Geomorphological and sedimentological analysis of a catastrophic flash flood in the Arás drainage basin (Central Pyrenees, Spain)

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Geomorphology 22 (1998) 265-283

Geomorphological and sedimentological analysis of a catastrophic flash flood in the Arfis drainage basin (Central Pyrenees, Spain) Francisco Guti6rrez *, Mateo Guti&rez, Carlos Sancho Departamento de Ciencias de la Tierra, Facultad de Ciencias, Pedro Cerbuna 12, 50009 Zaragoza, Spain Received 12 May 1997; revised 25 August 1997; accepted 23 September 1997

Abstract On August 7th, 1996, an intense and short-duration convective storm occurred over the 18.6-kin 2 Arfis drainage basin (Central Pyrenees, SpaJLn). This high relief basin is composed of three subbasins, Aso, Betts and La Selva, and feeds the Arfis alluvial fan, in the Gfillego river valley. This alluvial fan had been drained by an artificial channel (about 125 m3/s at bank-full capacity). More than 30 check dams in its feeder channel, the Arfis barranco, had been previously filled by earlier sediments. The heavies1: rain was over the Betts subbasin (total rainfall 178.4 ram; maximum rainfall intensity of 153 m m / h for a 10-min time interval was estimated). Most of the rainfall fell in a 70-min period. This storm resulted in high runoff, causing catastrophic damage and significant geomorphic changes in the drainage basin, especially in the Betts subbasin. The high discharge, concentrated in the Arfis barranco, destroyed most of the check dams, flushing out a great amount of debris. Major channel trenching and widening occurred in this barranco. When the confined sediment-laden flash flood reached the basin mouth, it sheet-flooded the southern sector of the Arfis fan depositing a massive amount of debris. On this fan 87 people lost their lives and the direct physical damage has been estimated at 55 million dollars. Two stages in the development of the flood have been differentiated from the sedimentological and morphological analysis of the flooded fan lobe. A first stage (peak discharge) of sheet-flooding deposited a coarse boulder lobe, burying the artificial channel at the fan head and causing a damming effect on the water flood. During the second stage (discharge decline) the flood made its way through the fan head, incising the previous debris accumulation and splitting into two main flow paths. © 1998 Elsevier Science B.V. Keywords: catastrophic storm; flash flood; alluvial fan; Central Pyrenees

1. Introduction The 18.6-km 2 Ar~is basin is a tributary watershed of the Gfillego River (Central Pyrenees, Spain) (Fig. 1). This basin is composed of three subbasins--the

* Corresponding author. Tel.: +34 76 761090; Fax: +34 76 761088; E-mail: [email protected]

Aso, Betts and La S e l v a - - a n d feeds the Arfis alluvial fan on the margin of the Gfillego valley. In the evening of 7 August 1996, an intense storm of short duration occurred in the Arfis drainage basin, resulting in high runoff and an extraordinary flood that caused catastrophic damage and significant geomorphic changes in both the drainage basin and on the alluvial fan. The well-known torrential activity of this geomorphic system had previously led to the

0169-555X/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved. Pll S 0 1 6 9 - 5 5 5 X ( 9 7 ) 0 0 0 8 7 - 1

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266

Basin divide ...................... Subbasin divide Contour line Ar&s alluvial fan

• Ar~s basin 10 °

~o i

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t

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Road Road cut

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148 m

1766 m Biescas \

/ N 1624 m

Las Nieves Camp-site

3kin

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Fig. 1. Geographical location and topographic map of the Arhs drainage basin.

F. Gutidrrez et al./ Geomorphology 22 (1998) 265-283

construction of control structures since the end of the last century. More than 30 check dams had been constructed in the steep feeder channel of the Ar~s alluvial fan (the Arfis barranco). These had been completely filled by sediment prior to the 1996 flood. The Ar~ts alluvial fan was drained by an artificial channel with a bank-full capacity of about 125 m a / s . Radar-derived rainfall data show that within the Ar~is basin the heaviest rain fell in the Betrs subbasin. This information is in agreement with both the spatial distribution of the geomorphic effects caused by the storm and eye-witness accounts. The road bridges over the Betrs and Aso barrancos were partially destroyed by the high discharge (Fig. 1). The water flood concentrated in the Ar~is barranco, the steep t~amk stream that drains the whole basin, and destroyed most of the check dams, flushing out the older sediments. Major channel trenching and widening occun'ed in this barranco. Garcia-Ruiz

