La valeur des terres humides urbaines : inestimable

Pourquoi les villes de l'Amérique du Nord dépensent-elles des dizaines de millions de dollars pour préserver leurs terres humides? Selon l'auteur Mark Darcy, la raison réside dans le fait que les coûts de remplacement des fonctions remplies par les terres humides comme la construction d'infrastructures pour les eaux de ruissellement et le traitement des eaux usées sont de l'ordre de 10 fois plus élevé.

En effet, la nature satisfait gratuitement les services que les villes ont le plus besoin comme : la lutte contre les inondations, l'alimentation des nappes souterraines, l'emmagasinage et la purification de l'eau, les habitats pour la faune et l'avifaune, la récréation et le tourisme. Les terres humides forestières captent, accumulent et purifient l'eau. Les terres humides constituent aussi notre assurance contre les changements climatiques.

Les terres humides urbaines valent donc la peine d'être préservées. Nos gouvernements ont donc l'obligation de ne pas payer pour des solutions inférieures lorsque la nature fournit des solutions inestimables gratuitement.

The Value of Urban Wetlands – Priceless

Mark D'Arcy
November 2008

hy do cities across North America spend tens of millions of dollars on preserving their wetlands? Because the costs of replacing the function of wetlands with stormwater and water treatment infrastructure are on the order of 10 times more.

Nature does for free what cities need most: flood control, groundwater recharge, the storage and purification of water, habitats for birds and animals, recreation, and tourism. Forested wetlands capture, store, and purify water. They provide habitat to a diverse group of plants and animals, including migratory birds. They represent the water we drink, the air we breathe, and the outdoor recreation spaces we enjoy. Urban wetlands act as a sponge and are integral to the watersheds of cities. Even in cities that get their drinking water from lakes or rivers, the integrity of watersheds surrounding the city help filter the urban runoff before it enters these bodies of water.

Larch swale in the UNB Woodlot, Fredericton.
(Photo: Mark D'Arcy)

Stormwater pipes are no substitute for intact wetlands and forests within our city limits. The importance of wetlands for flood control cannot be understated. One acre of wetland can hold 1-1.5 million gallons of floodwater. In June 2008, the 500-year flood in Cedar Rapids, Iowa, was a warning to the collision course between climate change and the loss of upstream wetlands to development. A severe rain event of 46 cm (18 inches) in 10 days saturated the soil to its carrying capacity and then the runoff swelled the banks of Cedar River. As wetlands are infilled and paved over, their rainfall capture ability is lost.

Wetlands are our insurance policy against climate change. The severity of runoff from heavy rain events is affected by the size and layout of wetlands and the surrounding vegetation. The Toronto Ravine System is not only a recreational jewel but it cushioned the city from catastrophic damage in October, 1954, when Hurricane Hazel dumped 12 cm (5 inches) of rain on the city in a day. Saint John received the same amount of rain in 2008 with Hurricane Hanna, resulting in severe flooding. The City of Saint John is now studying its drainage patterns in order to limit development in areas above which flooding problems exist.

But New Brunswickers can learn from these floods. The UNB Woodlot is a 3800-acre forested wetland situated at the top of Fredericton and is equal to the size of the south side of Fredericton and Lincoln in the valley below. Just like an egg cracked over a person's head, rain captured by the UNB Woodlot supplies the headwaters and tributaries of Corbett Brook, Phyllis Creek, Garden Creek, and Baker Brook watersheds, and smaller watercourses that flow through Fredericton and towards New Maryland. Not only do these forested wetlands contribute to our aquifer - the sole drinking water supply for the City of Fredericton - but they act as a giant sponge during severe rain events and slowly release the water to surrounding forests and aquifers that supply our drinking wells.

Pitcher plant in Regent Bog, UNB Woodlot, Fredericton.
(Photo: Mark D'Arcy)

If this water absorbing capacity of wetlands is fragmented or lost altogether by development, the taxpayers of Fredericton would be saddled with the cost to build replacement infrastructure for storm water management. Unfortunately storm water pipes and retention ponds are an inferior solution so we would also be saddled with flooding costs.

In this electronic age, the natural world is important for the healthy emotional and intellectual development of our children. Urban forests are magnets for exploration, adventure, hiking, mountain biking, and learning about nature. Wetlands are natural teaching classrooms for our children. Thousands of grade 4 children have visited the Ducks Unlimited Corbett Brook Marsh in the UNB Woodlot to learn about the diversity of plants and animals in these unique habitats. Wetlands also serve as outdoor labs and research for students in colleges and universities.

Beaver lodge in the UNB Woodlot, Fredericton.
(Photo: Mark D'Arcy)

The future climate scenarios for the Fredericton/New Brunswick region are sobering. Conservative computer models by Environment Canada predict major changes this century: severe rain events are expected to increase in number and severity with climate change; a 30% increase in winter precipitation; twice the amount of winter runoff; significant increase in freeze-thaw cycles (in winter); lower summer/fall runoff by one-half; a 3.1 to 5.9 *C increase in mean winter temperature; and a 2.4 to 5.1 *C increase in mean spring temperature. The combination of severe rain events with more winter melt will increase our risk of floods. Rain-saturated soil provided the perfect conditions for the great spring flood of 1973 in New Brunswick. Also, significant increases in rainfall and wind speeds of hurricanes will both be fueled by warmer water temperatures in the Atlantic. Notable hurricanes that caused flooding in New Brunswick include Carol (1953), Edna (1954), Gladys (1968), Belle (1976), Bertha (1996), and most recently Hanna (2008). And as this article is written, Hurricane Kyle was forecast to deliver up to 10 cm of rain over New Brunswick - a day after receiving already 5 cm of rain - but fortunately we did not take a direct hit.

Urban forested wetlands are well worth preserving. Our governments have an obligation not to pay for inferior solutions when nature provides a priceless solution for free.