Hydrologic Simulation Models

1. Hydrologic Simulation Models Hydrology and Floodplain Analysis, Chapter 5 CEVE 412 Dr. Phil Bedient

2. Introduction to Hydrologic Models Hydrology and Floodplain Analysis, Chapter 5.1

3. Factors in Hydrographs • In order to create an accurate model one first needs to understand all of the relevant factors in the watershed.

4. History of Computer Models • 1960s • Harvard, Stanford, and the U.S. Army Corps of Engineers pioneered hydrologic computer modeling. • The first available model was: Hydrologic Simulation Program – FORTRAN (HSPF) • 1970s • Increasing interest in water quality spurred more models.

5. History of Computer Models • Breakdown of computer modeling software packages.

6. Steps in Watershed Modeling Hydrology and Floodplain Analysis, Chapter 5.2

7. Steps in Watershed Modeling 1. Select model based on study objectives and watershed characteristics, availability of data, and project budget. 2. Obtain all necessary input data—rainfall data, digital topography, land use and soils, infiltration, channel characteristics, streamflow data, design floods, and reservoir data. 3. Evaluate and refine study objectives in terms of simulations to be performed under various watershed conditions.

8. Steps in Watershed Modeling 4. Choose methods for determining subbasin hydrographs and channel routing. 5. Calibrate model using historical rainfall, streamflow, and existing watershed conditions. Verify model using other events under different conditions while maintaining same calibration parameters.

9. Steps in Watershed Modeling 6. Perform model simulations using historical or design rainfall, various conditions of land use, and various control schemes for reservoirs, channels, or diversions, to the extent possible. 7. Perform sensitivity analysis on input rainfall, routing parameters, and hydrograph parameters as necessary. 8. Evaluate usefulness of the model and comment on needed changes or modifications.

10. Description of Major Hydrologic Models Hydrology and Floodplain Analysis, Chapter 5.3

11. Selected Simulation Models

12. HEC-HMS

13. HEC-RAS

14. SWMM

15. HEC-HMS Flood Hydrograph Theory Hydrology and Floodplain Analysis, Chapter 5.4

16. HEC-HMS Model Configuration

17. Capabilities of HEC-HMS • Simulation of watershed runoff and streamflow. • Determination of flood flows at various locations. • Evaluation of land use or topographic changes. • Simulation of flood control measures. • Computation of damage frequency curves.

18. Watershed Delineation and Organization • HEC-HMS uses parameters averaged in space and time to simulate the runoff process. • Each watershed is divided into relatively small homogeneous subbasins because of the limitations of UH theory. • 1-10 mi2 per subbasin

19. Precipitation • P is computed for each of those subbasins from either historical or synthetic storms. • Can be input in many different forms: • Incremental P for a time in each subbasin • Total cumulative P and time distribution • Historical gage data and areal weighting (Thiessen) coefficients for each subbasin • HEC-HMS can also do snowfall and snowmelt.

20. Example Watershed in HEC- HMS with precipitation

21. Loss Methods • Initial and Constant HEC exponential • SCS Curve Number • Holtan Method • Green and Ampt • Deficit/Constant • Soil Moisture Accounting

22. Subbasin Runoff Calculation • Clark hydrograph method (TC&R method) • Snyder unit hydrograph method • SCS method (CN method + SCS UH) • Kinematic wave for overland hydrograph • ModClark • User-specified S-graph

23. Family of 1hr UHs The larger the TC value the flatter the UH response. 4.3 mi2 80% Devel. SCS curve # 94

24. Baseflow Calculation • Three parameters: • Q0 flow in channel prior to start of rising limb • Ratio of recession flows at t=1 (RTIOR) • QR recession threshold Q0 = starting baseflow Q = baseflow rate at end of nΔT RTIOR = ratio of recession flows at t =1 hr increment apart.

25. Baseflow Calculation • For urban channels it can work to just use a low constant value. Brays Bayou, Houston

26. Flood Routing Flood routing analyzes the movement of a flood wave as it travels from reach to reach through a stream or river. Methods for Flood Routing in HEC-HMS • Muskingum • Modified Puls • Kinematic Wave • Muskingum – Cunge 8-point • Lag

27. Application of HEC-HMS Watersheds Hydrology and Floodplain Analysis, Chapter 5.5

28. History of HEC-HMS • HEC-1 Watershed Hydrology (1981) • There has since been HEC-2, HEC-3, HEC-4 • There have been numerous other HEC releases. • HEC-HMS, HEC-RAS, HEC-GeoRAS, HEC-SSP, HEC-RPT, HEC-ResSim, HEC-ResPRM, HEC-EFM, HEC-FDA, HEC-DSS • HEC-HMS (1998, 2006, 2010) • Major releases: 1998, 2006, 2010

29. HEC – HMS Projects • A project consists of three separate parts: • The Basin model • The meteorological model • The control specifications

30. Basin Model

31. Draw Basin Model

32. Define Rainfall – Met Model Hyetograph OR… Design Storm

33. Define Subbasin Parameters Infiltration (Loss) Unit Hydrograph

34. Can Add Reservoirs

35. Can Add Diversions

36. Control Specs and Run Model 1 2 Summary of Results 3

37. Detention Pond HMS Results A Subbasin

38. Outlet Results

39. Control Specifications

40. HEC – HMS Results • Result data for every object can be obtained. Example Output Hydrograph