Erosion in Coastal Seas and Navigable Rivers

Erosion in Coastal Seas and Navigable Rivers

 

Project leader: prof. Vlado Malačič, PhD (NIB)

 

Partners: Boris Petelin, PhD (NIB),

                 Branko Čermelj, PhD (NIB),

                 Tihomir Makovec (NIB),

                 Milijan Šiško (NIB),

                 prof. Vlado Malačič, PhD (NIB),

                 prof. Matjaž Četina, PhD (UL FGG),

                 prof. Dušan Žagar, PhD (UL FGG),

                 Gorazd Novak, PhD (UL FGG)

 

Code: J2-9444

 

Duration: 09.2018 - 08.2021

 

The authors acknowledge the basic project J2-9444 was financially supported by the Slovenian Research Agency.

 

Abstract

 

In coastal seas, as well as in navigable rivers, the issue of erosion of the bottom of water bodies or their banks is one of the most important challenges in coastal and riverine engineering. The basic principles of erosion of a substrate over which the volume of fluid (water) flows have long been known and are grounded on a surpassing of the bottom shear stress, a critical bottom stress value. The former is caused by the vertical shear of the velocity of currents and waves while the latter depends heavily on the nature of the substrate.

 

Three major possible mechanisms by which the moving vessels may erode sediments are: 1. erosion due to changes of a flow field by a ship’s movement in the absence of propeller’s propulsion 2. erosion due to propeller wake that reaches the sea bottom or near-shore area 3. erosion due to ship waves that reach near-shore zone. Only the first two mechanisms will be considered in the project.

 

We intend to complement field observations with state-of-the art equipment, e.g. Laser In-Situ Scattering and Transmissometery (LISST) instruments to measure the size distribution of suspended sediments (and their falling speeds)[1], and acoustic probes for the measurement of near-bottom turbulence[2] and for acoustic current meter profilers (ADCP), with the addition of turbidimeters. The microstructure probe[3] will make available vertical profiles of density and turbulence. Field observations will also be complemented by numerical modelling of fluid motion and sediment transport in the northern Adriatic[4]. Analytical expressions will also be utilized appropriately.

 

The original method of estimating bottom stress under the direct influence of vessel passage over (nearby) a spot at depth will be applied. There have been numerous research papers and reports stimulated by significant damage on moored vessels from ship passage. To our knowledge, studies relating a velocity (near) field around the vessel to bottom stress are extremely scarce, so this present study will thus be novel. There have been many studies concerning propeller wash, relating the propeller’s vorticity trail behind a ship, considered a jet, to bottom stress. Two aspects will be new in this study: First, the numerical merge of a narrow jet structure with an ambient velocity field due to circulation, which is feasible along the vessel’s path using the adaptive mesh concept, or the one-way nesting concept within the fixed ‘hot-spot’ area. A second topic is more a conceptual one: an in-depth study should remove doubts concerning tangential (and radial) velocity components in intertwined swirling vorticity trails, generated by each of the propeller blades, on bottom stress.

 

The proposed project will explore important mechanisms of ship-generated erosion of sediments on the sea. This research of the effects of ship motion will select from three major mechanisms the most important ones that matter at a specific site.

 

[1] http://www.sequoiasci.com/product/lisst-100x/
[2] http://www.nortek-as.com/en/products/velocimeters/vector; http://www.fondriest.com/pdf/sontek_adv_spec.pdf
[3] http://www.sea-sun-tech.com/marine-tech/offshore/mss-microstructureprobe/mss-90-profiler-microstructure-probe.html
[4] http://www.ccpo.odu.edu/POMWEB/; http://www.hydroqual.com/ehst_ecomsed.html

 

Relevance to the development of science

 

State of the art field observations, numerical simulations and hydrodynamic analysis of three key mechanisms (bypassing of vessels, the propellers jet during manoeuvring, arrivals to and departures from ports/berths) that affect erosion of shallow bottom will contribute to the development of engineering sciences in the field of environmental fluid dynamics. We will also introduce novel methods in the observation and analysis of these mechanisms. The assessment of their relative impact on the erosion in specific coastal zones will represent a significant contribution to science.

 

Here we give one of the findings that the signal on the seabed in the passage of the vessel is essentially due to the travel of a solitary nonlinear wave. The lecture given by dr. Vlado Malačič in 1997 at the International Center of Theoretical Physics (ICTP) in Trieste can be found here - a scan of the original lecture and a renewal of original.

 

Researchers - link to database SICRIS.

 

Information about the project - link to database SICRIS.