Controlling stormwater impacts
Along with high growth rates in the outer reaches of metropolitan areas, there is a growing housing market as high-income families move back to inner-ring suburbs and the central cities. To meet the housing needs of these groups, older structures, often located on small parcels (less than 10,000 square feet), are demolished and replaced by much larger houses that typically occupy the vast majority of the parcel area. Houses in inner-ring suburbs are prime for these activities because of the lack of undeveloped land in those established communities. Access to mass transit, such as rail transport, makes these areas ideal for redevelopment, as urban highway systems have become more congested and commuters prefer alternatives to auto transport.
This practice of demolishing and replacing structures is known as teardowns, knockdowns, bash and build, residential redevelopment,rebuilds, infill development, or scrape-offs, depending on the location. The term used by municipal officials and residents to describe the practice provides a clear indication of how well the process proceeds in a community. The term teardowns has a negative connotation, focusing on the destructive nature of the process. The term rebuilds, on the other hand, is a positive, optimistic expression.
Proponents of “bash and build” claim that property values increase because of the practice, the community benefits from new and better housing, and only a small percentage of the housing stock is affected.
Opponents claim that the practice changes the character of the community, homes with architectural and historic value are lost, and the larger new homes result in flooding, loss of open space, and removal of mature trees.
Across the country, communities are dealing with ever-increasing numbers of site developments involving teardowns. The statistics are eye-opening:
- Between 2000 and 2004, Dallas processed over 1,400 permits for demolition and rebuilding.
- Chicago suburbs have experienced rebuild permits ranging up to 9% of the building permits issued.
- Urbana, home of the University of Illinois, tallied 12.9% of its 2006 permits as rebuilds.
- Westport, CT’s Web site has an interactive page showing the latest teardown sites, with information regarding sale price and pending permits.
Throughout these and other metropolitan areas, communities are faced with many issues related to this practice, including property rights, resident concerns, increased tax revenues, affordable housing, neighborhood friction, and the inconvenience associated with new construction in established residential neighborhoods. Municipalities have dealt with these issues through revisions to building and zoning ordinances, demolition permitting, building height and width restrictions, and increased permit fees.
However, there are site-specific impacts of rebuild activity that are difficult to address with regulation. Stormwater management is a prime example. Issues such as aging and undersized sewers, topography changes, increased impervious area, and surface-water flow redirection can make this type of redevelopment even more contentious.
Redevelopment of individual parcels generally involves regrading the site and increasing the imperviousness of the parcel. These changes impact stormwater runoff rates, velocities, and flow patterns.
|Figure 1. Typical 8,200-square-foot lots
|Figure 2. Redeveloped 8,200-square-foot lots
Generally, the houses being demolished for rebuilds are small by today’s subdivision standards and are located on lots sized appropriately for the footprint. An example is shown in Figure 1.
On the redeveloped lot, the footprint of the house increases dramatically. In addition, accessory structures such as garages, sheds, patios, decks, and driveway extensions are constructed. Often, drainageways along side and rear lot lines are eliminated, and the public stormwater infrastructure is often undersized for existing and future stormwater flows. In some cases, there are no storm sewers to connect to. Figure 2 shows the area covered by structures and pavement on a redeveloped 8,200-square-foot lot.
In addition to changes in the plan view, changes in grade are typical. The new houses are generally built with basements. To minimize excavation costs, the basements are not completely buried, and the grades are changed to meet the new first floor. Figure 3 illustrates this concept.
The Village of Downers Grove, IL (pop. 50,000), is an established western suburban Chicago community experiencing significant redevelopment. Since 2003, the village has recorded the existing and proposed impervious area for residential additions and rebuilds.
These data were analyzed to look for trends in residential rebuild activity in the village. From 2003 through 2006, permit activity remained steady, averaging annually about 124 new single-family home permit applications. Table 1 illustrates the trends in residential building permits and shows that the average percentage increase in impervious area diminished over the four years. This is due, in part, to the early redevelopment of parcels with very small homes (easier to have a big impact on impervious-surface percentage). More recent redevelopment involves lots with larger existing homes (and higher existing impervious area), less open space, and more space limitations for development.
