How boundary conditions govern lacustrine delta growth and slope curvature: Insights from numerical modelling

ABSTRACT Lacustrine shallow‐water deltas exhibit complex clinoform geometries shaped by the dynamic interplay between upstream and downstream boundary conditions. However, quantitative frameworks for describing their evolution are limited. This study presents results from a numerical model (Delft3D) used to investigate the mechanisms of lacustrine delta growth and the resulting clinoform geometries under varying boundary conditions. A reference case was established to model the delta building‐out process at shallow‐ and deep‐basin depths. Geological and hydrodynamic models were established by varying the water level under steady river discharge and sediment supply. Slope profiles were extracted and fitted using a range of mathematical functions, including six newly applied sigmoidal equations to capture the full spectrum of clinoform geometries. The results revealed systematic changes in the clinoform shape in response to boundary conditions: rising water levels and low discharge favoured linear topsets and oblique foreset profiles, whereas falling water levels and high discharge promoted quadratic topsets and sigmoidal foreset profiles. Grain size further modulated these trends, with fine sand deltas exhibiting lower foreset curvature. However, basin depth exerted a key control over foreset concavity, with shallow‐water deltas displaying greater curvature (~1.5 to 3.5 × 10 −6 ) and a higher frequency of sigmoidal profiles (~50 to 55%), whereas deep‐water systems exhibited reduced curvature (~0.5 to 1.5 × 10 −6 ) and a lower frequency of sigmoidal profiles (~45 to 50%). Key morphodynamic metrics (foreset curvature, number of channel mouths, and their orientation) correlated positively with discharge and sand proportion and responded to minor depth variations, demonstrating that clinoform architecture records a subtle palaeobathymetric signal. Furthermore, upper foreset curvature in sigmoidal clinoforms proved to be a reliable proxy for palaeowater‐level reconstruction. Validated against modern satellite images and seismic data, these findings establish a quantitative, transferable framework for interpreting depositional conditions across lacustrine, continental margin, and mixed‐load shelf systems.