Abstract Runup deposits are veneers of alluvium that drape floodway valley side walls above the height of giant bars deposited during megafloods. Given sufficient sediment supply, the highest giant bars, deposited in re-entrants along the flood margins, tend to grow to close to the maximum time-averaged water level of the flood. However, considerable fluctuations in the water level, caused by sediment-charged floodwaters surging over shorter time-scales, are responsible for the higher runup deposits. Here, the theoretical calculations of the expected maximum runup heights are compared with surveyed heights of six runup deposits in the Chuja Valley, Altai, Siberia. The limitations and strengths of the theoretical approach are identified and modified parameters proposed that can be used to provide partial explanation for the differences between theory and observation. Conceptually, surging can be viewed as caused by four interrelated elements: (1) propagation of undular weir flow; (2) macroturbulence; (3) flow separation; and (4) standing, reflection and interference waves. The heights of the observed runup deposits primarily are related to the depth of the flood water above the bar tops and, to a lesser extent, the Froude number, but tend to lie below the maximum surge heights of the modelled flow. Changes in the effective geometry of the flow re-entrant, mediating flow patterns, as water depth increases is likely the cause of mismatch between theory and observation. Runup deposits may also lie at lower elevations than predicted because of sediment supply considerations and the return flow of surges ‘combing’ down deposits. Nonetheless, the difference between observed and predicted runup heights is often only a few tens of metres such that, for deep floods, runup deposits potentially are useful palaeostage indicators. The analysis also indicates that upper-stage plane beds do not dominate bar tops, rather bar top deposition was primarily to lower-stage plane beds, from dense suspensions.

中文翻译：

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大洪水爬坡沉积物发育的水力控制

摘要 Runup 沉积物是冲积层的饰面，它覆盖在大洪水期间沉积的巨型坝体高度上方的泄洪道谷侧壁。如果沉积物供应充足，沿着洪水边缘的折返口沉积的最高的巨型水坝往往会增长到接近洪水的最大时间平均水位。然而，由沉积物洪水在较短的时间尺度内涌动引起的水位相当大的波动是造成更高的沉积物的原因。在此，将预期最大爬升高度的理论计算与西伯利亚阿尔泰楚贾河谷的六个爬升矿床的调查高度进行了比较。确定了理论方法的局限性和优势，并提出了修改参数，可用于为理论和观察之间的差异提供部分解释。从概念上讲，涌动可以看作是由四个相互关联的因素引起的：(1) 不规则堰流的传播；(2) 宏观湍流；(3)流分离；(4)驻波、反射波和干涉波。观察到的上升沉积物的高度主要与坝顶上方的洪水深度有关，在较小程度上与弗劳德数有关，但往往低于模拟流的最大浪涌高度。随着水深的增加，折返流的有效几何形状发生变化，调节流动模式，这可能是理论与观察不匹配的原因。由于沉积物供应方面的考虑和“梳理”沉积物的回流回流，径流沉积物也可能位于低于预测的海拔高度。尽管如此，观测到的和预测的爬升高度之间的差异通常只有几十米，因此，对于深洪水，爬升沉积物可能是有用的古阶段指标。分析还表明，上层平面床并不支配棒顶，相反，棒顶沉积主要是从密集悬浮液到下层平面床。