Properties of macroflocs in the lower reaches
of the Gironde estuary
Andrew James Manning, Keith Richard Dyer, Malcom Christie
University of Plymouth, Institute of Marine Studies, Drake Circus, Plymouth PL4 8AA,
United Kingdom
amanning@plymouth.ac.uk
Introduction
Floccuktion is a dynamicallv active process which is directly affected by the
environmental conditions. This results in a continual process of aggregation
and disaggregation, and hence a continual change in floc properties. Turbulent shear within the water column has been identified as a prime contributor to particle collisions, but very little work has quantified its influence
on in situ floc formation (Manning & Dyer, 1999; Dyer & Manning, 1999).
An important lacet of cohesive sediment flocculation is its ability to continually alter the depositional characteristics of the particles in suspension.
Eisma et al. (1990) advocated that the population of larger aggregates,
refereed to as macroflocs (>120 pm), tended to have the main influence
on the mass settling flux within an estuary. However, the use of instruments such as “Owen tubes”, optical particle sizers, and in situ photography has provided very little information to date on the varying composition of these macroflocs wtithin different coastal estuaries.
Results are presented from a joint field experiment, which formed part of
the EC TMR SWAMIEE (Sediment and water movement in industriaüsed
estuarine environments) project. It was conducted at the seaward end of
the fluvial limit region of the Gironde estuary near Le Verdon, in France,
during which an unintrusivc in situ video camera svstem was utilised to
measure the flocs. This paper produces a detailed examination of the floc
population from a single floc sample.
Material and method
Flocs were examined using the new instmment Inssev (Jn situ settling velocity; Fennessy et ai, 1994; Manning & Fennessy, 1997). This is a microcomputer controlled two-chamber floc sampling system, which utilises an
underwater video camera and recording unit. The instmment was adjusted to sample flocs (1.6 metres above the sea bed. Tltis is due to the
velocity gradients generally being the largest in the near bed region (1020% of the water column depth), and approximately 80% of the turbulent energv generated bv the water flow occurs within this zone. Mehta
& Partheniades (1975) reported that this is where the strongest lift and
shear forces occur, and these can produce controlling influence on the
maximum floc size.
230
of the Gironde estuary
Andrew James Manning, Keith Richard Dyer, Malcom Christie
University of Plymouth, Institute of Marine Studies, Drake Circus, Plymouth PL4 8AA,
United Kingdom
amanning@plymouth.ac.uk
Introduction
Floccuktion is a dynamicallv active process which is directly affected by the
environmental conditions. This results in a continual process of aggregation
and disaggregation, and hence a continual change in floc properties. Turbulent shear within the water column has been identified as a prime contributor to particle collisions, but very little work has quantified its influence
on in situ floc formation (Manning & Dyer, 1999; Dyer & Manning, 1999).
An important lacet of cohesive sediment flocculation is its ability to continually alter the depositional characteristics of the particles in suspension.
Eisma et al. (1990) advocated that the population of larger aggregates,
refereed to as macroflocs (>120 pm), tended to have the main influence
on the mass settling flux within an estuary. However, the use of instruments such as “Owen tubes”, optical particle sizers, and in situ photography has provided very little information to date on the varying composition of these macroflocs wtithin different coastal estuaries.
Results are presented from a joint field experiment, which formed part of
the EC TMR SWAMIEE (Sediment and water movement in industriaüsed
estuarine environments) project. It was conducted at the seaward end of
the fluvial limit region of the Gironde estuary near Le Verdon, in France,
during which an unintrusivc in situ video camera svstem was utilised to
measure the flocs. This paper produces a detailed examination of the floc
population from a single floc sample.
Material and method
Flocs were examined using the new instmment Inssev (Jn situ settling velocity; Fennessy et ai, 1994; Manning & Fennessy, 1997). This is a microcomputer controlled two-chamber floc sampling system, which utilises an
underwater video camera and recording unit. The instmment was adjusted to sample flocs (1.6 metres above the sea bed. Tltis is due to the
velocity gradients generally being the largest in the near bed region (1020% of the water column depth), and approximately 80% of the turbulent energv generated bv the water flow occurs within this zone. Mehta
& Partheniades (1975) reported that this is where the strongest lift and
shear forces occur, and these can produce controlling influence on the
maximum floc size.
230