FEATURE STORY, JULY 2008

CUTTING THE DIAMOND
Resolving stormwater detention and grade issues to maximize a site’s usable area and return on investment.
Sherilyn R. Kosmos, PE

A rough diamond needs to be skillfully cut and polished to maximize its size, brilliance and return on investment. Similarly, many commercial lots in prime locations pose challenging grading and stormwater challenges, which require effective, often creative strategies to maximize usable area for development. When these strategies are successful, the developer has a real gem.

Site Assessment and Planning

The first step toward the grading and stormwater management challenges is an engineering site assessment. This should be performed during the due diligence phase, while the developer has the leverage to negotiate a lower purchase price to fund the investment in resolving these issues. Engineers will examine a sample site plan, assessing topography and opportunities to build a structure with a larger footprint if, for example, a retaining wall were constructed. They will estimate stormwater volume on the site based on annual rainfall estimates and total impervious area created by structures and surface parking, and the associated surface area required to manage the run-off.

At the same time, engineers can assess the availability of utilities, the cost to bring them to the site, and their capacity. They will also examine the zoning code for setbacks, and the likelihood of a need for a variance. In addition, they can identify impact fees related to connections with the water and sewer systems; if overlooked, these can make quite a dent in the developer’s budget.

Later, at the site planning stage, the engineer can suggest optimal placement of the building(s) and parking spaces to gain the most cost-effective use of the site given the building size and number of parking spaces.

Scaling the Walls

Effective grading strategies can maximize the usable area for construction and potentially reduce the volume of stormwater that must be retained on the site. A site with strong relief often has steeper slopes along the perimeter, and there is a limit to how steeply these can be banked in order to level the site for development. Construction of a retaining wall increases usable area, but often at a high cost. Another technique is to drop the grade through strategically placed landscaped areas within the parking lots. For example, say the site plan has an entrance drive with parking lots on both sides of the entrance and landscaped areas separating the parking lots from the drive, but one parking lot is 2 feet higher than the other. A possible solution is to enlarge the landscape area without losing parking, drop the grade through this area and slope the drive, avoiding the cost of retaining walls. Landscaped area also reduces the volume of stormwater run-off and, thus, the required size of a detention pond or underground system.

On the other hand, leveling a site using retaining walls may offer the benefit of increasing the area along the perimeter that can be utilized for parking, thus increasing the allowable size of the building. It is also possible to compensate for grade changes through strategic placement of individual buildings on the lot. Identifying the optimal solution depends on the specific zoning, economic and engineering issues associated with the site and the flexibility of the site plan.

For example, a major 716,000-square-foot retail development planned for a 62-acre site in Bolingbrook, Ill., posed grading challenges. Engineers at Woolpert, Inc. estimated that use of a retaining wall with backfill would allow approximately 33,000 square feet of additional retail to be provided. The cost of the wall installation would be approximately $278,000. If a monthly rent of $9.00 per square foot for this building were obtained (which is well below current market rate in this area), then the cost of the wall would be recovered in one month.

Going Underground

Stormwater management demands a significant amount of space for the required infrastructure. Developers must weigh the relative costs of surface versus underground detention against the potential value of the associated space for parking, building area, or a saleable outlot. (Some specialty users, will pay top dollar for a relatively small outlot if it is on the right location.)

In the Bolingbrook project, for example, engineers at Woolpert, Inc. calculated the sizes and costs of an above-ground detention basin versus an underground system. The cost of a conventional 1.5-acre above-ground detention basin was projected at approximately $40,000, while the projected cost of a 5-acre-foot underground system was $1 million. However, the land designated for the detention basin comprised a prime frontage outlot worth an estimated $12.50 per square foot — $820,000. By investing in an underground system, the developer was able to add a 150,000-square-foot anchor tenant to the project without the loss of smaller retail stores elsewhere on the site.

Another issue for both detention ponds and underground systems is water quality: water quality must be improved before it is discharged. As a result, manufacturers are developing a range of techniques to remove pollutants from the water, and these technologies add to the cost of stormwater detention systems. Some of these products contain PVC components, which have costs closely tied to the fluctuation of petroleum prices.

Up on the Roof

Just as landscaped surface area reduces the volume of stormwater run-off, so does a green roof. Believe it or not, a green roof can also double as “open space” in some jurisdictions, allowing for more parking spaces on the ground and an increased building area.

An extension of the existing roof, a green roof comprises a high-quality water proofing membrane, root repellent system, drainage system, filter cloth, lightweight growing medium and, ideally, low-maintenance plants. Although the green roof is a well-accepted concept in Europe, it is still relatively uncommon in the United States, and developers are often skeptical of its benefits relative to the costs. (Perhaps it is worth mentioning, however, that two recent projects by a major home improvement retailer utilized green roofs, demonstrating that even a “big-box” store can incorporate this site-maximizing technique.)

A green roof increases the building load and in turn, the associated cost of the structure. It also requires special attention in the placement of rooftop mechanical systems to avoid interference with the planted area. Thus, it creates another level of challenge for the architect and structural and MEP engineers. According to proponents of the technology, such as the industry group, Green Roofs for Healthy Cities (www.greenroofs.net), a green roof offers benefits in addition to stormwater management, including increased roof longevity, sound absorption, and decreased heating and cooling costs.

Certainly, there are trade-offs associated with every solution to grading and stormwater management challenges. The earlier in the planning process in which alternatives are evaluated, the more cost-effective the solution is likely to be. The developer who has flexibility in site planning, with a willingness to consider various strategies to maximize usable area, and the up-front capital to fund an investment in a mid- or long-term return on investment, has many ways of cutting the diamond to maximize its value.

Sherilyn Kosmos, P.E., is an Associate with Woolpert, Inc.

©2008 France Publications, Inc. Duplication or reproduction of this article not permitted without authorization from France Publications, Inc. For information on reprints of this article contact Barbara Sherer at (630) 554-6054.