact and their effects are therefore difficult to separate from each other: e.g., gullying by hydraulic erosion followed by gully wall collapse (mass failure). Lihologic conditions are important and badlands tend to develop on unconsolidated or poorly sorted materials such as shales, gypsiferous and salty-silt marls and silt-clay deposits of Tertiary and Quaternary age (Figure 2). Most badlands are situated on or near major mountain ranges, especially those that are still being uplifted. Badlands evolve by surface and subsurface erosion by water, including chemical erosion (soil dispersion due to the high concentrations of salts) and piping (Bryan and Yair, 1982; Imeson and Verstraten, 1988; Calvo-Cases et al., 1991). Characteristics of active badlands are high contemporary erosion rates, low surface permeabilities and high erodibility. Measured erosion rates in Mediterranean badlands vary widely, ranging between 5 ton/ha/year (0.45 mm/year) and 220-330 ton/ha/year (20-30 mm/year) (e.g., Benito et al., 1992; Bufalo and Nahon,1992). This wide range is the result of differences in climatic, lithologic and topographic characteristics at the various study sites, differences in spatial and temporal scales considered, as well as differences in measurement and calculation techniques used in the various studies. The off-site effects of badland erosion, i.e., runoff production and sediment pollution, may be more important than the on-site erosion effects. Despite the fact that badlands occupy only approximately 5% of southern Europe (Figure 2), they have received much attention in the erosion literature probably because of their spectacular nature. In other words, badlands are not representative of more than 90% of the Mediterranean part of Europe. In addition, badlands may not always be the net result of recent land degradation and some of them may have a long history (e.g., Wise et al., 1982). De Ploey (1992) calculated the age of badlands in the Mediterranean to range between 2700 and 40000 years. These results are opposed to the hypothesis of rapid badland development in historical times often imputed to massive forest clearance and soil degradation since Roman times. Such data also warn against an overestimation of erosion rates during the last two millennia. Badlands are often used to illustrate desertification in southern Europe, but is this justifiable? ’

factors: Given that soil erosion by water is, from a spatial point of view, the important erosion process in southern Europe (Figure 2), we discuss briefly the most important factors controlling water erosion (and associated floods) in Mediterranean

e

Erosion

most

environments.

Climate The Mediterranean climate is characterized by winter rainfall, dry hot summers and a distinctive regime of soil moisture. With respect to soil erosion by water (and mass movements) the following characteristics are important: 9

Mean annual rainfalls are generally less than 800 mm and the rainfall occurs between late September and May, sometimes with a bimodality, giving rains in September-October and March-May.


163

Figure 3 Highest

recorded rainfalls for southern

Europe (circles

and solid

line), the UK and the world (dotted lines). Rainfall data and corresponding sources

the UK

Europe are listed in Table 1. The curves for the world and extracted from Shaw (1994). Dh is rainfall duration expressed in

for southern are

hours

o

can be very intense, particular in the driest areas of Mediterranean Europe Obled and Tourasse 1994; IGN, 1995). These intense rains cause widespread (e.g., mass movements and flooding (e.g., All6e, 1984; Gallart and Clotetgullying, Peramau, 1988; Garcia Ruiz et al., 1996; Wainwright, 1996). Consequently, highintensity rainfall events do most of the geomorphological work in the drier areas (Thornes, 1976). The highest rainfall intensities have often been recorded in mountainous areas (orographic effect) within about 100 km from the coast. Table 1 and Figure 3 show some high rainfall intensities recorded in southern Europe. Based on these data,

Rains


164


165


166

the highest recorded rainfall depth (R, in mm) for southern Europe can be from the rainfall duration (D, in hours) using the equation (Figure 3):

o

9

o

predicted

For comparison, Figure 3 also shows the lines corresponding to the highest recorded rains in the UK and in the world. Intense rains exhibit an erratic temporal and spatial distribution and can be very localized, particularly in the driest parts of the European Mediterranean (e.g., Thomes, 1976; Rubio et al., 1983; Llasat and Puigcerver, 1992; Alonso-Sarria and Lopez-Bermudez, 1994; Garcia Ruiz et al., 1996) (Figure 4). Although very intense rains also occurred in historical times, there are indications that, at least for some parts of the Mediterranean, intense rainstorms have increased over the last 15 years. For instance, Llasat and Puigcerver (1994) reported an increase of extreme rainfall events for the Spanish Mediterranean basin (Figure 1). Rain data from southeast Portugal and central Italy suggest that rainfall in the Mediterranean has an erosivity, expressed by the specific kinetic energy, which is on average 20% higher than that predicted using the widely applied equations established in North America (Figure 5).

Soils Many soils in the Mediterranean exhibit a high susceptibility to water erosion because of their loamy to loamy-sand texture. Often soils have developed on lithologies that have low resistance to water erosion such as the salty marls of the intermountain basins around the Mediterranean Sea. The high temperatures in the summer and the low

Figure

4

Illustration of the

spatial distribution of intense rain falling over the

J6car basin (Spain) After Rubio et al., 1983


:

.

167

rainfall amounts cause relatively low organic matter contents in these soils. Many of the Mediterranean soils are therefore very susceptible to physical degradation (i.e., surface sealing, compaction and crusting). Dispersive soils, cracking soils and rocky outcrops are some of the features which are influential in the production of Hortonian overland flow in the Mediterranean. On the other hand, more than 60% of the land area in the Mediterranean consists of soils containing significant amounts of rock fragments in the top layer (Poesen and Lavee, 1994). These rock fragments should be considered as natural soil-surface stabilizers which often favour infiltration and reduce runoff and soil loss by rain and runoff.

Topography Throughout the European Mediterranean, steep slopes are common, particularly in mountain areas. In addition, slope aspect strongly affects the microclimate and therefore vegetation cover, soil development and erosion intensity. Southern and western slopes are warmer and have higher evaporation rates and lower water storage capacity than northern and eastern slopes. Rainfall may also be affected by slope aspect depending on the direction of winds during rainfall.

Figure

5

intensity. and Torri

between specific kinetic rainfall energy and rainfall extracted from Coutinho and Tomas (1995, Portugal), Zanchi central Italy) and USA (Renard et al., 1992)

Relationships Data

are

(1980,


168

strongly controlled by vegetation cover which in turn depends on biomass production. Dry Mediterranean environments are biologically marginal. Thus organic matter production (litter cover, roots and humus) is limited. All this indicates that even under natural conditions, the Mediterranean dry lands can be expected to be highly susceptible to water erosion (Thornes, 1995). Vegetation

Soil erosion is

Land use In addition to the climatic, lithologic-pedologic and topographic susceptibility of Mediterranean lands to soil erosion, land-use activities further affect the intensity of soil erosion significantly. Clearance of forest and matorral or reshaping badlands for agriculture, tillage of the top soil, rock fragment removal from the topsoil, shifting crops due to changing markets (e.g., Figure 6), abandonment and regeneration of the land, controlled burning and overgrazing are among the main human activities in Mediterranean landscapes. Many studies have shown that cultivated land, with its seasonal bare ground, is much more erodible than even a sparse semi-natural cover.

6 Evolution of area under cropland, horticulture and Almeria Province (1950-90, Grove 1996)

Figure

2

perennials

in

Catchment scale

Given the importance of water erosion in Mediterranean environments we discuss in this section the various water erosion processes, their intensity and their main controlling factors. However, one should keep in mind that water erosion strongly interacts with other soil degradation processes which are also desertification processes: i.e., biological soil degradation (i.e., the reduction in top soil organic matter), physical soil degradation (i.e., adverse changes in top soil porosity, bulk density, structural stability and permeability) and salinization (i.e., the accumulation of soluble salts on or at various depths in the soil profile or the exposure of saline rocks). In addition, attention is also paid to three other soil erosion processes operating within Mediterranean catchments.


