Example 3
HEC-HMS Simulation
3 Different Loss Models
Purpose
• Illustrate different loss models in a HECHMS precipitation-runoff simulation
– The example is “minimal” in the sense that
only a small set of HEC-HMS capabilities are
employed
– Realistic parameter values are employed from
public-domain references, but the example is
fabricated for pedagogical simplicity.
Learning Objectives
• Learn how to copy a “project” so can modify
without damage to original files.
– Reinforce the concepts of “Projects” as a datastorage paradigm.
• Learn how to import observations.
– Use of measured rainfall and discharge. Where
available can use to “calibrate” a watershed model.
• Learn how to parameterize different loss models
– Initial loss and constant rate loss model
– NRCS CN runoff generation (loss)
– Green-Ampt loss model
Problem Statement
• Simulate the response of the Ash Creek
watershed at Highland Road for a the
XXXX historical conditions.
– Use Example 2 as the base “model”, modify
by substitution of the real rainfall time series
and the observed runoff time series.
– Treat the entire watershed as a single subbasin.
Background and Data
• Watershed Outlet
– Highland Road and
Ash Creek, Dallas,
TX.
– Area is residential
subdivisions, light
industrial parks, and
some open
parkland.
– White Rock Lake is
water body to the
North-West
Physical Properties
• Watershed
Properties
– AREA=6.92 mi2
– MCL=5.416 mi
– MCS=0.005595
– CN=86
– R=0
This was used in IaCl specification
-- will examine source of number
-- will also need GA values
Historical Data
• Precipitation and
Runoff
– May 20, 1978
– Total depth about
3-inches, close to
Example 2 depth
(much different
time scale)
Historical Data
• Precipitation
– Note start time
• Runoff
– Note start time
Start Time
• Start time is important
– The start time of rainfall time series and runoff time
series should be same
– A common error is time mismatch, usually obvious in
the simulation but not always.
Using “Real” Data
• Need to prepare the data
– Uniform time steps (no 17 hour gap)
– Rainfall and runoff data should have same start
times, use zero-padding to make happen
• Data become “time-series” elements
– Rainfall => With a “rain-gage” in HEC-HMS
– Runoff => With a “discharge-gage” in HEC-HMS
Building The Model –
Create a New Project
•
Use Example 2 as the base model
– Start Example 2
– Select “Save-As”
– Save to a new project name and file
Building The Model –
Create a New Project
•
Use Example 2 as the base model
– Start Example 2
– Select “Save-As”
– Save to a new project name and file
Name
Path
Check these to create duplicate files
Building The Model
Verify the copy
• Run a simulation
– Convince self that
have a working copy
Data Preparation
• Now prepare the external hyetograph from
the historical data
– Use Excel to prepare the time series
– Specify a long enough time window in the
time-series manager
– Use HEC-HMS “fill” to fill in the 17 hours
Data Preparation: Rainfall
• Use Excel to prepare
the time series
• Specify a long
enough time window
in the time-series
manager
– 5-20-1978 to 5-221978 should cover the
rainfall and allow an
entire day for runoff
– Can refine if needed
Data Preparation:Rainfall
• Specify a long
enough time window
in the time-series
manager
– 5-20-1978 to 5-221978 should cover the
rainfall and allow an
entire day for runoff
Data Preparation: Rainfall
• Use Excel to prepare the
time series
– Identify non-uniform time
step sections
– line up with input table in
HEC-HMS
– One-time only is easiest to
enter blocks, then use
HMS tools to fill in values
– Many storms, worth writing
code to interpolate
(external to HMS)
Not every 15 minutes here
Data Preparation: Rainfall
• Use Excel to prepare the
time series
– Identify non-uniform time
step sections
– line up with input table in
HEC-HMS
– One-time only is easiest to
enter blocks, then use
HMS tools to fill in values
– Many storms, worth writing
code to interpolate
(external to HMS)
Data Preparation: Rainfall
• One-time only is
easiest to enter
blocks, then use HMS
tools to fill in values
– Enter value first line
(not displayed)
– Highlight fill block
– Right-click block and
select fill method
Data Preparation: Rainfall
• One-time only is
easiest to enter
blocks, then use HMS
tools to fill in values
– Enter value first line
(not displayed)
– Highlight fill block
– Right-click block and
select fill method
Completed data fill (zero padding in t
Data Preparation: Rainfall
• Continue for remaining non-uniform spaced blocks.
– When complete, plot the time-series and Excel and HEC-HMS; should
look the same (padded the Excel file to start/end same elapsed time)
• This plot is QA/QC only, once data are entered, won’t need further
Excel plots.
Loss Model Parameters
• IaCl model in TxDOT 0-4193-7
Data Preparation: Runoff
• Use Excel to prepare
the time series
• Specify a same time
window as rainfall
• Copy, paste, fill by
same process.
Data Preparation: Runoff
• Create discharge gage
– Time series manager.
