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9 24 2004 (1)


Effects of riparian tree management on flood conveyance study of Manning's Roughness in vegetated floodplains with an application on the Embarras River in Illinois

 Volume/Number:  2002  
 Time Period Covered:    
 Issuing Agency:  Illinois State Water Survey 
 Description:  Riparian forests have been proposed by the Technical Advisory Subcommittee of the Upper Embarras River Basin Commission in its alternatives for mitigating flood damages in the Village of Villa Grove and nearby farmlands. In order to evaluate potential reduction in flood stages in Villa Grove, methods for accounting for flow resistances induced by the riparian forests are needed in the hydraulic model for the Upper Embarras River. This project has been designed to better apply the available knowledge in practical field applications, particularly, how to evaluate the vegetal roughness in terms of Manning's andlt;EMandgt;nandlt;/EMandgt; coefficient for specified planting scenarios. Approaches presented in this report are literature review on Manning's roughness with emphasis on vegetative roughness, and evaluation and selection of methods for computing vegetative roughness due to riparian forests. The Petryk and Bosmajian (1975) method was selected for evaluating Manning's andlt;EMandgt;nandlt;/EMandgt; for mature trees because parameters could be reasonably obtained with available general field information. Using this approach, effects of riparian forest on floods were evaluated with the scenarios that the two-year floodplain has two densities of trees. The study reach was the channel between Villa Grove and Camargo. Also investigated were the options of having uniform tree density for the whole reach or half of the reach. An interface has been developed for implementing the computed andlt;EMandgt;nandlt;/EMandgt; values to a HEC-RAS hydraulic model, and capacity curves were developed to illustrate the effects on flood conveyance among these scenarios. The capacity curves thoroughly included possible boundary conditions and were presented in simple nomographs that relate discharge and downstream elevations to a specified flood elevation in Villa Grove. Therefore it was easier to evaluate the resulting effects of different alternatives. 
 Date Created:  9 24 2004 
 Agency ID:  CR-2002-02 
 ISL ID:  000000000861   Original UID: 999999994341Control Number: 0038 
  Documents:  20061002184728_ISWSCR2002-02.pdf