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| Titel |
Tidal currents and bedload transport at the mouth of a rock-bound estuary during low river discharge conditions (Guadiana Estuary, Portugal) |
| VerfasserIn |
E. Garel, A. Pacheco, Ó. Ferreira |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029258
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| Zusammenfassung |
| The present study documents the poorly-described hydro-sediment dynamics of narrow
bedrock-controlled estuaries during periods of low-river discharge. The results also contribute
to assess the geomorphological evolution of these systems, when affected by drastic flow
regulation.
The Guadiana Estuary is a narrow rock-bound mesotidal estuary, 80 km in length, located
at the southern border between Spain and Portugal. Until recently, the river inputs to the
estuary displayed high (annual and seasonal) variability, characterized by periods of droughts,
and episodic flood events with (monthly-averaged) fluvial discharge as high as 5,000 m3s-1
(160 m3s-1 in average, for the period 1947/2001). This pattern has ceased in February 2002,
with the impoundment of the main river by the large Alqueva dam, 60 km upstream from the
estuary head. At present, the daily-averaged river discharge is generally kept low throughout
the year (< 50 m3s-1). In the absence of significant flood events to expel massively
sediment out of the estuary, concerns have been raised about sand infilling at the
mouth and increased erosion at the adjacent coastline. For the assessment of the
sediment balance of the estuary under present hydrodynamic conditions, this study
examines the tidal currents and bedload transport at the entrance of the estuarine
channel.
Current measurement transects were performed across the 600 m-wide channel entrance
using a ship borne Acoustic Doppler Profiler (ADP, operating at 1.5 MHz frequency) during 2
entire tidal cycles, at spring (17 September 2008, 3.0 m tidal range) and at neap tide (21
October 2008, 1.6 m tidal range). Surficial sediment samples were also collected across the
channel during the spring tidal cycle. The bed sediment consists of well-sorted medium
sand with mean grain size ranging from 0.5 to 0.3 mm (with coarser material at the
deepest part of the channel cross-section). Tidal currents were analysed along 6
sub-sections to take into account these grain size variations. The friction velocity
and bed shear stress were computed based on the mean depth-averaged velocities
of each sub-sections and considering a power law vertical velocity profile. The
transport rate of sand was then estimated using Nielsen (1992) formula for bedload
transport. The transport of sand in suspension was not considered in the study, as
the skin friction velocities were lesser than the estimated settling velocities of the
grains.
Maximum velocity values (about 1.2 and 0.8 m.s-1 at spring and neap, respectively) were
observed near the surface of the deepest sub-section of the channel. The tidal prism was about
1.5 times larger at spring (39x106 m3) than at neap (25x106m3), whereas the fresh water
inputs during both tidal cycles were comparatively negligible. Maximum depth-averaged, bed
and surface current velocities were ebb-directed at both neap and spring tides, for each of the
6 channel sub-sections. No significant lateral variation of the tidal flow was observed,
in relation with the narrowness of the channel. Vertical residual velocity profiles
were also directed downstream at both neap and spring tide. At neap, however, the
(ebb-directed) residual velocities were slower near the bed and faster near the surface, when
compared to the spring tide. These differences were induced by the reinforcement
of the estuarine circulation, in relation with enhanced stratified conditions during
neap periods (weak currents and reduced mixing). The net bedload transport of
sand was also directed downstream for all channel sub-sections. The transport rates
of the entire channel were estimated to be of about 30 and 10 m3 for the spring
and neap tidal cycles, respectively. Extrapolation of these extreme (i.e. neap and
spring) rates yielded a potential seaward export of sand of approximately 15,000
m3yr-1. This study suggests that the Guadiana estuary departs from typical estuaries
where landward net transport of sediment is generally described. The outputs of
the study are important with respect to the long-term (decades) geomorphological
evolution and sustainable management of the estuary mouth and adjacent coastline. |
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