About

Professor Heidi Nepf leads the Nepf Environmental Fluid Mechanics Lab at MIT, where the research focuses on the interaction of flow with aquatic vegetation and the resulting feedbacks to sediment transport, chemical flux, and ecosystem function. The lab develops models for physical processes that determine how vegetated habitats, such as seagrasses, salt marshes, and mangroves, provide coastal protection, mitigate anthropogenic nutrient and pollutant loads, and serve as blue carbon reservoirs. The overarching goal of this work is to apply these models to improve the management of natural resources and the design of green infrastructure. Recent projects from the lab include studies demonstrating the economic benefits of marshes in coastal protection by reducing the need for seawall heightening, investigations into the limited carbon storage capacity of patchy seagrass meadows due to patch migration, and research identifying sediment characteristics that influence microplastic accumulation and resuspension. Through these efforts, Professor Nepf's work advances understanding of sediment transport in vegetated regions and the role of restoration and green infrastructure in coastal and aquatic environments.

Research topics

• Mechanics
• Geotechnical engineering
• Environmental science
• Geology
• Physics
• Biology
• Geography
• Ecology
• Geomorphology
• Soil science
• Atmospheric sciences
• Thermodynamics
• Mathematics
• Classical mechanics
• Geometry

Selected publications

• A unified framework for bed shear stress and boundary layer thickness for rough and smooth beds in bare and vegetated channels

  Journal of Fluid Mechanics · 2026-04-13

  articleSenior author

  Bed shear stress is a key parameter governing sediment transport and fluxes at the sediment–water interface. In vegetated channels, predicting bed shear stress, especially for rough beds, remains a challenge. This study developed a unified theoretical model for bed shear stress that smoothly spans conditions from bare bed to vegetated bed for both smooth and rough beds. Building on phenomenological turbulence theory, the model relates bed shear stress to the characteristic velocities of the larger energy-containing eddies and the smaller, near-bed eddies, with the new assumption that the bottom boundary layer (BBL) thickness controls the larger, energy-containing eddy length scale. The BBL was defined as the region within which the bed shear stress contributed significantly, compared to vegetation drag, and a force balance predicted that the BBL thickness scales with the ratio of bed shear stress to vegetation drag. In the limit of zero vegetation density, the BBL thickness equals the water depth, and the bed shear stress model reduces to the classical bare bed formulation. With increasing vegetation density (drag), the thickness of the boundary layer decreases, and the bed friction coefficient increases, which is consistent with previous observations. For rough beds, the bed friction coefficient increases with bed roughness, but is not dependent on the mean velocity. In contrast, for smooth beds, the bed friction coefficient decreases with increasing mean velocity. The coupled models for bed shear stress and BBL thickness were compared against 114 physical and numerical experiments from multiple previous studies.

  PublisherDOI

• Quantifying the Impact of Temporal and Spatial Heterogeneity in Turbulence on Bed‐Load Transport Over Vegetated Beds Under Wave‐Current Conditions

  Geophysical Research Letters · 2026-03-30

  articleOpen accessSenior author

  Abstract Laboratory experiments measured velocity and bedload transport in vegetated channels under current‐dominated, combined wave‐current conditions. Waves and vegetation, respectively, introduced temporal and spatial heterogeneity in the velocity, which significantly impacted sediment transport rate. Compared to pure‐current, the addition of waves only moderately increased (<40%) the time‐mean turbulent kinetic energy (TKE) but introduced temporal peaks in TKE that triggered an exponential increase in sediment transport. Similarly, local regions of intense TKE, reflected in the TKE spatial variance, enhanced channel‐average sediment transport. The TKE spatial variance decreased with increasing area density of vegetation stems, such that the influence of spatial variance was strongest in sparse canopies. The impacts of spatial and temporal heterogeneity were incorporated into a new model for bedload transport, which significantly improved prediction. The enhancement in sediment transport from both temporal and spatial heterogeneity is most significant when hydrodynamic forcing is near the critical threshold for sediment motion.

  PublisherDOI

• Near-bed velocity law and bed shear stress in vegetated flows

  Journal of Fluid Mechanics · 2026-01-10 · 1 citations

  articleSenior author

  By generating drag and turbulence away from the bed, aquatic vegetation shapes the mean and turbulent velocity profile. However, the near-bed velocity distribution in vegetated flows has received little theoretical or experimental attention. This study investigated the near-bed velocity profile and bed shear stress using a coupled particle image velocimetry and particle tracking velocimetry system, which enabled the acquisition of flow-field measurements at very high spatial and temporal resolution. A viscous sublayer with a linear velocity profile was present, but this sublayer thickness was much smaller in vegetated flows than in bare flows with the same channel velocity. However, the dimensionless viscous sublayer thickness was the same in vegetated and bare flows, $z_v^+ = z_v \langle u_*\rangle / \nu = 6.1 \pm 0.7$ . In addition, in vegetated flow, the horizontally averaged velocity profile above the viscous sublayer did not follow the classic logarithmic law found for bare beds. This deviation was attributed to the violation of two key assumptions in the classic Prandtl mixing length theory. By modifying the mixing length theory for vegetated conditions, a new theoretical power law profile for near-bed velocity was derived and validated with velocity data from both the present and previous studies, with mean percent errors of 4.9 % and 7.8 %, respectively. Using the new velocity law, the spatially averaged bed shear stress (and friction velocity) can be predicted from channel-average velocity, vegetation density and stem diameter, all of which are conveniently measured in the field.

