THE CATASTROPHIC FLOOD EVENT OF THE 18TH OF OCTOBER, 1973, IN THE RAMBLA OF ALBUÑOL ( LA RABITA, GRANADA, SPAIN).
The catastrophic flooding on the 18th of October, 1973, in the dry stream (rambla) ofAlbuñol caused the loss of 46 lives, the destruction of 91 houses and serious damages to
141. A layer of mud (2 to 3 metres thick) covered the streets of La Rabita ( the village most damaged). The roads and the bridges of comunication between the town and the rest of the province were destroyed along with 250 hectares of cultivated land. The damages were estimated at 300 millions of pesetas.
A heavy rainstorm (600 mm) produced the rapid development of a high density flow in the dry stream, with an elevated volume of sediments (1/3 of the discharge), that was the responsible factor of the high discharge reachedduring the flood (2580 m3/s).
The object of this study is not only the identification of the episode and the analysis of the disaster’s social impact, but also to analyse the different techniques to study the flooding in this type of fluvial system. The historic reconstruction of the event includes an analysis of the social impact and restoration works of the affected area. In addition the hydrologic analyses of the flooding that was based on the study of the greatest floods in small basins has been carried out.
Finally, a morphodynamic and sedimentologic reconstruction was
undertaken. The sedimentological study of deposits corresponding to the flash flood shows that the dynamic nature of these alluvial channels has important implications for floodplain management policy.
The potencial for dramatic changes in geomorphic and hydrologic
characteristics of alluvial channels must be considered in establishing procedures for flood hazard evaluation.
Spain is periodically devastated by catastrophes. The measures taken to solve this natural risk are territory management policy by risk maps and the solutions of the emergencies once the catastrophe has happened. The flood of the 18th of October of 1973,in the dry stream (rambla) of Albuñol, constitutes a basic example of catastrophic flooding
– a catastrophe of a low frequency and high magnitude event exceeding the balance of the
fluvial system (Schumm, 1973) in a Mediterranean dry streams.
V.R.Thorndycraft, G. Benito, M. Barriendos and M.C. Llasat (2003). Palaeofloods, Historical Floods
and Climatic Variability: Applications in Flood Risk Assessment (Proceedings of the PHEFRA
Workshop, Barcelona, 16-19th October, 2002).
E. Romero Cordón
2 THE HISTORIC RECONSTRUCTION OF THE EVENT
The rain started in the morning of the 18th October of 1973 after six months of drought.
In total 600 mm fell, according to the Comisaria de Aguas del Sur de España records. At about nine o’clock in the evening ( the river had half a metre of water) the rate of rain increased suddenly and the lights of the village went off. At midnight the river, about fifty
metres wide and seven metres deep, overflowed. The majority of the habitants took shelter in the castle tower, the graveyard and nearby hills to the village, but the villagers that lived in two blocks of flats near the river and those living in the fishing district (Rambla de la
Mochila) were left isolated. The rainfall became heavily torrential causing the formation of two massive waves over the river. The first wave went through the village at about half past three in the morning, razing crops, cars, houses, etc. The second wave pulled out a building,
near the river, from its foundations and swept it to the sea with twenty seven people inside.
At dawn the water had disappeared. The flood caused the following damages:
• The loss of 46 lives in the villages of Albuñol, La Rabita, El Pozuelo and in some
• Destruction of 91 houses and serious damages to 141.
• The streets of the villages La Rabita, Albuñol and El Pozuelo were left covered with a
layer of mud (2 to 4 metres thick). Several cars were left buried.
• The water supply and drainage network were seriously damaged. Roads were
destroyed (the C-333 and N-340), 30 km of country lanes ruined, 250 ha of cultivated
land were devastated and covered by a layer of debris 2 to 3 m thick.
• Several industries were completely destroyed (Distillery, Gravel pit, etc).
• Creation of a delta over the sea about 200 metres long.
ADMINISTRATION TYPE OF DAMAGES EVALUATION
Devastated land recovery (3.265 has.) 173.638.000
Recovery of irrigation systems 58.000.000
National Institute of
Land Reform and
Development 30 Km of paths destroyed 12.000.000
Conditioning of the road Granada – La Rabita 7.083.000
New road Public Works between La Rabita and Albuñol 100.000.000
Bridge construction 35.000.000
Confederacion Channelling of the Albuñol rambla 450.000.000
Hydrografica del Sur Recovery of irrigation systems 16.000.000
House construction and access conditioning 34.400.000
Building repairs 3.177.000
Repairs in the La Rabita Church 3.165.000
Delegacion de Vivienda
Restitution of the electrical layout 4.536.000
Delegacion de Industria
Delegacion de Education
and Science Child maintenance until school conditioning 14.180.000
Council Structures 1.600.000
TOTAL COST 958.649.000
Table 1. Evaluated damages (in pesetas) caused by the flood event
The catastrophic flood event of 1973 in the Albuñol rambla
3 HISTORICAL RECONSTRUCTION
3.1 Damage evaluation
The evaluation of the catastrophic damages elaborated by the Gobierno Civil of Granada (1974) shows the resultant social impact. Table 1 shows the evaluation of the damages made by each participant public organisations.
3.2 Aid organisation during the first days after the catastrophe
Work carried out during the first days of the emergency (Table 2) show the actions needed to be taken in case of similar emergencies and the organisations who are responsible for implementing it.
