Runoff Coefficients of High-flow Events in Undisturbed New England Basins

The New England region in the Northeast U.S. receives high annual precipitation as rain and snow, which results in floods that endanger people and infrastructure. Owing to the complexity of hydrologic systems, increases in the frequency and intensity of large precipitation events do not always translate into increases in surface runoff measured as event flow at the basin outlet. However, recent studies have recognized positive trends in the frequency and magnitude of high-flow events in New England. For high-flow events of equal or greater than 2-year daily runoff, the runoff coefficients, or the fraction of precipitation converted into surface runoff during an event, were determined for 28 undisturbed New England basins with natural flow conditions. Results indicated that runoff coefficients increase in magnitude and variability with distance from the Atlantic coast toward the north and west. The average runoff coefficient of high-flow events across all basins is 0.90, while there exist many high-flow events with runoff coefficients greater than one. Also, runoff coefficients were generally stationary showing that flood events in undisturbed basins have remained proportional to precipitation inputs, despite increases in extreme precipitation, possibly due to shifts in evapotranspiration, snowpack, and soil moisture. Flood management efforts should continue to focus on large springtime precipitation events, which generate the highest runoff coefficients. Finally, this study can serve as a reference point for future exploration of the flood susceptibility of basins with anthropogenic alterations like dam construction or land use change

The physical role of transverse deep zones in improving constructed treatment wetland performance

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 254-274).Velocity heterogeneity is often present in wetland systems and results in some influent water remaining in the wetland for less than the expected residence time. This phenomenon, known as short-circuiting, alters the distribution of the chemical and biological transformations that occur within the wetland and decreases performance in constructed treatment wetlands. In this thesis, field observations, experiments in a laboratory physical model, and mathematical modeling are used to explore the ability of transverse deep zones to mitigate the negative effect of short-circuiting on constructed wetland performance. Field observations were used to quantify short-circuiting in a 360-acre constructed treatment wetland in Augusta, Georgia. In each of the three marsh sections examined, between three and six narrow flowpaths were found that together carried 20-70% of the flow at a velocity at least ten times faster than the rest of the marsh. One known method for offsetting the deleterious effect of short-circuiting flowpaths is to include several transverse deep zones within each wetland cell. To study the physical mechanisms behind this proposed strategy, laser-induced fluorescence (LIF) was used within a laboratory scale model of a short-circuiting wetland with a transverse deep zone. Water exiting a fast flowpath formed a jet that initially entrained co-flowing fluid and spread laterally but then, due to the drag present within the system, reached a final width that depended on the width of the upstream flowpath. Finally, the understanding of flow patterns gained by the field and laboratory experiments were combined into an analytical streamtube model.(cont.) Modeled results revealed that a transverse deep zone can offset the adverse impact of short-circuiting flowpaths through two separate mechanisms. When lateral mixing is present within the deep zone, it dilutes the water that has traveled through the fast flowpath. In addition, deep zones likely reduce the probability that fast flowpaths will align throughout the entire wetland, which increases the probability that all water will receive some treatment even when no lateral mixing is present within the deep zones. These results indicate that deep zones may improve performance when properly sized and located within a constructed treatment wetland.by Anne F. Lightbody.Ph.D

Field and laboratory observations of small-scale dispersion in wetlands

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2004.Includes bibliographical references (p. 156-160).Estimating longitudinal dispersion in wetlands is a necessary first step in predicting the behavior of dissolved species and suspended particles. However, many processes are involved, and they can interact in nonlinear ways. Relevant processes include turbulent diffusion, which describes net solute flux created by turbulent eddies. Other dispersive processes result from the retardation of a portion of the solute relative to the rest of a cloud. This retardation can be provided by trapping in the vortex structure behind stems (hold-up dispersion), velocity deficits well downstream of stems (stem-wake dispersion), or transverse gradients in longitudinal velocity (shear dispersion). To better understand the relative magnitude of these various dispersive processes, measurements were taken of velocity, vertical diffusion, and longitudinal dispersion in both the laboratory and the field. Laboratory flume experiments were conducted using an emergent canopy of rigid cylinders with different cylinder densities over depth. Field experiments were conducted in a natural salt marsh. Drag due to local stem density was found to control horizontal velocity in both the lab and field studies over most of the depth. The resulting non-uniform velocity profile generated shear dispersion, which controlled dispersion at longer distances (> 250 cm) downstream of a slug release. For distances < 250 cm downstream, wake shear dispersion was found to be most important.by Anne F. Lightbody.S.M

Ecogeomorphic feedbacks and flood loss of riparian tree seedlings in meandering channel experiments