267

et al. (1996) calculated peak discharges of 300 m 3 / s in the Betrs barranco, 100 m 3 / s in the lower reach of the Aso barranco, and 400 m a / s and 500 m a / s at the upper reach of the Ar~is barranco and close to the fan apex, respectively. These authors attribute this rapid increase in the peak flood discharge, in spite of the small increase in the contributing area, to the large amount of sediment load released by the breakage of the check dams. When the confined sediment-laden flood reached the mouth of the basin, it spread over the southern sector of the alluvial fan depositing a massive amount of debris at the fan head, burying the artificial channel (Fig. 2). The Las Nieves camp site had been sited on this fan portion since 1988. The flood swept people, tents, cars and caravans from the camp site (Fig. 3). Massive debris (coarse boulders to gravels) and sand deposition occurred on the alluvial fan (Fig. 2) and along the west side of the Gfillego River. The high discharge supplied by the Ar~s basin caused

Fig. 2. View of the Ar-Lsalluvial fan twelvedays after the 7th August 1996 event, when most of the depositionaland erosionalfeatureshave been removed.Note the sharp bend of the Ar~s feeder channel at the basin mouthjust upstreamof the artificial channellization. Las Nieves camp site correspondsto the area planted with poplars. The coarse-boulderlobe deposited at the fan head shows a sharp distal end defined by the upper part of the camp site. The arrow marks the damagedtrack bridge over the natural channel of the fan. In the foregroundthe channeUization of the Sia alluvial fan. Most of the features recognizablein these photograph are represented in Fig. 6.

268

F. Guti~rrez et al. / Geomorphology 22 (1998) 265-283

Fig. 3. Vehiclesand debris deposited by the flood waters downstreamof the camp site. Photographtaken on August 9, 1996 from the N-260 road.

local flooding in the G~llego valley and backflooding in the channelized Sfa alluvial fan, located on the opposite side of the valley. The Sfa channel deposited a debris sheet by overbank floods on the right side of the channel. In this catastrophic event, 87 campers lost their lives and the direct physical damage in the Ar~s basin and alluvial fan has been estimated at 55 million dollars. On the day of the disaster the camp site, with a capacity of 900 people, was occupied by 630 people. At the time of the event, around 19h30, only about 150 people were present. In terms of human lives lost, this flood has been the largest natural disaster in Spain in the last 23 years. Outside the basin, the road connecting the villages Aso, Yosa and Betrs with the valley was blocked by a debris flow close to the junction with the A-136 road (Fig. 1). These villages lost road contact after the event. The N-260 road that crosses the Arfis alluvial fan was destroyed by the flood (Fig. 1) and about 4 km to the north of Biescas debris accumulations blocked the A-136 road. These road blocks cut

off Biescas village and the Las Nieves camp site from the rest of the valley. Electricity and phone cuts were also caused by the storm. All these circumstances made the rescue operations difficult during the night after the event. In the Pyrenees, similarly to other alpine regions, high-intensity rainfall events are quite common, especially in summer when moist and warm air masses close to the ground provide adequate atmospheric conditions for the development of convective storms. The prediction on a local scale of such meteorological events is almost impossible due to their haphazard spatial and temporal distribution. These cloudbursts cause catastrophic phenomena like mass movements, debris flows, torrential flows and floods, to be accentuated by the high relief, steep slopes and the presence of large amounts of unstable material (e.g. glacial deposits) that characterize alpine environments. Numerous storm-related catastrophic phenomena, in many cases involving severe damage in terms of loss of human lives and property have been reported in the Pyrenees (Martf-Bono and

F. Guti~rrezet al. / Geomorphology22 (1998)265-283

Puigdeffibregas, 19813; Garcla-Ruiz et al., 1983; Bru et al., 1984; Bravard, 1988; Antoine, 1988, 1989; Clotet et al., 1989; Meunier, 1990). In the last decades, as development in alpine regions has rapidly expanded, damage from such phenomena has dramatically increased. In spite of land-use planning and control measures, damage is commonly high in recently developed areas. This situation can be attributed to several circumstances, such as the following: (a) the limited available precipitation record and the understanding of these catastrophic phenomena do not allow a precise risk evaluation for a safe development; (b) developments have been expanded to more dangerous areas due to the lack of suitable space elsewhere; (c) in some cases natural hazards are considered as a second-order criterion in land planning and development; (d) these storm-generated phenomena have a higher frequency in summer, when there is a greater tourist population that tend to occupy recently developed areas; (e) some recent developments like camp sites are highly vulnerable.

2. The Ards drainage basin and alluvial fan 2.1. Characterization of the Ar6s drainage basin

The Ar~s drainage basin, 18.6 kin 2 in area, has a spoon-like shape and is composed of three subbasins drained by the Aso barranco (10.6 km2), Bet6s barranco (4.2 km 2) and La Selva barranco (3.8 km 2) (Fig. 1). Downstreana of the junction of these three streams, the fifth-order Ar~s barranco collects the water of the whole catchment and feeds the Arfis alluvial fan. The basin, with a relief between 2190 m and 910 m, hangs more than 200 m above the G~llego valley due to glacial overdeepening. A great part of the catchment has gradients higher than 20%, and close to the divides higher than 60%. The area has a Mediterranean mountain climate with marked seasonal temperature contrast. The meteorological station at Aso (at 1240 m a.s.1.) has a mean annual temperature of 8.75°C with 1282 mm mean annual precipitation. Although summer is the driest season, it is also the period when heavy storms are more frequent. The previous maximum daily

269

rainfall over an 18-year record (1971-1994) reached 98 mm. The Ar~s drainage basin is developed in Eocene turbidites of the South-Pyrenean Zone. These sediments consist of rhythmic sequences of thinly bedded sandstones and clays (Remacha, 1983), intercalated with thick limestone beds (megaturbidites, Labaume, 1983). The whole sequence shows a complex structure with tight folds and imbricated thrusts (Barnolas, 1996). This Eocene bedrock is partially covered by Quaternary deposits linked to the Pleistocene glaciation of the G~llego valley (Fig. 4). In the lower part of the basin, around Yosa, the GS.11ego glacier deposited two lateral moraines which dammed the Ar~s watershed, generating three glacio-lacustrine basins (Barrere, 1966; Serrano, 1991; Marti-Bono, 1996) (Fig. 5) aged between 50,000 and 20,000 years B.P. (Serrano, 1991; Bordonau, 1992). The outer and inner moraines are respectively about 400 and 350 m above the G~llego valley floor. The lower one is split into two ridges which define two episodes of glacial retreat (Serrano, 1991) (Fig. 4). The morainic deposits locally reach more than 100 m in thickness and are composed of large blocks and boulders embedded in a matrix of dominantly gravel and sand with low silt and clay contents. The infill of the moraine-dammed basins is clay-rich fine-grained sediments with scattered clasts supplied by the surrounding slopes and moraines. The development of the drainage system and the Ar~s alluvial fan took place after the G(dlego valley deglaciation and the later capture of the lacustrine basins. Several depositional and erosional landforms have been developed associated with the drainage network (Fig. 4) like debris cones in the Aso and Bet6s subbasins, small stream terraces in the Aso barranco and gully systems dissecting the lower moraine deposits. The drainage network is characterized by high-gradient channels except for the reaches where the Aso and Bet6s streams run through the former lake basins (Fig. 5). Downstream, the channels steepen and the Ar~s barranco descends with a high slope (16%) to the apex of the Ar~s alluvial fan. The drainage basin has a well-developed vegetation cover, mainly of pineforest, covering 55% of the catchment. Above the forest limit at 1600 to 1950 m (Fig. 5), the hillslopes are dominated by summer pastures with a thin soil cover, showing solifluction

270

F. Guti~rrez et al. / Geomorphology 22 (1998) 265-283 Peffas de Aso =L 2190 m

Morainicridge [•--•--I ~

Outermorainedeposits

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alluvialfan and debriscones Divide

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Fig. 4. Geomorphological map of the Ar~isdrainage basin and alluvial fan.

F. Gutidrrez et al./ Geomorphology 22 (1998) 265-283

271

Fig. 5. General view of the Aso subbasin. The flat bottom of the basin correspondsto the Aso glacio-lacustrinebasin (see Fig. 4). At the background the G~llego;glaciervalley. Photographtaken on October 19, 1996 from the highest point of the Ar~s basin (Pefiasde Aso, 2190 m a.s.1.).

and numerous shallow soil slides (Garcla-Ruiz and Puigdefftbregas, 1982) (Figs. 4 and 5). In the glacial lake basins, the areas close to the streams are used for crops and pastttres. In spite of the high vegetation cover, the morphometric attributes of the drainage basin favour a rapid runoff concentration and the generation of flash floods with high erosion and transport capacity from major precipitation events (Patton and Baker, 1976; Patton, 1988; Blair and McPherson, 1994a,b). 2.2. Human impact on the Ar6s drainage basin

Anthropogenic changes in vegetation and channel control measures in the Ar~s basin have modified the hydrological behaviour of the catchment and the dynamics of the Ar~s alluvial fan. Between the 10th and 12th centuries cattle raising led to the transformation of the highest parts of the basin from forest into pasture (Montserrat, 1992). Much later, from the

last third of the 19th century to recent years, a high proportion of the crop lands on the slopes and in the former lake basins were abandoned and are being colonised by scrub and forest (Lasanta, 1989). According to Borderas (1930), the Arfis alluvial fan was considered the most dangerous of the region due to the frequent floods and debris accumulations that used to affect the fan surface. The control measures in both the Arfis barranco and alluvial fan go back to the end of the last century. In the summer of 1929 two heavy storms occurred (1 lth June and 15th July). For the first storm 50.1 m m of rain fell in 90 min and the peak discharge estimated in three sections were of the order of 120 m 3 / s (Borderas, 1930). Both storms resulted in the flooding of the Ar~s fan in spite of the existing check dams and channelization of the alluvial fan. These floods spread over the road for 1300 m, causing the loss of one life, the destruction of the fan channel and the isolation of the upper G(tllego valley. With the aim of

