Hydrologic Simulation Models
CEVE 412Dr. Phil Bedient
Hydrology and Floodplain Analysis, Chapter 5
Introduction to Hydrologic Models
Hydrology and Floodplain Analysis, Chapter 5.1
Factors in Hydrographs In order to create an accurate model
one first needs to understand all of the relevant factors in the watershed.
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.
History of Computer Models
Breakdown of computer modeling software packages.
Steps in Watershed Modeling
Hydrology and Floodplain Analysis, Chapter 5.2
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.
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.
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.
Description of Major Hydrologic Models
Hydrology and Floodplain Analysis, Chapter 5.3
Selected Simulation Models
HEC-HMS
HEC-RAS
SWMM
HEC-HMS Flood Hydrograph Theory
Hydrology and Floodplain Analysis, Chapter 5.4
HEC-HMS Model Configuration
Capabilities of HEC-HMS
1. Simulation of watershed runoff and streamflow.
2. Determination of flood flows at various locations.
3. Evaluation of land use or topographic changes.
4. Simulation of flood control measures.5. Computation of damage frequency
curves.
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
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.
Example Watershed in HEC- HMS with precipitation
Loss Methods
Initial and Constant HEC exponential
SCS Curve Number Holtan Method Green and Ampt Deficit/Constant Soil Moisture
Accounting
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
Subbasin Runoff Calculation
Family of 1hr UHs
The larger the TC value the flatter the UH response.
4.3 mi2 80% Devel.SCS curve # 94
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 baseflowQ = baseflow rate at end of nΔTRTIOR = ratio of recession flows at t =1 hr increment apart.
Baseflow Calculation For urban channels it can work to
just use a constant value.
Brays Bayou, Houston
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
Application of HEC-HMS Watersheds
Hydrology and Floodplain Analysis, Chapter 5.5
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
HEC - HMS A project consists of three
separate parts:› The Basin model› The meteorological model› The control specifications
Basin Model
Meteorological Model
Control Specifications
HEC – HMS Results Result data for every object can be
obtained.
Example OutputHydrograph