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Recent human impacts and change in dynamics and morphology of ephemeral rivers

机译：近期人类的影响以及短暂河流的动力学和形态变化

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Ephemeral streams induce flash-flood events, which cause dramatic morphological changes and impacts on population, mainly because they are intermittent and less predictable. Human pressures on the basin modify load and discharge relationships, inducing dormant instability on the fluvial system that will manifest abruptly during flood events. The flash-flood response of two ephemeral streams affected by load supply modification due to land use changes is discussed in a combination of geomorphic and hydraulic approaches. During the Rivillas flash flood, intensive clearing on the basin led to high rates of sediment flowing into an artificially straightened and inefficient channel. The stream evolved from a sinuous single channel into a shallow braiding occupying the entire width of the valley floor. Misfits and unsteady channel conditions increased velocity, stream power and sediment entrainment capacity and considerably magnified flood damage. Resulting morphosedimentary features revealed a close relationship with the valley floor postflood hydraulic model, and pre-event awareness would have made it possible to predict risk-sensitive areas. In the second case, the Azohía stream, modelling of current pre-flood channel conditions make it possible to determine channel narrowing and entrenchment in the lower alluvial fan stretch. Abandonment of intensive agriculture, basin reforestation and urbanization diminish load contribution and trigger channel incision. This induces an increase in slope and velocity in the bankfull channel, producing renewed erosive energy and thus activating upstream propagation of incision and bank undermining. The absence of water-spreading dynamics on the alluvial fan in favour of confinement in a single channel produces an unstable dynamic in the system, also offering a false sense of stability, as long as no large magnitude floods occur. When modelling flood-prone areas and analysing hydraulic variables, it is important to detect possible anthropic disturbances that may affect basin load budgets in order to anticipate catastrophic consequences resulting from inappropriate fluvial management before the occurrence of an extraordinary event.
机译：短暂的河流诱发了洪水泛滥事件，这导致了形态上的巨大变化并影响了人口，主要是因为它们是断断续续且难以预测的。流域上的人为压力会改变负载和流量关系，在河流系统上引起休眠的不稳定状态，这种不稳定状态在洪水事件中会突然显现出来。结合地貌方法和水力方法，讨论了由于土地利用变化而受到负荷供应变化影响的两个短暂河流的洪水响应。在里维拉斯山洪暴发期间，流域的密集清理导致大量沉积物流入人为拉直且效率低下的渠道。溪流从蜿蜒的单通道演变为浅的编织层，占据了谷底的整个宽度。不合适和不稳定的河道条件增加了速度，水流动力和夹带泥沙的能力，并大大加剧了洪水的破坏。产生的形态沉积特征揭示了与谷底洪灾后水力模型的密切关系，并且事前意识将使预测风险敏感区域成为可能。在第二种情况下，Azohía溪流通过对当前洪泛前河道条件进行建模，从而可以确定冲积扇下部河道中的河道变窄和盘根。集约化农业的放弃，流域的植树造林和城市化减少了负荷贡献并触发了渠道切割。这导致堤岸通道中的坡度和速度增加，产生新的侵蚀能量，从而激活切口的上游传播和堤岸破坏。只要没有大范围的洪水泛滥，冲积扇上就没有将水限制在单个通道内的动力，这会在系统中产生不稳定的动力，而且还提供了一种错误的稳定性感。在对易发洪水地区进行建模并分析水力变量时，重要的是要检测可能影响流域负荷预算的可能的人为干扰，以便在异常事件发生之前预测由于不当河流管理而造成的灾难性后果。

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