Fundamentals of Engineering Design
CASC2D has been employed to simulate a number of study-watersheds with considerable success.
Like any other hydrologic model, CASC2D is founded on assumptions of the relative importance of
different hydrologic processes. Recent experiences with CASC2D have shown that in regions of
infiltration-excess (Hortonian) runoff production, CASC2D is quite accurate at predicting runoff, even at
internal locations within the watershed (Johnson et al., 1993; Ogden et al., 1998). The continuous
simulation capability of CASC2D has been found to be particularly good for reducing the uncertainty in
estimating initial soil-moisture conditions, and for improving calibration uniqueness (Ogden and Senarath,
1997; Ogden et al., 1998). CASC2D has also proven to be valuable for studying extreme runoff events.
The model was recently applied to study the extreme flood on the Rapidan River, Virginia, on June 27,
1995 (Smith et al. 1996), for the purpose of examining geomorphological changes; and the extreme urban
flood event in Trenton, New Jersey (Stock, 1977) for the purpose of recommending stormwater
management improvements. CASC2D is currently being applied to evaluate the impact of radar-rainfall
estimation errors in a study funded by ARO, and in an NSF-sponsored study of the devastating flood that
severely impacted Fort Collins, Colorado, on June 28, 1997.
The overland erosion/sediment transport capabilities of CASC2D were evaluated in detail by
Johnson (1997). In upland areas, the method was shown to calculate sediment yield well with the
acceptable range of -50% to +200%. Compared with actual field observations of annual sediment yield,
CASC2D predictions were generally within 20% of observed values.
Further information on CASC2D capabilities and availability can be obtained from the U.S. Army
Engineer Waterways Experiment Station web site, http://chl.wes.army.mil /software/.
METHODS FOR ASSESSING HISTORICAL RIVER STABILITY
The channel stability assessment phase of the systems approach requires developing an
understanding of the total system dynamics. This understanding will allow the investigator to discriminate
between channel reaches that are degradational, aggradational, or in a state of equilibrium, and also to
catagorize channel banks as stable or unstable. A geomorphic investigation of the entire watershed is
beneficial, and the detail of the geomorphic investigation depends on the level of effort required for the
systems analysis. Through the geomorphic study, system responses, past and present, are determined, and
all pertinent data are assimilated to form a picture of the overall system stability.
Various tools that facilitate the stability assessment are developed from the gathered information.
The following sections present four typical tools used to assess channel stability. These include specific
gage analysis, comparative thalweg analysis, analysis of cross section geometry, and aerial photography.
A detailed field investigation is also extremely important in assessing channel stability because the physical
characteristics of the stream are indicators of the dominant geomorphic processes occurring in the basin.
Section 5.2 describes recommended procedures and methods for field investigations.