1. Watershed Modeling using HEC-HMS and EPA-SWMM
©T. G. Cleveland, Ph.D., P.E.
10 July 2012
Lesson 3

2. Outline Precipitation Hyetographs Design Storms
Precipitation in HEC-HMS
Exercise: Minimal HMS Model and Texas DDF

3. Precipitation
Driver of rainfall-runoff models
Event
Continuous

4. Hyetographs
The plot of depth versus time (or intensity versus time) is a hyetograph.

5. Hyetographs
The plot of depth versus time (or intensity versus time) is a hyetograph.

6. Design Storms
Precipitation pattern defined for use in the design of hydrologic system
Serves as an input to the hydrologic system
Can by defined by:
Hyetograph (time distribution of rainfall)
Isohyetal map (spatial distribution of rainfall)

7. Design Storm Estimates
Could use observed data and prepare your own Depth-Duration-Frequency relationship
Outside scope of this course.
Use existing Depth-Duration-Frequency (DDF) or Intensity-Duration-Frequency (IDF) tools for a study area
These produce point estimates!
If area on the large side, consider ARF.

8. Module 11: Design Storms WRI 99-4267 ARF for Texas Design Storms
A design storm for a point is the depth of precipitation that has a specified duration and frequency (recurrence interval).
The effective depth often is computed by multiplying the design-storm depth by a “depth-area correction factor” or an “areal-reduction factor.”

9. ARF in Texas
Region of Unit Hydrograph applicability

10. ARF in Texas

11. Concept of IDF for Design
Estimate intensity for 5-yr return period for a 30-minute duration
i ~ 2.75 inches/hour

12. Design Storms for Texas
TP Maps of storm depths for different storm durations and probabilities

13. Design Storms for Texas
HY-35 Maps of storm depths for different storm durations and probabilities
TP40, HY35 both have interpolation guidance to construct values between mapped values.

14. Design Storms for Texas
TxDOT spreadsheet that tabulates information in the maps. Beware it is units dependent!

15. Design Storms for Texas
Link is good (verified 5 AUG 11)
Reports intensity instead of depth. Multiply by time to recover depth.
Author added this row, not in on-line version

16. Design Storms (elsewhere)
NOAA Atlas-14
On-line collection of interactive maps.
Select a location, and the atlas generates DDF information.

17. Design Storms for Texas
What the spreadsheet and the maps represent is a hyperbolic model that relates time and intensity.
The values e,b, and d parameterize the model.
The value Tc has meaning of averaging time, although usually treated as a time of concentration.

18. Design Storms for Texas
D moves this “knee” LEFT/RIGHT
The values e,b, and d parameterize the model.
The shaded polygon is a hull that encloses TP-40 and HY-35 for Harris Co., TX (barely visible open circles)
The “design equation” curve is the EBDLKUP.xls curve for Harris Co., TX
B moves this curve UP/DOWN
E changes slope of the curve

19. Design Storms for Texas
D moves this “knee” LEFT/RIGHT
Aside:
The “blue” cloud is a simulation using the empirical hyetographs and PP1725 for Harris Co.
The solid red dots are maximum observed intensity regardless of location (some dots are from Texas)
The empirical curves represent an alternative model.
B moves this curve UP/DOWN
E changes slope of the curve

20. Design Storms for Texas
DDF Atlas
Alternative to TP40, HY35, and EBDLKUP.
Includes more recent data (20 years) than these other tools
Provides guidance for interpolation and extrapolation
Works in depth – the native unit in HMS

21. Rainfall Depth
Look up depths by recurrence interval, STORM duration, and location.

22. Local Information
DDF for Austin, TX

23. Local Information IDF for Houston, TX
Most Metropolitan areas in Texas (USA) have similar DDF/IDF charts and tables published.
Serve as a basis for Design Storms

24. Design Precipitation Hyetographs
Ultimately are interested in entire hyetographs and not just the depths or average intensities.
Techniques for developing design precipitation hyetographs
SCS method
Triangular hyetograph method
Using IDF relationships
Empirical Hyetographs (Texas specific)
This slide adapted from:

25. SCS Method
SCS (1973) analyzed DDF to develop dimensionless rainfall temporal patterns called type curves for four different regions in the US.
SCS type curves are in the form of percentage mass (cumulative) curves based on 24-hr rainfall of the desired frequency.
This slide adapted from:

26. SCS Method
SCS (1973) analyzed DDF to develop dimensionless rainfall temporal patterns called type curves for four different regions in the US.
SCS type curves are in the form of percentage mass (cumulative) curves based on 24-hr rainfall of the desired frequency.
If a single precipitation depth of desired frequency is known, the SCS type curve is rescaled (multiplied by the known number) to get the time distribution.
For durations less than 24 hr, the steepest part of the type curve for required duration is used
This slide adapted from:

