Helping local jurisdictions cope = with multiple runoff requirements

By Douglas Beyerlein

Implementing improved engineering techniques and drastic changes in where and how land is developed and how people live and move across the land are necessary to achieve the goals in = the federal Clean Water Act—to preserve, maintain, and restore the benefi= cial uses of our nation’s waters.
—Washington State Department of Ecology (2001a)

For the 19 counties of western Washington, stretching from the Columbia River in the south to the Canadian border in the north, stormw= ater mitigation rules have changed. Today, through National Pollutant Discharge Elimination System (NPDES) permits and certifications under Section 401 of = the Clean Water Act, the Washington State Department of Ecology requires flow duration analysis of runoff from new proposed development. Postdevelopment flows cannot exceed predevelopment conditions for flows in the erosive flow range of half the two-year flood to the 50-year flood. Gone are the days of just matching the predevelopment two- and 10-year flood frequencies.=

In the state of Washington, the need for new stormwater requirements, standards, and methodologies is the result of state and feder= al regulations. State regulations under the Growth Management Act and the Water Pollution Control Act as well as under the federal Clean Water Act, Safe Drinking Water Act, and Endangered Species Act call for significant changes= in the way we manage urban stormwater runoff. The adoption of a flow duration–based standard and methodology allows us to accomplish these goals.

Similar efforts are beginning in = parts of California to meet NPDES municipal separate storm sewer system (MS4) per= mit requirements as they pertain to hydromodification impacts (changes in runoff peaks and volumes).<= /p>

Important in meeting the new flow duration–based requirements is the use of low-impact development (LID) approaches to decrease the amount of stormwater runoff generated from land-= use changes. These LID approaches can take on many forms, ranging from pervious pavement to roof gardens (green roofs), and they can be incorporated in flow duration–based modeling to determine their effectiveness in reducing stormwater peaks and total volume. This, in turn, reduces the stormwater detention storage required to meet predevelopment runoff conditions.=

Background
The Washington S= tate Department of Ecology realized the difficulty for engineers to meet a flow duration–based standard, a requirement that is part of the 2001 Stormwater Management Manual for Western Washington<= /st1:place> (Washington State Department of Ecology 2001b). To facilitate the needed fl= ow duration analysis, the Department of Ecology contracted with Aqua Terra Consultants to develop a computer modeling package to model predevelopment = and postdevelopment runoff and perform the flow duration analysis. This software package became the Western Washington Hydrology Model (WWHM).

The purpose of the WWHM is to pro= vide a visually oriented interactive tool to optimally size stormwater control facilities (for both water quantity and water quality) to mitigate the stormwater and water-quality impacts of land-use changes on local streams a= nd rivers. The model can also be used to review development plans for complian= ce with stormwater regulations, promote sustainable development practices, and educate the engineering community and public on the impacts of land-use changes.

The model uses the Washington Sta= te Department of Ecology’s flow duration standard to prevent erosive flo= ws from increasing with increased runoff from new development. Flow duration is defined as the percent of time flow exceeds a specific value.

Analysis and studies conducted by= Booth and Jackson (1997) show that there is a range of stream flows that produces= the majority of erosion in western Washington streams. This erosive flow range is from approximately one-half of the two-= year flood to the 50-year flood. Modeling work by Beyerlein (1999) demonstrated = that standard mitigation methods focusing solely on frequency events, such as the two-year and the 10-year flood, do not address the erosive flow issue and consistently undersize stormwater control facilities. This is because the t= otal volume of stormwater runoff increases with the level or intensity of land development (Figure 1). Single-event flood frequency–based methods (s= uch as SCS curve numbers, the Santa Barbara Urban Hydrograph, and the Rational Method) do not address this problem and consistently undersize stormwater storage requirements (Figure 2). The solution is to use continuous simulati= on hydrologic modeling.=