169

Water erosion processes: Different water erosion processes operate at various spatial scales in the landscape: i.e., inter-rill, rill erosion, gully erosion and piping (Figure 7). Inter-rill and rill erosion, i.e., the detachment and transport of soil particles by raindrop impact, sheet flow and concentrated overland flow in small erosion channels, usually occur on field parcels and these processes have received a lot of attention in Mediterranean Europe. This is seen in the many studies where these processes have been quantified in various environments using either field runoff plots ( < 0.01 ha) (Tables 2 and 3) or erosion models such as USLE, RUSLE, EUROSEM and the MEDALUS model. Tables 2 and 3 indicate that mean annual soil losses due to inter-rill and rill erosion in the Mediterranean are fairly low. For instance, for cultivated land, mean soil losses are a

Table 2

runoff

Mean annual soil loss (SL, ton/ha/y) by inter-rill and rill erosion measured plots ( < 0.01 ha) in various Mediterranean environments

Table continued


on

on

page 170

170

Table 2

Continued

Note: Rc

=

mean

surface rock

fragment

cover.

Table 3 Mean annual soil loss (SL, ton/ha/year) by inter-rill and rill erosion measured standard runoff plots ( < 0.01 ha) in Mediterranean Europe for different land uses

Notes: n = number of

.

year data; al., 1997.

plot

Source: Kosmas et

SD

=

standard deviation.

, ’

&dquo;

.

1.


on

171

diagram illustrating where various erosion subprocesses by corresponding erosion forms occur in cultivated landscapes of the Mediterranean region (A runoff generation area, B inter-rill erosion, rill erosion, D C sedimentation zone (colluvium), E ephemeral gully erosion, F terrace bank, G pipe inlet, H pipe outlet, I bank gully) After Poesen, 1995, partly based on Farres et al., 1993 Figure

7

Block

water and the

=

=

=

=

=

=

=

=

=

less than a few tons/ha/year, with the exception of years with extreme rainfall, whereas for abandoned agricultural land, rangeland and shrubland this figure drops below 0.5 ton/ha/year. These relatively low figures can be explained by 1) the presence of a significant rock fragment cover at the soil surface (Poesen et al., 1994); 2) in some cases, the high clay content of the soil (e.g., plot data from Italy); 3) the presence of a permanent vegetation cover in the case of abandoned agricultural land, rangeland and shrubland; and 4) the lack of extreme rain events during the observation period. The latter points to a problem when assessing soil losses by water erosion in Mediterranean environments: i.e., assessing water erosion rates in an environment which is dominated by large lowfrequency rainstorms, too rare to occur in the duration of the average research project. The reported soil loss data from various studies in the Mediterranean also point to the fact that the reduction of the area of cultivated land (land abandonment) and the expansion of shrubland (matorral) in the former fields will cause a drastic decrease in inter-rill and rill soil loss and therefore in sediment production (e.g., Quine et al., 1994; Garcia-Ruiz et al., 1995). However, it should be stressed that soil losses measured on runoff plots in the field only tell part of the erosion story. Other water erosion processes, such as (ephemeral) gullies and bank gullies (Figure 7) operate within agricultural catchments at spatial scales which exceed those occupied by an individual runoff plot. Once a concentrated flow channel exceeds a critical cross-section value (about 900 cm2) the eroded channel is termed a gully (Poesen et al., 1996b). Ephemeral gullies typically form where overland flow concentrates in the landscape, i.e., in natural drainage lines (e.g., colluvium-filled hollows) or along or in linear landscape elements (e.g., drill lines, plough furrows, parcel


172

access roads). This erosion process is largely controlled by flow intensity. Ephemeral gullies are erased yearly by tillage operations (Poesen, 1993). On the other hand, bank gullies form where overland flow crosses an (earth) bank (e.g., a terrace, a road bank or river channel bank) and processes such as piping play a crucial role in the initiation of this gully type. Initiation and development of pipes are controlled by the presence of (desiccation or tension) cracks and biopores in silty-clayey material. Other factors encouraging the action of subsurface flows and the growth of a pipe network are high hydraulic gradients (e.g., near the banks of irrigated terraces), the presence of sodium in the adsorption complex of the soil, prolonged periods of drought to encourage desiccation cracks, and irregular intense rainstorms to activate the pipes (e.g., Harvey, 1982; Martin-Penela, 1994). Obviously, sparse or no vegetation on the bank increases the risk of bank gullying because of the erosional power of surface and subsurface runoff and the increased erodibility of the bank because of the lack of a dense root mat which binds the subsurface soil particles. Compared to inter-rill and rill erosion, gully erosion in the Mediterranean has received much less attention in the literature. Also, no widely applicable model exists for this soil degradation process as compared to inter-rill and rill erosion. Yet, there are indications that gullies are a very important sediment source in Mediterranean environments. This is based on the following data and observations. Mean long-term sediment yield figures, based on reservoir sedimentation data in southeast Spain, typically range between 1.5 and 4 ton/ha/y and can even go up to 10 ton/ha/y (Lopez-Bermudez, 1990; Romero Diaz et al., 1992; Sanz Montero et al., 1996 Figure 8). These figures are equal to or higher than sediment yield due to inter-rill and rill erosion, obtained from runoff plot measurements on upland areas (see Tables 2 and 3). Sediment produced by inter-rill and rill erosion is often deposited at the foot of the slope or in depressions within the landscape and does therefore not reach the river channel. Hence, other sediment-generating processes in catchments must play an important role in the production of sediments which are transported by (ephemeral) rivers and which cause reservoir sedimentation. Gully and channel erosion are very likely to be such processes. Sediment produced by gully erosion has more chances to reach the river channel since gullies often act as an important link between upland areas and river channels. Consequently, sediment produced by gullying together with river channel sediments will be also responsible for flooding and rapid reservoir sedimentation. In other words, it is highly likely that most of the sediment in these reservoirs originates from gully and (river) channel banks. Recent field observations seem to confirm the importance of gullies as a dominant sediment source in Mediterranean environments (e.g., Ternan et al., 1994; Faulkner 1996). Field data collected recently indicate that ephemeral gullying contributes 80-83% of total sediment produced on cultivated uplands of southeast Portugal and rangelands of southeast Spain (Table 4; Poesen et al., 1996b). No information on the contribution of bank gullies in Mediterranean Europe to sediment yield is presently available. The figures corresponding to ephemeral gullying compare well with published data for other arid and semi-arid environments: e.g., 58% in Argentina (Coronato and Del Valle, 1993), 60-81 % in Arizona, USA (Osborn and Simanton, 1989) and 80% in Niger (Heusch, 1980). One of the reasons for the important contribution of ephemeral gullies to sediment yield from Mediterranean uplands is thought to be the fairly high rock-fragment contents of the soils which reduce inter-rill and rill erosion but not necessarily overland flow production. This overland flow causes (ephemeral) gully erosion to occur more downslope in topographic

borders,


173

8 Mean long-term specific sediment yield for 60 catchments in calculated using reservoir sedimentation data After Sanz Montero et al., 1996

Figure

Spain,

concavities (hollows) and in linear landscape elements partly due to the clear-water effect (Poesen et al., 1996b). The data from southeast Spain clearly indicated that, when cultivated land is abandoned, erosion rates due to inter-rill and rill flow are reduced due to the development of a permanent vegetation cover and erosion pavements, but that gully erosion may become very important, particularly when the land is grazed regularly. From the previous discussion it becomes clear that assessments of water erosion rates in Mediterranean Europe should not be limited to runoff plot measurements or to Table 4 Mean annual soil loss Mediterranean environments

Source: Poesen et

on

upland

areas

by

various water erosion processes in two

al., 1996b.


