A sign posted at The Rock Creek Woods Apartments after the Sept. 1, 2021, flooding. Credit: Steve Bohnel

Editor’s note: This story is part two of a two-part series examining the Sept. 1, 2021, flooding event at Rock Creek Woods Apartments and Congressional Towers in Rockville. Today’s story focuses on the findings of a consultant’s report prepared for the county that examines the cause of the flooding at Rock Creek Woods. Friday’s story explored how the residents and the management teams of both apartment complexes have handled the flooding aftermath.

In the wake of the disastrous flooding a year ago at the Rock Creek Woods Apartments just outside the Rockville city limits – which caused 12 ground-floor apartments in two buildings to fill with water and led to the death of one young man as he sought to rescue his mother – an engineering consultant hired by the Montgomery County Department of Transportation (MCDOT) produced a nearly 200-page report assessing what had happened and why.   

The report details several infrastructure-related issues that contributed to the flooding, as remnants of Hurricane Ida, exacerbated by the effects of climate change, moved through the area in the pre-dawn hours of Sept. 1, 2021. The storm dumped an intense burst of 2.56 inches of rain in a mere 30 minutes, between 3:30 and 4 a.m., at the Rock Creek Woods complex along Twinbrook Parkway.   

Two tributaries of Rock Creek that flow through the property – hence the area name of “Twinbrook” — are directed through two box culverts underneath the apartment complex. The authors of the report, Ashton-based Mercado Consultants, determined that one of those culverts, near the northwest corner of the property, became overwhelmed by the volume of rainfall early that morning – as did a rectangular inlet built into the top of the culvert that was designed to drain water from adjacent parking lots.

As a result, the two apartment buildings in the northwest corner of the property – “constructed in a depression that resembles a bowl,” as the consultants described it – became engulfed in water more than 8 feet deep.

A year later, a flood sensor – one of three dozen such devices obtained from the U.S. Department of Homeland Security – is tentatively scheduled to be installed at the Rock Creek Woods complex this month by the county’s Office of Emergency Management and Homeland Security. These devices – 30 of which already have been put in place this year at locations around the county – notify county departments and agencies of rising water within 30 seconds, expediting issuance of alerts via cellphone and other wireless devices to residents in the event of future flooding episodes.

But, even as an interdepartmental, multi-year effort is now underway to identify and devise solutions to major flood risks around Montgomery County, MCDOT officials – while indicating agreement with the conclusions of the consultants’ report they commissioned – have no immediate plans to upgrade or replace portions of the stormwater infrastructure overwhelmed by last year’s flooding rainfall at the Rock Creek Woods complex. (The ground-level apartments inundated by water during that episode are currently not occupied, according to the management of the complex.)

Tim Cupples, MCDOT’s acting deputy director for transportation policy, emphasized the extraordinary nature of what occurred at Rock Creek Woods – considered a so-called 300-year storm.

“A 300-year event—another way of thinking of that is something that has a one-third of a [1] percent of a chance of occurring in a [given] year,” Cupples said in an interview, adding, “I’m not aware of anything that could or should be done to address an event like this” with regard to the infrastructure at the site.

Cupples’ comments are echoed in the consultants’ report prepared for MCDOT earlier this year. The report noted that the box culvert overwhelmed by water last Sept. 1 “has functioned without overtopping for more than 55 years, even during Hurricane Agnes in 1972, which documented hourly rainfall intensities of over 200-years frequency in the Rock Creek watershed. This fact is a testament to the historical rarity of the September 1, 2021, rainfall event… .”

But there’s a “however” that follows, as the report continues: “…The occurrence of the frontal zone [of September 1, 2021] and the resulting extreme high rainfall intensities should not be seen as an occasional event in the future … Montgomery County should consider enacting updated technical specifications for culvert design with regard to modern climate patterns. One recommendation is the adoption of a shorter (6- or 12-hour)rainfall duration for analysis, as traditional 24-hour rainfall distributions are based on rainfall monitoring data prior to the broad academic and political acknowledgment of climate changes.”

Such statements bespeak a dilemma faced by officials in Montgomery County and elsewhere.

As the impact of climate change is regularly being felt locally (storms in early July dumped 6.5 inches of rainfall on Silver Spring and up to 8 inches in parts of Bethesda in a short period, according to county climate change officer Adriana Hochberg), regulatory agencies at multiple levels are in the midst of assessing the duration and intensity of the storms yielded by climate change — and what changes to recommend for the capacity of future stormwater infrastructure.

When the Rock Creek Woods Apartments were constructed in the 1960s, the “gray infrastructure” on the site was designed to handle a so-called 10-year storm (one with a 10% likelihood of occurring in a given year), the consultants’ report noted. “Most agencies have standardized the design of culverts to pass a 10- to 25-year, 24-hour event,” the report said. “This is partially due to the fact that drainage systems across the United States were — and are — currently designed only to manage the most frequently occurring storm events.”

Referring to the existing stormwater drainage system under the apartment complex, Cupples – an engineer by profession — asserted: “If we were to design it today, it would get a very similar design. The problem  – and the nation is grappling with this – is ‘What should the design storm be going forward?’” (In meterological and engineering circles, “design storm” refers to the rainfall intensity – be it a 10-year storm or 300-year storm – that infrastructure should be designed to handle.)i

In contrast to several hundred years of rainfall data available for analysis, Cupples added: “We don’t have a whole lot of data since the 1960s or 1970s, or even the last 10 to 20 years. But what it does seem to indicate, though, is we’re not necessarily getting more rain in the course of 24 hours, but it arrives all at the same time — within a short segment in that 24-hour period.

