UW LID Workshop Permeable Pavement Modeling June 2008 Doug Beyerlein, P.E. Clear Creek Solutions, Inc.

Clear Creek Solutions’ Stormwater LID Expertise Clear Creek Solutions, Inc., provides complete range of hydrologic and stormwater modeling services. Clear Creek specializes in continuous simulation hydrologic modeling. We have 30+ years of experience modeling complex hydrologic and stormwater problems. We created the Western Washington Hydrology Model Version 3 (WWHM3) for Washington State Department of Ecology. We teach WWHM and HSPF workshops.

Presentation Introduction WWHM: Western Washington Hydrology Model Permeable Pavement Implicit Modeling Permeable Pavement Explicit Modeling Modeling Results Summary Questions & Answers

Permeable pavement:

Permeable Pavement Reduces Runoff Volume: 1.Infiltration to native soil. 2.Evaporation.

Hydrologic Modeling Continuous simulation: WWHM (HSPF) Continuous simulation hydrology models the entire hydrologic cycle for multiple years.

Western Washington Hydrology Model (WWHM) Developed for the State of Washington Department of Ecology. Project Manager: Dr. Foroozan Labib Department of Ecology PO Box Olympia, WA (360)

Western Washington Hydrology Model (WWHM) Developed for the 19 counties of western Washington. Part of Ecology’s Stormwater Management Manual

WWHM3 Available free from the Washington State Department of Ecology web site:

WWHM3 WWHM helps the user design facilities to meet the Washington State Department of Ecology’s flow control standards.

WWHM3 Ecology’s flow duration standard: based on erosive flows. Erosive flow range: ½ of the 2-year to the 50-year

Disclaimer: Permeable pavement by itself may not fully meet Ecology’s flow control standards… but permeable pavement will reduce the size of a flow control facility (stormwater pond, vault, etc.).

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

Permeable Pavement Flow Paths Surface Runoff Underdrain Flow Surface runoff and underdrain flow are subject to Ecology’s flow control standard (1/2 of 2-yr to 50-yr).

Permeable Pavement Flow Paths Infiltration to native soil is dependent on native soil characteristics. Infiltration to native soil

WWHM: Permeable Pavement Modeling Options Permeable pavement can be modeled implicitly or explicitly:  PSAT (Puget Sound Action Team) recommends how to implicitly represent permeable pavement in WWHM2  WWHM3 explicitly represents permeable pavement

WWHM: Permeable Pavement Modeling Options Implicit modeling: Permeable asphalt and concrete Public facility (road or parking lot): 1.Above grade without underdrain: Model as lawn. 2.Above grade with underdrain: Model as impervious. 3.Below grade without underdrain: (a) Lawn or (b) impervious surface routed to infiltration facility. 4.Below grade with underdrain: (a) Impervious or (b) impervious to infiltration facility.

WWHM: Permeable Pavement Modeling Options Implicit modeling: Permeable asphalt and concrete Private facility (driveway, parking lot, sidewalk, etc): 1.Below grade without underdrain: 50% lawn, 50% impervious. 2.Below grade with underdrain: impervious.

WWHM: Permeable Pavement Modeling Options Implicit modeling: Non-concrete grid/ lattice Public facility (road or parking lot): 1.Above grade without underdrain: Model as lawn. 2.Above grade with underdrain: Model as impervious. 3.Below grade without underdrain: (a) Lawn or (b) impervious surface routed to infiltration facility. 4.Below grade with underdrain: (a) Impervious or (b) impervious to infiltration facility.

WWHM: Permeable Pavement Modeling Options Implicit modeling: Non-concrete grid/ lattice Private facility (driveway, parking lot, sidewalk, etc): 1.Below grade without underdrain: 50% lawn 50% impervious. 2.Below grade with underdrain: impervious.

WWHM: Permeable Pavement Modeling Options Implicit modeling: Paving blocks Public facility (road or parking lot): 1.Above grade without underdrain: 50% lawn, 50% impervious. 2.Above grade with underdrain: Model as impervious. 3.Below grade without underdrain: (a) 50% lawn, 50% impervious or (b) impervious surface routed to infiltration facility. 4.Below grade with underdrain: (a) Impervious or (b) impervious to infiltration facility.

WWHM: Permeable Pavement Modeling Options Implicit modeling: Paving blocks Private facility (driveway, parking lot, sidewalk, etc): 1.Below grade without underdrain: 50% lawn 50% impervious. 2.Below grade with underdrain: impervious.

WWHM Permeable Pavement Modeling (Implicit) Table 1. Permeable Pavement Reduction of Total Runoff Volume Site Total Runoff (in/yr) Reduction (in/yr) Reduction (%) 100% Impervious % 50% Lawn 50% Imp % 100% Lawn %

WWHM Permeable Pavement Modeling (Implicit) Table 2. Permeable Pavement Reduction of 100-Year Flow Site 100-Year Flow (cfs/ac) Reduction (cfs/ac) Reduction (%) 100% Impervious % 50% Lawn 50% Imp % 100% Lawn %

WWHM Permeable Pavement Modeling (Implicit) Table 3. Permeable Pavement Reduction of Storage Volume Site Pond Storage Volume (ac-ft/ac) Reduction (ac-ft/ac) Reduction (%) 100% Impervious % 50% Lawn 50% Imp % 100% Lawn %

WWHM: Permeable Pavement Modeling Options Explicit modeling (current):  Assumes infiltration rate into pavement is greater than rainfall rate (no surface ponding until pavement and subgrade layers are filled).  Assumes evaporation from subgrade layer.  Represented in WWHM3 by land use basin element (surface) and gravel trench bed element (subsurface layers).

WWHM: Permeable Pavement Modeling Options Explicit modeling (future):  Separate permeable pavement element.

WWHM Permeable Pavement Modeling (Current)

WWHM Permeable Pavement Modeling (Explicit) Table 4. Permeable Pavement Reduction of Total Runoff Volume Site Infiltration (in/hr) Total Runoff (in/yr) Reduction (in/yr) Reduction (%) Impervious % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable %

Permeable Pavement Modeling (Explicit) Figure 1. Permeable Pavement Reduction of Total Runoff Volume

WWHM Permeable Pavement Modeling (Explicit) Table 5. Permeable Pavement Reduction of 100-Year Flow Site 100-Year Flow (cfs/ac) Reduction (cfs/ac) Reduction (%) Impervious % Permeable – No infiltration % Permeable – Infiltration 0.01 in/hr %

WWHM Permeable Pavement Modeling (Explicit) Table 6. Permeable Pavement Reduction of Storage Volume Site Pond Storage Volume (ac-ft/ac) Reduction (ac-ft/ac) Reduction (%) Impervious % Permeable – No infiltration % Permeable – Infiltration 0.01 in/hr %

Permeable Pavement Modeling Results Summary: Permeable pavement works best for flow control when there is infiltration to the native soil. Underdrain flows must still be mitigated to flow control standards. Permeable pavement can reduce stormwater runoff volume, but additional mitigation may still be required to meet Ecology’s flow control standards.

For more information on WWHM3 permeable pavement modeling go to:

Questions? Contact: Doug Beyerlein Joe Brascher