The Value of Using Unofficial Measurements of Rainfall: The Dublin Storm and Flood of June 1963

Rainfall measurements are vital for the design of hydraulic structures, climate change studies, irrigation and land drainage works. The most important source of design rainfall data comes from convective storms. Accurate assessment of the storm rainfall requires a fairly dense network of raingauges. In 1963, such a storm took place over Dublin in Ireland. However, the existing raingauge network was insufficient to identify both the depth and pattern of rainfall. An appeal was made by Met Eireann for additional unofficial rainfall data. The result was remarkable in that the estimated maximum rainfall depth was found to be more than double the official value and that the resulting depth area analysis suggested a rainfall volume over a large area much bigger than the original isohyet map indicated. This result has huge implications for the estimation of maximum rainfall and dam safety assessment, especially in countries where the raingauge network has a low density. This paper first provides a description of the synoptic conditions that led to the storm, second an analysis of the rainfall data and how the unofficial measurements produced a very different depth area relationship;third, the social consequences of the resulting flood are described. Fourth, the storm is then placed in the context of other storms in the British Isles Finally the implications for rainfall measurement, gauge density and an example of how revised estimates of probable maximum precipitation (PMP) have been used to improve the safety and design standard of a flood detention dam are discussed.

New Estimates of 24-Hour Probable Maximum Precipitation (PMP) for the British Isles

Estimates of PMP are needed in order to estimate the spillway design flood for dams which must be capable of safely passing the probable maximum flood (PMF). For over forty years the standard estimates of the Flood Studies Report (FSR) have almost always been used. However, since then several studies have shown that these values are too low, and new estimates for South West England and then Britain were published. This paper extends these studies to include the whole of the British Isles. The study uses 6570 station years’ data in a new method of frequency analysis that identifies part of the data which represents the long term situation. Further analyses to support the results of this approach have been produced from the in situ maximisation of 12 historic storms using World Meteorological Organisation methods. The results are broadly consistent with those obtained from frequency analysis. Values of 24 hour PMP in Britain range from 600 mm in upland areas of the Lake District, to 400 mm in parts of East Anglia. This range of values is present in Wales and parts of South West England. For Ireland the highest values are over the upland areas, such as in the Wicklow Mountains (550 mm) and SW Ireland (600 mm), while in the lowlands values around 350 mm can be expected. The question of uncertainty in estimating PMP is considered, most of which is due to the temporal and spatial shortcomings of the data. The implications for the probable maximum flood (PMF) and spillway design flood mean that widespread reassessment of dam safety should be carried out at once.

Saving Lives: Timely Flash Flood Warnings in the UK

Flash floods are a major cause of death and destruction to property on a worldwide scale. In the UK sudden flooding has been the cause of the loss of over 60 lives during the last century. Forecasting these events to give enough warning is a major concern: after the 2004 flood at Boscastle, Cornwall UK the Environment Agency (2004) stated that it was not possible to provide a warning in such a fast reacting and small catchment. This is untrue since the Agency had already implemented a real time non-linear flow model as part of a flood warning system on the upper Brue in Somerset UK. This model is described in this paper as it has been applied to the Lynmouth flood of 1952, and briefly for the Boscastle catchment, both of which have an area of about 20 km2. The model uses locally measured SMD and saturated hydraulic conductivity data. With the addition of further parameters the model has been successfully used nationwide.

Causes of Big Bushfires in Australia: Higher Temperatures and Rainfall or More Fuel?

The 2019-2020 bushfires in Australia caused the loss of 34 lives and an estimated 100 bn AU$ damage. This has sharpened the apparent division between Australians who believe that the increasing number of bushfires is due to climate change, and those who suggest that fuel loads must be managed more carefully. Bushfires whose area equals or exceeds 1 mHa have been analysed in this paper. The results show that the number, duration, and size have increased over the period 1850-2020, but that since 1953, there has been a downward trend in the number of big bushfires. There is a range of temperatures of about 20°C that are associated with the fires, with a modal temperature of 30°C - 32.5°C. Using an analysis of the maximum temperatures for the period 1970-2020 as a standard for comparison with bushfires for the period 1850-2020, shows that during bushfire events the standardised temperatures have a downward trend. This is most clearly shown by the application of the Fisher Exact Test. This suggests that the fuel load in forests is a key factor for bushfires. The role of pre-bushfire rainfall shows a rapid rise in the area burnt when the rainfall exceeds 150 mm month-1 which would lead to more plant growth and hence fuel load. The role of traditional burning over the whole of Australia is described based on documentary evidence. A tentative cost benefit analysis has shown that a comprehensive program of wildfire management is cost effective. The recommendations of previous research, National Inquiries and more recent management practices, have all failed to prevent the 2019-20 fire disaster in Australia.

Models, the Establishment, and the Real World: Why Do So Many Flood Problems Remain in the UK?

In spite of the proliferation of research and Government Reports on floods in the UK, there are about 5 million properties located in flood prone areas. Many of the houses have been built during the past 50 years with little regard to the existing and future flood risk. Decision makers, who are part of the Establishment, rely on the outputs of models which are produced by so called “experts”. But this information has been found to be flawed. However, the Establishment is largely unwilling to allow local knowledge when assessing flood risk, even though prevailing methodology suggests otherwise. This situation has grown worse since the formation of the Environment Agency in 1996. The public engagement with flood risk science and its application needs to proceed along a co-production of knowledge model. Three examples of the difficulties of making realistic assessments of fluvial flood risk are described. The problem of surface water flooding is also considered since it too has local causes which may not be accounted for in standard methodology. Finally, more open discussion and co-operation in identifying flood problems in the UK is called for.

A 249-Year Record of Floods at Appleby in Westmorland, UK

An analysis of nearly 250 years of flood records on the river Eden at Appleby-in-Westmorland has enabled a flood frequency relationship to be established. The most severe floods were in the late 18th and early 19th century. With such a long history of flooding, some remedial measures would have been expected but the local people have, to some extent, adapted to the flood hazard by means of temporary and permanent flood proofing methods such as a cemented board across a doorway and removable flood boards. These measures were overwhelmed during the 2015 flood, as were the flood gates installed by the Environment Agency in 1998. A higher level of protection from floods at Appleby is called for.

Downward Trend in the Frequency of Destructive Hailstorms and Maximum Hailstone Size in England and Wales

By their definition, destructive hailstorms cause a great deal of damage to property and have been known to be fatal to humans. At present the long term changes in the frequency of these storms are unknown. Records of destructive hailstorms from many different sources in England and Wales go back for several centuries. Recently discovered records of over 30 destructive hailstorms have allowed a better description of hailstorm history since 1780. The data were grouped into 30, 40, and 50 year non-overlapping time periods in relation to time. Increases in their frequency in the 17th and 18th centuries were probably due to an increase in population and better recording. Since 1900 the frequency has significantly declined so that in the present 50 year time period only 22 have been recorded as compared with an average of 49 in both halves of the 19th century. The decline in frequency may be partly due to the big increase in particulate matter from aviation, which leads to a much higher concentration of ice and cloud condensation nuclei, resulting in fewer destructive hailstorms.