Design Storms in HEC-HMS

1
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
• 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

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
  dont need both.
• 0-4194-3 Empirical Hyetographs

7
Precipitation Depth

• Using EBDLKUP
• 24 hr, 25 yr Depth 10.01 inches
• 6 hr, 25 yr Depth 6.75 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 (AREA1 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 1200 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
• SCS 24-hr is built-in, specify storm type and
  depth.
• SCS 6-hr is processed externally
• Results
• 24 hr, Qp 9340 cfs, Tp 1152 , V 10.01 in.
• 6 hr, Qp 8905 cfs , Tp 252 , 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 6.5 inches Or 0 9.5 inches
0 6 hours Or 0 24 hours
28

• We wont 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
SCS-3 EBDLKUP 10.01 3723 1200
DDFEmpirical 9.49 2219 0030
DDFFrequency 9.00 4356 1205
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.