Extraordinary floods related to warm periods: geoarchaeological record of the Roman theatre of Guadix (SE Spain)

The old Roman town of Guadix is placed in the SE part of the Iberian Peninsula (Granada, Spain), being crossed by the Guadix River and some wadis locally called ramblas. There are numerous documentary records reporting multiple flash floods that have periodically partially inundated Guadix. The last extraordinary flood, which is still remembered by the population, took place in October 1973, causing severe damage in the historical city. More recently, there have been other floods in Guadix (e.g. September 2007, August 2009 and 2018), although with much less damage than the 1973 flood.

Going further back in time, geomorphological, geoarchaeological, sedimentological and palaeohydrological studies of the detrital deposits covering the ruins of the Roman theatre of the ancient city of Colonia Iulia Gemella Acci have succeeded in reconstructing the periodic recurrence of catastrophic floods in this city over the last two millennia. More specifically, by means of archaeological remains, radiocarbon dating and laser stimulation optical luminescence, at least three major flood events have been identified: two dating back to the beginning of the 1st century A.C. (period of the Pax Romana Empire); and the other one between the end of the 11th century (Islamic Zirid dynasty) and the beginning of the 12th century. The moments in which such extreme hydrological events occurred coincided with the end of two warm periods: the Roman Warm Age and the Medieval Warm Period. The meteorological fluctuations associated to climate changes during warming periods, such as the current one associated with climate change, produce this kind of extraordinary events, which were able to mobilise large amounts of solid load (sands, silts and clays), and produced changes in the planform configuration of the river system (channel avulsions, meander cut-offs and captures among tributaries) and alluvial fans in the tributaries mouths.

Therefore, flood risk management in the current town of Guadix, whose hazard and risk maps have been drawn up according to the criteria of the European Flood Directive, should take into account: (i) the millennial recurrent character of catastrophic floods in Guadix, well above the 500-year return period officially considered; (ii) the importance of the mobilisation of solid loads in flood hazard and risk, which can obstruct drains and reduce the capacity of channels and bridges; (iii) the geomorphological dynamics, capable of modifying the position of the channels and thus the hazard and risk zones; (iv) the role of non-permanent tributaries of the Guadix River during storms and heavy rain related to cuasi-stationary convective cells. Only in this way flood hazard and risk maps will be useful in territorial and urban planning, civil protection during emergencies and the reduction of damage from future catastrophic floods. In this regard, the present study highlights how geoarchaeological and paleoenvironmental records can be a highly valuable source of data for the improvement of flood hazard assessments.

This work has been developed in the framework of the RISKCOAST project: “Development of tools to prevent and manage geological risks on the coast linked to climate change” (Ref: SOE3/P4/E0868) funded by the Interreg SUDOE programme.