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In high elevation alpine catchments, first-order streams are often constituted by steep and narrow channels bound by cliffs. These channels frequently have a structural control imposed by fractures and faults in bedrock and typically constitute temporary streams where snowmelt processes strongly influence runoff. Rocky headwater basins show a different hydrological response with respect to soil-mantled basins and their hydrology is poorly known due to the lack of widespread monitoring sites. Herein we present the preliminary results achieved through an experimental hydro-meteorological monitoring network setup in a 0.1 km2 rocky headwater basin located on the southern flank of the Sella Group in the Dolomites (North-Eastern Italy). Elevation ranges between 2700 m, at the outlet, and 3174 m, with an average value of 2950 m. Geology is constituted mainly by the Norian "Dolomia Principale" (Dolomite) featuring a complex structural setting. The monitoring network, active since 2009, is designed with three rain gauges with a time rate of 5 minutes. Two are located at 2609 and 2597 m (close to the outlet), and the third is located on the divide in the central part of the basin (2911 m). Runoff at the outlet is monitored by a pressure transducer. The time interval was set at 5 minutes in the summer months in order to capture the stream response due to intense and spatially-concentrated rainfall events. In winter the pressure transducer is maintained active with a time rate of 30 minutes so as to be ready to register in spring the stream response due to snowmelt. In the summer 2011, some precipitation, runoff, snow and spring water samples for isotopic analysis ( 18 O and 2 H) were collected aiming to better characterize the origin of subsurface water and the main sources to runoff. Preliminary results show ephemeral presence of runoff, mostly occurring during snowmelt (from May to early July) and after intense summer rainstorms. The lag-time from precipitation centroid to runoff peak (on the order of about 1 hour) is rather long for a very small and steep basin. This is likely due to the presence of fractured bedrock that creates a shallow, subsurface reservoir and increases the catchment response time. Isotopic data of water samples well reflect the precipitation signal indicating reduced evaporation processes during the transfer of the water input to the outlet. Moreover, the isotopic composition of streamflow is close to that of the sampled springs but less depleted than snowmelt samples, revealing a possible contribution to runoff of snowmelt water mixed with subsurface water previously stored in the system.

Flash floods are a major natural hazard under different climatic conditions. In the European Alps, flash flood hazard often results in major risk due to the increasing presence of elements at risk (urban areas and transportation routes) in valley floors and on alluvial fans endangered by flash floods. The assessment of the hazards related to flash floods requires collection of reliable quantitative data on these phenomena. Due to the space scale of flash flood occurrence, which mostly occur in ungauged catchments, hydrometeorological data from monitoring network need to be integrated by data collected by means of post-flood surveys. This paper presents methods for flash flood data collection and discusses the basic characteristics of flash floods in alpine catchments based on the analysis of a sample of 6 flash floods (20 catchments), which occurred from 1999 to 2006 in Italy, Austria and Slovenia.

The 2 June 2008 flood-producing storm on the Starzel river basin in South-West Germany is examined as a prototype for organized convective systems that dominate the upper tail of the precipitation frequency distribution and are likely responsible for the flash flood peaks in this region. The availability of high-resolution rainfall estimates from radar observations and a rain gauge network, together with indirect peak discharge estimates from a detailed post-event survey, provides the opportunity to study the hydrometeorological and hydrological mechanisms associated with this extreme storm and the ensuing flood. Radar-derived rainfall, streamgauge data and indirect estimates of peak discharges are used along with a distributed hydrologic model to reconstruct hydrographs at multiple locations. The influence of storm structure, evolution and motion on the modeled flood hydrograph is examined by using the "spatial moments of catchment rainfall" (Zoccatelli et al., 2011). It is shown that downbasin storm motion had a noticeable impact on flood peak magnitude. Small runoff ratios (less than 20%) characterized the runoff response. The flood response can be reasonably well reproduced with the distributed hydrological model, using high resolution rainfall observations and model parameters calibrated at a river section which includes most of the area impacted by the storm.

International audience

Flash floods are one of the most significant natural hazards in Europe, and can cause damage to buildings and infrastructure and pose a high risk to life. Existing inventories of past floods give information for other geographical regions or lack quantitative information on the meteo-hydrological and hydraulic characteristics of the event. The lack of centralised national and international databases for storing meteo-hydrological, hydraulic and socio-economic data relating to past flood (and particularly flash flood) events within Europe means that it is difficult to calibrate and validate research results on a range of previous floods from different climatic regions. To address this apparent gap in availability of centrally stored, high quality data and with the aim of supporting research on risk assessment and ?ash flood forecasting, a data base has been developed that collates flash flood data from seven European hydrometeorological regions and that could be extended in the future to other areas. Compilation of the database required the development of a data template and flood event selection rules. Initial analyses of the data highlight spatial variations of flood hazard magnitude, with highest speci?c peak discharges in the Mediterranean region. There is a strong seasonality to the occurrence of events, with events occurring mostly in the autumn in the Mediterranean region and in the summer in middle and eastern European regions. There have been flash floods in France and Spain on a basin scale of greater than 500 km2 . This work has been carried out for HYDRATE, which is a currently ongoing EC funded project that is aiming to improve techniques for flash flood forecasting. The work presented here represents some of the first outputs from this project.

This work introduces a fast algorithm based on Singular Value Decomposition to compute the Nonuniform Fourier Transform. This approach is compared to proven techniques like the ones based on interpolation and least square approximation. Nonuniform Fourier exponentials are approximated through a set of optimum spaces obtained by modulating a single space. For a fixed precision, the space dimension is smaller with respect to the previous approaches, resulting in a computational cost reduction. Furthermore, the proposed formulation involves only real-complex multiplications rather than complex-complex ones. As a counterpart, the amount of projections to be computed is higher with respect to proven approaches. So, the proposed algorithm results to be optimum for dense nonuniformly sampled frequencies.

Hydrate project Work package 4 report, December 2007