River Restoration From Boundary Layer to Watershed: Integrating Physical and Biological Science Over Space and Time VI
Presiding: J Jack, University of Louisville; A C Parola, Stream Restoration Institute, University of Louisville
NB31C-01 INVITED 08:30h
Restoration of Sinuous Channels with Pool Riffle Morphology in Floodplains Impacted by Mill Dams
The association of fine-grained alluvial sediments with agricultural period land-use in the Piedmont of the mid-Atlantic United States is well documented; however, the impact of small dams, primarily for mills has not been considered fully. Research of historic documents has revealed over 500 mills and associated dams in Lancaster County, Pennsylvania, directly impacting 300 km of valley. Examination of historic sediments indicated fine-grain sediment deposits up to 5.8 m. Although many dams have been partially or completely breached, the legacy effects of these structures persist and, in some cases, control channel grade. The high frequency of small dams on some streams has severely altered the interaction of the floodplain and channel, channel dimensions and transport characteristics, bed and valley profiles, bed substrate, and bank materials. Channel response to contemporary land-use can be highly affected by the legacy effects. Stream assessments and restoration designs should consider the important legacy effects of past mill operations. In design of stream restorations, several questions arise about dominant discharge, bank heights, local base level control, floodplain levels, potential excavation requirements and potential long-term channel lateral and vertical responses. These questions have been addressed in several stream restoration projects completed by the author.
NB31C-02 INVITED 08:45h
Similarity of Cross-Sectional Characteristics of Costal Plain Streams of the Southeastern United States
The Coastal Plain physiographic region of the southeastern United States encompasses over 1.2 million km2. Analysis of data from six separate geomorphic studies conducted in five of the 19 states having a Coastal Plain region suggests that streams of the Coastal Plain region have very similar correlations betweein bankfull cross section area (R2 = 0.92, n = 123) and discharge (R2 = 0.85, n = 77) with drainage basin area, despite geographic and hydrologic differences. Analysis of these data also shows that bankfull cross section area for southeastern Coastal Plain streams is 47% less than that indicated for other eastern US rivers by Dunne and Leopold (1978). Furthermore, estimates of bankfull discharge have an average return period of 1.14 years on the annual series. These bankfull flows range from 6.2-90% of the 1.5-year flow, often considered to represent the bankfull flow condition, and 4.4-75% of the 2-year flow, which some have suggested for restoration design. The strong correlation of these data supports the concept of using regional morphologic data for initiating assessments and should be considered as an important component of restoration design for streams of the southeastern Coastal Plain.
NB31C-03 09:00h
Mechanical and Hydrologic Effects of Riparian Vegetation on Critical Conditions for Streambank Stability: Upper Truckee River, California
The Upper Truckee River is the single largest contributor of sediment to Lake Tahoe with a large proportion of the suspended-sediment load coming from eroding streambanks. Recent advances in quantifying streambank processes highlight the combined effects of hydraulic erosion at the bank toe with geotechnical stability of the upper part of the bank and resulted in the development of a deterministic model of bank-toe erosion and streambank stability (Simon et al., 1999). The use of riparian vegetation in schemes of bank stabilization and stream restoration have become popular but are often implemented on a trial and error basis because of a lack of quantifiable information on the mechanical and hydrologic effects of vegetation on bank stability. This study, conducted along an unstable reach of the Upper Truckee River, combines field data with numerical modeling to quantify (1) hydraulic and geotechnical driving and resisting forces that control bank failures, (2) the mechanical and hydrologic effects of vegetation on shear strength, and (3) the critical conditions for bank stability with and without indigenous riparian species. Tests were conducted using three top-bank treatments: bare (control), Lemmon's willow, and young Lodgepole pine. The susceptibility of the bank toe to erosion by hydraulic forces was quantified by conducting submerged jet tests of in situ material to determine the erodibility coefficient (k) and the critical shear stress of the material. Drained, shear-strength parameters (cohesion and friction angle) of the banks were determined from borehole shear tests at various depths. Pore-water pressure and matric suction were monitored at three depths (30, 100, and 150 cm) with digital tensiometers to calculate changes in apparent cohesion for the period (September 2003 - May 2004) and to differentiate between the hydrologic effects of the two species. Root reinforcement of the two species was quantified by determining the relation between root-tensile strength and root diameter and integrating this with root distribution. Bank failures occurred during winter and spring, brought on by repeated basal melting of snow packs and rain-on-snow events. Lemmon's willow provided an order of magnitude more root-reinforcement (5.5 kPa) than the young Lodgepole pines (0.5 kPa). This difference is not related to differences in root strength, but to the far greater number of roots associated with Lemmon's willow. The hydrologic effects of the species varied spatially and temporally, were beneficial (drier) at depths of 100 and 150 cm but disadvantageous (wetter) near the surface (30 cm). In total, these effects were generally smaller in magnitude than the mechanical effects. Lemmon's willow provided a significant increase in bank strength to streambanks along the Upper Truckee River. Model runs conducted with the hydrologic and mechanical effects of Lemmon's willow included, showed no failures during the simulation period. Overall, Lemmon's willow provided a significant increase in bank strength, reducing the frequency of bank failures and delivery of fine-grained sediment to the study reach of the Upper Truckee River. The use of Lemmon's willow in a bank-stabilization scheme was found to be numerically equivalent to reducing the angle of the entire bank 10-15 degrees.
