Extreme Floods in Small Mediterranean Catchments: Long-Term Response to Climate Variability and Change

Abstract

Climate change implies changes in the frequency and magnitude of flood events. The influence of climate variability on flooding was evaluated by an analysis of sedimentary (palaeofloods) and documentary archives. A 500-year palaeoflood record at Montlleó River (657 km2 in catchment area), eastern Spain, revealed up to 31 palaeofloods with a range of discharges of 20–950 m3 s−1, and with at least five floods exceeding 740–950 m3 s−1. This information contrasts with the available gauged flood registers (since year 1971) with an annual maximum daily discharge of 129 m3 s−1. Our palaeoflood dataset indicates flood cluster episodes at (1) 1570–1620, (2) 1775–1795, (3) 1850–1890, and (4) 1920–1969. Flood rich periods 1 and 3 corresponded to cooler than usual (about 0.3 °C and 0.2 °C) climate oscillations, whereas 2 and 4 were characterised by higher inter-annual climatic variability (floods and droughts). This high inter-annual rainfall variability increased over the last 150 years, leading to a reduction of annual maximum flow. Flood quantiles (>50 years) calculated from palaeoflood+gauged data showed 30%–40% higher peak discharges than those using only instrumental records, whereas when increasing the catchment area (1500 km2) the discharge estimation variance decreased to ~15%. The results reflect the higher sensitivity of small catchments to changes on flood magnitude and frequency due to climate variability whereas a larger catchment buffers the response due to the limited extent of convective storms. Our findings show that extended flood records provide robust knowledge about hazardous flooding that can assist in the prioritization of low-regret actions for flood-risk adaptation to climate change.

New Approach to Estimate Extreme Flooding Using Continuous Synthetic Simulation Supported by Regional Precipitation and Non-Systematic Flood Data

“…As previously mentioned, non-systematic data were collected from previous studies reporting documentary floods and palaeoflood analysis [27,40]. Historical floods were collected from scientific and technical reports; local history books and non-systematic compilations by historians, such as Balbás [43], Fogués [44], and Fontana Tarrats [45]; and from recent compilations by Camarasa Belmonte and Segura Beltrán [34], Sánchez Adell et al [46], and Beltrán Manrique [47].…”

“…The Rambla de la Viuda documentary flood evidence reports a total 19 large floods since CE1378, although continuous and homogeneous data started in 1959. Palaeoflood records were reconstructed on the basis of sedimentary evidence at two sites (Figure 2), namely in Rambla de la Viuda upstream of the Maria Cristina Reservoir [40] and in the Monlleó River [27]. Palaeoflood data result from the analysis of palaeostage indicators consisting of stratigraphic sequences of fine-flood sediments (mainly sand and silt), each representing an individual flood event [40].…”

“…Further information on extremes was obtained from a regional study of maximum daily precipitation [39] that was used for the additional calibration of the weather generator. In addition, non-systematic flood data were collected from documentary datasets and palaeoflood studies [27,40] in two sites, which were integrated for the validation of the GW-hydrological model methodology.…”

“…The fitted potential equations have a coefficient R of Sperman higher than 0.8 at both locations. Finally, palaeoflood information from [27,40] was converted into daily values using the fitted potential relationship.…”

“…The fact that longer records provide less biased estimates, as indicated by Metzger et al [23], calls for the inclusion of historical and other non-standard flood data [25,26]. These include historical observations, interval data (estimates of flood peaks within specified ranges of magnitude and time), and palaeoflood records [27]. Palaeoflood data provide physical evidence of flooding over secular time scales and typically consist of fine-texture sediment layers accumulated on valley sides [28] and durable botanical evidence, such as scars produced by impacting floating debris and stones on riparian trees [29].…”