Drag of Clean and Fouled Net Panels – Measurements and Parameterization of Fouling

Abstract

Biofouling is a serious problem in marine aquaculture and it has a number of negative impacts including increased forces on aquaculture structures and reduced water exchange across nets. This in turn affects the behavior of fish cages in waves and currents and has an impact on the water volume and quality inside net pens. Even though these negative effects are acknowledged by the research community and governmental institutions, there is limited knowledge about fouling related effects on the flow past nets, and more detailed investigations distinguishing between different fouling types have been called for. This study evaluates the effect of hydroids, an important fouling organism in Norwegian aquaculture, on the forces acting on net panels. Drag forces on clean and fouled nets were measured in a flume tank, and net solidity including effect of fouling were determined using image analysis. The relationship between net solidity and drag was assessed, and it was found that a solidity increase due to hydroids caused less additional drag than a similar increase caused by change in clean net parameters. For solidities tested in this study, the difference in drag force increase could be as high as 43% between fouled and clean nets with same solidity. The relationship between solidity and drag force is well described by exponential functions for clean as well as for fouled nets. A method is proposed to parameterize the effect of fouling in terms of an increase in net solidity. This allows existing numerical methods developed for clean nets to be used to model the effects of biofouling on nets. Measurements with other types of fouling can be added to build a database on effects of the accumulation of different fouling organisms on aquaculture nets.

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“…Gansel, McClimans, and Myrhaug (2012) conducted physical-model tests on six cy lindrical nettings using PIV technique to study the average flow fields around net cages with an attempt to analyze the biofouling effect. Gansel et al (2015) measured drag forces on both clean and biofouled nettings in laboratory experiment and the relationship between net solidity and drag was assessed. Lader et al (2015) performed field tests to investigate the growth characteristics of hydroids growing on a net and conducted laboratory experiments to study the hydrodynamic drag on the fouled twines by using fabricated models of net twines with artificial hydroid fouling.…”

Drag on Nets Fouled With Blue Mussel (Mytilus Edulis) and Sugar Kelp (Saccharina Latissima) and Parameterization of Fouling

“…Marine biofouling, the undesirable accumulation of organisms on submerged surfaces, has a number of adverse effects in marine aquaculture, including reduced water exchange across nets and increased net drag [1][2][3][4]. Shortly after submergence of nets fouling organisms settle on the surface, and, given suitable conditions, both mobile and sessile biofouling can then build up very fast.…”

“…Consequently, biofouling also affects the loads on moorings, it can cause net deformation and it can dramatically change the hydrodynamics around and inside fish cages (e.g. [13,14], reviewed in [2,4,15]). Potential effects of the accumulation of biofouling should therefore be considered when evaluating water exchange across aquaculture nets and when calculating hydrodynamic forces on cages and moorings.…”

“…However, diverse and complex fouling communities complicate such an evaluation, and while methods exist to quantify overall fouling type and amount, and net aperture occlusion, information on hydrodynamic effects of specific fouling types on nets is scarce. In order to develop better models describing the hydrodynamic effects of net fouling and to build a better basis for the implementation of net fouling into regulations and protocols for marine fish farms like the Norwegian Standard NS9415 [16] or NYTEK-forskriften [17], hydrodynamic effects of important growth types should be investigated ( [1,4]). Fouling communities on aquaculture sites are often dominated by mussels, hydroids, ascidians and algae (e.g.…”

“…These organisms differ in size, form and flexibility and will affect the flow past nets differently. [4] investigated the effects of hydroid fouling on aquaculture nets, and the authors suggested a method to parameterize fouling to allow the use of existing clean net models to describe effects of fouling on nets. This study aims to 1) quantify the effects of different degrees of mussel and seaweed fouling on aquaculture nets on net drag and to 2) parameterize these effects so that existing numerical models can be used to evaluate the effects of mussel and seaweed fouling.…”