Growing Forced Bars Determine Nonideal Estuary Planform

The planform of estuaries is often described with an ideal shape, which exponentially converges in landward direction. We show how growing topographically forced nonmigratory (i.e., anchored) bars determine the large-scale estuary planform, which explains the deviations observed in the planform of natural estuaries filled with bars compared to the ideal planform. Experiments were conducted in a 20-m long, 3-m-wide tilting flume, the Metronome. From a narrow, converging channel a self-formed estuary developed characterized by multiple channels, braided bars, a meandering ebb channel, and an ebb delta. Bars hardly migrated due to the alternating current, but the bar width increased with increasing estuary width. At locations where the estuary width was narrow, major channel confluences were present, while the zones between the confluences were characterized by a higher braiding index, periodically migrating channels, and a relatively large estuary width. At the seaward boundary, confluences were forced in place by the presence of the ebb tidal delta. Between confluences, bars were topographically forced to be nonmigratory. Diversion of flow around forced midchannel bars caused bank erosion. This resulted in a planform shape with a quasiperiodic widening and narrowing at the scale of forced bars. Observations in natural systems show that major confluence locations can also be caused by inherited geology and human engineering, but otherwise the estuary outline is similarly affected by tidal bars. These observations provide a framework for understanding the evolution of tidal bar patterns and the planform shape of the estuary, which has wide implications for navigation, dredging, and ecology. Plain Language Summary Estuaries, which are also called river mouths, form the transition from rivers to the ocean. A common concept to describe the shape of these tidally influenced estuaries is the concept of an ideal estuary, which describes estuaries as perfectly converging channels. However, in natural systems we often observe planform shapes that deviate from this converging shape. In this article, we show with experiments in a tilting flume that estuaries without any hard boundaries (such as bedrock geology) evolve into planforms with an irregular shape rather than the perfectly converging shape. We identified a mechanism in which sand bars build up in the center of the estuary, after which the flow is diverted around the bars. This process results in local widening where tidal bars initially formed, while the estuary remains relatively narrow at zones between the bars. This mechanism helps to understand the dynamic behavior of estuaries, in which bars form valuable ecological habitat and in which channels provide access to million dollar harbors.