|Minimising harbour siltation|
MarCom Working Group 102 (2008). Minimising harbour siltation. PIANC Report, 102. PIANC = AIPCN: Brussel. ISBN 2-87223-169-2. 75 pp.
Part of: PIANC Report. PIANC = AIPCN: Brussels, more
Harbours; Siltation; Marine
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This report focuses on fine sediments with emphasis on muddy cohesive material in rivers, estuaries and marine coastal environments. Many millions of cubic metres of such mud are dredged annually from harbour approaches, fairways and basins in order to safeguard navigation. Volumes are expected to increase due to continued economic growth and increases in vessel draught. Dredging is necessary at both large commercial and small craft harbours and its cost can threaten the financial viability of both. Dredging and placement (e.g. upland, off-shore), through induced large-scale unnatural circulation of mud, may exacerbate sediment starvation of aquatic systems, or result in wasteful repetitive return. Owing to dredging costs and environmental impact concerns, there are compelling reasons to seek to reduce dredging volumes by minimising harbour siltation (MHS). This report summarizes old, new, proven and promising methods for MHS. For reasons outlined above, emphasis is placed on methods that keep sediment in the system (KSIS) to the extent practicable.
Harbour basin and channel geometries often create relatively quiescent areas in estuarial/coastal systems that would have remained in equilibrium in the absence of port development. Hence, many ports experience siltation. While complex, harbour basin/channel siltation is governed by basic physical processes. The practitioner should recognize that understanding local sediment regime must always underpin choice of appropriate sediment management systems.
Section 1 of the report introduces the MHS topic. Section 2 discusses harbour basin fairway/channel configurations and includes hydrodynamics, sediment types, sediment sources and physical geometry. Section 3 presents simple formulae, some of which are suitable for engineering applications, in order to facilitate understanding of natural systems. Section 3 also presents the basic physical processes that govern siltation in harbour basins and channels. Regarding harbour basins, siltation is dictated by the following flow mechanisms:
• horizontal entrainment • tidal filling • salinity- (or thermal-) induced density currents • sediment-induced density currents
In contrast, siltation in channels/fairways generally results from the reductions of flow velocities that arise when the channel/fairway is deepened relative to natural (i.e. equilibrium) conditions. Section 4 presents successful and unsuccessful MHS case histories. It summarizes MHS according to the following methods:
1. KSO: Keep sediment out - minimise sediment flux to the harbour area.
2. KSM: Keep sediment moving - minimise sediment settling in the harbour area.
3. KSN: Keep sediment navigable - take advantage of transitional properties of fluid mud or prevent it consolidating so that ships can navigate through the suspension (i.e. nautical depth principle).
The report distinguishes between passive and active measures. The former involves non-mechanical structures (e.g. harbour entrance geometry, training structures, nautical depth, etc.); the latter, mechanical devices (e.g. jet pumps, water injection, etc.). Section 5 summarizes field data collection methods, Section 6 numerical and physical model technology.
The technologies reviewed fall naturally within the deterministic constraints of KSIS and grouped by KSO, KSM and KSN, into 6 categories. These 6, the report’s main findings, are set out in the Conclusion. The most important issue high-lighted by our studies is the high potential for saving and earning for port and shipping operators, coupled with the benefits intrinsic to these sustainable generic sediment management systems.
Though the methodologies described here are generic, as they are based on physical principles, this report is not a recipe book. A proper understanding of the hydro-dynamic and sediment system is essential to develop efficient MHS strategies. Without such understanding measures may cause adverse effects. This is applicable to MHS improvements at existing harbours but especially relevant to planning/siting of new harbours.