Title

Geomorphic Problems with Natural Design Methods Applied to Stream Channel Restoration: Examples from a Comprehensive Survey of Projects in North Carolina and Implications for the Susquehanna Watershed

Start Date

23-9-2006 11:00 AM

End Date

23-9-2006 2:00 AM

Description

Federal, state, local, and private funds are increasingly being committed to stream restoration projects in the United States, to stabilize stream channels, reduce bank erosion, and improve aquatic habitat. The majority of these projects utilize a method known as Natural Channel Design, which relies on the classification of channels into Rosgen type stream reaches and assigns a prescriptive restorative treatment. Hallmarks of this strategy employ in-channel rock structures such as cross-vanes, J-hooks, and rock weirs as well as bank treatments such as rootwads, revetments, and live stakings. Little information exists about the performance of these projects. We are finishing a comprehensive 3-year survey of 400 restoration projects for the state of North Carolina completed within the past 8 years. The evaluation of these widely-used techniques can provide insights into the physical performance of restoration designs. The data are intended to provide guidelines for the design of restoration projects, and can be applied to similar projects in the Susquehanna watershed. Field surveys show that at about a third of examined sites, more than 70% of in-stream structures have sustained significant damage so that structures no longer perform their intended function; numerous others exhibit less severe damage. Some structures were rendered non-functional after the first overbank flood. In some cases, structures were installed, damaged, and repaired, only to be damaged again. Damages vary, but a common problem was the erosion around the landward side of J-hooks and cross-vanes during overbank flows, accelerating bank erosion locally and stranding the rock structures as ineffective midchannel islands. Approximately 30 % of the damaged structures were affected by either burial or bar development, which led to channel migration and flanking of the structure along newly eroded banks. Channels with high failure rates are associated with easily erodible bed and bank materials (in comparison to stream power) and/or streams with high rates of bedload transport. A preliminary review of design documents shows that analyses of the past or current geomorphic conditions were rarely undertaken, and analyses of sediment transport continuity through the restored reach were virtually non-existent. In-stream structures are designed to handle bankfull or lower flows. During overbank flows (which occur on-theaverage every 1-2 years), the structures become mere roughness elements. In some cases they may even accelerate bank erosion and channel migration. Moreover, many projects fail to consider changes in sediment and water yield that have occurred, or are occurring, due to historical land use changes and/or major flood events in upstream portions of the drainage basins. In these cases, streams may be in a protracted phase of morphologic adjustment, rendering restorative structures virtually useless A similar evaluation of restoration projects is needed in the Susquehanna watershed. The large number of sites in North Carolina exhibiting damaged structures within a few years of construction strongly suggests that more rigorous geomorphic and hydrological analyses are required to improve the performance of restoration projects. Even though this will initially require more time, money, and trained personnel, improvements in the success of stream restoration projects are possible by integrating geomorphic knowledge of river channel behavior with the planning and designing of restoration projects.

Keywords

stream restoration, Natural Channel Design, North Carolina, Susquehanna watershed

Type

Presentation

Session

Environmental Protection and Restoration, Renee Carey

Language

eng

This document is currently not available here.

Share

COinS
 
Sep 23rd, 11:00 AM Sep 23rd, 2:00 AM

Geomorphic Problems with Natural Design Methods Applied to Stream Channel Restoration: Examples from a Comprehensive Survey of Projects in North Carolina and Implications for the Susquehanna Watershed

Elaine Langone Center

Federal, state, local, and private funds are increasingly being committed to stream restoration projects in the United States, to stabilize stream channels, reduce bank erosion, and improve aquatic habitat. The majority of these projects utilize a method known as Natural Channel Design, which relies on the classification of channels into Rosgen type stream reaches and assigns a prescriptive restorative treatment. Hallmarks of this strategy employ in-channel rock structures such as cross-vanes, J-hooks, and rock weirs as well as bank treatments such as rootwads, revetments, and live stakings. Little information exists about the performance of these projects. We are finishing a comprehensive 3-year survey of 400 restoration projects for the state of North Carolina completed within the past 8 years. The evaluation of these widely-used techniques can provide insights into the physical performance of restoration designs. The data are intended to provide guidelines for the design of restoration projects, and can be applied to similar projects in the Susquehanna watershed. Field surveys show that at about a third of examined sites, more than 70% of in-stream structures have sustained significant damage so that structures no longer perform their intended function; numerous others exhibit less severe damage. Some structures were rendered non-functional after the first overbank flood. In some cases, structures were installed, damaged, and repaired, only to be damaged again. Damages vary, but a common problem was the erosion around the landward side of J-hooks and cross-vanes during overbank flows, accelerating bank erosion locally and stranding the rock structures as ineffective midchannel islands. Approximately 30 % of the damaged structures were affected by either burial or bar development, which led to channel migration and flanking of the structure along newly eroded banks. Channels with high failure rates are associated with easily erodible bed and bank materials (in comparison to stream power) and/or streams with high rates of bedload transport. A preliminary review of design documents shows that analyses of the past or current geomorphic conditions were rarely undertaken, and analyses of sediment transport continuity through the restored reach were virtually non-existent. In-stream structures are designed to handle bankfull or lower flows. During overbank flows (which occur on-theaverage every 1-2 years), the structures become mere roughness elements. In some cases they may even accelerate bank erosion and channel migration. Moreover, many projects fail to consider changes in sediment and water yield that have occurred, or are occurring, due to historical land use changes and/or major flood events in upstream portions of the drainage basins. In these cases, streams may be in a protracted phase of morphologic adjustment, rendering restorative structures virtually useless A similar evaluation of restoration projects is needed in the Susquehanna watershed. The large number of sites in North Carolina exhibiting damaged structures within a few years of construction strongly suggests that more rigorous geomorphic and hydrological analyses are required to improve the performance of restoration projects. Even though this will initially require more time, money, and trained personnel, improvements in the success of stream restoration projects are possible by integrating geomorphic knowledge of river channel behavior with the planning and designing of restoration projects.