Improving Design Guidance for In-Stream Structures Used in Stream Restoration
Hickman, Elizabeth Louise
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Vane-type in-stream structures and step pool storm conveyance (SPSC) are more ecologically friendly alternatives to traditional stream channel stabilization and stormwater conveyance techniques. Vane-type structures have been widely accepted as elements of stream restoration projects and are regularly implemented in streams throughout the United States. However, these structures commonly experience partial or total failures of function or stability, often due either to improper installation or misapplication. This study undertook a thorough review of the available design guidance for the single-arm vane, j-hook vane, cross vane, and w-weir, which revealed that the existing guidance is composed of non-standardized recommendations largely based on practitioner experience and rules of thumb. Existing guidance was synthesized with current structure research and practitioner surveys to create factsheets for each of the four structures and the SPSC, with the intent of improving structure application and offering concise general guidance. This study also endeavored to improve the design of the SPSC by determining the most accurate of several common prediction methods for Manning's roughness coefficient n, used in SPSC design velocity calculations. This was done by using Rhodamine WT dye tracer experiments to determine n values during storm flows in two SPSC structures in Annapolis, MD, which were then compared to predicted n values. Values of Manning's n determined in the SPSCs at low flows (0.28-12) often exceeded the predicted n values (-0.17-3.9) by several orders of magnitude. Though the applicability of these results is limited, an increase in design n to 0.1-0.2 is still recommended.
General Audience Abstract
Vane-type in-stream structures are stone or wood structures installed within a stream channel for purposes such as streambank stabilization or aquatic habitat creation. Step pool storm conveyance (SPSC) is a technique which converts an existing steep stream or gully into a step-pool channel. Both of these techniques are more ecologically friendly than many traditional stream channel stabilization or stormwater conveyance techniques such as riprap or concrete storm drains. Vane-type structures in particular have been widely accepted as elements of stream restoration projects and are regularly implemented in streams throughout the United States. However, these structures commonly experience partial or total failures, either through structural collapse or failure to function properly. This is often either because they were improperly installed or because they were installed at a stream site where they were inappropriate or unnecessary. A review of the available guidance for the design of these structures revealed that the existing guidance is composed of non-standardized and sometimes contradictory recommendations which are largely based on designer trial and error and rules of thumb, rather than on the results of scientific experiments or modeling. The goal of this study was to improve the success of vane-type in-stream structures and the SPSC by providing factsheets offering clear and concise general design guidelines and sound recommendations for structure application. Flow studies of two SPSC structures in Annapolis, MD were also conducted to improve the design of that structure by measuring its flow characteristics in the field.
- Masters Theses