Modeling Stormwater Runoff at the Property Scale

Assignment 1 - SOIL 516 22 October 2015

MODELING STORMWATER RUNOFF AT THE PROPERTY SCALE

By: PJ Bell

INTRODUCTION

Despite the fact that it is arguably the world’s most precious resource, water is often neglected by the planners, developers, and architects who design our communities and cities. Urban sprawl has resulted in a massive amount of impervious surfaces, fundamentally changing the ways that water moves across the landscape (see Figure 1). Luckily, there are a number of best management practices that can help to improve these condi-tions.

This report uses various models and tools created by the District of North Vancouver, British Colum-bia in order to assess the impact of residential development on watersheds. Part One examines a particular site in North Vancouver, looking at changes in surface conditions between 1992 and 2009. Part Two then uses a modeling tool to ex-amine the effect of these surface conditions on water and assess the impact of various best man-agement practices that are meant to improve stream health.

PART ONE: HISTORICAL SITE COMPARISON

The site that I selected is located at 2458 Bad-ger Road, in the Deep Cove neighbourhood of the District of North Vancouver (see Figures 2, 3, and 4 for location and site details). Badger Road is bordered on the west and north by undisturbed forest, while Deep Cove is located about 300 me-tres to the east (see Figure 3).

The nearest major stream is Gallant Creek, which begins on the east side of Mount Seymour and drains into Deep Cove (The Pacific Streamkeep-ers Federation, 2009). Gallant Creek is relative-ly undisturbed in its upper reaches, but once it approaches the Deep Cove neighbourhood it is heavily impacted by urbanization (The Pacific Streamkeepers Federation, 2009). The creek has been armored on the bottom and sides for 100 metres upstream of Badger Road and it is cul-verted under Panorama and Gallant Roads (The Pacific Streamkeepers Federation, 2009).

Figure 1: Pervious vs. ImperviousSource: Partnership for Water Sustainability in British Columbia, 2015

Based on historical aerial photographs from the District of North Vancouver’s GEOweb applica-tion, the site was undisturbed in 1992 (see Figure 5). The site has an area of 653 square meters and it was estimated that in 1992, trees covered ap-proximately 75% of the site, while the remaining 25% was covered in mixed vegetation (i.e. bushes, native grasses, and other forms of undergrowth) (see Figure 7).

The GEOweb application’s measuring tool was un-available for use on historic aerial photographs, so these numbers are simply estimates. It was difficult to determine the exact ground cover due to the low resolution of the 1992 photograph, but I was able to use the better quality photographs from 1997 and 2003 as comparisons because the site was not developed until 2006.

In 2006, a detached single family home was con-structed at 2458 Badger Road, significantly alter-ing the site’s surface cover (see Figure 6). Table 1 compares the area and percentage of five dif-ferent surface coverings, including impervious surfaces (rooftop and concrete) and pervious surfaces (mixed vegetation, lawn, and trees). Us-ing GEOweb’s measuring tool, it was determined

Figures 2 and 3 (top): Context MapsSource: Google, 2015

Figure 4 (left): Site MapSource: District of North Vancouver, 2015

Figure 5 (left): 1992 Site and Figure 6 (right): 2009 SiteSource: District of North Vancouver, 2015

that in 2009, the site contained 55.8% impervious surfaces and only 44.2% pervious surfaces, as op-posed to in 1992 when 100% of the site was per-vious (see Figure 8) (District of North Vancouver, 2015). This represents a 55.8% increase in imper-vious surfaces from 1992 to 2009.

PART TWO: MODEL-BASED ANALYSIS

MODEL SETUP

The District of North Vancouver’s Water Balance Model Express (WBME) tool allows a user to exper-iment with different best management practices (BMPs) that change the movement of water over their site, providing an excellent educational op-portunity for homeowners. It works by extrapo-lating site-specific data to the watershed scale and assessing the overall “stream health score” of the area based on the surface types and site interventions. Unfortunately, the site still seems to be under development and there are a number of issues that prevent the tool from functioning properly, which will be discussed at the end of this section.

In order to create a project in the WBME appli-cation, users are required to select the Hastings Creek Watershed as the project base. My site (2458 Badger Road) is located outside of this wa-tershed, so some of the underlying factors such as retention volume, infiltration area, and base flow release rate may differ (Page et al., 2013).

This is simply a limitation of the WBME applica-tion.

