Intuitively, it seems that a high-rise building will generate less rainwater runoff than a single-story building of the same floor area. This is because the high-rise will have less roof area (impervious area), resulting in less runoff. Less intuitive, however, is the fact that a higher-density residential development also will generate less runoff than a lower-density residential development with the same number of residences. Although the higher-density development will have a greater percentage of impervious area per acre of development, the total impervious area per residence actually will be less. Thus, each residence will generate less runoff, thereby having less of an impact on the community’s water resources.
This fact has been documented by the EPA in a report entitled “Protecting Water Resources with Higher-Density Development” (EPA publication 231-R-06-001). Consistent with the City of Grand Rapids’ desire to promote low-impact development (LID) and smart growth initiatives, city staff expanded upon the EPA report by evaluating the water resource impacts for higher densities and different types of development, with the intent of determining the appropriate density thresholds to define high-density development and establishing a standard evaluation method for granting a waiver or reduction in the city’s stormwater regulations for high-density developments.
Evaluation of Development Types and Densities
As in the EPA report, the city’s runoff evaluation is based on the runoff caused by impervious areas such as roofs, roads, driveways, and sidewalks per unit of development, rather than simply the percent of impervious area. Therefore, a residential development is evaluated on the average impervious area per residence, a parking facility development on the average impervious area (exposed impervious surface area) per parking space, and an office or commercial development on the average impervious area (roof area) per gross floor area. The impervious area for a higher-density development is compared with the impervious area of a predefined, low-density development with an equal number of development units (residences, parking spaces, or gross floor area). The runoff reduction is estimated by subtracting from one the ratio of the site’s actual impervious area (AIsite) divided by the impervious area (AiLD) of a low-density development having the same number of units, and converted to a percentage, as shown in Equation 1: Percent Runoff Reduction = (1 – Aisite / AiLD) x 100%.
Impact-Reduction Goal
A previous evaluation of vegetated roof systems conducted by Grand Rapids city staff had determined that vegetated roof systems provided more than an 80% reduction in runoff when compared with a standard roof. The 80% runoff reduction justified granting a waiver of the stormwater management requirements, because the impact of a vegetated roof system closely resembled that of natural pervious areas. Therefore, it seems reasonable to use the same 80% runoff reduction as the threshold for the granting of a waiver for high-density developments.
Residential Development
Figure 1 shows the runoff reductions from the EPA report and from Grand Rapids’ extended analysis. The resulting curves are very similar for the densities evaluated in both of the analyses, the only difference being that Grand Rapids’ analysis was extended to include much higher densities. The curves show a rapid rise in the percent of runoff reduction between the densities of one residence per acre and five residences per acre, followed by smaller runoff reductions indicated by a flattening of the curve as densities increase beyond five residences per acre. Although there were only three different densities evaluated in the EPA report, the same rapid rise and flattening of the curve is evident.
The EPA report effectively demonstrates that higher-density developments will result in less impact on the overall watershed. However, the density of one residence per acre for the base low-density residential development does not seem appropriate when applied to urbanized communities for consideration of waiving the stormwater management requirements. Generally, urbanized communities have very few residential areas with densities less than five residences per acre. And, given the dramatic difference in runoff impact (150%) for developments of one residence per acre versus five residences per acre, the definition of a low-density residential development is five residences per acre for this analysis, rather than one residence per acre as defined in the EPA report.
Typically, a residential development with five residences per acre will have an average of 4,700 square feet of impervious area per residence. The impervious area includes roof, patio, sidewalk, and driveway. Figure 2 shows the relative percent of runoff reduction for higher densities compared with an equivalent low-density residential development of five residences per acre with the same number of residences. The 80% runoff reduction for residential developments is obtained at a density of 38 residences per acre.
Parking Facility Development
The configuration for low-density parking facilities was defined as a 1-acre surface parking lot with a 10-foot vegetation buffer on three sides, two parking spaces on each side of a single 20-foot-wide drive aisle, and a driveway at each end. The parking spaces were assumed to be 9 feet wide by 20 feet long. The low-density parking facility has an optimum density of 134 parking spaces per acre with a total impervious area of 36,850 square feet, resulting in a unit density of 275 square feet of impervious area per parking space. The higher-density parking facilities were defined as being a 1-acre multi-level parking facility. A rule of thumb, which was provided by Mark DeClercq of Walker Parking Consultants, for estimating parking deck area needed to accommodate a given number of parking spaces is 350 square feet of deck area per parking space. Assuming a parcel size of 1 acre, the parking spaces per level were calculated by taking 43,560 square feet per acre and dividing by 350 square feet per parking space, which yields 124 spaces per parking level. Because only the upper level of a multi-level parking facility receives rainfall, the runoff will remain constant regardless of the number of parking levels.