Figure 1. Rainfall Distribution, Seattle, WA

Figure 2. Comparison of Single-Event and Continuous Simulation-Sized Ponds

Thom Hooper of the National Ocean= ic and Atmospheric Administration (NOAA) Fisheries in Seattl= e explains the problem from a fisheries perspective: “In western Washington, broad habitat degradation is perhaps the most significant contribution to the dec= line of wild salmon stocks. Salmon have evolved to habitat conditions dependent = on the recruitment of clean gravel and flow of uncontaminated cold fresh water= for spawning, egg development, and successful fry emergence. Altered landscapes have adversely affected the delivery of sediment and gravel and significant= ly altered stream hydrology and water quality. Loss of forest cover, increases= in impervious surfaces, and increased drainage networks have had a contributing role. New development and redevelopment should be done in such a way as to prevent further degradation of stream channels. Implementation of the Weste= rn Washington Hydrology Model to engineer stormwater abatement measures is a promising tool to help accomplish this.”

King Count= y (the Seattle metropolitan area), WA, was first to require flow duration analysis for proposed new development. The King County Runoff Time Series (KCRTS) model = is the required continuous simulation methodology under the 1998 King County Surface Water Design Manual (King County 1998) for si= zing stormwater detention and infiltration flow control facilities. KCRTS is specific to King County and uses pregenerated HSPF (Hydrological Simulation Program–FORTRAN) runoff time series to perfo= rm the flow duration analysis.=

Although KCRTS led the way in the= use of flow duration analysis to size stormwater detention ponds, its computati= onal limitations and menu-driven interface did not make it a useful tool for flow duration–based stormwater mitigation modeling throughout the 19 count= ies of western Washington. Recognition of KCRTS’s limitations led to the development of the WWHM. Dr. Foroozan Labib of the Department of Ecology’s Water Quality Program says that the need for a new tool was evident when “Ecology concluded that to protect stream channels in western Washington from accelerated erosion, it must adopt a flow duration standard for urban stormwater discharges to streams. Achievement of this standard requires the= use of continuous simulation runoff modeling.”

The Department of Ecology’s= goal was to develop a continuous simulation hydrologic model for western Washington to pe= rform flow duration analysis. The components of this goal included the ability to=

simulate the predevelopment runoff for a user-defined site using HSPF and a long-term meteorological record;
simulate the postdevelopment runoff based on user-specified development conditions;
provide runoff time series input to commercial software package= s to size stormwater mitigation facilities;
shield the complexities of HSPF from the general user; and
allow access to the full capabilities of HSPF for the more adva= nced user.

WWHM version 1.0, produced in 200= 1, achieved all of these goals. After this first version was introduced to the engineering and regulatory communities, the Department of Ecology sponsored= a series of training workshops throughout western Washington. At the workshops participa= nts had the opportunity to try the software and provide feedback on their initi= al experiences. Some 300 engineers, planners, and reviewers attended. Feedback= was generally positive, although it was widely recognized that the WWHM needed = to include the ability to size stormwater ponds without the user having to emp= loy a separate third-party software program, as was the case with version 1.0.<= /span>

WWHM version 2 (WWHM2) corrected = this limitation and added an AutoPond feature that automatically sizes and optim= izes a stormwater detention pond to meet flow duration requirements. WWHM2 also added a new user interface, more flexibility in selected predevelopment and postdevelopment land-use conditions, multiple basins and ponds, and water-quality calculations. The user’s manual (Washington State Department of Ecology 2003) documents all of the new features and provides detailed directions for all of the options in the model.

WWHM2 workshops were held in the = summer of 2003. More than 400 engineers attended the four-hour training sessions. = The feedback was overwhelmingly positive. Erik Davido, principal/president of Davido Consulting Group in Sea= ttle, says that “WWHM2 takes a complex model [HSPF] and makes it a quick and useful tool for sizing stormwater ponds.”