174

predictions using empirical or process-based models which only account for inter-rill and rill erosion. There can be no doubt that gully erosion is an important erosion process in these environments and that in particular cases it is the dominant water erosion process. Consequently, future studies of water erosion should incorporate the impact of gullying on soil losses and on sediment yield. However, at present there is no standard methodology to measure the intensity of this process (Poesen et al., 1997a). Also, no universally applicable model for gully erosion exists. Hence, this erosion process should receive more attention in studies dealing with soil erosion assessments under environmental change. Limited data suggest that land-use changes have a significant impact on the topographic threshold for gullying (Poesen et al., 1997a). Also, extreme rainfalls have been reported to cause important gullying in Mediterranean Europe (e.g., All6e 1984; Garcia Ruiz et al., 1996; Wainwright, 1996). Much work still needs to be done to understand and to model the conditions under which gullies start to form, develop and fill in. b

Other

erosion processes operating at the catchment scale: In addition to the processes discussed above, other erosion processes also cause significant soil loss in Mediterranean Europe: i.e., tillage erosion, trampling erosion and erosion due to soil quarrying and land preparation (e.g., terracing).

important

water erosion

Tillage erosion

Weinblum and Stekelmacher (1963)

were

among the first in the Mediter-

to describe quantitatively the effects of various tillage implements on the downslope movement of soil on hillside terraces (Israel). Over the last years, it has ranean

become clear that frequent tillage operations on steep slopes throughout the Mediterranean may lead to significant net downslope soil fluxes and, hence, soil losses on topographic convexities and on the upper part of field plots (e.g., Revel et al., 1990; Poesen and Lavee, 1994; Guiresse and Revel, 1995; Poesen. 1995; Revel and Guiresse, 1995; Poesen et al., 1997b). In some parts of the Mediterranean tillage erosion has increased over the last decades because of 1) the increase of the capacity for deep ploughing by using heavy caterpillar tractors instead of animal power (horses or donkeys); and 2) expanding almond groves on steep slopes since 1970 (e.g., Figure 6) (Poesen et al., 1997b). Tillage erosion is controlled by factors such as slope of the land, tillage equipment, depth, direction, speed and frequency of tillage, soil type, structure and moisture content of the top soil at the time of tillage. Field experiments in the Guadalentin basin (southeast Spain) revealed that mean soil losses in almond groves due to tillage erosion (3-5 tillage passes with a duckfoot chissel per year) can easily amount to 22-39 ton/ha/year (1.5-2.6 mm/year) for contour tillage and up to 54-88 ton/ha/ year (3.6-5.9 mm/year) for up- and downslope tillage for a field, 50 m long and having a slope of 20% (Poesen et al.,1997b). Such figures are at least one order of magnitude larger than reported rates of soil loss by inter-rill and rill erosion in similar environments (Tables 2 and 3). The main consequences of tillage erosion are strong truncation of soil profiles on topographic convexities and at the upper boundary of field lots with the creation of specific patterns of shallow soils and surface rock-fragment cover, the creation of earth banks at the bottom end of field lots and the rapid infilling of narrow valley bottoms (Poesen et al., 1997b). Because of tillage deposition at footslope positions, buried Ap horizons have been observed in an intensively cultivated area (southeast Spain) by Boer et al. (1996). Together with water erosion, tillage erosion may be held responsible for the areal increase in lithosols, as seen in the Alentejo (southeast Portugal) (Poesen and Lavee, 1994; Poesen and Bunte, 1996). Contrary to water erosion, the off-site effects of


175

tillage erosion remain rather limited since most soil translocation often occurs within the field lot. Nevertheless, tillage erosion should also be considered as a land degradation process when assessing the impact of land-use changes (e.g., the shift from matorral to almond cultivation).

Trampling cover

erosion

However, grazing direct

It is well known that grazing causes a reduction of the vegetation grazing increases the susceptibility of the land to water erosion. by sheep and goats on steep, rocky slopes may cause a significant

and therefore

displacement of both rock fragments and fine earth as shown by field experiments

in Turkey (Govers and Poesen, 1997). Therefore, the increase of sheep and goat numbers in certain parts of the Mediterranean may contribute considerably to soil degradation of fragile hilly and mountainous areas of Mediterranean Europe. Apart from field observations and a limited number of experiments, few data are available to document the

impact

of this soil

degradation

process.

Soil degradation due to quarrying and reshaping of slopes The expansion of greenhouses in coastal areas of the Mediterranean for horticulture (e.g., Figure 6) has led to intense terracing and soil quarrying in these environments. One should keep in mind that such activities, together with soil removal for infrastructural works or for land preparation (e.g., levelling of badlands), cause locally important denudation rates and should therefore be considered as soil degradation processes since they reduce soil quality drastically. For instance, Constantini (1992) reports that where excessive earth-moving activities in Tuscany (Italy) have led to the outcrop of Pliocene marine deposits, vineyards have low productivity both in terms of quantity of grapes as well as wine quality. Data are scarce on the impact of this process as well as on its extent in Mediterranean environments.

soil erosion: Various soil erosion models have been tested and/or Mediterranean Europe: e.g., USLE, RUSLE (e.g., Coutinho and Tomas, 1995) and EUROSEM (Morgan et al., 1997) at the field plot scale, MEDALUS model (Kirkby, 1995; Thornes et al., 1996) at the hillslope scale and MEDRUSH model (Kirkby, 1995; Thornes et al., 1996) or SHETRAN (Bathurst et al., 1996) at the catchment scale. Given that runoff and erosional events in the Mediterranean are sporadic, intense, infrequent, difficult to study, their effects difficult to anticipate and the effects of climatic and vegetation change on runoff and erosion difficult to predict, modelling has to augment the traditional field approaches (Thornes et al., 1996; Wainwright, 1996). Modelling soil erosion is not a substitute for detailed process investigations but can help to overcome the limitations of timescales in understanding the effects of environmental change on soil erosion of Mediterranean hillslopes and catchments. For this purpose, the MEDALUS model and the MEDRUSH model have been built. The MEDALUS model was found to perform moderately well for runoff and very well for sediment yield for event-based simulations (Thornes et al., 1996). Given that the response of Mediterranean lands to environmental change shows a high spatial variability (e.g., Mulligan, 1996) more research is needed to improve and validate these erosion models for a variety of Mediterranean conditions. While all these models definitely have a strong potential, they still do not include all important soil erosion processes in Mediterranean environments such as, for instance, gully and channel erosion. Therefore, more efforts should be made to incorporate these processes into erosion models.

