After the Deluge: Flooding on New Providence

by Larry Smith

Several years ago I sat in on a meeting where a government planner displayed some startling maps. They showed the areas of New Providence that would be severely affected by storm surge from a direct hit by a major hurricane.

A category three storm would heavily impact the airport and Lake Killarney area, the entire south coast (except for Clifton) and major over-the-hill housing areas. A category five storm would flood the entire island, except for the coastal ridge running from Clifton to East End and the central ridge running from Marathon to Lake Cunningham.

"These maps show the potential risk areas from a major hurricane strike," the planner explained. "But they also demonstrate that we don't have good high land to build on, and the land that is available now is not desirable."

According to the 2004 Water Resources Assessment of the Bahamas produced by the US Army Corps of Engineers, flooding has always been a major concern during our summer rainy season. But such conditions are becoming more frequent and severe due to the stronger storms and sea level rise produced by climate change.

The Met Office has indicated that 1 1⁄2 inches of rainfall in six hours or less, typical of summertime, can cause flooding in New Providence. Once the ground becomes saturated flooding occurs quickly, since seepage to the sea is a slow process and is the only natural way to deal with the problem.

"This flooding contaminates the soil and groundwater with seawater, sewage, petroleum products, pesticides, as well as any other objectionable substances that are not adequately secured or stored," the report said. In fact, it is estimated that most of the water recharging Nassau's fresh water lens today has passed through a septic tank, making the groundwater heavily polluted.

The report said wetland areas played a beneficial role in pollution and flood control: "Wetland retention and creation offer a more sustainable approach to flood management." 

But destruction of wetlands and building in low-lying areas has only exacerbated the problem. "In New Providence," the report said, "land is expensive, and wetlands and seasonal ponding areas are being developed without concern of floods, until the rainy season hits. Building on low-lying areas, as well as urbanization, which limits space for storm water drainage and decreases natural infiltration areas, intensifies flooding issues."
 
One of the lowest-lying areas of the island is Pinewood Gardens, a housing estate just north of the Malcolm's Creek wetlands on the southern coast. This area's poor drainage has been made worse by  high-density construction (often without proper water and sewerage infrastructure) as well as by "indiscriminate dumping and encroachment" along Malcolm's Creek. 

In 1972 the land now known as Pinewood Gardens was subdivided into 3500 lots. The principal behind the original development was a foreign investor named Robert Petrie. When Petrie's company went into liquidation, the subdivision was acquired in 1983 by Arawak Homes, headed by former politico Franklyn Wilson.

The Pinewood area - and other eastern districts - received more than a dozen inches of rain over the 24-hour period beginning on Tuesday afternoon (May 21). To put that in perspective, over the same period, a total of 4.35 inches of rainfall was recorded across the entire island. And for the past 30 years rainfall on New Providence has been under 5 inches on average for the entire month of May.

The Met Office alerts were more in the way of announcements rather than warnings. The first severe weather alert was issued at 3.15 pm on Tuesday, warning of thunderstorms, wind gusts, lightning and tornadic activity. Out east, the storms ramped up almost immediately, with continuous heavy rain until the next day.

At 6 pm the Met Office warned of "moist and unstable weather…for the next 24 hours". There was no mention of flooding, which affected thousands of people living in congested inner city neighbourhoods, some of whom had to abandon their homes. Meanwhile, ZNS complained that NEMA's office was closed on the night of the storm and - strangely - the government made no official statement in the immediate aftermath of the disaster.

This is something we cannot comprehend. It is incumbent upon the leadership of any country to take account of serious events that negatively affect large numbers of citizens. Not only that - it just makes plain good sense. The prime minister should have appointed Pinewood representative Khaalis Rolle to oversee matters from a political perspective, while broadcasting an early statement on the storm aimed at reassuring the public.

But in reality there is not much that can be done about the flooding - too much of New Providence is just too low-lying. People are being allowed to build in these areas, and it will only get worse as sea level rises. The best solution is to restrict construction in extremely low-lying areas, require build-up of lots or homes, and install water and sewerage systems to avert health issues. 

According to architect Pat Rahming, "where and how you build are decisions that should be approached rationally. If the land is low it means you should build with the realization that at times it will flood, as the people of my old neighbourhood did. We should begin the design of a building by finding out about the site we are building on. If you want a cheap lot, it will come with some expensive problems, unless you recognize why the lot is cheap in the first place and design accordingly."

Such considerations will become more important as time passes. As the Corps report said, "Rising sea levels are expected to erode the land, expand wetlands, and cause migration and loss of mangroves. Flooding from extreme rainfall events and movement of seawater inland, due to rising sea level and eroding coastal defenses, will be experienced."

Traditional drains don't work when there is no relief to exploit. Drainage wells do work but there has to be enough head to drain the water, and they have to be serviced regularly. High tides can also reduce the head to the extent that the wells do not work.

In 2004 there were about 400 disposal wells  in Nassau and Freeport drilled to a depth of 150 feet or more. These wells work due to the high permeability of the limestone substratum, but fail once the floodwaters reach the head of the wells.

Several studies have been done on this issue over the years, but the solutions other than deep drainage wells are considered too costly. These usually require collection systems with pumping stations to move the water to another location where it can be more effectively disposed of. 

A drainage canal to the sea at South Beach has also been proposed. But apart from the huge cost (including the acquisition of existing land and houses), this would also be a two-edged sword because high tides, storm surges and rising sea levels could make the problem worse without construction of expensive flood barriers. 

"There are no practical solutions to flooding for places like Pinewood Gardens," one engineer told me. "Lots of Holland is below sea level but they live on clays that keep the sea out of the protected areas. Our limestone is far too permeable and as sea level rises it will come up under our feet. Areas like those around Lake Killarney will be totally submerged as sea level rises, which poses a real threat to our wonderful new airport".

 

Comments

[larry smith]

NEMA Warning issued today (May 28): Residents throughout the archipelago should brace themselves for excessively prolonged rainfall throughout the day and lasting through Friday night.

A broad area of moist unstable air extending southwest to northeast from the Caribbean Sea will generate heavy rainfall and thunderstorms across the entire archipelago. Conditions will be favourable for flash flood warning in low-lying areas.

Winds are expected to reach 30 miles per hour or more in embedded thunderstorms. Tornadoes are likely to be spawned from the thunderstorms. A severe thunder storm watch or warning with the possibility of tornadoes will be issued when it is indicated by the weather radar or sighted by spotters.

The Department of Meteorology is appealing to residents in flood prone areas to be prepared to evacuate at a moment’s notice. Residents are advised not to drive, walk or play in flood waters as the depth can easily be misjudged. All efforts should be made to protect lives and property.

All advice and notifications by the National Emergency Management Agency (NEMA) should be heeded.

[larry smith]

http://www.bahamapundit.com/2010/03/the-impact-of-rising-sea-levels-on-the-bahamas.html

[larry smith]

"For example, the Lynden Pindling airport now being redeveloped at great expense will flood as the water table rises in response to higher sea level. The College of the Bahamas in Oakes Field is barely a foot above sea level and already floods when it rains, so this will only get worse. In fact, experts say that inland inundation and salinisation will become huge issues because our groundwater is tidal and directly linked to sea level.

And of course, these forecasts do not take account of storm surges or other coastal effects. So they give only a partial picture of vulnerability."