Example 9
Design Storms in HEC-HMS
Purpose
• Illustrate the steps to create a design storm in HEC-HMS. – The example will create a variety of design
storms for a particular Texas location.– Focus on HOW to construct the hyetograph
(for design storms requiring external processing) and the two built-in methods
Learning Objectives
• Generate an input hyetograph design storm using several different methods.– External processed storms
• Generate an SCS and Frequency Storm using HEC HMS– Internal processed storms
• Generate rapid generic HMS models for creating input data (for later export).
Problem Statement
• Generate a 24-hour, 25-year design storm for Harris Co. Texas using– SCS Design Storm Approach and EBDLKUP– 0-4194-3 Empirical Hyetograph
• Generate a 6-hour, 25-year design storm for Harris Co. Texas using– SCS Design Storm Approach and EBDLKUP– 0-4194-3 Empirical Hyetograph
Problem Statement
• Generate a 24-hour, 25-year design storm for Harris Co. Texas using– Frequency Storm and DDF Atlas
Required Tools
• TP-40, HY35, DDF Atlas, or EBDLKUP– This example will use both the DDF Atlas
and EBDLKUP to illustrate use of the two tools, you don’t need both.
• 0-4194-3 Empirical Hyetographs
Precipitation Depth
• Using EBDLKUP– 24 hr, 25 yr Depth = 10.01 inches– 6 hr, 25 yr Depth = 6.75 inches
Rapid HMS Model
• Create a new project– Basin model
• Dummy subbasin• No loss• No UH transform
Rapid HMS Model
• Create a new project– Meterological model
• SCS Storm
Rapid HMS Model
• Meterological model– SCS Storm
• Select Type• Insert Depth
Rapid HMS Model
• Control Specifications– Time Window
• 24 hours for SCS storm
Rapid HMS Model
• Run the model
Rapid HMS Model
• We will want the SCS 24-hour storm for the later work, so lets get a copy from HMS.– Observe that element time series has no rain – storm
is produced directly, but we can convert the 1 sq.mi. discharge into watershed inches/hour in Excel
HEC-HMS Output
• Convert the No-transform hydrograph into the SCS Type 2 storm (AREA=1 sq. mi.)
SCS Type-2 Storm
6-Hour Storm
• Now we will figure out the 6 hour SCS storm.– Idea is to use the most intense part of the
storm.– Use the 6 hours centered on 12:00 of the
storm, rescale these to the correct depth and we have a 6-hour storm.
SCS 6-hour
SCS 6-hour, Unscaled
• Pick the 6-hour period.– Then set remainder to
zero– Compute total depth– Adjust to get the
required total depth of 6.75 inches
SCS 6-hour, Unscaled
• Pick the 6-hour period.– Then set remainder to
zero– Compute total depth– Adjust to get the
required total depth of 6.75 inches
SCS 6-hour, Unscaled
• Pick the 6-hour period.– Then set remainder to
zero– Compute total depth– Adjust to get the
required total depth of 6.75 inches
SCS 6-hour, Scaled
• Pick the 6-hour period.– Then set remainder to
zero– Compute total depth– Adjust to get the
required total depth of 6.75 inches
SCS 6-hour, Scaled
• Cut and past into HMS – Time series data
manager
HEC-HMS Model
• Run the model
HEC-HMS Model
• Summary– SCS 24-hr is “built-in”, specify storm type and depth.– SCS 6-hr is processed externally
• Results– 24 hr, Qp = 9340 cfs, Tp = 11:52 , V= 10.01 in.– 6 hr, Qp = 8905 cfs , Tp = 2:52 , V = 6.75 in– Recall the Qp are not true “runoff” in this example –
they represent “excess precipitation” expressed in units of watershed discharge for a 1 sq. mi. watershed.
Using DDF Atlas
• Repeat the example using the DDF atlas– Need two maps; 25 yr – 24 hr and 25 yr – 6 hr.
• Use DDF atlas to find depths would produce nearly identical results– 25 yr, 24 hr ~ 9-10 inches– 25 yr, 6 hr ~ 6-7 inches depth
• Building an HMS model would be the same for SCS Type 2 storm.
