*3.5. E*ffi*ciency Tests of HDM and STM*

The computational time step ( Δ*t*) is set to 90 s and is equally divided into 9 sub time steps in the backtracking of the point-wise ELM. The 1-day unsteady flow and sediment transport, under an upstream bankfull discharge (45,000 m<sup>3</sup>/s) and a downstream spring tide, is tested to clarify the speedup property of the modeling system. The total runtime of the HDM and the STM is 1550.7 s in sequential runs, and is reduced to 131.1 s in parallel runs (*nc* = 16) with a speedup of 11.8. The unsteady flow and sediment transport in 1999 are then tested. It takes the modeling system 12.2 h (using 16 cores) to complete the simulation of a 1-year unsteady flow, sediment transport, and riverbed evolution in the Yangtze Estuary.