During floods, fluvial forces interact with riparian plants to influence evolution of river morphology and floodplain plant community development. Understanding of these interactions, however, is constrained by insufficient precision and control of drivers in field settings, and insufficient realism in laboratory studies. We completed a novel set of flume experiments using woody seedlings planted on a sandbar within an outdoor meandering stream channel. We quantified effects on local sedimentation and seedling loss to scour and burial across realistic ranges of woody plant morphologies (Populus versus Tamarix species), densities (240 plants m-2 versus 24 m-2), and sediment supply (equilibrium versus deficit). Sedimentation was higher within Tamarix patches than Populus patches, reflecting Tamarix’s greater crown frontal area and lower maximum crown density. Plant dislodgement occurred rarely (1% of plants) and was induced in plants with shorter roots. Complete burial was most frequent for small Tamarix that occurred at high densities. Burial risk decreased 3% for Populus and 13% for Tamarix for every centimeter increment in stem height, and was very low for plants \u3e50 cm tall. These results suggest that Tamarix are proportionally more vulnerable than Populus when small (\u3c20 cm tall), but that larger plants of both species are resistant to both burial and scour. Thus, plant morphological traits and development windows must be considered in addition to physical drivers when designing process-based restoration efforts on regulated rivers such as flow releases to benefit native tree species

Flow and scour constrainst on uprooting of pioneer woody seedlings

Scour and uprooting during flood events is a major disturbance agent that affects plant mortality rates and subsequent vegetation composition and density, setting the trajectory of physical-biological interactions in rivers. During flood events, riparian plants may be uprooted if they are subjected to hydraulic drag forces greater than their resisting force. We measured the resisting force of woody seedlings established on river bars with in situ lateral pull tests that simulated flood flows with and without substrate scour. We quantified the influence of seedling sizes, species (Populus and Tamarix), water-table depth, and scour depth on resisting force. Seedling size and resisting force were positively related with scour depth and water-table depth--a proxy for root length--exerting strong and opposing controls on resisting force. Populus required less force to uproot than Tamarix, but displayed a greater increase in uprooting force with seedling size. Further, we found that calculated mean velocities required to uproot seedlings were greater than modeled flood velocities under most conditions. Only when plants were either shallowly rooted or subjected to substrate scour (≥0.3 m) did the calculated velocities required for uprooting decrease to within the range of modeled flood velocities, indicating that drag forces alone are unlikely to uproot seedlings in the absence of extreme events of bar-scale sediment transport. Seedlings on river bars are most resilient to uprooting when they are large, deeply rooted, and unlikely to experience substrate scour, which has implications for ecogeomorphic evolution and river management

Effects of Added Vegetation on Sand Bar Stability and Stream Hydrodynamics

Vegetation was added to a fully developed sandy point bar in the meander of a constructed stream. Significant changes in the flow structure and bed topography were observed. As expected, the addition of vegetative resistance decreased the depth-averaged streamwise velocity over the bar and increased it in the open region. In addition, the secondary circulation increased in strength but became confined to the deepest section of the channel. Over the point bar, the secondary flow was entirely outward, i.e., toward the outer bank. The changes in flow led to changes in bar shape. Although the region of the bar closest to the inner bank accumulated sediment, erosion of the bar and the removal of plants by scouring were observed at the interface between the planted bar and the open channel.National Science Foundation (U.S.) (Grant No. EAR 0738352

Riparian Vegetation and Sediment Supply Regulate the Morphodynamic Response of an Experimental Stream to Floods

Feedbacks between woody plants and fluvial morphodynamics result in co-development of riparian vegetation communities and channel form. To advance mechanistic knowledge regarding these interactions, we measured the response of topography and flow to the presence of riparian tree seedlings with contrasting morphologies in an experimental, field-scale, meandering stream channel with a mobile sand bed. On a convex point bar, we installed seedlings of Tamarix spp. (tamarisk) and Populus fremontii (cottonwood) with intact roots and simulated a bankfull flood, with each of eight runs varying sediment supply, plant density, and plant species. Vegetation reduced turbulence and velocities on the bar relative to bare-bed conditions, inducing sediment deposition when vegetation was present, regardless of vegetation density or species. Sediment supply also played a dominant role, and eliminating sediment supply reduced deposition regardless of the presence of vegetation. Unexpectedly, plant density and species architecture (shrubby tamarisk vs. single-stemmed cottonwood) had only a secondary influence on hydraulics and sediment transport. In the absence of plants, mobile bedforms were prominent across the bar, but vegetation of all types decreased the height and lateral extent of bedforms migrating across the bar, suggesting a mechanism by which vegetation modulates feedbacks among sediment transport, topography, and hydraulics. Our measurements and resulting insights bridge the gap between laboratory conditions and real dryland sand-bed rivers and motivate further morphodynamic modeling