Surface signatures of aquatic ecosystems

Characterizing and monitoring the spatial extent, health, and general characteristics of aquatic ecosystems like seagrass beds presents a challenge that evolves both in space (due to fragmentation of these habitats) and time (due to tides, seasons, and ecosystem growth). We are interested in the hydrodynamic signatures that these aquatic ecosystems leave on the water surface, as this may serve as a proxy for characteristics of the system, as well as the flow it is exposed to.

At UNH, we are collaborating with colleague Prof. Gabe Venegas to connect observations of free-surface perturbations over flexible seagrass mimics to acoustic properties beneath the surface, with scattering generated by small bubbles on the model blades (to approximate photosynthetic bubbles in the marine environment). A key component of this work is exploring linkages between free-surface slope and acoustic fields, and the extent to which we can predict the latter based on knowledge of the former.

Ongoing work at UNH is funded by the Office of Naval Research award no. N00014-21-2-2669.

Related publications:
Mandel et al. (2019). On the surface expression of a canopy-generated shear instability. Journal of Fluid Mechanics 867: 633-660 [pdf][doi]
Mandel et al. (2017). Characterizing free-surface expressions of flow instabilities by tracking submerged features. Experiments in Fluids (11), 153 [pdf] [doi]

Plumes and particles in stratification

Plumes and particles in the marine environment can interact strongly with the ambient ocean stratification. We are interested in the trapping of oil droplets and oceanic particles in intrusion layers, as well as the surfacing of subglacial discharge (freshwater) plumes, and how this fundamental problem in fluid mechanics relates to dynamics observed in the field.

Related publications:
Saeed, Z., Weidner, E., Johnson, B.A., and Mandel, T.L. (2022). Buoyancy-modified entrainment in plumes: Theoretical predictions. Physics of Fluids 34: 015122 [doi]
Mandel, T.L., Zhou, D.Z., Waldrop, L., Theillard, M., Kleckner, D., and Khatri, S. (2020). Retention of rising droplets in density stratification. Physical Review Fluids 5: 124803 [pdf] [doi] [data]

Seasonal variation in the biomechanical properties of salt marsh plants

Marsh vegetation, such as Spartina alternifora, reduces flow velocities and helps sediment settle within the marsh. Plant stiffness plays an important role in this process. In coastal flood modeling, if vegetation is parameterized with a single value that is constant and time in space, we may over- or under-estimate the protective benefits of marsh plants.

In this project, we are studying how seasonal plant life-cycle changes (such as stem stiffness, plant height, and plant density) affect sediment delivery into salt marshes. We have been sampling plants from several locations along the NH Seacoast and measuring the Young's modulus -- so far collecting data points for the summer and fall.

This work is funded by New Hampshire Sea Grant development funding.

Measuring wave fields by tracking bottom features

In clear waters such as coral reefs, fixed roughness features at the sea bed are optically distorted by the air-water interface. We are currently developing methods to measure surface wave fields by quantifying this apparent distortion.

To learn more about our bench-scale test tank and wavemaker (image on left), check out Matthias Page's wavemaker design: