Flood patterns in a catchment with mixed bedrock geology and a hilly landscape: identification of flashy runoff contributions during storm events

[摘要] With flash flood events having been repeatedly observed in Central andWestern Europe in recent years, there is a growing interest in how catchmentphysiographic properties and hydrological conditions are eventuallycontrolling rapid and concentrated hydrological responses. Here we focus ona set of two nested catchments in Luxembourg (Europe) that have been exposedin 2016 and 2018 to flash flood events and study their seasonal runoff timetransfer distributions. Both catchments are of similar size ( ∼  30 km 2 ) and have analogous hydrological distance distributions, buttheir geological bedrock and landscape features are notably different. Theupper catchment (KOE) is dominated by a low land area (38 % of thecatchment is located less than 30 m above the river network) consisting ofvariegated marly bedrock (middle Keuper Km3) and moderately steep Luxembourgsandstone outcrops (lower Liassic Li2). The lower catchment (HM) has itsdrainage network deeply cut into the Luxembourg sandstone, with half of itbeing covered by marly plateaus (Lower Liassic Li3, located between 80 and100 m above the river network) featuring heavy clay soil. Based on datagenerated from a dedicated hydro-meteorological monitoring network, wecalculated for 40 rainfall–runoff events observed between August 2019 andJuly 2021 the corresponding net rainfall transfer time distributions (TTDs)from the hillslopes to the catchment outlet. We then compared the TTDproperties and related them to the catchment's hydrological state andrainfall properties. We observed a marked seasonality in TTDs for both catchments. The KOEcatchment reacts fastest during the winter period (December–February),while its response time is most delayed and spread out during periods ofcatchment recharging (October–November) and drying (March–May). The HMcatchment exhibits similar TTDs during the mid-October to mid-April period,but they diverge markedly during the remaining part of the year, withopposite variations. During the mid-April to mid-October period, the averageresponse time increases progressively in the KOE catchment. This behavioris in stark contrast to the HM catchment, where response times aresignificantly shorter (peak discharge delay time decreases by − 70 %  ±  28 %) and more concentrated (runoff volume occurring in 1 hincreases by + 48 %  ±  87 %) during the mid-April to mid-October,in comparison to the extended winter period. This opposite seasonality leadsus to consider different control factors of the runoff transfer processes inrelation with the topographic and geological layout of the catchment areas.In the KOE catchment, we found the TTD to be essentially driven by onset andcessation of hydrological connectivity on the flat marly terrain – thelatter operating like a variable contributing area in terms of deep soilstorage dynamics (except for one summer event). The HM section exhibitscontrasted TTDs throughout the year, suggesting threshold-dependenthydrological processes. More specifically, particularly quick runofftransfers seem to dominate under dry conditions (mid-April to mid-October).Correlation analyses compared to the literature on runoff generation on theone hand and our descriptive knowledge of the catchments on the other handsuggest multiple causes for the triggering of these rapid flows. Thefractured marly plateaus, but also the hydrophobic forest litter formingduring dry conditions on steep slopes, stand as our main hypotheses in thisrespect. Moreover, the absence of a riparian zone, preventing any dampeningof (observed) abrupt and massive flows during extreme precipitation events,also seems to be a key feature of the rapid runoff transfer. For improving our understanding and forecasting capabilities in Luxembourg(and more broadly in the nearby regions of Germany, Belgium, and France withsimilar physiographic and climate conditions), we recommend further studiesfocusing on catchments with fractured bedrock and limited riparian zones.Special attention may equally be given to the hypothesized responses ofhydrophobic soil surfaces on steep hillslopes and marly soils to heavyprecipitation events occurring after extended dry spells.

[学科分类] 妇产科学