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

Alluvial dolines are abundant in Quaternary terraces and pediments ove rlying Neogene gypsum in the Zaragoza area (central Ebro basin). Spati al analysis and experimental simulation show that sulphate content in the groundwater, grain size of the detrital cover, topography of the T ertiary/Quaternary boundary, annual variation of the water table and t hickness 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 equatio n and a hazard map. Both experiments and field data show a high corres pondance between two basic mechanisms of evacuation and subsidence (dr agging slow subsidence and collapse of cavities) and two basic types o f morphology and internal structure of dolines (basin doline-funnel st ructure and well doline-vault structures, respectively). A genetic cla ssification of dolines and some evolutionary patterns based on these c riteria are developed. Historical changes observed in doline distribut ion, as well as field surveys of urban damage and microtopographic pro files 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.