Predicting developed in c


176

III 1

Flooding Runoff and sediment

generation research has taken place in semi-arid areas and in humid areas on conditions which runoff is generated. As with so many aspects of this subject, the literature

Much under tends to be focused on either semi-arid areas, where it is assumed that overland flow is generated almost immediately from rainfall because of the high intensities relative to the low infiltration capacities of the soil (Horton overland flow), or on humid areas where infiltration rates tend to be high, water flows predominantly in subsurface layers and overland flow is only generated after prolonged wet conditions when saturation of the soil layers is reached (saturation overland flow). Very few researchers have examined the change from humid to subhumid to semi-arid regimes but the expectation would be that soil characteristics and thickness are altered such that infiltration capacities tend to decrease across that climatic gradient and, as has already been shown, rainfall intensities tend to increase from humid temperate to semi-arid areas. Thus responses to rainfall tend to be more rapid and greater flows are generated from the rainstorms as climate becomes more arid. However, the rainstorms decrease in frequency, duration and spatial extent. Added to this the flows generated in a particular area rarely persist far downstream, beyond the storm area, because of losses through the dry river beds (water transmission losses). This results in flow and floods in more semi-arid areas becoming much more variable than in humid areas both spatially and temporally. This poses particular challenges both in analysis of the processes and in managing the flood and channel

problems. The mechanisms of runoff

generation

in semi-arid

areas

have been

widely

reviewed

(Pilgrim et al., 1988; Scoging, 1989; Thornes, 1994). Scoging (1989) summarizes some of the major theories and key articles relating to runoff generation. The figures for percentage runoff from rainfall vary widely for individual storms and between catchments but events in which runoff amounts are 80% of the rainfall are recorded. This is far higher than in humid areas. Factors influencing runoff generation include both terrestrial characteristics within the catchment and their distribution (soils, vegetation, slope, etc.) and the meteorological conditions. The state of the system at the time of a storm is of great significance. As has been shown in the section on soil erosion, much of the empirical work on the mechanisms and generation of overland flow (uplands) and of runoff (channel), within the Mediterranean and elsewhere, has concentrated at the plot scale. Some studies have begun to show how topographic features at a larger scale, such as convexities and concavities, do have a significant influence on runoff generation (Anderson et al., 1984; Yair and Lavee, 1985). Other studies have focused on the basin scale, using catchment characteristics to model and predict flows and flood frequencies and much of the latter work simply tests predicted flows against measured outputs. Some models (see below) are now incorporating areal differentiation and routing of flows down through basins and channels but there is still a considerable lack of empirical work to validate such models and to provide understanding of the exact nature of runoff mechanisms and flood generation beyond the hillslope scale. One of the difficulties, as Obled and Tourasse (1994: 493), in discussing uncertainties in flood forecasting techniques applied in France, point out is that progress has been made in understanding runoff-generating mechanisms especially in the humid areas but ’in the Mediterranean area, there may be no relationship between flow prior to the event (sometimes zero) and the initial state of the catchment


177

area’. Hortonian overland flow depends on the state of ground surface and that is only accessible by point measurements; investigation is therefore not easily amenable to remote-sensing techniques. The evidence is that runoff generation is highly variable and localized in space and thus even analysis of distributed characteristics at basin scale is often not sufficiently fine to detect these key areas. The same comments apply to an even greater extent to the generation of sediment in flood flows. Much more empirical work is needed in the Mediterranean region on the spatial distribution of runoff and sediment and its transfer down through the drainage systems. Much more data are needed on how far flows are transmitted and the conditions influencing this. A major question with regard to sediment is how far it is transported and particularly the degree of connectivity

between slopes and channels. One of the exceptions to this lack of study of areal differentiations is that by ConesaGarcia (1990) of basins near Cartagena (southeast Spain) in which he shows that much of the sediment in autumn is being generated from restricted parts of the basin (Figure 9) but that it moves relatively short distances and leads to zones of sedimentation. Poesen et al. (1996b) have shown that a very high proportion of sediment is eroded from gully systems in the Mediterranean region. Balasch et al. (1992) and Llorens and Gallart (1992) have investigated in the eastern Pyrenees the hydrological and sediment dynamics of small areas, including gullies, and have also included modelling of the effects of agricultural terraces and abandoned land. The research points towards the importance of linearities and connectivity of channels in generating water and sediment that reaches main streams. Work in other regions, e.g., at Walnut Gulch in southern Arizona, confirms the lack of delivery of sediment to the main channels from diffuse erosion on slopes (Parsons and Wainwright, 1995). The localized nature of flow in flood events is illustrated by the 1980 event in Rambla Honda near Almeria, southeast Spain (Harvey, 1984) when most of the flow was generated in one major tributary. 2

Runoff

modelling

The literature contains many reviews of runoff models and development of flood analysis methods (e.g., Todini, 1988; Rossi et al., 1994), but most are not orientated to the Mediterranean or to a particular climatic zone. Exceptions include the review by El Hames and Richards (1994) of rainfall-runoff modelling in arid lands. It is not possible in this article to provide a complete review of runoff modelling though much hydrological work worldwide and in the Mediterranean is focused on runoff modelling. Some of the major approaches and developments relevant to the present theme will be outlined. Many models are based on deterministic approaches, i.e., on fundamental understanding of the component processes. Models specifically developed for the Mediterranean include the MEDRUSH model in the MEDALUS program. Other models such as SHE, IHDM4 and TOPMODEL were not originally applicable to zones in which Hortonian overland flow is the predominant form of runoff generation but are being adapted. Various empirical models are available and have been applied, e.g., the USA soil conservation runoff curve number technique in which curves for runoff in relation to rainfall, allowing for vegetation type and cover, are provided. However, this method and many other such models are only applicable to small basins (best for < 1 kM2 but up to 25 km2, not more). Many models are based on the unit hydrograph which is predicated on Hortonian overland flow. Developments of geomorphological unit hydrograph models are now being pursued in which network and morphometric characteristics are


178

9 Spatial patterns of sediment production in autumn within two small catchments in southeast Spain (A catchment of Rambla Cabezo Gordo, B catchment of Rambla Benipila) After Conesa-Garcia, 1990

Figure

=

=

incorporated but these require detailed field data. The trend is away from crude methods lumping basin characteristics towards distributed models which recognize spatial variability and distribution of characteristics. Data acquisition and manipulation is aided by remote-sensing sources and use of GIS and DTMs but questions of scale of analysis still arise.


179

3

Flood

routing

Once flood flows are generated, whether in modelling or in reality as gauged flows, then, in order to provide forecasts downstream or to produce more general predictions of flood frequency at downstream locations, some knowledge and modelling of how flood flows are transmitted downstream are needed. Again, several approaches have been taken to this problem of flood routing of which the simplest calculate successive inputs, outputs and stores given channel conditions in a hydrological approach; others incorporate more complex hydraulics (Todini, 1991). The choice of model mainly depends on the purpose of use. Particular problems pertain to flood routing in ephemeral and dryland channels, most notably that arising from transmission losses through the bed. In most ephemerally flowing channels large amounts of water are lost through the channel bed because flow is generated quickly and often from beyond that area and so the channel bed is dry and soil moisture stores are low. Characteristically, peak flows in such channel systems decrease downstream and run out within the basin. Only the highest peaks tend to persist for long distances down catchments. Details of morphology and morphological variation downstream such as is typically found in ephemerally flowing streams have mostly not been incorporated in the flood-routing models but development of this is now taking place, e.g., within the MEDALUS program. Transmission losses have been studied and modelled in some detail because of their importance to groundwater replenishment as well as to peak flows. In summary, it is now possible to cascade process components to give a watershed runoff model but this needs to be subdivided for subcatchments because of localized storms. Use of DTMs and GIS is enhancing these models but there is still a lack of connectivity in terms of real characteristics and generation of flows through to the channel and then down through the basin, allowing for spatial patchiness of occurrence of rainfall and of terrain characteristics. Development of high-quality, realistic runoff models applicable to the conditions found in the Mediterranean region is important for prediction and forecasting of floods. Once suitable models have been developed then outcomes under different scenarios can be tested and assessments of sensitivity of basins to climatic and land-use change can be made. 4

Meteorological

conditions

are obviously generated by rainfall but we have already seen how the soil and catchment characteristics and their states mediate between the rainfall and the runoff giving rise to very large variations in runoff as a proportion of rainfall. The largest floods tend to be generated under conditions of high-intensity rainfall and / or long duration and where peak flow from several tributaries coincides, i.e., where storms are spatially extensive. Heavy rainfalls are more likely to arise under particular meteorological conditions and floods are more likely to be generated at particular times of the year. The relations are not simple. For example, Lanza et al. (1994: 431) state ’In Mediterranean regions winter weather conditions can lead to extreme rainfall over large regions that enter the Italian peninsula once or twice a year; on the other side, in a given basin, flooding phenomena take place once in 25-100 years, according to the size of the basin and the structural river training in the reaches of interest’. The meteorological conditions under which floods are generated have been investigated for individual floods and to an increasing extent are being investigated for

