H41A-0121
Boundary Shear Stress Along Vegetated Streambanks
Sediment, a leading cause of water quality impairment, damages aquatic ecosystems and interferes with recreational uses and water treatment processes. Streambank retreat can contribute as much as 85% of watershed sediment yield. Vegetation is an important component of stream restoration designs used to control streambank retreat, but vegetation effects on streambank boundary shear stress (SBSS) need to be quantified. The overall goal of this experiment is to predict boundary shear stress along vegetated streambanks. This goal will be met by determining a method for measuring boundary shear stress in the field along hydraulically rough streambanks, evaluating the effects of streambank vegetation on boundary shear stress in the field, and developing predictive methods based on measurable vegetative properties. First, three streambank vegetation types (herbaceous, shrubbery, and woody) will be modeled in a flume study to examine both boundary shear stress measurement theory and instruments for field use. An appropriate method (law of the wall, Reynold's stresses, TKE, or average wall shear stress) and field instrument (ADV, propeller, or Pitot tube) will be selected, resulting in a field technique to measure SBSS. Predictive methods for estimating SBSS, based on common vegetation measurements, will be developed in the flume study and validated with field data. This research is intended to improve our understanding of the role of riparian vegetation in stream morphology by evaluating the effects of vegetation on boundary shear stress, providing insight to the type and density of vegetation required for streambank stability. The results will also aide in quantifying sediment inputs from streambanks, providing quantitative information for stream restoration projects and watershed management planning.
H41A-0122
Engineered Logjam Technology: A decade of application and development of science based design guidelines.
Over 11 years ago the first engineered logjams (ELJs) were constructed in the Upper Cowlitz River (Abbe et al 1997). Nine years ago, the North Fork Stillaguamish River project was presented at the 1998 AGU Fall Meeting (Abbe et al 1998). Over the last decade, tribes, governmental agencies, private land owners and non-profit organizations have used ELJs to restore river habitat, limit channel incision, and provide bank protection for property and infrastructure. ELJs have been constructed throughout the Pacific Northwest, Alaska, California and as far away as New South Wales, Australia. The development of ELJ technology was founded on the premise of applying scientific methods to: (1) assess project sites (e.g, hydrology, hydraulics, sediment, vegetation, channel dynamics), (2) understand the mechanics of wood debris, (3) emulate natural processes and forms, (4) adapt solutions to situations constrained by human development (e.g., channelization, flow regulation) and (5) educate human communities about fluvial systems (e.g., address real and perceived views about wood debris and fluvial systems, considering direct and indirect effects on property and public safety). The case for a scientific standard of practice is supported by the successful performance of ELJ projects that employed a scientific analysis of site conditions, structure stability and the hydraulic and channel response to proposed structures. ELJ structures have successfully survived 100-yr flood events; delivered measurable increases in the amount of aquatic habitat, periphyton and invertebrate populations, and floodplain connectivity; created preferential habitat for juvenile salmon; and provided effective bank protection. I summarize the physical performance of several ELJ projects built from 1996 to 2006 and present a general scientific standard of practice for ELJ technology and wood debris management.