The next step was to select the soil type for the given site. Based on generalized soil maps and types for the surrounding area, I assumed that the site would contain Podzolic soil, which is dominant in areas with sandy deposits that re-ceive over 700mm of rain per year (University of Saskatchewan, n.d.; Land & Food Systems, UBC, 2004). Podzolic soils have a texture that is coars-er than clay and contain sand, but they are not simply made of sand (Agriculture and Agri-Food Canada, 2013). For these reasons, I chose “Sandy Loam” for the site’s soil type. I selected a soil depth of 100mm because the Canadian System of Soil Classification guide contained a diagram indicating a depth of 100mm, and the area sur-rounding Gallant Creek (which is in close proxim-ity to Badger Road) has shallow soil depths (Agri-

Table 1: Surface Cover in 1992 and 2009Source: District of North Vancouver, 2015

Figure 7 (top): 1992 Surface CoverFigure 8 (bottom): 2009 Surface CoverSource: District of North Vancouver, 2015

culture and Agri-Food Canada, 2013; Page et al., 2013).

SITE CHARACTERISTICS & SCORE: 1992

Due to limitations with the WBME application, it was impossible to properly assess the stream health score of my site in 1992. The model is built to assess areas that have been developed rather than undisturbed sites, and as a result, it requires that the site contain some form of im-pervious hardscape such as pavement or rooftop space. While installing one square metre of con-crete—which I did just to let the model function—should not greatly influence stream health, there is a more important issue. In order to simplify the model, there is no option for undisturbed, native vegetation; landscaping “such as a lawn or flower garden” is the only option for vegetation (Part-nership for Water Sustainability in British Colum-bia, 2015).

Basic landscaping can have a very different im-pact on stream health than native, undisturbed forest vegetation. Studies have shown that de-veloped turf sites—our typical lawns—are often nearly as impervious as roads (Otto et al., 2002). This is because landscaping often results in the grading of the site, the removal of topsoil, ero-sion, and soil compaction from heavy machinery and the filling of depressions (Otto et al., 2002). The end result is a relatively hard surface with low absorptive capacity, which does little to pre-vent runoff. As a result, the initial stream health score for my 1992 site was 0.04, as shown in Fig-

ure 9.

In reality, however, a natural, undisturbed site should result in a “green” stream health score between 0.75 and 1. Under natural conditions, a portion of rainfall will not reach the ground due to interception and evaporation by the canopy and litter (see Figure 10) (The Federal Interagen-cy Stream Restoration Working Group, 2001). Much of the rain that does make it to the ground will infiltrate the soil, with only a small amount of runoff occurring (Federal Interagency Stream

Figure 9: 1992 Steam Health Score Source: District of North Vancouver, 2015

Figure 10: Forest Rainfall PathwaysSource: The Federal Interagency Stream Restoration Working Group, 2001

Restoration Working Group, 2001). When water is absorbed by the soil, it is naturally filtered and purified before reaching groundwater supplies, which helps to maintain overall stream health (Frankenberger, 2000). Water that is absorbed is also stored and moves much more slowly towards an outlet point, which lowers the risk of flooding when storms and large snow melts occur as well as the erosion that occurs due to flooding (Arnold & Gibbons, 1996).

SITE CHARACTERISTICS & SCORE: 2009

In 2009, elements such as rooftops, pavement, and a lawn were added to the WBME calculations for the Badger Road site, as indicated in Table 1. It is worth noting that the model simplifies these elements by categorizing them as either imper-vious, neutral (landscaping), or absorptive (cis-tern). As mentioned above, landscaping can have a far different effect than natural forest, so if there were any natural vegetation on the site, it would behave differently than a lawn. Addition-ally, rooftops and pavement do not necessarily have the same impact on the hydrological cycle, especially at the greater watershed scale.

The important aspect is to determining how much of the rainfall on a site actually makes it into a stream or stormwater collection system (Arnold and Gibbons, 1996). Generally, rooftops are less effective at producing runoff than road-ways (Arnold and Gibbons, 1996). This is due to the fact that in a suburban residential area, rooftops typically drain onto lawns or other per-meable forms of landscaping, whereas roadways typically channel runoff directly into stormwater systems (Arnold and Gibbons, 1996). A study by the City of Olympia, Washington in 1994 estimat-ed that low-density residential areas are approx-imately 40% effective at producing runoff, while dense commercial and industrial areas are clos-er to 100% effective (Arnold and Gibbons, 1996). For the sake of this exercise, however, it is much more simple to group these together as impervi-ous surfaces.

