Filters and Sponges

The undecided edge between land and water teems with life.  Wetlands and marshes, reefs and mangrove forests, estuaries and tidal flats demonstrate the physical result of ebb and flow.  They embody the indeterminacy of the physical status between liquid and solid, dissolving into vapor on foggy mornings.  Productive ribbons along coastlines create shadow nurseries for finfish and crustaceans, filter suspended material, stabilize and trap sediment, and assimilate dissolved nutrients.  The living plants here are unusual species, halophytes, which thrive in salty conditions; botanical amphibians that can survive choking mud and desiccating heat to naturally break storm surge and absorb wave action.
Coastal wetlands are bioshields for the continental edge. Marshes and maritime forests protect coastal development and infrastructure from hurricanes and other storm surges. Where they do not exist, such as in south Florida, property damage caused by storms is much more devastating than in Louisiana, where a buffer of coastal wetlands separates cities from the Gulf of Mexico. “By serving as a buffer to destructive marine forces and the episodic impact of storms, Louisiana’s coastal wetlands help to protect the vast infrastructure of nationally significant oil and gas facilities.”  The marshes protect other infrastructure, too, such as roads – and more than 400 million tons of water-borne cargo, including ten federal navigation channels that provide access to port facilities handling 25% of U.S.exports.  
Most threats to the wetlands seem like a personal challenge to continued occupation.  A New York Times article explored the barrier islands of North Carolina, and the changes due to rising sea levels – 7” in the last 100 years, with an additional 2’-3’ rise by 2100 – and the communities affected by coastal erosion.  “Dr. Riggs predicted, major storms will turn many parts of the Banks into underwater shoals or flats that are above water only at low tide.

If Highway


12 were abandoned and the islands allowed to find their natural equilibrium, he writes, the resulting villages would be ‘situated like a string of pearls on a vast network of inlet and shoal environments.’”[i]  Loss of the connecting roadways would lead to isolation, accelerating decay, and the abandonment of the community, first by the hundreds of thousands of tourists each year, and then by the inhabitants.

