In October 2016, the LIFE15 ENV/IT/000391 Marina Plan Plus officially started to prevent harbour silting. Co-financed by EASME (Executive Agency for Small and Medium-sized Enterprises), Marina Plan Plus is one of the 15 European projects, selected among hundreds of other projects presented by all European countries, that in 2016 received funding in the Water Sector.
Trevi, world leader in subsoil engineering, is the coordinator of the research team that also includes Bologna University, Cervia Municipality and ICOMIA (International Council of Marine Industry Association).
The Project received € 1.5 million funding from Easme, it will last 39 months and will be completed by December 31, 2019.
The main objective of the Research Team is to improve easy access to Cervia Harbour by keeping the seabed at the harbour’s entrance at an optimal depth, thus allowing navigation of incoming and outgoing vessels.
To achieve this goal, many activities – such as lab tests, bathymetries and sediment analyses – will be carried out in order to install, within one year, an experimental plant specifically designed to keep seabed’s depth constant at the harbour’s entrance. Thanks to its features and size, said experimental plant will have no impact on navigation through the harbour’s entrance, as well as on wharf activities.
State of the art
The dredging process involves the removal of sediment in its natural deposited condition by using either mechanical or hydraulic equipment:
− Hydraulic dredging: removal by cutter heads, dustpans, hoppers, hydraulic pipeline, plain suction and side casters, usually for maintenance works. Sediment is removed and transported in liquid slurry form.
− Mechanical dredging: removal of loose or hard compacted materials by clamshell, dipper, or ladder dredges, either for maintenance or new-works. Mechanical dredges remove bottom sediment through the direct application of mechanical force to dislodge and excavate the material at almost in situ densities.
After the sediment has been excavated, it is transported to the placement site or disposal area. Water quality at the dredging and disposal sites is a particularly important consideration in the choice of dredging equipment. Hydraulic dredging can limit disturbance and resuspension of sediments at the dredging site, and it is often the first choice when dredging occurs in enclosed waterbodies or in locations near aquatic resources that would be sensitive to increases in suspended solids or turbidity. However, since hydraulic dredging entrains additional water that is many times the volume of sediment removed, water management and water quality must be controlled. In contrast, mechanical dredging creates little additional water management concern at the disposal site; therefore mechanical dredging is the first choice when disposal site capacity limitations are a primary concern. However, mechanical equipment often creates more disturbance and resuspension of sediments at the dredging site. Dredging operation negative impacts are:
− Impact on water quality: dredging affects turbidity, suspended solids, and other variables that affect light transmittance, dissolved oxygen, nutrients, salinity, temperature, pH, and concentrations of trace metals and organic contaminants if they are present in the sediments.
− Impact on sediment: increased post-dredging sedimentation in the newly deepened areas for new work projects, local changes in air-water chemistry, and possible slumping of materials from the sides of the dredging areas.
− Resuspension of contaminants: dredging will re-suspend contaminants if contamination is present in the surface sediments. Dredging of contaminated sediments does present the potential for release of contaminants to the water column and for the uptake of contaminants by organisms.
− Impact on biological resources: Short-term impacts include local changes in species abundance or community diversity during or immediately after dredging. Long-term impacts include permanent species abundance or community diversity changes. Direct impacts would be directly attributable to the dredging activity, such as a direct loss of mudflat habitat or a temporary turbidity induced reduction in productivity in an eelgrass bed immediately adjacent to a dredging site.
In the EU, approximately 140 million tons (dry weight) of dredged material is disposed of in coastal areas every year. The impacts that dredged material can have on the seafloor are diverse, ranging from physical differences in sediment structure to significant reductions in the numbers of species that live there since disposal of dredged material on the seabed can disrupt sediment-dwelling animals, with potential knock-on effects further up the food chain.
Need for improved operations and maintenance techniques and equipment for sediment bypassing is a profitable sector for research and innovation. Sand bypassing systems have been created to artificially bypass the littoral drift. Sand bypassing systems realize a strategic approach, which is different from dredging: dredging removes sediment once critical conditions are reached, while the sand bypassing systems perform sediment removal continuously, without reaching, therefore, critical conditions for navigation. Different systems have been developed around the world. Among the other technologies, jet pump has a great potential as primary component in sand bypassing system, since it requires limited personnel, is able of great portability and can be assembled at reasonable cost: moreover, the technology is reliable since has been applied starting from 1976 for coastal application. A jet pump is a device that transfers momentum from a high speed primary jet flow to a secondary flow. The primary jet flow contacts the suction fluid at the nozzle exit and drags it into the jet pump, thus starting up and sustaining the secondary flow of suction fluid from surrounding water mass. If present, solid particles are entrained in the secondary flow, thus being introduced in the mixing chamber, where jet stream and suction fluid are further mixed, exchanging momentum and recovering pressure. The slurry then pass through a diffuser and into a discharge pipe for delivery to a discharge point (or into a booster).
