There has been a lot of talk about how we protect West Chelsea from the next inevitable inundation. Any flood-mitigation project for West Chelsea will need to be compact enough to be stored for long periods cheaply, strong enough to resist 72,000,000 gallons of water, yet simple enough to fully deploy within 24 hours.

With a Midwest river flood, you can confidently predict that high-water seven days upstream is going to hit you downstream in seven days. With a hurricane, no one is certain of anything until hours before. With a midwest flood, high water can persist for days under sunny, blue skies. In a hurricane, the waters rise then recede within a matter of hours under intense storm conditions.

Sandy's Track courtesy NOAA.

courtesy NOAA.

Consider Sandy: while it was predicted four days in advance that she would make a left over the Jersey shore, no one was sure where or when landfall would occur until it actually did, leaving us fewer than 24 hours to prepare.

Any solution will require extremely fast reaction times, instantaneous access to materials, tight organization and coordination of resources. That’s a tall order when it involves over 150 individual property owners, 335 galleries and other businesses, 3.064 million square feet of surface real estate, and gale-force wind and rain, all while satisfying the permitting requirements of the City.

$16,000,000 : Sandy’s Cheapest Seats.

We have to do something. With 550,000 SF of ground floor gallery space, at about $30/SF to repair the physical space, the cost just to get the 130 ground floor galleries open again was over $16,000,000 collectively. This does not include loss of art, records, equipment and furnishings nor general infrastructure recovery to the buildings themselves. Nor does it include loss of business costs, not just to ground floor galleries, but to upper floor neighbors when the power went off for up to two weeks because salt-water blew out basement transformers. The actual cost was many millions more.

West Chelsea needs to be hardened, but how?

New York City is drawing up brilliant plans for protecting the city in a probable future of rising tides and worsening storms, but even in the best of cases, implementation is tens of billion of dollars, and years if not decades away. If surge-floods hit us again next year or in three years, or this year, until those oyster beds, artificial wetlands, seawalls, Verrazano Narrows Floodgates or whatever the end solutions turn out to be, are put in place, West Chelsea is on its own.



West Chelsea Dry.

Before we can have any discussion of the methodology and material solutions involved, we have to determine the approach, the strategy for flood-mitigation. Any discussion needs to be made in the context of real estate since we’re talking about property and buildings, some more porous than others, some 19th century others dazzlingly new, all interconnected.

First, manageability is key, which decreases as complexity increases, as in the diagram below:


Second, the approach needs to address the core causes of damage to property in a flood event:

Over-flow – the inflow of water into the property through the facade and strore-front as a a direct result of high-water above grade.
Back-flow – the inflow of water into the property through sewers, drains and utility conduits from increased water pressure below grade in overwhelmed street drains and lines.
Hydrostatic Pressure – inflow of water into the property, usually through the basement floors and walls, caused by increased water pressure in the water table, which is high in West Chelsea under normal conditions. Seepage through concrete is more likely to occur the longer standing water is present, which is shorter during a hurricane.

The Individual Tenant Level.

The Tenant level is by far the cheapest strategy, requiring at most a few thousand dollars worth of plastic sheets, a few sandbags, duct-tape, plywood, tools and a generator. Duct-tape between the door frame and the door on the the outside will stop over-flow since the windows are waterproof. Back-flow and hydrostatic pressure are more difficult to address, though there are products that help, like this handy little item from Panseal, Inc. designed to mitigate sewage back-flow, shown below. But the water is still in the street. If the water is in the streets, its in the storm drains and sewers. With the water under pressure, it will flow under pressure into buildings through conduits and subterranean pipes unimpeded.

toilet Panseal

Even with giant toilet stoppers, this is still the least reliable, if for no other reason than there are usually several tenants occupying ground-floor spaces in a single building. If one Tenant doesn’t harden their space, or doesn’t harden it properly and floods, everyone floods. The demising walls between spaces are just aluminum studs screwed to the floor and covered with sheetrock, and sometimes plywood. They’re fireproof but not waterproof. Organizing all tenants would be difficult at best.


The Building Level.

