Draft review on

Unconfined aquatic placement is the proposed method in the application. It is a reintroduction of dredged sediments in the water system from which they originate such that the returned sediments become again part of the natural sediment cycle. This involves placement of sediments on flat or gently sloping waterbed in the form of a mound (see Figure 3A). The placement sites can be dispersive or non-dispersive (retentive) depending on whether the sediment is transported out of the site by currents and/or wave action (Bray 2008).

While the type of placement influences the long-term fate of dredged sediments in the receiving environment several short-term behaviours occur during and shortly after sediment discharge. These are convective descent, collapse and mound formation, and passive dispersion.

Convective decent is describes the fall of the concentrated sediment cloud (Bray 2008). The sediments fall as a high-density plume with nearly all sediments contained within the cloud; descent velocity is determined by negative plume buoyancy, drag and momentum (ERDC 2008). Some sediment enters the water column, the exact quantity depends on sediment type, plume density, descent velocity and distance between release and sediment bed (ERDC 2008). When the descent plume reaches the bottom it forms a horizontal surge as the vertical momentum is transferred to horizontal momentum called dynamic collapse (Bray 2008). The surge head will slow as it expands across the sediment bed, depositing sediments to the bed and entraining some into the water column due to currents and turbulence cause by the surge head (ERDC 2008). As discharge continues a mound develops, characteristics of which depend upon sediment type. Passive dispersion describes the motion of the sediments entrained in the water column. Advection and diffusion by currents and particle settling govern sediment motion and sedimentation rates (ERDC 2008). Only a small portion of suspended particles, 5-20%, is exposed to currents capable of transporting sediments outside of designated discharge areas (Bray 2008).

Descent characteristics are highly dependent upon discharge characteristics (Figure 2). The material from direct pipeline discharge as in the application is a liquid slurry which may contain clay balls, gravel or coarse sand material. The coarse material quickly settles, while the mixture of process water and fine particles descends to the bed to form a fluid mud mound. Some fine material may remain in suspension as a turbidity plume.

Sediment plume modelling was conducted at the proposed MOF dredge site to evaluate the short- term and long-term distribution of disturbed sediments. Plume modelling of the dredging area by ASL Environmental Services predicted that up to 2 mm of sediment will re-settle in the majority of the surrounding area up to a radius of 3 km from the MOF. Maximum sediment deposition could reach 11.3 mm within 500 m of the dredge site (excluding redeposition at the MOF where dredging occurs) in areas of low water current along Lelu Island. A plume dispersion model for the marine berth dredge area was not available. It is anticipated that the resulting plume range, concentration of suspended solids and subsequent sediment deposition upon completion of dredging activities will be greater.

Longterm behaviour of sediments occurs over months or years and includes mound consolidation, resuspension and erosion, and the transport and deposition of eroded materials. Mound consolidation occurs as the self-weight of the sediments expel pore-water and mound elevation decreases. Reduction in mound size depends upon sediment characteristics; fine-grained sediments can face upwards of a 50% reduction in deposit thickness (Bray 2008).