Protocols: morpho-bathymetric and morpho-sedimentary mapping and biological inventory management

Avant-propos

Foreword

These protocols aim to provide:

  1. standardised implementation of operations at sea related to initial baseline and follow-up studies;
  2. a guide to facilitate issuing guidance and recommendations provided by Ifremer to public decision-makers (ministries, prefectures, decentralised State services) and thus harmonise responses on a national scale.

The initial baseline and follow-up studies of a marine extraction site should include, as a minimum, the following:

  • Morpho-bathymetric mapping;
  • Morpho-sedimentary mapping;
  • A biological inventory.

1 - Morpho-bathymetric mapping

The bathymetric survey is carried out using a well-calibrated, accurate single-beam sounder (capable of measuring changes in level of 30 cm); the objective is to have reliable measurements for comparisons with subsequent survey(s) and for establishing differentials.

The size of the areas authorized for extraction is currently in the order of a few km², the scale 1/5 000 (five thousandth) is the relevant scale for the bathymetric map. This requires a spacing of profiles of 50 m, to which will be added perpendicular routes spaced by 150 m. It will therefore be possible to check, at the crossing points of profiles and after reduction of the observed tide, the similarity of measurements obtained.

To obtain soundings related to the hydrographic zero, correction of recorded bathymetric measurements is made using the observed tide (predicted tide to be avoided), available in the nearest port, taking into account, if necessary, the time and height correction for the attached port.

The immersion of a tide gauge on site during the operation is recommended.

Three maps are produced:

  • A map of sounder positioning (bathymetric minute);
  • An isobath map (equidistance 0.5 m);
  • A map of differentials (for monitoring purposes).

Nota: The type of projection, ellipsoid and geoid models used for the maps must be specified. The bathymetric survey will be carried out at the same time as the morpho-sedimentary survey.   

2 - Morpho-sedimentary mapping

The morpho-sedimentary survey uses side-scan sonar. The frequency of the sonar is defined for the finest coverage of the different sedimentary facies.

Spacing of routes (profiles) must allow for significant overlay of the sonograms, in the order of 10%, between two neighbouring passages to guarantee complete “sonar” coverage.

This survey is supplemented by sediment samples taken from the grab (see § 3) in order to associate the lithological nature and particle size class with each acoustic facies detected by the side-scan sonar. The location of the sampling points, inside and outside the perimeter, will be determined after analysis and examination of the mosaic of "sonar" images.

The points outside the area will be used as control stations, for comparison during subsequent monitoring.

Two documents are produced:

  • a morpho-sedimentary map, established at a scale of 1:5,000, showing the distribution and nature of the superficial formations (mud, sand, ...), and the associated morphology (megawrinkles, ribbons, slugs, ...) witness the mobility of sediments;
  • Differential map (as part of tracking).

Nota: The type of projection, ellipsoid and geoid models used for the maps must be specified. 

Production of the sonar mosaic image is recommended. The morpho-sedimentary survey will be carried out simultaneously with the bathymetric survey.

3 - Biological inventory

The inventory of benthic macrofauna and macroflora, in and around the area concerned (see § 4), aims to determine biological diversity and abundance and monitor their evolution during exploitation.

The sampling stations will be scattered throughout the different morpho-sedimentary strata defined from the sonar mosaic image. As far as possible, underwater video surveillance will complete the morpho-sedimentary survey and identify the largest epibenthic animal and plant species.

The number of sampling stations and their distribution will depend on observed heterogeneity; this number will necessarily be smaller in the case of homogeneous seabed morphology. These stations will be coupled to the sediment sampling points.

Additional stations may be installed.

Sampling uses a grab suitable for fauna samples. We recommend the following brands of grab:

  • Smith-Mac Intyre Grab, sampling area: 1/10 m2 
  • The Hamon grab, sampling area: 1/4 m2

Nota: Van Veen or Shipeck grabs, for chemical and particle size analysis, should not be used for biological sampling.

At each sampling point, a minimum of four replicates (Smith-Mac Intyrer) or two when using the Hamon model are recommended to account for the distribution of species at a given point and the distribution of larger species.

Depending on the sediment nature, samples are sieved on a 1 mm (preferably round) mesh sieve for fine or silted sediments, or 2 mm for coarse sediments. The reject, fixed to formalin (diluted to 5 %), will be analysed including:

  • Organic sorting;
  • A species-level determination for the most characteristic individuals of the site, including species of commercial interest;
  • Gender determination for other species;
  • Enumeration of identified individuals.

 Results are presented as follows:

  • tables showing the geographical position of the station, the sounder (i.e., reduced water depth at the observed tide), the sedimentary surface (biotope), the number of individuals per m² and the standard deviation for each sample point, identification of species of commercial interest;
  • small-scale maps of the quantitative distribution of dominant and commercially relevant species;
  • a synthetic map of the major bio-sedimentary units.

4 - Scope covered by the surveys

For substantive understanding of the evolution of the seabed over time (objective sought through the initial reference state and follow-up studies), bathymetric and morpho-sedimentary surveys (carried out simultaneously) incorporate, at a minimum, a 500 m wide strip around the site of operation.

In the case of sites operating in the vicinity of sensitive coastal areas, the perimeter of surveys may be extended.

5 - Positioning and navigation

A precise navigation system is essential for positioning the routes and sampling points, to guarantee the recurrence of measurements and observations during subsequent surveys and studies.

Currently, the differential GPS is operated with accuracy of a few meters. The now-available kinematic GPS reaches an absolute horizontal centimetric and vertical sub decimetric precision.

6 - Summary of documents 

  1. Map of routes and sampling stations (sediment, fauna and flora);
  2. Sounder position map (bathymetric minute);
  3. Isobath map (equidistance 0.5 m);
  4. Map of differentials (for monitoring purposes);
  5. Morpho-sedimentary map;
  6. Map of morpho-sedimentological differentials (for monitoring purposes);
  7. Quantitative distribution maps of dominant and commercially relevant species;
  8. Synthetic map of the main biosedimentary units;
  9. A report describing the tools used (bathymetric sounder, side scan sonar, grab, navigation system, tide gauge...), conditions of implementation, computer methods for establishing differentials, results obtained (bathymetry, morpho-sedimentology, fauna and flora), analysis and interpretation of results (comparison of sedimentary evolution/faunistic evolution);
  10. Optional: a mosaic sonar image.

7- Frequency of monitoring operations

The evolution of the seabed affected by the exploitation of marine materials is controlled by the combined action of natural phenomena and anthropic phenomena specific to exploitation.

To differentiate the respective actions of these two types of phenomena, and to observe the resulting evolution as objectively as possible, without the preponderance of sudden meteorological episodes, for example, a sufficiently long period is essential for quality results.

We therefore propose that monitoring operations be carried out at five-yearly intervals.