1. SDHM Hydromodification Overview
Doug Beyerlein, P.E.
Joe Brascher
Clear Creek Solutions, Inc.

2. Why HMP Modeling? Impacts of land use changes:
Increase in impervious surface
Decrease in amount of vegetation
Grading and compaction of soils
Construction of drainage facilities

3. Why HMP Modeling? Effects of land use change on stormwater runoff:
Less infiltration and evapotranspiration
More surface runoff (increased volume)
Runoff leaves the site faster (increased peak flows)
Runoff occurs more often (increased duration)
Runoff conveyed directly to creek (increased connectivity)

4. What is HMP Modeling?
HMP modeling is quantifying the hydrologic impacts caused by land use changes in terms of:
(1) frequency
(2) duration

5. Frequency
Frequency is the probability of a certain size event occurring:
2-year (50% probability)
5-year (20%)
10-year (10%)
25-year (4%)
50-year (2%)
100-year (1%)

6. Frequency
Traditionally we have used single-event design storms to size facilities based on frequency.
Design storm assumption: 25-year rainfall causes 25-year flood.

7. Single-event design storm methodology doesn’t work for HMP design because:
Single-event flow frequency standards are based on inappropriate assumptions.
Single-event modeling cannot compute flow durations (percent of time flows exceed a specific value).

8. Duration
Flow Duration Analysis: Percent of time the flow exceeds a specific flow value.

9. Durations Flow duration standard: based on erosive flows.
Western Washington: 50% of 2-yr to 50-yr

10. Durations Flow duration standard: based on erosive flows.
Santa Clara, San Mateo, Alameda: 10% of 2-yr to 10-yr

11. Durations Flow duration standard: based on erosive flows.
San Diego County: 20% of 5-yr to 10-yr

12. Based on continuous simulation hydrology: HSPF (SDHM) HEC-HMS SWMM
HMP Modeling Options
Based on continuous simulation hydrology:
HSPF (SDHM)
HEC-HMS
SWMM

13. HYDROLOGIC SIMULATION PROGRAM - FORTRAN
HSPF:
HYDROLOGIC SIMULATION
PROGRAM - FORTRAN
Continuous simulation model
Natural and developed watersheds and water systems
Land surface and subsurface hydrology and quality processes
Stream/lake hydraulics and water quality processes
Time series data management and storage
Time series data statistical analysis and operations
Core watershed model in EPA BASINS and Army Corps WMS
Development and maintenance activities sponsored by U.S. EPA and U.S. Geological Survey

14. HSPF: History 1966 – Stanford Watershed Model by Linsley and Crawford
1972 – HSP by Hydrocomp
1973 – ARM (Agricultural Runoff Management) Model for EPA by Hydrocomp
1974 – NPS (Non-Point Source) Model for EPA by Hydrocomp
1979 – HSPF (combining HSP, ARM, and NPS) for EPA by Hydrocomp

15. HSPF: Glossary (Greek to English)
UCI: User Control Input (file type)
WDM: Watershed Data Management (file type)
PERLND: Pervious land area
IMPLND: Impervious land area
RCHRES: Conveyance/routing reach
FTABLE: Stage-storage-discharge table
SURO: Surface runoff
IFWO: Interflow (shallow, subsurface runoff)
AGWO: Active groundwater/base flow

16. HSPF: File Types UCI file: User Control Input (text file)
Land area (soil, vegetation, slope, impervious) values
Conveyance parameter values
Display of annual peak flows
Duration analysis
WDM file: Watershed Data Management (binary file)
Precipitation time series
Potential evapotranspiration time series
Simulated runoff time series

17. HSPF Made Easy: SDHM San Diego Hydrology Model (SDHM).
SDHM is HSPF in an easy to use package.

18. HSPF Made Easy: SDHM
HSPF UCI and WDM files are automatically created by SDHM and are transparent to the user, but are available for review.

19. SDHM
Available free to local municipal agency staff from
Available to consultants for a fee.

20. SDHM Based on the same software platform as BAHM and WWHM3.
Customized for San Diego County.

21. WDM Data: Precipitation
Requirements:
continuous record (no data gaps or missing data)
30-50 years of data
hourly or smaller time interval
Example: San Diego Airport National Weather Service data
Note: Need to scale precipitation data to project site. Take ratio of station mean annual precip and project site mean annual precip.

22. WDM Data: Potential Evapotranspiration
Requirements:
continuous record (no data gaps or missing data)
30-50 years of data
daily or smaller time interval
Example: local pan evaporation data
If using pan evap data need to include pan evap coefficient to convert to PET data.

23. WDM Data: Irrigation? Requirements:
continuous record (no data gaps or missing data)
30-50 years of data
daily or smaller time interval
Example: Representative of urban landscape watering patterns.

24. Simulation Period 30-50 years
dependent on start and end dates of precip
and PET time series
should start in a dry period
(water year: Oct 1 – Sep 30)

25. PERLNDs (Pervious Land Areas)
Designation of PERLND types is based on
soil type (A, B, C, D)
vegetation type
forest/trees
scrub/shrub
natural grasslands
bare earth/dirt
urban landscape vegetation (irrigated)
land slope
flat (0-5% slope)
moderate (5-10% slope)
steep (10-20% slope)
very steep (>20% slope)

26. PERLNDs (Pervious Land Areas)
Simulate the water movement and storage in the different components/pathways in the hydrologic cycle.