This strong positive response by = the engineering community encouraged the Department of Ecology to fund version = 3, which is currently under development.<= o:p>

Jim Scanlin, program manager of t= he Alameda (California) Countywide Clean Water Program, also found that the WWHM approach to stormw= ater design would meet his program’s NPDES MS4 permit requirements. “= ;Our member agencies initially considered a design storm approach, but as we tra= cked the progress of studies by the Santa Clara Valley Urban Runoff Pollution Prevention Program, the results pointed to the flow duration approach as the most technically sound way to address the hydromodification management plan permit requirements. Our next concern was finding a way to implement flow duration design that would be practical for the developers and municipal reviewers, which led us to look at adapting the WWHM.”

WWHM2 is available free of charge= from the Washington State Department of Ecology Web site at www.ecy.wa.gov/programs/wq/stormwater/wwhm_training/wwhm/=
wwhm_v2/instructions_v2.html. More WWHM2 information is available at www.wwhm2.com/.=

Methodology
The WWHM uses HS= PF (Bicknell et al. 2001) as its computational engine. HSPF is the US Geologic= al Survey (USGS) and Environmental Protection Agency continuous simulation hydrology software package, maintained by Aqua Terra Consultants. HSPF, sin= ce its introduction in 1980, has become the industry standard for hydrologic modeling.

The HSPF continuous simulation mo= del is preferred over single-event hydrology models because of its ability to comp= ute and keep track of all of the individual components of the hydrologic cycle, including surface runoff, interflow, groundwater, soil moisture, and evapotranspiration. Surface runoff (also known as overland flow) is the fir= st runoff component to reach the stream and is usually the major source of urb= an flood, as it is generated mainly by impervious surfaces. Interflow travels laterally through the upper soil layers until it reaches a conveyance system (Linsley et al. 1982). Interflow moves more slowly than surface runoff but = can still cause flooding problems. Stormwater is the sum of surface runoff and interflow. Groundwater replenishes the aquifer and/or provides base flow to= the stream. Evapotranspiration is the sum of evaporation from land surfaces and= transpiration from plants. Urbanization decreases groundwater and evapotranspiration and increases stormwater runoff (Figure 1).

The HSPF hydrology parameter valu= es used in the WWHM are based on calibrated watersheds located in western Washington. The = USGS Water-Resources Investigations Report 89-4052, Characterization and Simulation of Rainfall-Runoff Relations for Headwater Basins in Western King and Snohomish Counties, Washington (Dinicola 1990), is the source for much of the information. The WWHM values have been checked and validated on numerous ot= her small and large watersheds throughout western Washington. That said, however, Labib = notes that “Ecology encourages local governments to obtain locally calibrat= ed parameters to be used in the WWHM for watersheds in their jurisdictions.= 221;

The WWHM uses one or more long-te= rm local precipitation gages for each of the 19 counties in western Washington= and then scales the precipitation to the user’s site using published NOAA rainfall map data.

The WWHM computes stormwater runo= ff for a site selected by the user. The model runs HSPF in the background to gener= ate 40 or more years of hourly runoff data. Stormwater runoff is computed for b= oth predevelopment and postdevelopment land-use conditions. The WWHM routes the= postdevelopment stormwater runoff through a stormwater control facility of the user’s choice.

Using the predevelopment peak flo= od value for each water year, the model computes the predevelopment two- throu= gh 100-year flood frequency values. The postdevelopment runoff two- through 100-year flood frequency values are computed from the outlet of the proposed stormwater facility. The model routes the postdevelopment runoff through the stormwater facility. As with the predevelopment peak flow values, the maxim= um developed flow value for each water year is selected by the model to compute the developed two- through 100-year flood frequency.

The actual flood frequency calcul= ations are made using the federal standard Log Pearson Type III distribution descr= ibed in Bulletin 17B (United States Water Resources Council 1981). This standard flood frequency distribution is provided in USGS program J407, version 3.9A= -P, revised August 9, 1989. The Bulletin 17B algorithms in program J407 are included in the WWHM calculations.

