Attenuating Waves with Kelp Farms

Project Description

Coastal cities are vulnerable to risks associated with storms. Typical storm surge and wave damage includes beach erosion, flooding, and impairment of coastal infrastructure. Often hard engineering structures are used to mitigate these influences, though they have adverse impacts to adjacent coastlines and can be viewed unfavorably by society. Soft (or green) engineering structures are a viable alternative, typically in the form of shorelines buffered with vegetation (i.e. living shoreline). The goal of the project is to explore the use of seaweed aquaculture farms to attenuate storm driven waves and assess if they can serve as a coastal defense alternative. This goal will be achieved through an investigation into current erosion mitigation standards, as well as laboratory experiments, field measurements, and the development of innovative analytical and numerical modeling tools.

A three layer, generalized analytical model has been developed to predict how bottom-rooted, suspended, and floating vegetation attenuates wave energy from pure wave and wave/current conditions. This analysis provides a preliminary understanding as to how farm parameters influence wave attenuation. Key parameters influencing the siting and orientation of kelp farms acting as wave attenuators have been identified. Farm characteristics will be assessed to yield the greatest mitigation potential. A 2D cable model was developed to predict how a single vegetation blade sways under wave action. This 2D model was extended to 3D in order to predict how a suspended kelp farm will move under a wavy environment. As a next step, the 3D kelp model will be coupled with a computational fluid dynamics model to simulate coupled fluid-structure interaction. The model will simulate the dynamic response of the kelp farm under wave and current action and the feedback between the kelp farm and the hydrodynamics of the surrounding flow field. In order to accurately incorporate kelp into the numerical model, geometric and material properties of kelp are needed to understand how these structures interact in a fluid environment.

Since the morphology of kelp is site-specific and highly dependent on species, the approach described represents an initial effort to understand the wide range of geometric and material characteristics that may exist. Since macroalgae is known for its compliance, it is necessary to examine the flexural rigidity of kelp as the product of the modulus of elasticity (E) and the second area moment of the cross-section (I). The flexural rigidity (EI) of the macro-algae samples are determined from performing cantilever beam tests. An initial set of tests was conducted with multiple samples. The geometric and material properties of the kelp are used as inputs to the numerical models that are being developed. These characteristics are used to build physical models for tank tests under various wave conditions. Tank test experimental results are compared with numerical simulations for validation purposes.

Project personnel deployed instrumentation at Isle au Haut, ME, which is an island community located at the mouth of Penobscot Bay. South winds in this area have a large fetch and can build up waves to over a meter in height. Researchers deployed four lines at the site of a future kelp farm with instrumentation measuring current speed, wave speed, and wave height. Wind speed and direction will be collected from a local personal weather station. Kelp was seeded on the lines over the winter of 2017-2018 and the same sensors will be deployed after a grow-out period to determine the effects of the kelp curtain on these parameters at this location. These observations will provide direct attenuation rates across the farm, which will be used to compare model results.

Results and Accomplishments

The main findings during 2017-2018 were: (a) seaweed aquaculture farms have the potential to reduce wind wave energy by 30% to 50% under storm conditions, and (b) wave attenuation by offshore, suspended longline, aquaculture farms are not affected by rising sea level, such as resulting from high tide, storm surge, etc. Traditional living shoreline vegetation is bottom rooted, and the effectiveness on wave attenuation decays during large storm-tides.

Researchers developed a numerical model that can effectively simulate the whip-like motion of vegetation blades in waves, and determined the mechanisms for the asymmetric swaying of the blade under symmetric wave actions is due to spatial asymmetry in wave orbital velocities and the interaction between the deformation of the blade and the vertical component of the wave orbital velocity.

These results suggest that kelp aquaculture farms could be a sustainable measure to stabilize a shoreline from storm wave damage. These findings provide a first indication that kelp aquaculture farms can be used as a supplement to existing living shorelines, suggesting they may be a tool to stabilize an eroding shoreline. They are still effective in large storm tide conditions, during which bottom rooting shoreline techniques become less effective.

Summary of Data Being Collected

Data	Type	Quantity	Location
Seaweed Young’s modulus E	Physical property	Sufficient sample measurements to achieve statistical certainty	UNE
Second moment of the cross-section area of seaweed	Geometrical Character	Sufficient sample measurements to achieve statistical certainty	UNE
Field sensor data	Physical wave data, wind, line stress	Every 2 seconds	Isle au Haut, Maine
Kelp growth	Length	Monthly during growing season	Isle au Haut, Maine