The basis for an extraction design is the amount of marine sand needed and the quality. Other practical considerations are also important, for instance legislation, economical aspects (sailing distance) and the method of extraction. This leads to a reconnaissance of potential areas that can provide these conditions. The selected extraction site can be characterized by location (orientation, depth), hydrodynamics (waves, currents), sediment (transport, -type, -distribution, percentage organic matter, stratigraphy), morphology (sand waves or sand ridges) and ecology (assemblage). Given the characteristics of the area, a preliminary design of the ecosystem-based sand extraction can be determined, considering a number of technical, physical, and ecological parameters. The life span of the design, the costs for implementing, safety issues and potential of ecosystem-based sand extraction should be determined .

Figure 4: Flow chart showing the process used to accumulate requirements for designing the bed forms at existing extraction sites.

 

 

Technical parameters

Trailer Suction Hopper Dredgers (TSHD's) are the most efficient tool for sand extraction and they vary in size and capacity. To avoid unnecessary high cost and loss of time, several technical parameters have to be taken into account when designing an ecosystem-based extraction site.

 

Technical parameter

Description

Reference

Dredging depth

The maximum dredging depth of the TSHDs should be considered to avoid excessive cost and loss of time. Only a few TSHDs can create deep troughs.

Rijks, 2011

Manoeuvrability

The manoeuvrability of the ships to create specific shapes should be considered to avoid excessive cost and loss of time. The sharper the shapes, the more time it will take to create.

Rijks, 2011

Capacity

The capacity of the ships and the specific sailing lengths to fill the hopper should be taken into account to avoid excessive cost and loss of time. The smaller the shapes, the less likely TSHDs will reach an optimal dredging volume during one track and thereby increasing dredging effort and cost.

Rijks, 2011

Hydraulic conditions

For economic and technical reasons shapes should preferably not lie perpendicular to the main current directions as ships and dredging operations are limited under these circumstances, increasing time and costs.

Rijks, 2011

 

Physical parameters

Physical parameter

Description

Reference

Location

The location determines the characteristics of the extraction site including hydrodynamics, metocean and geotechnical conditions, sediment transport, morphology and ecology.

Rijks, 2011

Shape

The shape of the extraction site is often determined by other user functions. The shape will determine what kind of ecosystem-based bed form design can be planned in the extraction site.

Rijks, 2011

Sediment

The extraction site needs to contain sufficient sand of the desired grain size. The natural variation in grain sizes and percentage fines can be used in the creation of different landscapes and related ecological habitats. If needed, overflow from dredging equipment can be stimulated to ensure that more fines are left behind in certain areas for the development of specific habitats.

Rijks, 2011

Morphology

The occurrence and dimensions of naturally existing morphological bed forms in the wider extraction area can be copied in the extraction site although they may need another orientation with regards to the tidal current.

Rijks, 2011

Infrastructure

The presence of wrecks, cables and pipelines determine the shape of the extraction site and ecosystem-based bed forms.

Rijks, 2011

 

Ecological parameters

Ideally, the dredged seabed has variation in substrate composition (grain size, mud and organic matter content)and containing series of bed forms with variation in steepness and orientation with regards to the tidal current. Each combination will be tailor made to create a certain ecological habitat. This means that the relation between seabeds, via hydrodynamics and physics, and ecology must be clear when determining the goals of ecosystem-based landscaping. Knowledge on the relationship between marine organisms and sediment characteristics, water depth, hydrodynamics, sediment transport and morphology is the subject of a PhD study within the Building with Nature programme. Preliminary results indicate that the sediment characteristics and time-averaged bed shears stresses are important parameters determining ecological response and therefore of use in the design of ecosystem-based seabeds (De Jong et al. 2015).

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