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