Chapter 2. Hydrology
39
Three-dimensional models are also becoming available and are useful for
modeling secondary circulation associated with the confluence of tributaries
(Bradbrook et al. 1998). While these conditions exist along ephemeral streams,
they are more localized phenomenon and are not of great importance in under-
standing flow at the reach or watershed level. Three-dimensional models are
important, however, for identifying the limitations of two-dimensional models
and for identifying the needs of future model development (Bradbrook et al.
1998).
A number of hydraulic models, including HEC-RAS and several two-
dimensional models, have been accepted by FEMA for use in the National Flood
Insurance Program (Table 6). Despite the limitations of hydraulic modeling, the
accepted FEMA models may serve as a starting point for identifying the models
most applicable for identifying the OHWM along ephemeral streams and for
identifying the limitations that must be overcome to create reliable hydraulic
models for arid regions. Wide use of complex two- and three-dimensional models
is unlikely in the near future because of the detailed data collection required and
the difficulty in mastering the large-scale computer models (Leopardi et al.
2002). Additionally, it is unlikely that a single model will suffice for every
situation, and it is important that model performance be evaluated with respect to
specific field problems (e.g., delineation of the OHWM) as they are encountered
(Lane 1998).
Usefulness of Hydrology and Hydraulics for Delineating the OHWM
Hydrological parameters such as precipitation inputs and watershed charac-
teristics determine the amount of runoff entering a stream channel. The hydrau-
lics of the flow in the stream exerts a force on the channel boundary, which in
turn shapes the morphology of the channel. Although hydrology and hydraulics
are extremely important in modeling and predicting flow in channels, they are
unable to determine the ultimate channel morphology and physical features that
develop along a stream in response to a flow event of a given size (e.g., ordinary
or extreme flows). The development of channel morphology is not only depend-
ent on the flow conditions but also on sediment inputs and boundary conditions
along the channel, two aspects not adequately addressed in current hydrologic
and hydraulic models.
While hydrologic and hydraulic models may be insufficient to predict the
types and location of OHWM features, they can be extremely useful for recon-
structing the flow conditions responsible for developing the physical features
associated with the OHWM (see Chapter 3 for a list of these potential features).
Channel cross sections and estimates of Manning's roughness coefficient, along
Previous Page
