By Shanda Davenport, P.E., CFM, AICP
“Take an umbrella- it might rain!” How many times have we heard that?
These days it seems to be happening more and more. Is it really raining more? Or is it raining heavily more often? In the coastal plain of the east coast, that question keeps coming up. The City of Virginia Beach has been conducting an analysis to develop a plan to protect against the impacts of sea level rise. But, as we worried and fretted as to whether or not we were on the right curve or projection from the myriad of possibilities and probabilities associated with sea level rise, portions of the City were getting flooded by rainfall in ways and in locations that we have not experienced in the past.
We know that sea level rise is a major concern for coastal Virginia and particularly for the Hampton Roads region. The five long-term water level observation stations in southeast Virginia, highlighted in green in the table below, are in the top 10% of the highest relative sea level rise rates in the nation.
TABLE 1 – NOAA TIDE STATIONS WITH THE HIGHEST SEA LEVEL RISE TRENDS IN THE NATION.
|STATION NAME||RECORD LENGTH (YR)||SEA LEVEL RISE (FT/CENTURY)||RANK|
|EUGENE ISLAND, LA||35||3.2||1|
|GRAND ISLE, LA||69||3.0||2|
|GALVESTON PIER 21, TX||112||2.1||4|
|CHESAPEAKE BAY BRIDGE TUNNEL, VA||41||2.0||5|
|SABINE PASS, TX||58||1.9||6|
|OCEAN CITY INLET, MD||41||1.8||7|
|LEWISETTA, VA||46||1.7||10||NEW CANAL, LA||34||1.7||11||COLONIAL BEACH, VA||38||1.6||12||NORTH SPIT, CA||39||1.5||13||SEWELLS POINT, VA||89||1.5||14||CAPE MAY, NJ||51||1.5||15||DUCK, NC||38||1.5||16||APRA HARBOR, GUAM||23||1.5||17||FREEPORT, TX||36||1.5||18||BAY WAVELAND, MS||38||1.4||19||CORPUS CHRISTI, TX||33||1.4||20|
In 2016, the City experienced three rainfall events that were greater than a 100-year storm event! Widely used, the nomenclature of a 100-year storm is probably not very useful to most people. However, it’s really about chance – a storm that has a 1% chance of happening in any year. These storm events in the City of Virginia Beach in 2016 caused us to pause and consider the context for evaluating risk in the future. A shift in this understanding created momentum for addressing the question:
If we are assessing impacts from future water levels, i.e. sea level rise and tidal conditions, shouldn’t we also look at future rainfall patterns?
We began working in-house and with consultants, such as Dewberry Engineers headquartered in Fairfax, Virginia, to look into the possibility of providing information about future precipitation patterns (for more information, see the study overview, City of Virginia Beach Comprehensive Sea Level Rise and Recurrent Flooding Response Plan). We know that designers and engineers must consider the impacts of rainfall in every design solution that is undertaken, but with changing rainfall patterns this is even more essential.
Surface water has long been defined as a “common enemy” and that attitude prevailed in site designs of the past.1 Water was quickly collected and concrete and pipes moved that water invisibly from sites to “somewhere” downstream. This approach worked fine as long as rainfall was as expected and there was a downstream where it could go. However, as we have become more aware of the impacts of development, there has been a move to retain water and mimic the natural systems that were replaced. As designs become more complicated, there is a need to better understand rainfall. Today’s more complex designs work to replicate natural systems that exist prior to development. We are no longer concerned solely with draining a developed site; we also seek to emulate natural infiltration and flow volumes and timing. Some of these changes have come about as our understanding of stormwater systems has grown and some because of regulatory changes which mandate that stormwater is controlled. These design challenges coupled with the diminishing supply of greenfield sites has made our reliance on correct and specific data much more important. Today, it is very likely a failure of a stormwater system will cause cascading failures.
The work within in the City of Virginia Beach that has been undertaken by Dewberry ultimately encompasses three main efforts:
- a historical analysis of rainfall data was undertaken to determine if shifts in rainfall had occurred in the past;
- historical patterns were compared with downscaled future climate models for correlation;
- and, finally, research was undertaken to determine how the future conditions might be integrated into design standards.
The typical source of rainfall data used in our region is from National Oceanic and Atmospheric Administration (NOAA) Atlas 14. However, in the course of the analysis, we discovered that by including rainfall data beyond the 2004 data included in the NOAA publication, there was already a statistically significant increase in heavy rainfall and that the listed values underreported rainfall. This is important because that means that infrastructure that is being designed and built today will not perform as expected during its service. This is true not just for things like pipes and ditches, but also for best management practices and design features meant to enhance water quality.
The study then looked at multiple downscaled climate models and compared recent historical rainfall patterns with future projections. Comparisons were made using statistical methods and from that information, projections of future storm amounts was computed. The key to the projections was selecting a model where future projections match past events. Mid-term projections, around 2045, showed increases in storm amounts of 10-25% and longer-term values increasing as much as 35%. This means that a storm that would be classified as a 100-year storm today may only be a 50-year storm by mid-century.
What do we do with this information? Making sense of the data showing increased rainfall needs to be tempered by the uncertainty of any future projections. Around the country, design professionals are wrestling with this changing environment. Some communities and agencies, such as the state of New York and US Army Corp of Engineers have adopted guidance on climate change and rainfall changes, but most agencies have yet to integrate these changes into standards. The City of Virginia Beach is looking to integrate this knowledge of rainfall changes into its Standards and Specifications. The City has already implemented a policy requiring applications for conditional changes in zoning or use to include the increased rainfall amounts when preparing a preliminary stormwater analysis. The idea is that if changes, like a rezoning of property are made, it is reasonable to want to know how resilient the development might be in the future.
Designing for a changing environment seems like a paradigm shift, but that’s not completely accurate. We have always needed to consider how natural and built systems evolve. What is different is that we are now recognizing that we also need to consider that there may be changes to natural processes as well. This means just as soil settles, plants grow, and materials weather and age; we as seeing changes to things like rainfall events and ocean and bay levels. This adds yet another element of consideration as we transform our environment. Our systems are complex and if we are to be good stewards of our resources and continue to create the highest and best uses for the health, safety and welfare of all, we must use the best data and tools available. If you don’t plan for change, you are not planning for the future.
For more information on the Comprehensive Sea Level Rise and Recurrent Flooding Analysis and Response Plan, visit http://www.vbgov.com/pwSLR.
1Virginia applies the modified common law rule applicable to surface water Mullins, 2 6 Va. at 589, 311 S.E.2d at 112. Under this rule, “surface water is a common enemy, and each landowner may fight it off as best he can, provided he does so reasonably and in good faith and not wantonly, unnecessarily or carelessly.”
Ms. Davenport is a Stormwater Engineer for the City of Virginia Beach in Stormwater Technical Services where she brings innovative, yet grounded responses to challenging problems. She has a background in both public and private sector work. She earned degrees in both Landscape Architecture and Civil Engineering from North Carolina State University and is licensed as both a Professional Engineer and Certified Planner. Ms. Davenport currently manages a three-million dollar capital improvement project to assess the City’s vulnerability to sea level rise and develop actionable hard and soft solutions to recurrent flooding. She is a member of the Association of State Floodplain Managers and is a Certified Floodplain Manager. She has previously served as floodplain manager, CRS administrator and chief damage assessment officer for a community on North Carolina’s Outer Banks. She grew up on the Outer Banks, yet has now spent enough time inland to understand the utility of both socks and umbrellas.