272

F. Gutidrrez et al. / Geomorphology 22 (1998) 265-283

avoiding these problems, the engineer Borderas (1930) elaborated a project to improve the control measures in the Ar~is barranco. He proposed the reparation of the fan channel, the construction of eleven new check dams made of sandstone blocks (maximum height 8.5 m, thicknesses 4 m) and ten second-order check dams in the higher reaches of the Arfis barranco, two check dams in both the Yosa and

Bet6s tributaries close to their junction, and the application of slope stabilization measures in the Ar~is barranco. These control works complemented the reforestation (dominantly with pines) of more than 5 km 2 of the catchment. These works were finished around 1950. Before the 7th August 1996 event, the Ar~is barranco (1100 m in length) had more than 30 check

~

Crop fields x Damaged structure Sandy bouldery gravel Buildings ---Embankment Sandy cobble and pebble gravel

~

Pebbly sand

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Coarse boulders

--* Flow direction - - - Flood limit Scours and scars ~ Rotational slide ~* Pines

925

e0 o,c

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Fig, 6. Map of the acdve deposidonal lobe of the Ar~s alluvial fan showing the main sedimentological (textural facies) and geomorphologica] features generated in the flood of August 7, ]996. The contour lines correspond to the fan topography previous to the flood, (Asterisks

indicate locations of previous flood deposits.)

F. Gutidrrez et al. / Geomorphology 22 (1998) 265-283

dams giving a stepped profile to the barranco. The Aso barranco had two check dams just upstream of the La Selva barranco junction. All the check dams had been filled with sediment long before the 1996 event. Large pines had grown on the sediments surfaces. Dendrochronological methods suggest ages around 40 years indicating that some dams had been filled soon after their construction (Garcia-Ruiz et al., 1996).

2.3. The Ar6s alluvial fan The Arfis alluvial fan (Fig. 4) is 0.55 km 2 in area, (0.8 km average radial length and 4.6 ° average radial slope) drained by a straight and stepped artificial channel built of masonry on the fan surface, approximately along the medial line of the fan (Fig. 2). This is the only alluvial fan in this sector of the Gfillego valley on which no village has been built. The northern sector of the fan is occupied by numerous crop fields bounded by stone walls (Fig. 6) more than 2 m in thickness close to the fan apex. This northern portion of the fan seems to have been inactive for a long time span. Prior to the 1996 event, the active depositional lobe was situated in the southern portion of the alluvial fan. In this sector the fan was incised by a channel (see contour lines in Fig. 6). The recent trenching of the fan, a common feature of the alluvial fans in this sector of the Ggdlego valley, may be explained by the decrease in sediment yield resulting from the construction of the check dams in the feeder channel and the increase in vegetation cover in the drainage basin (G6mez, 1996). The artificial channel, built on the fan surface at the turn of the last century, interrupted the incision trend of the fan. The aerial photographs taken in 1946, 1957 and 1981 show how in the southern portion of the fan natural channels have changed through time, and vegetation (mainly Buxus sempervirens, Hippophae rhammoides, Genista scorpius and Salix eleagnos) has rapidly colonised tlxis depositional lobe. In 1988 the camp site Las Niew'.s was opened on this portion of the fan (Fig. 4). In 1996, the Spanish Geological Survey produced a 1:25,000-scale flooding hazard map of the studied area, assigning level 7 (in a 0 to 9 hazard scale) to the Arfis alluvial fan (Rios et al., 1996).

273

3. The 7th August Arfis flood

3.1. Rainfall On the day of the event, the National Institute of Meteorology (I.N.M.) detected cold air currents at high altitudes and moist warm Mediterranean air masses near the surface. The highly unstable atmospheric conditions produced by the convergence of both air masses led to the generation of localised convective storms. Daily rainfall totals, recorded at several raingauges located in the surroundings of the basin and rainfall data derived from radar imagery (Riosalido et al., 1996) provide isohyet plots of daily rainfall totals and rainfall intensities for 10-min time intervals. The radar station that recorded the storm is about 75 km to the south of the Ar~s basin at a height of 829 m a.s.1. (Monte Oscuro, Sierra de Alcubierre, Zaragoza). This radar station, with a coverage radius of 240 kin, has a scanning interval of 10 min providing averaged information for 2 km × 2 km pixels. The radar visibility in the Arfis basin was good for the day of the event. The daily total isohyet map derived from the radar record (not corrected by the raingauge data) and daily totals from raingauges reflect a spatially restricted storm located over the Arfis basin (Fig. 7), with a total of 160 nun of rainfall recorded at Biescas, the raingauge closest to the basin. The estimated daily rainfall totals within the Arfis basin range from 43.2 mm in the basin head (pixel centred in x = 15, y = 103) to 178.4 mm close to the northeastern divide of Bet6s subbasin ( x = 19, y = 99) (Fig. 7). For the whole area affected by the storm, 192 mm is the maximum value recorded by the radar in a pixel centred about 2 km to the northeast of the basin (x = 19, y = 103). The plotted radar rainfall intensities for 10-min intervals at three points in the basin indicate that high intensity rain fell in a 70-min period, from 18h00 to 19h10 (Fig. 8). The maximum estimated 10-min rainfall intensity reached 153 m m / h from 18h50 to 19h00 in the area around Bet6s. Rainfall intensity changed very sharply during the storm. All these data indicate that within the Ar~s basin, most of the rainfall occurred in Bet6s subbasin.