“The first step is understanding how climate change is changing rainfall patterns, and then the next step is ‘OK, what do we do about it?’ — because we spent 100 years building a system that is designed around an understanding of rainfall patterns, and it’s not something that’s going to be fixed overnight.”

While these deliberations move ahead in scientific and regulatory circles, Montgomery County in the past year has started to undertake a “comprehensive look at what’s the potential in the county for buildings to flood, and what are the possible remedies,” Hochberg, also acting director of the county’s Department of Environmental Protection (DEP), said in an interview.

This exercise – recommended by the county’s Climate Action Plan, published a couple of months before the flooding at the Rock Creek Woods Apartments – led to creation late last year of an informal task force consisting of several county departments, DEP and MCDOT among them, followed by the hiring of an outside contractor last spring.

“This is a multi-year, multi-phase process of getting a better understanding of what’s the existing regulatory lay of the land, what are different departments doing and where are the gaps – and then moving forward to identify what are the regulatory [and] infrastructure changes that need to be done,” the task force’s coordinator, Stan Edwards, who heads DEP’s Energy, Climate and Compliance Division, explained in an interview.

The first phase of the process will culminate at the end of this year or in early 2023 with the release of a report compiled by the contractor hired by the task force. Publication of the report will be preceded by a series of public sessions this fall – the dates and locations of which are pending — intended to obtain feedback from county residents on flooding concerns.

Release of the report will lead to what Edwards described as the second and third phases of the process. “In Phase 2, we are doing detailed on-the-ground engineering assessments of specific areas to understand where there might be flooding vulnerabilities and what the infrastructure is, and then in Phase 3, we start to implement solutions to specifically identified flooding issues,” he said.

While cautioning that “Phases 2 and 3 will take a fair amount of time and cost a fair amount of money” over “a number of years,” Edwards noted: “…We don’t have to do all of Phase 2 to start Phase 3 – if we do a Phase 2 assessment in an area, and identify a flooding concern, we can start to implement solutions to that. And, right now, we are not waiting for the conclusion of Phase 1 to start some Phase 2 assessments.”

He was referring to the county’s efforts to tap into a program in which the U.S. Army Corps of Engineers provides planning assistance to states, with the cost split 50-50 with the federal government. County officials have asked the federal agency to help assess four specific parcels of acreage in the county – one in the Rockville area, the others in Wheaton and portions of the East County. Edwards said he hopes to hear back from the Army Corps this month on how many of these studies can be undertaken in the immediate future.

The four proposed studies involve one area in a flood plain where there have been past problems during intense rainfalls; the other three, while not in a flood plain, are in built-up areas potentially subject to urban flooding due to a large quantity of impervious surfaces unable to absorb water, Edwards said, adding: “We wanted to look at two different types of flooding. We all recognize that in flood plains there’s the danger – we want to study some areas where the danger is not as recognized.”

In the meantime, he said the county is developing a new website on flooding – slated to be launched in the next couple of months — “trying to help people understand that the fact you’re not in a flood plain or near a flood plain doesn’t mean you’re not prone to flooding in the event of a severe storm – and that, for example, every home in Montgomery County is eligible to get flood insurance.”

In fact, the Rock Creek Woods Apartments complex is not currently classified as being in a flood plain by the Federal Emergency Management Agency (FEMA). The consultants’ report prepared for MCDOT noted that flood insurance studies conducted in the late 1970s – and carried over to updated maps in the first decade of this century – listed the Rock Creek Woods complex as being in an area with a minimal flood risk.

FEMA is expected to complete its latest update of maps for this area by the end of 2023, and a recent draft shows the Rock Creek Woods Apartments as being located in a flood plain.

Such assessments aside, the county considers the Rock Creek Woods Apartments to be located in a flood plain. In fact, under current county regulations, construction of the apartment complex would not be permitted if the land on which it is located were vacant today, according to William Musico, floodplain administrator for the county’s Department of Permitting Services.

As did the MCDOT consultants’ report, Musico described the configuration of the land in question as resembling a bowl, adding: “The property is in a low-lying area, meaning there is no overland relief – and the only way for flood waters to get out is a storm drain … . We would not permit that categorically now.”          

As the countywide assessment of areas subject to flooding and possible remedial solutions moves ahead, Hochberg said, “We’re assessing all options, and not crossing anything off the list at this early juncture.” At the same time, she added, “We need to recognize this [as] being very costly and time-consuming work because we are in a county that is already built out.”

Echoing concerns by MCDOT officials that changes to the stormwater infrastructure at flood-prone locations such as Rock Creek Woods are accompanied by the risk of unintended consequences, Hochberg cautioned: “Something that our DOT colleagues speak about quite eloquently is that if you tweak one part of the system, then you have to understand what are the downstream impacts.

“Water flows downstream, and you just can’t fix it in one spot – you really have to look at it in a holistic way. And that’s what makes it so challenging.”

Louis Peck, a contributing editor for Bethesda Magazine, can be reached at: lou.peck@bethesdamagazine.com.