The mixing of river plumes into the coastal ocean influences the fate of river-borne tracers over the inner-shelf, though the relative importance of mixing mechanisms under different environmental conditions is not fully understood. In particular, the contribution to plume mixing from bottom generated shear stresses, referred to as tidal mixing, is rarely considered important relative to frontal and stratified shear (interfacial) mixing in surface advected plumes. The effect of different mixing mechanisms is investigated numerically on an idealized, tidally pulsed river plume with varying river discharge and tidal amplitudes. Frontal, interfacial, and tidal mixing are quantified via a mixing energy budget to compare the relative importance of each to the overall buoyancy flux over one tide. Results indicate that tidal mixing can dominate the energy budget when the tidal mixing power exceeds that of the input buoyancy flux. This occurs when the non-dimensional number, RiE (the estuarine Richardson number divided by the mouth Rossby number), is generally less than 1. Tidal mixing accounts for between 60% and 90% of the net mixing when RiE < 1, with the largest contributions during large tides and low discharge. Interfacial mixing varies from 10% to 90% of total mixing and dominates the budget for high discharge events with relatively weaker tides (RiE > 1). Frontal mixing is always less than 10% of total mixing and never dominates the budget. This work is the first to show tidal mixing as an important mixing mechanism in surface advected river plumes.

Tide-surge interaction creates perturbations to storm surge at tidal frequencies and can affect the timing and magnitude of surge in tidally energetic regions. To date, limited research has identified high-frequency tide-surge interaction (>4 cycles per day) in coastal areas, and its significance in fluvial estuaries (where we consider it tide-surge-river interaction) is not well documented. Water level and current velocity observations were used to analyze tide-surge-river interaction at multiple tidal and overtide frequencies inside of a shallow estuary. Near the head of the estuary, higher frequency harmonics dominate tide-surge-river interaction and produce amplitudes more than double that of wind and pressure-driven surge. Bottom friction enhanced by storm-induced currents is the primary mechanism behind the interaction, which is further amplified by within-estuary resonance. High-frequency tide-surge-river interactions in estuaries present a significant threat to human life, as the onset of flooding (in <1.5 hr) is more rapid than coastal storm surge flooding. Commonly used storm surge forecasting models neglect high-frequency tide-surge-river interaction and thus can markedly underestimate the magnitude and timing of inland storm surge flooding.Plain Language Summary Storm surges are a threat to life and property on the coast. How storm surges interact with tides varies by region and is not well understood, particularly in estuaries. This tide-surge interaction, which we identify as tide-surge-river interaction in estuaries with a strong river influence, can affect the timing and magnitude of storm surges and so is important to understand. This study calculated storm surge and tide-surge-river interaction in a large estuary with strong tides after collecting water levels in the system for one Fall/Winter season. Results show that tide-surge-river interaction can more than double storm surges relative to the nontidal influenced surge and create rapid oscillations to water level that are hard to predict. Enhanced current velocities during storms from wind and surge propagation can cause tide-surge-river interaction, which can be further amplified by estuary geometry. Common surge models do not accurately resolve these high-frequency tide-surge-river interactions.Key Points:• Tide-surge-river interaction was shown to be largest in the sixth and eighth diurnal bands • High-frequency tide-surge-river interaction can have amplitudes more than double that of low-frequency surge • Enhanced bottom friction and resonance were the primary mechanisms causing tidesurge-river interaction at the D 6 and D 8 bands

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Storm surges create coastal flooding that can be damaging to life and property. In estuaries with significant river influence (fluvial), it is possible for tides, storm surge, and river discharge to interact and enhance surges relative to the immediate coast. These tide-surge-river interactions were previously identified in a fluvial Maine estuary as higher frequency (>four cycles per day) oscillations to storm surge which were proposed to be incited by enhanced friction and resonance during certain windstorm events (Spicer et al. 2019). The relative contributions to tide-surge-river interaction from atmospheric forcing variables (wind, barometric pressure, and externally generated surge) remains unclear. This work seeks to decompose and analyze a recent windstorm surge event to better isolate the effects of atmospheric forcing on tidesurge- river interaction. Results show total storm surges in the fluvial estuary to be two times larger than at the estuary mouth because of tide-surge-river interaction. Analysis indicated at least 50% of the magnitude of tide-surge-river interactions are created by non-tidal forcing, in the form of wind, enhancing frictional energy in the estuary. The remaining tide-surge-river interaction is likely a result of changes in tidal wave propagation speed due to surge deepening the mean estuary water level.

The discharge of river-borne water into the coastal ocean is subject to a variety of physical processes which mix fresh and salty water. Mixing dictates pollutant, nutrient, and sediment fate and so can influence coastal ecological health. In general, plumes mix into the coastal ocean by wind, waves, tidal processes, and frontal shear and convergence at plume interfaces (Horner-Devine et al., 2015). Although great advances have been made in recent years observing and modeling plumes, there is still significant uncertainty regarding the importance of mixing mechanisms spatially and temporally in plumes exposed to different environmental conditions.Coastal winds are a notable environmental condition controlling plume dynamics. Winds can modify circulation patterns over the shelf, which in turn modulate river plume dynamics outside the estuary. Extensive numerical and