1. DES 606 : Watershed Modeling with HEC-HMS
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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2. Outline for Module 2 Hydrologic Principles Overview of HEC-HMS
Watershed
Hydrologic Cycle
Overview of HEC-HMS
Projects
Components
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3. Hydrologic Principles
The Watershed
The Hydrologic Cycle
Precipitation
Losses
Storage and Routing
Runoff
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4. Watershed
The fundamental unit in surface water hydrology is the watershed.
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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5. Watershed Watershed physical properties are characteristics such as:
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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6. Watershed 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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7. Watershed
Physical properties are those things than can be measured from a topographic map.
Area, slope, length
Descriptive properties are everything else.
Soil texture (and infiltration rate)
Fraction developed
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8. Watershed
An analysis or even design will likely start with watershed delineation.
Aerial imagery
Topographic map
Sewer drawings
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9. Watershed 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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10. Watershed 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 cost-sharing with the U.S. Geological Survey to do so.
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11. Manual delineation example in course reference
Topowatershed.pdf
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12. Watershed
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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13. Watershed
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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14. Watershed Minimal Description
Watershed boundary on a map
Outlet
Subsurface storm sewer network
Area that drains to the outlet
Main channel length
Outlet to highest point in watershed
Slope(s)
Descriptive (any or all)
Soil type
Fraction developed/impermeable etc.
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15. Example/Exercise 2
Harden Branch Creek, Concho County Texas
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16. Hydrologic Cycle Precipitation (Input) Loss Runoff (Output) Module 2
Subsurface storm sewer network if appropriate and available.
Runoff (Output)
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17. Precipitation Precipitation Meteorology Data
Rainfall (by far most important in Texas)
Snow, Sleet, Hail
Meteorology
Synoptic storms
Cyclonic storms
Data
NWS, local networks, SAO, NCDC (historical)
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18. Losses Losses Infiltration Evapotranspiration 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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19. Transformation
Transform the spatially distributed precipitation input to the outlet
Unit Hydrograph
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20. Storage and Routing Storage Routing
Reservoirs, ponds, depressions store water and release later in time (as compared to the input)
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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21. Rainfall-Runoff Process
Precipitation
Meterology, Climate
Watershed
Losses
Transformation
Storage
Routing
Runoff
Fraction of precipitation signal remaining after losses
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22. HEC-HMS Overview History HMS is a complex and sophisticated tool
Evolved from HEC-1 as part of “new-generation” software circa 1990
Integrated user interface to speed up data input and enhance output interpretation
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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23. HEC-HMS
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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24. HEC-HMS
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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25. Summary Watershed is fundamental unit
Area, Length, Slope, etc.
Exercise 2: Delineate and Measure Area
Hydrologic Cycle and Processes
Rainfall is the input function
Losses and Storage are watershed functions
Runoff is the excess rain redistributed in time
HEC-HMS is a computer program for rainfall-runoff modeling
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