H41A-0123
Evaluating the effectiveness of floodplain restoration on the North Fork John Day River, Northeast Oregon, USA
Over the last decade hundreds of river restoration projects intended to maintain, protect, and restore watersheds, rivers, and habitat for native species in the Pacific Northwest have been implemented. By some counts, investment in watershed restoration exceeds hundreds of millions of dollars annually yet the effectiveness of these efforts remains an elusive question (Roni, 2005). Remote sensing and GIS technologies show great promise for large-scale river monitoring, however most natural resource organizations who implement these projects have limited budget and staff and would benefit from simple, low cost monitoring techniques that use readily available imagery. We used 1:24000 digitized orthorectified resource imagery from 1995, and National Agriculture Imagery Program (NAIP) digital orthophotography from 2005 to assess the effectiveness of floodplain restoration on a 16 km reach of the North Fork John Day River. Between 1993 and 1997 this section was restored by mechanically removing, reshaping, and revegetating cobble-boulder tailings piles left from dredge mining. The project was intended to directly improve floodplain function (i.e. inundation, riparian habitat) and indirectly improve instream habitat (pools, spawning) by reconnecting the active river channel with a reconstructed floodplain surface. Project effectiveness was not well documented initially in terms of quantifying floodplain functional area improvement or channel condition and response at the river-reach scale. Our objectives were to field-verify remote sensing measurements of response variables to test the applicability of available remote sensing imagery for project effectiveness monitoring, and to quantify adjustment in river response variables, using a "before-after" case study approach. Bracketing restoration activities with 1995 and 2000 imagery, we developed and tested methods for acquisition and processing of digital imagery and identified a core set of response variables to sample. Methods for sampling and data capture to characterize floodplain and channel conditions before and after treatment, and for quantifying effects of treatment were also developed. Response variables included: channel sinuosity, depositional area (bars), active channel width, floodplain area, and dominant vegetation type. We found mixed results in the post-project NAIP image interpretation due to resolution and sampling methods, but small, positive changes in some river response attributes (sinuosity, vegetation cover) were observed. We found complex mixed response in other variables (active channel width, floodplain area), presumably due to the compounding effects of previous habitat enhancement work and limited response and recovery time. Evaluating restoration treatment effectiveness on larger river systems presents numerous technical challenges. Such evaluations should consider river management history, original project objectives and design criteria, and the occurrence of channel-altering flows since treatment.
H41A-0124
Development and application of a hydrodynamic and water quality model for riverine floodplain environments
A two-dimensional, depth-averaged hydrodynamic and water quality model was developed to better understand the circulation, mixing, and water quality processes taking place during riverine floodplain inundation and to aid in the design of effective ecosystem restoration plans. The model utilizes several computational methods designed to accurately compute flow and transport in the contrasting river channel and floodplain environments. Model performance was tested using benchmark simulations, and the model was then applied to a restored floodplain on the lower Cosumnes River, CA, USA to examine the distribution of hydraulic residence times and implications for phytoplankton exports to the downstream Sacramento-San Joaquin Delta. Average residence times were found to be highly spatially and temporally variable and correlated with observed phytoplankton concentrations. Results from hypothetical management scenarios indicated that if the floodplain were allowed to drain faster there would be no significant effect on phytoplankton exports. The model is currently being applied to assess the potential for floodplain remediation on the Upper Truckee River, CA to reduce loadings of nitrogen, phosphorus, and suspended sediment to Lake Tahoe, CA-NV, USA.
H41A-0125
A Multi-Reach Comparison of Geomorphic Complexity and Hyporheic Exchange in Agricultural and Urban Streams
The influence of hyporheic flow on the thermal, nutrient and hydraulic regimes of streams is well known, but quantifying the components of geomorphic complexity that have the greatest influence on hyporheic exchange is a challenge. To explore this relationship, we surveyed three unique segments on each of two streams, one in a Colorado Front Range urban setting and the other in a mountainous agricultural region in north central Colorado. Each segment was chosen for its distinctive geomorphic setting and historical human modification; the urban stream showing various levels of stabilization and planform alteration, and the agricultural stream subject to variable cattle grazing practices. All segments were surveyed using a detailed protocol for characterizing physical complexity in terms of the spatial distribution of habitat units with distinct combinations of geomorphic, substrate, and hydraulic attributes. Results from this research may shed light on restoration strategies that could enhance hyporheic flow.
H41A-0126
Sediment Transport through Road Culverts Retrofit for Fish Passage
A critical component of watershed restoration includes improved mobility within the watershed for fish and other aquatic organisms. At the large scale, this effort includes installation of fish ladders around dams and at smaller scales replacement or retrofit of road culverts or installation of roughened channels to mitigate steep channel slopes. A project to quantify changes in culvert hydraulic performance and hydraulic capacity for three culvert shapes and six fish passage retrofit designs was initiated in June 2005. In 2007, laboratory physical model experiments were conducted to evaluate sediment transport and trapping characteristics of these designs over a range of flows. Generally, experimental results indicate trapped sediment in culverts retrofit to improve fish passage decreases the effectiveness of the retrofit due to sediment deposition in areas with lower velocities (where fish can rest). Other observations include: 1. Trapped sediment reduced the effective culvert barrel roughness and, thus, decreased water depths and increased velocities through the culvert, compared to clear water experiments with the retrofit baffles. 2. High flows (culvert barrel water depth/culvert height greater than 0.5) successfully cleared trapped sediment under conditions of minimal transport from upstream 3. Preliminary results indicate moderate flows (culvert barrel water depth/culvert height between 0.25 to 0.5) in combination with moderate sediment feed rates caused the greatest accumulation of trapped sediment These experiments highlight the importance of including sediment accumulation in design and analysis, and potentially impact design recommendations for culverts retrofit for fish passage and other similar fish passage improvement structures.
