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
Introduction to Hydrologic Models

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
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
Steps in Watershed Modeling

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
Description of Major Hydrologic Models

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
HEC-HMS Flood Hydrograph Theory

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.
80% Devel.
SCS curve # 94
The larger the TC value the flatter the UH response.

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 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
Application of HEC-HMS Watersheds

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 A project consists of three separate parts: The Basin model
The meteorological model
The control specifications

30. Basin Model

31. Meteorological Model

32. Control Specifications

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