The predevelopment two-year flood frequency value is multiplied by 50% to set the lower limit of the erosive flows. The predevelopment 50-year flood frequency value is the upper limit. Flow duration analysis is conducted for 100 flow levels between the lower erosive zone limit and the upper limit. The model counts the number of hours that predevelopment flows exceed each of the flow levels during the entire = more than 40-year simulation period. The model does the same analysis for the postdevelopment mitigated flows. =

Using the Department of Ecology f= low control standard, there are three criteria by which flow duration values are compared:

If the postdevelopment flow duration values exceed any of the predevelopment flow levels between 50% and 100% of the two-year predevelopment peak flow values (100% threshold), the flow control standard requirement has not been met.
If the postdevelopment flow duration values exceed any of the predevelopment flow levels between 100% of the two-year and 100% of the 50-year predevelopment peak flow values more than 10% of the time (110% threshold), the flow control standard has not been met.
If more than 50% of the flow duration levels exceed the 100% threshold, the flow control standard has not been met.

Model Input
WWHM input is relatively simple. The user must locate the project site on the appropriate county map (Figure 3). The user can zoom in or out on the map to find the e= xact location. The WWHM uses this information to select the appropriate precipit= ation record and multiplier for this location.

Figure 3. Project Site Location

The user then goes to the Scenari= o Generator screen to specify land-use, vegetation, and soils information. For western Washington, the vegetation categories are forest, pasture, and landscape (lawn). Predevelopment vegetation is assumed to be forest unless the user c= an document that the natural vegetation for the site was something else (for example, pasture). Western Washington has three major soil categories: outw= ash (SCS A and B soils), till (C and some D soils), and hydric/saturated wetland soils. Postdevelopment land use can include roofs; streets, sidewalks, and parking areas; landscaped areas; pasture; and pond in addition to forest. In other parts of the country the model would include other soil and vegetation categories representative of those regions.

The user inputs the number of acr= es of predevelopment land use in each of the different land categories and does t= he same for the proposed development. For residential development, there is the option to include LID practices such as roof runoff infiltration or dispers= al and porous pavement. These LID credits reduce runoff and stormwater facility size.

The user selects the type of stor= mwater control facility to include in the analysis. The available types are standa= rd trapezoidal pond; tank (cylindrical, arched); vault; and irregular-shaped p= ond. The user can select one, two, or three orifices and a riser with a flat or notched weir (notch types include rectangular, V-shaped, and Sutro). The facility can include infiltration, if appropriate for the site. The facility can be either manually sized to meet the Department of Ecology’s standards or the user can use the WWHM’s pond optimization feature (AutoPond) to size the facility. An example of the WWHM pond information in= put form is shown in Figure 4.<= /span>

Figure 4. Pond Information

AutoPond uses a complex set of ru= les to select pond dimensions and outlet orifice diameters and heights. Once AutoP= ond has made an initial selection of pond and orifice sizes, the WWHM runs HSPF= to generate the 40-plus years of hourly runoff. The runoff is routed through t= he stormwater control facility and a flow duration comparison is made with the predevelopment flows. If the postdevelopment flow duration results do not p= ass the Department of Ecology flow control standard criteria, AutoPond changes dimensions and tries again. If the postdevelopment flow duration results pa= ss the standard, AutoPond tries to make the pond smaller. This process produces the smallest (and most efficient) pond possible to meet the standard. At any time during this process the user has the option to stop AutoPond and make manual changes, if desired.=

The user has the option of adding= a water-quality facility either upstream or downstream of the stormwater cont= rol facility. Placing the water-quality facility upstream takes advantage of the flow moderation it provides to the control facility. This will result in a smaller stormwater control facility. Conversely, the water-quality facility will have to be made larger to handle the greater variations in flows than = if it is downstream of the control facility (which then moderates the flows to= the water-quality facility).

An example of a water-quality fac= ility is a sand filter. The user must input the hydraulic conductivity of the fil= ter material, the filter material depth, and the dimensions of the filter facil= ity and its outlet structure. The WWHM routes the runoff through the filter and computes the percentage of runoff filtered by the facility.

