Abstract
Abstract. Historical records are an important source of information on extreme and rare floods and fundamental to establish a reliable flood return frequency. The use of long historical records for flood frequency analysis brings in the question of flood stationarity, since climatic and land-use conditions can affect the relevance of past flooding as a predictor of future flooding. In this paper, a detailed 400 yr flood record from the Tagus River in Aranjuez (central Spain) was analysed under stationary and non-stationary flood frequency approaches, to assess their contribution within hazard studies. Historical flood records in Aranjuez were obtained from documents (Proceedings of the City Council, diaries, chronicles, memoirs, etc.), epigraphic marks, and indirect historical sources and reports. The water levels associated with different floods (derived from descriptions or epigraphic marks) were computed into discharge values using a one-dimensional hydraulic model. Secular variations in flood magnitude and frequency, found to respond to climate and environmental drivers, showed a good correlation between high values of historical flood discharges and a negative mode of the North Atlantic Oscillation (NAO) index. Over the systematic gauge record (1913–2008), an abrupt change on flood magnitude was produced in 1957 due to constructions of three major reservoirs in the Tagus headwaters (Bolarque, Entrepeñas and Buendia) controlling 80% of the watershed surface draining to Aranjuez. Two different models were used for the flood frequency analysis: (a) a stationary model estimating statistical distributions incorporating imprecise and categorical data based on maximum likelihood estimators, and (b) a time-varying model based on "generalized additive models for location, scale and shape" (GAMLSS) modelling, which incorporates external covariates related to climate variability (NAO index) and catchment hydrology factors (in this paper a reservoir index; RI). Flood frequency analysis using documentary data (plus gauged records) improved the estimates of the probabilities of rare floods (return intervals of 100 yr and higher). Under non-stationary modelling flood occurrence associated with an exceedance probability of 0.01 (i.e. return period of 100 yr) has changed over the last 500 yr due to decadal and multi-decadal variability of the NAO. Yet, frequency analysis under stationary models was successful in providing an average discharge around which value flood quantiles estimated by non-stationary models fluctuate through time.
Highlights
Throughout Europe, national legislations on flood hazard assessment are based on flood-frequency analyses which estimate discharges associated with different return periods
This paper aims at addressing stationary and nonstationary flood frequency analyses of historical floods, and to test their performance in flood hazard analyses
Water stage data related to documentary descriptions and epigraphic marks were interpreted in terms of stage indicators as follows: (1) sites or landmarks reached by the flood were assumed to provide an exact discharge level, (2) flooded areas provided a minimum flood stage, (3) non-flooded areas or landmarks (e.g. Royal Palace surrounded by water) were interpreted as a non-reached maximum flood stage, and (4) the relative importance of the event with respect to previous floods was quoted as a range of discharge in the case of two recorded levels
Summary
Throughout Europe, national legislations on flood hazard assessment are based on flood-frequency analyses which estimate discharges associated with different return periods (usually 10, 25, 50, 100 and 500 yr). The straightforward handling of these simple statistical methods that makes them so widely used hides important uncertainties as well as scientific and technical problems which have been extensively discussed in the literature (Merz and Blöschl, 2008) This conventional method of addressing flood hazard assessment can be improved by including information of past floods (historical floods and palaeofloods), which should be accomplished using rigorous procedures of data collection and statistical modelling. Long records of historical extreme floods have been applied successfully in hazard analysis together with the more traditional empirical, statistical and deterministic methods to estimate the largest floods (Stedinger and Cohn, 1986; Francés et al, 1994; England et al, 2003) Information on these extreme floods is highly demanded by planners and engineers and yet seldom registered in the observational record due to its short time length (Enzel et al, 1993; Benito et al, 2004)
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