  PublisherDOI

• A Within Canopy Deposition Model Capturing the Influence of Microplastic Size, Biological Cohesion, and Vegetation‐Generated Turbulence

  Geophysical Research Letters · 2026-03-28

  articleOpen accessSenior author

  Abstract Laboratory experiments investigated how the presence of biofilm and particle size influenced deposition in both bare and vegetated channels. Both abiotic and biotic sediment beds were considered, with the biotic beds containing Extracellular Polymeric Substances (EPS). As current magnitude and turbulent kinetic energy increased, particles were more easily resuspended, leading to reduced deposition rate. At the same turbulence level, depositing particles smaller than the sediment grains exhibited higher deposition rates than larger particles because the smaller particles were more effectively shielded from the flow by settling deeper into the spaces between sediment grains. Increasing the concentration of EPS decreased the deposition rate for all particle types, but this trend was more pronounced for smaller particles, because the enhancement in particle exposure due to EPS was more significant. A new model was developed to describe microparticle deposition probability as a function of EPS concentration, flow characteristics, and particle properties.

  PublisherDOI

• Author Correction: Marsh restoration in front of seawalls is an economically justified nature-based solution for coastal protection

  Communications Earth & Environment · 2025-12-08

  articleOpen accessSenior author
  PublisherOA PDFDOI

• Floating Treatment Island in a Stormwater Pond: Numerical Insights into the Effects of Root Zone Permeability on Pond Hydraulics and Nutrient Removal

  Journal of Environmental Engineering · 2025-06-02 · 1 citations

  articleSenior author

  This study used computational fluid dynamics to investigate how the root zone density affected the hydraulics and nutrient removal of a full-scale stormwater pond retrofitted with a floating treatment island (FTI). The root zone density was represented by permeability, which describes the capacity of fluid to flow through the porous root zone of the FTI. Permeability ranged between 10−20 and 10−2 m2. Nutrient removal was represented by the first-order reaction model, and the FTI occupied 11% of the pond volume. The simulations showed that low permeability caused strong flow deflection around the FTI, resulting in a modified flow structure within the pond that reduced short-circuiting and mixing, compared to ponds without FTIs or with high permeability FTIs. Importantly, higher limits of mass removal were achieved for FTIs with higher permeability, suggesting that increasing removal does not require increasing the number of plants placed in the FTIs, thus reducing the cost of building FTIs. The inverse relationship between root zone density and treatment is explained by the fact that, for higher root density, most pond water bypasses the root zone, receiving no treatment. The relationship between mass removal in the pond system and root zone permeability is described for a comprehensive range of permeability values.

  PublisherDOI

• Continual migration of patches within a Massachusetts seagrass meadow limits carbon accretion and storage

  Communications Earth & Environment · 2025-02-20 · 4 citations

  articleOpen accessSenior author

  Seagrass meadows facilitate the capture and storage of organic carbon, but spatial variability in carbon has been observed among and within meadows. Here we combine sediment and seagrass data with aerial images collected near the Annisquam River, Massachusetts, USA to examine spatial variations in carbon retention across a patchy seagrass meadow. Tidal velocities were reduced within patches and elevated in bare regions, which was expected to promote carbon accumulation within the patches. However, organic carbon was not correlated with the spatial distributions of seagrass or velocity. Historical aerial images showed continual patch movement, with vegetation persistence of less than a decade throughout the meadow. The highest carbon stock occurred in the largest area of recent vegetation persistence. Though present for >45 years, the meadow accumulated negligible carbon, likely due to the migration of patches. Overall, we provide insight into a potential limitation on carbon accretion and storage in patchy meadows. The spatial variation in sediment organic carbon stocks in a patchy seagrass meadow is linked to the temporal stability of the patches, according to an analysis of sediment samples and historical aerial imagery collected in Massachusetts, USA.

  PublisherOA PDFDOI

• Turbulence in a channel with a patchy submerged canopy: the impact of spatial configuration

  Journal of Fluid Mechanics · 2025-03-07 · 14 citations

  articleSenior author

  This study investigates how the spatial configuration of submerged three-dimensional patches of vegetation impacts turbulence. Laboratory experiments were conducted in a channel with submerged patches of model vegetation configured with different patch area densities ( $\phi _{p}$ ), representing the bed area fraction occupied by patches, ranging from 0.13 to 0.78, and different spatial patterns transitioning from two dimensional (channel-spanning patches) to three dimensional (laterally unconfined patches). These configurations produced a range of flow regimes within the canopy, from wake interference flow to skimming flow. At low area density ( $\phi _{p}\lt0.5$ ), turbulence within the canopy increased with increasing $\phi _{p}$ regardless of spatial configuration, while at high area density ( $\phi _{p}\gt0.5$ ), the relationship between turbulence and $\phi _{p}$ depended on the spatial configuration of the patches. For the same patch area density, the configuration with smaller lateral gaps generated stronger turbulence within the canopy. The relative contributions of wake and shear production also varied with the spatial configuration of the patches. At low area densities, wake production dominated over shear production, while at high area densities, shear production was more dominant due to an enhanced shear layer at the top of the canopy and reduced mean velocity within the canopy. A new predictive model for the channel-averaged turbulent kinetic energy (TKE) was developed as a function of channel-averaged velocity, canopy geometry, and patch area density, which showed good agreement with the measured TKE.