As shown in Figure 4, the physical changes resulting from rebuilds can include larger impervious area, higher finished grade, steeper slopes to meet existing grade at lot lines, and regrading (or elimination) of side- and rear-yard swales. Impacts on stormwater can include:
- A reduction in the time of concentration of runoff
- An increase in the volume of runoff from the site
- An increase in flow velocities (across paved surfaces instead of grassed areas)
- Changes to or elimination of overland flow paths
- Changes to or elimination of drainageways along side and rear lot lines
- New discharges from sump pumps, often to poorly graded surface drainageways
To quantify the aggregate impact of the increased imperviousness of residential rebuilds on stormwater runoff, a study was performed for the Village of Downers Grove. A 338-acre typical suburban watershed with primarily residential land use was analyzed to compare existing flow rates in a receiving stream compared to the flow rates when the average impervious area on a residential lot increases from 10% to 50%. This watershed was selected because it is nearly 100% developed, is predominantly residential, contains a mixture of lot sizes, is drained by storm sewers and open channels, and has no additional upstream flows impacting the hydrology or hydraulics of the study area.
To perform the analysis, parcels were categorized by land use and size; the average imperviousness for each category, the total existing imperviousness, and the increase in imperviousness by various percentages for each category were determined; existing peak flow rates were calculated using the hydrological model HEC-HMS; and the increase in peak flows for discreet changes in impervious cover were determined.
To estimate the existing percent imperviousness in the study watershed, parcel data and aerial photography were used. Parcels were divided into seven categories, roughly based on zoning requirements. Data about these categories, including number of lots and impervious and pervious areas, are shown in Figure 5.
Five scenarios were modeled to estimate the potential impact of an increase of average percent imperviousness in the basin. To estimate the future percent imperviousness, the average impervious area per lot was increased by 10% through 50% for the four categories of residential parcels (only those parcels exceeding 7,500 square feet). It was assumed that nonresidential parcels would not change. Table 2 summarizes the percent impervious for each of the five scenarios.
To estimate the existing runoff from the study watershed, the HEC-HMS model was used. The watershed was divided into nine sub-basins, based on contour information and storm sewer atlases. Time of concentration was estimated using the TR-55 method. The SCS Unit Hydrograph/Curve Number method was used to estimate flows, with an average percent impervious of 33.1%. A curve number of 69 was used for all pervious areas. A curve number of 98 was used for impervious areas, thus calculating a weighted curve number for each sub-basin. The 10-year recurrence interval storm was used to make comparisons between existing and future peak flows. The HEC-HMS model was run for the one-, two-, three-, six-, 12-, 18-, and 24-hour-duration storms to determine the impacts of different types of storms on peak runoff.
To estimate the proposed runoff from each of the five scenarios (increased residential impervious area of 10% to 50%), the HEC-HMS model was run with the same parameters except percent impervious and time of concentration. For each scenario, the weighted curve number was changed to reflect the increase in percent impervious and times of concentration were reduced slightly to account for an anticipated lower sheet flow component.
As shown in Table 2, the results demonstrate that, on average, for each 10% increase in impervious area of residential lots in the basin, the average percent impervious throughout the basin is predicted to increase by 6.7% and stormwater runoff (peak flows) will increase by 7.0%.
Using the results of the analysis, the impact of residential redevelopment on peak flows can be anticipated. Recent parcel redevelopment has resulted in an average increase in impervious area of 54%. Assuming that 50% of the lots redevelop with an average increase of 50%, the average basin percentage impervious area would be 38%, which translates into an increase in peak flow of about 16%. Another scenario could be that 20% of the lots are redeveloped with a 50% increase in impervious area, resulting in an increase in peak flows of about 6.3%. Various scenarios are shown in Table 2. Figure 6 shows the differences in the hydrographs when developing 50% of the lots with an average increase of 100% impervious area.
Dealing With the Impacts of Rebuilds on Stormwater
Based on the results of the analysis, the Village of Downers Grove is including an evaluation of the potential increased impervious area resulting from future rebuilds in the comprehensive watershed planning efforts currently under way. It is anticipated that recommendations for stormwater improvements will address this issue on a watershed basis.
It is obvious that the process of teardown and rebuild can negatively impact a municipality’s stormwater system, especially individual residents who are directly impacted. To gain an understanding of the stormwater issues and how various municipalities are dealing with them, information was gathered from a select group of engineering staff from municipalities impacted by the teardown phenomena.
Responses to the following questions were requested from various municipal engineers, planners, and public works directors:
- What terminology is used to describe the process involving purchase and demolition of a house followed by construction of a larger new house in the same location?
- In your community, what percentage of issued building permits is for teardown/rebuild activities?
- What mechanisms do you use to control teardown/rebuild activities? Do these mechanisms control stormwater on a site-by-site basis?
- What are the major stormwater issues you experience on teardown/rebuild sites?
