The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. I. Sedimentology and morphology
The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. I. Sedimentology and morphology
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DOI：
10.1046/j.1365-3091.2000.00290.x
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发表时间：
2000-02
期刊：
影响因子：
3.5
通讯作者：
Carling;Gölz;Orr;Radecki‐Pawlik
中科院分区：
文献类型：
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作者：
Carling;Gölz;Orr;Radecki‐Pawlik
在德国莱茵河的一个供应有限的河段，研究了大型孤立沙丘在砾石滞后层上移动的动力学。本文考虑了河床沉积物、沙丘几何形状、河床迁移速率和沙丘的内部结构。在另一篇论文中考虑了水动力和泥沙输运数据。平坦滞后层的卵石和鹅卵石（D50为10 mm）很少被夹带。沙丘由分选良好的中至粗砂组成（D50为0.9 mm）。小鹅卵石通过“过桥”在沙丘上移动，但它们的大小和形状有一定程度的选择性。在7 ~ 1 m和5 ~ 10 m区域，沙中波纹种群（D50 < 0.6 mm）和大小沙丘被明显的断裂分隔开。波纹和小沙丘可能有弯曲的顶线，但主要表现为二维平台。相比之下，大型沙丘主要是三维的barchanoid形式。小沙丘背面的波纹很少发展到最大的陡度。小沙丘可以在砾石床上或在大沙丘压力侧的边界层内作为次级沙丘达到平衡几何形状。次级沙丘在穿越大沙丘的上斜面时，往往形成座头状剖面，在穿越峰顶区域时，高度减少，长度增加。但是，长度超过5米的次生沙丘很少。平衡纹波和长次生沙丘的缺乏可能与母质层上可供河床发育的偏移长度有限有关。长度在20米到100米之间的大沙丘不接近平衡几何。沙丘高度的主要控制因素是深度限制，而不是泥沙供应限制；在水深约4米的地方，沙丘高度很少超过1米。沙丘速度随平均流速的平方而增加，但这种一般关系掩盖了更微妙的形态动力学。在河流上升阶段，由于峰顶堆积，沙丘高度增加，减缓了下游进程，使沙丘形状变陡。在稳定或下降阶段，一个扩展的峰顶平台与背风侧的快速下游迁移和沙丘高度的降低有关。实际上，这些逐渐减少的沙丘单位体积的增加是由于次级沙丘移动经过一个停滞的压力趾而增加背风堆积的过程。提出了一个沙丘生长和减少的六阶段模型来解释观测到的形态变化。该模型展示了内边界层的发展以及水表面与这些以层载为主的沙丘的波峰的相互作用如何导致沙丘具有平缓的背风面和弱流动分离的特征。这一发现意义重大，因为其他关于大河沙丘的研究将这种形态反应归因于悬浮载荷运输的优势。
The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply‐limited reach of the River Rhine, Germany. Bed sediments, dune geometry, bedform migration rates and the internal structure of dunes are considered in this paper. Hydrodynamic and sediment transport data are considered in a companion paper. The pebbles and cobbles (D50 of 10 mm) of the flat lag layer are rarely entrained. Dunes consist of well‐sorted medium to coarse sand (D50 of 0·9 mm). Small pebbles move over the dunes by ‘overpassing’, but there is a degree of size and shape selectivity. Populations of ripples in sand (D50 < 0·6 mm), and small and large dunes are separated by distinct breaks in the bedform length data in the regions of 0·7–1 m and 5–10 m. Ripples and small dunes may have sinuous crestlines but primarily exhibit two‐dimensional planforms. In contrast, large dunes are primarily three‐dimensional barchanoid forms. Ripples on the backs of small dunes rarely develop to maximum steepness. Small dunes may achieve an equilibrium geometry, either on the gravel bed or as secondary dunes within the boundary layer on the stoss side of large dunes. Secondary dunes frequently develop a humpback profile as they migrate across the upper stoss slope of large dunes, diminishing in height but increasing in length as they traverse the crestal region. However, secondary dunes more than 5 m in length are rare. The dearth of equilibrium ripples and long secondary dunes is probably related to the limited excursion length available for bedform development on the parent bedforms. Large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry. A depth limitation rather than a sediment supply limitation is the primary control on dune height; dunes rarely exceed 1 m high in water depths of ≈4 m. Dune celerity increases as a function of the mean flow velocity squared, but this general relationship obscures more subtle morphodynamics. During rising river stage, dunes tend to grow in height owing to crestal accumulation, which slows downstream progression and steepens the dune form. During steady or falling stage, an extended crestal platform develops in association with a rapid downstream migration of the lee side and a reduction in dune height. These diminishing dunes actually increase in unit volume by a process of increased leeside accumulation fed by secondary dunes moving past a stalled stoss toe. A six‐stage model of dune growth and diminution is proposed to explain variations in observed morphology. The model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload‐dominated dunes can result in dunes characterized by gentle lee sides with weak flow separation. This finding is significant, as other studies of dunes in large rivers have attributed this morphological response to a predominance of suspended load transport.