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HEC-ResSim has been developed by the
Hydrologic Engineering Center of the US Army Corps of
Engineers to aid engineers and planners performing water
resources studies in predicting the behavior of reservoirs
and to help reservoir operators plan releases in real-time
during day-to-day and emergency operations. The following
describes the major features of HEC-ResSim:
Graphical User Interface
Designed to follow Windows® software development
standards, HEC-ResSim’s interface doesn’t require
extensive tutorials to learn to use. Familiar data entry
features make model development easy, and localized “mini
plots” graph the data entered in most tables so that
errors can be seen and corrected quickly.

A variety of default plots and reports, along
with tools to create customized plots and reports,
facilitate output analysis.

Map-Based Schematic
HEC-ResSim provides a realistic view of
the physical river/reservoir system using a map-based
schematic with a set of element drawing tools. Also,
with the hierarchical outlet structure, the modeler can
represent each outlet of the reservoir rather the being
limited to a single composite outlet defintion.
Schematic
- The program’s user interface allows the user to draw
the network schematic either as a stick figure or an
overlay on one or more geo-referenced maps of the
watershed.

Drawing Tools - HEC-ResSim represents a system of reservoirs as
a network composed of four types of physical
elements: junctions, routing reaches, diversions,
and reservoirs. By combining these elements, the HEC-ResSim modeler is able to build a network
capable of representing anything from a single
reservoir on a single stream to a highly developed
and interconnected system like that of California’s
central valley.

A reservoir is the most complex element of the
reservoir network and is composed of a pool and a
dam. HEC-ResSim assumes that the pool is level
(i.e., it has no routing behavior) and its hydraulic
behavior is completely defined by an
elevation-storage-area table. The real complexity of
HEC-ResSim’s reservoir network begins with the dam.
Hierarchical Outlet Structure - The dam is the root of an outlet hierarchy or
“tree” which allows the user to describe the
different outlets of the reservoir in as much detail
as is deemed necessary. There are two basic and two
advanced outlet types. The basic outlet types are
controlled and uncontrolled. An uncontrolled outlet
can be used to represent an outlet of the reservoir,
such as an overflow spillway, that has no control
structure to regulate flow.
Controlled outlets can
be used to represent any outlet capable of
regulating flow, such as a gate or valve. The
advanced outlet types are power plant and pump, both
of which are controlled outlets with additional
features to represent their special purposes. The
power plant has the ability to compute energy
production. The pump is even more specialized
because its flow direction is opposite that of the
other outlet types, and it can draw water up into
the reservoir from the pool of another reservoir.
The pump outlet type was added to enable the user to
model pump-back operation in hydropower systems,
although hydropower is not required for its
operation.
Rule-Based Operations
Most reservoirs are constructed for one or more of
the following purposes: flood control, power
generation, navigation, water supply, recreation,
and environmental quality. These purposes typically
define the goals and constraints that describe the
reservoir’s release objectives. Other factors that may
influence these objectives include: time of year,
hydrologic conditions, water temperature, current
pool elevation (or zone), and simultaneous
operations by other reservoirs in a system.
HEC-ResSim is unique among reservoir simulation
models because it attempts to reproduce the decision
making process that human reservoir operators must
use to set releases. It uses an original rule-based
description of the operational goals and constraints
that reservoir operators must consider when making
release decisions. As HEC-ResSim has developed,
advanced features such as outlet prioritization,
scripted state variables, and conditional logic have
made it possible to model more complex systems and
operational requirements.

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