Disentangling spring-neap SPM dynamics in estuaries

Abstract. Suspended particulate matter (SPM) concentrations in estuaries have been observed to vary strongly over the spring-neap cycle through complex interactions between trapping and re-suspension. However, a systematic framework for analysing the processes causing this spring-neap SPM variability in general is missing. In this study we set-up such a framework, consisting of three tiers. First, by studying the sediment transport capacity, it is identified how the locations of sediment trapping change over the spring-neap cycle. Second, it is studied how the transport capacity affects the sediment stock and bottom pool of sediment. This bottom pool only adapts gradually to the changing transport conditions, incorporating a lag or memory effect. Using a two-timescales analysis it is shown this slow movement of the bottom pool is the leading source of such lag effects. Third, the SPM concentration is explained from an almost instantaneously balanced exchanged between the bottom pool and the water column through re-suspension and deposition. We demonstrate the use of this framework on two model cases implemented in the idealised width-averaged iFlow model: an idealised test case where the sediment dynamics does not affect the water motion and a case representative of the Loire estuary, with strong feedback between sediment and the water motion through sediment-induced damping of turbulence. The first is illustrative as it allows a full understanding in terms of cause-and-effect between water motion, transport and SPM concentration. In the more realistic Loire case, the ETM dynamics cannot be explained in terms of cause and effect, but can explain the trapping locations and timing of maximum concentrations in a systematic way in terms of the governing physical mechanisms.