DRS CONOR CAHALANE, AVRIL BEHAN and EUGENE MCGOVERN outline how satellite imaging offers a solution to the problem of how to survey large areas to formulate future flood management strategies.
December 2015 and January 2016 saw extensive flooding in communities along the banks of the Shannon, with areas such as Limerick, Athlone and Portumna particularly affected as the year drew to a close. The cost estimated by local authorities for infrastructure repair and clean-up could reach €100 million, and the cost to businesses and livelihoods is as yet unknown. The inconvenience, hardship and risk to the public throughout this period of prolonged rainfall was substantial, with an increased number of incidents reported near water bodies and on road networks.
The unpredictability of flooding presents serious problems when attempting to devise management strategies, and therefore accurate, regular mapping and monitoring of the affected areas is vital. Planning for flood defences, drainage works and possible relocations in advance of future flood events requires extensive survey and mapping. Gaining daily access to monitor tens of thousands of properties along the Shannon catchment is clearly not possible using conventional ground survey methods, and although aerial platforms can survey large areas quickly and efficiently, flights can be cancelled during periods of bad weather. However, a space-age alternative now exists: the current generation of Earth observation (EO) satellites offers an ideal, low-cost mapping technology, providing rapid, regular, wide-area coverage.
Satellite imagery – a potential solution
EO satellites broadly fall into two classes: those that record the sun’s energy reflecting from the earth’s surface; and, those that transmit their own energy and measure what comes back. These satellites can survey large strips during each orbit, with some capable of surveying areas the size of Leinster in just a few minutes.
Other satellites, such as the commercially available Worldview 3, focus on narrower strips but can therefore provide imagery with a spatial resolution as low as 0.31m2 (each pixel represents an area on the ground about the size of a box of cornflakes). Imagery of the whole island is captured every 10-15 days by the individual satellites, and geomatics surveyors can enable more frequent monitoring by utilising data from multiple satellites.
Additionally, as some of these platforms (NASA’s Landsat 5 for example) have been operational since the mid-1980s, this comprises a source of free, archival imagery for surveyors to use when investigating past events.
A recent EO development of particular relevance to Ireland is the ongoing Sentinel project, organised by Copernicus. Copernicus is an EU programme aimed at developing European information services based on satellite data, implemented by the European Commission and with support provided by the European Space Agency (ESA) for the space component. Preliminary market estimates are that for every Euro put into Copernicus by a member state, seven to ten Euro are returned in economic value. Sentinel-1a (Figure 1a) successfully entered orbit in April 2014, Sentinel-2a (Figure 1b) in July 2015, and Sentinel-3a on February 16, 2016.
The unpredictability of flooding presents serious problems when attempting to devise management strategies, and therefore accurate, regular mapping and monitoring of the affected areas is vital. Planning for flood defences, drainage works and possible relocations in advance of future flood events requires extensive survey and mapping.
Sentinel-1a is particularly suited for mapping countries like Ireland. Unlike the optical systems that rely on sunlight and cloud-free days, Sentinel-1a is a synthetic aperture radar (SAR) satellite. This means that it emits its own energy, which is capable of penetrating cloud cover. SAR imagery cannot be compared with the more familiar aerial orthophotography, as instead of colour photographs the resulting imagery is a measure of how much energy returns to the sensor (Figure 2a). SAR images can be used for terrain mapping, classifying land cover, monitoring urban subsidence/deformation, flood mapping, tracking oil slicks, etc. Sentinel-2a is a multispectral satellite, which although capable of producing colour imagery similar to aerial orthophotography (Figure 2b), can also record information that a human eye cannot see, such as thermal and near-infrared energy.
Multispectral satellites can be used for mapping, land cover classification, urban planning, flood monitoring, assessing water quality, precision agriculture, forestry, mapping the seabed, etc. Each of the Sentinel satellites produces approximately 1.7 terabytes of free data each day.
Earth observation satellites in action
A recent demonstration of the application of SAR imagery in an Irish context was the severe flooding in December 2015. Because extreme weather events regularly coincide with heavy rainfall and extensive cloud cover, there are unlikely to be many clear sky days. Figure 3a displays a multispectral satellite image from early December 2015 captured by one of NASA’s satellites, Landsat 8.
The inundated regions in north Munster were completely obscured by cloud cover, and it was not until December 24 that a gap in the clouds coincided with an overpass of Landsat 8 (Figure 3b).
SAR satellites such as Sentinel-1a are therefore essential, as it is their ability to penetrate cloud that enables mapping and monitoring during periods of cloud cover.
By measuring the strength of the energy that returns to the satellite, SAR enables the automatic identification of water and the delineation of flood boundaries for measurement of the flood extents. This data can then be combined with pre-event colour imagery to aid with visualisation and, because a single satellite is rarely sufficient for monitoring major flood events, geomatics surveyors can incorporate data from multiple satellites.
The process of delineating flooded areas in Limerick using data from two SAR satellites (December 10-13, 2015) is shown in Figure 4. In times of major events such as the recent flooding, the Copernicus Emergency Services programme provides regular, localised geospatial data, derived from multiple satellites, free of charge to all actors involved in crisis management.
Government agencies and businesses should be aware of the potential of satellite-derived spatial data for providing rapid, up-to-date mapping. If used properly, this can enable a changing situation to be closely monitored, and measures for mitigating the event can then be developed.
Archival satellite imagery can be used for assessing performance during management of past events, planning flood defences or redefining flood plains for County Development Plans, enabling effective use of public funding. Other applications such as coastal defence, seabed mapping, precision agriculture, forestry, geology, fishery protection, climate monitoring, urban deformation monitoring and biodiversity protection can all benefit from satellite imagery.
Dr Conor Cahalane
Dr Avril Behan
Dr Eugene McGovern
The authors are members of the Geomatics Division Working Group on Remote Sensing and Earth Observation.
Satellite concept art provided by the European Space Agency. Flood polygons provided by Copernicus.