Data Preparation: Runoff
• Specify time window
Data Preparation: Runoff
• Use same process as
for rainfall
– Insert blocks
– Use fill tool to insert
missing values
– Plot results to
compare
Data Preparation: Runoff
• Continue for remaining non-uniform spaced blocks.
– When complete, plot the time-series and Excel and HEC-HMS; should
look the same (padded the Excel file to start/end same elapsed time)
• This plot is QA/QC only, once data are entered, won’t need further
Excel plots.
Example 3: IaCl Loss Model
• Leave remainder of model unchanged
– Represents the IaCl model
– Represents the NRCS DUH transformation model
• Run the simulation using the real rainfall (in
contrast to a hypothetical input)
– Compare simulation output with discharge gage
– Assess how well the estimation methods worked
– Try different loss models (Run 2 and Run 3)
Example 3: IaCl Loss Model
• Instruct the program
to plot the observed
gage with the
simulation gage.
– Basin/Options
Example 3: IaCl Loss Model
• Dotted curve is observed
runoff, solid is simulation
– Timing ~ 70 minutes late
– Peak ~ 50% low
– Volume ~ 20% high
• One could “calibrate” but
that is for a later module.
• Stipulate that simulation is a
bit off, and explore different
loss models.
Example 3B : Green-Ampt
• Requires some added knowledge about
the Ash Creek locale
– Soil types and tables of values
– Prior study (if lucky – in this example
available)
Example 3B : Green-Ampt
• Soil Types
– Texas available from
TAMU
– Nationwide from
NRCS
• This example will
download the NRCS
map, it is more
assessable to
hydrologists, the TAMU
database is specialized
for soil scientists.
Example 3B : Green-Ampt
• NRCS Soil Map
Zoom to this area
Read description
Example 3B : Green-Ampt
• NRCS Soil Map : Type 2
Example 3B : Green-Ampt
• NRCS Soil Map : Type 2
– Loamy and clayey soils
Example 3B : Green-Ampt
• Compare
description with
published soil
behavior
– Use middle
description.
– Other arrows
indicate
reasonable
bounding ranges
Loss Model: Green-Ampt
• Parameter estimation
– Initial water content. 0.187
– Saturated water content: 0.464
– Saturated hydraulic conductivity: 0.04 in/hr
– Soil suction: 8.27 inches
Example 3B: Green-Ampt
• Results
– Timing ~ 70 minutes
late
– Peak ~ 50% low
– Volume ~ 9% high
(good!)
Example 3A : NRCS Loss Model
• Example 3A will substitute the NRCS Loss
Model for the IaCl model
• Clone the project again (Save As …) to
preserve structure and reduce chance of a
data specification error
• Change the loss model specification, enter
curve number and re-simulate.
Example 3A : NRCS Loss Model
• Curve number selection
– Determine hydrologic soil classification
– Uses same soil map as in Green-Ampt
– Soil Group C or D appropriate based on
saturated hydraulic conductivity.
Example 3A : NRCS Loss Model
•Make an assessment of “open space”,
residential, and commercial industrial.
•Then decide fraction impervious for a
composite number.
•Subjective, but most analysts will be
within +/- 10.
Example 3A : NRCS Loss Model
• Look up CN for the different parts, I choose
lowest value in C group soil.
– 10% of area is the stream drainage, essentially open
space CN ~ 79, %IC=0
– 30% of area is commercial-business (note the general
aviation airport is included) CN ~ 91, %IC ~ 85
– 60% of area is some kind of residential, CN ~ 83,
%IC~40
• Composite these to a value of CN=90 for the
watershed, IC is already considered.
Example 3A : NRCS Loss Model
• Clone the project
again (Save As …) to
preserve structure
and reduce chance of
a data specification
error
• Change the loss
model specification,
enter curve number
and re-simulate.
Example 3A : NRCS Loss Model
• Results with different
loss model.
– Timing ~ 70 minutes
late
– Peak ~ 50% low
– Volume ~ 50% high
HEC-HMS Example 3
• Learning Points
– Copy entire projects to keep different models
organized.
– Used Excel to prepare data for import into
Time-Series-Manager; allows use of
measured values where available.
– IaCl, Green-Ampt, NRCS CN perform
differently but require similar data preparatory
effort.
HEC-HMS Example 3
• Learning Points
– Used external data sources
• NRCS soil maps (internet)
• Texas A&M Soil Database (didn’t use, but know
available)
• Used a Green-Ampt soil property correlation in
SWMM (but from the soils literature)
• Used TxDOT hydraulic design manual for CN
estimation. NEH 630 Chapter 9-10 would have
produced similar values.
HEC-HMS Example 3
• Learning Points
– Assembly of external data sources is vital to
the hydrologist
• Most practicing hydrologist’s offices are a mess of
old reports – that’s where they find the data.
• Many useful external data sources are available in
PDF reports from a variety of sources, need to get
in the habit of citing the data source should one
need to defend input value choices.
HEC-HMS Example 3
• Learn more
– HEC HMS user manual
– FHWA-NHI-02-001 Highway Hydrology
• Next example
– Calibration tools