River floods


180

incorporation into flood-forecasting systems to give early warning. Some analysis of the synoptic and meteorological conditions giving rise to extreme rainfalls has also been carried out, e.g., Llasat and Puigcerver (1992) in Catalonia, and development of typologies of events is taking place, e.g., Alonso-Sarria and Lopez-Bermudez (1994). As yet, little work of the type done in the western USA (Hirschboeck, 1988) has been carried out in the Mediterranean in which frequency and magnitudes of floods in relation to climatic conditions and climatic fluctuations have been analysed. An exception is the notable work of Benito et al. (1996) in which they have compiled long historic flood records for basins in Spain and analysed the conditions under which they occurred. They also demonstrate the variability of flood incidence over the centuries. Floods in Spain are most likely to occur under west or northwesterly zonal flow where frontal systems cross the Iberian Peninsula. In southeast Spain floods particularly occur in autumn under conditions of a cold pool developing (’gota fria’). Floods generally occur in the winter months throughout the Mediterranean but the month in which peaks are most frequent does vary (IGN, 1995; Benito et al., 1996). For example, in southeast Spain it is in October; on the Tiber it is December. 5

Flood

frequency Estimation of flood frequency or return period of specific size floods is necessary for flood prediction and for design of structures as well as for understanding the impact of flood events and the longer-term evolution of channels and landscape. Almost any major construction scheme associated with channel, water or flood management involves estimation of extreme flood frequency or return periods of certain magnitude. For example, dam schemes in Spain require spillways to be designed to the 500-year capacity

flood. Calculation of flood frequency or return period usually entails compilation of as long a record of flows as possible, abstraction and ranking of peak flows and then plotting the distribution in relation to extreme probability. The problem in any region is that a long record is needed to be able to calculate extremes and even then the size of very rare events (e.g., the 500-year flood) must be extrapolated. The principle relies on homogeneity of relations and frequency over the period, yet it is increasingly obvious that, on timescales of centuries and even decades, all basins have been subject to climatic fluctuations and almost all to anthropogenic effects too. A major problem relating to the modelling and prediction of runoff and floods in the Mediterranean is the lack of data, particularly routine measurements of rainfall and of discharge with which to test models or to establish empirical relations. The problem is particularly severe in semi-arid regions such as the Mediterranean because the frequency distribution of events, i.e., the large but infrequent rainfalls and floods and the very high spatial and temporal variability, means that an even longer record is needed than in perennially flowing areas. Likewise, the spatial variability and localized nature of rainfall and flow mean that a closer density of instrumentation is needed rather than the sparser one generally found. A major approach to calculating flood frequency and to developing a method applicable to ungauged catchments is the calculation of regional flood frequency curves. Many attempts have been made in all types of regions to produce such curves. For example, work of this type has been going on for the Mediterranean region under the AMHY group of the UNESCO FRIEND project (Prudhomme, 1995). Flood frequency in relation to various basin attributes and statistical distributions has been analysed by Ferrari et al. (1993) for south Italy. Mimikou (1987) has constructed regional curves relating flood


181

magnitude to morphoclimatic variables for northwestern and western Greece. A wider flood frequency analysis for arid and semi-arid areas has been carried out by Farquharson et al. (1992). They have used MAF (mean annual flood) as a scaling factor but they, and others, suggest this may not be suitable or meaningful. Peak flows in relation to supposed MAFs can be hundreds of times in magnitude in semi-arid climates. Flood frequency can be extended backwards by use of palaeoflood indicators including slackwater deposits and historical records. It has mainly been applied in western USA (Baker et al., 1983) but is just beginning to be applied in Europe to the Mediterranean region (Benito et al.,1996). El Hames and Richards (1994) advocate combining this type of evidence with physical modelling to test models. Compilations of historical evidence of the chronology and frequency of floods show considerable variation over time The historical analysis shows the high sensitivity of flood magnitude and frequency to climatic variation and the variability calls into question the use of standardized flood frequency and extreme event analysis as return periods. Pavese et al. (1992) identify perturbed and quiet phases of climate in the flood record from Alessandria in the Italian Piedmont. L6pez-Berm6dez (1973) has compiled a chronology of major floods in the Segura basin and analysed historical documentation of the impacts of floods. Molina Sempere et al. (1994) took this record extending from AD 826 to 1990 and identified three periods of higher frequency of floods which they associated with deforestation and agricultural change (Figure 10). Almost all the compiled chronologies show a marked increase in frequency in recent times, e.g., compilations for the main rivers on the Iberian Peninsula (Benito et al., 1996; Figure 11) and for the Arno (La Barbera et al., 1992). This is probably mainly an artifact of increased reporting (though that is important in terms of increased perception of floods) but questions are arising, when examining the gauged flows as well as historic records, as to whether the frequency of large floods is increasing and to what extent this may be due to climatic change or to land use and channel management changes or both. The intensity of debate has not quite reached that surrounding the ’Arroyo question’ in the western USA but is highly pertinent to the whole issue of desertification in the Mediterranean region. Knox (1993) has shown from evidence in the northern Mississippi that small climatic changes can

Figure 10 Evolution of flood frequency in the between AD 826 and 1990 After Molina Sempere et al., 1994

Segura River (southeast Spain)


182

Figure 11 Historical floods (per decade) drainage basins After Beruto et al., 1996

in the

Spanish

Mediterranean

large and sometimes abrupt changes in flood magnitudes especially in the submargins. Deforestation and channel management are well documented as increasing runoff in catchments and the increasing pace of urbanization, in spite of long histories of development in the Mediterranean, is now being shown to be increasing peak flows and decreasing response times (Inbar and Sala, 1992). Analysis of floods in Catalonia over the past 25 years reveals nine flood episodes have been recorded of which five occurred between 1982 and 1988. Analysis of these events shows that the worst floods were not necessarily engendered by the most important rainfalls; that depends essentially on the localization of the area affected and on the maximum intensities recorded (Llasat and Puigcerver, 1992). Sediment has been almost completely neglected in flood prediction and in flood routing. There are assessments of sediment yield at catchment scale but not of sediment concentrations in flows or of locations of origin. It will be highlighted later that this is a cause

humid


183

major gap in managing flood impacts. Sediment yield studies can incorporate a morphological or topological element, e.g., work in Italy by Ciccacci et al. (1992). Degree of incision of the fluvial system and therefore its history can affect processes and sedimentation and even susceptibility to desertification (Ibanez et al., 1996). Flood

forecasting Flood forecasting differs from flood prediction of general frequency of events in being a warning that an event will actually occur. It therefore is usually made in real time and as part of channel management since it is often the basis for taking action. Because of its importance in alleviating flood damage and danger, much work has gone into the development of methods of forecasting. Most of these involve developing and testing runoff generation and flood routing models in application to particular catchments and specific event characteristics. For example, in Portugal a real-time forecasting system has been developed which comprises a runoff-generating and a flood-routing component 6

(Oliviera and Ford, 1991). This showed, that in

one storm 50% of the catchment within 1 and 100% of the catchment within 2.5 hours. The hour of rainfall responded in small catchments in the and short time interval before flow rapid response Mediterranean mean that forecasts in real time from measured flows upstream or even from telemetered rainfall data give insufficient time for flood warnings. This makes it imperative in small catchments that forecasts of flooding are based on forecasts of rainfall rather than actual rainfall or gauged flows. Forecasting schemes using radar rainfall signals have been developed and tested, e.g., in the Amo basin (Becchi et al., 1994), and a large European project, named STORM, has focused on this issue (Roth, 1996). Geomorphological and hydrological characteristics of cells are input, and flows are routed through the channel network. The model was tested with DTM grids at both 100 m and 400 m intervals. As might be expected, grid-based models work much better for spatially nonuniform rainfall than do lumped models. Much research is now going into analysis of the sensitivity and critical parameters in use for radar rainfall, e.g., Pessoa et al. (1993) for the Sieve basin. More complex, expert systems and systems integrating various forms of data to forecasts floods based on meteorological conditions are also being developed as part of the Amo project (Carrara et al., 1992; Lanza et al., 1994).