F. Guti~rrez et al. / Geomorphology 22 (1998) 265-283

274 125

120

115

110

105

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VillanOa (47.7) sa (30.0) .

Castiello (28.3) . Besc6s ( 3 ~

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Jaca (30.5) 90

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5

10

15

20

25

30

Fig. 7. Dally rainfall totals from raingauges (in bold) and isohyet map of dally rainfall totals derived from radar imagery in the Ar~s basin area for August 7, 1996. Note the restricted spatial distribution of the storm. Figure redelineated from the I.N.M. unpublished report of the storm of August 7, 1996 (Riosalido et al., 1996).

This picture was confirmed by eye-witness accounts from inhabitants of Bet6s village, who identified two storms above Aso and Santa Elena, 4 km to the north of Biescas, around 18h30. The two merged above Bet6s at about 19h00, the time of heaviest rainfall. On the day of the event the soils of the basin were fairly wet favouring runoff, since weather had been quite rainy for several days prior to the event. From

the 25th July to the 2nd August a rainfall total of 71.5 m m was recorded at the Biescas raingauge.

3.2. Geomorphic effects and damages caused by the storm in the Ards basin The high runoff generated by the storm produced conspicuous geomorphic changes and severe damage to property in the Ar~s drainage basin. The study of these geomorphic changes caused by the Ar~is storm

275

F. Guti~rrez et aL / Geomorphology 22 (1998) 265-283 2 0 0 "1 o

Yosa

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Fig. 8. Radar-derivedrairffall intensity plot. The radar provides averaged information for pixels of 2 km × 2 km with a scanning interval of 10 min. Yosa, Aso and Betds correspondto the closest pixels to these villages centred in points x = 15, y = 97; x = 17, y = 99 and x = 19, y = 99 of Fig. 7, respecti~:ely. Plot elaborated from data of the I.N.M. report (Riosalido et al., 1996).

provides an ideal opportunity for assessing the role that such extreme events play in landform development in upland envh'onments (Harvey, 1986). Different types of geomorphic response have been identified. On open grass hillslopes above the forest, shallow soil slides were reactivated showing fresh surfaces in their scars. Significant erosion occurred in gully systems with slope erosion and trenching in the gully beds. Some low-order steep channels were extensively scoured. Pines in the beds and on the slopes of these channels were stripped and knocked down by the high-velocity sediment-charged water flow. Major sediment deposition took place on some debris cones at the base of the scoured channel systems. Numerous small slides and slumps were formed in terraced crop fields. In many cases the clay-rich slides disintegrated and transformed into small debris flows. This geomorphic response shows how in certain areas the rainfall threshold that determines slope stability was exceeded (Calne, 1980).

Damage to property was particularly severe at Betds village. This village at the foot of two debris cones (Fig. 4) hindered the flow laden with debris from the scoured channels and talus of the terraced crop fields. The village was flooded to a depth of at least 1 m and massive cobble and boulder accumulation occurred. Crop fields and tracks were extensively damaged and some livestock was lost. Further downstream the asphalt of the road bridge over the Betds stream was disrupted by the flood that spilled over the bridge (Fig. 1). Damage also affected crop fields at Yosa and Aso and the road bridge over the Aso stream was washed out (Fig. 1). Most of these geomorphic changes occurred in the Betds subbasin and in the adjacent area extending from Aso to Yosa village. Outside that area the response was absent or less intense. This spatial pattern coincides with the area of the basin where the most intense rain was recorded by the radar (Fig. 7), suggesting that geomorphic changes caused by a

276

F. Gutidrrez et al. / Geomorphology 22 (1998) 265-283

Fig. 9. Destroyed dams in the Arfis barranco. The large block in the foreground was not entrained by the flood and was forming part of the destroyed dam. In the centre, one of the check dams that resisted the flood. In the background, thick deposits of the outer moraine. In the circle two people for scale. Photograph taken on August 9, 1996.

