Fundamentals of Engineering Design
5.1.5.2 Computation of Project Hydrology - Hydrology Models
Although there are many different computer models that can be utilized in developing project
hydrology, only HEC-1 (USACE, 1985) and CAS2D (Julien et al., 1995) will be discussed. HEC-1 is
probably the hydrologic model most familiar to USACE personnel, and CAS2D is the one of the surface-
water hydrologic models selected to be included in the watershed modeling system under development by
the USACE.
5.1.5.2.1 HEC-1
Ponce (1985) describes HEC-1, subtitled Flood Hydrograph Package, as a program designed
to be used for the simulation of flood events in watersheds and river basins (USACE, 1985). The river
basin is represented as an interconnected system of hydrologic and hydraulic components. Each
component models an aspect of the precipitation-runoff process within a portion of the basin referred to
as subbasin. Component description requires the knowledge of a set of parameters and mathematical
relationships describing the physical processes. The result of the modeling is the computation of streamflow
hydrographs at desired locations within the river basin.
A river basin is represented as an interconnected group of subbasins. Within each subbasin, the
hydrologic processes are represented by average parameter values. For hydrologically nonhomogeneous
subbasins, further subdivision may be necessary to ensure that average parameter values are representative
of each subbasin entity.
HEC-1 is an event model; therefore, it has no provision for soil moisture recovery during periods
of no precipitation, with simulations being limited to a single-storm event. The model calculates discharges
only, although stages can be indirectly determined through ratings supplied by the user. Alternatively, the
results of HEC-1 can be used as input to HEC-2, which calculates stages based on discharge by using
steady gradually varied flow principles. In HEC-1, stream channel routing is accomplished by hydrologic
methods. Therefore, the model does not account for the dynamic effects that are present in rivers of mild
slope. Reservoir routing is based on the modified Puls technique, which may not be applicable in cases
where reservoirs are operated with controlled outflow.
HEC-1 model components simulate the rainfall-runoff process as it occurs in a river basin.
Mathematical relations are intended to represent individual meteorological, hydrologic, and hydraulic
processes encompassing the rainfall-runoff phenomena. The processes considered in HEC-1 are (1)
precipitation, (2) interception/infiltration, (3) transformation of effective precipitation into subbasin runoff,
(4) addition of baseflow, and (5) flood hydrograph routing, either in stream channels or reservoirs.
Rough estimates of HEC-1 model parameters can usually be obtained from individual experience
or by other empirical means. Calibration using measured data, however, is the preferred way of estimating
model parameters. With rainfall-runoff data from gaged catchments, the mathematical optimization
algorithm included in HEC-1 can be used to estimate some model parameters. Using regional analysis,
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