- What tools and/or policies would be useful for your community in managing stormwater on teardown/rebuild sites?
Municipal staff responsible for permit reviews use a variety of methods to attempt to minimize the adverse stormwater impacts of rebuilds. These methods include (paraphrased from survey responses):
- Municipal staff verifies that drainage patterns are improved or maintained.
- Downspouts and sump pumps are connected directly to the storm sewer to minimize surface runoff.
- Grading, drainage, and tree protection are approved prior to building permit.
- Individual site grading plans and runoff calculations are reviewed.
- Impact fees are assessed to replace and/or install storm sewers and storage facilities.
- Controls are placed on amount of fill or height of new foundation (relative to adjacent existing structures).
- The site plan is reviewed with accurate village-wide topography, storm sewer mapping, and flooding/drainage complaint records.
- Dry wells are required with volume and infiltration capacity to accept increased runoff.
The results of the survey provide insight into the problem and outline the creative approaches taken by municipal staff to minimize the stormwater impacts of teardowns.
The HEC-HMS model provides a rough estimate of what the impact of residential development will be; it is possible that the actual impact will be more than estimated. Residential development, whether in the form of additions or teardowns, impacts the land in ways other than from increased impervious cover:
- Changes in topography can alter existing drainage paths, causing increased flows onto neighboring properties.
- Times of concentration can be reduced due to a shorter flow path over pervious surfaces or from yard drains being directly connected to the storm sewer. Modeling the reduced times of concentration in the HEC-HMS analysis would have yielded greater increases in peak flows resulting from teardown/rebuild activities.
- Deeper basements constructed with imported granular material will provide groundwater with quick access to the stormwater conveyance system through sump pump connections.
The key to limiting the higher rates and volumes of stormwater runoff is to take a proactive stance to limit the impact of redevelopment on runoff characteristics. This is most effectively accomplished through a zoning ordinance, a stormwater ordinance, and active enforcement of the two. Ordinance language can include requirements such as the following:
- Maximum percent impervious on lots
- Strict setback requirements
- Maximum yard slopes
- Restrictions on altering topography
- Site plan submittal requirements, including a stormwater management plan, prepared and sealed by a registered professional engineer
- Maintenance of adequate overflow routes
It is also imperative that a thorough plan review be completed and site inspection be performed to ensure that the construction meets the new requirements. Special permit fees can be assessed that specifically address teardown/rebuild activity. These new fees may be adjusted by municipalities to accomplish the following goals:
- Pay for increased staff time (review and site inspection) to administer the teardown/rebuild program.
- In cases where teardown/rebuild activity is difficult to control, and if the municipality wants to preserve the character of the community, the fees could be used to slow the rate of redevelopment, creating an economic disincentive to do so.
- Pay for the maintenance and/or replacement of existing stormwater infrastructure that is unable to manage stormwater runoff from redeveloping areas.
- It is tempting for municipalities to use the teardown/rebuild phenomena as a way to generate higher tax revenues, as the reconstructed homes will trigger a higher property valuation. However, these changes place a significant burden on the stormwater system, often creating new drainage problems that affect older properties in the vicinity of the redeveloping areas. Furthermore, as teardown/rebuild activity takes over a community, the socioeconomic composition of a community is forever changed, permanently changing the character of the community.
- Given that most communities welcome the practice of redevelopment as an environmentally friendly alternative to urban sprawl, the practice will likely thrive throughout the country. At the municipal level, it will be important to ensure the following:
- Develop and enforce specific requirements to protect existing drainage infrastructure (zoning and engineering requirements).
- Work with contractors, architects, and their engineers to develop best management practices for single-family residential redevelopment, striving to minimize the impact of redevelopment on stormwater runoff (including incentives for low-impact development practices).
- Impose appropriate development fees (i.e., permit fees, impact fees) to address the impact of redevelopment on the municipality’s infrastructure and to fund new enforcement programs.
- Develop subwatershed plans to predict the impact of redevelopment (per local standards) on peak flow rates and develop a capital improvement plan to replace key stormwater components to convey (and/or detain) the increased peak flow rates and volumes.
Author's Bio: Michelle Lewis, P.E., is with Clark Dietz Inc. in Chicago, IL.
Author's Bio: David Barber, P.E., is director of Public Works for the Village of Downers Grove, IL
Author's Bio: Melcy Curth Pond, P.E., is with Clark Dietz Inc. in Chicago, IL.
Author's Bio: Greg Kacvinsky, P.E., is with Clark Dietz Inc., in Champaign, IL.