THE COUPLING OF A TWO-DIMENSIONAL HYDRODYNAMIC/SEDIMENT ROUTING MODEL WITH AN 
UPLAND WATERSHED EROSION MODEL IN A WATERSHED

Thanos Papanicolaou1 and A. Bdour1
The University of Iowa, Iowa Institute of Hydroscience and Engineering, Iowa 
City, Iowa 52242

In an effort to generate new knowledge and improved understanding of the complex 
interrelationships between watershed and instream parameters and the scale 
integrity influences on channel morphology, an integrated watershed 
hydrologic/sedimentation framework for medium size watersheds is developed.  
This framework provides advanced analytical techniques and numerical models for 
simulating upland (macro level) and instream (micro level) processes in an 
integrated fashion.  The framework is developed based on the premise that 
watershed-wide parameters have cumulative impacts on stream ecology and 
therefore, watershed modeling should facilitate integration of spatial and 
temporal scales in order to provide meaningful answers from the physical and 
statistical point of view.  First, a statistical analysis is employed to 
classify the watershed upland and instream affecting parameters and to 
quantitatively describe the impacts of these parameters on stream ecology, as it 
is expressed with the Index of Fish Density (IFD).  Second, the GeoWEPP soil 
erosion model is employed to simulate the hydrologic, and sediment entrainment 
phenomena at the uplands of the watershed.  Long-term averages and different 
frequency distributions analyses are performed to investigate the temporal 
variability in upland soil erosion processes.  Third, to accurately address the 
scale and coupling issues, a thorough investigation for the particle transit 
time is performed using the state of the art concepts, including the hypsometric 
curve approach and the particle virtual velocity approach.  It is demonstrated 
that a fine sediment particle moves from the uplands to the mouth of the 
watershed within a relatively short period of time (few days).  Fourth, this 
work also involves enhancing capabilities of an existing instream two-
dimensional (2-D) hydrodynamic and sediment transport model that was originally 
developed to simulate the transport of uniform sediments.  The new version of 
the model, EnSEDZL model, incorporates the Parker and Wilcock’s sediment 
transport equations to predict multifractional bedload transport rates.  
Supplemented by several empirical functions for predicting bedload transport 
capacity, hiding function, reference transport rate, etc.  The streambed 
elevation changes obtained from an overall mass balance equation.  The upland 
soil erosion model is eventually combined with the instream numerical model, by 
matching the return period for a rainfall storm event for the upland soil 
erosion processes with instream flow that has the same return period for 
instream sediment transport processes, to accurately determine the upstream 
boundary conditions for the instream sediment transport modeling.  Finally, 
modeling results are compared with 13-year detailed field data and against the 
predictions of commercial and private models, including the USACE models (RMA2 
and SED2D) and the 3ST1D model developed in-house.

Keywords: Watershed, Stream Scales, Sediment Transport, Upland Erosion, Coupling