Insights into the impacts of inflow discharge variation on cross-sectional topography in the upper neck area of a laboratory-scale subaerial delta from SWJ–LSTM simulations

Abstract

Rivers significantly influence delta morphology and sedimentation patterns. However, the dynamic effects of rivers on the upper neck areas of subaerial deltas, which are the complex zones connecting main channels to distributary networks, remain understudied. In this research, the impacts of discharge variation on cross-sectional topography within the upper neck area of a laboratory-scale subaerial delta were examined via an integrated shallow water jet (SWJ)–long short-term memory (LSTM) modeling approach that synergistically couples SWJ equations incorporating analytical velocity distributions and parameterized bedload transport relationships with LSTM networks and gradient boosting for data-driven enhancements. Laboratory experiments, which provide detailed topographic measurements, were used for model calibration and validation. We investigated stepwise, periodic, and stochastic discharge alteration scenarios. The results revealed a fundamental pattern of spatially differentiated morphodynamic sensitivity within the upper neck area. The section farthest upstream consistently exhibited relative stability. In stark contrast, the mid-sections (spanning approximately 20%–30% of the total delta length from the inlet) emerged as the primary loci of morphological change, consistently demonstrating robust switching behaviors between pronounced erosion and deposition regimes under varying discharge regimes. In contrast, the section farthest downstream showed a more integrated and dampened response. This distinct switching mechanism within defined mid-sections, rather than diffuse variability, constituted a key finding regarding the mechanism by which the upper neck area could fundamentally process discharge fluctuations. Specifically, discharge decreases typically led to localized scouring and enhanced channelization, particularly within these active mid-sections. Conversely, increases in discharge induced increasingly complex responses involving erosion and deposition, with the specific outcome being dependent on the precise location within these mid-sections and on the nature of discharge alteration. The core components of the morphological evolution of the delta were further evaluated by the finding that the magnitude, rate, and timing of discharge changes (e.g., rapid exponential changes and slow logistic decreases), along with the amplitude of periodic fluctuations, significantly governed the intensity and characteristics of this switching behavior and the resultant morphology. Increasingly pronounced effects were observed under rapid exponential changes, slow logistic decreases, and large periodic amplitudes. Under stochastic discharge, the mean reversion rate and long-term mean volatility of discharge exerted complex, spatially variable influences on the mean bed elevation change, highlighting their critical roles in shaping morphology, whereas the volatility had a more subtle and discharge-dependent impact. Thus, this research revealed not only variability but also a spatially organized response framework featuring critical zones and specific mechanisms, such as mid-section switching, governed by identifiable hydraulic parameters. The findings offered practical insights into delta management, climate adaptation, and environmental assessment, strengthening our understanding of fluvial–deltaic interactions and supporting ecosystem sustainability.