NB31C-04 09:15h
Stream Restoration Flow Estimation for Kentucky Streams
Stream restoration analysis and natural stream design include components of channel analysis related to flow magnitude and estimation of bank-full flow return period events. The more frequent flood events, associated with return periods less than 5 years, are important in restoration project planning and channel stability. Concepts proposed by Costa and O'Connor (1995) including components of flow duration, stream power per unit area, flood energy, and other characteristics indicate flood effectiveness as a source of geomorphic change in channels and floodplains. This study considers stream gage sites in the Cumberland, Tennessee and Mississippi watersheds of Kentucky. The magnitude and variability of recorded flood events is described, as well as the frequency of observed peak flow record. The importance of the higher frequency events is presented in the context of channel forming flow, bank-full discharge, effective discharge, and mean annual discharge (FISRWG 1998). Comparison of historical flow records, estimated bank-full flow derived from observed conditions is presented as a link between these information resources.
NB31C-05 09:30h
Wilson Creek Valley Restoration: Transformation of a Bedrock Channel to an Alluvial Stream
Wilson Creek Valley Restoration: Transformation of a Bedrock Channel to an Alluvial Stream The historically high economic value of alluvial valleys associated with agriculture, transportation infrastructure and habitation in the eastern US has resulted in the modification of virtually all alluvial stream valleys sufficiently suitable for subsistence agriculture. In stream valleys with thin alluvial veneers over bedrock, anthropogenic modification that includes increases in channel cross sectional dimensions, reduction in channel sinuosity, flood flow confinement, removal of form drag elements, and relocation of streams may transform alluvial stream reaches into bedrock reaches through channel incision processes. The effects of channel incision on infrastructure adjacent to deep alluvial systems are well known; however, exposure, flow quarrying, transport and deposition of broken bedrock pieces in valleys adjusted for transport of gravels represent serious fluvial hazards not widely recognized. In addition, the transformation from alluvial regimes to one dominated by bedrock involves changes of several important stream and wetland habitat features including substrate modification, reduction in bed topographic variation, reduced frequency of floodplain inundation, and a general groundwater lowering that adversely affects hyporheic zones. The restoration of Wilson Creek valley in central Kentucky transformed the stream channel from its anthropogenically imposed low sinuosity, confined, and relocated bedrock regime to a sinuous alluvial stream position near the valley center. The restoration demonstrates the ecological effectiveness of restoring valleys to accommodate streams to a form similar to their historic alluvial form. In addition to sediment transport, many other important aspects of stream design were considered to restore riverine habitat to a valley modified for agricultural purchases more than 80 years ago. An important restoration design component was the proximity of valley groundwater levels and aquifer thickness to support high stress period (low flow) pool and hyporheic zone habitat. Pre- and post-restoration biological data indicate an unanticipated rapid re-colonization of the restoration and an increase in fish species only months after restoration completion. Continued detailed post-restoration biological and geomorphic monitoring including bedload sampling is in-progress.
NB31C-06 09:45h
Structural Responses of a Stream Community to a Channel Relocation Using a Natural Channel Design Approach
Streams have been heavily impacted by historical and contemporary management practices. Restorations are seen as a way to enhance stream ecosystem integrity, but there are few restoration sites where pre- and post-restoration data are available to assess "success." In 2003, a channelized reach of Wilson Creek (Kentucky, USA) was relocated using a natural channel design approach. We compared the structural and functional responses of the stream pre- and post restoration/relocation at sites within Wilson and two reference streams. Despite the construction disturbance, water chemistry parameters such as nitrate and turbidity were nearly identical at sampling stations above and below the relocation for 2003-2004. Macroinvertebrate colonization of the relocation sites was rapid, with communities dominated by Cheumatopsyche, Perlesta and Baetis. Assessments of CPOM transport indicated that the new stream channel is more retentive of leaf and woody debris material than the pre-restoration Wilson sites or unrestored reference stream sites. The restoration of suitable habitat and the presence of "source populations" for colonization may compensate for even large-scale (but short-term) construction disturbance. More research is needed to assess the balance between the disturbance impacts of restoration installation and the long term benefits of stream ecological improvement.