Hydromorphological modelling of the September 9th 2018 flash flood event at Medieval Sta. María de Huerta Monastery (Soria, Spain). A detailed hazard analysis for flood risk mitigation proposal.

The occurrence of flash-floods affecting the Sta. María de Huerta Medieval Monastery is a recurrent process over time. The most recent examples date from 2015 and 2018, but evidences of previous historical flood episodes are available in documentary record (three flood events between 1761 and 1772). Although the Sta. María de Huerta Monastery is closed to the Jalón River, a tributary of Ebro River, flash-flood occurrence is mostly linked to a small tributary (Torrehermosa ravine) with a drainage basin smaller than 5 km². During the afternoon of September 9^th (2018), high intensity rainfall related to a summer convective cell caused a cumulative rainfall volume of roughly 100 l m^-2. Rainfall duration was less than two hours and caused a flash-flood breaking the brick-wall in the south boundary of the monastery, so the flood finally affected the main monastery building. A wave of water and suspended sediment load (mainly silt and clay in size) penetrated through the entrance doors into the building, reaching a height above one meter in some of the rooms (church, cloister, refectory). The flash-flooding event finally only caused tangible damage that required cleaning of walls and furniture, as well as the replacement of some glass panes. However, if the occurrence of the flash-flood had coincided with the celebration of liturgical events, the damage to the attendees could have been significant.

For the detailed hydromorphological analysis of the September 9^th 2018 flash-flood event, the first step was the incorporation of the monastery building structure (outdoor and indoor building walls, rooms doors…) with their related flat grounds, and surroundings (perimeter wall, secondary buildings…) into a detailed LiDAR derived DEM. Then, the hydrological module of Iber hydrodynamic software was used for hydrograph and sedimentograph simulation. The outcomes of this hydrological simulation were subsequently used in hydraulic modeling, where suspended sediment transport was into consideration. For the hydraulic analysis, two independent scenarios were considered: the first linked to the initial situation (the one existing on September 9^th, 2018); and the second considering a proposal of measures for flood hazard mitigation including the installation of flood gates in the monastery doors.

Results for the scenario representing conditions existing on September 9^th 2018 show a flooding extension covering the full monastery building with flow depths ranging from 0.25 meters until more than 1.0 meters, depending on the considered return period. Flow velocity values into the monastery do not increase flood hazard. These results, and specifically those located at the position of the entrance doors, were used to design the minimum dimensions of the flood doors. The results of the second scenario show only residual flows into the monastery building that are in the same range as model uncertainties. These results show how detailed hydromorphological analysis approaches can be applied to flood risk reduction in cultural heritage buildings, improving the design of mitigation measures.

Funded by spanish National Research Agency, research project PID2020-116896RB-C22 “Innovación en la respuesta preventiva a eventos direccionales hidrometeorológicos extremos en el patrimonio cultural”