Roles of Bank Material in Setting Bankfull Hydraulic Geometry as Informed by the Selenga River Delta, Russia

Abstract

A semi‐empirical bankfull Shields number relation as a function of slope, bed, and bank sediment grain size is obtained based on a field data set that includes the delta of the Selenga River, Russia, and other rivers from around the globe. The new Shields number relation is used in conjunction with continuity, flow resistance, and sediment transport equations to deduce predictive relations for bankfull width, depth, and slope of sand‐bed rivers. In addition, hydraulic geometry relations are obtained specifically for the Selenga River delta. Key results of this study are as follows: (1) bankfull width is strongly dependent on water discharge and is directly related to bank sediment size; (2) bankfull shear velocity is weakly dependent on bed sediment size and is inversely related to bank sediment size; (3) sand‐bed deltas with multiple distributary channels maintain smaller bankfull Shields numbers than is typical of alluvial rivers. This analysis is the first of its kind to include bank sediment size into a predictive bankfull Shields number relation to obtain relations for bankfull hydraulic geometry. The relations presented here can be utilized in morphodynamic models that explore how fluvial and deltaic systems respond to a range of imposed conditions, such as variable base level, sediment, and water supply.

Model for the Formation of Single‐Thread Rivers in Barren Landscapes and Implications for Pre‐Silurian and Martian Fluvial Deposits

“…In the cohesive limit, τ *;bank ∝ΩRe −2 p;bank , such that the equilibrium bed slope for a given W and h is expected to be proportional to d −3 bank in the cohesive regime (e.g., Figure 4); conversely, at constant S, W/h is expected to increase with increasing d bank . The latter prediction is qualitatively consistent with the analysis of the vegetated Selenga River delta of Dong et al (2019). In summary, the equilibrium bankfull geometry of single-thread rivers forming in unvegetated cohesive sediments is expected to be a strong function of bank grain size.…”

“…In the case of the experiments of Braudrick et al (), we find that a moderate increase in the bank‐erosion threshold ( σ * = 1), consistent with the moderate effect of short and shallow‐rooted plants on bank strength (e.g., Micheli et al, ; Polvi et al, ), makes predicted W / h (shaded green field in Figure b) match experimental data. We conduct a similar exercise for natural vegetated single‐thread rivers using the compilation of Dong et al (), which includes data from single‐threaded reaches of the Selenga River delta (Russia), a set of gravel‐bedded rivers from England, the Llano River (United States), the Fly River (Papua New Guinea), and the Siret River (Hungary). Channel reaches from Dong et al () have W / h ~ 4–136, S ~2 × 10 −5 − 2 × 10 −2 , and d bank ~ 5–465 μm.…”

“…We conduct a similar exercise for natural vegetated single‐thread rivers using the compilation of Dong et al (), which includes data from single‐threaded reaches of the Selenga River delta (Russia), a set of gravel‐bedded rivers from England, the Llano River (United States), the Fly River (Papua New Guinea), and the Siret River (Hungary). Channel reaches from Dong et al () have W / h ~ 4–136, S ~2 × 10 −5 − 2 × 10 −2 , and d bank ~ 5–465 μm. The compiled data from vegetated rivers plot within the braided‐stability zone of the ( d bank , S ) space (Figure ) and are predicted to have W / h > 200 in the absence of vegetation, indicating that vegetation is likely an important bank‐strengthening agent for these rivers.…”

Predicting Water and Sediment Partitioning in a Delta Channel Network Under Varying Discharge Conditions

“…These nodal relations are evaluated using standard statistics for linear correlation: coefficient of determination ( R 2 ), normalized root‐mean‐square error (nRMSE), and Pearson's correlation coefficient (PCC). Moreover, to determine the relative predictabilities of the Type I and Type II relations, Akaike information criterion (AIC) is used (see details in Appendix ; Anderson & Burnham, 2004; Dong et al., 2019). Regression methods that minimize ϵ are not used, as the goal of this study is to evaluate the relative predicative quality among different hydraulic variables on flow partitioning to develop the aforementioned general framework, rather than to optimize the nodal relation for a specific delta system.…”

“…In particular, the Selenga River mainstem is classified as a first‐order channel and the delta system bifurcates downstream into two second‐order channels, and so on, with a total of nine identified channel orders (Figure 1c). Within this network, channel geometry, bed and bank material sizes, vegetation type, and bank morphology vary spatially (Dong et al., 2016, 2019; Il'icheva et al., 2015; Pietroń et al., 2018). Specifically, both median bed and bank sediment grain size fine downstream, over ∼35 km of distance, from gravel at the delta apex to very fine sand and silt at the shoreline.…”