Field studies of estuarine turbidity under different freshwater flow conditions, Kaipara River, New Zealand. (5th November 2017)

• Record Type: Journal Article
• Authors: Mitchell, Steven B.; Green, Malcolm O.; MacDonald, Iain T.; Pritchard, Mark
• Is Part Of: Estuarine, coastal and shelf science. Volume 198:Part B(2017)
• Page Start: 542
• Page End: 554
• Publication Date: 2017-11-05
• Subjects: Estuarine sediments -- Turbidity -- Tides -- Salinity gradients; Estuarine oceanography -- Periodicals; Coasts -- Periodicals; Estuarine biology -- Periodicals; Seashore biology -- Periodicals; Coasts; Estuarine biology; Estuarine oceanography; Seashore biology; Periodicals; 551.461805
• DOI: 10.1016/j.ecss.2016.06.009
• Languages: English
• ISSNs: 0272-7714

Abstract

Abstract: We present a first interpretation of three days of measurements made in 2013 from the tidal reaches of the Kaipara River (New Zealand) under both low and high freshwater inputs and a neap tidal cycle. During the first day, we occupied two stations that were approximately 6 km apart in a tidal reach that runs for 25 km from the river mouth to the upstream limit of tidal influence. During the second day, longitudinal surveys were conducted over a distance of 6 km centred on the upstream station. The data reveal a turbidity maximum in the form of a high-concentration 'plug' of suspended mud that was advected downstream on the ebbing tide past the upper (HB) measurement station and which exchanged sediment with the seabed by settling at low slack water and by resuspension in the early flooding tide. The data suggest that fine sediment is transported landwards and trapped in the upper part of the tidal reach under these low-flow conditions. On the third day of measurements we repeated the experiments of the first day but later in the year, for a much higher freshwater flow. This interpretation of our data set highlights the potential contribution of a range of processes to the generation of the observed suspended-sediment signals, including resuspension of local bed sediment, advection by the tidal current, settling of suspended sediment over a long timescale compared to the advection timescale, advection of longitudinal gradients in suspended sediment, and suppression of vertical mixing by density stratification of the water column. The level of temporal and spatial detail afforded by these measurements allows a much clearer understanding of the timing and importance of vertical stratification on the transport of suspended particulate matter than is generally possible using fixed-point sensors. Highlights: Observations of salinity and suspended sediment over depth and time show a turbidity maximum in a system in New Zealand. An unusual and very clear turbidity maximum was observed under low flow but which is flushed downstream under high flows. We show the importance of tidal asymmetry and vertical salinity gradient in trapping sediments, depending on fresh water flow. Sediment suspension can be seen to be controlled by both local resuspension and advection of previously suspended material. Estuarine turbidity is shown by measurement to be very dependent on fresh water flow.