DES 606 : Watershed Modeling with HEC-HMS

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 Module 2

2. Outline for Module 2 • Hydrologic Principles • Watershed • Hydrologic Cycle • Overview of HEC-HMS • Projects • Components Module 2

3. Hydrologic Principles • The Watershed • The Hydrologic Cycle • Precipitation • Losses • Storage and Routing • Runoff Module 2

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. Module 2

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. Module 2

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. Module 2

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 Module 2

8. Watershed • An analysis or even design will likely start with watershed delineation. • Aerial imagery • Topographic map • Sewer drawings Module 2

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. Module 2

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. Module 2

11. Manual delineation example in course reference • Topowatershed.pdf Module 2

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 Module 2

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. Module 2

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. Module 2

15. Example/Exercise 2 • Harden Branch Creek, Concho County Texas Module 1

16. Hydrologic Cycle Precipitation (Input) Loss Runoff (Output) Module 2

17. Precipitation • Precipitation • Rainfall (by far most important in Texas) • Snow, Sleet, Hail • Meteorology • Synoptic storms • Cyclonic storms • Data • NWS, local networks, SAO, NCDC (historical) Module 2

18. Losses • Losses • Infiltration • Hortonian Loss Model • Green-Ampt Loss Model • NRCS Runoff Generation Model • Initial Abstraction, Constant Rate Model • Evapotranspiration • Thornwaithe • Energy Balance Models Module 2

19. Transformation • Transform the spatially distributed precipitation input to the outlet • Unit Hydrograph Module 2

20. Storage and Routing • Storage • 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 Module 2

21. Rainfall-Runoff Process • Precipitation • Meterology, Climate • Runoff • Fraction of precipitation signal remaining after losses • Watershed • Losses • Transformation • Storage • Routing Module 2

22. HEC-HMS Overview • 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 • HMS is a complex and sophisticated tool • Intended to be used by a knowledgeable and skilled operator • Knowledge and skill increase with use Module 2

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 Module 2

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 Module 2

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 Module 2