H33J-01 INVITED
Closing the Gap Between Sediment Budgeting and Stream Restoration Planning
A wooden boat builder will tell you that the majority of a project work load is often associated with the setup and preparation. The same reality applies to stream restoration intended to achieve watershed sediment yield reductions, with project site selections comprising a substantial share of the activities necessary to achieve long- term basin-wide objectives. Sediment budgeting strategies that have been long standing pursuits of the geomorphology community can directly and indirectly provide support to a framework for stream restoration targeting and benefit assessment. A level of detail suitable to support decisions for the prioritization of stream restoration investments in either headwater or lower alluvial valley reaches can be attained using a suite of data from small pond and reservoir sedimentation assessments, short-term storm sampling, drainage network measurements, and rainfall-runoff modeling. Analyses of hydrograph sediment concentration trends, watershed flow patterns, and channel adjustment mechanics that provide the details necessary for defensible sediment supply and delivery estimates can also guide the selection of strategies to alter them. Examples will be provided from an investigation of first-order watershed sediment production in the Piedmont region of Maryland (USA) to illustrate how stream restoration site targeting and design concepts can be derived from spatially scaled watershed sediment budgeting exercises.
H33J-02
Connectivity and Variability as Drivers for Lowland River Backwater Restoration
Rehabilitation, protection, and management of floodplain aquatic habitats that are seasonally or periodically connected to the main channel are difficult in landscapes with intensively developed or cultivated floodplains. Here backwater hydrology is often perturbed due to flow regulation by levees or upstream dams, main channel incision, and backwater sedimentation. Additional issues include water quality degradation, aquatic plant infestation, and extreme variation in water temperature and habitat volume during hot, dry months. The pre-rehabilitation status of three severed meander bends along the Coldwater River in northern Mississippi was documented for two years by monitoring the bendways and adjacent river reaches. Backwaters contained up to 2.5 m of recent sediment and experienced near anaerobic conditions with water temperatures greater than 30 deg C during late summer. One of the sites was selected for short-term rehabilitation treatment. An approach for assessing aquatic system status proposed by Kondolf and others (http://www.ecologyandsociety.org/vol11/iss2/art5/) was used to design a rehabilitation project featuring water control structures that trap floodwaters and increase backwater depth and volume. The Kondolf approach evaluates aquatic system status by locating the system in four-dimensional space representing connectivity in the three spatial dimensions and temporal variability. Project design using a simple simulation model and initial ecological responses to rehabilitation are described. The model allows simulation of backwater surface elevation, mean depth and surface area and computes volumes of water exchanged with the main channel for any imposed annual hydrograph with a timestep of 0.0001 day. Backwater control structure (weir) design and operation may be varied to achieve connectivity and variability objectives. Rehabilitation effects on water quality and fish are described.
H33J-03
Pulsed Discharge Through Wetland Vegetation as a Control on Bed Shear Stress and Sediment Transport Affecting Everglades Restoration
The ridge and slough landscape is a patterned peatland within the Florida Everglades in which elevated ridges of emergent vegetation are regularly interspersed among open-water sloughs with floating and submerged vegetation. Landscape features are aligned parallel to the historic flow direction. Degradation of patterning over the past 100 years coincides with diminished flow resulting from drainage and construction of levees and canals. A goal of restoration is to increase flow velocities and redistribution of particles and solutes in attempt to preserve remnant patterning and restore degraded portions of the ridge and slough landscape. To explore different management strategies that could induce sediment redistribution in the ridge and slough landscape, we simulated velocity profiles and bed shear stresses for different combinations of surface water stage, water surface slope, and vegetation community structure, based on field measurements and laboratory experiments. A mixing length approach, in which the minimum of stem spacing and distance from a solid boundary determined eddy scale, was used to simulate velocity profiles and bed shear stress in vegetated arrays. Simplified velocity profiles based only on vegetation frontal area above the bed and the Karman-Prandtl logarithmic law near the bed closely were used to approximate solutions of the one-dimensional Navier-Stokes equations for large-scale simulation. Estimates of bed shear stress were most sensitive to bed roughness, vegetation community structure, and energy slope. Importantly, our simulations illustrate that velocity and bed shear stress cannot be increased substantially in the Everglades simply by increasing surface-water stage. This result comes directly from the dependence of velocity and shear stress on vegetation frontal area and the fact that emergent vegetation stems protrude through the water column even during times of relatively deep water in the Everglades. Since merely increasing water depth is not likely to increase water velocity and entrainment, it is necessary instead that restoration focus on increasing energy slope as a means to entrain sediment within sloughs and redistribute it to ridges. Surface-water gravity waves caused by hurricanes or pulsed releases of water from impounded areas may be the most effective mechanism for achieving sediment redistribution in the Everglades and other wetland and riparian environments with abundant emergent vegetation.