H41A-0127
Flume experiments of log incipient motion in rivers
In natural rivers the transport phenomena of floating debris involve a large number of problems that are relevant to both environmental and technical aspects. A large part of the research developed has been motivated by linkages between fish habitat and geomorphological processes and forms influenced by large woody debris, but drifts reduce also the capacity of bridge openings, contribute to scour around piers and abutements, and increase lateral forces on bridges. Large woody debris (LWD) is defined as a log having at least 10 cm mid-point diameter, being 2 m in length while the term CWD (coarse woody debris) usually refers to smaller pieces. Key pieces are individual logs with rootwads that are less likely to move than other wood pieces during a bankfull flow. They play a relevant role in snags amassing because key pieces constitute the first step of the woody accumulation process. To identify hydraulic thresholds for movement and transport of key pieces for different log geometries we developed a series of experiments in a flume model 0.30 m wide and 5.0 m long with about 10% slope transversal to the main flow direction. The log, with and without rootwads, is assumed fallen on the sloping bank and the hydraulic threshold is analyzed as a function of log geometry and hydrodynamic action. The key pieces are simulated by wood cylinder ended with an octagon (to take into account the rootwad) of different lengths and thicknesses. Besides the acquisition of water level and discharge as usual, the video recording of the experiments permits the identification of the log position at the threshold of motion into the flume. A conceptual model, based on the 3-D stationary equilibrium of gravity, buoyancy, friction and hydrodynamic forces acting on a rigid body, was developed to allow the interpretation of scale flume model experiments. From the preliminary analysis of laboratory experiments it seems that the condition of incipient motion is mainly controlled by the ratio between the rootwad size and the log length.
H41A-0128
Case Study: Channel-forming discharge on the Dolores River and Yampa River, Colorado
Channel-forming discharges of both the Yampa and Dolores Rivers have been estimated in the past by various methods. In this study we estimated channel-forming discharge for three study sites on the Yampa River and Dolores River in Colorado via four different methods and compared the results with results from previous studies. The first method estimated the bankfull discharge utilizing GPS survey data, GIS mapping and HEC-RAS modeling to determine the flow that begins to inundate the floodplains or reaches the top of banks. The second method determined the effective discharge using discharge and sediment data at USGS gaging stations to determine the flow that transports to most sediment over a long period of time. The third method involved development of flood-frequency curves from annual peak flow data and determination of the two-year frequency flow. The final method estimated the flows necessary for marginal transport and for significant motion of the bed material using the Shields equation and the average boundary shear stress. On the Dolores River, the channel-forming discharge estimates are in the range of 74 to 96 m3/s corresponding to 1.8 to 2.5-year frequencies in the post-dam flow regime. The effective discharge was not estimated on the Dolores River because of a lack of sediment data. On the Yampa River, the flow that inundates most of the floodplain areas and the flow that begins to mobilize the bed material range from 311 to 368 m3/s corresponding to 2.5 to 5.4 year frequencies. The effective discharge on the Yampa River, however, was estimated to be 230 m3/s and 1.4-year frequency, which differs from previous studies that suggested that the bankfull and effective discharges are equivalent. The difference between estimates of bankfull and effective discharge on the Yampa River raises questions regarding which method is appropriate to estimate channel-forming discharge in a gravel- bed river.
