Chapter 5

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Hydrology and Floodplain Analysis,
Chapter 5
Hydrology and Floodplain
Analysis, Chapter 5.1

In order to create an accurate model
one first needs to understand all of the
relevant factors in the watershed.

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.

Breakdown of computer modeling
software packages.
Hydrology and Floodplain
Analysis, Chapter 5.2
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.
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.
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.
Hydrology and Floodplain
Analysis, Chapter 5.3
Hydrology and Floodplain
Analysis, Chapter 5.4
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.
1.
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

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.

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Initial and Constant
HEC exponential
SCS Curve Number
Holtan Method
Green and Ampt
Deficit/Constant
Soil Moisture
Accounting

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
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
4.3 mi2
80% Devel.
SCS curve #
94
The larger the TC value the flatter the UH
response.

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.

For urban channels it can work to just
use a constant value.
Brays Bayou, Houston
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
Hydrology and Floodplain
Analysis, Chapter 5.5

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
A
project consists of three
separate parts:
› The Basin model
› The meteorological model
› The control specifications

Result data for every object can be
obtained.
Example Output
Hydrograph

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