Search and rescue of people Guardia Civil, Army Wounded evacuation to Motril Hospital (10 of a total of 25 seriously ill) Helicópteros ejército S.A.R.
Accommodation and social assistance Delegación Provincial de Acción Social y
Victims evacuation to Motril and Adra Cofradía de pescadores (por barco)
Medical assistance Delegación de Salud,
Wreckage clearance and cleaning of village (94
Guardia Civil, Bomberos, volunteers OJE,
Search and identification of mortal victims (46) Guardia Civil, Grupo de submarinistas del Ejercito
Distribution of clothes and first aid equipment Sección Femenina
Food distribution and organisation of a sufferers
dining room Guardia Civil, Sección Femenina
Incineration of dead animals Guardia Civil, volunteers
Provisional electrical installation Army
Repairs of the water supply network Motril City Council
Table 2. Work carried out in the first days after the catastrophe
4 CONDITIONING FACTORS: INTRINSIC BASIN CHARACTERISTICS, HYDROLOGICAL AND GEOMORPHOLOGICAL ANALYSES
The importance of the great volume of material in suspension (see Table 3) as the leading factor of the high river flow reached is conditioned by:
• Basin topography with dominant slopes between 20 and 40 %.
• Poor development of soils with a tendency to slide off a rocky subsoil.
• The use of crops incompatible with the retention of the ground.
• The Flow Basin of the Albuñol rambla lies over metapelitic rocks overlaying
carbonated materials, which outcrop at the base of the Aldayar and Agijón ramblas.
The intense crushing suffered by the materials increase their facility to be altered, therefore they are an important source of solid material for the floods.
Basin area 116 km2 Maximum flow 2580 m3/s
Relief rate (Rr) 0.088 Liquid flow 1518 m3/s
Weigh slope 423 m/km Solid flow 1062 m3/s
Average slope 30-40% Drained total volume 17 Hm3
Elongation relation (Re) 0.51 Specific flow 13.4 m3/s/km2
Index of circularity (Cc) 0.45 Escorrentia coefficient 0.62
Index of compaction (Ic) 1.39 Stream flow velocity 3.44 m/s
Solids concentration 41% Period of return > 500 years
Index of torrentially 50 Calculated flow for 600
mm precipitation 2831 m3/s
Drainage density 10 km/km2<
Concentration time (Tc) 4
Table 3. Morphometric characteristcs of the basin and hydraulic parameters of the flood
5 MORPHODYNAMIC AND SEDIMENTOLOGICAL RECONSTRUCTION
5.1 Production zone
The production zone is identified by a predominance of erosive landforms (scars of
sliding, landslides and important development of grooves) that occur in the basin. As depositional landforms the solifluction mudflows and the accumulation of fell boulders are frequent.
During the flood in the gauges excavated in limestone (2 to 3 m wide) the water raised tens of metres favouring rock falls that once combined with water multiplied the erosive power.
5.2 Transference zone
The BRAIDED PLAIN covered most of the river valley. The morphological
adjustment of the braided channel to the flood, is changed into a channel surface increment that constitutes the braided plain. This erosive adjustment is produed as a consequence of the great magnitude of river flow and the high energy that had to be dissipated throughout the base of a relatively narrow valley.
The FLOODPLAIN takes up sheltered areas from the principal river flow, producing small terraces in the braided plain margins. The overflow deposit originated by the flood over those terraces was a layer of well graded sand and gravel with silt (10 to 30 cm thick).
On the surface boulders, of 2-4 m diameter, can be seen that have been pulled up from the walls that protect the crops, and transported tens of metres. Linked to the boulders crescent mark structures can be seen.
ALLUVIAL FANS form where streams confined by narrow valleys emerge onto a plain or mayor trunk valley. The development of a small fan in the Barranco de la Mochila opening should be noted, located to the northwest of the La Rabita village, which buried a great amount of houses before going into the sea were it originated a fan delta.
The catastrophic flood event of 1973 in the Albuñol rambla
The ACCUMULATION ZONE is formed by a delta which is the result of the
amalgamation of a series of lobes whose growth, during the flood, advanced 200 m into the sea (Figure 1) with a thickness of 4 m in the apical zone.
Two types of deposits can be distinguished:
1) Sheet flow deposits: produced as a consequence of the flow expansion and a fast diminution of the water sheet. They constitute the principal deposit type in the construction of the different deltaic lobes. The development of those is, at the same time, in relation with
the pass of the flow through La Rabita bridge and with the various destruction phases.
2) Overflow deposits: formed when the wall that was protecting La Rabita failed and the fine materials that were in suspension were deposited.
Figure 1. Distribution of erosive and depositional landforms caused by the 1973 flood at the Albuñol delta.
From the study of the 1973 catastrophic flood in La Rabita it can be deduced that the ramblas working system takes a drastic occupation of the valley bed, therefore it is important to take it into account for the territorial ordination of thoseareas looking for the
maintenance of natural conditions.
The areas with high rates of erosion as well as those with a high sedimentation rate (alluvial fans), should be considered as areas with a significant risk, not only for low magnitude floods but also for greater floods.
The risk mapping should be carried out from an analysis estimation of the probable discharge, and bearing in mind the hydraulics and hydrologic characteristics together with the record of deposits.