Sedimentary Dynamics and Decadal-Scale Changes in the Macrotidal Aulne River Estuary, Brittany, France
ABSTRACT
Moskalski, S.; Floc'h, F.; Verney, R.; Fromant, G.; Le Dantec, N., and Deschamps, A., 2018. Sedimentary dynamics and decadal-scale changes in the macrotidal Aulne River estuary, Brittany, France.
Studies of macrotidal estuaries have been dominated by large, high-turbidity systems like the Gironde and Severn, but some macrotidal estuaries are different from these systems. The goal of this study was to document the state of hydrosedimentary processes in the macrotidal, low-turbidity Aulne River estuary and, where possible, compare it to a previous study. Study sites from a 1977 study were revisited in 2013. Observations of velocity, salinity, and suspended sediment concentration were analyzed and compared to the 1977 study, with particular attention given to hourly data. The results illustrate the impact of shallowing on hydrosedimentary processes. The estuary is strongly dominated by spring–neap variability and shallowing, with increasing tidal distortion in the landward direction and a turbidity maximum zone created by sediment resuspension. The maximum flooding and ebbing velocity were stronger and tidal velocity distortion was more pronounced than in 1977. The salinity intrusion moved farther landward during low discharge in 2013 than in 1977, but not during average and high discharge. Other hydrosedimentary processes remained consistent with the previous study. This study also illustrates potential problems that can occur when attempting to use historical analyses.
Map of the study area with locations of 2013 study sites (Sites 1–4), 1977 study sites (Sites 1B–4B), and 1977 locations used for longitudinal profiling (Sites A–G, stars). The insets show the position of the study site within France and the Bay of Brest.
Relationship between YSI SSC and BI from Bin 2 of the AWAC at Site 1 in February.
Depth, near-bottom velocity, near-bottom SSC, and near-bottom salinity for Site 1 (a–d) and Site 3 (e–h) in February. Spring and neap datasets both start with hour 0 at midnight. Negative velocity is seaward. Salinity data are expressed according to the practical salinity scale.
Depth, near-bottom velocity, near-bottom SSC, and near-bottom salinity for Site 1 (a–d) and Site 3 (e–h) in September. Spring and neap datasets both start with hour 0 at midnight. Negative velocity is seaward. Salinity data are expressed according to the practical salinity scale.
All recorded high and low tides at Brest between 1977 and 2013. Thick lines are linear fits to the heights of high tides and heights of low tides. High tides: y = 3.115 × 10−6(x) + 3.989. Low tides: y = 1.135 × 10−5(x) − 6.355. For the hypothesis that the slope of the regression line was zero, p = 0.009 for highs and p = 8.15 × 10−21 for lows.
Mean annual discharge at Pont Pol from 1977 through 2013. The years 1977 and 2013 are highlighted with large black circles. A linear fit to the data yielded the equation y = 0.016(x) − 10. For the hypothesis that the slope of the regression line was zero, p = 0.844, and the hypothesis was not rejected.
Daily total precipitation at Brest (a) and river discharge at Châteaulin (b) during 1977 and 2013. The black dashed rectangles indicate the dates of 2013 instrument deployments. The light gray vertical lines indicate the dates of 1977 data collection that were used for comparison with 2013 data (Table 1).
Near-surface and near-bottom velocity in 1977 and 2013 during spring tide conditions. Near-surface data are from 0.5 m below the water surface, and near-bottom data are from 1 m above the bottom. The 2013 data were smoothed with a 30-minute frequency-domain lowpass filter with a Hann window. Negative velocity is seaward.
Longitudinal profiles of salinity during low discharge and low tide, in 2013 (solid) and 1977 (dashed), during spring tide. The date each site was surveyed is noted at the top of the figure (this took place on two spring tides). The tide range at Brest was 6.4 m in 2013 and 6.7 m in 1977. The 1977 data were redrawn from Bassoullet (1979). Salinity data are expressed according to the practical salinity scale in 2013 and parts per thousand from 1977.
Longitudinal profiles of salinity during low discharge and high tide, in 2013 (solid) and 1977 (dashed), during spring tide. The date each site was surveyed is noted at the top of the figure (this took place on two spring tides). The tide range at Brest was 6.4 m in 2013 and 6.7 m in 1977. The 1977 data were redrawn from Bassoullet (1979). Salinity data are expressed according to the practical salinity scale in 2013 and parts per thousand from 1977.
Near-bottom SSC during spring tide and average river discharge (28 m3 s−1), measured at Sites 3 and 3B. Data from Site 3 were collected 26 February 2014, and data from Site 3B were collected 3–5 May 1977 (Table 1). SSC data are from 1 m above bottom in 1977 and Bin 1 of the AWAC in 2013 (1 m above bottom).
(a) Results of bathymetry comparison between 1977 and 2013. The thin gray transect lines (Lines 1–21) represent data that could not be used. The thick gray bars indicate the zones of shallowing and deepening relative to 1977. (b) Measured bathymetry for Line 25 in 1977 and 2013. (c) Measured bathymetry for Line 37 in 1977 and 2013. (d) Measured bathymetry for Line 47 in 1977 and 2013.
