RESIDENTIAL CLUSTER DEVELOPMENT:
	Storm Water Management	3
	Robert D. Sykes, ASLA

Summary

Residential cluster developments offer local governments an excellent opportunity to manage storm water more effectively than they can in conventional developments. This publication reviews the fundamentals of storm water management, highlighting the problems conventional developments have in this area, and identifying the benefits of cluster design in developing a natural system of storm water management. There is also a description of an existing subdivision that benefitted from this new design.

Fundamentals of Storm Water Management

In every location there are two storm water management systems, the major and the minor. Three considerations largely shape the design of these systems: flooding, convenience and water quality. Paths taken by runoff from very large storms are called major systems. Where these systems are specifically designed, flooding is usually avoided. But where the effects of large storms are not specifically considered or planned for, flood damage can be substantial.

Systems designed with convenience in mind quickly remove runoff water from areas such as streets and sidewalks because they're difficult to use when covered with water. Convenience facilities like storm sewers, technically referred to as minor systems, quickly remove the peak flow of a runoff resulting from typical small storms.

Apart from temporary measures to control sediment in construction areas, water quality concerns in residential areas focus on the polluting substances washed from paved surfaces and carried into streams or other bodies of water during storms. Pollutants carried in runoff include sediments, nutrients, chemicals, disease-carrying organisms and heavy metals. Sources of these pollutants include grass clippings, leaves, eroded soil, fertilizer particles, oil and gasoline drippings, animal droppings, and metal flecks from vehicles. Detention ponds remove most of these suspended substances from runoff by temporarily holding it until the particles settle out (see Figure 1). Regulations require such ponds.

Storm Water Management
in Conventional Developments

Since World War II, conventional zoning has typically led to the development of residential subdivisions that completely blanket a parcel with evenly-spaced lots. This results from zoning provisions that require minimum lot sizes and widths, and from local governments requiring developers to construct streets that serve every lot. In most cases, these streets must have curbs, gutters and storm sewers (see Figure 2).

The adverse effects of storm water management in traditional developments mainly occurs because of changes made to the character of the land surface. Developments introduce roofs and large areas of pavement, referred to as impervious surfaces, which substantially reduce the amount of rainfall soaking into the soil and substantially increase the amount of runoff.

Because pavements and roofs have much less surface area to wet in a rainstorm compared to plant-covered lands, more water is also free to run off these simpler, impervious smooth surfaces. Because they are smoother, water also runs off them faster. Instead of flowing off slowly over a long period of time, a larger volume of water arrives downstream at the same time much like rush hour on highway networks. More water running more quickly causes "traffic jams" of water downstream that we commonly call flooding.

This addition of impervious surfaces associated with urbanization can significantly contribute to lowering the water table, both locally and regionally.*  This can skew the balance of water over time into a feast-and-famine moisture pattern between storms and dry periods.

Development under conventional zoning does little to minimize, much less prevent, these ill effects. Curbs hold water in the roadway, requiring storm sewers to let it out. The round pipes used for storm sewers move masses of water very efficiently. But instead of moving runoff slowly over natural surfaces so it soaks in, runoff moves rapidly once it's inside storm sewers, with no opportunity to infiltrate the soil. The high speed of flow keeps pollutants suspended in the runoff. Constructed ponds are then required to remove pollutants and reduce peak flows. Since storm sewers are designed to flow without pumps, they tend to be put in the lowest portions of the landscape which are natural drainage-ways. Streets then follow this drainage pattern. Thus when storm sewers overflow, the street's smooth, uninterrupted, impervious surfaces become the paths flood flows follow.

The proverbial "cookie cutter syndrome" that results from conventional zoning (houses spread evenly over an entire site) leads to a large amount of pavement so the streets connect to all the houses. In addition, large lots and front yard set-backs necessitate even more pavement to connect garages and front doors to streets. With all this pavement connected, there is much less opportunity for runoff to soak into the ground. In short, conventional development carries with it a subtle but powerful bias toward maximizing both the quantity and speed of runoff.

Storm Water Management in
Cluster Developments

Cluster zoning allows the same number of houses on a site as conventional zoning (see Figure 3). However, it allows developers to put the houses on smaller lots and requires the preservation of large areas of a site as open space where houses can never be built. These two provisions give local governments and developers the flexibility needed for good design and modern storm water management.

The layout for a clustered housing development can be arranged so that the steep slopes, natural drainage-ways, and areas of prime vegetation fall where the open space is (see Figure 4). And by clustering lots closer together and facing them on open spaces, shorter roads (and less pavement) are necessary. Smaller, narrower lots also help reduce the need for pavement in driveways and walks, as in Figure 3.