Model Output
The WWHM produces model output in both graphical and tabular form. The major graphical output= of interest is the flow duration plot of predevelopment flow and mitigated postdevelopment flow. All of the mitigated postdevelopment flow values must= be on or to the left of the predevelopment values.

Numeric output is provided in tab= ular form. The WWHM produces a project report that lists all of the input information. This includes the precipitation station and multiplier used, b= oth predevelopment and postdevelopment land-use types and acreages, and the dimensions and specifications of the flow control facility.

Any changes to the default parame= ter values or flow control standard criteria are noted in the project report. T= he project report also lists the number of hours the predevelopment and postdevelopment flows exceed each of the 100 flow duration levels and wheth= er or not the flow control facility passes or fails the flow control standard = for that level. Failure at any one of the 100 levels means the facility fails to meet the Department of Ecology’s standard.

The WWHM also computes the flow r= ate required by the Washington Department of Ecology for water-quality treatment based on treating 91% of the total annual runoff volume. The water-quality treatment facility (which is separate from the flow control facility) can be either an online system or an offline system, depending on the user’s preference.

The user has the option of saving= the project file to disk. This project file can be later read into the WWHM by = the user or a reviewer to check or further modify the project.

Low-Impact Development Practices<= br> LID practices ha= ve been recognized as opportunities to reduce or eliminate stormwater runoff at the source before it becomes a problem. LID practices typically use natural approaches to stormwater management. These include compost-amended soils, bioretention, permeable pavement, rainwater harvesting, green roofs, rain gardens, and spray irrigation. All of these approaches reduce stormwater runoff. But how much? Where does the rainfall go that does not become runof= f? And how much stormwater detention storage is still required?

Figure 5. LID Generator (WWHM3 Prototype)

The answers to these questions de= pend on the site’s climate, rainfall patterns, soils, and amount and densi= ty of development. The WWHM can be used to answer these questions because it tracks all of the precipitation’s destinations (stormwater, groundwat= er, soil storage, and evapotranspiration). By doing so all of the impacts to the environment can be documented.

WWHM2 explicitly includes the fol= lowing LID practices:

Roof runoff dispersion on adjacent pervious land
Roof runoff infiltration into adjacent soils =
Impervious-surface paving blocks

There are certain Department of Ecology–specified requirements for the use of any of these LID practi= ces. For example, to use the roof runoff dispersion LID practice, the user must document that the downspouts will be directed onto splashblocks and that a vegetated flow path of at least 50 feet will be maintained between the discharge point and any property line, structure, steep slope, stream, wetl= and, lake, or impervious surface. No erosion or flooding of downstream propertie= s is allowed.

WWHM2 can also be used to implici= tly model compost-amended soils and green roofs. In these two cases the user mu= st currently make changes in the WWHM HSPF parameter values to represent these= LID practices. At Aqua Terra we have used WWHM2 to analyze the effectiveness of green roofs in western Washington (Beyerlein et al. 2004). We found that th= ey typically retain 30% to 50% of the annual rainfall and reduce standard stormwater storage requirements by 20% to 30%, based on the Department of Ecology’s flow duration standard.

Interestingly enough, our modelin= g also found that if the soil is removed from the green roof (turning it into a &#= 8220;blue roof”), rainfall retention increases. Six inches of rooftop storage w= ill fully mitigate the roof’s impervious-area effects on stormwater runof= f, and no additional stormwater detention storage is required to meet the flow duration standard.