storm may be a useful tool to infer qualitatively the rainfall spatial pattern of extreme storm events. The storm-derived flood caused intense channel trenching and widening along the lower reach of the Bet6s barranco, where very large boulders were entrained. In contrast, no major channel changes occurred in the Aso and La Selva barrancos. The strength of the flood in the Aso barranco was significantly lower than in the Bet6s barranco in spite of the larger contributing basin area. The only two check dams located in the Aso barranco, just upstream of the junction with the Bet6s barranco, were two of the few weirs that resisted the flood. These observations together with radar rainfall data and the indirect peak discharges estimated by Garcfa-Ruiz et al. (1996), indicate that the Bet6s barranco supplied a great proportion of the discharge to the Arfis trunk stream. The high discharge concentrated in the Arfis barranco caused the destruction of most of the check dams, releasing the sediments which they stored

(Fig. 9). The dam footings were in either turbiditic bedrock or in till deposits and in some cases very large boulders in the channel bed formed part of the dam (Fig. 9). The failure of the dams took place mainly by undermining of the outer part of the dam footings, by the high shear stress and the water overpressure (Garcfa-Ruiz et al., 1996). Major channel trenching and widening occurred in the Arfis barranco (Fig. 9). Numerous small failures (metric in scale) were formed at the banks by undercutting during the flood and due to stress release as the water level declined after the flood. Cfincer (1996) suggests that landslides from the channel banks could have caused a temporary damming of the flood, affecting the peak flood discharge. However, no evidence of large mass movements capable of damming the flood have been found along the whole of the Ar~s barranco. A lag of coarse boulders mixed with logs and weir fragments was selectively deposited along the Arfis barranco (Fig. 9). The boulders show fresh

F. Guti~rrezet al. / Geomorphology 22 (1998) 265-283

277

Fig. 10. Boulderjam fo:rmedupstream of a group of pines growing on the infiU of some check dams located in the Ar~s barrancojust upstream of the fan apex (see Fig. 2). The hammer is 26 cm. Photographtaken on August 28, 1996. percussion marks and display crude imbricated fabrics, with the long axes perpendicular to the flow direction. Finer-grained particles were deposited during flood recession. At some locations two different terraced levels of boulder lags can be differentiated, one on top of the sediments filling the dams (pre-dam breach deposit) and another inset within the dams infill (post-dam breach deposit). Boulder-log jams and several types of boulder bars were formed at locations of energy dissipation (Fig. 10). A fanshaped expansion boulder bar was formed at the mouth of the Betts barranco. A complex boulder bar (lateral point-expartsion bar) was also formed at the inside bank of a bend located just upstream of the fan apex (Fig. 6). Deposition at this location was controlled by a decline in slope, channel widening, the bend of the channel and the obstruction produced by a group of pin¢;s that had grown on the infill of some check dams. These bars are characteristic depositional bedforms formed by extreme water floods in steep, deep arid narrow non-alluvial channels (Baker, 1984).

3.3. Flood development on the ArSs alluvial f a n - sedimentological and geomorphological features The descriptions of survivors give a first-hand picture of the development and activity of the flood at the camp site. During a very intense rainstorm, a shallow water sheet started to flood the camping area. Suddenly, a great torrent of water and sediment, more than 1 m deep rushed into the camp site with a roar. Most of the people did not have enough time to take refuge in any of the existing buildings. During the 10 rain that the flood lasted, the highvelocity and turbulent flow of water and sediment violently swept away people, cars and caravans. This description, taken from numerous eye-witness accounts, makes evident the flashy character of the flood. The people that survived the disaster are mainly those who were able to take shelter in one of the buildings or those who climbed the poplars planted in the camping area (Figs. 6 and 11). Most of those who died were drowned, trapped or buffed in the

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Fig. 11. View of the southernportion of the Ar~s alluvial fan head mantledby debris deposited during the flood of August 7, 1996. Note the sharp boundarybetween the coarse-boulderfacies and the sandy, boulderygravel. Photographtaken on August 27, 1996. sediment and hit by moving debris. In subsequent days bodies were found in the G(dlego river, at the dams at SabifiSxtigo and Jabarella, about 15 and 20 km respectively downstream of the Arfis alluvial fan. Around 50% of those killed were women and 25% children. It seems that physical strength was a factor in survival. According to eye-witness accounts, the flood in the camp site lasted around 10 min, starting at around 19h30, 30 min after the highest-intensity rainfall recorded by the radar. These data give a rough idea of runoff concentration time and the flashy character of the flood. Despite most depositional and erosional features being disturbed by the rescue and repair works, an attempt was made to map the spatial distribution of these features (Fig. 6). Because no aerial photographs of the fan after the flood were available, the map was produced by direct surface survey in the field, using a 1 : 5000 topographic map which shows the pre-flood fan topography. A rough differentiation of four textural facies based on the size of the largest

clasts (reflecting flow competence) was made. These were: (a) coarse boulders ( > 1 m intermediate axial length); (b) sandy, bouldery gravel; (c) sandy cobble and pebble gravel; (d) pebbly sand. Flow directions were inferred on the basis of several different criteria, such as clast imbrication, elongated scours and depositional bedforms, oriented jetsam and flattened vegetation. The distribution of the different textural facies and their geometrical relationships reveal that flooding occurred in two main stages. In the first stage, when the peak flood discharge reached the basin mouth, the sediment-charged flow heavily eroded (by more than 4 m) the lined outer bank of the channel (Figs. 2 and 6). A protruding bedrock spur located on the left bank at the fan apex deflected the confined flow towards the south. Beyond the fan apex the unconfined flow spread over the southern sector of the fan depositing a lobe of coarse boulders on the fan head. The upper reach of the artificial channel built on the fan surface was buried by an accumulation of boulders and cobbles. The coarse-boulder facies distribution shows how the