Therefore, while the number of parking spaces increases with each additional parking level, there is less runoff per parking space. Figure 3 shows the relative percentage of runoff reduction compared with a surface parking lot with the same number of parking spaces. The 80% runoff reduction occurs when there are six levels of parking, which translates into a density of 744 parking spaces per acre of land area used by the facility.
Commercial and Office Development
The configuration for the low-density commercial/office development was defined as a single-story commercial or office building. The runoff was estimated for the building area being 100% impervious. Because a building’s density is related to the floor space, the ratio of floor space area divided by the building roof area was used as the indicator for density. The 80% threshold is reached by buildings with a gross floor area five times greater than the building roof area or, in other words, by buildings that are at least five stories in height. Figure 4 shows the relative percentage of runoff reduction compared to a single-story building of equal floor space.
Mixed-Use Development
Most high-density developments involve a mix of uses. Such developments typically have retail use at street level, parking decks in the back and beneath the development, office spaces on the second and possibly third levels, and residential use on the remaining upper floors. Therefore, an empirical method is needed for evaluating the runoff impact reduction for such mixed-use developments.
The evaluation is based on the average impervious area per unit of the predefined low-density development unit, which is shown in Table 1.
The equivalent low-density impervious area (AiLD) for each use type is estimated by taking the number of proposed development units (i.e., residences, parking spaces, or gross floor area) and multiplying by the “typical impervious area” for each development unit. The equivalent low-density impervious area (AiLD) for a residential development is calculated by multiplying the number of proposed residences by 4,700 square feet per residence. The AiLD for a parking facility is calculated by multiplying the number of proposed parking spaces by 275 square feet per parking space. The AiLD for all other uses such as office, retail, and hotel is simply the proposed gross floor area. The development’s total equivalent low-density impervious area (AiLD) for the mixed uses is the sum of the AiLD for each of the different uses. The percent runoff reduction is estimated using Equation 1, incorporating the development’s actual site impervious area (Aisite) (less any vegetated roof areas) and the development’s total equivalent low-density impervious area (AiLD).
The goal for granting a waiver from the stormwater management requirements is for at least an 80% reduction in runoff relative to an equivalent low-density development.
Conclusion
The granting of a waiver from certain stormwater management requirements for high-density developments makes good sense from an overall watershed perspective. High-density developments provide more spaces for living, working, and commerce while reducing the total disturbed land area, requiring fewer streets, and minimizing the overall volume of runoff discharged into the surface waters of the watershed. In spite of these environmental benefits for the watershed, the increased peak discharge from high-density development into local and downstream drainage systems must be considered. If the local and downstream drainage systems can accommodate that discharge, as is the case for the Grand Rapids downtown area, then a waiver can be granted. However, if the local and downstream drainage system cannot accommodate that discharge, public officials need to consider improvements to the drainage system (including offsite detention) to accommodate the granting of such a waiver. Therefore, although high-density development should be fostered as a watershed best management practice, consideration must be given to the capacity of the local and downstream drainage systems to accommodate and manage the peak discharges before a waiver or reduction in the stormwater management requirements is granted.
Currently, the City of Grand Rapids has granted waivers for two high-density developments. These two developments, involving a total of 1.2 acres of redevelopment, avoided the creation of over 14 acres of impervious area elsewhere in the watershed. This more-than-tenfold reduction was due, in large part, to the city’s policy that deliberately recognizes the value of high-density development and rewards such development by reducing or waiving certain stormwater requirements. Combined with other low-impact development policies and practices like promoting the Green Building Council’s Leadership in Energy and Environmental Design certification, vegetated roof covers, porous paving, rain gardens, and runoff capture and reuse, the City of Grand Rapids is quickly redeveloping into a community with a healthy environment in which to live, work, and play.