• Use these values instead in the empirical hyetograph approach
Rainfall Depth
Generate a Hyetograph
• Dimensionless Hyetograph is parameterized to generate an input hyetograph that is 6 or 24 hours long and produces the 25-year depth.– For this
example, will use the median (50th percentile) curve
0 – 6 hours Or0 – 24 hours
0 –
6.5
inch
es
Or
0 –
9.5
in
ches
• We won’t actually use the graph, instead use the tabular values in the report.– This column scales TIME– This column scales
DEPTH
• We saw this same chart in example 2
Dimensional Hyetograph
Dimensional Hydrograph
• Use interpolation to generate uniformly spaced in time cumulative depths.
• This example will use the HMS fill feature
Input Hyetograph
• Cut-paste-fill to create the hyetograph
• Considerable time required (will illustrate “live”)
Empirical 24-hr, 25-yr
• Cut-paste-fill to create the hyetograph
Data Preparation
• Discovered in this example that using the dimensionless hyetograph requires a tedious cut-paste-fill process to put the data into the uniform spaced time series structure.– Need a better way, that is some kind of
interpolator that will take non-uniform spaced paired data and produce uniform spaced data.
Interpolation in Excel
• Use Excel to interpolate by use of INDEX and MATCH functions. – Takes a bit of programming, but will make
empirical hyetographs easier to manage and will save time.
Interpolation in Excel
• Copy the dimensionalized hyetograph to a different worksheet (as values).– Use MATCH and INDEX to locate the nearest values
in the dimensional TIME and DEPTH to the arbitrary TIME
– Equation to interpolate depth is
)(
))(( edinterpolatedinterpolat
lowhi
lowlowhi
TT
TTDDD
Interpolation in Excel
)(
))(( edinterpolatedinterpolat
lowhi
lowlowhi
TT
TTDDD
6-hr, 25 yr Empirical
• Now that we have an interpolator, we can prepare a six hour storm with less data entry effort in HMS.– Depth ~ 6-7 inches, lets use 7– Duration is 6 hours
• Back to the Excel sheet (we already built)
6-hr, 25 yr Empirical
Change these values as appropriate
Copy to the interpolate sheet
6-hr, 25 yr Empirical
Change these values as appropriate
6-hr, 25 yr Empirical
Copy the interpolated series into HEC-HMS
Copied the interpolated depths here
Frequency Storm
• HEC-HMS has a “frequency” storm option built-in to the meterological manager.
• It requires a set of depths for different times in a storm (kind of like the empirical hyetograph).
• It is a way to directly enter DDF values into HMS without the interpolation issues.
• Will illustrate with the 24-hour Harris County example.
Frequency Storm
• From the DDF atlas we will need a series of depths
Frequency Storm
• From the DDF atlas we will need a series of depths
Read these from the Atlas Maps pp 47-54
Frequency Storm
• Run the model
Comparison of Results
• Several different design storms– SCS, Empirical Hyetograph, Frequency
Storms
• Different durations– Compare the 24-hour
• Anticipate different results because storm “shapes” are different.
• Anticipate about same total depths
Comparison of Results
Design Storm Model
Total Depth IPeak Tpeak
SCS-3 +EBDLKUP 10.01 3723 12:00
DDF+Empirical 9.49 2219 ~ 00:30
DDF+Frequency 9.00 4356 12:05
Summary
• Illustrated a 24-hour SCS storm parameterized using EBDLKUP
• Illustrated how to “export” that storm from HMS and convert into a 6-hour storm
• Illustrated how to use the DDF Atlas and Empirical Hyetograph to generate 24-hour and 6-hour storms.
• Illustrated the Frequency storm parameterized by the DDF Atlas
Summary
• Storm depths similar (anticipated result)
• Time of peak intensity different for Empirical Hyetograph – Anticipated– empirical are front-loaded storms– SCS and Frequency are “balanced” about the
½ storm duration
Summary
• As an aside, the choice of 1-minute time steps was dumb – but this example was about storms and not how well the hypothetical 1 sq. mi. converted those storms into excess.