Physics Based, Integrated Modeling Of Hydrology And Hydraulics At Watershed Scales

Open Access
Author:
Huang, Guobiao
Graduate Program:
Civil Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 21, 2006
Committee Members:
  • Gour Tsyh Yeh, Committee Chair
  • Christopher J Duffy, Committee Member
  • Derek Elsworth, Committee Member
  • Arthur Carl Miller, Committee Chair
  • Andrew Scanlon, Committee Member
Keywords:
  • overland flow
  • channel flow
  • Watersheds
  • integrated models
  • numerical models
  • subsurface flow
  • finite element methods
Abstract:
This thesis presents the major findings in the development of the hydrology and hydraulics modules of a first principle, physics-based watershed model (WASH123D Version 1.5). The numerical model simulates water movement in watersheds with individual water flow components of one-dimensional stream/channel network flow, two-dimensional overland flow and three-dimensional variably saturated subsurface flow and their interactions. First, the complete Saint Venant equations/2-D shallow water equations (dynamic wave equations) and the kinematic wave or diffusion wave approximations were implemented as three solution options for 1-D channel network and 2-D overland flow. Different solution techniques are considered for the governing equations based on physical reasoning and their mathematical property. The nature and propagation of these approximation errors under more complex 2-D flow conditions are evaluated within WASH123D with comparison of simulation results of several example problems. The accuracy and applicability of dynamic-wave, diffusion-wave and kinematic-wave models to real watershed modeling is discussed with simulation results from numerical experiments. Second, the physics-based coupling between surface water and subsurface flow was investigated. Generally, there are two cases based on physical nature of the interface: continuous or discontinuous assumption, when a sediment layer exists at the interface, the discontinuous assumption may be justified. As for numerical schemes, there are three cases: time-lagged, iterative and simultaneous solutions. Numerical experiments are used to compare the performance of each coupling approach for different types of surface water and groundwater interactions. Third, The Method of Characteristics (MOC) in the context of finite element method was applied to the complete 2-D shallow water equations for 2-D overland flow. The search for genuinely multidimensional numerical schemes for 2-D surface water flow is an active research topic. We consider the Method of Characteristics (MOC) in the context of finite element method as a good alternative. The intrinsic difficulty in implementing MOC for 2-D overland flow is that there are infinite numbers of wave characteristics in the 2-D context, although only three independent wave directions are needed for a well-posed solution to the characteristic equations. We have implemented a numerical scheme that attempts to diagonalize the characteristic equations based on pressure and velocity gradient relationship. This new scheme was evaluated by comparison with other choice of wave characteristic directions in the literature. Example problems of mixed sub-critical flow/super-critical flow in a channel with approximate analytical solution was used to verify the numerical algorithm. The circular dam break problem was solved with different selections of wave characteristic directions and the performance of each selection was evaluated based on accuracy and numerical stability. Finally, the physics-based, integrated watershed model was tested and validated with the hydrologic simulation of a pilot constructed wetland in South Florida. For this field problem, strong surface water and groundwater interactions are a key component of the hydrologic processes. This study demonstrates the need and the utility of a physics-based modeling approach for strong surface water and groundwater interactions.