Simple flushing time calculations for estuarine systems can be used as proxies for eutrophication susceptibility. However, more complex methods are required to better understand entire systems. Understanding of the hydrodynamics driving circulation and flushing times in small, eutrophic, temperate estuaries is less advanced than larger counterparts due to lack of data and difficulties in accurately modelling small-scale systems. This paper uses the microtidal Christchurch Harbour estuary in Southern UK as a case study to elucidate the physical controls on eutrophication susceptibility in small shallow basins. A depth-averaged hydrodynamic model has been configured of the estuary to investigate the physical processes driving circulation with particular emphasis on understanding the impact of riverine inputs to this system. Results indicate circulation control changes from tidally to fluvially driven as riverine inputs increase. Flushing times, calculated using a particle tracking method, indicate that the system can take as long as 132 h to flush when river flow is low, or as short as 12 h when riverine input is exceptionally high. When total river flow into the estuary is less than 30 m³ s⁻¹, tidal flux is the dominant hydrodynamic control, which results in high flushing times during neap tides. Conversely, when riverine input is greater than 30 m³ s⁻¹, the dominant hydrodynamic control is fluvial flux, and flushing times during spring tides are longer than at neaps. The methodology presented here shows that modelling at small spatial scales is possible but highlights the importance of particle tracking methods to determine flushing time variability across a system.

河口系统的简单冲洗时间计算可以用作富营养化敏感性的代理。但是，需要更复杂的方法才能更好地了解整个系统。由于缺少数据和难以对小型系统进行精确建模，因此对小型，富营养化，温带河口的水动力驱动循环和冲洗时间的了解不如大型河口。本文以英国南部克利斯特彻奇港小潮汐河口为例，阐明了浅浅盆地富营养化敏感性的物理控制方法。河口已配置了平均深度的水动力模型，以研究驱动环流的物理过程，并特别着重于了解河流输入对该系统的影响。结果表明，随着河道投入的增加，循环控制从潮汐驱动变为河流驱动。使用粒子跟踪方法计算的冲洗时间表明，当河流流量较低时，系统可能需要长达132小时的冲洗时间，而当河流输入量特别高时，则可能需要12小时的冲洗时间。当流入河口的总河流少于30 m³  s ^-1，潮汐通量是主要的水动力控制因素，这导致在潮汐期间冲水时间较长。相反，当河流输入大于30 m ³  s ^-1时，主要的水动力控制是河流通量，并且在春季潮汐期间的冲刷时间比在午间时长。此处介绍的方法论表明，在小空间尺度上进行建模是可能的，但强调了粒子跟踪方法对确定整个系统中冲洗时间可变性的重要性。