7

Flood events and

impacts

Flood events in the Mediterranean region, and in semi-arid regions in general, are characterized by an extremely rapid rise in the discharge (Figure 12). Plots of hydrographs show an almost vertical rising limb and the flood wave coming down dry river channels in these flash floods has frequently been described as a ’wall of water’. The rapid rise in water level is one of the reasons that these events tend to be so devastating and dangerous. Secondly, such events tend to be of short duration, events often lasting only a few hours though the actual duration varies with the synoptic conditions giving rise to the rainfall. The floods are often very localized and typically peak flows decrease downstream from the zone of the storm due to transmission losses. Overall, the peak discharge in flood events is related to size of catchments. Actual magnitudes of peak flow are indicated in Figure 13. These flood events are also characterized by extremely high sediment loads with sediment concentrations far in excess of any recorded in humid region rivers. For example, Laronne and Reid (1993) reckon that bedload can be 10 to 100


184

.

Figure 12 Evolution of water discharge in Rambla Nogalte (southeast Spain) during an extreme rainfall event After Navarro HervAs 1991

Figure 13 Peak flow discharge against catchment area for Mediterranean basins (based on various sources, see references). Envelope curve indicates highest peak flow discharge for a given drainage area fold that in humid regions. Sediment loads of 40% of the volume of flow have been recorded in events such as the extreme flood in 1973 on the Nogalte in southeast Spain

(Conesa-Garcia, 1995).

Very many descriptions and analyses of individual extreme flood events in the Mediterranean exist but many are as local or institutional reports and produced to


185

evaluate damage. Compilation and evaluation of physical effects of floods are quite difficult but analyses of physical characteristics and impacts of floods in the academic and wider literature include the studies of the Vaison-la-Romaine event in 1992 in southeast France (Arnaud-Fassetta et al., 1993; Wainwright, 1996) and a major publication from the region of Catalonia 50 years after the catastrophic flood of 1940 in that area (Universitat de Perpiny~, 1990), studies of the 1973 event in southeast Spain (Thornes, 1976; Navarro Herv6s, 1985), an event in 1980 near Almeria (Harvey, 1984) and the 1996 event in Biescas (Garcia Ruiz et al., 1996). Events on the Amo have also been intensively studied particularly following the 1966 event in Florence. Many of the major floods have occurred in the mountainous areas of the general Mediterranean region and have characteristics relating to both types of environment, e.g., floods in the north Italian Alpine Piedmont, and in the Pyrenees. The geomorphological impacts of floods have not been considered very much on Mediterranean streams. For example, Molina Sempere et al. (1994: 277) say ’Very few bibliographic references exist concerning the dynamics of floods in Spanish semiarid streams. Only scattered information is available about the changes of the physical and chemical parameters after a flood’. A few case studies of individual events have been carried out but few systematic studies in which the overall impacts of flood events are assessed. One of the few exceptions is that of Conesa-Garcia (1995) in which he classifies types of flood according to their impacts and the degree of change they produce and then analyses the characteristics of three streams in southeast Spain in terms of these categories. On those streams the largest floods which transform the channels and produce large amounts of scour and deposition are calculated to occur with a return period of 2-6 years. More moderate floods which produce some channel adjustment occur with a return period of about 1.4 years. Evidence from the Nogalte (southeast Spain), other streams in the Mediterranean and elsewhere, particularly the western USA, indicate that, in the very largest floods, the channels tend to braid, very intense lateral erosion takes place and whole gravel bars are destroyed and constructed. Bank erosion is particularly important and commonly causes more problems than the flooding (Pearthree, 1987). Minor floods can produce minor modification and some bank slumping and failure. The study by Conesa Garcia (1995) and others shows how, even within one region, the frequency of floods, and their impacts vary, depending on catchment characteristics and type and amount of sediment load. Vita-Finzi (1969) observed that floods can produce sediment from gullies and small tributaries that form small fans in the main channels. Thornes (1976) and others report a very marked spatial variability in scour and deposition. A complexity of cut and fill and a tendency to maximum sediment storage in the middle part of basins have been found from many semi-arid areas around the world (e.g., Patton and Schumm, 1981; Sutherland and Bryan, 1991). Harvey (1984) analysed the geomorphic effects of the 1980 event on a small catchment, Rambla Honda, near Almeria. He found there were spatial variations in the impacts but particularly marked differences in response between the trenched, confined channel and the nontrenched unconfined part of the channel on the downstream fan surface. Scour predominated in the upstream part of the system and deposition downstream. Overall, some general patterns of position of scour and fill and of locations of intense bank erosion and zones of deposition in semi-arid stream channels are beginning to emerge but more systematic analysis is needed. This requires detailed data from individual flood events but is vital to adequate modelling and prediction of flood impacts and for effective channel management.


186

Generally, it is the very large events which cause major channel change and channel widening by erosion. The long-term impact of such events depends on subsequent events but the morphology of a channel created by an event can have a positive feedback effect on the impact of subsequent flows (Hooke, 1996). Models of longer-term development of channels and floodplains and interaction of factors including human action are now taking place within the MEDALUS program. It has also been found that increased sediment loads occur in events after the major flood (Thornes, 1976). Over time, there may be a gradual narrowing of the channel by deposition during moderate events but this will depend on the sequence of events and the interacting factors, particularly vegetation. Conceptual frameworks for analysis of impacts of floods are moving away from simple threshold models of metamorphosis by large events and towards realizing the importance of sequences of events and positive feedbacks. It is now widely accepted that ideas of equilibrium forms of channels are not applicable to ephemerally flowing streams. This has important implications for the management and design of channels. Direct measurements of sediment load during floods are rare except for some instrumented catchments in the USA, Italy and Israel. On Virginio Creek, a tributary of the Arno in Tuscany, bedload has been measured continuously during flood events and some

information is available

1987). Measurements those of

a

perennial

on

the effects of different-size floods (Tacconi and Billi,

stream in Israel produced bedload rates which were 100 times stream in Britain and the highest rates recorded in the literature on a

(Laronne et al., 1992). In general, the literature tends to examine streams in terms of ephemeral or perennial flow and to assume they have different characteristics. Vidal-Abarca et al. (1992) did try to differentiate between types of stream within the Segura basin on the basis of morphometric properties. They identified perennially, seasonally and ephemerally flowing parts of the network within the Segura basin, showing that even within one small region streams of various characteristics and behaviour can occur. Sala (1983) studied fluvial processes in a subhumid region and showed that the frequency of bank erosion processes lies between that of humid and semi-arid areas. In a later article from the Catalan region Batalla and Sala (1995) show that 65% of the sediment load is carried as bedload and 31 % is carried in 2.2% of the time. The proportions of total sediment load carried as bedload are generally far higher in semi-arid areas than in humid regions. Laronne and Reid (1993: 149) in analysing bedload measurements from a stream in Israel say that their results ’suggest that areas at risk of shifting from sub-humid to semi-arid conditions as a result of prospective global changes in climate may suffer severe sedimentation problems’. Review of impacts of flood events highlights how important is sediment in causing problems in flood events, mainly because of erosion (e.g., Pearthree, 1987) and because structures are not designed for the high sediment loads which occur (e.g., Marchi et al., 1991). Much evidence points towards the important contribution of channel bed and banks to that sediment load. This has implications for channel management.

