MEASURING FLUXES IN TIDAL ESTUARIES - SENSITIVITY TO INSTRUMENTATION AND ASSOCIATED DATA ANALYSES

This study examines the feasibility of quantifying net fluxes across a macrotidal estuary employing a range of available instrumentation. Th e observational programme in the Mersey Estuary used the following arr ay of instruments to measure tidal currents: bottom mounted and towed ADCPs (acoustic Doppler current profiler); bottom and (floating) platf orm-mounted electromagnetic current meters; and moored rotary meters. In 1992, observations extending over a 15-day spring-neap tidal cycle included near-continuous half-hourly towed ADCP transects across the 1 .5 km wide Mersey Narrows. The ADCPs also recorded acoustic backscatte r, used here to indicate concentrations of suspended particulate matte r (SPM). Tidal analyses of data of this type are complicated by the ef fect of the change in cross-sectional area between low and high waters . Here, an appropriate vertical reference frame is suggested that opti mizes the tidal analyses (especially where drying occurs) and allows b oth the comparison of moored and floating instruments, and subsequent synthesis into cross-sectionally integrated parameters. Formulation of a related cross-sectional grid scheme for analyses of the towed ADCP data is also described. After careful adoption of the above frameworks , coherent horizontal and vertical distributions were identified from analyses of the towed ADCP data for amplitude, phase and direction of N-2, M-2, S-2, M-4, MSf and Z(0) tidal ellipses. Encouraging compariso ns were found between tidal current constituents derived from the vari ous instrument arrays. Similarly, coherent cross-sectional distributio ns for both tidal and residual fluxes of water and SPM were obtained. Residual current time series recorded at fixed positions showed good c ross-correlation, but limited correlation against local wind data. By contrast, the residual time series for the principal mode representing the towed ADCP current data was uncorrelated with these fixed positio n data. It is possible that extraneous noise inherent in the towed ins truments is not effectively filtered in the tidal analysis, and contam inates the residual current time series. Although the spatial distribu tions of the various estimates of Z(0) for the net water flux are cohe rent, the values (mean over 15 days) are clearly unrepresentative. Thu s, despite the scale of the observational programme, it is concluded t hat where net fluxes are less than a few percent of tidal oscillations , these can not be determined by direct measurements of the sort descr ibed, even with state-of-the-art instrumentation. This conclusion esse ntially confirms experience elsewhere. Alternative methods of determin ing net fluxes include interpretation of measurements (of the present cross-sectional kind) with the aid of numerical models, or for 'conser vative' riverine tracers, calculation from longitudinal profiles.