WWHM version 3 (WWHM3) will come = with an LID generator. The model will include an extensive selection of LID practices from which to choose to minimize stormwater runoff from new development. The user will be able to select a specific LID practice and, u= sing the LID generator, see how the stormwater runoff changes (Figure 5). By comparing the effectiveness of the different LID practices, the user can se= lect the one that generates the least amount of stormwater runoff and design the site and stormwater flow control facility accordingly. Or the user can sele= ct the lowest-cost LID practice and, using WWHM, compute the size of the stormwater facility needed to still meet the flow duration standard. By mak= ing these comparisons, the user can determine the least-cost solution while sti= ll meeting stormwater standards. The analysis is almost instantaneous.<= span style=3D'color:#666666'>

The specific LID practices to be included in WWHM3 and its LID generator are dependent on what the Departmen= t of Ecology and/or other agencies want to encourage developers to use and are willing to fund for addition to WWHM3. Our preliminary discussions with California Bay Area counties indicate that they will probably be interested= in different LID practices from those used in the Pacific Northwest. Their ver= sion of the software will be configured to meet their specific needs

Other Uses
The model’s flexibility allows it to be used to solve hydrologic problems far beyond its original intent. For example, by changing the flow duration criterion, the = user can compute flow durations to size low-flow fish passage culverts. <= span style=3D'color:#666666'>

For a project for the City of Eve= rett, WA, I used the model to produce monthly stormwater and base flow volumes for nine streams draining to Port Gardner Bay, an arm of Puget Sound. Taylor Associates of Seattle used this information together with representative pollutant concentrations based on land use for each flow type to compute the total pollutant loading to the bay.

Hal Mullis of Earth Tech in Belle= vue, WA, is using WWHM2 to evaluate alternative facility sites with varying port= ions of the study basin draining to each site. He notes, “The model works = well for quickly exporting the resulting flood frequency and flow duration data = for plotting in Excel. In this way I am able to easily view and compare results= for numerous alternatives. An added bonus is its utility in quickly building an HSPF model that can be later modified or expanded.”

The Washington State Department of Transportation is funding the addition of wetland hydroperiod analysis (analysis of wetland seasonal water-surface fluctuations) to WWHM3 to deter= mine the biological impacts of using wetlands to mitigate highway runoff. Other transportation agencies are looking at using the model to design instream structures.

References
Beyerlein, D.C. 1999. Why standard stormwater mitigation doesn’t work. pp. 477–479. Proceedings for the Ameri= can Water Resources Association Conference: Watershed Management to Protect Declining Species, Seattle.

Beyerlein, D.C., J.T. Brascher, and S.R. White. = 2004. Green roof hydrology. Everett and Tumwater, WA: Aqua Terra Consultants.=

Bicknell, B.R., J.C. Imhoff, J.L. Kittle Jr., T.H. Jobes, and A.S. Donigian Jr. 2001. Hydrological Simulation Program–Fortran, User’s Manual for Version 12. Mountain View, CA: Aqua Terra Consultants.

Booth, D.B. and C.R. Jackson. 199= 7. Urbanization of aquatic systems: Degradation thresholds, stormwater detecti= on, and the limits of mitigation. J Amer Water Res Assn.

Dinicola, R.S. 1990. Characteriza= tion and simulation of rainfall-runoff relations for headwater basins in western King and Snohomish Counties, Washington. Water-resources investigations report 89-4052. Tacoma, WA: US Geological Survey.

King County,= Washington. 1998. Surface Water Design Manual. King County Department of Natural Resources and Parks, Water and Land Resources Divisio= n. Seattle, WA.

Linsley, R.K., M.A. Kohler, and J= .L.H. Paulus. 1982. Hydrology for engineers. 3rd ed. McGraw-Hill Book Co. =

United States Water Resources Cou= ncil. 1981. Guidelines for determining flood frequency. Bulletin #17B of the Hydrology Committee. Washington, DC.

Washington State Department of Ec= ology. 2001a. Stormwater management manual for western Washington. Vol. 1: Minimum technical requirements and site planning. Publication no. 99-11. Olympia, W= A.

Washington State Department of Ec= ology. 2001b. Stormwater management manual for western Washington. Volume 3: Hydrologic analysis and flow control design/BMPs. Publication no. 99-13. Olympia, WA.

Washington State Department of Ec= ology. 2003. Western Washington Hydrology Model version 2.1. User’s manual. Publication no. 03-10-036. Olympia, WA.

Douglas Beyerlein, P.E., is a vice president with Aqua Terra Consultants in Everett, WA.