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:
Bheeroo, V. & Mandel, T.L. (2023). Comparison of Schlieren-based techniques for measurements of a turbulent and wavy free surface. Experiments in Fluids 64: 114. [doi]
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]

Surfacing and trapping in buoyancy-driven environmental systems

Plumes and particles in the marine environment can interact strongly with the ambient ocean stratification. We are interested in the trapping of 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]

Connecting seed and pollen transport in turbulent canopy flow with genetic distributions

Genetic diversity in Zostera marina influences both short- and long-term resilience to environmental change. However, the effects of hydrodynamics on the genetic diversity and structure on eelgrass populations is poorly understood. We are collaborating with Dr. Cynthia Hays (project PI), a seagrass geneticist at Keene State College, Dr. Tom Lippmann, a coastal oceanographer and modeler at UNH, and Dr. Theresa Oehmke, a postdoctoral fellow at UNH with expertise in the transport of particles in turbulence, to study this interdisciplinary problem.

In this project, the Ocean Hydrodynamics Lab is conducting experiments to study the transport of anisotropic seed particles in a model eelgrass bed, to measure the distribution of settling locations. We will be developing parameterizations of seed distribution based on release location and flow/meadow characteristics, and working with our genetics colleagues to help explain observed spatial patterns.

This work is funded by New Hampshire Sea Grant. Photo by Tim Briggs, NH Sea Grant.

Sediment fate and transport in the Hampton-Seabrook Estuary

Salt marshes are highly effective sediment traps and historically accrete and increase in elevation over time. However, current rates of sedimentation in the Hampton-Seabrook Estuary are not enough to keep up with sea level rise, which will lead to transitions in low and high marsh habitats and ecosystem services. Inspired by the idea of a "mud motor," in which sediments are injected at flood tides to build up marsh, we are studying small and large scale sediment transport and dispersion HSE and in an idealized laboratory setup.

Collaborators: Dr. Tom Lippmann (lead PI), Dr. Diane Foster, Dr. Mike Palace, & Dr. David Burdick.

This work is funded by the U.S. Army Corps Engineer Research & Development Center and U.S. Coastal Research Program.

Seasonal variation in the biomechanical properties of marsh and dune 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: