News reports documenting Houston’s record-breaking rainfall and disastrous flooding, generated by Hurricane Harvey, often attempt to explain why Houston is chronically vulnerable to floods. But these reports tend to be incomplete and inaccurate.
Moreover, while the media understandably focus on how people are coping with the storm’s life-threatening, property-destroying consequences, we hear little about what — if anything — might be done to avert such disasters in the future.
Commentators primarily attribute Houston’s flooding vulnerability to the city’s extraordinary amount of impervious surface — roofs, paved roads and parking lots. Excessive imperviousness is in turn blamed on the city’s rapid growth in population and physical size, catalyzed by Houston’s go-go economy, lack of zoning and imprudent urban planning.
But adversely comparing Houston’s imperviousness to other cities is misleading. Large amounts of impervious surface area exist in metropolitan regions such as Dallas, Los Angeles, Atlanta, Chicago and New York City, as well as Washington.
In fact, news reports tend to overlook the most fundamental reasons for Houston’s susceptibility to extreme rainfall and catastrophic flooding: its almost dead-level topography, constrained stormwater-management capabilities and adverse soil conditions.
Houston’s stormwater system simply cannot handle the volume of rainwater dumped on the city by Harvey. Making matters worse, the amount of surface water that Houston’s open, vegetated land can absorb is limited. The high clay content of soils impedes absorption. And close to bayous, bays, rivers and the Gulf of Mexico, the underground water table is not far below grade, which is why most homes in Houston are built without basements.
Consequently, many Houston neighborhoods have limited capacity for retaining and evacuating rainwater. Runoff collected in drainage swales or fed into underground pipes ultimately has nowhere to go except into slowly draining creeks and bayous. A heavy, persistent rainstorm can rapidly fill natural waterways and storm systems to overflowing, with rainwater backing up onto neighborhood streets and properties.
Some areas in the Mid-Atlantic region, especially near the Chesapeake Bay, face similar problems. But much of metropolitan Washington is favored with varying topography and adequately sloping land. And, of course, we get much less rain than Houston.
Nevertheless, Houston is a reminder that deficient and aging infrastructure is a challenge for many American counties, towns and municipalities. Failure of a stormwater system is critical, as it immediately and adversely threatens the health, safety and property of citizens using and depending on the system.
Regardless of topography, any community’s stormwater system can be overtaxed and fail if the volume of water running off and pouring into the system exceeds the system’s engineered capacity. And runoff volume generated by a rainstorm is affected by how a community has been and continues to be planned, designed and built.
Civil engineers and hydrologists design stormwater-management systems in a logical way. They compute the total amount of impervious area — paved surfaces and roofs — within the tributary drainage area of the system. Next they apply factors representing probable, peak rainfall rates to arrive at maximum predicted runoff. Then they figure out where and how to collect and route runoff flows, whether into storm drains, pipes, culverts, drainage swales, retention basins or natural streams. Expected flow rates also determine sizes of stormwater structures.
Problems arise when flow predictions are exceeded, as Houston is experiencing with Harvey. But excessive flow and system failure can occur in any jurisdiction where growth and development have led to more density and more impervious, water-shedding surface area than foreseen by system engineers.
Thus, to ensure acceptable stormwater-system performance, jurisdictional agencies and officials must ensure that the type, timing and amount of real estate development are in sync with the capacity and configuration of the jurisdiction’s storm system. Many communities are at risk because of overbuilding or for allowing building in areas with inadequate infrastructure.
Governments also must be diligent about upgrading and expanding such systems in a timely fashion as growth and development increases beyond what was first planned and anticipated. Regrettably, this is too often not the case.
Jurisdictional policies and regulations should require that all new development or redevelopment be designed to minimize stormwater runoff. This is especially critical if the existing stormwater system’s capacity is already stretched to the limit.
Rainwater can be collected, slowed down and retained temporarily on green roofs and vegetated open spaces, or in cisterns to be recycled for uses other than drinking. Pervious paving can be used for sidewalks, terraces and parking areas, enabling rainwater to percolate into soils below.
Implementing all of these stormwater-management strategies and tactics would be of value in reducing future flooding risk in Houston. Yet Houston teaches a very important lesson we keep relearning and sometimes forgetting at great cost: Cities and towns, as well as buildings, should never be built on flood plains.
Roger K. Lewis is a practicing architect, a professor emeritus of architecture at the University of Maryland and a regular guest commentator on “The Kojo Nnamdi Show” on WAMU (88.5 FM).