Department of Industrial Engineering (DIN) of Bologna University, in collaboration with Plant Engineering Srl (Bologna, Italy), developed and tested an innovative plant for seabed maintenance characterized by the fact that the main element, called “ejector”, is an open jet pump (i.e. without closed suction chamber and mixing throat) with a converging section instead of a diffuser. Plant Engineering Srl, which is the patent owner, signed an agreement with Trevi for patent licensing and will be involved in the project as technology supplier by Trevi.
The ejector suction effect is due by the behaviour of a fluid jet in free outflow from a hole (nozzle diameter d) towards an open environment. A jet under these conditions increases its flow, from inlet to outlet section, due to the flow absorbed within the jet itself from the surrounding environment: the high velocity of the jet creates a low-pressure area out of the nozzle leading the pumping of the second flow toward this minimum pressure point. Consequently, there is an exchange of momentum between the two streams resulting in a uniform mixed stream flowing at an intermediate velocity between the primary and secondary flow ones. The ejector is used as a fixed device placed on seabed and works on a limited area whose diameter depends on the sediment characteristic as, for example, the angle of repose. By ejectors integration in series and in parallel it is possible to create a seaway. Each ejectors suck a mixture of water and sediment whose composition depends on the geometrical characteristics of the ejector (in particular the diameter d of the nozzle), the water feeding flow rate and the characteristics of the sediment and seabed.
The plant is designed to realize a low solid/liquid mixture, normally between 1-5%, thus resulting in a very low concentration of solid. As a result, no turbidity or resuspension is produced by plant operating, both near the ejectors and at discharge pipeline outlets. Plant discharge pipeline outlets are placed in favour of marine current to enable a natural removal of the sediment. Therefore, the ejectors simply move the sediments which are naturally transported by marine currents from a critical position for the maintenance of a certain water depth to a another one in which sediments can be resumed by the same current to be taken elsewhere or otherwise in a position where sediments does not constitute obstruction to navigation. The plant operates with a zero mass balance since the sediments that is transported by marine current is the sediments that the plant moves away from port inlet.
The plant works continuously (24 hours a day and 7 days a week), so it can guarantee navigability throughout the year. Finally, the plant can be used as a dredge substitute also in the sediments removal operations, which foresee sediment treatment. Several technologies has been developed for sediments treatment, including contaminated ones, like soil washing or wet oxidation, and recovery, like the technology developed within the LIFE project SEDI.PORT.SIL, where a technology for silicon extraction from dredged sediments has been tested. The plant can be easily integrated with these kinds of technology: the sediments are more dilute than the one produced by dredging but the continuous working allows reducing treatment plant size.
Two pilot plants have been realized and tested in the past:
− The first one (co-financed by Emilia-Romagna Regional Programme PRIITT 2003-2005) foresaw the realization of a first experimental plant in Riccione (Italy);
− The second one (co-financed by Emilia-Romagna regional operative programme POR-FESR 2007-2013, “Support for collaborative research projects of SMEs with research laboratories and centers for innovation”) foresaw the realization of a small-scale prototype plant in Portoverde (Italy).
The first plant (2005) was designed and build only for ejectors testing. This experimental plant worked all Summer long and guaranteed a water depth at the Riccione port inlet of about 3 meters. The second plant (2012) was the first small-scale industrial application. After some improvements both on plant setting and on ejector geometry, the plant worked continuously from September 2013 to April 2014 and guaranteed more than 2 meters of water depth at the Portoverde port inlet. The second plant was characterized by higher automation grade, lower management costs and lower energy consumption (about 40%) than the first one. Starting these experiences, MARINAPLAN PLUS aims to realize a first demo industrial plant. The large-scale industrial dimension is important to assess operating and management costs as well as for monitoring plant impacts on marine flora and fauna and on undersea noise. The demo plant will be realized in the Port of Cervia, Italy.