Estimating bed material fluxes upstream and downstream of a controlled large bifurcation ‐ the Mississippi‐Atchafalaya River diversion

Abstract

Bed material transport at river bifurcations is crucial for channel stability and downstream geomorphic dynamics. However, measurements of bed material transport at bifurcations of large alluvial rivers are difficult to make, and standard estimates based on the assumption of proportional partitioning of flow and bedload transport at bifurcations may be erroneous. In this study, we employed a combined approach based on observed topographic change (erosion/deposition) and bed material transport predicted from a one‐dimensional model to investigate bed material fluxes near the engineering‐controlled Mississippi‐Atchafalaya River diversion, which is of great importance to sediment distribution and delivery to Louisiana's coast. Yang's (1973) sediment transport equation was utilized to estimate daily bed material loads upstream, downstream, and through the diversion during 2004–2013. Bathymetric changes in these channels were assessed with single beam data collected in 2004 and 2013. Results show that over the study period, 24% of the Mississippi River flow was diverted into the Atchafalaya River, while the rest remained in the mainstem Mississippi. Upstream of the diversion, the bed material yield was predicted to be 201 million metric tons (MT), of which approximately 35 MT (i.e., 17%) passed through the bifurcation channel to the Atchafalaya River. The findings from this study reveal that in the mainstem Mississippi, the percentage of bed material diversion (83%) is larger than the percentage of flow diversion (76%); Conversely, the diversion channel receives a disproportionate amount of flow (24%) relative to bed material supply (17%). Consequently, severe bed scouring occurred in the controlled Outflow Channel to the Atchafalaya River, while riverbed aggradation progressed in the mainstem Mississippi downstream of the diversion structures, implying reduced flow capacity and potential risk of a high backwater during megafloods. The study demonstrates that Yang's sediment transport equation provides plausible results of bed material fluxes for a highly complicated large river diversion, and that integration of the sediment transport equation with observed morphological changes in riverbed is a valuable approach to investigate sediment dynamics at controlled river bifurcations.

Impact of water-sediment diversion and afflux on erosion-deposition in the Luoshan-Hankou reach, middle Yangtze River, China

“…If Hu et al (2016) had focused on the whole Jingjiang reach, we believe that the two |ψ| values would have been more comparable. Similar erosion-deposition promotion events caused by water-sediment diversion and afflux along the mainstream have also been observed in other rivers, such as the Yellow River (Pan et al, 2015), Mississippi River (Kemp et al, 2014;Wang and Xu, 2020), and Amazon River (Li et al, 2020).…”

Artificial bifurcation effect on downstream channel dynamics of a large lowland river, the Atchafalaya

“…The much gentler slope (i.e., −0.7 × 10 −4 , 0.2 × 10 −3 , and 0.4 × 10 −3 ) of the water surface in the Atchafalaya River natural channel under high, medium, and low flow conditions has played a critical role in sediment siltation and channel aggradation. Wang and Xu (2020) reported that in the mainstem Mississippi, the percentage of bed material diversion (83%) is larger than the percentage of flow diversion (76%). Consequently, riverbed aggradation progressed in the mainstem Mississippi River downstream of the diversion structures with the disproportional flow–sediment ratio.…”

“…Results indicated that Yang's (1973) formula was most robust because its accuracies were least sensitive to the fluctuation of water depth, dimensionless shear velocity, dimensionless unit stream power, Froude number, and sediment concentration. In a recent study, Wang and Xu (2020) estimated daily BMLs at the Mississippi–Atchafalaya River diversion during 2004–2013 using Yang's Equation (Yang, 1973).…”

“…At present, shorelines of many deltas in the world are experiencing land loss due to declining sediment supply and accelerating rates of sea‐level rise (Blum & Roberts, 2009; Syvitski & Saito, 2007). Because large river deltas are densely populated and are transportation hubs and industrial and commercial centers, artificial bifurcation have been implemented, or are being considered, as a viable engineering tool to divert flow and sediment for combating coastal land loss, especially on those densely populated deltas such as the Yellow River delta (Carlson et al, 2021; Syvitski & Saito, 2007) and the Mississippi River Delta (Shaw et al, 2021; Wang & Xu, 2020).…”

Capture Timescale of an Uncontrolled Mississippi–Atchafalaya Bifurcation with Future Lower River Strategy Implications

“…Surveys were conducted by the USACE at multiple reaches in these locations to monitor diversion changes between 1880 and 1951. More recent studies have shown these patterns to persist in these reaches even after the construction of the ORCS (Knox and Latrubesse, 2016; Wang and Xu, 2018, 2020; Xu et al ., 2021).…”