West Chelsea will flood every time the Hudson River takes in more water than it can hold, especially seawater, which the Hudson mostly is anyway. We conjecture that surge-flooding as a consequence of storms will happen more often in the future, though how often no one can say. FEMA called Sandy a “Hundred Year Flood”. Comparing their predictive maps at the time with the actual event, it was worse than forecast. So much worse that FEMA has since redrawn their flood-risk maps to include much more “dry” land east of Tenth Avenue.

Still, Frankenstorm Sandy was an unusual confluence of conditions statistically unlikely to be repeated: record-low barometric pressure that generated a record-high storm surge; a surge that hit during both high tide and a lunar high tide; a monster storm that moved huge quantities of water despite relatively low wind speeds; an aberrant last-minute hard left-turn that funneled the surge into New York harbor; on top of sea-levels that have risen locally several inches during the last century.

Surge-tide flooding near the scale of Sandy has only been seen a few times in the past, notably in 1624, 1788, 1821, and 1893 and to a lesser degree in 1953, 1960 and 1992. The chart below, courtesy of The World Climate Report with 2011 and 2012 added here, illustrates the frequency of recent storms and intensity historically. Since record-keeping has improved over the past 100 years, it’s difficult to say with more certainty than conjecture that storm-surge flooding will happen more often or when it will happen next, but it is apparent it definitely will happen again.

~WCH_StormSurge_NYC_FIN

 

~WCH_dry_element

 

West Chelsea is vulnerable for the same reason most Zone A areas are: they’re “made land” by filling an area with dirt once occupied by water, landfill. They are difficult zones to protect from flooding because they are lower than the natural land and usually covered with buildings. West Chelsea is further complicated by the history of Manhattan’s madcap development, the unique underwater topography just off-shore and the terrain of the original 1609 shoreline. With its rolling hills, rocky shores, streams and salt marshes, the landscape of what we call Chelsea must have been astonishingly beautiful 400 years ago, though the gallery district would have been mostly in the Hudson back then. Wild Mannahattes was quickly “tamed” by early New Yorkers. In their frenzied rush to grid and level the island without the benefit of environmental impact studies, an inconceivable concept in the 19th century, they bequeathed to us the inundated galleries of October 2012.

 

Archaic-Shoreline-w-streets

 

The map above, courtesy of The Welikia Project, depicts the original 1609 coastline superimposed on the street grid we know as West Chelsea. The forested band that breaks up the street grid reveals the higher elevations of a band of small hills between 20 and 30 feet high, populated at the time by Lenape natives and plentiful wildlife. During the 18th century, the family of Clement Clark Moore acquired the land, deforested it and traded the red oak, chestnut and maple trees for apple orchards.

WCH-Chelsea HouseThe family mansion “Chelsea House” (left, courtesy of the NY Historical Society) sat on a hill on the south side of present-day West 23rd Street, between Ninth and Tenth Avenues, overlooking the Hudson River to the west. The landscape sloped steeply down from the hill to the river, terminating in low marshes around West 18th and Tenth Avenue. The slope was more gradual northward, which had small streams that had been depositing sediment for centuries as they drained low tidal marshes, the light-green area at the top-center above, a feature now occupied by the Post Office facility, West Chelsea North and where Hudson Yards and Midtown West are being developed.

By the 1820s, as a result of the Commissioner’s Plan of 1811, “street graders” were carving streets straight through hills and leveling valleys as they moved relentlessly forward with the grid. The leveling of the area was accomplished in phases over the next 40 years. The first dirt to be “infilled” into the Hudson extended the shoreline just beyond Tenth Avenue south of West 23rd Street, bordering the river at mid-block. By 1835 the hills and orchards were platted into blocks and lots over the newly made land and homes were being constructed by well-to-do New Yorkers.  By the time of Mr. Moore’s death in 1860, the area was fully built and extended to Eleventh Avenue, which was not much more than an access road and train tracks servicing the distilleries, factories and piers crowding the waterfront.