27. SCS Method
If a single precipitation depth of desired frequency is known, the SCS type curve is rescaled (multiplied by the known number) to get the time distribution.
This slide adapted from:

28. SCS Method
For durations less than 24 hr, the steepest part of the type curve for required duration is used (i.e. 6-hour as shown)
HEC-HMS has SCS built-in, but does not rescale time – storm must be 24-hours (or analyst rescales external to the program)
1.0
0.0
This slide adapted from:

29. SCS type curves for Texas (II&III)
SCS 24-Hour Rainfall Distributions
T (hrs)
Fraction of 24-hr rainfall
Type II
Type III
0.0
0.000
11.5
0.283
0.298
1.0
0.011
0.010
11.8
0.357
0.339
2.0
0.022
0.020
12.0
0.663
0.500
3.0
0.034
0.031
12.5
0.735
0.702
4.0
0.048
0.043
13.0
0.772
0.751
5.0
0.063
0.057
13.5
0.799
0.785
6.0
0.080
0.072
14.0
0.820
0.811
7.0
0.098
0.089
15.0
0.854
8.0
0.120
0.115
16.0
0.880
0.886
8.5
0.133
0.130
17.0
0.903
0.910
9.0
0.147
0.148
18.0
0.922
0.928
9.5
0.163
0.167
19.0
0.938
0.943
9.8
0.172
0.178
20.0
0.952
0.957
10.0
0.181
0.189
21.0
0.964
0.969
10.5
0.204
0.216
22.0
0.976
0.981
11.0
0.235
0.250
23.0
0.988
0.991
24.0
1.000
Not much difference in the two curves in dimensionless space!
This slide adapted from:

30. SCS Method Steps Given Td and frequency/T, find the design hyetograph
Compute P/i (from DDF/IDF curves or equations)
Pick a SCS type curve based on the location
If Td = 24 hour, multiply (rescale) the type curve with P to get the design mass curve
If Td is less than 24 hr, pick the steepest part of the type curve for rescaling
Get the incremental precipitation from the rescaled mass curve to develop the design hyetograph
This slide adapted from:

31. Example 9 – SCS Method
Find - rainfall hyetograph for a 25-year, 24-hour duration SCS Type-III storm in Harris County using a one-hour time increment
a = 81, b = 7.7, c = (from Tx-DOT hydraulic manual)
Find
Cumulative fraction - interpolate SCS table
Cumulative rainfall = product of cumulative fraction * total 24-hour rainfall (10.01 in)
Incremental rainfall = difference between current and preceding cumulative rainfall
This slide adapted from:

32. SCS – Example (Cont.)
If a hyetograph for less than 24 needs to be prepared, pick time intervals that include the steepest part of the type curve (to capture peak rainfall). For 3-hr pick 11 to 13, 6-hr pick 9 to 14 and so on.
This slide adapted from:

33. Triangular Hyetograph Method
Time
Rainfall intensity, i h ta tb Td
Td: hyetograph base length = precipitation duration
ta: time before the peak
r: storm advancement coefficient = ta/Td
tb: recession time = Td – ta = (1-r)Td
Given Td and frequency/T, find the design hyetograph
Compute P/i (from DDF/IDF curves or equations)
Use above equations to get ta, tb, Td and h (r is available for various locations)
This slide adapted from:

34. Triangular hyetograph - example
Find - rainfall hyetograph for a 25-year, 6-hour duration in Harris County. Use storm advancement coefficient of 0.5.
a = 81, b = 7.7, c = (from Tx-DOT hydraulic manual)
3 hr
3 hr
Rainfall intensity, in/hr
2.24
6 hr
Time
This slide adapted from:

35. Alternating block method
Given Td and T/frequency, develop a hyetograph in Dt increments
Using T, find i for Dt, 2Dt, 3Dt,…nDt using the IDF curve for the specified location
Using i compute P for Dt, 2Dt, 3Dt,…nDt. This gives cumulative P.
Compute incremental precipitation from cumulative P.
Pick the highest incremental precipitation (maximum block) and place it in the middle of the hyetograph. Pick the second highest block and place it to the right of the maximum block, pick the third highest block and place it to the left of the maximum block, pick the fourth highest block and place it to the right of the maximum block (after second block), and so on until the last block.
This slide adapted from:

36. Example: Alternating Block Method
Find: Design precipitation hyetograph for a 2-hour storm (in 10 minute increments) in Denver with a 10-year return period 10-minute
This slide adapted from:

37. Empirical Hyetograph
Dimensionless Hyetograph is parameterized to generate an input hyetograph that is 3 hours long and produces the 5-year depth.
For this example, will use the median (50th percentile) curve
Rescale Depth
Average Intensity
Rescale Time

