Floods on alluvial fans: implications for reworking rates, morphology and fan hazards

Abstract

Flood events are the agents of change on alluvial fans. However, most alluvial fan experiments have used constant flows to model fans and the channels upon them. Here, we present results from a series of alluvial fan experiments with different patterns of flow variation (i.e. different hydrograph shapes). We conducted experiments with 1) constant flow, 2) alternating high and low flows, 3) a moderate flood peak that decayed slowly, alternating with a constant low flow, and 4) a high flood peak that decayed rapidly, alternating with a constant low flow. Importantly, all experiments had the same mean flow and sediment supply, but the different hydrographs generated fans with different slopes. In addition, higher peak flows led to increased lateral migration rates and increased erosion and deposition. These results challenge the notion that a single representative flow can be used to approximate the geomorphic effects of a range of flows in a natural stream. Moreover, the data suggest that hydrograph shape can govern the geomorphic impact of a flood event. Our findings indicate how altered basin hydrology (for instance, through changes to land cover) could influence geomorphic change and natural hazards on alluvial fans.

Análise De Inundações Em Bacias Montanhosas No Sul Do Brasil Por Meio De Monitoramento E Modelagem

“…Através da mediana é possível observar que em 50% do período as vazões eram muito baixas. Observa-se também que, os picos de vazões são mais acentuados em relação às vazões de base que, logo após a recessão, toda a água é escoada rapidamente, isso é um regime fluviométrico bem típico da região de leques aluviais (Bowman, 2019;Vasconcellos et al, 2021;Blackburn et al 2021;Leeman et al 2022). Com base nos dados de EMBRAPA ( 2004) e USDA-SCS (1985), a Figura 6 apresenta os mapas dos grupos hidrológicos do solo, do uso e ocupação do solo e de escoamento superficial (CN) da bacia do rio Malacara, utilizados para o cálculo da precipitação de projeto (vazão máxima).…”

Secondary Geomorphic Processes and Their Influence on Alluvial Fan Morphology, Channel Behavior and Flood Hazards

“…Our experiments attempt to replicate the effect of secondary events by imposing periods of moderate flows and no sediment supply between primary fan‐building events, and they look specifically at changes in the duration of those secondary events. Because our secondary events had no sediment feed they were strictly erosional; other experiments using variable flows maintained upstream sediment supply, thus blurring the distinction between fan‐building events and fan‐modifying events (e.g., Leenman et al., 2022). We add to the work of Miller et al.…”

“…Physical models have long been used in alluvial fan research, with experimental studies taking place as early as the 1960s (Hooke, 1967, 1968). Recently there have been a number of experimental studies of alluvial fan processes and forms (Clarke et al., 2010; Eaton et al., 2017; Hamilton et al., 2013; Leenman & Eaton, 2021; Leenman et al., 2022). While many of these studies focus on fluvial processes, recent work by de Haas et al.…”

“…Leenman et al. (2022) found that for variable discharge experiments, the shape of the flood hydrograph has important impacts on both lateral channel migration and overall morphologic change.…”