Clustering enables a better relationship between impervious surfaces and natural drainageways, too. Roads can be placed along ridge lines, with houses just off the ridges on the ridge 'brow.' This means that most pavement and roofs are located as far from the preserved natural drainage system as possible (see Figure 5). Runoff from impervious surfaces now flows slowly over pervious, vegetation-covered areas, soaking into the soil, which filters out some of the pollutants before the storm water reaches lakes, rivers and streams. Greater use is made of drainage devices such as ditches and swales. (Similar to ditches, swales are typically short, shallow and wide depressions covered with vegetation.) Costs are lowered because curbs and storm sewers are no longer needed. More rainfall is directed toward the replenishment of ground water. The development generates a smaller volume of runoff moving more slowly toward the bodies of water receiving it.

Management of Storm Water Facilities in
Cluster Developments and Open Spaces

Preservation of natural drainage systems and the use of overland swales for storm water require a different approach to maintenance and repair than traditional storm sewer systems. Most of all, this type of storm water network requires maintenance of living plants and occasional removal of sediment. This in turn requires a management organization designed and funded for that purpose. Public works departments are one alternative for managing these natural drainage systems. There are three other management options: (1) homeowners associations, (2) storm water utilities, and (3) water quality cooperatives.

• Homeowners Associations
  A homeowners association is initially established by the land developer as a nonprofit organization. Through deed restrictions, all homeowners are members of the association and bound to the subdivision. This means the association can set rules and assess membership fees for the care of commonly owned property, including storm water systems and open spaces.

• Storm Water Utilities
  In Minnesota, a local government may establish a storm water utility for the maintenance of storm water infrastructure. Many cities have done so in the last fifteen years to take care of storm water detention ponds. A storm water utility can assess the costs of its services to property owners that benefit from the storm water facilities it owns or for which it is responsible. Two Minnesota examples are the cities of Lake Elmo and Marine-on-St. Croix.

• Water Quality Cooperatives
  Individuals can also form water quality cooperatives that own and care for the storm water infrastructure discussed above. Cooperatives are non-profit, member-owned organizations that provide services to their members and are financed through a membership fee structure. These organizations can also be used to join together several homeowners associations by using the Master Association feature of the Minnesota Common Interest Ownership Act. By joining, these groups effectively improve their bargaining position when securing technical management or other services for their residents.

The Woodlands Case Study

Typical residential street in The Woodlands, Texas. (Photo courtesy of Professor Davis G. Pitt, Department of Landscape Architecture, University of Minnesota)

Nationally, one of the best-known developments to use residential clustering for a natural system of storm water management is The Woodlands New Community outside Houston, Texas. This 20,000-acre town was planned and designed by Wallace, McHarg, Roberts and Todd, Landscape Architects and Planners, Philadelphia, Pennsylvania. The site is flat and heavily wooded, with extensive areas of poorly-drained soils. Clustering was included in the firm's comprehensive plan to preserve the site's natural drainage system, avoid environmentally-critical areas, work with existing topography, and maintain prevailing hydrological conditions.

The Woodlands' general plan used the existing natural drainage system to provide the town's major storm water system. Major roads and dense development were located along ridge lines, while preserving the natural flood plains as parks and open space. Rather than an underground storm sewer system, the minor storm water system is made up of open space and roadside and lot-line swales. The minor system focused on getting small rainstorms to soak into the soil.

In its original plan, engineers compared the capital cost of the natural drainage system to that of a conventional system and found that the natural approach saved over $14 million. In addition, a conventional storm water management approach would have cleared thousands of trees, increased runoff 180 percent, degraded downstream water quality, and caused a daily water table draw-down of 15 million gallons. The plan avoided or sharply reduced the impact of all these problems.**

The ultimate measure of The Woodlands' approach occurred one April day in 1979 when a record storm dropped nine inches of rainfall on the Houston area in less than five hours. No houses in The Woodlands experienced any flooding. But neighboring areas, with conventional storm water management systems, were hit hard by flood damage.

Authors

Robert Sykes
Associate Professor
Department of Landscape Architecture
(612) 625-6091
sykes002@maroon.tc.umn.edu

For More Information

Thomas Wegner
Extension Educator
University of Minnesota Extension Service 
Hennepin County
(612) 374-8400
twegner@extension.umn.edu

Sources for Figures 2, 3, 4, and 5: Protecting Water Quality in Urban Areas: Best Management Practices for Minesota, Ch. 3, Minnesota Pollution Control Agency, Division of Water Quality, St. Paul, MN, Oct. 1989.

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