EFFECT OF FLOODPLAIN VEGETATION ON FLOOD MANAGEMENT: CASE STUDY  OF
THE UPPER EMBARRAS RIVER

Matthew J. Hoffman 1 and David Soong 2.
1 Department of Civil and Environmental Engineering, University of Illinois at Urbana-
Champaign, Urbana, IL 61801; and 2 Illinois State Water Survey, 2204 Griffith Drive, 
Champaign, IL 61820.

Recently buffer strips have been considered as one of the preferred Best Management 
Practices (BMPs) for watershed management in controlling sediment and pollutant 
loads from entering streams.  Buffer strips also are known to be natural habitat. 
Though still are research agenda, documentation regarding the benefits on water 
quality and biology is available and some design specifications are outlined.  However, 
little detailed analysis has been done to investigate the hydraulic effects of buffer 
strips.

Buffer strips are applied to floodplains.  Changing floodplain characteristics can affect 
flood stages and thus impact local economics, alter flood frequency and therefore 
floodplain insurance, and impact the extent of riparian biomass and prominent stream 
geomorphologic properties.  This paper demonstrates how vegetated floodplains can 
affect stream carrying capacity by translating floodplain vegetation into Manning's 
roughness coefficients.  In that sense, the sizes of the tree, growing seasons (leaf 
masses), planing arrangement, and placement can all affect the n values and therefore 
flood stages.

The authors applied different techniques for determining floodplain n values 
augmentated by trees and shrubs.  Among different techniques they concluded proper 
methods for young and mature trees.  In addition, the sensitivity of flood plain n 
values to vegetation characteristics was considered.  The authors were able to 
identify what information is critical for obtaining accurate capacity predictions.

The results were applied to a HEC-RAS model for the Upper Embarras River in Central 
Illinois.  The authors tested a scenario considering the hydraulic implications if 
restoration of riparian buffer strips was from young to mature forest.  The hydraulic 
capacity of the same reach at different points in vegetation succession was 
determined.  In this way, the effect on flooding of riparian forest buffer strip creation 
near population centers can be estimated.  The HEC-RAS model results were then 
developed Hydraulic Performance Graphs and flood levels resulting from different 
vegetation schemes are mapped.  This methodology can be applied for purposes such 
as flood mitigation and wetland determination.

Keywords:  floodplain, vegetation, buffer strips, hydraulic capacity, hydraulic 
performance graph
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