Relationship between field observations of flood sand deposits and water depth applied to the identification of flood hazard areas (Sella River, NW Spain)

Erosion and deposition processes produce morphological forms over the channel and floodplains. Those forms depend on flow characteristics such as high velocity and deep water. By taking into account geomorphological criteria, the floodplain area presenting erosive and sedimentary morphologies is called Preferential Flow Area (ZFP). The ZFP was defined in the modification of the Hydraulic Public Domain Regulation (Royal Decree 9/2008, 11 January) and it is one of the units identified in the Spanish Flooding Zone Mapping System (SNCZI). The relationship between flood deposits and water flood deep is usually simulated by using physical models and hydraulic modelling. Nevertheless, there is little knowledge about the true relation by using field survey data. In order to improve the knowledge about the relations between geomorphological criteria based of flood forms and the ZFP, several field works were developed after floods in the lower part of the Sella River in Asturias (NW Spain). The present study considers the flood on 7 March 2009 with 5 year return period. 98 water flood data observed on field with 11 flood sand deposits thickness measured over floodplains was considered. The average value of sand deposits thickness was 0.14 m, with maximum data of 0.40 m, with an average value of water deep over these flood deposits of 1.54 m. An important correlation between these values was found and the linear and exponential model exhibit a good fitting explaining 81% and 86% of the deep-water variability respectively. By taking to account these results, flood sand deposits over 10 cm thickness could be used to identify representative areas of high flood hazard and the Preferential Flow Area.