1. Design Storms in HEC-HMS
Example 9
Design Storms in HEC-HMS

2. 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

3. 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).

4. Problem Statement
Generate a 24-hour, 25-year design storm for Harris Co. Texas using
SCS Design Storm Approach and EBDLKUP
Empirical Hyetograph
Generate a 6-hour, 25-year design storm for Harris Co. Texas using

5. Problem Statement
Generate a 24-hour, 25-year design storm for Harris Co. Texas using
Frequency Storm and DDF Atlas

6. 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.
Empirical Hyetographs

7. Precipitation Depth Using EBDLKUP 24 hr, 25 yr Depth = 10.01 inches

8. Rapid HMS Model Create a new project Basin model Dummy subbasin
No loss
No UH transform

9. Rapid HMS Model
Create a new project
Meterological model
SCS Storm

10. Rapid HMS Model
Meterological model
SCS Storm
Select Type
Insert Depth

11. Rapid HMS Model Control Specifications Time Window
24 hours for SCS storm

12. Rapid HMS Model
Run the model

13. 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

14. HEC-HMS Output
Convert the No-transform hydrograph into the SCS Type 2 storm (AREA=1 sq. mi.)

15. SCS Type-2 Storm

16. 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.

17. SCS 6-hour

18. 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

19. 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

20. 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

21. 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

22. SCS 6-hour, Scaled
Cut and past into HMS
Time series data manager

23. HEC-HMS Model
Run the model

24. HEC-HMS Model Summary Results
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= 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.

25. Using DDF Atlas Repeat the example using the DDF atlas
Need two maps; 25 yr – 24 hr and 25 yr – 6 hr.

26. Rainfall Depth
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

27. 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 – 9.5 inches
0 – 6.5 inches Or
0 – 6 hours Or
0 – 24 hours

28. We saw this same chart in example 2
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

29. Dimensional Hyetograph

30. Dimensional Hydrograph
Use interpolation to generate uniformly spaced in time cumulative depths.
This example will use the HMS fill feature

31. Input Hyetograph Cut-paste-fill to create the hyetograph
Considerable time required (will illustrate “live”)

32. Empirical 24-hr, 25-yr
Cut-paste-fill to create the hyetograph

33. 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.

34. 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.

35. 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

36. Interpolation in Excel

37. 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)

38. 6-hr, 25 yr Empirical Change these values as appropriate
Copy to the interpolate sheet

39. 6-hr, 25 yr Empirical
Change these values as appropriate

40. 6-hr, 25 yr Empirical Copy the interpolated series into HEC-HMS
Copied the interpolated depths here

41. 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.

42. Frequency Storm
From the DDF atlas we will need a series of depths

43. Frequency Storm From the DDF atlas we will need a series of depths
Read these from the Atlas Maps pp 47-54

44. Frequency Storm
Run the model

45. 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

46. Comparison of Results Design Storm Model Total Depth IPeak Tpeak 10.01
SCS-3 +EBDLKUP
10.01
3723
12:00
DDF+Empirical
9.49
2219
~ 00:30
DDF+Frequency
9.00
4356
12:05

47. 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

48. 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

49. 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.