Saturday, February 17, 2018

Nature’s balancing act

Manzanilla’s delicate ecosystem...

MANZAN-DISTRUCTION--CHASE

COLLAPSE: A house on the Manzanilla-Mayaro Road which suffered the worst damage following the floods.

Mark Fraser


Manzanilla Beach is part of a barrier beach system about 20 kilometres long located in Cocos Bay, embraced by two prominent headlands at Manzanilla Point and Radix Point, and separates the Atlantic Ocean on the east from the Nariva Swamp on the west.

This “barrier beach” impounded the east-flowing rivers (L’Ebranche, Nariva and Ortoire) and created the extensive Nariva freshwater swamp—declared an international wetland reserve in 1993 under the Ramsar Convention (1988).

Nariva Swamp is the largest in the country and home to a wide variety of flora and fauna (including the endangered West Indian manatee) and forms a distinct wetland ecosystem.

The viability and longevity of this wetland depends on the maintenance of the Manzanilla barrier beach.

It is on this barrier beach that the Manzanilla/Mayaro Main Road and other coastal infrastructure have been built and impacted by the recent flooding event.

Manzanilla’s coastline is constantly changing as it is exposed to the dynamic marine processes of the Atlantic. The underlying geology of Cocos Bay is composed of sediments which form a wide gentle low-lying plain.

The inherent geology as well as the high energy marine environment of the Atlantic have created conditions which promote coastal erosion.

Because of the fairly low topography of the Manzanilla sand bar, with some sections below sea level, there is a threat that coastal erosion could breach the barrier beach.

There is also significant recorded erosion along several parts of the coastline in the vicinity of L’Ebranche Nariva and Ortoire River mouths—attributable to fresh water outflow and tidal inflow dynamics.

At several points along Manzanilla Beach, the sand bar has been eroded from fresh water outflow and sea water inflow; creating points where salt water is able to directly penetrate and alter salinity in the Nariva Swamp (Environmental Management Authority 2001).



The flooding event



While the threat of coastal erosion and coastal flooding from storm surges exists, flooding from the swamp’s freshwater discharge was not previously considered a high threat. Although documented cases of freshwater discharge flooding sections of the roadway has occurred in the past, it is evident that this event supersedes all previous occurrences.

So what led to this extreme flooding event?

Under normal conditions wetlands act as sponges that soak up, retain and slow the release of water. However, due to the prolonged and continuous rainfall that occurred, the hydrological capacity of the swamp was exceeded, resulting in large volumes of water flowing over the land surface.

The water’s flow was obstructed on reaching the barrier beach and forced to flow adjacent to the roadway, following the lowest elevation. This flow became channellised along the road resulting in severe erosion which undercut the road causing it to collapse. The water followed this new flow path and upon reaching topographic low points along the barrier beach, eventually cut channels out to the sea, which caused damage to property. The resultant communication disruption this event has created, will potentially have negative economic implications for the southeastern communities that have been cut off.



Rehabilitation: no quick fix



While there may be many issues surrounding what led to the event itself, at present, there are three pressing matters that require addressing. The first issue involves the reconstruction of the damaged portions of the Manzanilla/Mayaro Main Road in an attempt to restore connectivity between the communities to the north and south. The approach needs careful consideration of the environmental setting of the area.

It is evident that a road diversion may be temporarily constructed while construction on the main road progresses. This temporary road however will have to be located some distance away and parallel to the existing collapsed roadway, closer to the swamp.

This engineering would have to consider the effects of the hydrologic regime of the area, so that the natural delicate balance in the wetland is maintained.

Consideration also needs to be given to the temporary construction of a seawall or similar type hard structure which might be used during rehabilitation of the main road. Seawalls and other hard structures reflect wave energy, and usually lead to adjacent beach erosion, which is of particular importance in this high energy marine environment. In this regard, the use of hard and soft approaches may yield more desirable results.

Secondly, the erosion channels cut across the beach by the flood waters have introduced a new twist to the flooding debacle. While these channels are allowing flood waters to drain out to sea, owing to the low topography of the channels, high tides have already begun dumping sea water into the erosion channels. If these channels are not repaired, then marine erosion will attack the breached sections of the beach, and induce further damage to the roadway.

The question is whether repair to these channels should be allowed to take place naturally, or whether human intervention is needed. Improper engineering solutions can have serious negative implications for the barrier beach and the Nariva Swamp. It is imperative that credence be given to proper scientific understanding of the dynamism of this coastal area, instead of hasty reactive solutions.

The third issue relates to damaged property, and whether or not reconstruction should be permitted as opposed to establishing appropriate setbacks.

Placed in the context of vulnerability, this is by far the greater issue that needs addressing.

The environmental setting of Manzanilla makes it highly vulnerable to several natural disasters such as earthquakes, storm surges from hurricanes and flooding.

The barrier beach upon which the main road and homes were built, is just a few metres above sea level (with some sections actually below sea level), and maintains a delicate balancing act between the Atlantic Ocean on the eastern side, and the freshwater Nariva Swamp on the west.

The beach shifts its position from time to time in response to nature’s forces. Historically, the swamp has flooded onto the barrier beach, and is nature’s way of relieving itself of excess water. In the absence of human infrastructure, nature would undoubtedly repair itself over time, and these effects may have gone unnoticed.

But, should these homes be reconstructed on the same sites where the effects of flooding have been witnessed?

The reality is that the entire Manzanilla beach is vulnerable, and infrastructural development should not have been permitted in the first place, particularly along the sections below sea level. And, put in the context of climate change, these events would possibly become more frequent.

Rather than simply adopting a reactive approach to these disasters, it would be prudent to start thinking along the lines of a national set-back policy, not only for coastal areas, but also for inland flood-prone areas, such as swamps and riverine settings. A setback policy would consider the vulnerabilities that exist in specific areas, and suggest an appropriate distance that infrastructure must be built away from the perceived risk.

These policy approaches should be informed by rigorous data collection and computer-based simulation modelling to account for extreme events that have longer recurrence intervals. Such approaches will prove to be more sustainable in the long term, and actually result in cost savings both in the public and private sectors.



Dr Junior Darsan is a coastal geomorphologist, Department of Geography, The University

of the West Indies



Dr Matthew Wilson

(hydrologist) and Hamish

Asmath (PhD candidate)

contributed to this article