38. Tabular values in the report.
This column scales TIME
This column scales DEPTH

39. Dimensional Hyetograph

40. Dimensional Hyetograph
Use interpolation to generate uniformly spaced cumulative depths.

41. Other Design Storms Frequency Design Storm
Enter a frequency (probability)
Enter intensity “duration” (lengths of pulses)
Enter storm “duration”
Enter accumulated depths at different portions of the storm (dimensional hyetograph)
Enter storm area (HMS uses this value for its own ARF computations)

42. Summary - I
Design storms are precipitation depths for a location for a given storm duration and a given probability.
DDF Atlas
EBDLKUP.xls, TP40, HY35
Design hyetographs are the time-redistribution of these depths.
SCS
Triangular
Empirical

43. Summary - II
Intensities are average intensities that produce to observed depth.
DDF, IDF curves convey same information. Depth is the natural (and measured) variable.
Area Reduction Factors may be appropriate for larger watersheds represented by point gages.
Theissen weights are for spatial distribution of gages
ARFs are computed externally and applied to the time series before areal weighting.

44. HEC-HMS Overview History HMS is a complex and sophisticated tool
Evolved from HEC-1 as part of “new-generation” software circa 1990
Integrated user interface to speed up data input and enhance output interpretation
HMS is a complex and sophisticated tool
Intended to be used by a knowledgeable and skilled operator
Knowledge and skill increase with use

45. HEC-1 (Predecessor to HEC-HMS)
Separate (individual) programs in 1967 (L. R. Beard)
Unified into a single program in 1973
Revised in 1981: kinematic wave
PC full version in 1988
Revised 1991: Extended memory support
Final release 1998
32 years development until final release

46. HEC-HMS Evolved from HEC-1 Project begun in 1990
HEC-HMS “released” in 1998
Current version is 3.5
21 years of development to date.
Include the HEC-1 period and have nearly 50 years of development – The program “engine” is mature!

47. HEC-HMS Purpose Replacement for HEC-1
Foundation for future hydrologic software
Improved interface (GUI), graphics, and reporting.
Newer hydrologic computation methods imbedded
Integration of hydrologic capabilities

48. Rainfall-Runoff Process
Precipitation
Meterology, Climate
Watershed
Losses
Transformation
Storage
Routing
Runoff
Fraction of precipitation signal remaining after losses

49. Land Surface and Vegetation
HEC-HMS
Hydrologic Cycle Components in HEC-HMS (circa 2008)
Snowfall
Evapo-
transpiration
Rainfall, P(t)
Snowpack
Snowmelt
Infiltration Loss
Land Surface and Vegetation
Runoff
Runoff
Percolation Loss
Channels
Reservoirs
Discharge, Q(t)

50. HEC-HMS
Conceptualizes precipitation, watershed interaction, and runoff into major elements
Basin and sub-basin description
Supply how the system components are interconnected
Loss model
Supply how rainfall is converted into excess rainfall
Transformation model
Supply how the excess rainfall is redistributed in time and moved to the outlet

51. HEC-HMS
Conceptualizes precipitation, watershed interaction, and runoff into major elements
Meterological model
Raingage specifications and assignment to different sub-basins
Time-series models
Supply input hyetographs
Supply observed hydrographs
Simulation control
Supply instructions of what, when, how to simulate

52. HEC-HMS Precipitation Abstractions Routing
Fraction of precipitation that does not contribute to runoff (and ultimately discharge)
Routing
Watershed routing
Stream (Channel) routing
Reservoir (Storage) routing

53. HEC-HMS Data management All files arranged in a “Project”
Graphical User Interface (GUI)
Multiple input files
Multiple output files
Time-series in HEC-DSS
All files arranged in a “Project”
Paths to individual files
Can entire project folders and have them run elsewhere

54. HEC-HMS, IO Files project-name.hms basin-model-name.basin
List of models, descriptions, project default methods
basin-model-name.basin
Basin model data, including connectivity
meterologic-model-name.met
Meterologic model data

55. HEC-HMS, IO Files control-specifications-name.control run-name.log
Run log; messages, warnings, etc. during a run.
project-name.run
List of runs, includes recent execution time.

56. HEC-HMS
Introduction to HMS
Minimal Model
Rainfall models

57. Exercise Exercise 3 Interpret Texas DDF Atlas
Explore HEC-HMS Minimal Model

58. Hydrologic Models Require engineering judgment
Experience helps
Results can be difficult to interpret
Require accurate input data
Judgment here too, some data have marginal influence on results, other data are vital.
Require quality control procedures

59. Documenting Your Work
This exercise is a bit more complex than filling out a table (although that is part of the exercise).
This exercise you will complete the table and hand in a report with relevant screen captures of the HMS model and the written component for the last problem.