, suspended-sediment concentrations (SSC) at 15 and 30 cm above the bed (cmab) increased by 1-2 orders of magnitude, and vertical gradients in SSC were strong enough to produce turbulence-limiting stratification, with gradient Richardson numbers exceeding 0.25. Simultaneously, turbulent stresses (decomposed from wave motions) increased by an order of magnitude. The apparent contradiction of energetic turbulence in the presence of strong stratification was reconciled by considering the turbulent kinetic energy (TKE) budget: in general, dissipation and buoyancy flux were balanced by local shear production, and each of these terms increased during wave events. The classic wave-current boundary layer model represented the observations qualitatively, but not quantitatively since the velocity profile could not be approximated as logarithmic. Rather, the mean shear was elevated by the Stokes drift return flow and wind-generated surface stress, which diffused sediment upward and limited stratification. Our findings highlight a pathway for waves to supply energy to both the production and destruction of turbulence, and demonstrate that in such shallow depths, TKE and SSC can be elevated over more of the water column than predicted by traditional models.

Observations of tidal trapping in a channel connected to large volumes along its perimeter showed that the exchange between them is driven by advection due to tidal flows. Therefore, quantifying the longitudinal dispersion of scalars in the channel that results from tidal trapping was not possible using traditional frameworks, which assume that the exchange is a diffusive process. This study uses the concentration moment method to solve analytically for the dispersion coefficient of a solute in a tidal channel which exchanges advectively with volumes along its edges. This constitutes a new framework for analyzing the longitudinal dispersion that results from tidal trapping in systems such as a branching tidal channel or the breached salt ponds of San Francisco Bay. A comparison of dispersion coefficients from traditional, diffusion-driven frameworks for tidal trapping, the new advective framework derived in the present study, and observations show that the new formulation is best suited to this environment.

No abstract

Regulatory goals for the California Delta attempt to restore natural ecosystems through various water management efforts. Defining management criteria for restoration is challenging, given that the earliest data describing the hydrology of the region follow many decades of change associated with agricultural development, channel modification, and flood control. This study explores the hydrology of the 1850–1920 period by synthesizing new reconstructed precipitation, basin inflows, land use change, and levee construction time series, in a semidistributed hydrologic model. The model demonstrated that it is impossible to simultaneously reproduce estimated historical flood extents, frequencies, and durations given contemporary topography. Bounding cases were constructed to span potential water budget partitioning and suggest that the state of this region by the 1920s was hydrologically similar to that of the natural regime (i.e., flow experienced was depressed due to drought, but within the bounds of variability of the natural Delta system), partly due to the flow augmentation provided by flood control infrastructure and enhanced channel conveyance. The model suggests that levee construction, rather than land use change, had the greatest impact on Delta hydrology. This and other reconstructions, however, suggest that decreases in annual Delta outflows accelerated after 1920. Future efforts to reconstruct Delta hydrology should focus on improving information about the historical topography and channel geometry of the Central Valley river network, with a view to refining understanding of the natural and historical flood regime.

The water balance dynamics in lowland watersheds are influenced not only by local hydroclimatic controls on energy and water availability, but also by imports of water from the upstream watershed. These imports result in a stochastic extent of inundation in lowland watersheds that is determined by the local flood regime, watershed topography, and the rate of loss processes such as drainage and evaporation. Thus, lowland watershed water balances depend on two stochastic processes—rainfall and local inundation dynamics. Lowlands are high productivity environments that are disproportionately associated with urbanization, high productivity agriculture, biodiversity, and flood risk. Consequently, they are being rapidly altered by human development—generally with clear economic and social motivation—but also with significant trade‐offs in ecosystem services provision, directly related to changes in the components and variability of the lowland water balance. We present a stochastic framework to assess the lowland water balance and its sensitivity to two common human interventions—replacement of native vegetation with alternative land uses, and construction of local flood protection levees. By providing analytical solutions for the mean and PDF of the water balance components, the proposed framework provides a mechanism to connect human interventions to hydrologic outcomes, and, in conjunction with ecosystem service production estimates, to evaluate trade‐offs associated with lowland watershed development.