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sheetflood expanded to an arc of about 30° (given by the down-fan limit of the boulders) and that expansion to the north was limited by the channelization (Fig. 6). The abrupt distal end of the boulder lobe was controlled by the upper edge of the camp site. The sudden stoppage of the boulders was promoted by a wire fence and the terraced topography of the camp site, the presence of obstacles and the rapid flow attenuation. This boulder deposit rests on the pre-flood fan surface and has a sheet-like geometry. Elongate boulders are well imbricated with the long axes oriented perpendicular to the flow direction and with a - b planes dipping up-fan (Fig. 12). The largest clasts in this boulder lobe have intermediate axial lengths of 1.2 m at the distal end of the lobe. Fragments of guardrail transported from the destroyed Yosa bridge (Fig. 1) were found in the lower part of the boulder lobe. Down-fan from the coarse-boulder lobe, where the sheefflood decelerated, sediments ranging from medium boulder to sand-sized were deposited. Sedimentation was controlled largely by vegetation and

279

the structures that obstructed the flow. Sandy, bouldefy gravel facies mantled an extensive surface of the fan (Fig. 6). Clasts show well imbricated fabrics. Pebbly sand sheets with no distinct bedforms were deposited in patches of dense scrub vegetation. The poplars in the camp site were badly scarred although the flow was not capable of knocking them down (Fig. 11). Streamlined trains of imbricated vehicles, logs and boulders were formed behind obstacles as upright trees and poles. Numerous severely damaged vehicles were transported to the distal fan area, downstream of the road (Fig. 3). The roadways of the camp site were scoured and acted as preferential flow paths. The swimming pool behaved as a sediment trap being filled mainly by fine-grained deposits. Numerous shallow failures were formed in the slopes of the terraces in the camp site (Fig. 6). The highly turbulent flow generated scours up to 1.5 m deep on bare surfaces and next to obstructions like the road whose asphalt cover was torn off. Several high-water marks found on the buildings and trees of the camp site indicate a flow depth around 1 m

Fig. 12. Channel fill of sandy, boulderygravels incised in the coarse-boulderlobe upstream of the camp site. Note the imbricationof the boulders and the associatedbattered logs. Photographtaken on August 20, 1996.

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which agrees with descriptions of the flood made by eye-witnesses. In the second stage of the flood, as discharge declined, the boulder deposit at the fan head caused the damming of the flood waters. Then the flood eroded the boulder barrier splitting into two main flow paths (Figs. 6 and 11). A channel was eroded through the southern margin of the fan following a former natural channel clearly distinguishable by the pre-flood contour lines (Fig. 6) and in aerial photographs taken prior to the event. The other flow path taken by the flood was the artificial masonry channel. A similar situation of flood damming produced by sediment deposition in the proximal fan area has been described by Blair (1987) for the Roaring river alluvial fan in the Rocky Mountain National Park, Colorado. According to Wells and Harvey (1987), incision processes are common during the late flood stages in alluvial fans. The flood on the southern edge of the fan incised a deep channel into the sediments below the first flood deposit and caused large-scale lateral erosion.

The confined flow also breached a small distributary channel through the boulder lobe (Fig. 12) which converged with two elongated scours at the upper end of the camp site (Fig. 6). Down-fan from the camp site the channelized flow expanded, destroying the road and track bridges built over the natural channel. Further downstream the flood became reconfined and flowed southward between the valley margin and an embankment on the right bank of the GS_llego river. At the confluence of the natural channel with the G~illego River a large gravel bar was formed. The channels incised during this second stage were filled by a well imbricated sandy, bouldery gravel deposit (Fig. 12). At the fan head this facies shows a sharp boundary with the coarseboulder facies (Fig. 13). In some reaches the sediments filling the main channel are inset within sediments deposited before the event, whereas at other locations they onlap the earlier boulder deposits. The artificial channel was the other flow path opened through the boulder barrier by the flood. The masonry in the stepped bed of the channel was

Fig. 13. Boundary between the coarse-boulder facies and the later sandy, bouldery gravel in the fan head. Hammer is 26 cm long, placed on imbricated boulders (arrowed). Photograph taken August 20, 1996.