It wasn’t until the early 1900s that the shoreline was filled to it’s present location, primarily with infill brought from excavating the foundations of the London Terrace apartments in the 1930s. In the process of expanding the shoreline, widely varying river-depths were encountered within the 12-block area. The river bottom just off-shore above West 25th had an average estimated depth of about 3 feet, which gave the river-bottom contour a more gentle slope all the way out to Twelfth Avenue, where the water was about 24 feet deep. The sub-riverine terrain south of West 25th dropped off more sharply with a steeper slope, at an average depth of 9 feet closer to shore, reaching 20 feet at about half the distance than farther north.

This required more dirt to fill in the deeper water, which settled as it became saturated with water and compacted. This compacting process lowered the elevation of the area containing the most landfill, so that today the settled area south of 24th Street is more downhill than West 26th and above, contributing to the more severe inundation experienced there in 2012.

You can see the drop off clearly in this bedrock elevation map (data courtesy of NYCEM) superimposed over the 1609 coastline map, below. The bottom slopes gradually where the lines are spaced further apart, and more steeply where they’re close together.

 

~WCH_Archaic-Shoreline-bedrock

 

Gradually the fashionable 1840s homes along the river gave way to manufacturing plants, railroad tracks, housing for immigrant workers, and early 20th-century warehouses. All that remain are the homes in the Chelsea Historic District east of Tenth Avenue and a few random buildings like 502 West 27th Street. Concrete foundations have stabilized the settled landfill, newer buildings are generally more elevated than older ones. Developed streets with masses of utility conduit, pipes and unseen infrastructure feeding newer construction are higher than streets serving older buildings. The water-table is a mere 2′ to 6′ below grade, depending upon the street.

17th-century Mannahattes and 400 years of environmental meddling cast a long shadow over 21st-century Manhattan.

The two maps below illustrate this. On the left (data courtesy of the USGS), the extent of surge-tide flooding in West Chelsea is shown in blue superimposed over the contour of the bedrock. On the right (data courtesy of The New York Times), the range of water depths due to surge-tide flooding are shown block-by-block in West Chelsea.

Comparing the two maps, its interesting to note the surge-tide flooding and the observed water depths from flooding closely conform to the bedrock contours: deeper in the south, more shallow in the north.

 

~WCH_FloodExtent_Depths_Rev

 

When I walked West Chelsea in the days immediately following Sandy, I was surprised that the highest watermark I measured, the bathtub ring left by the Hudson, was almost eye-level to my 6’4″ frame in some places on West 19th, yet just over knee-high on West 25th, with little or no flooding north above West 27th Street. Water depths were greater or lesser from one end of the same street to the other, and between the north and south sides of the street.

The south-north differential in elevations between West 18th Street and West 29th Streets from Tenth to Eleventh Avenues produced varying degrees of flooding, as shown in this schematic, with high-water marks as recorded on site (not to scale).

 

WestChelsea-HW

 

In 2013, FEMA revised its flood-risk maps, below, reflecting the “new normal”.

 

Archaic-Shoreline-w-RevFEMA2013

 

 

~WCH_dry_element

 

We can take a page from how flooding is handled in the Midwest, as seen in these images from Minnesota and Iowa (left: FEMA Photo Library, by Andrea Booher / right: USAToday, by M. Spencer Green AP). The televised helicopter shots of vast tracks of farmland transformed into muddy brown lakes out west, that we see year after year, say more about the resilience of the farmers who deal with it, than about the power of the Mighty Miss and Mo.  Folks in NYC have a unique set of challenges living in a city this size, such as materials storage and transportation, acquisition of materials and limited space in which to maneuver.

 

So, what does all this mean? One thing is that we can’t stop West Chelsea from surge-flooding any more than midwest farmers can hold back their rivers. Or, conversely, we may be able to stop surge-flooding here the same way they hold back rivers out there. The next post examines what we can learn from the Heartland. Read Part II here.