1. Northern California LID Hydrology and Hydraulics
Doug Beyerlein, P.E.
Clear Creek Solutions, Inc.

2. Clear Creek Solutions, Inc.
This presentation was originally given at the ACEC LID Seminar in Sacramento, CA, in November 2008 by
Doug Beyerlein, P.E.
Clear Creek Solutions, Inc.

3. LID Hydrology and Hydraulics Modeling
There is nothing magical about LIDs. Water must go somewhere. Water must either:
Infiltrate into the soil.
Evaporate/transpire into the atmosphere.
Runoff.

4. Why LID Hydrology and Hydraulics 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)

5. Why Modeling?
We use modeling to quantify the hydrologic impacts of LIDs in terms of:
(1) Frequency
(2) Duration
(3) Volume

6. 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%)

7. 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.

8. Single-event design storm methodology doesn’t work for LID modeling because:
Single-event flow frequency standards are based on inappropriate assumptions.
Single-event modeling does not compute flow durations for hydromod requirements (flow duration is the percent of time flows exceed a specific value).
Single-event modeling does not include the long-term effects of evapotranspiration.

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

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

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

12. Volume
Annual runoff volume reduction due to increased infiltration and evapotranspiration.

13. Accurate simulation of LID hydrology requires continuous simulation modeling
Represent all of the components of the hydrologic cycle.
Include a full range of soil, vegetation, and topographic conditions.
Reproduce observed streamflow for both small and large drainages.
Use long-term local hourly precip to generate long-term hourly runoff.

14. LID Hydraulics Represents all conveyance systems, including LIDs.
Routes runoff using linear reservoir routing.
Represents conveyance systems with a table of stage-storage-discharge values (BAHM SSD Table or HSPF FTABLE).
Includes rainfall and evaporation on open water surfaces.
Includes infiltration (if turned on by user)

15. Sample Stage-Storage-Discharge Table

16. Potential LID Techniques/Facilities
stormwater infiltration ponds/basins
bioretention cells
planter boxes
porous pavement
green roofs
rain gardens
sand filters

17. LID Hydrology and Hydraulics Modeling Results
porous pavement
green roofs
bioretention cells / planter boxes / rain gardens

18. LID Modeling Options Based on continuous simulation hydrology:
HSPF (WWHM, BAHM, SDHM)
HEC-HMS
SWMM

19. 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

20. 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

21. HSPF Made Easy: BAHM
BAHM: Bay Area Hydrology Model using San Jose rainfall data

22. LID: Porous Pavement

23. Porous Pavement Flow Paths
Evaporation from pavement
Rain on pavement
Surface Runoff
Infiltration through pavement
Infiltration to gravel subgrade
Underdrain Flow
Infiltration to native soil

24. LID: Porous Pavement Modeling Assumptions:
Porous pavement thickness of 6 inches
Gravel subgrade thickness of 18 inches
Evaporation from gravel subgrade
Infiltration into native soil
No underdrain

25. Northern California: Flow Frequency Native Soil Infiltration (in/hr)
LID: Porous Pavement
Northern California: Flow Frequency
Site
Native Soil Infiltration (in/hr)
10-Yr Flow (cfs/ac)
Reduction (cfs/ac)
Reduction (%)
Impervious
0.603
0.000
0.0%
Porous
0.330
0.273
45.3%
0.001
0.216
0.387
64.2%
0.002
0.155
0.448
74.3%
0.003
0.057
0.546
90.5%
0.004
100.0%
0.005

26. Northern California: Flow duration curves
LID: Porous Pavement
Northern California: Flow duration curves

27. LID: Porous Pavement Northern California: Flow Duration Hours Site
Native Soil Infiltration (in/hr)
Flow Duration hours (0.1 cfs/ac)
Reduction (cfs/ac)
Reduction (%)
Impervious
1449
0.0%
Porous
181
1268
87.5%
0.001 90
1359
93.8%
0.002 38
1411
97.4%
0.003 15
1434
99.0%
0.004 3
1446
99.8%
0.005
100.0%

28. LID: Porous Pavement Northern California: Annual Flow Volume Reduction
Site
Native Soil Infiltration (in/hr)
Total Runoff (in/yr)
Reduction (in/yr)
Reduction (%)
Impervious
12.371
0.000
0.0%
Porous
1.882
10.489
84.8%
0.001
0.953
11.418
92.3%
0.002
0.414
11.957
96.7%
0.003
0.112
12.259
99.1%
0.004
0.030
12.341
99.8%
0.005
100.0%

29. LID: Green Roof Green/vegetated/eco-roof
Water is stored in the soil prior to runoff.

30. Green roofs:

31. LID: Green Roof Modeling Assumptions:
Green roof vegetation is ground cover
Roof surface flat (< 1% slope)
Flow path length of 50 feet to a drain
No infiltration into building
No underdrain