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locally scoured and the banks were breached at some locations. Numerous coarse boulders were deposited all along this channel. A fan-shaped bar of medium to coarse boulders with a steep avalanche lee side (1.5 m high) was deposited in the G~llego river at the mouth of the artificial channel. On the left margin of the artificial channel small and low-relief lobes of sandy cobble and pebble gravel were formed. In some cases these streamlined depositional bedforrns occur just downstream of elongated scours. In some areas the sharp and (in plan view) digitated boundary between the gravel and fine-grained facies was controlled by the scrub vegetation distribution. Garcla-Ruiz et al. (1996) in an early report indicate that the sediments released by the breakage of the check dams and the erosion of the till deposits formed a debris flow. However, none of the sedimentological and morphological features of the deposits found within the Arfis barranco and alluvial fan correspond to a debris flow but to a water flood process (Costa and Jarret, 1981; Wells and Harvey, 1987; Costa, 1988; Blair and McPherson, 1992, 1994a,b; Coussot and Meunier, 1996).

4. Previous flood deposits

The fan surface has been searched for large boulders deposited by previous floods in order to obtain palaeohydrological information for the Ar~s alluvial fan. The size of the largest clasts provides a conservative estimate of the maximum competence of the stream during previous flood events, since it can be assumed that clasts of all sizes provided by the till deposit have always been available in the Ar~s barranco bed (Costa, 1983). At points 1, 2 and 3 (Fig. 6) coarse boulders of grey calcareous sandstone with a reddish patina were found among the scrub vegetation. The boulders show clear imbrication with the long axis oriented perpendicular to the slope and with a - b planes clipping up-fan. These deposits show the distinctive sedimentological features of water flood deposits (Costa, 1988). The measured intermediate axial lengths of the largest clasts are 1.55, 1.1 and 0.7 m at [~ints 1, 2 and 3, respectively. These values may reflect a down-fan sorting of the clast sizes.

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At the distal end of the boulder lobe deposited during the event of 7 August 1996 the largest boulders reached 1.2 m in intermediate axial length. This size is very close to the value measured at point 2 at a similar radial distance from the fan apex. These values suggest that a previous flood with a maximum competence similar to that of the flood of 7 August 1996 occurred in relatively recent times. It has to be taken into account that these old boulders may have been deposited prior to the installation of the control structures in the Ar~s basin and on the alluvial fan. In that case the measured values would not be directly comparable, since they reflect the sedimentary response of two different morpho-hydrological systems. On the other hand, the breakage of the check dams and the release of the sediments that they stored confers to the flood of 7 August 1996 the status of an exceptional event in the evolution of the Ar~s alluvial fan.

5. Final considerations

- The distribution and magnitude of the geomorphic changes triggered by an intense storm of short duration may supply useful information on the spatial pattern of rainfall intensity in a small mountainous drainage basin. - The high-intensity storm over the Ar~s basin of 7 August 1996 produced a flash flood of short duration (around 10 min) on the alluvial fan as revealed by eye-witness accounts. - Vulnerable check dams full of sediments in mountain torrents may be a potential danger since their breakage by a flood can incorporate a large load in the water flood, amplifying the geomorphic work and effectiveness of the hydrological event. These highly sediment-laden floods when impacting on human settlements may have catastrophic consequences. - On alluvial fans fed by drainage basins capable of supplying large amounts of debris, floods may be dammed by massive aggradations at the fan head. Once the flood is obstructed, it can make its way through the fan-head accumulation taking an unpredictable path. This possibility should be taken into account when designing control structures and evaluating flood hazards on alluvial fans.

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As Costa (1978) states for the foothills of Colorado, the available precipitation and discharge records for small drainage basins do not allow accurate evaluation of rainfall intensity-frequency relationships or the return periods of extreme floods. The same considerations can be applied to the studied region. The traditional hydrological methods applied for flood-frequency analysis should be complemented by palaeohydrological studies based on geomorphological, chronostratigraphical and sedimentological techniques. These studies can provide quantitative and objective information about the chronology and magnitude of previous non-recorded extreme floods. The integration of both methods would improve the difficult task of understanding the temporal evolution of alluvial fans. - Detailed historical, geomorphological and sedimentological analysis of alluvial fans and their drainage basins may help to understand and infer the present behaviour and evolutionary trend of these geomorphic systems. These studies are essential for adequate hazard zoning and land-use planning of alluvial fans. -

Acknowledgements

The authors would like to thank the technical staff of the Instituto Nacional de Meteorolog~a in Zaragoza for providing unpublished data about the radar report of the storm event. We also express our gratitude to Santiago Rios and Antonio Bamolas of the Instituto Tecnol6gico y Geominero in Spain for supplying unpublished studies of the area. Finally, thanks to Ilmo. Sr. Pablo Munilla, Director General del Medio Natural, Diputaci6n General de Arag6n, for providing the report of M. Borderas.

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