After inputting the correct surface features, as well as soil type and depth, it was determined that in 2009, 2458 Badger Road had an abysmal stream health score of 0.02 (Partnership for Wa-ter Sustainability in British Columbia, 2015). This is significantly worse than the “green” score (0.75 to 1) achieved by the site in 1992, and it is due to several factors. First, the sandy loam soil type on the property is not very efficient at retaining moisture, as demonstrated in Figure 11 (Partner-ship for Water Sustainability in British Columbia, 2015). This creates increased runoff, more non-point source pollution, and reduced groundwater quantities (Otto et al., 2002).

Second, and most importantly, is the large in-crease in impervious surfaces. While impervious surfaces themselves do not generate pollution, they create a number of issues relating to the hydrological cycle (Arnold and Gibbons, 1996). As an area becomes increasingly urbanized, the pro-portion of impervious surface increases, altering the movement of water (The Federal Interagen-cy Stream Restoration Working Group, 2001). As Figure 12 demonstrates, the greater the propor-tion of impervious surfaces on a site, the more runoff—and subsequently less infiltration—there is (The Federal Interagency Stream Restoration Working Group, 2001).

Figure 11: Soil MoistureSource: Partnership for Water Sustainability in British Columbia, 2015

As means of a brief summary, Schueler (1994) has identified the following problems caused by im-pervious surfaces:• Increased runoff• Stream warming (impervious surfaces absorb

heat and warm up the runoff, which is com-pounded by the loss of tree cover)

• Stream bank erosion and habitat destruction due to more severe and frequent flooding

• Declining water quality due to nonpoint source pollution

• Lower stream biodiversity

Many researchers over the years have echoed and added to Schueler’s insights, with research from the past three decades showing a strong correla-tion between the amount of impervious surfaces in a drainage basin and the health of its receiving stream (Arnold and Gibbons, 1996; Frankenberg-er, 2000). According to Frankenberger (2000), the key factor in determining the negative impact of development on a watershed is the number and

extent of impervious areas that are directly con-nected to the stormwater system via sewers and other piping systems, because this correlates to the amount of pollution and flooding caused by runoff.

IMPLICATIONS OF DEVELOPMENT AT THEWATERSHED SCALE

These water management issues have import-ant implications not only at the site level, but at the greater community and watershed scales as well. A common development pattern in North America is that of urban sprawl, which is defined by low-density, automobile-dependent suburbs where places such as home, work, and school are spaced relatively far apart and separated by wide roads, parking lots, and buildings (Otto et al., 2002). Sprawl also typically means the loss of forests, meadows, wetlands, and small streams—natural systems that trap and absorb water. This type of development massively increases the amount of impervious surfaces in a community,

Figure 12: Relationship Between Impervious Cover and Surface RunoffSource: The Federal Interagency Stream Restoration Working Group, 2001

fundamentally altering the movement, cleanli-ness, and availability of water in an urban water-shed (Otto et al., 2002).

The availability of water is not only a human problem, as it affects all other flora and fauna in a watershed as well. From the human perspec-tive, though, Otto et al. (2002, p. 5) explain that “[w]hen we sprawl, we threaten our freshwater resources at the very time our demand for them is increasing.” When water cannot infiltrate the soil, groundwater reserves are not recharged. This can lead to emptying water reserves and, eventually, having to find alternative sources of water, which can further damage natural areas.

The health of a watershed depends directly upon the proportion of impervious surfaces it contains. Schueler (1994, p. 8) emphasizes this in the fol-lowing passage: “[t]he many independent lines of research reviewed here converge toward a com-mon conclusion: that it is extremely difficult to maintain predevelopment stream quality when watershed development exceeds 10 to 15% im-pervious cover.” Multiple studies suggest that aquatic biological systems can begin to degrade when impervious levels reach 10%, with partic-ularly sensitive systems seeing negative impacts at even lower percentages (Dorworth & McCor-mick, 1998). When the proportion of impervious surfaces reach 25% or higher, a watershed will have “severely impacted streams” (Dorworth & McCormick, 1998, p. 2). Figure 13 displays this threshold graphically.

