Module 2: Hydrology Principles, HMS Overview
Theodore G. Cleveland, Ph.D., P.E, M. ASCE, F. EWRI
26-28 August 2015
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Hydrologic Principles
 Watershed
 Hydrologic Cycle
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Overview of HEC-HMS
 Projects
 Components
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The Watershed
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The Hydrologic Cycle
 Precipitation
 Losses
 Storage and Routing
 Runoff
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The fundamental unit in surface water hydrology
is the watershed.
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A watershed is defined as the area on the
surface of the earth that drains to a specific
location.
 The watershed is defined both by the drainage
location as well as topographic features that govern
flow to that point.
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Watershed physical properties are
characteristics such as:
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Area
Main channel length (if a main channel exists)
Slope (requires the specification of path),
Soil moisture/permeability, and similar
measurable characteristics.
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Watersheds also have descriptive
properties such as:
 %-developed
 %- polluted, and so forth.
▪ These properties are certainly physical, but are called
descriptive because there will be analyst
interpretation in the specification of the values.
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Physical properties are those things than
can be measured from a topographic map.
 Area, slope, length
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Descriptive properties are everything else.
 Soil texture (and infiltration rate)
 Fraction developed
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An analysis or even design will likely
start with watershed delineation.
 Aerial imagery
 Topographic map
 Sewer drawings
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What is the process of delineating
watersheds?
 Manual delineation involves drawing lines on a
topographic map, and connecting the slope or
ridge tops.
▪ Assuming the water will drain away from those points,
the watershed is delineated by enclosing a polygon.
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What is the process of delineating
watersheds?
 Automated delineation involves some technical
skills with GIS-like databases.
▪ Digital Elevation Model (DEM)
▪ Stream network, and stream outlets.
▪ The DEM and stream outlets can either be downloaded or
constructed.
▪ If a new DEM must be constructed, consider costsharing with the U.S. Geological Survey to do so.
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Manual delineation example in course
reference
Topowatershed.pdf
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Watershed physical and descriptive
characteristics determined after delineation.
 Areas and lengths
 Slopes
▪ Change in elevation along a path
 Special concerns
▪ Sewers can cross topographic watershed boundaries
▪ Flat terrain – flow paths hard to define
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Watershed physical and descriptive
characteristics determined after delineation.
 Descriptive characteristics
▪ Google Earth/aerial imagery can be used to define
cover types and fractions.
▪ Soil maps for selected properties.
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Watershed boundary on a map
 Outlet
 Subsurface storm sewer network
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Area that drains to the outlet
 Main channel length
 Outlet to highest point in watershed
 Slope(s)
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Descriptive (any or all)
 Soil type
 Fraction developed/impermeable etc.
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Harden Branch Creek, Concho County Texas
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Precipitation (Input)
Loss
Runoff (Output)
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Precipitation
 Rainfall (by far most important in Texas)
 Snow, Sleet, Hail
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Meteorology
 Synoptic storms
 Cyclonic storms
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Data
 NWS, local networks, SAO, NCDC (historical)
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Losses
 Infiltration
▪ Hortonian Loss Model
▪ Green-Ampt Loss Model
▪ NRCS Runoff Generation Model
▪ Initial Abstraction, Constant Rate Model
 Evapotranspiration
▪ Thornwaithe
▪ Energy Balance Models
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Transform the spatially distributed
precipitation input to the outlet
 Unit Hydrograph
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Storage
 Reservoirs, ponds, depressions store water
and release later in time (as compared to the
input)
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Routing
 Moving water from one location to another on
the watershed occurs over a path (route).
 Routing develops the temporal relationship of
input to the outlet from this process
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•
Precipitation
– Meterology, Climate
• Watershed
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Runoff
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Losses
Transformation
Storage
Routing
Fraction of precipitation
signal remaining after losses
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History
 Evolved from HEC-1 as part of “new-
generation” software circa 1990
 Integrated user interface to speed up data
input and enhance output interpretation
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HMS is a complex and sophisticated tool
 Intended to be used by a knowledgeable and
skilled operator
 Knowledge and skill increase with use
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Conceptualizes precipitation, watershed
interaction, and runoff into major elements
 Basin and sub-basin description
▪ Supply how the system components are
interconnected
 Loss model
▪ Supply how rainfall is converted into excess rainfall
 Transformation model
▪ Supply how the excess rainfall is redistributed in
time and moved to the outlet
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Conceptualizes precipitation, watershed
interaction, and runoff into major elements
 Meterological model
▪ Raingage specifications and assignment to different
sub-basins
 Time-series models
▪ Supply input hyetographs
▪ Supply observed hydrographs
 Simulation control
▪ Supply instructions of what, when, how to simulate
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Watershed is fundamental unit
 Area, Length, Slope, etc.
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Exercise 2: Delineate and Measure Area
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Hydrologic Cycle and Processes
 Rainfall is the input function
 Losses and Storage are watershed functions
 Runoff is the excess rain redistributed in time
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HEC-HMS is a computer program for rainfallrunoff modeling
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