H31J-01 INVITED
Toward Predictive Stream Channel Design
A professional practice of stream restoration links restoration objectives and actions via predictive relations connecting the project site to its watershed via the supply of water, sediment, and nutrients. At present, most stream channel design is based on analogy: a template is sought in a nearby or idealized channel that the designer judges to be suitable. But if a disturbed stream is adjusting to changes in essential controlling factors, an appropriate template will not exist, a predicament is commonly encountered in practice. More critically, an analogy approach cannot provide true prediction because it provides no basis for linking cause and effect in a logically complete and testable framework. At best, the design can be tested after the fact for its ability to transport the supplied sediment with the available flow. The alternative approach explicitly incorporates the essential drivers – the supply of water and sediment – in the design process. The tools available for an explicit, predictive design process have advanced in recent years, particularly in terms of the treatment of mixed grain sizes and ready implementation of hydraulic flow and sediment routing. Yet particular challenges remain. Forecasts of sediment supply and its uncertainty remain difficult and time consuming. Improved guidance for selecting a design discharge requires explicit connections between the physical and ecological components of the project. Methods for incorporating uncertainty in channel design are overly simplistic. This talk outlines recent advances in the logic and tools for predictive stream channel design and examines challenges in its development and implementation. A primary challenge for those engaged in research is developing a closer understanding of the problems faced by those engaged in restoration practice. Some design problems will be addressed via improved predictive tools spun off from research. Other design problems will be resolved by the clever specification of project objectives that effectively account for uncertainty and acceptable risk. Advances in restoration practice will require close collaboration through which research results find their way into practice and practice helps define the most pressing research priorities.
H31J-02
Quantifying Reductions of Mass-Failure Frequency and Sediment Loadings from Streambanks using Toe Protection
Streambank erosion represents an important form of channel adjustment and a significant source of sediment in disturbed streams, often contributing 60-80% of the suspended sediment load. Mass failures regularly occur by a combination of hydraulic processes that undercut bank toes and geotechnical processes causing bank collapse by gravity. Little quantitative information is available on the effectiveness of bank treatments on reducing erosion. To evaluate potential reduction in sediment loadings, the hydraulic and geotechnical processes responsible for mass failure were simulated under existing and mitigated conditions using a Bank-Stability and Toe-Erosion Model. Two critical erosion sites were selected from each of three watersheds that contribute the greatest amounts of fine sediment by streambank processes in the Lake Tahoe Basin: Upper Truckee River, Blackwood and Ward Creeks. Blackwood and Ward Creeks represent west-side, steep (0.008-0.03 m/m), coarse-bedded systems where 10-15 m-high terraces sporadically abut the channel. The Upper Truckee River represents flatter (0.002 m/m) sections that meander through grassed meadows. To provide for the driving, hydraulic forces, the 1995 annual hydrograph was selected as a typical high-flow year. The rain-on-snow event of January 1-2, 1997 (a 50- year event in some parts of the basin) was also added. Stage data from gauging stations were discretized into individual events to use with channel gradient to calculate boundary shear stress. Bank-material strength data were collected for each layer using a borehole shear-test device. Species-specific root-reinforcement values were applied based on root distributions using a fiber-bundle model. Hydraulic erosion was simulated using an excess shear-stress approach in the toe-erosion sub model. The new geometry was then exported into the bank-stability sub-model to test for stability of the bank under peak flow and drawdown conditions. In this way, BSTEM was used iteratively for all flow events under both existing conditions and with stone-toe protection. Volumes of material eroded by hydraulic and geotechnical processes were tracked for each event and summed to make comparisons between existing and mitigated conditions. Under existing conditions, total streambank erosion ranged from 472 m3 to 5260 m3 of which 35 m3 to 900 m3 were fine grained (silts and clays). On average, 13.6% of the material was eroded by hydraulic shear, the remainder by mass failures, which occurred about 5 times over the period. Iterative simulations with 1.0 m-high rock-toe protection showed a dramatic reduction in average loadings (87%; std. error = 4.2%). Failure frequency was reduced in most cases to a single episode, coinciding with recession of the January 1-2, 1997 rain-on-snow event. Thus, an almost 90% reduction in streambank loadings was realized by virtually eliminating the erosion of only 14% of the material that was entrained by hydraulic forces. Thus, simulations show average load reductions of about an order of magnitude (2070m3 to 127 m3 for total erosion; 292m3 to 21.2 m3 for fines). Results stress the critical importance of protecting the bank toe-region from steepening by hydraulic forces that would otherwise entrain previously-failed and in situ bank materials, thereby allowing the upper bank to flatten (by failure) to a stable slope.
