Author(s): Yuanjian Wang; Xudong Fu; Yuanfeng Zhang; Gary Parker
Keywords: Time variation; Bankfull characteristics; Hydraulic geometry; Multiple-thread; Single-thread; Yellow River
Abstract: Purely alluvial rivers construct their own channels and floodplains. The channel-floodplain complex is characterized by a bankfull condition, for which the river flow is on the verge of spilling onto its floodplain. The bankfull characteristics of a river are typically quantified in terms of bankfull discharge Qbf, bankfull width Bbf, cross-sectionally averaged bankfull depth Hbf, and bankfull velocity Ubf = Qbf/ (HbfBbf). Both dimensioned and dimensionless relations for these parameters have been developed for a wide range of alluvial river types. Two problems, however, arise in such treatments. The first of these concerns multiple-thread rivers. A) While bankfull discharge is not too difficult to define in single-thread rivers with floodplains, its definition for multiple-thread (braided or wandering) streams is highly ambiguous. The several compilations listing bankfull discharge for multiple-thread streams, however, typically lack specific information as to the criteria used to identify it. B) Such compilations typically list a single set of bankfull parameters for each stream. As such, they represent a snapshot of channel geometry and discharge at a single time. The implicit assumption is that bankfull discharge should not vary strongly in time within a quasi-graded reach subject to quasi-invariant hydrologic regime. The sand-bed Yellow River, China, provides a perfect setting to test these ideas. Here we consider two reaches: a) a multiple-thread reach from Huayuankou to Gaocun (Huayuankou Reach), and a b) meandering reach farther downstream, Taochengpu to Lijin (Lijin Reach). Channel configuration is subject to rather rapid temporal variation at Lijin, and highly rapid temporal variation at Huayuankou. We develop a new methodology which allows for the objective determination of bankfull characteristics in both meandering and multiple-thread streams. Our results show that the meandering Lijin reach has bankfull characteristics that are generally similar to other large, sand-bed streams around the world, although bankfull depth is somewhat low. While temporal variation is notable, it more or less falls within the scatter of single snapshots of other sand-bed rivers. The wandering Huayuankou reach, however shows not only a much smaller bankfull depth than corresponding single-thread streams, but also shows a temporal variation of width over an order of magnitude. This research forms the basis for a better characterization of the bankfull geometry of sand-bed streams in general.