H41A-0129
Using Two-Year Recurrence-Period Discharge to Develop Regional Hydraulic Geometry Curves
Knowledge of bankfull discharge, Qbf, and channel characteristics (i.e., width, Wbf, depth, Dbf, and cross-section area, Abf) is essential for planners, engineers, geomorphologists, environmentalists, agricultural interests, developments situated on flood prone lands, surface mining and reclamation activities, and others interested in floods and flooding. In conjunction with estimating Qbf, Wbf, Dbf, and Abf, regionalized hydraulic geometry relationships, which relate Qbf, Wbf, Dbf, and Abf to drainage basin area ( Ada), are often used. The utility of regional curves is due to the ease with which Ada can be obtained for any geographic location. Among other things, regional curves are useful for identifying and confirming field indicators of bankfull discharge and channel characteristics, stream assessments, and planning studies. This study seeks to improve upon the common practice of predicting Qbf, Wbf, Dbf, and Abf using Ada exclusively. Specifically, we hypothesize that predictions of Qbf, Wbf, Dbf, and Abf can be improved by including estimates of the two-year recurrence-period discharge ( Q2) in regression models for predicting Qbf, Wbf, Dbf, and Abf. For testing this hypothesis, we use Qbf, Wbf, Dbf, and Abf measurements from 30 reports containing data for streams that span 34 hydrologically homogeneous regions in 16 states. Corresponding values of Q2 were compiled from flood-frequency reports prepared in accordance with guidelines for conducting flood-frequency studies developed by the Interagency Advisory Committee on Water Data. By comparing statistical measures (i.e., root mean squared error, coefficient of determination, and Akaike's information criterion) we determined that predicting Qbf from Q2 rather than Ada yields consistently better estimates of Qbf. Also, preliminary analyses to determine if using Q2 improves predictions of Wbf, Dbf, and Abf are promising. Other principal findings are (1) data are needed for at least 12 sites in a region for reliable hydraulic geometry model selection and (2) an approximate range of values for Q2/ Qbf is 0.10-3.0.
H41A-0130
A Numerical Model for Meandering River with Mass Balance
An updated computer model for meandering rivers has been developed using the Linearization Analyses of Johannesson and Parker (1989) and Sun et al. (2001a, b). The model simulates the bed topography, flow field, and bank erosion rate in a curved channel with an erodible bed. Sediment mass balance is preserved to update the channel bed elevation, which is represented by a 2D cell with high elevation area, water surface elevation area, and low elevation area. The model is used to simulate a laboratory channel migration where the upstream channel migration is slowed down due to channel erosion and high bank height.
H41A-0131
Flow Measurements through natural and degraded regions of the Everglades
The $8 Billion Comprehensive Everglades Restoration Plan (CERP) will attempt to preserve the ecological richness of the Everglades, a unique "river of grass" and UNESCO world heritage site. However, the natural flow conditions in the Everglades are complex and poorly understood. A better understanding of flow dynamics is important not only as a target for restoration designs, but also to elucidate the mechanisms by which ridge and slough structures (low elevation land and shallow channels, respectively) are maintained by the flow. Current hypotheses include direct transport of sediment or the effects of nutrient transport on soil chemistry. We perform a set of tracer releases using Sulfur Hexafluoride (SF6) to examine these hypotheses, as well as provide information of immediate utility for any possible restoration plans in the South Florida Water Management District's WCA-3A. SF6 tracer releases offer the ability to both visualize and quantify the flow dynamics over a large area (roughly 30 ha). We measure SF6 levels to high accuracy using a uniquely rugged and portable gas extraction and chromatography unit. Using these techniques, we compare the flow in an area with relatively little degradation with areas both upstream and downstream of a typical canal/levee obstruction.
