Watershed Modeling using HEC-HMS and EPA-SWMM

1. Watershed Modeling using HEC-HMS and EPA-SWMM ©T. G. Cleveland, Ph.D., P.E. 10 July 2012 Lesson 2

2. Outline • Hydrologic Principles • Interpolation of irregular spaced data • Exercise: Interpolate an empirical hyetograph • Digitizing Charts and Maps • Exercise: Delineate and measure a watershed using a digital planimeter.

3. Hydrologic Principles • The Watershed • The Hydrologic cycle • Precipitation • Losses • Runoff

4. Watershed • The fundamental unit in surface water hydrology is the watershed. • It can be as small as a parking lot draining to an inlet, or continental sized (Bedient et al., 2008). • A watershed is defined as the area on the surface of the earth that drains to a specific location. • The watershed is therefore defined both by the drainage location as well as topographic features that govern flow to that point.

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.

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.

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

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

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.

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.

11. Manual delineation example in course reference CD • Topowatershed.pdf

12. Watershed • Watershed physical and descriptive characteristics determined after delineation. • Areas and lengths • Use Acrobat Pro tools to find areas and lengths on map image, convert to actual lengths using map scale. • Slopes • Change in elevation along a path • Special concerns • Sewers can cross topographic watershed boundaries • Flat terrain – channels hard to define

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.

14. Watershed • Minimal descriptions • 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

15. Watershed • Minimal descriptions • Slope(s) • Descriptive (any or all) • Soil type • Fraction developed/impermeable etc.

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

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)

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

19. Transformation • Transform the spatially and temporally distributed precipitation input to the outlet • Unit Hydrograph is an example of a lumped transform model

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

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

22. Hydrologic Principles • Watershed is fundamental unit • Area, Length, Slope, etc. • Rainfall is the input function • Losses and Storage are Watershed functions • Excess rainfall is fraction of rainfall signal remaining after storage and losses are satisfied • Runoff is the excess rain redistributed in time

23. Irregular Spaced Data • Use linear interpolation to map irregular spaced data to fixed time or distance steps. • Seemingly simple task, but of immense importance. • Illustrate by example – won’t actually need to use this method until we get to historical data representation. • Important point is that data need preparation outside of the modeling software!

24. Digitizing Maps and Charts • Delineation first. • Use a tool to find X-Y pairs of area of interest AND A REFERENCE area of known dimension. • Describe enclosing polygon, measure its area, then scale to the reference area. • Charts are similar except reference area has no meaning.

25. Exercise • Exercise 2 • Delineate and measure a small watershed in Virginia (later we will pretend it is in Texas)

26. Documenting Your Work • Like the prior exercise you are completing a table. As before complete the exercise and fill in the table, then print the report and hand-in.