32. Northern California: Flow Frequency
LID: Green Roof
Northern California: Flow Frequency
Site
Soil Depth (in)
10-Yr Flow (cfs/ac)
Reduction (cfs/ac)
Reduction (%)
Impervious
0.603
0.000
0.0%
Green 2
0.372
0.231
38.3% 3
0.358
0.245
40.6% 4
0.351
0.252
41.8% 5
0.346
0.257
42.6% 6
0.343
0.260
43.1% 7
0.340
0.263
43.6% 8
0.338
0.265
43.9% 9
0.335
0.268
44.4% 10
0.332
0.271
44.9% 11
0.327
0.276
45.8% 12
0.323
0.280
46.4%

33. Northern California: Flow duration curves
LID: Green Roof
Northern California: Flow duration curves

34. LID: Green Roof Northern California: Flow Duration Hours Site
Soil Depth (in)
Flow Duration hours (0.1 cfs/ac)
Reduction (cfs/ac)
Reduction (%)
Impervious
1449
0.0%
Green 2
336
1113
76.8% 3
305
1144
79.0% 4
277
1172
80.9% 5
251
1198
82.7% 6
231
1218
84.1% 7
211
1238
85.4% 8
182
1267
87.4% 9
169
1280
88.3% 10
156
1293
89.2% 11
148
1301
89.8% 12
137
1312
90.5%

35. Northern California: Annual Flow Volume Reduction
LID: Green Roof
Northern California: Annual Flow Volume Reduction
Site
Soil Depth (in)
Total Runoff (in/yr)
Reduction (in/yr)
Reduction (%)
Impervious
12.371
0.000
0.0%
Green 2
7.173
5.198
42.0% 3
6.607
5.764
46.6% 4
6.175
6.196
50.1% 5
5.812
6.559
53.0% 6
5.486
6.885
55.7% 7
5.185
7.187
58.1% 8
4.904
7.467
60.4% 9
4.642
7.729
62.5% 10
4.397
7.974
64.5% 11
4.168
8.203
66.3% 12
3.953
8.418
68.0%

36. Rain Garden/Bioretention
Rain garden/bioretention/landscape swale
Water infiltrates into the soil before runoff.

37. Bioretention/rain garden/landscape swale:

38. Bioretention: Planter Box

39. LID: Bioretention Modeling Assumptions:
Drainage area is 1 acre of impervious surface
Bioretention area is 5% of impervious area draining to it
Top layer of bioretention area is amended soil
Amended soil thickness of 24 inches
Amended soil infiltration rate equals 2 inches per hour
No underdrain

40. Northern California: Flow Frequency Native Soil Infiltration (in/hr)
LID: Bioretention
Northern California: Flow Frequency
Site
Native Soil Infiltration (in/hr)
10-Yr Flow (cfs/ac)
Reduction (cfs/ac)
Reduction (%)
Impervious
0.633
0.000
0.0%
Bioretention
0.598
0.035
5.5%
0.001
0.005
0.594
0.039
6.2%
0.01
0.05
0.542
0.091
14.4%
0.1
0.535
0.098
15.5%
0.5
0.516
0.117
18.5% 1
0.488
0.145
22.9% 5
0.485
0.148
23.4% 10 50

41. Northern California: Flow duration curves
LID: Bioretention
Northern California: Flow duration curves

42. LID: Bioretention Northern California: Flow Duration Hours Site
Native Soil Infiltration (in/hr)
Flow Duration hours (0.1 cfs/ac)
Reduction (cfs/ac)
Reduction (%)
Impervious
1555
0.0%
Bioretention
1523 32
2.1%
0.001
1437
118
7.6%
0.005
1334
221
14.2%
0.01
1244
311
20.0%
0.05
974
581
37.4%
0.1
833
722
46.4%
0.5
577
978
62.9% 1
468
1087
69.9% 5
399
1156
74.3% 10 50

43. LID: Bioretention Northern California: Annual Flow Volume Reduction
Site
Native Soil Infiltration (in/hr)
Total Runoff (in/yr)
Reduction (in/yr)
Reduction (%)
Impervious
12.990
0.000
0.0%
Bioretention
12.900
0.090
0.7%
0.001
11.882
1.108
8.5%
0.005
11.096
1.893
14.6%
0.01
10.327
2.663
20.5%
0.05
7.733
5.257
40.5%
0.1
6.519
6.471
49.8%
0.5
4.367
8.622
66.4% 1
3.594
9.396
72.3% 5
3.063
9.927
76.4% 10 50

44. Summary
Porous pavement can provide 100% reduction in runoff volume, peak flows (frequency), and durations at very low infiltration rates (< 0.01 in/hr).

45. Summary Green roofs reduce runoff volume by 40-70%
runoff durations by 70-90%
peak flows by 40-50%

46. Summary Bioretention reduces
volume and durations by 10-20% for poor draining soils
volume and durations by 50-70% for well drained soils
peak flows by 5-20%

47. LID Hydrology and Hydraulic Modeling
For more information Go to:

48. LID Modeling
Questions? Contact: Doug Beyerlein