H41A-0132
Adjustment Needed for Helley-Smith Bedload Samples Collected at low Transport Rates on Coarse Gravel Beds
River restoration frequently requires measurements of bedload transport as part of a site specific rating curve or as input for model calibration. A Helley-Smith type (HS) sampler is commonly used for these measurements, but it can have biased results. Several studies have shown that HS samplers deployed directly on a coarse gravel bed interact with the bed sediment. The interaction may take the form of perching on top of large particles (which prevents collection of particles moving in close contact with the bed), dislocating and scooping pebbles, creating scour around the sampler, and sucking sediment into the sampler due to a hydraulic efficiency >1. This interaction can lead to an assessment of transport rates that are either too high or too low. By contrast, studies that deployed a HS sampler on a sill report good correspondence with transport rates collected using other devices. This study examined whether elimination of direct bed contact with a HS sampler improves its sampling accuracy. Paired bedload samples were collected (either simultaneously or immediately following each other) with a 3- inch, thin-walled HS sampler in two mountain gravel-bed streams. One set of samples was collected conventionally by placing the sampler for 2 minutes each onto 15 evenly spaced locations across the stream directly on the bed. The other set was collected by placing the sampler for 5 minutes each onto 6 metal ground- plates installed flush with the streambed at about even intervals across the stream, i.e., mimicking placement of a HS sampler onto a sill. The time of ground contact per cross-section (15 x 2 min vs. 6 x 5 min) was the same for both sets of samples. Results from both streams showed that the HS sampler deployed on ground-plates measured smaller transport rates than the one deployed directly on the bed. The difference was most pronounced for the lowest flows and more pronounced for gravel than for sand bedload. At 50% of bankfull flow, gravel transport rates obtained when the sampler was deployed on the bed was 3 times greater than on the ground-plates at one stream and 150 times greater at the other. At near bankfull flow, transport rates from both deployments yielded similar results. The difference in sampling results can be explained as a function of transport rates. Both deployment versions yielded similar results for transport rates larger than 10 g/m/s because bed interactions contribute only a minor amount to the total sample volume. At low transport rates, these interactions dominate the sampling outcome, such that at rates of 1 and 0.1 g/m/s, the HS deployed on the bed yielded transport rates 1 and 3 orders of magnitude above those collected with the HS sampler deployed on ground-plates. Because deployment of the HS on ground-plates eliminates bed interaction and its distorted transport rates, an adjustment factor (correction function) is proposed for transport rates measured with the HS sampler set directly on a coarse gravel bed. A correction of sampling results measured during low transport would enable more accurate calibration of site specific models when only low flow measurements are available.
H41A-0133
IRBM for the Rio Conchos Basin as a Restoration and Conservation Tool
The Rio Conchos basin is the main water supply for the people of the State of Chihuahua and the middle and lower Rio Bravo in northern Mexico. Flowing for about 850 km from the highlands of the Sierra Tarahumara towards the wide valleys of the Chihuahuan Desert, the river presents recurrent periods of water stress and its basin of 6.7 million of hectares experiences a wide spectrum of problems such us long drought periods, water over allocation and extraction, water pollution, severe soil use changes. Besides, drastic soil moisture reduction is forecasted by effects of climate change. These natural and anthropological harmful situations impose a serious stress for this important and beautiful river and the rest of the basin hydrological resources. The WWF-Gonzalo Rio Arronte Alliance and its partners USAID, The Coca Cola Company and RICOH are implementing since 2004 an Integrated River Basin Management (IRBM) strategy to recover the natural integrity of the Rio Conchos in the form of environmental flow. The strategy includes the five basic working lines: i) development of river basin scientific knowledge, ii) strengthen of local institutional capacities, iii) development of demonstrative projects, iv) strengthen of indigenous communities, v) education and communication. Although the implementation of the IRBM program is expected to show main results until the year 2050, some interesting results have been obtained. The strategy has provided i) new basic knowledge about the basin dynamic events such as soil change use rates, baseline values of biological integrity, water economic values, among others; ii) strong program acceptance by government and main water users (farmers), and the integration of a working group formed by government, academia and NGO's; iii) local acceptance and understanding of benefits about basin management (soil recovery, reforestation, ecological sanitation) through demonstrative projects; iv) social organization; v) few advances in education and communications have been obtained. Some of the negative results of program implementation are lack of positive acceptance by powerful groups and a couple of NGO's currently working in the basin, and low credibility from indigenous governors (leaders of the Tarahumara group). http://www.wwf.org.mx/wwfmex/prog_cuencas.php
H41A-0134
Numerical Prediction For Channel Bed Changes Near Groyne In Experimental Flume
Numerical modeling for groynes in a rectangular section flume was developed to predict channel bed changes and to investigate the best performing groyne installation interval. Five different porous groynes were simulated in this study to evaluate hydraulic influences on the maximum scour depth induced by the groyne. Channel surface elevation and velocity changes near groyne were measured using surface topology digital imaging system and three-dimensional acoustic doppler velocimeter. Three-dimensional solutions governed by the Reynolds averaged Navier-Stokes and continuity equations were calculated using a commercial computational fluid dynamics code, which uses the finite volume method. For considering turbulent open channel flow in this computations, k-e model and Reynolds normalization group model were employed. Permeability of the groyne was reproduced by changing the gap between 2 cm diameter of cylinders. The approach water depths and the approach velocity acquired from the physical model were treated as the boundary conditions for the numerical model. Positive velocity boundary and the continuative boundary, which consists of zero normal derivatives at the boundary for a smooth continuation of the flow through the boundary, were set for inflow and outflow of the domain (x-direction). Atmospheric pressure boundary and no-slip wall condition were assigned at the top and the bottom of the domain (y-direction), respectively. Computed maximum scour and deposition depth and channel bed changes near the groyne were compared with the physical model measurements for validation of the numerical model. Calibration statistics of a mean error and a normalized root mean squared value was provided with 95% confidence interval plot. The numerical model computations showed very positive agreement with the physical model measurements. The relationship between the maximum scour depth and groyne porosity was generated. It was found that the numerical model could complement the physical model, and this numerical model will provide valuable information for design of groyne placement interval.