It is a constant battle against the sea to postpone but never halt completely the shifts along the edge.  The constant toil of dredging and beach renourishment, of building levees, breakwaters, seawalls, groins, piers, jetties, and dunes signify the impermanent quality of the line between water and land.  With the added pressure of climate change, chances of erosion and flooding increases.
One way to respond to the threat of rising waters is to plan now for their eventual extent by creating space along the coastlines for water to occupy.“Immediately after the tsunami (in 2004), the Sri Lanka government through its Coast Conservation Advisory Council declared a conservation buffer zone along 1,000 kilometers of coastline prohibiting rebuilding and new construction.  The buffer varied in width: in the south it was 100 meters and along the east coast it was 200 meters.  Many viewed the ban, an attempt to safeguard the country from future flooding disasters, as a government land grab to promote the lucrative tourism trade.  The resulting public outcry ultimately caused officials to relax the blanket post-tsunami ban; it is now 15 to 40 meters.”[ii]
Managed retreat from the coastal edge was a topic during the post-Katrina political discussions, but was abandoned faster than a flooding building.  The result illustrated a linear park at the western edge of Mississippi, a green buffer with boardwalks and bayous at the lowest lands on the coast, and an extension of the already-protected cheniers and marshes stretching to the Pearl River which forms the dividing line with Louisiana.  But residents were not ready to hear that their beachfront lots were the target of a buy-out program to avoid future government flood program payments… and communities didn’t look forward to the loss of the highest property tax dollars, plus the prospect of inheriting maintenance on the empty lots.  (The program proceeds quietly and voluntarily, the impetus fueled by older people who just cannot afford the emotional and financial toll of life on the beachfront, accompanied by a sharp rise in insurance and construction costs.)
Wetlands across the globe show evidence of a system well into collapse.  In the 1600’s, the continental United States had 220 million acres of wetlands, but by 1997 there were less than half of that, a total of 105.5 million acres remaining.  Only about 5% of these wetlands are along coastlines; the remaining 95% are inland marshes, meadows, forests, and swamps.  
At inland locations, most floods come from the clouds above.  Planning space for stormwater runoff is the reasoning behind requirements for retention ponds as an alternative to structured drainage, but most examples fail to contribute to the aesthetics of the community, or to ecological functioning.  Ringed with chain link, home to invasive species and windblown litter, retention ponds at the edge of big-box parking lots may provide space for water to infiltrate, but they certainly don’t come close to replicating authentic wetlands, which (along with tropical rain forests) are among the most biologically productive habitats on earth, supporting nearly 200 species of amphibians, 5,000 plant species, one-third of all native bird species, and a similar diversity of plant life.
After devastating spring floods in 2011, people along the Mississippi River better understand the value of wetlands as flood control.  Because of the loss of wetland areas for catchment and percolation, damage to buildings, infrastructure, and industries along the river banks was estimated at $6 to 9 billion.  The bottomland hardwoods and wetlands along the river could store at least 60 days of floodwater in the early 19thcentury, but can now store only 12 days of floodwater.  This loss of capacity led to dramatic rises along the hard-edged river, with more episodes expected in the future.
Drainage woes plague many towns, especially in flat areas, coastal plains, or valleys threatened by flooding.  Storm drainage, whether surface or subsurface, must handle the overflow and keep water moving to safe discharge areas.  Constructed culverts and drains are durable, but also susceptible to obstruction.  Surface drainage, such as swales, filter non-point source pollutants and handle higher capacities during significant rainfall. 
Water plazas in cities are the newest idea for creating a multi-functional open space to be used as a park, a grassy field, a stepped amphitheatre, playground, skate park or labyrinth in fine weather; giant catchment areas, mega-sponges, when the rains arrive.  Rotterdam, a city where the elevation is below sea level, is flirting with the idea.  No single catchment area is big enough – even Central Park cannot drain New York City alone.  A system of smaller units, spread across neighborhoods and linked, will help to offset the accelerated and intensified stormwater in cities.

The solution may be as simple as replacing structures with soils.  Not to form dead-end canals which lead water into sites, but to create more places for percolation.  Water in the city reintroduces habitat for wildlife, and provides space for children to dream, and adults to meditate.  Water, that least mysterious element of two hydrogen molecules and one of oxygen, has secret powers within the urban environment that can lead to more vibrant and congenial social spaces where neighbors talk, and adults play, and children imagine entirely different worlds. Meanwhile, the focus of all this dreaming and thinking and planning, the water silently percolates through sand and clay to the infinite cracks and fissures below, swelling the earth with fecundity once more.
Green filters are “non-structural” methods for restoring capacity along coastal edges and within cities to handle the increasing demands of storm waters: storm surge, flood, excessive rain, tidal flow, and sea level rise.  Creating green buffer zones allows water to safely pool before affecting the lives and works of man.  Until recently, wetlands were seen as wastelands.  Their productivity remained unmeasured – they occupied the undervalued edges, the barriers to contiguous development patterns, and the places of last resort for the poor and the latecomers to the urban environment.  The pressures of low densities and high population growth have nudged growth ever outward, knocking down the mangrove forests and bottomland hardwoods, filling estuaries with concrete and asphalt.
Wetlands are critical to humans’ continued occupation of the fragile edges between land and water.  They endure the most extreme conditions of flux, and provide greater value (by diverting, filtering, recharging, and evaporating stormwater) than much of the grey infrastructure which underpins cities.  Wetlands may be the most valuable lands on earth, without a single “improvement” made by man.  Man should not build with permanence on their indeterminate ground. 

[i] “A Lifeline Built on Shifting Sands.”  CorneliaDean, New York Times, 5 March 2012.
[ii]“Lessons from Sri Lanka” in Birch, 247.