Hydraulic modelling of the flood prone area in a basin with a historical report of urban inundation: The Arunca River case (Central Portugal)

Floods, considered major hazardous processes due to the rising number of events with high socio-economic impacts causing widespread disturbance, are frequent in the Arunca River drainage basin (Central Portugal) as a result of climatic, morphological, geological, hydrological and anthropogenic factors. The vulnerability of the area has increased in recent decades, mainly due to the disturbances introduced by man (e.g., channel artificialisation and a reduction in the infiltration capacity of the floodplain and cover in urban areas). This paper describes how the 1-D hydraulic model HEC-RAS, using a higher resolution topographic surface including hydrogeomorphological details and other features influencing hydraulics, was applied to four reaches/sections spanning the upper, middle and lower basin of the Arunca River as well as urban and rural areas, in order to determine the flood prone areas for a return period of 100 years. The HEC-RAS results were then compared with the existing flood prone areas. The analysis made possible a new cartographic representation of the flood prone areas in four sections, which represent the most hazardous areas of the basin due to urban occupation (the large concentration of residential, industrial and commercial areas) and communications infrastructures (national and regional roads and national railway). The comparison of the previous flood prone areas represented in the River Mondego Hydrological Basin Plan maps with the new cartographic representation stresses the great variations in the upstream sections (over 20%), due to more significant anthropogenic changes, in contrast with the downstream sections (under 4%). The results of the water height maps emphasise that in the downstream sections measurements of over 2 meters are dominant; 93 ha of Section D, the section furthest downstream, has a water column height of over 4 meters. An analysis of the elements exposed to flooding reveals a total of 391 residential buildings, essentially located in the two major towns (Pombal and Soure). In the downstream sections, the flooded area affects mainly farmland and its corresponding infrastructures. It is also significant to note that in all the modelled sections it is not possible to cross the floodplain area in the event of flooding. These disruptions would have a serious effect on regional and municipal socio-economic flows and connections. The hydrologic-hydraulic modeling, with new relevant data and a detailed DTM, in association with the incorporation of hydraulic and block structures, and anthropogenic morphological and land use changes, has enabled new flood prone areas to be defined and the water height to be mapped for a 100-year return period. This study can serve as a support element in plan-ning updates, including the Master Plans for the Soure and Pombal municipalities and the Mondego Hydrographical Basin Plan.