H41A-0135
Modeling Water Temperatures in the Colorado River Below Glen Canyon Dam, Arizona to Assess the Influence of Operational and Environmental Factors on Downstream Thermal Dynamics
The closure of Glen Canyon Dam in 1963 transformed the seasonally warm Colorado River into a consistently cold river owing to the hypolimnetic releases associated with deep penstock withdrawal structures within Lake Powell. This regulation effect has substantially altered the thermal regime of the downstream riverine environment which, in turn, has greatly impacted the biota of the river corridor, particularly native fishes and the aquatic food base. Given the significance of water temperature as one of the primary controls on many biological processes in aquatic ecosystems, the capability to predict downstream water temperatures allows resource managers the ability to assess potential future impacts of environmental and operational factors. To support adaptive management below the dam, we developed a suite of water temperature models for the mainstem and nearshore environments. The modeling suite consists of a one-dimensional temperature model for the mainstem, completely mixed "pond" models for nearshore, low-velocity "backwater" environments that are thought to be important rearing habitat for juvenile native fish, and multi-dimensional models for kilometer scale reaches in order to evaluate water temperatures in backwaters as well as other less defined shoreline habitats. The calibrated mainstem model has been used to evaluate water temperatures under a variety of scenarios for water releases from Glen Canyon Dam, such as flows with daily fluctuations (i.e. electricity load-following) compared to steady flow releases, and across a range of annual release volumes and temperatures associated with variability of basin hydrology and reservoir storage. Simulation results suggest that daily dam operations result in negligible differences in mainstem water temperature throughout the river corridor (average difference < 0.1°C). However, storage conditions in the upstream reservoir, which are controlled by decadal-scale upper Colorado River basin climate, can have a significant influence on mainstem temperatures. For example, simulated average temperature differences between low and high release volumes for a two week period with typical, mid-summer meteorological conditions ranged from 0.5°C approximately 24 kilometers below the dam to 2.6°C approximately 386 kilometers below the dam. Similar simulation efforts are currently underway with the nearshore temperature models to assess the effects of dam operations and environmental factors on thermal dynamics in backwaters and other nearshore environments.
H41A-0136
Channel Reconfiguration Impacts on Solute Transient Storage Dynamics in the Provo River, Utah
The Provo River Restoration Project has reconfigured 19-km of previously channelized and diked river length of the Provo River, Heber Valley, Utah. Reconfiguration has resulted in increased number and frequency of pool-riffle sequences and increasing river sinuosity. We expected that this increase in channel morphologic heterogeneity would result in enhanced transient storage of stream water due to 1) greater pool area that could accommodate enhanced in-channel dead zone storage, and 2) enhanced hyporheic exchange that would occur in response to greater hydraulic head variability throughout the reconfigured reaches. We measured channel geomorphology and estimated transient storage parameters in three reconfigured and three channelized river reaches to test our prediction. Transient storage parameters were estimated from simulations of Rhodamine-WT breakthrough curves in each of these six reaches using a 1-D solute transport model that accounts for transient storage. Despite substantial channel alterations, we found that estimates of mean residence time in storage, and mass transfer rates were not significantly different between reconfigured and channelized reaches. We did, however, find a significant increase (186%) in estimates of relative storage capacity (ratio of storage volume to main channel volume) in reconfigured reaches, compared to channelized reaches. Statistical correlation between geomorphic parameters and relative storage capacity suggests that reconfiguration may have resulted in increased in-channel storage in larger, more frequent reconstructed pools, and increased hyporheic exchange through shallow, rapidly exchanging flowpaths in steep reconstructed riffles. Thus reconstructed morphology did not yeild substantial increases in hyporheic exchange, as we had proposed. We observed substantial compaction of reconfigured channel beds during the construction process, suggesting that hyporheic exchange was likely limited by a shallow, low-permeability boundary.
