100-Year Storms in Virginia

Storm surges along the coastline are serious threats. Analysts at insurance companies have to calculate the expected number of claims that will be paid each decade, and define the minimum amount that that the companies should charge for insuring against specific risks.

Virginia Beach is at risk of a storm surge flooding the city during a Category 3 hurricane. The level of flooding in the city would vary, depending upon:

  1. the intensity of the storm (on the Saffir-Simpson Hurricane Scale, a Category One hurricane has winds 74-95 mph while a Category 5 hurricane has winds greater than 155 mph)
  2. whether or not it hit at high tide
  3. the amount of "floodproofing" done in advance of the storm's arrival

In the maps below, "green" indicates dry land and "blue" indicates water-covered areas. Normally, Back Bay is separated fom the Atlantic Ocean by a thin beach and a line of sand dunes:

Hurricane surge impacts on southeastern Virginia Beach
Source: Hampton Roads Emergency Management Committee Hurrican Flooding and Tidal Surge maps

Insurance companies with policies covering damages from hurricane surge impacts on southeastern Virginia Beach can't control the weather. They can only control the "floodproofing." In some areas, engineers could build floodwalls around a building, complex, or a neighborhood that would protect against water that rise 1, 2, maybe even 4 feet.

That would cost vast sums of money. Usually the costs for such projects (such as the widening of the beach by dredging sand offshore and pumping it onshore at te resor portion of Virginia Beach) are spread over a wide number of people, by using government (tax) dollars. Occasionally, a special land use district is defined, and the property owners in that particular district pay a higher-than-normal tax to cover the costs of the higher-than-normal protection. Property owners at Sandbridge funds their beach replenishment through a special tax.

beach cottage
beach cottage on stilts (at Corolla, NC)

Another form of floodproofing is to move structures away from the water. This move can be vertical or horizontal. At Sandbridge (the beachfront community north of False Cape State Park and Back Bay National Wildlife Refuge) the houses are built on stilts. The first floor is 10-14 feet above the ground; the posts in the ground can get wet in a "northeaster" or mild hurricane, but the kitchen and bedrooms stay dry. Residents and those renting a vacation cottage have to walk up and down steps every time they go outside, though they can park their cars in the shade under the house...

Moving the house away from the flood reduces the risk of damage. On the opposite side of Virginia, the entire town of Grundy in Buchanan County was moved to the other side of the Levisa River. The Federal government and Virginia Department of Transportation (VDT) jointly financed a large percentage of the $28 million cost of creating a new townsite above the 1977 flood elevation, and rebuilding US 460 to serve as serve as a levee to provide flood protection . The project reduced the threat of future property damages from major floods each decade by moving Grundy both vertically and horizontally, rather than by controlling the water.

red polygon indicates site for new Grundy, before move across river
red polygon indicates site for new Grundy, before move across river
Source: US Army Corps of Engineers )

In the last 50 years, hurricanes coming through Virginia have flooded shoreline communities, but the greatest damage has occurred far inland from Hampton Roads.

Every year or two, it is normal for a storm to dump rain so fast that it causes minor flooding. Every century, it is normal to experience a particularly strong storm. When 10-12 inches of rain falls in just a few hours, the water piles up faster than the streams can drain the watershed. The "100-year storms" are not guaranteed to occur just once every century, however - they have a 1% chance of occurring each year. Virginia had two 100-year storms just three years apart, when hurricanes Camille (1969) and Agnes (1972) brought massive amounts of water into the atmosphere over Virginia.

The historic Floods and Droughts in Roanoke, Virginia and the impact of Hurricane Camille in 1969 show that rainfall patterns are not steady. A "100" year flood is expected once every 100 years on the average, but there is no guaranteee of a 100-year gap between the floods. Every year, there is 1% chance of the 100-year flood flood occurring. A 1,000 year flood has only a .1% chance of occuring. During Tropical Storm Lee on September 4, 2011, seven inches of rain fell in just 3 hours at Fort Belvoir in Fairfax County. That may have a 1,000 year flood - but it could occur again at any time.1

Cub Run in Fairfax County, bankfull but not flooding after Tropical Storm Lee in September, 2011
Cub Run in Fairfax County, "bankfull" but not flooding after Tropical Storm Lee in September, 2011

In the last decade, study of such floods are revealing how important they are in shaping the topography of streams in the Blue Ridge. We are discovering that streams are "underfit," meaning that the erosive power of the small streams does not appear to be sufficient for carving such wide valleys and moving the giant bolders in the streambeds. However, once every few centuries, it appears that very-localized storms can drop so much rain in one watershed that one storm can transform hillsides for centuries.

In 1995, a 500-year storm reshaped Paine Run in Augusta County, Staunton River in Madison County, and Moormans River in Albemarle County.

Stauton River in Madison County, south of President Herbert Hoover's getaway at Rapidan Camp
Stauton River in Madison County, south of President Herbert Hoover's getaway at Rapidan Camp
Source: US Geological Survey, Fletcher 7.5x7.5 topographic quad (2010)
(NOTE: a stretch of the Roanoke River between Smith Mountain Lake and Clarksville is also called the Staunton River, which can be quite confusing. The "Staunton River" shown on the map above is in Madison County. If your Virginia Atlas and Gazetteer is getting dog-eared by now from finding the locations of these place names used over the last month in this class... good. You're getting a good return on your investment...)

Floods are natural events; we can't prevent them all. Debris flows and catastrophic landslides are reshaping the Blue Ridge and the Appalachians, as water works its magic on the landscape. The gradual drip-drip-drip of everyday rainstorms causes little erosion. The gradual erosion from chemical and physical weathering, caused by summer thunderstorms and winter freezing and thawing, will inevitably reduce the Blue Ridge to a flat Piedmont - but each decade or two, somewhere in the Appalachians, a big storm will move a chunk of the mountains downhill towards the Atlantic or the Gulf of Mexico. The rare (every 500-years or so, on the average) events are literally the earth-movers where we can see mountain-shaping happen overnight. The "singularities" of rare events punctuate the normal pattern of gradual erosion and speed up the process of flattening the mountains...

As described in a Resource Assessment of the June 27 and 28, 1995 Floods:2

Although bedrock exposures are abundant along the ridge crest and on Old Rag Mountain, most of Shenandoah National Park is covered in a thick mantle or regolith that includes: soil, weathered rock (saprolite), transported debris (colluvium), soils and rock reworked by streams (alluvium), large areas of broken rock formed by freezing and thawing of water (blockfields), and remains of ancient landslides and debris flows (stone streams)... Heavy rains cause sheet washing and thick mantles of soil and saprolite slide or creep downhill toward streams. Streams erode colluvium and carry material further downhill. Long continued accumulation of colluvium in the upper reaches of streams or hollows promotes the sudden onset of landslides and debris flows during heavy rains.

After the storm:3
Bubbling mountain streams, once 15 to 30 feet wide and shaded by streamside trees, were "blown-out," resulting in 150 to 200 foot-wide channels filled with rocks, boulders, and debris dams of de-barked trees.

There's one small item to remember, however. The Blue Ridge has not been flattened yet, and not only because the billion-year old granitic core (and 600-million year old lava coating) resists erosion. The mountains are still being uplifted, perhaps by whatever geologic force that is moving the eastern part of the continent towards the west at the speed your fingernails grow, and the particles of quartz are still eroding and washing downstream. The Cheerios are being stirred even now; don't assume that erosion will win every battle, and that the Virginia mountains will have eroded to a lower height at the end of your lifetime.

Links

References

1. "National Weather Service: Ft. Belvoir rain was more than 1 in 1,000 year event, 'off the charts'," Washington Post, September 12, 2011, http://www.washingtonpost.com/blogs/capital-weather-gang/post/national-weather-service-ft-belvoir-rain-was-1-in-1000-year-event/2011/09/12/gIQA7PtRNK_blog.html (last checked September 18, 2011)
2. Karish, John, Blount, Tom, Krumenaker, Bob, "Introduction," Resource Assessment of the June 27 and 28, 1995 Floods, Natural Resources Report NPS/SHEN/NRR-97001, National Park Service (Shenandoah National Park), April 1997, www.nps.gov/shen/ps/nr/mi/fr95/fr95.htm (last checked September 15, 2002)
3. Resource Assessment of the June 27 and 28, 1995 Floods


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