Based on the simple calculation rules for the computation of depth-averaged velocities and time-averaged bed shear stresses recommendations can be obtained for a minimum excavation depth of ecological sand extraction sites along the Dutch coast. Given a pre-extraction water depth and depth-averaged peak flow velocity, the minimum excavation depth resulting in time-averaged bed shear stresses of about 0.4 N m-2 in the bottom of the sand extraction site can be determined from figure 5. This figure gives minimal excavation depths (resulting in time-averaged bed shear stresses in the bottom of the pit of about 0.4 N m-2) as a function of water depth for depth-averaged peak flow velocities of 0.55, 0.65 and 0.75 m s-1. Given the extraction volume, the length and width of the ecological mining pit can be chosen subsequently.
Figure 5: Left panel: Data on infauna (biomass in g m-2, number of species and dominant species), epifauna (biomass in g m-2, number of species and dominant species), fish (biomass in g m-2, number of species and dominant species), sediment (grain size in µm, volume percentage of fines) and extraction depth (0, 2, 8, 20, 24 m below the seabed). The red line is the estimated bed shear stress of the case studies 1: 20 m deep regions without sand extraction, 2: shallow sand extraction (2 m) near barrier island Terschelling, 3: intermediate deep sand extraction (8 m) in the turning basin of the Euromaasgeul shipping lane, 4: deep sand extraction (20 and 24 m).
Right panel: Calculated extraction depths needed to reach bed shear stress values of each case study (2 m: 0.41, 8 m: 0.17, 20 m: 0.08 and 24 m: 0.04 N m-2), for areas with different depth averaged flow velocity (solid lines: 0.65, large-dash lines: 0.7, intermediate- dash lines: 0.75 and small dash lines: 0.8 m s-1) and initial water depth at the x-axes (15, 25, 30, 35, 40 m).
With the design graph presented in figure 5, the minimum excavation depth of can easily be determined. The total area of the mining pit can then be computed by dividing the required sand extraction volume by the excavation depth. From the total area of the sand extraction site, the length and width of the extraction site can be selected based on available space and restrictions.
- Step 1: Determine the water depth and depth averaged peak-flow velocity in the extraction area. Along the Dutch coast, the depth-average peak flow velocities (M2 velocity amplitudes) are between 0.55 and 0.75 m s-1 (Fig. 12 of Tool – Ecosystem-based Design Rules for Sand Extraction Sites)
- Step 2: Determine the minimum excavation depth using the design graph (Fig. 5).
- Step 3: Compute the surface area of the mining pit by dividing the required extraction volume by the determined sand extraction depth. From the total surface area of the sand extraction site, the length and width of the site can be selected based on available space and restrictions.