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This page was updated on 18th April 2005.

INSSEV is the pseudo-acronym for IN Situ SEttling Velocity instrument. It was developed in the early 1990s to obtain data on suspended sediment near the bed of estuaries. The main reason was because numerical simulation of estuarine sediment transport was unable to resolve the complexities of the settling rate of flocculated particles. Flocs are aggregations of very fine clay particles. Clay mineral particles are typically less than 2 microns; large flocs commonly exceed 1 mm in size and can be composed of 10,000 clay particles. The aggregations become less dense as they become larger and therefore can be broken up by excessive turbulence. Low level turbulence actually assists flocs to grow in size as the rate of interparticle collision increases.

INSSEV is always used in conjunction with an array of current velocity (3 axes) sensors to quantify turbulence at the time of the INSSEV recordings.

INSSEV allows suspended sediment to settle in a specially constructed two chamber system. As the flocs settle they are recorded on video and this provides the data on floc size and settling velocity. By applying equations that incorporate other master variables it is possible to calculate the bulk density of individual flocs. No other in situ instrumentation has managed to gather this type of data for floc sizes down to 7 microns in size. Whole sample calculations allow concentration to be inferred which can then be compared with data from water bottle samples and turbidity sensors.

As well as supplying large data sets for many estuaries in North West Europe, INSSEV has also identified many other features of flocculated suspended sediments. It has long been known that flocs are partly held together by organic compounds such as mucopolysaccharides; these are visible on the video recordings as long stringy flocs. INSSEV has also captured the first ever images of a copepod eating a floc and spitting out the bits it didn't require! It was probably digesting the bacteria and other organic material, then spitting out the clay minerals.

Why can't data on floc size spectra be obtained from samples returned to laboratories? For years this was the main method, until it was realised that the floc size and density spectra changed on sampling and bringing to the surface. The time delay also caused the samples to be organically corrupted. Flocs populations are known to change over time scales of minutes. Only in situ measurement can provide the data on floc populations that is needed to understand estuarine sediment transport.

Because small flocs settle so slowly it has been found that in excess of 95% of suspended sediment (dry mass) reaches the bed of estuaries as large flocs. On occasions this figures exceeds 99%. Hence, it is essential to obtain the floc size and density spectra, which is where INSSEV data are so valuable. Unfortunately, numerical models of estuarine sedimentation are still very coarse in this respect. For years modellers have requested mean sizes and mean settling velocities. This continues today and is the principle reason why estuarine sedimentation is poorly simulated. Attempts have been made to arrive at relationships between size, settling velocity and turbulence. INSSEV data have been used in this context to define such relationships but it is only a partial solution to the spectral problem. There is still a need to examine the way that the morphology of flocculated particles influence rapid changes in the the settling flux of estuarine suspended sediment.

Simplified forms of sediment transport software have now cascaded down to consulting engineers and there is a growing problem of inaccurately simulated estuarine sites associated with large construction projects. Many sites are now assessed with only partial field data, but computer simulation packages are then used to present crisp images of what is supposed to be happening in the estuary. Such presentational packages might appear plausible at planning inquiries, but the consequences for the sustainability of estuaries are alarming.

INSSEV is still in action after 14 years of use. The high resolution camera and computer controlled robotics that operate the hydrodynamic sampling chambers are unchanged. What was built as a second prototype has given many more years of service than most production oceanographic instruments.

Currently, it is expertly operated and maintained by Dr Andy Manning of University of Plymouth. The instrumentation is only available for hire, or in collaborative projects, with Dr Manning as the instrument operator and INSSEV floc data analysyt.
To use it, contact:
Dr Andy Manning       amanning@plymouth.ac.uk       01752 233723
School of Earth,Ocean and Environmental Science
Portland Square A504
University of Plymouth
Drake Circus
PLYMOUTH
PL4 8AA

Numerous papers have been published in international journals. Use a search engine to type in "cohesive sediment fennessy" or "cohesive sediment manning", or any other words like "flocs", "flocculation", "inssev", "settling velocity". These key words should find most of the papers, and others where INSSEV data has been cited.
The following reference explains the construction of the instrument as well as discussing early data sets:

Fennessy, M. J., K. R. Dyer and D. A. Huntley (1994) INSSEV: an instrument to measure the size and settling velocity of flocs in situ. Marine Geology, 117: 107-117.

Fennessy, M. J., K. R. Dyer, D. A. Huntley and A. J. Bale (1997) Estimation of settling flux spectra in estuaries using INSSEV. In: Burt, N., R. Parker and J. Watts (Eds.) Cohesive Sediments, Proc. 4th Nearshore and Estuarine Cohesive Sediment Transport Conference (INTERCOH'94), Wallingford, Wiley & Son, Chichester, pp. 87-104.