ALLUVIAL DOLINES IN THE CENTRAL EBRO BASIN, SPAIN - A SPATIAL AND DEVELOPMENTAL HAZARD ANALYSIS

Alluvial dolines are abundant in Quaternary terraces and pediments overlying Neogene gypsum in the Zaragoza area (central Ebro basin). Spatial analysis and experimental simulation show that sulphate content in the groundwater, grain size of the detrital cover, topography of the Tertiary/Quaternary boundary, annual variation of the water table and thickness of the detrital cover are the main factors controlling their development. Taking into account these variables a theoretical spatial hazard model has been elaborated, expressed as a mathematical equation and a hazard map. Both experiments and field data show a high correspondance between two basic mechanisms of evacuation and subsidence (dragging slow subsidence and collapse of cavities) and two basic types of morphology and internal structure of dolines (basin doline-funnel structure and well doline-vault structures, respectively). A genetic classification of dolines and some evolutionary patterns based on these criteria are developed. Historical changes observed in doline distribution, as well as field surveys of urban damage and microtopographic profiles allow us to evaluate the present day activity of dolines. Local subsidence rates measured in urban areas range from 2.5 to 10 cm/year, although doline generation and reactivation in the whole area tend to be compensated by filling by human activity.