IV

Channel management

Various

be taken to channel and flood management. These are suma paper by Yevjevich (1994a) and by Penning-Rowsell (1996; Figure 14). Management of flooding is usually divided into two major groups, structural and nonstructural solutions. Yevjevich’s paper was published in a book entitled Coping

approaches

can

marized, for example, in


187

Figure 14 Structural and nonstructural approaches to flood alleviation After Penning-Rowsell, 1996 with Floods (Rossi et al., 1994) which was the result of a NATO conference on this In the Preface to that book Rossi (p. xi) explains the reasons for the meeting:

subject.

arose from appraisal of two major changes which have been affecting the flood hazard mitigation activities after many decades of lack of significant innovations, namely the development of new technologies for real-time flood forecasts and warning (based on weather, radar and satellites) and the tendency to shift from structural to nonstructural means, also due to the greater awareness about the environment conservation and the adverse impacts of hydraulic works.

It

As Rossi states, major changes in attitude and strategies taken to alleviate floods have occurred in recent years. There is a strong move away from structural and engineering solutions towards ’softer’ and ’greener’ solutions. This has come about not only because of the environmental pressures but also from a realization that the engineering solutions themselves cause problems and may not be a long-term sustainable solution. Thus, in the UK, for example, the government policy on river management is now one of ’working with nature’ and in various parts of northern Europe restoration of rivers from a channelized state to a more natural form is taking place. This is a recognition that rivers in their natural state have a greater ability to adjust to a range of natural events. Given the


188

high variability of both form and process in the drier region rivers of the Mediterranean then such an approach should be even more applicable in this region. Structural solutions are still being implemented in the Mediterranean region (though such solutions will probably always be the most appropriate in certain situations of high risk and high economic value) but there is a growing realization of the need to develop alternatives and the realization that this requires greater knowledge and understanding and data. For example, channelization of a 70 km reach of the Segura has taken place. It is designed to take the 50-year flood. It cannot eliminate the flood effect and there is now a problem of increased development in the floodplain (Molina Sempere et al., 1994). Nonstructural solutions are needed but problems include that ’historical series of rainfall intensity do not exist, hydrologic dynamics of soils are not known, and importance of vegetation is not measured’ (Molina Sempere et al., 1994: 277). Similarly, following the devastating Vaison-la-Romaine flood in 1992 in southeast France, Masson (1993) questions the previous approach to hydraulic modelling, flood-risk prediction and planning strategies. He recommends that a much more geomorphological, naturalistic and holistic approach should be taken in which the real features of the channel and floodplain are considered, particularly by use of geomorphological mapping and aerial photographs. Graf (1988) has pointed out that there are major differences between arid region floods and humid region floods which have to be realized for management purposes. These differences are frequently not accounted for which is one reason why faulty management occurs in arid and semi-arid regions. 1

Structural

measures

Water and channel management has long been practised in the Mediterranean as witnessed by the Roman aqueducts and remains of dams. Vita-Finzi (1969) investigated and documented the history of many of these in his superb work on Mediterranean valleys. He says of these works (Vita-Finzi, 1969: 14-15): ’What is certain is that, to ensure success, dams had to be built at several points in the various basins, for otherwise the spates’ (i.e., flash floods) ’could not be fully controlled’. Some dams were flood-diversion dams to serve the needs of agriculture. One dam and diversion channel was even built to divert silt from a harbour. Many ancient dams are left hanging now because of head-cut retreat and channel incision. Even stone dykes to protect against floods were constructed in Classical antiquity (Vita-Finzi, 1969). Similarly flood defences date back centuries in some cities. Valencia had major works in the seventeenth century though a new flood channel was constructed in the middle of the twentieth century (Carmona Gonzalez,

1986).

Water-harvesting techniques, some practised from ancient times, were not designed to mitigate the effects of floods but to enhance the water resources for agricultural purposes. However, they obviously have an effect on peak flows, particularly moderate events. The decline of traditional harvesting techniques, such as canadas, boqueros and acequias de canon in southeast Spain (Giraldez et al., 1988), must therefore have an effect on flood regimes. These have been modelled (e.g., Lee, 1988) but rarely quantified directly. The decline of such practices will tend to increase the flood hazard and exacerbate the effects of floods. One form of channel management and water management commonly used in semiarid areas and the Mediterranean region is the construction of dams. Many are constructed for water resources purposes, some small ones as check dams to control


189

sediment and some as flood control reservoirs. High water supply and good protection against floods have opposing requirements in terms of state of reservoir storage. A particular and severe problem with reservoirs for either purpose in semi-arid areas is that, because of the high sediment loads, the reservoirs are quickly infilled with sediment and capacity is reduced. Many reservoirs have become completely filled with sediment. The time that took varies with the catchment but, in an extreme case in southeast Spain, a reservoir at Nijar was infilled in six years. This was mainly because erosion was accelerated by incision upstream. The absence or weakness of calculations of sediment yields and sediment transport has meant that lifetimes of reservoirs have frequently been overestimated. Vita-Finzi (1969) in the Mediterranean and Graf (1983) in Arizona have also demonstrated how dams induce aggradation upstream which then encourages extensive stream migration once a sediment wedge is formed. Reservoirs and channel controls that prevent movement of sediment can have other negative consequences downstream, including channel degradation from clear-water effects and lack of sediment supply to the coast and therefore increased dominance of marine influences and greater salt-water penetration. In the case of the Ebro the sediment yield into the sea is 1 % of what it was before construction of the dams (Guillen and

Palanques, 1992). As well as having

dams and diversion channels upstream of urban areas, another structural solution to urban flood problems is that of flood retention basins. These store water during the flood and allow it to be more gradually released into the urban sewer system after the peak flow. An example is in the city of Rethymnon on Crete. An optimal solution is to combine the hydraulic function with some other activities of public interest such as sport facilities (Ganoulis, 1994). These retention basins also increase the reliability of the sewer system because suspended material is retained in the basins. Flood retention basins may also be constructed to enhance groundwater replenishment but have the additional advantage of reducing peak flow. One of the main forms of channel management is channelization, that is a redesign of the channel which usually involves straightening, alteration of the cross-section and construction of smoother and more resistant boundaries. This has the effect of transmitting water more quickly downstream and may therefore evacuate water from a vulnerable area but it may increase the flood hazard downstream. Likewise, there are consequences on erosion and on sedimentation in channels. It is often found that an initial channelization of a reach leads to progressive channelization because of erosion downstream. The effects of channelization have been blamed for exacerbating the problems in some of the recent floods in northern Italy. Brookes (1988) has set out the case against channelization and some of the alternatives which can be considered. Efforts are now being made to retain meanders, roughness features and vegetation in channels and some channelization schemes are even being reversed with the more natural features restored. There is some limited documentation of the effects of channel and water management on the morphology of channels in the Mediterranean region. For example, Roux et al. (1989) and colleagues have shown the transformation of streams in the Rhone basin, and Castaldini and Piacente (1995) demonstrate that channel management on the River Po increased markedly in the 1970s and that now much of the river is engineered. Bank protection has led to deposition in other places and to a progressive narrowing of the low-water channel. The natural evolution of the river is now considerably hindered and therefore has less ability to adjust in large flood events.


