Fluvial Dynamics and Risk Conditions in River Chiani Valley (Central Italy).

The paper presentes the results of a survey of morphological, sedimentary and human characteristics of the Chiani river bed and alluvial plain (Central Italy), in order to define the evolutionary trend of the river bed - alluvial plain system, the risk conditions and to supply criteria about a correct "management of the watercourse. The River Chiani can be subdivided in two reaches characterized by different features. The first one in the Upper Valley ("upstream reach), presents a wide alluvial plain and the whole reach was conditioned by channelizations and embankments came out in the past when the whole "Valdichiana" area was reclaimed. The second one in the Middle and Lower Valley (downstream reach").shows opposite characteristics: bead cutting, vertical and lateral erosional processes are in progress the slopes are steep: there is an alterne or straight channel - beadcutted in bedrock-and mobile reaches. moderately braided, where a narrow alluvial plain was developed. Risk conditions affect the end of this reach at the confluence with the River Paglia, where floodings frequently occurred. These threaten the outskirts of Orvieto and need urgent restoration and maintenance works. The headcutting trend of the downstream reach" is especially due to human activities: a check dam, built at the beginning of this reach, worked as a sedimentary trap for the bedload of the whole upstream reach the slope stabilization in the last decades (reforestation and building of check dams through the tributaries of the River China), lowered the sediment supply to the main stream, quoting activity in the river bed was came out from 60 until few years ago. a reduction of the width of cross section was carried out, as it is proved by historical cartographic documents (Gregorian Land Register dated 1826). The consequences were: an intense sedimentary deficit and a surplus of velocity and stream power, due to the reduction in width of the cross sections. The stream reacted headcutting the river bed and causing vertical and lateral erosional processes. Moreover, the peak discharges were in the past dissipated by inundation of the small alluvial plains along the whole downstream reach they worked as natural flooded areas. At present this phenomenon doesn't occur: the peak discharges can pass through the headcurted cross sections, the alluvial plain is at a higher level and it is not floodable. The peak discharge dissipate their power along the final reach near the confluence with the River Paglia, where the river bed of Chiani cannot headcut (this is its base level) and it is free to flood an urbanized area, where there is the maximum of risk conditions. The works interesting this final reach cannot leave out of consideration a restoration of a state in equilibrium. The best solution seems to restore to the alluvial plains, located along the downstream reach their function of natural flooded areas, by river training works which will rise the thalweg and will let the controlled) floods of areas characterized by law on no urbanization this will have, as a useful consequence, the cutting of the peak discharge in the area in risk conditions at present.