Supply‐limited bedform patterns and scaling downstream of a gravel–sand transition
Supply‐limited bedform patterns and scaling downstream of a gravel–sand transition
复制标题
供应有限的河床形态和砾石-砂过渡下游的扩展
作者：
J. Venditti;J. Nittrouer;M. Allison;R. Humphries;M. Church
在沙子供应有限的实验室流中，观察到出现了一套独特的沙床形态。随着河床上的沙子供应量逐渐增加，人们会观察到沙带、离散的新月形，最终形成跨越河道的沙丘；但从天然河道对这一序列的观察相对较少。此外，很少有观察到现场砂子供应从有限到充足的转变。在不列颠哥伦比亚省弗雷泽河的砾石-沙子突然过渡下游，沙子供应有限但不断增加，形成的河床形态采用高流量下获得的多波束测深仪进行了检查。这是研究供应有限的河床形态的理想地点，因为由于河流坡度的断裂，沙子从砾石-沙子上游的冲刷荷载过渡到下游的河床物质荷载。紧邻下游，可以观察到沙丘和孤立的沙丘。大多数床型场在槽中都有间隙，这与沙子在平坦的不动或弱移动的砾石床上移动一致。线性的沿流河床形态场（在局部较厚的线性沙沉积物上形成的一系列横向沙丘）表现出具有叠加河床形态的沙带特征。再往下游，河道横跨沙丘，河床完全由沙子组成。该数据集中没有出现沙丘的深度缩放。只有在河道床上积聚了大量沙子的地方，沙丘的尺度才会与之前的数据汇编一致。观察结果表明，沉积物供应在河流河床结垢过程中发挥着重要但经常被忽视的作用。
A distinct suite of sand bedforms has been observed to occur in laboratory flows with limited sand supply. As sand supply to the bed progressively increases one observes sand ribbons, discrete barchans and, eventually, channel spanning dunes; but there are relatively few observations of this sequence from natural river channels. Furthermore, there are few observations of transitions from limited sand supply to abundant supply in the field. Bedforms developed under limited, but increasing, sand supply downstream of the abrupt gravel–sand transition in the Fraser River, British Columbia, are examined using multi‐beam swath‐bathymetry obtained at high flow. This is an ideal location to study supply‐limited bedforms because, due to a break in river slope, sand transitions from washload upstream of the gravel–sand transition to bed material load downstream. Immediately downstream, barchanoid and isolated dunes are observed. Most of the bedform field has gaps in the troughs, consistent with sand moving over a flat immobile or weakly mobile gravel bed. Linear, alongstream bedform fields (trains of transverse dunes formed on locally thick, linear deposits of sand) exhibit characteristics of sand ribbons with superimposed bedforms. Further downstream, channel spanning dunes develop where the bed is composed entirely of sand. Depth scaling of the dunes does not emerge in this data set. Only where the channel has accumulated abundant sand on the bed do the dunes exhibit scaling congruent with previous data compilations. The observations suggest that sediment supply plays an important, but often overlooked, role in bedform scaling in rivers.