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

Abstract

Artificial river bifurcations have been widely created for different purposes. Yet, long‐term dynamics in the bifurcated channels and natural channels downstream of the artificial bifurcation nodes are not well investigated. Herein, we employed an approach combining three decades (1977–2006) of bathymetric survey records, geographic information system (GIS) application, and bed material transport modeling to investigate riverbed dynamics in an artificially bifurcated channel and the downstream natural channel of the Atchafalaya River, the largest distributary of the Mississippi River. Decadal changes in the riverbed volume were determined, and erosion/deposition patterns in these channels were assessed. Hydraulic properties under low, medium, and high flow conditions were quantified to analyze main factors affecting the long‐term trend of channel dynamics. Our study found that flow from the Atchafalaya River into the artificially bifurcated Wax Lake outlet channel (WLOC) steadily increased. Hydraulic gradient, stream power, and shear stress under all flow conditions of the WLOC were greater than those of the Atchafalaya River natural mainstem channel (ARMC) downstream of the artificial bifurcation. On average, 52% of the total bed material loads (BMLs) from the Atchafalaya entered the steeper and straight WLOC with 41% of the river's flow from 1977 to 2006. Despite the higher sediment inflow, the WLOC degraded with an average rate of −0.12 × 105 m3 km−1 yr−1, while the ARMC downstream of the bifurcation aggraded substantially with an average rate of 0.38 × 105 m3 km−1 yr−1 over the three decades. The disproportional flow–sediment ratio and channel erosion‐aggradation patterns below the artificial bifurcation were apparently a reflection of the difference in the river hydraulic properties. These findings imply that lowland alluvial river diversions for introducing sediment to build new land in world's sinking deltas need to develop sustainable solutions in order to avoid causing severe bed scouring in one but excessive deposition in the other channel downstream of the bifurcation.

Lowland rivers: Geomorphology, human impacts, and management

“…While originally designed to transport 30% of the Atchafalaya River streamflow, the channel has rapidly evolved (Fisk, 1952;Roberts, 1998). Using historic hydrographic surveys over about a 30-year period, from 1977 to 2006, Tang et al (2022) found that Wax Lake Outflow Channel now transports 41% of Atchafalaya River discharge but 52% of its bedload.…”

“…While originally designed to transport 30% of the Atchafalaya River streamflow, the channel has rapidly evolved (Fisk, 1952; Roberts, 1998). Using historic hydrographic surveys over about a 30‐year period, from 1977 to 2006, Tang et al (2022) found that Wax Lake Outflow Channel now transports 41% of Atchafalaya River discharge but 52% of its bedload. Interestingly, while the artificially created Wax Lake Outlet channel is degrading and increasing in size the natural Atchafalaya River mainstem outlet channel is aggrading.…”

“…The 10 special issue contributions herein reviewed concern important fundamental and applied topics within lowland river science, specifically highlighting three key themes. These include ‘natural’ river channel dynamics within settings that vary from the standard body of knowledge (Guha et al, 2022; Rogério et al, 2022; Sinha et al, 2022), geomorphic response to channel engineering (Mossa & Chen, 2022; Marteau et al, 2022; Tang et al, 2022; Wu et al, 2022), and flood basin management in response to environmental change (Keim et al, 2022; Phillips, 2022; Valentine et al, 2022). The categories are not mutually exclusive, and most contributions have linkages across multiple themes.…”