H31J-03
Remote Sensing of Fire Effects on Tamarisk Water Use and River Restoration
Millions of dollars have been spent in New Mexico to remove tamarisk from riparian lands along the Rio Grande River, and restore previously monotypic tamarisk areas with native species such as cottonwood and willow. Elevated evapotranspiration (ET) rates of tamarisk are a main factor in motivating the invasive species' control. One of the tamarisk removal techniques is through the use of controlled burning. In this study, LANDSAT and MODIS satellite imagery was used in conjunction with the Surface Energy Balance Algorithm for Land (SEBAL) computer model to compare ET and NDVI of tamarisk covered riparian areas before and after three recent fires: 1. Mitchell Fire of April 9-16, 2005, which burned 450 hectares, 2. Marcial Fire of May 3-10, 2006, which burned 2,250 hectares, and 3. Bosquecito Fire of June 6-9, 2006, which burned 260 hectares. By comparing the remote sensing results to field point measurements, we evaluate the spatial and temporal ET and vegetation recovery after fires. Our results demonstrate: 1. SEBAL can be used as a low cost effective tool to quantify changes in riparian water use caused by river restoration projects, 2. Tamarisk ET rebounds much faster after fire than previously thought; one year after fire the tamarisk density and ET returns to pre-fire conditions. This study will ultimately advise hydrological and ecological managers around the globe on tamarisk regeneration rates and changes in ET post fire, and will evaluate the efficacy of fire as a restoration method in river restoration science.
H31J-04
Interactions Between Fluvial Processes and Vegetation in a Newly Created River Channel
Following floodplain aggregate extraction, a new river channel was cut in a sinuous, intact ribbon of the floodplain of the River Cole, West Midlands, UK at a point where the river emerges from its urban headwater catchment. The new channel had a trapezoidal cross section of approximately 10m width with no artificially created "geomorphic" forms and no reseeding or planting of vegetation. This paper describes the evolution of fluvial landforms and vegetation cover and composition over the first five years following the diversion of the river into the new channel. A close relationship between vegetation and geomorphology was found across a range of spatial scales. This led to the establishment of a geomorphologically and ecologically complex system within the first three years, with 145 taxa of viable seeds and plants identified. This research illustrates the crucial role of vegetation as well as fluvial processes in driving river morphology. It also indicates that in rivers of modest energy the provision of an initial, sinuous corridor is sufficient to induce rapid development of fluvial features and vegetation cover without the need to construct bed forms or to seed the banks.
H31J-05
Effects of Removal of Riparian Vegetation on Levee Stability on the Sacramento River
A new policy of the US Army Corps of Engineers requires that all levee vegetation be removed from federal levees in California. This directive requires levees to be cleared of all vegetation to preserve channel capacity and allow access for inspection and repair. The case for leaving vegetation in place on levees has largely been an environmental one, with concerns regarding removal of habitat and aesthetics. However, stability factors should also be considered. A previous study by Shields and Gray (1992) investigated the effects of vegetation on sandy levee integrity along the Sacramento River, just one such river that is affected by this vegetation-removal policy. Their study showed that even low root concentrations allowed for more stable bank conditions under worst-case conditions for bank stability. In the years since this initial study, modeling of root-reinforcement and streambank stability has improved greatly. This study used geotechnical data collected along the Sacramento River to model the effects of woody and herbaceous vegetation on levee stability using the Bank Stability and Toe Erosion Model developed at the National Sedimentation Laboratory and the root-reinforcement model, RipRoot. Model runs were carried out for a 4 m high levee with 2H: 1V and 3H: 1V slopes, and vegetation growing at different locations on the levee. Levee material was assumed to be a homogeneous, sandy soil, with very low cohesion (0.84 kPa). Three hydrologic conditions were modeled: baseflow conditions, peak of hydrograph, and the most critical bank condition during the receding limb of a hydrograph. Roots were assumed to grow perpendicular to the soil surface, with the additional cohesion due to roots only being added to soil layers in which the roots extended beyond the potential shear surface in that layer. Values for root-reinforcement were calculated using the RipRoot model, using typical root densities, depths and tensile strength measurements for different riparian species measured at sites across the USA. Values of 3, 15, and 20 kPa were added to the banks to represent young saplings, bunch grasses and mature trees respectively. Results showed that the levees were stable without the reinforcing effect of vegetation under all conditions, except under drawdown conditions which are the most critical. In those cases, root-reinforcement increased levee- stability significantly. A 2H: 1V levee had a factor of safety of 0.33 under these conditions without vegetation and a 3H: 1V levee a value of 0.54 (values <1 are unstable). With the addition of vegetation to the levee sides, factor of safety values increased to >1 under all conditions. Reinforcement added by mature trees and bunch grasses provided highest factor of safety values of up to 8.16 and 5.13 for 2H: 1V and 3H: 1V slopes respectively. The findings suggest that root-reinforcement of levees should be taken into account before complete removal of vegetation is carried out along rivers such as the Sacramento River. In cases where levees are composed of largely uncohesive materials, root-reinforcement provides significant support to the soil matrix, whilst additionally reducing shear stresses acting on the soil from flowing water and protecting the levee from rainfall impact and runoff. In deciding the case for removal of levee vegetation, these positive influences of vegetation should be weighted carefully against the desire for increased channel capacity and any possible negative influences of plant roots on levee integrity.