A strategy of mitigation at the building level has essentially the same problems as described above, only larger. Adjacent buildings all need to conform to the same standards and even when they do, the water is still in the street. 511-541 West 25th Street experienced just how destructive even a comparatively small amount of water in the street can be. While they only had about 6″ of water in the basement, salt water had entered through the electrical conduits fed by the main electric line under the street, pouring over the transformers, knocking out power for two weeks, shutting down business.

A better example of the inefficiency of flood-hardening at this level: a building that was seriously impacted by Sandy is spending almost $1,000,000 to seal just that building with extensive modifications. As you can read in Part III, that amount represents 37% of the cost to barrier the entire district. If each of the 158 buildings in West Chelsea followed their lead, the total cost would balloon to over $150 million from $2.7 million, and the water is still in the street.

Hardening Entire Blocks.

The block level would be easier to barricade: by enclosing each of West Chelsea’s 15 blocks with dikes, we seal all the buildings and commercial units at once. But apart from requiring 26,000 linear feet – almost 5 miles – the water is still in the street. Preventing over-flow at all three levels is comparatively easy, but as long as water floods the streets, combating back-flow and hydrostatic pressure is exponentially more difficult.

Put another way:

consider that there are about 788,780 SF of streets and sidewalks in West Chelsea, roughly 26% of the total surface area. If the water is 3 feet deep, that’s 2,336,340 cubic feet of water in the streets, 17,474,823 gallons, which, at 8.34 pounds per gallon, weighs 145,740,024 pounds, or 72,870 tons of water.

That’s like having three USS Intrepids wash up in the middle of West Chelsea.

Now, factor in that sea water at 60˚ F exerts 0.44 psi per foot, and though the math may give you a headache, you can understand why the water should stay out of the streets.

From this it would appear there are two basic rules to successfully prevent surge-flooding in West Chelsea:


Rule #1 ~ Keep It Simple.
Rule #2 ~ Keep The Water In The River.



The Great Flood Wall of West Chelsea.


Why not construct a single mile-long barrier – 5,300 linear feet – from high ground in the south at West 18th, running north along Twelfth Avenue, to high-ground uptown at West 29th? One flood wall is the easiest to manage and deploy. Enclosing the entire district satisfies Rule #1, and more importantly, Rule #2 as well: It keeps the water in the river.

It also minimally impedes emergency vehicular traffic, leaving the West Side Highway unblocked with an easy detour of uptown Tenth Avenue and downtown Eleventh Avenue traffic west to Twelfth Avenue at 18th Street and 30th Street respectively. Traffic within the district could move around freely.

As this picture demonstrates, even with a perfectly functioning barrier that allowed no water inside the district, there is still pressure in sewers and storm drains, which causes geysering of water in the street behind the dike, on the dry side. geyseringSandbags could be used to dike manholes and drain grates, but the pressure – while a fraction of what inundated streets would produce – will still exist and would find another way out. The best strategy is to let it happen and manage it, by pumping the water out and over the dike, back into the Hudson. The cost depends upon the strength of the pumps and the water flow. An excellent calculator can be found here.


So, what’s the best material to use to construct this mighty barrier?

Down On The Bayou

The obvious barrier material is sandbags. Build a sandbag levee around West Chelsea, like they do down in Louisiana and in the Heartland. On the face of it sandbags present the least expensive solution, at around $600,000. However, a sandbag solution is DOA in NYC for several reasons:

1 – The U.S. Army Corps of Engineers doesn’t allow sandbag levees higher than 4 feet, ours would need to be almost 7 feet in places to mitigate Sandy’s worst affects.

That kills the sandbag idea from the get-go, but let’s follow it through because it demonstrates clearly the nature of the challenges we face.