SITE MANIPULATION USING BEST MANAGEMENT PRACTICES

I attempted two different combinations of BMPs on the Badger Road site from 2009. In Option A, I began by changing all of the soil in the site’s landscaped area from sandy loam to silt, and I made it 450 mm deep instead of 100 mm (see Fig-ure 14). Silt was the best soil listed for retaining water, and deeper soil has a greater absorptive capacity (Partnership for Water Sustainability in British Columbia, 2015). Next, I replaced all of the standard pavement on my site with porous pavement, which allows water to percolate into the base material (see Figure 15) (Partnership for Water Sustainability in British Columbia, 2015). The base material of this pavement needed a depth of at least 350 mm in order to move the device gauge into the green zone (the optimal position). Finally, I removed one square meter of landscaping and installed a small cistern with storage capacity, which was connected to my building (see Figure 16). This device is meant to absorb the water falling on hardscapes and re-lease it in a controlled manner (Partnership for Water Sustainability in British Columbia, 2015).

These interventions resulted in a stream health score of 0.5. They would not be very feasible, however, because they require all soil to be re-moved and exchanged for better, deeper soil, as well as all pavement to be replaced with porous pavement. These best management practices would be very expensive, although if built on an undeveloped property, the process would be much simpler and likely quite effective.

For Option B, I attempted to keep the scenar-io more realistic by maintaining the original soil type, soil depth, and pavement. I reduced the amount of landscaping on the site from 43% to 21% in order to make room for BMP devices. I in-stalled a small rain garden with storage and con-nected it to my building in order to catch and store runoff (see Figure 17). I maxed out the depth of the rain garden and base material in or-der to improve the device’s absorptive capacity. I then installed a small infiltration swale with stor-age and connected it to the pavement on my site Figure 13: Imperviousness vs. Degradation

Source: Dorworth & McCormick, 1998

Figure 14 (top): Option A - Landscape PropertiesFigure 15 (middle): Option A - Porous PavementFigure 16 (bottom): Option A - CisternSource: Partnership for Water Sustainability in British Columbia, 2015

(see Figure 18). Next, I added an infiltration without storage that took up 20.5% of my site, followed by a cistern with storage that took up 18% of the site (See Figures 19 and 20). By add-ing all four of these BMPs, I was only able to attain a stream health score of 0.48.

PROBLEMS WITH WATER BALANCE MODEL EXPRESS TOOL

As mentioned at the beginning of this section, there are a number of problems with the WBME tool. Multiple classmates reported experienc-ing many, if not all, of these same issues. The issues are listed below:• Stream health is given as a score out of one,

but regardless of whichever intervention is attempted—including removing virtually all impervious surfaces from a site—the score never goes higher than 0.5.

• If a BMP device that is connected to a hard-scape is made larger than 1% of the lot, the infiltration area gauge shows that the area is much too large. However, you need to make the device larger than 1% in order for the device’s volume gauge to move into the green space. I was unable to get both gaug-es into the green area simultaneously.

• Orifice Size is listed as a title on the left side underneath the stream health gauge, but there is no information that pops up. It seems to be hidden under the page, but scrolling down does not reveal it.

• Often, the informational pop-ups that pro-vide details about a device will freeze on the screen, getting in the way of function-ality and requiring a page refresh.

• You can only connect one device at a time to a given hardscape, even if there is still excess water to be absorbed. All other BMP devices need to be left unattached to any-thing, which seems to defeat their purpose.

• If you add a device that exceeds the size of your lot, there are excessive pop ups that warn you every time you adjust the size of the device, until you are back under the al-lotted amount.

BETTER PLANNING FOR THE FUTURE

As Frankenberger (2000, p. 9) explains, “[t]here is no doubt that it is easier to plan for good stormwa-ter management before development takes place rather than retrofitting existing development to reduce stormwater impacts.” Retrofits are often complicated, expensive, and less effective than applying sound planning principles in the first place. Due to the limitations of the WBME tool, it was difficult to determine how effective these BMPs were at improving stream health, although perhaps it is true that even after adding every BMP tool, streams will still only be about half as healthy as they were prior to development.

Figure 17 (top left): Option B - Rain GardenFigure 18 (bottom left): Option B - Swale with StorageFigure 19 (top right): Option B - SwaleFigure 20 (bottom right): Option B - CisternSource: Partnership for Water Sustainability in British Columbia, 2015

Moving forward, planners, developers, and archi-tects need to commit to the protection of the natural environment, taking advantage of ecosys-tem services rather than destroying them. Too of-ten, humans attempt to replace natural systems like wetlands and forests with human-made sys-tems like stormwater systems. Our stormwater systems treat precipitation as a waste product, when in actuality, it should be treated like the precious resource that it is (Otto et al., 2002). Our attempts at engineering the landscape often fail, revealing the superiority of the natural sys-tem that existed in the first place. We need to utilize these natural systems in innovative ways and attempt to mimic them in our designs, such as when we build constructed wetlands and arti-ficial swales (Dorworth & McCormick, 1998).

