By Lewis Link, Ph.D., M.SAME, and Brig. Gen. Gerald Galloway, Ph.D., P.E., F.SAME, USA (Ret.)
Since its establishment as a mill town on the banks of the Patapsco River in the late 1700s, Ellicott City, Md., has experienced some degree of flooding on the average of every 12 years. The most recent floods, occurring in 2016 and 2018, broke standing records and produced extensive losses to businesses and residences in the historic part of the town. These damaging events left many questioning their ability to recover and the status of their future.
This year, two landmark studies were published on urban flooding in the United States—one by the National Academies, “Framing the Challenge of Urban Flooding in the United States”; and the second by the University of Maryland and Texas A&M-Galveston, “The Growing Threat of Urban Flooding, A National Challenge.” Both studies point out the dilemma faced by, literally, thousands of communities across the country.
Each local area has their own situation experiencing the growing threat of increased flooding stemming from decades of continuing urbanization, encroachment on floodplains, and crumbling storm sewer and drainage infrastructure that has not kept pace with growth and modern standards. Couple that with a recent trend of more frequent and severe storms, particularly in the Northeast, and what results in a dangerous equation for cities and towns nationwide.
Perhaps, though, their greatest frustration is that as a nation we have not done much to address this risk, until in the aftermath of a major disaster hitting. We have placed the responsibility in the hands of individual communities.
As pointed out by the National Academies and University of Maryland/Texas A&M-Galveston studies, some of the greatest difficulties include finding flood hazard assessment methods adequate for the complexity and variety of urban situations, development of stronger coordination across agencies, and dramatically increasing local to state to federal resource commitments. There also is a need to define and implement customized solutions for the wide variety of situations, not a “standardized approach.”
In addition, while severe storms fall on rich and poor alike, socially vulnerable populations are struggling the most to deal with this increasing threat.
Ellicott City fits this dilemma exactly. While its location at the confluence of multiple streams and creeks was a distinct advantage as a mill town, today, as an urban center, the steep slopes and largely impervious surfaces are all too effective at collecting rainfall and accelerating its movement into the overtaxed storm drainage system. When the capacity of the drainage and storm sewer infrastructure is exceeded, Main Street becomes the alternative path to the Patapsco River, which itself, during such events, is likely higher than normal and becomes a barrier to the efficient discharge of the urban storm water.
In fact, the dominant mode of flooding has evolved from high water on the Patapsco backing up into the city to heavy runoff from intense rainfall surging through the city on its way to the confluence of rivers.
One of the many challenges faced by Ellicott City, located just 11-mi west of Baltimore, is the number of historic buildings that reside in town, especially along the currently favored drainage way for floodwaters. Of particular concern is a group of structures at the bottom of the Historic District and near the confluence of the smaller rivers into the Patapsco. Can they be preserved, or must they be removed to make room for more efficient drainage?
These structures are a contentious component of another major issue: how to improve drainage to allow more water to be efficiently conveyed through the city when it is needed. Ultimately, the most pressing demand, however, is to agree on a source of dedicated funding to implement options developed by planning that has been going on since the 2011 flood, which resulted from heavy rainfall during Tropical Storm Lee.
Planning activities, primarily supported by Howard County, have resulted in ideas to mitigate flooding. Options include development of large retention ponds to capture runoff before it enters the downtown drainage system; systematic improvements to the storm drainage system to include larger culverts and removal of structures that impede flow during a flood event; and flood-proofing concepts, particularly for historic structures vulnerable to flooding developed by the Baltimore District of the U.S. Army Corps of Engineers. One option proposed augmenting the current drainage system with a large underground tunnel to convey water under the city.
All these approaches are expensive and would require a large amount of dedicated funds. In many cases, even individual projects, let alone the systems approach that is really necessary to reduce the current flood risk, require resources far beyond what the local community has.
Howard County has announced a second phase of initiatives to deal with the Ellicott City flood problem. The plan provided five options that range in cost from $63 million to $175 million, and require from four years to seven years to complete, depending on the option chosen.
On May 13, 2019, County Commissioner Calvin Ball announced that Option 5 was selected. This decision includes removing four historic structures at the bottom of Main Street that are constricting drainage, building a large tunnel under the historic area to more efficiently convey runoff to the Patapsco River, and adding upstream retention ponds. Option 5 is estimated to cost $113.5 million to $140.5 million and require five years for implementation.
Analysis by Howard County consultants shows that these measures would not eliminate flooding in the Historic District, particularly for severe events as those experienced recently. Collectively, however, they would significantly reduce flood risk and provide tremendous relief against less-rare events. The source of funding was not announced, but the commissioner stated that the effort would be approached one year at a time.
ADDRESSING FLOODING RISK
During a seminar sponsored by the SAME Mid-Maryland Post held in March 2019 in Ellicott City, local business owners expressed their exasperation with respect to the availability and delivery of federal funding for recovery and risk mitigation. They have exhausted their personal resources and brought their businesses back, but with the realizations that they are likely not able to do it again, following the doublebarrel flooding they experienced in 2016 and 2018.
Worse, they are fully aware that the flood risk has not been addressed and they continue to be at significant risk of continued flooding. As the University of Maryland/Texas A&M-Galveston study cites, small communities often fall through the cracks in the resource system. Their losses are not great enough to be considered of federal interest and they are often not competitive for large grant programs.
PLANS FOR TOMORROW
There are certainly challenges for county and community authorities struggling with how to address problems of such a large scale with limited resources. An additional issue looming on the horizon for Howard County is how the mitigation in Option 5 will stand the test of time.
Community storm drainage systems are commonly designed to deal with an event expected on average once in 10 years. The 2016 rainfall event in Ellicott City that led to major flooding was assessed by one source to be a 1 in 1,000 year event; by other experts it was estimated to represent a 1 in 275 year event. Either way, it was relatively rare. Option 5 would appear to go well beyond the normal 1/10 criteria.
However, the latest National Climate Assessment, an interagency report released in November 2018, stated that the Northeast has experienced a 71 percent increase in severe rainfall events since 1958. If that trend continues, events like the ones Ellicott City faced in 2016 and 2018 will become more common. Whatever is implemented in the near term for flood mitigation will have to consider what might occur in the future. This means anticipating new amendments to the mitigation plan that can be added incrementally if and as needed.
An incremental adaptive approach is exactly how The Netherlands is dealing with the large uncertainty presented by climate change and sea level rise in a country that has extensive land well below sea level. It requires considering steps two and three before completing step one, so that all steps are compatible and efficient to implement. And that assumes that continued urbanization of the area will be closely managed.
Ellicott City embodies a drama playing out across the nation. Even military communities are not exempt. Ongoing flooding in Nebraska involves both Offutt AFB and an Army National Guard facility. Will recent disasters spur officials to attempt to remedy problems that have been building over decades, which are amplified by the difficultly of managing growth and evolving public infrastructure to accommodate man-made and natural changes?
Urban flooding in smaller communities is a national systemic issue and must be addressed more comprehensively and with greater emphasis on adaptation and resilience. Small communities represent one of our vulnerable populations and it is time to do something.
Lewis Link, Ph.D., M.SAME is Research Professor, and Brig. Gen. Gerald Galloway, Ph.D., P.E., F.SAME, USA (Ret.), is Research Professor and Glenn L. Martin Institute Professor of Engineering, Center for Disaster Resilience, Department of Civil & Environmental Engineering, University of Maryland. They can be reached at email@example.com; and firstname.lastname@example.org.
[Article first published in the July-August 2019 issue of The Military Engineer]