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"Kochel, R. Craig"
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Assessment of channel dynamics, in-stream structures and post-project channel adjustments in North Carolina and its implications to effective stream restoration
Site assessment and monitoring data were analyzed for 26 stream restoration projects in North Carolina where the channel was reconfigured. Post-project changes in channel capacity were highly variable from site to site, but more than 60% of the projects underwent, on average at a given site, at least a 20% change in channel capacity. An analysis of site and basin geomorphology revealed that large post-construction adjustments were associated with highly dynamic stream channels characterized by a combination of high sediment transport capacity, large sediment supply, and/or easily eroded bank materials. In-stream structures along dynamic, reconfigured channels also exhibited high incidences of damage. Thus, the design and construction of channels in a state of equilibrium, which do not exhibit excessive erosion or deposition along highly dynamic rivers is currently problematic. In light of these findings, a conceptual framework based on geomorphic parameters is put forth to assess the likelihood of project success early in the design process to (1) eliminate high risk sites from consideration of channel reconfiguration and (2) improve upon the implemented management strategies that are ultimately used. It is also argued that where space permits an enhanced natural channel, adjustment approach is likely to be more effective than projects based on natural channel design.
Journal Article
Use and performance of in-stream structures for river restoration: a case study from North Carolina
In-stream structures including cross-vanes, J-hooks, rock vanes, and W-weirs are widely used in river restoration to limit bank erosion, prevent changes in channel gradient, and improve aquatic habitat. During this investigation, a rapid assessment protocol was combined with post-project monitoring data to assess factors influencing the performance of more than 558 in-stream structures and rootwads in North Carolina. Cross-sectional survey data examined for 221 cross sections from 26 sites showed that channel adjustments were highly variable from site to site, but approximately 60 % of the sites underwent at least a 20 % net change in channel capacity. Evaluation of in-stream structures ranging from 1 to 8 years in age showed that about half of the structures were impaired at 10 of the 26 sites. Major structural damage was often associated with floods of low to moderate frequency and magnitude. Failure mechanisms varied between sites and structure types, but included: (1) erosion of the channel bed and banks (outflanking); (2) movement of rock materials during floods; and (3) burial of the structures in the channel bed. Sites with reconstructed channels that exhibited large changes in channel capacity possessed the highest rates of structural impairment, suggesting that channel adjustments between structures led to their degradation of function. The data question whether currently used in-stream structures are capable of stabilizing reconfigured channels for even short periods when applied to dynamic rivers.
Journal Article
Paleoflood hydrology
The difficult task of estimating recurrence intervals for large floods has long plagued hydrologists because statistical measures fail when return intervals of floods exceed the length of historical data sets. Sediments deposited in the backwaters of large floods may accumulate thick sequences in tributary mouths. Stratigraphic and sedimentologic studies of these sequences combined with radiocarbon dating have established a 10,000-year paleoflood record for the lower Pecos and Devils rivers in southwestern Texas. This technique is rapid and relatively inexpensive and can be used where historical records are short or entirely absent.
Journal Article
U.S. Mid-Atlantic Barrier Island Geomorphology
Quantitative analysis of 15 geomorphic attributes of the mid-Atlantic barrier coast has revealed that there are systematic variations in the geomorphic arrangement of coastal features. Our analysis of attributes (measured at 1 km intervals along the 800 km study reach from Cape Henlopen, Delaware, to the North Carolina border) has resulted in an atlas of coastal geomorphic types that can be classified at scales ranging from tens of kilometers (regional scale) to several kilometers (local scale). Similar data collection of coastal geomorphic attributes could be made for other coastal areas and could be quantitatively compared to the mid-Atlantic coast through the use of principal components analysis.
Journal Article
The Role of Overwash on a Mid-Atlantic Coast Barrier Island
The importance of storms and their overwash in barrier island transgression is assessed quantitatively by determining net sediment budgets of four representative Assateague Island washover sites for the 1982-83 season. The results show that most of the annual accretion occurred during one large storm which had a return interval of 10 to 15 years. The combined effects of eight subsequent storms with return intervals of less than 2.5 years accounted for less sedimentation. Change due to eolian redistribution was insignificant.
Journal Article
Relative Role of Overwash and Aeolian Processes on Washover Fans, Assateague Island, Virginia-Maryland
There has been considerable controversy in recent years about the relative importance of overwash vs. aeolian processes in the vertical accretion or erosion of barrier islands. Most studies have used sediment budget data sets of two years or less in length. Analysis of coastal climatic data suggests that there are significant secular variations in storm intensity, frequency, and duration over time scales of 10's of years. Annual variations in climatic variables such as the magnitude and frequency of precipitation, high winds, and overwash have a considerable effect on accretion or erosion of washover fan surfaces along mid-Atlantic barrier islands. Conclusions and predictions based on data sets from very stormy years may be dramatically different than those based on data from relatively calm periods. Sediment budget data for six fans along Assateague Island are presented for a four-year period that includes an exceptionally stormy year, an exceptionally calm year, and two years having average storm history. Accretion due to overwash processes was exceedingly dominant during the stormy year, but still significant in normal years. In contrast, aeolian deflation and deposition dominated during the calm year. Overall, Assateague Island fan surfaces accreted significantly over the course of the study, by an island-wide average of 22 cm. Individual fans accreted by as much as 90 cm. Observed variations in fan surface changes depended primarily upon the frequency of precipitation, frequency of overwash, and topography of the local fan area. The importance of overwash or aeolian processes with regard to fan accretion and erosion appears to depend upon variability and cyclicity of climatic factors. The effects of two exceedingly large storms resulted in extensive accretion on fan surfaces that was greater than the sum of 15 moderate storms during the four years. Longer observations periods may be needed to project future changes on barrier island surfaces.
Journal Article
Beach Cusp Destruction, Formation, and Evolution during and Subsequent to an Extratropical Storm, Duck, North Carolina
Many studies have debated whether beach cusps are erosional or depositional features. The April 12-14, 1988, extratropical storm provided an opportunity to view the direct effects of one of the largest storms of the past decade upon beach sedimentology and morphology on barrier islands near Duck, North Carolina. Prior to the storm, the beach at Duck was characterized by a well-defined pattern of beach cusps with horn-to-horn spacings averaging 35 m. Storm-induced alterations were dominated by an initial period of beach erosion that remobilized the upper 30 to 50 cm of beach sediment, followed by aggradation. Net aggradation was most prominent along the middle beachface and within the pre-storm cusp bays. These morphologic adjustments resulted in the destruction of cusps, which were replaced with a post-storm planar beachface composed of horizontally bedded fine- to coarse-grained sediments. Within 24 hrs of storm subsidence, new beach cusps formed sequentially along the coast in the direction of longshore transport. Initial cusp formation resulted from beach erosion and the creation of bays in the planar storm-beach surface at positions of preferential post-storm runup. The initial cusp horns were composed of truncated horizontal beds of the planar beach accreted during the storm. After their formation, the cusps sequentially migrated downdrift. Migrating horns were composed of a coarse-grained sediment wedge that thickened toward horn crests, suggesting formation by deposition. It is concluded from these observations that beach cusps are both erosional and depositional in nature.
Journal Article