190

2

Nonstructural

measures

types of nonstructural approaches to flood alleviation can be taken, as shown in 14. Much work has gone on, particularly under the EC EUROFLOOD project, into Figure assessment of the flood risk hazard and the mitigation of the hazards. This project encompasses various aspects of the problem from assessment of the impacts of floods to public perception and to operation and effectiveness of flood-warning schemes (PenningRowsell and Fordham, 1994). The project involves assessment of economic and other

Various

values and the problems of quantifying and categorizing effects and risks. The focus of the project is on the effects of climate change and the research is designed to evaluate hazard and vulnerability analysis within a framework of both the physical system and human activities and behaviour. One example of such a study is that from Setubal, Portugal where concern with the public perception of the hazard is involving use of GIS in flood-control planning and the communication of information to the public. Urbanization has increased the flood hazard in many small catchments vulnerable to flash floods in Portugal (Correia et al., 1994). Much effort in many countries and regions of Europe is being put into flood-hazard and flood-risk mapping. This usually entails calculation of the size of particular recurrence interval flows then calculation of the locations and areas these floods will inundate. Flood maps are produced. Marco (1994) reviews the situation and methods in several countries of Europe and exemplifies flood-risk mapping in the Valencia region. He is critical of the European Union for not showing initiative in setting flood-risk map standards. The difficulties of estimating return periods have already been highlighted. Kresan (1988) points out additional problems in arid region channels, including the mobility and the morphology of the channel causing flood-prone zones to alter from one event to another. Graf (1988), Kresan (1988) and others have pointed out that most of the hazard and damage from arid and semi-arid region floods come from erosion and the sediment effects rather than from the flooding itself, particularly where channels are confined (as so often in urban areas). Alluvial fans on which channels are unconfined do also present severe problems of flooding and channel switching and require very careful management and planning (Rhoads, 1986; Marco 1994; Schick, 1995). As has been shown earlier, flash floods in small basins cause a particular problem with flood-warning schemes because the response time is insufficient for action to be taken. Effort is therefore going into forecasting of rainfall rather than forecasting of flow once the rain is falling. Such developments are exemplified by the work in the Arno basin in

Tuscany. Overall, the move in channel management is away from structural solutions towards nonstructural approaches and even do-nothing strategies. As Yevjevich (1994b: 39) says: Knowledge of flood characteristics, flood impacts and conditions of floodplains and the other flood-prone areas is a powerful tool for minimising losses and maximising benefits in the do-nothing alternative of coping with floods. The more exhaustive this knowledge, the better people will adjust to that alternative. The future application of the do-nothing approach to floods will likely be based not only on this fundamental information on floods, but also on the results of research on how plants, animals and humans interact in flood-prone areas. In summary, almost all the climatic and land-use changes likely to affect the Mediterranean region in the medium term lead to a greater variability in the flood hazard but also to greater magnitudes of flooding. The greater urbanization especially in the coastal fringes of the Mediterranean, decline in traditional water-harvesting techniques, increase in irrigation and increase in channelization, as well as increased aridity, all increase the risk of floods.


191

recommendations

3

Policy

The

major recommendations in terms of policy arising from this review are as follows:

integrated and holistic approach to flood and channel management should be taken within basins. Nonstructural and do-nothing solutions should always be considered and should be adopted as far as possible since they would appear to have a greater long-term

1) An

2)

sustainability. 3) In evaluation of the flood hazard and flood mitigation 4)

measures as much historical data on floods and channel behaviour should be collected as possible. The high spatial and temporal variability of floods and flood impacts and of feedback effects leading to longer phases of particular characteristics should be taken into

account.

5) The hazards created

6)

V

by erosion and sedimentation should be considered along with those of inundation and flow of water and structures should be designed for high sediment loads. It should be recognized that exact predictions are not possible and scientific results be given to decision-makers as a range of possibilities and probabilities with consequences of extremes indicated. Research gaps

From the previous analysis, a series of research needs in the fields of soil erosion and flooding in the Mediterranean can be defined in order to help guide future research. It is believed that such research could greatly enhance a better understanding and prediction of the impact of environmental change on erosion and flooding in Mediterranean environments. At the same time, such research would be invaluable for developing appropriate soil conservation and channel management measures. 1

Erosion

prediction of the impact of global change on soil erosion requires a sufficient understanding of the scale-dependent nature of erosion processes. More information is needed on relevant erosion processes for a given spatial scale and the interactions between erosion processes operating at a hierarchy of spatial scales. Research is also needed on the interactions between soil erosion and the various other soil degradation processes (e.g., physical degradation, biological degradation, mass movements). This kind of understanding can only be gained through monitoring and experimentation at a number of different scales. The results of this research are of utmost importance for model Accurate

calibration and validation. Most erosion research has concentrated on the two extremes of the catchments: either plot studies on uplands or sediment yield data for large catchments and reservoirs. Gullying and sedimentation within catchments have received little attention, yet these processes are important links between uplands and channels. More research is needed to understand how environmental change affects these processes. There is a lack of long-term records on soil erosion, particularly for the Mediterranean where erosion processes are dominated by extreme events too rare to occur in the

improvement,


192

duration of the average research project. Therefore, long-term monitoring and experimentation of key soil erosion processes operating at a range of scales are required. When assessing the impact of land-use changes on erosion rates, attention should be also paid to less known processes such as tillage erosion, trampling erosion and erosion due to soil quarrying, terracing and land preparation (reshaping former badlands). Prediction of soil losses or sediment yields based on, for instance, a USLE approach is not sufficient since such a model only accounts for inter-rill and rill erosion. Recently developed erosion models still need to be extended, refined and validated in order to apply them to a wider range of typical Mediterranean situations (e.g., cracking clay soils) or to incorporate other important erosion processes (e.g., gully erosion, mass move-

ments). 2

Flooding A major impediment to further understanding of flood occurrence and flood impacts and therefore the basis for management, is the lack of data on rainfall, on flows and on their effects. Efforts should be made to instigate greater measurement and monitoring and systematically to compile historic data on these events. These need to be public authority functions since academic institutions can rarely sustain such activity over the number of years for which this is necessary. Much more work is needed on the physical impact of floods and the spatial variability and patterns of those impacts. Increased understanding of the interaction between vegetation, flood flows and human activity in valley floors is needed to assess sensitivity of channels and floodplains to desertification. More research is needed on the actual nature and frequency of linkage between upstream and downstream areas, between slopes and channels, and into the dynamics and propagation of the runoff and sedimentproducing zones. More work is needed at the catchment scale but involving details of spatial distribution of processes. Research on modelling flow generation and flood routing should be progressed but much more research is also needed on modelling sediment dynamics and the sources, transport, deposition and storage of sediment within channel systems in a range of magnitude of events. Further research is required into the impacts of land-use change and of channel management on the characteristics and effects of flooding. Detailed work is needed on the physical, social and economic impacts of alternative strategies of channel management. This should examine both the short and long-term consequences and the implications for sustainability of channel management policies. A range and continuum of behaviour from perennial to ephemeral streams should be recognized, and more work directed towards the implications of change from subhumid to semi-arid climates. The particular problems posed by the juxtaposition of mountain areas to drier lowlands in the Mediterranean should also be considered. The complexity of natural behaviour as well as the high variability needs to be acknowledged and incorporated into any modelling.

Acknowledgements Mrs

Jennifer Mant, Dr Dirk Oostwoud Wijdenes, Dr Bas van Wesemael, Dr Elena de Luna, Dr Claude Cosandey, Dr G. Govers and Dr Esther Bochet are thanked for helping


193

with literature collection. Dr Frank Law (Institure of Hydrology, UK) also kindly provided some materials. We would also like to acknowledge the many researchers working in the MEDALUS project for putting publications and papers at our disposal. Financial support provided by the European Commission Environment and Climate Research Programme (MEDALUS III project, contract ENV4-CT95-0118) is appreciated. This study is a contribution to the Soil Erosion Network of the Global Change and Terrestrial Ecosystems Core Research Programme which is part of the International

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