Christopher Burgess, JP, PE, PhD
A significant proportion of the population lives along the coastline, along with tourism and national assets such as critical facilities and roads. In an era of climate change, with sea level rise anticipated, shoreline recession has serious implications. Aerial and satellite imagery for the 1968 to 2010 period were analyzed to determine historical shoreline positions from a baseline. Both short-term changes between the intervals and long-term changes were estimated and the annualized rate of erosion or accretion determined. Both point to an underlying erosion rate of 0.26 metres per annum with individual rates of up to 0.76 metres per annum for particular beaches. The rates determined are either consistent with or lower than regional and international observations. The importance of increased storm intensity as well as Global Sea Level Rise was examined. Risk assessment indicates that shoreline recession over a 50 year planning horizon has an estimated value of USD114.0 Million for roads, hotels and other critical facilities. Recommendations for a comprehensive vulnerability and risk assessment programme and a national beach monitoring programme were made with a view to assisting in the prioritizing of mitigation programmes.
Figure 1 – Location of nine (9) beach stations used for examining shoreline retreat for the period 1968 to 2010
Figure 2 – Observed short-term (end-point) and Long-term shoreline retreat for eight beaches in Jamaica for the period 1969 to 2010
Several contributory factors can lead to shoreline erosion. Namely, increase sea levels that allow for storm waves to access more of the sandy shoreline; increase storm intensity and frequency that lead to more episodic erosion events and negative anthropogenic impacts such as sand mining and inappropriate coastal works.
Whilst it is accepted that increased sea levels will result in increased shoreline retreat there is limited gauge data in the Caribbean to support regional sea level rise rates estimates. Simpson, et al (2010) however supports the tracking of global trends of 3.1 mm per year for the 1993 to 2003, and notes that recent satellite altimeter data for 3.4 mm/year in the region. This is support by detail global analysis of satellite altimeter data and tide gauge data that shows a rate of sea level increase of 2.8 mm per year for the period 1995 to 2004 by Chung-Yen Kuo (2006). Chung-Yen also concluded that there is no evidence at this time and insufficient data as well to detect any acceleration of sea level rate of increase. In summary the observations and suggestions of regional and international practitioners is consistent with the IPCC (2007) projections of a 0.3 metres sea rise in the 2000 to 2100 period.
The estimation of shoreline retreat as a result of sea level rise is usually estimated with the Bruun Rule. Titus (1985) illustrated how a 30 centimeter rise in sea level would result in 30 metres of erosion for a beach in Maryland, U.S.A using the Bruun Rule. Several similar exercises have been undertaken for beaches in Jamaica. Such as Old Harbour Bay, St. Catherine, CEAC (2009); Negril, Westmoreland, CEAC (2012); and Plumb Point, Kingston, CEAC (2011). These findings are shown in Table 1. In summary, sea level rise can result in shoreline retreat from 0.17 to 0.46 metres per year. The contributory role of sea level rise in shoreline retreat (versus other factors, such as increased wave intensities) can therefore account for between 20 to 100% of observed rates with typical values of 30 to 50% being estimated for Jamaican shorelines.
The IPCC projections for climate change in tropical areas indicate storms are for becoming more intense than in previous years. Their 2007 report indicated that climate modeling studies are showing that storm events are becoming more intense. It further indicated that the future hurricanes or cyclones will become more intense based on the observable trends over the last 30 years. This prediction is consistent with the analysis of the number of category 4 and 5 storms that have tracked pass or over Jamaica in the past 130 years by querying the National Oceanographic and Atmospheric Administration Hurricane Center database on hurricane tracks for storms that pasted within 400 kilometres of Jamaica shorelines. Nodes off the north, south, east and west coast was used queried and the results are shown in figure 5. The number of category 4 and 5 storms has increased from 10 to 15 storms per twenty year interval to 30 to 35 storms per twenty year interval. This doubling of storm occurrences can result in shoreline retreat as beach profiles adjust to more stable configuration and react to the increased wave heights.
Figure 5 – Occurrences of Category 4 and 5 hurricanes that have passed within 300 kilometres of Jamaica’s shoreline since 1890 to 2010, in twenty years intervals
The observations and analysis point to a wide scale erosion scenario across the island at this time, based upon the past forty years of data. Eight of the nine beaches were observed to undergoing shoreline retreat for the period 1968 to 2010, at an average rate of 0.26 metres per annum. Over a 50 year period this will result in total losses of 12.5 to 38 metres of shoreline, putting more of the population, road, hotels and other national assets at increased risk from hurricane waves and swell waves. An estimate of the risk of this shoreline recession relative to the national assets was determined to be USD114 Million over a 50 year period. This estimate is justification for national investment to adapt to climate change and to mitigate the consequences. Otherwise it is very likely that the risk will be realized incrementally or in severe events.
Rationalizing a response to shoreline retreat involve a formal national programme of: monitoring, vulnerability and risk assessment and prioritizing of investment/adaptation measures. A beach profile monitoring programme is a starting point and such a programme using established protocols for data collection and reporting. Additionally, a national vulnerability assessment should be undertaken using at least a 50 year projection for the identification of vulnerable assets for prioritization of mitigation and adaptation efforts. Such a preliminary study could possibly utilize a regionalized rate as a first step to identification of critical areas. Lastly, consideration should be given to the review and revision of setback rules in coastal developments to take shoreline retreat rates into account.