There are a number of ways that communities and cities can be designed to protect water. En-tire books have been written on this topic, so it is impossible to adequately describe all of these

methods in this report. However, there are a few key ideas that are worth mentioning, such as the application of smart growth principles and nat-ural resource-based planning. Otto et al. (2002) recommend protecting open space (especially critical and sensitive habitat), managing growth by investing in existing communities (in order to limit sprawl), and encouraging smart growth de-velopment such as cluster zoning, as explained in Figure 21. Arnold and Gibbons (1996) suggest that reducing road widths is one of the best ways to re-duce imperviousness. Frankenberger (2000) em-phasizes the need to reduce surface pollutants, create buffer areas around streams, and, crucial-ly, to plan at the watershed scale rather than the ecologically arbitrary municipal level. These are just a handful of the many strategies that exist for protecting water resources moving forward. Hope-fully, communities across Canada will put more emphasis on these strategies moving forward.

Figure 21: Clustering to Reduce Overall Site ImperviousnessSource: Arnold and Gibbons, 1996

WORKS CITED:

Agriculture and Agri-Food Canada (2013). Canadian System of Soil Classification, 3rd Edition: Chap-ter 10: Podzolic Order. Retrieved from http://sis.agr.gc.ca/cansis/taxa/cssc3/chpt10.html

Arnold, C. L., & Gibbons, C. J. (1996). Impervious Surface Coverage: The Emergence of a Key Envi-ronmental Indicator. Journal of the American Planning Association, 62(2), 243-258. http://dx.doi.org/10.1080/01944369608975688

District of North Vancouver (2015). GEOweb. Retrieved from http://geoweb.dnv.org/index.html

Dorworth, L., & McCormick, R. (1998). Impacts of Development on Waterways. Retrieved from https://www.extension.purdue.edu/extmedia/ID/ID-257-W.pdf

Frankenberger, J. (2000). Land Use and Water Quality. Retrieved from http://www.researchgate.net/publication/228026470_Land_Use_and_Water_Quality

Google (2015). Google Maps Application. Retrieved from https://www.google.ca/maps

Land & Food Systems, UBC (2004). Soil Classification: Canadian Soil Orders. Retrieved from http://soilweb.landfood.ubc.ca/classification/

Otto, B., et al. (2002). Paving Our Way to Water Shortages: How Sprawl Aggravates the Effects of Drought. Retrieved from http://www.smartgrowthamerica.org/documents/DroughtSprawlRe-port09.pdf

Page, N., Lilley, P., Matsubara, D., & Tokgoz, N. (2013). Hastings Creek Watershed: Ecology and Hydrotechnical Assessment. Retrieved from https://www.dnv.org/sites/default/files/edocs/hastings-creek-watershed-assessment.pdf

Partnership for Water Sustainability in British Columbia (2015). Water Balance Model Express. Re-trieved from http://dnv.waterbalance-express.ca/

Schueler, T.R. (1994). The Importance of Imperviousness. Watershed Protection Techniques 1(3), 100-111. Retrieved from http://scc.wa.gov/wp-content/uploads/2015/06/The-Importance-of-Im-perviousness_Schueler_2000.pdf

The Federal Interagency Stream Restoration Working Group (2001). Stream Corridor Restoration: Principles, Processes, and Practices. Retrieved from http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044574.pdf

The Pacific Streamkeepers Federation (2009). Gallant Creek Watershed Profile. Retrieved from http://www.pskf.ca/ecology/watershed/northvan/gallant02.htm

University of Saskatchewan (n.d.). Soils of Canada: Orders: Podzolic. Retrieved from http://www.soilsofcanada.ca/orders/podzolic/

http://soilweb.landfood.ubc.ca/classification/

http://soilweb.landfood.ubc.ca/classification/

https://www.dnv.org/sites/default/files/edocs/hastings-creek-watershed-assessment.pdf

https://www.dnv.org/sites/default/files/edocs/hastings-creek-watershed-assessment.pdf

http://dnv.waterbalance-express.ca/

Modeling Stormwater Runoff at the Property Scale

Documents

Transcript of Modeling Stormwater Runoff at the Property Scale