Fundamentals of Engineering Design
parameters obtained in this way can be transferred to ungaged catchments of similar hydrologic
characteristics (USACE, 1985).
Information on the availability of HEC-1 can be obtained from the U.S. Army Corps of Engineers
Hydrologic Engineering Center internet site, http://www.waterengr.com/ hecprog2.htm.
Ogden (1998) describes CASC2D as a fully-unsteady, physically-based, distributed-parameter,
raster (square-grid), two-dimensional, infiltration-excess (Hortonian) hydrologic model for simulating the
hydrologic response of watersheds subject to an input rainfall field. Major components of the model
include: continuous soil-moisture accounting, rainfall interception, infiltration, surface and channel runoff
routing, soil erosion and sediment transport. CASC2D development was initiated in 1989 at the U.S.
Army Research Office (ARO) funded Center for Excellence in Geosciences at Colorado State University.
The original version of CASC2D has been significantly enhanced under funding from ARO and the U.S.
Army Corps of Engineers Waterways Experiment Station (USACEWES). CASC2D has been selected
by USACEWES as its premier two-dimensional surface water hydrologic model, and is one of the surface-
water hydrologic models support by the Watershed Modeling System (WMS) under development at
Brigham Young University.
CASC2D is a state-of-the-art hydrologic model that takes advantage of recent advances in
Geographic Information Systems (GIS), remote sensing, and low-cost computational power. Compared
with the USACE standard practice surface water hydrology model HEC-1, CASC2D offers significant
improvements in capability. HEC-1 requires the division of study watersheds into sub-catchments that are
assumed to be hydrologically uniform, while CASC2D allows the user to select a grid size that
appropriately describes the spatial variability in all watershed characteristics. Furthermore, CASC2D is
physically-based; CASC2D solves the equations of conservation of mass and energy to determine the
timing and path of runoff in the watershed. More traditional approaches such as HEC-1 rely on more
conceptualizations of runoff production. The physically-based approach is superior when the modeler is
interested in runoff process details at small scales within the watershed. Physically-based hydrologic
models are also superior when trying to predict the behavior of ungaged watersheds where calibration data
do not exist.
The following paragraphs describe CASC2D input requirements and simulation capabilities. These
descriptions are intended for general informational purposes. For more detailed descriptions, see Julien
et al. (1995) and Ogden (1997).
An explicit, two-dimensional, finite-difference, diffusive-wave scheme is used to route overland flow
in CASC2D (Julien et al., 1995). The Manning equation is used to calculate overland flow velocities,
requiring input of a map of spatially-varied Manning roughness coefficient values for overland flow. The
routing scheme is shown conceptually in Figure 5.6.