  PublisherDOI

• Biological Cohesion of Sediment Bed Diminishes Net Deposition of Fine Non‐Cohesive Particles Over Bare Bed and Within Model Emergent Canopies

  Geophysical Research Letters · 2025-05-12 · 1 citations

  articleOpen accessSenior author

  Abstract This study investigated how Extracelluar Polymetric Substances (EPS) produced by microorganisms influenced particle deposition to a sediment bed. The particle deposition decreased with increasing EPS, because the EPS filled the pore spaces between individual sediment grains, reducing the porosity of the sediment bed. With decreased porosity, newly deposited particles could not settle in between the grains of the bed, so that particles were more exposed to the flow, making resuspension easier and leading to decreased deposition. For the same level of bio‐cohesion, increasing the near‐bed turbulence diminished deposition. For the vegetated channel, as bio‐cohesion increased, particles were easily resuspended around individual stems due to the enhanced exposure effect, expanding the regions where deposition was excluded and leading to a more heterogeneous spatial distribution of deposition. The effect of EPS was negligible for the smallest velocity magnitude, for which all particles deposited, and for largest velocity magnitude, for which most particles were resuspended.

  PublisherOA PDFDOI

• Initiation of Sediment Resuspension by Combined Wave‐Current Conditions in an Artificial Seagrass Meadow

  Journal of Geophysical Research Earth Surface · 2025-06-01 · 1 citations

  articleOpen accessSenior author

  Abstract Laboratory experiments examined the impact of current on ripple formation and the onset of wave‐driven resuspension within an artificial seagrass meadow modeled after Zostera marina. Within the meadow, the current was less than or equal to the wave velocity. Meadows were constructed with three shoot densities: 247, 455 and 962 stems/m 2 , and each shoot had six flexible blades. The sediment bed, consisting of spherical grains, was initially 1.4 cm thick, allowing ripple and scour hole formation. The formation of wave‐orbital ripples was dependent on meadow density and current magnitude. Over bare beds and sparse meadows, ripples were present and not impacted by the addition of current, such that the wave velocity resuspension threshold with current was the same as that in pure wave conditions. In medium‐density meadows, the addition of current reduced ripple height due to plant‐generated turbulence. As current increased, ripple size and ripple‐generated turbulence decreased, requiring a higher wave velocity to resuspend sediment. That is, for medium density meadows, the critical wave velocity increased as the current velocity increased. Finally, in dense meadows, no ripples formed and resuspension was driven by a critical value of plant‐induced turbulence, which was proportional to the total velocity (current plus wave velocity), such that as the current velocity increased, the critical wave velocity decreased. A model predicting the critical wave velocity for the dense meadow was derived based on the assumption that resuspension was driven by a critical level of stem‐generated turbulence.

  PublisherDOI

Recent grants

• Predictive Models for Wave Damping by Flexible Aquatic Vegetation

  NSF · $441k · 2017–2022

• Predicting In-Canopy Velocity and Retention Time for Aquatic Canopies

  NSF · $383k · 2008–2012

• Collaborative Research: Dispersion of Particles Within and Above Plant Canopies

  NSF · $372k · 2011–2016

• Impact of vegetation geometry and distribution on bedload transport

  NSF · $408k · 2019–2024

• Thermally-Driven Exchange Flows in Regions of Vegetation

  NSF · $452k · 2005–2010

Frequent coauthors

• Chao Liu

  30 shared

• Isabella Schalko

  Massachusetts Institute of Technology

  29 shared

• Marcelo Chamecki

  University of California, Los Angeles

  28 shared

• A. Lightbody

  27 shared

• Xiaoxia Zhang

  Shenzhen University

  23 shared

• Yukie Tanino

  22 shared

• Jiarui Lei

  National University of Singapore

  20 shared

• Elizabeth Follett

  University of Liverpool

  20 shared

Labs

• Nepf LabPI

Education

• Ph.D., Civil Engineering

  Massachusetts Institute of Technology

  1995

• M.S., Civil Engineering

  Massachusetts Institute of Technology

  1991

• B.S., Civil Engineering

  University of California, Berkeley

  1988

Awards & honors

• IAHR M. Salim Yalin Lifetime Achievement Award 2023
• Fellow, American Geophysical Union 2018
• ASCE, Hunter Rouse Hydraulic Engineering Award 2019
• Chi Epsilon Honor Member 2017
• 18th Harold Jan Schoemaker Award 2013