H33J-04
Geometric Scaling of Step-pool Channels
Step-pool channels are a ubiquitous element of mountain watersheds. As communities continue to expand into the mountainous regions of the world, an increased understanding of step-pool properties and characteristics is essential to support informed land-use decisions. A surge in recent research has explored fundamental characteristics of step-pools in part from the perspective of utilizing step-pools in stream restoration or stabilization efforts. Much of this research has focused on applying hydraulic and energy theory to elucidate controls on step-pool geometry. These approaches are based on the concept that step-pool form evolves to provide a maximum resistance to flow. We describe a new perspective on step-pool geometric scaling. Our work suggests that step-pool geometry is the result of interactions between channel geometry and step-pool hydraulics, notably head loss through the step-pool reach. We will illustrate that at the reach level of organization, the ratio of bankfull width to head loss can be described by average reach slope. Step-pool wavelength and height are in turn well described by bankfull width, again expressed as ratio to head loss through the step-pool reach. These results provide for a fundamental understanding of step-pool form in relation to watershed-scale systems. Our results also suggest that average head loss and bankfull width through a step-pool reach are perhaps the most important scaling variables of step-pool geometry. The existing literature has thus far failed to consistently demonstrate a linkage between geometry and reach-average slope. Our work has been tested against data from other existing studies and reveals that data from different physiographic regions of the Western U.S. and within different climatic zones agree well with our data. Because we use data from naturally developed step-pool reaches across a spectrum of geologies and climates, our results are applicable for use with channel evolution models and perhaps more importantly in the design of engineered step-pool reaches. We compare engineered step-pool geometry to our data.
H33J-05
Evaluation of Pool-Riffle Maintenance Processes in an Incised Urban Channel Using a 3D Hydrodynamic Model: Implications for Stream Restoration
Channel adjustments due to watershed urbanization include incision, widening, and loss of pool-riffle structure, a result of modifications in hydrology, local hydraulics, and geomorphic processes. In theory, pool-riffle sequences are maintained hydraulically by channel-scale helical flow patterns, both in meandering and straight channels. Through the use of FLOW-3Dâ, a three-dimensional (3D) computational fluid dynamics model, flow structures were characterized in an incised straight channel with woody bank vegetation impeding near-bank flood flows. The 3D model was applied to a 110-m section of Beaver Creek in Knox County, Tennessee, which included a survey of channel topography and 122 bank trees. Three model simulations were conducted during bankfull flow stage; they were: 1) channel with bank trees, representing the channel's current state, 2) trees removed from the channel, and 3) a restoration design using three clusters of original trees in the channel to promote flow acceleration-deceleration patterns. In addition, one simulation was conducted to evaluate model performance, in which an acoustic Doppler velocimeter was used to collect instream 3D velocity data during a one-half bankfull flow event. Overall, simulations suggest flow structures along the study reach are highly influenced by bank trees, do not scale to the channel, and may possibly explain pool-riffle loss in incised channels with woody bank vegetation. In the restoration design simulation, flow acceleration-deceleration patterns were successfully induced along the channel. This simulation illustrated the importance of considering bank structure and hydraulic roughness in designs to promote pool-riffle maintenance, rather than relying solely on bed weir structures.
H33J-06
Quantifying the Hydrological Effects of Stream Restoration in A Mountain Meadow
Stream restoration activities are increasingly abundant. Particularly common are "pond and plug" type stream restoration projects, in which (a) alluvial materials are excavated from the floodplain, forming ponds; (b) excavated alluvial materials are used to plug incised channels; and (c) channels are restored to the floodplain surface. A commonly stated objective of these efforts is to restore hydrological processes to the riparian systems. However, little research has been conducted to quantify the restoration of these hydrological processes. Direct comparisons of pre- and post-project hydrological observations are often misleading due to inter-annual climate variability. To overcome this issue and accurately quantify the hydrological effects of restoration, we developed, calibrated and validated a MIKE SHE hydrological model of a 230 ha mountain meadow along a 3.6 km restored reach of Bear Creek in northeastern California. We then applied the model to simulate the pre- and post-restoration scenarios by altering the floodplain topography and stream channel networks. A comparison of model results indicate that channel and floodplain topographic modifications associated with stream restoration activities (a) raise groundwater levels, (b) reduce the magnitude of flood peaks, (c) increase floodplain inundation through increased channel-floodplain connectivity, (d) increase recharge and subsurface storage, (e) increase evapotranspiration and (f) reduce total runoff. This study supports and quantifies the hypothesis that "pond and plug" type stream restorations restore hydrological processes to riparian systems. In addition, this study can be used to improve quantitative objectives for "pond and plug" type stream restoration activities in similar environments.
H33J-07
Restoration in Urban Streams Impacted by Legacy Sediments
Degraded urban streams are ideal candidates for restoration; as these restorations can provide multiple benefits ranging from increased accessible green space to enhanced nutrient processing. However, connections between the channel and floodplain are poorly understood in urban systems. In particular, urban valleys impacted by legacy sediment are challenging to rehabilitate. For example, the excavation of legacy sediment is expensive and can require the destruction of established riparian vegetation. The hardening of stream banks slows fluvial dynamics and may inhibit nutrient processing occurring in self-adjusting alluvial stream reaches. Further complications arise from the hydrologic changes that accompany urbanization, which can create unexpected hydrodynamic conditions divergent from intended channel and floodplain design. Here, we analyze data from Eastern U.S. Piedmont streams to characterize the role of floodplains in reach and watershed-scale sediment flux. A series of collocated historical cross-section resurveys and radio-isotopic reconstruction of over-bank sedimentation rates are compared. The data indicate that floodplain storage rates in some urban areas are comparable to or larger than rates of sediment remobilization from channel widening. This observation suggests urban floodplains with thick legacy deposits can remain a sediment sink even after channel entrenchment. The contemporary storage capacity has fundamental consequences for stream restoration designs, including the potential for rapid refilling of excavated bottoms and unsatisfactory reductions in watershed sediment yields.