2 – Even if they were permitted, the ratio of base to height of sandbag walls is 3:1, which means a 7 foot dike would need to be 21 feet wide at the base, occupying 1/3 of the street and sidewalk, suddenly an unwieldy structure built with almost three-quarters of a million sandbags. Here is how that’s figured:

  • Because the height of the dike at West 18th Street would need to be at least 7 feet, and only about 2 feet at West 29th Street, the average height of the barrier should be approximately 5 feet.
  • The base of the dike should be 3 times the height since water is heavy and doesn’t compress. For example, the force exerted by standing water on the base of a 5 foot dike is 310 lbs/square foot, running water more than that. The equation for determining the number of sandbags 12′ X 18″ X 3″ needed for a pyramidal dike whose base is 3 times the height is:
  • N = (3 x H)+(9 x H x H)/2 – per linear foot
  • where N = Number of bags needed; and H = the height.
  • So, a 5 foot high 3:1 flood barrier around all of West Chelsea, running 5,300 linear feet, would require 130 bags x 5,300 linear feet, or 689,000 sandbags.


  • Sandbags cost about $0.30 sans sand, or $210,000 for all the bags needed, including some contingency. Add $6,600 for enough plastic sheets to line the river-side of our dike.

3 – To be able to quickly deploy a sandbag levee would require easy-access to the sand. 72 sandbags contain about 1 cubic yard of sand , so 689,000 sandbags would require 9,570 cubic yards of sand. Sand weighs about 2,700 pounds per cubic yard, giving us 25,839,000 pounds or 12,920 tons to procure:

  • Sand costs about $8.00 per cubic yard. Sand adequate to the task would cost about $76,560.
  • How many dump truck loads would we need? A typical NYS-legal tandem-axel dump can carry 13 cubic yards, so 736 dump truck loads would be required. I’m not sure there are 700 dump trucks available at one time in the entire metro area, so let’s assume we could get 200 trucks to deliver that much sand within 24 hours. At $975 per day per truck, our cost for trucking the sand is $195,000.
  • 200 trucks each running 4 loads within 24-hours in hurricane conditions is hardly feasible, so sand would have to be stockpiled. To stockpile 13,000 tons of sand to deploy within 24 hours, when raw land in West Chelsea costs $600/SF and higher, is a huge challenge, as well as a huge pile. To give you an idea of how much sand that is, it would make a pile 200′ X 100′ X 32′ tall, looking something like this, potentially sitting there for years until it was needed:


In terms of logistics alone, sandbags are absurdly impractical. Yet there are other problems:

4 – Sandbags are permeable, absorbing water and compacting, which increases their sealing capability. They also function as unintended filters, trapping all the sewage, volatile hydrocarbons and nasty chemicals unleashed by the flooding as the water passes through them. One can’t just dump this used sand back on Jones Beach when the sun comes out; now it’s toxic waste that must be handled as an environmental hazard, greatly increasing the cost.

5 – Provided one sand bag could be filled and placed every three minutes, it would take 1,000 volunteers over 35 hours working non-stop to build a sandbag levee. In hurricane conditions, that’s not happening.

Sandbags are a messy, expensive, labor-intensive, and a hopelessly 20th-century means of barrier construction, wholly unsuitable for urban use. The good news is, there are other options being used out west with great success. The bad news is they all require sand or dirt, or are disposable and expensive. All except one.

The following table compares the base price of representative product types. The costs do not include labor, transportation and storage, or disposal costs.



The cheapest product, cheaper even than sandbags, is TrapBag, which has been used on Fire Island. Unfortunately, it as well as HESCO and RDFW all require earth-fill, which as has been demonstrated above, requires stockpiling somewhere nearby, a solution that will not work.

AquaBags, and a similar product called FloodSax, are interesting because they’re self-inflating: just add water to the SAP filler and… poof, instant sand-less sandbag. They require no earth fill. They stack flat when dry, facilitating compact storage until needed. The problem with this solution is they’re three times the cost of sandbags yet still share the same limitations: with a pyramidal cross-section when deployed they, like sandbags, are not permitted to be any higher than four feet, and they are a one-time-use product. True, we may not need them more than once, but if we do face increasing frequency of storms in the future, then we may as well dump $1.4 million into the Hudson every time we have a hurricane.

We need a cost-effective, easily stored, reusable solution that does not require fill. Only one product design satisfies all of those criteria: Flood Fence.

In Part III, we examine the fencing option, and how it can be implemented in the near future. You can read it here.