A First-Order, Morphological Response Model (FORM) for predicting hydrologically induced bathymetric change in coastal-plain estuaries

Estuarine hydrodynamic modeling is complex, and predicting bathymetric outcomes from imposed hydrological changes requires models to encompass time periods spanning years, decades, or longer. Within this work, a first-order response model (FORM) was developed that doesn't require temporal or process components and represents a considerable computational saving compared with two-dimensional/three-dimensional morphodynamic models. FORM enables a final regime to be predicted using knowledge of only the initial regime and the causal change in the tidal prism. It builds on the power width/distance relationship (y = axⁿ) for a simple estuary and allows for the addition of storages or tributaries. FORM was validated using data from 2004 and pre-1968 for the Petitcodiac River, New Brunswick, Canada, where considerable bathymetric change has occurred because of the Moncton Barrage. Linear regression of the modeled upper Tamar River estuary, Tasmania, Australia, revealed a significant and positive relationship between the predicted and actual estuarine widths (R² = 0.89 p < 0.001, slope = 1.0085). Historical channel widths, representing various phases of estuarine geomorphology and development were modeled for 1830 and 1890 and were validated against data from an 1830 navigation chart. As a further validation of the 1806 result, the volume of silt accretion associated with the bathymetric change was calculated. The result (13.4 × 10⁶ m³) was within the range of the observed volume of accretion since 1806 (10.0 × 10⁶–15.0 × 10⁶ m³). The model extends the applicability of existing regime models to similar coastal-plain estuaries composed of multiple tributaries, channels, and storages, regardless of synchronicity.