H31J-06
Sediment provenance in the Minnesota River basin- Using lakes to determine river sediment source types
The Minnesota River and its tributaries have suspended sediment loads that exceed EPA standards. Lake Pepin, a natural reservoir for the Minnesota River, Upper Mississippi River, and St. Croix River watersheds, currently has an accumulation rate of approximately 10 times pre-settlement rates and is the focus of restoration efforts. The Minnesota River watershed is >80% row-crop agriculture and has a significant amount of tile and ditch drainage networks. Although it only accounts for 38% of the Lake Pepin watershed, it contributes 80-90% of all sediment that reaches Lake Pepin. Before effective mitigation programs can begin in the watershed, sediment sources must be identified. The main sources for river sediments, as constrained by this study, are: (1) bank and bluff erosion of sharply incised stream channels and near-channel gully and ravine erosion at the bluff/upland interface, all of which are referred to as non-field sources, and (2) topsoil erosion from agricultural fields. Our goal is to apply a new sediment-tracing technique using the atmospherically deposited radionuclides 210Pb and 137Cs to apportion sediment to one of these original landscape positions. 210Pb and 137Cs are good fingerprinting radioisotopes because they are atmospherically deposited and are highly particle reactive, thus sediments from sources with significant atmospheric exposure, such as fields, have higher concentrations. Conversely, sediments from sources with minimal atmospheric exposure, such as banks, have negligible concentrations. The total amount of 210Pb and 137Cs in river sediments is determined by measuring concentrations on sediments deposited in backwaters and artificial impoundments. The signature of the radionucludes from field sources is determined by measuring flux in a series of reference lakes that have no significant channelized inputs. The reference lake system works on the assumption that the same processes that carry field sediment to lakes are the same in river systems. Sediment fingerprints are compared by using a simple mixing model to apportion sources. Preliminary results indicate that >70% of the total suspended sediments in the highly agricultural Minnesota River tributary watersheds are from near-channel, non-field sources. This agrees with the Lake Pepin sediment cores, which integrate the entire watershed, with 75% from non-field sources. Because the Lake Pepin sediment accumulation rate has increased ten-fold since European settlement in Minnesota and the non-field contribution is >70%, it can be inferred that the increase in suspended sediment is not a natural phenomenon.
H31J-07
A Conceptual Framework for Post-Project Assessment Applied to the Provo River Restoration Project, Utah
Comprehensive assessment of stream restoration projects necessitates evaluation of: 1) the actual pre-project condition in relation to the public perception of impairment, 2) the project concept, goals, and objectives, 3) the project design, 4) the project as actually built, and 5) the project performance. We applied this framework in assessing the recently completed Provo River Restoration Project (PRRP). Begun in 1999 and completed in 2007, the PRRP's budget was 10 million for construction of approximately 16 km of channel and adjacent floodplain wetlands. We analyzed project planning documents, design documents, and made field measurements of the Provo River channel before and after channel re-alignment. Although the impaired, pre-project channel was never explicitly measured by restoration designers, our measurements demonstrate that the bed material organization and floodplain inundation frequency was perturbed from those attributes typical of channels in similar physiographic settings. Project designers did not develop quantitative project goals, and there were no metrics by which performance success was to be measured. Design documents demonstrate that the realigned channel has the potential to re-establish channel and floodplain connection. Surprisingly, there were significant differences between design and as-built channel geometry. These discrepancies have the potential to adversely impact project performance in re-establishing the ecosystem benefits provided by a naturalized channel/floodplain connection. However, field construction of a channel whose capacity is larger than designed has been compensated by a hydrologic regime whose common floods have been larger than anticipated. In addition, the original channel design did not explicitly consider sediment supply, which has the potential to rearrange reconstructed channel elements in the downstream part of the project. Collectively, this analysis demonstrates the degree of uncertainty and ambiguity associated with channel restoration design, the potentially significant differences between design and as-built channels, and the critical need to evaluate project performance by metrics such as changes in the distribution and characteristics of spawning gravels, frequency of floodplain disturbance, impact on in-stream temperature, and changes in hyporheic function.