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Publication Number:  FHWA-HRT-14-064    Date:  August 2014
Publication Number: FHWA-HRT-14-064
Date: August 2014


Fish Passage in Large Culverts With Low Flow


This report documents a series of physical and numerical modeling efforts designed to support the development of a design procedure for characterizing the variation in velocity within non-embedded and embedded culverts. Physical modeling of symmetrical half-section circular culverts was conducted to provide data with which CFD modeling could be validated. Use of CFD allows more extensive test matrices to be evaluated in less time and budget than physical modeling.

The initial CFD modeling featured two-phase numerical computations that successfully reproduced the physical modeling results. To simplify the CFD modeling, single-phase modeling and truncated single-phase modeling were evaluated with good results. For the embedded culvert runs, a successful strategy for representing natural bed material within the culvert was developed.

Once the CFD modeling of the physical data was completed and validated, the CFD modeling was used to analyze the full culvert cross-sections (3-ft CMP) rather than symmetrical half sections. Subsequent test matrices included CFD runs scaled up to larger culvert sizes. One series of runs maintained Froude number-based scaling (6-ft CMP), and one series tested larger sizes without the scaling constraint (8-ft CMP). These full culvert cross-sections were used to develop and test the proposed design method.

The velocity distribution model that formed the basis of the design methodology is described by Chiu and Chiou and then Chiu.(30,31) Using the 42 CFD runs for a 3-ft diameter culvert, the five parameters necessary for the velocity model (βi, δy, ε, M, δi) were estimated. Based on geometric and hydraulic parameters available to a designer, relations were developed to estimate those parameters for design purposes. The approach was successfully validated on CFD runs for 6-ft and 8-ft diameter culvert models.

The proposed design procedure allows a designer to estimate the velocity throughout a cross-section. These data may be depth-averaged to provide a distribution of velocity and depth across the culvert cross-section that may be used to evaluate fish passage. Although developed for circular culverts, the parameters used in the method are such that the procedure should be applicable to rectangular and other shapes. Two design examples are provided to illustrate the method and the required computations.

Future work is recommended to further validate and improve the recommended design methodology. Further CFD runs should capture the following qualities:


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