1. Introduction Storms 2. The Impact of Storms on the coast 3. Storm types 4. Coastal Sectors 5. Sorm Location and Seasonality 6. Storm Severity 7. Storm Frequency and grouping 8. The design storm event 1. Introduction A storm is a major meteorological disturbance generally caused by low atmospheric pressures. Storms are of a temporary nature and are characterised by strong winds, raging seas and possibly heavy rain. An intense storm close to the coast can severely erode the shoreline and damage coastal developments. Even storms located a long distance offshore can generate heavy ocean swells which will travel, reach and attack the coast. Storm can wreak damage along the entire lenght of the coastline. The storm Xynthia, in the end of February 2010, hit badly several European countries and mainly the coast of 1
A basic understanding of coastal meteorology is an important component in coastal design, planning and management. Perhaps the most important meteorological consideration relates to the dominant role of winds in wave generation. However, any other meteorological processes (e.g., direct wind forces on structures, precipitation, wind-driven coastal currents and surges, the role of winds in dune formation, and atmospheric circulations of pollution and salt) are also important environmental factors to consider in man s interactions with nature in this sometimes fragile, sometimes harsh environment. 2. The impact of storms on the coast Storm winds can cause direct damage to coastal developments, e.g. unroofing buildings, uprooting trees, bringing down power lines and even causing the collapse of buildings. This direct wind damage can extend many kilometres inland. The windborne sediment transport associated with strong winds accelerates the migration of coastal sand dunes and the smothering (overwhelm) of coastal developments. Storms are responsible for the generation of large and potentially destructive waves. 2
The significant increase in coastal water levels caused by storm induced wind and wave setup intensifies wave damage to beaches and coastal structures and coastal inundation (flooding) problems. The elevated water levels and large waves associated with storms are a major cause of beach erosion. Storm damage to beaches and coastal structures. Australia 3
Coast of France. Storm Xynthia. Febrary 2010. Coastal inundation. Storm Xynthia 4
Storm Xynthia Storm Xynthia 5
Storm Xynthia 3. Storm Types Storms along the coast are usually classified by the meteo office in several different types. The picture shows characteristic synoptic patterns of some storms with regard to coastal processes in the NSW coast. 6
Synoptic chart Source CEM 7
Tropical Cyclone Easterly trough low Continental Low Southern Secondary Low Inland trough low Anti-Cyclone Intensification Countries reporting fatalities (February 2010) 8
4. Coastal Sectors (considering storms) The coastline can be divided into sectors on the basis of storm types generally experienced in each sector. The following picture shows coastal sectors for the NSW coast. They correspond approximately to Bureau of Meteorology forecasting districts. Coastal Sectors for Storms. NSW coast 9
5. Storm Location and Seasonality The ocurrence of the storm types in each of the four coastal sectors is shown on the following picture for the NSW coast. These results are based on an analysis of 50 years of storm data. Tropical cyclones are experienced mainly in the North Coast sector. The majority of storm events on the North and Mid- North Coast are due to locally formed Easterly Trough Lows and Tropical Cyclones. Southern Secondary Low predominates in Central and South Coasts sectors. Storms in NSW. (50 years of data) 10
Storms by type of storm in NSW. (50 years of data) The various storm types generally display a distinct seasonality, i. e. certain types of storm are more likely to occur during a particular period of the year. 11
Seasonal occurence of all storm types for each of the four coastal sectors. (*) Take into account that we are in Australia. 6. Storm Severity. The pressure on the central point of the storm is a good indicator of its severity and the likely strenght of the winds it will generate. However, the impact of a storm on the coast depends upon its location, movement and the extent and the duration of the strong winds. The resulting wind speed, storm surge level and wave height can all be used as measures of storm severity. An example of classification of storm severity based on the significant wave height is shown on the table. 12
Storm Classification based on the significant wave height. Offshore buoy at Nazaré on the Portuguese Coast. 13
Sequence of storms from October to December 2010, offshore Nazaré on the Portuguese Coast. (www.hidrografico.pt).significant wave height (m) versus time (days) It is important to stress that in several coasts of the world storms are felt only by the height of the waves arriving into the coast (as a strong swell). The path of the storm is thousands of kilometres way on the ocean. This is the case of the Portuguese west coast. 14
Waves Hs>5m arriving from a offshore storm 7. Storm frequency and grouping. Minor to moderate storms occur frequentely alons avery coastline. Such storms do not have major impacts on the coast or coastal developments. Severe storms of categories A and X, in which the significant wave height exceeds 5m, can be expected to occur in average, in the NSW coast, four times per year. Obviously, these number varies from coast to coast. The occurrence of major storms over the period 1880 to 1985 in the NSW coast is shown on the following picture. 15
Ocurrence of Major Storms. New South Wales Coastline. 1880-1985 What conclusions can be drawn from the last picture? Looking back only twenty five years (from 1985) we might arrive to the conclusion that this period was calm, only a small number of storms occuring. This shows clearly the importance of the record lenght. We may be looking at a window that is just to small! One that does not represent reality! 16
There are a number of other factors which affect the impact of storms on the coastline, particularly with respect to their erosive capability. Storm duration, the incidence id of spring tides (tides of high amplitude) during and immediately after a storm are important factors. There is no single satisfactory measure of the damage potential of a storm or series of storms! 8. The design storm event. Typically, coastline hazard is assessed in terms of the likely impact of a design storm event. The selection of an appropriate design event is governed by many factors, includings, safety aspects, likely damage and social disruption, all of which h depend d upon the type and nature of the development. It should be noted that longer term coastal changes, such as shoreline recession and sea level rise, also affect the damage potential of the design storm event. Recession exposes additional development to storm hazards; elevated sea levels allow larger storm waves to attack the coast. 17
Typically, a design storm event is specified in terms of its Annual Exceedance Probability (AEP), e.g. the 5% storm event. There is a 5% chance of such a storm occuring in any year. On average five such storms would be expected to occur in a periord of 100 years, i.e. the average recurrence interval (or return period) of a storm of this severity is 20 years. The coast can esperience the design storm event ent at any time. Long periods of relative calm, as experienced sometimes in certain locations, can give people a false sense of security. Questions 1. What is a storm? 2. What are the main impacts of storms on the coastline? 3. Why is it important for coastal management to classify storms types that can hit the coast? 4. Do all storm types occur in all seasons? 5. Give a measure of storm severity? 6. What kind of information can be derived from storm frequency? Why is it important to have a long series? 7. Why are storms more hazardous if they occur during spring tides? 8. A storm with a AEP of 10% has a recurrence interval of how many years? 18