27. IMPLNDs (Impervious Land Areas)
Designation of IMPLND types is based on
land slope
flat (0-5% slope)
moderate (5-10% slope)
steep (10-20% slope)
very steep (>20% slope)

28. Three types of runoff IFWO (interflow) AGWO (groundwater/base flow)
SURO (surface runoff)
IFWO (interflow)
AGWO (groundwater/base flow)
Stormwater runoff
is usually defined as
the sum of SURO+IFWO
(surface runoff +
interflow).

29. RCHRES (Conveyance reach system)
Precipitation on Reach Surface
Evaporation
Outflow through exit 1
Volume of Water in a Reach
Exit 2
Inflow
(upstream
sources,
direct drainage)
Exit (N = up to 4)
Exit can represent infiltration through bottom of reach

30. RCHRES (Conveyance reach system)
RCHRES can represent any type of conveyance/storage system:
open channel
stormwater pond
stormwater vault
pipe system
bioretention cell
planter box
porous pavement

31. RCHRES (Conveyance reach system)
All conveyance/storage systems are described using an FTABLE.
An FTABLE is a table with:
Stage (feet)
Surface area (acres)
Storage volume (acre-feet)
Discharge (cfs)

32. RCHRES (Conveyance reach system)
Open channel:

33. RCHRES (Conveyance reach system)
Stormwater pond:

34. RCHRES (Conveyance reach system)
Stormwater pond discharge:
Riser and notch discharge based on weir equation.
Orifice discharge based on orifice equation.

35. RCHRES (Conveyance reach system)
Riser notch options:
Rectangular
V-Notch
Sutro

36. RCHRES (Conveyance reach system)
Stormwater vault:

37. RCHRES (Conveyance reach system)
Pipe system:

38. RCHRES (Conveyance reach system)
Bioretention cell:
Need HSPF Special Actions to check on the ability to discharge from the surface storage to Layer 1.

39. RCHRES (Conveyance reach system)
Planter box:
Need HSPF Special Actions to check on the ability to discharge from the surface storage to the soil layer.

40. RCHRES (Conveyance reach system)
Porous pavement:
Storage in the pavement layer and gravel subgrade. Infiltration to the underlying native soil is optional.

41. Downstream Point of Compliance
Point of Compliance (POC) is at the downstream exit from the project site.

42. Two Model Scenarios Existing Post-development
Compute stormwater runoff for both existing and
post-development conditions.
Existing Post-development

43. Flow Frequency
To compute frequency the maximum hourly annual existing flow values are needed.

44. Flow Frequency Computational Methods
Weibull Plotting Position
P = (m-a)/(N-a-b+1) where P = probability
m = rank
N = number of years
a = constant
b = constant
Return period, Tr (years) = 1/P

45. Duration Analysis
Flow Duration Analysis: Percent of time the flow exceeds a specific flow value.

46. Duration Analysis Comparison
Flow Duration Table for flows from 20% of the 5-year to 100% of the 10-year frequency value.

47. Duration Analysis Comparison
Flow Duration Curves for flows from 20% of the 5-year to 100% of the 10-year frequency value.

48. Other Analysis: Drawdown
For vector management: Want to know the water level drawdown/retention depth (or stage) for 1-5 days plus maximum stage and drawdown time.
Use stage time series.

49. SDHM Steps for sizing hydromod facilities:
Locate project site on county map.
Input existing land conditions.
Input future developed land conditions.
Create hydromod mitigation facility.
Create Point of Compliance.
Automatically size hydromod mitigation facility.
Run HSPF.
Compute flow durations.
Compute drawdown (optional).

50. SDHM 1. Locate project site on county map:
Place red dot on county map

51. SDHM 2. Input existing land conditions:

52. SDHM 3. Input future developed land conditions:

53. SDHM 4. Create hydromod mitigation facility:
User can select from a menu of different types.

54. SDHM 5. Create Point of Compliance:

55. SDHM 6. Automatically size hydromod mitigation facility:
SDHM will automatically size many facilities.

56. SDHM 7. Run HSPF:

57. SDHM 8. Compute flow durations:
Automatically computes frequency and duration analysis for POC.

58. SDHM 9. Compute drawdown (optional)

59. SDHM Create project report:
Includes all input and output info plus analyses.

60. SDHM: LID/BMP Options SDHM includes the following LID/BMP options:
Dispersion of impervious surface runoff on adjacent pervious surface (example: roof runoff to lawn).
Bioretention/rain gardens/landscape swales to retain and infiltrate stormwater.
Planters/dry wells/infiltration trenches/porous pavement to retain and infiltrate stormwater.
Vegetated/grassy (dry) swales/sand filters to provide water quality enhancements.

61. SDHM Project files created by SDHM:
WHM: Binary project file.
WH2: Text project file.
WDM: Binary Watershed Data Management file.

62. Free download for agency staff and consultants at
SDHM User Manual:
Free download for agency staff and consultants at

63. SDHM
Questions? Contact: Doug Beyerlein Joe Brascher