The use of a morphological acceleration factor in the simulation of large-scale fluvial morphodynamics

Abstract

Numerical simulation of fluvial morphodynamic processes can identify important dynamics at time and space scales difficult to observe in the field. However, simulations involving large spatial scales and/or the long timescales characteristic of morphodynamic processes are often untenable because of long computation times. The morphological acceleration factor (morfac) applies a scalar multiplier to the sediment continuity equation, and is often applied in morphodynamic simulations to reduce computational time. Although the use of morfac in coastal simulations is relatively common, its applicability in field-scale fluvial models is generally confined to steady-flow simulations over reach-scale spatial domains. Here we explore the viability of using morfac to simulate large-scale, long-term morphodynamics in a gravel-bed river. We use Delft3D to simulate a 60-d period with a significant discharge event in the Nooksack River, Washington. We systematically adjust morfac values (ranging from 5 to 50) to compare with a baseline condition of no acceleration. Model results suggest that morfac-based modification of the inflow hydrograph time-series reduces peak flow magnitudes and flood wave celerities. Higher morfac values result in greater flood-wave attenuation and lower celerity, reducing the simulated morphological impact at locations farther downstream. In general, relative error compared to the model baseline increases farther downstream because of the altered flood-wave propagation. However, error analysis also shows a positive relationship with channel slope and a negative relationship with channel width; hence, errors increase in the steeper, confined reaches of the upper part of the domain. Even for the lowest morfac values absolute cumulative volume change errors are on the order of 10%, indicating that the use of morfac in fluvial simulations is best restricted to short-term and/or smaller-scale modeling efforts. Wider channel sections with lower gradients are shown to produce lower morfac-related errors. Therefore, morphological acceleration as examined here is potentially more appropriate for limited reaches rather than large-scale domains.