H41A-0137
Influence of Hyporheic Flow and Geomorphology on Temperature in Large, Gravel-bed River, Clackamas River, Northwestern Oregon
The hyporheic zone influences the thermal regime of rivers, acting as a temperature buffer by storing and releasing heat throughout the day. We quantified the amount of hyporheic exchange in a 24-km reach of the lower Clackamas River (median discharge = 115 m3/s; minimum mean monthly discharge = 25 m3/s in August) in northwestern Oregon, and through use of a simple mixing model, estimated how much hyporheic exchange cools the river during hot summer months. Hyporheic exchange was primarily identified by temperature anomalies, which are patches of water that demonstrate at least a 1°C temperature difference from the main channel. Forty hyporheic temperature anomalies were identified through field investigations and TIR (Thermal-Infrared-Radiometry) imagery in Summer 2006. The location of anomalies is associated with gravel bar morphology, specifically features like bar channels and bar heads that act as preferential pathways to hyporheic flow. Further field characterization and groundwater modeling on three Clackamas gravel bars indicate residence times of hyporheic water can vary from hours to weeks and months, largely determined by hydraulic conductivity and how recently the gravel bar formed or was reworked. Upscaling of modeled discharges and hydrologic parameters from these bars to the other anomalies on the Clackamas network shows that hyporheic discharge comprises a small fraction (< 1%) of mainstem discharge, making overall river cooling effects small. However, the presence of cooler patches of water within rivers act as thermal refugia for fish, making the creation or enhancement of hyporheic exchange an attractive method of river restoration.
H41A-0138
On the Ability of a 2D Model for Predicting Contraction Scour
Contraction scour is often encountered in natural rivers due to flow area contraction or river restoration structures. Such scour may be predicted better with multi-dimensional models. However, it has been reported that the two- dimensional (2D) depth-averaged model is inadequate for modeling the contraction scour. Two potential problems of the 2D model have been identified in previous studies: inability to account for the three-dimensional (3D) effect and deficiency in the turbulence model. The finding led to efforts to use 3D models to model the contraction scour. This study aims to investigate whether other factors play important roles in the poor performance of 2D models. In the process, an improved prediction using the 2D model is obtained and factors influencing the model prediction are identified. The motivation of the study stems from the fact that a 3D model is still too complex, and many engineering applications still have to rely on 2D models. Our study shows that the 2D model is adequate for the contraction scour modeling, and comparable results with those of 3D modeling may be obtained. In addition to the 3D effect and turbulence models, other factors play important roles such as those related to the sediment transport model. Through a parametric study, the relative importance of a number of parameters is studied and suggestions are given with regard to the proper calibration and verification.
H41A-0139
Use of Ground-based LiDAR in River Restoration and Ecohydrology
Rivers and their associated landforms have complex topography and vegetation that is difficult to capture with traditional survey techniques such as total stations or GPS due to interference from vegetation and relatively low point resolution. Ground-based LiDAR techniques offer much higher point spacing, increased speed of data acquisition, and decreased cost compared to traditional survey techniques but as yet have not been evaluated for use in river restoration or ecohydrologic applications. For this project, we evaluated the efficiency and utility of ground-based LiDAR over a wide variety of river restoration and hydrology projects. We found that ground-based LiDAR is an extremely useful technique that could greatly improve the science of river restoration through improved survey quality of bare ground surfaces, built restoration structures, and vegetation. In particular, ground- based LiDAR is well-suited for quickly and accurately documenting as-built conditions of restoration projects. Surveys of as-built conditions would greatly improve monitoring efforts, most of which currently rely only on construction design documents, not what was actually built. Ground-based LiDAR also could be used for compliance checks by funding agencies with a minimum of effort and time. In summary, ground-based LiDAR is an excellent monitoring tool for a broad range of geomorphic, hydrologic and vegetation applications, all of which are fundamental to the study of river restoration.