Why we concern floods?
Flood has become one of the most serious nature disasters in the recent years. With the fact of global warming, more rainfall may become a real problem in these years, which may cost more future flood damage. The trend of floods occurring in the recent years keeps increasing in this case. A recent report made by the UN authority says that in the last 20years, 157,000 living lost are caused by floods. (Richard D, 2016) It says over 2.3 billion people, at about 56 percentage of all the people affected by the weather related disasters, are affected as a result of flooding from 1995 to 2015. Flood brings not only economic losses, but death as well. Especially in Australia, a country located in the ocean and may suffer from extreme weather conditions frequently, which is the main reason that causing floods.
How to overcome this hazard?
Using flood mapping to monitor flood conditions is the main result. Remote sensing (RS) technology along with geographic information system (GIS) has become the key tool for flood monitoring in recent years. Due to the increasing losses in flood, not only the financial losses, but also death of affected flooding people, the precise prediction of flooding disaster, as well as the mapping after flood occur is needed on time. The developments including in this field is mainly from optical to radar remote sensing in these years because radar remote sensing can provide nearly all the weather conditions while optical can not offer. Also there is another alternative method which is Satellite remote sensing, that mainly using the improved data to contribute to dealing floods. It also needs to concern hydrological parameters, as well as climatic data because floods are always from the extreme weather conditions.
What data is needed?
SAR data is the major parameters! As well as the GPS, visible and near infrared or thermal infrared imageing, multi parameter SAR, and microwave imaging data.
SAR data can provide the flood mapping with all the weather conditions, InSAR data is able to measure the displacements with a very high accuracy topographic mapping details. It is the major data needed in flood mapping. Also GPS data is needed in order to make sure the real-time data can be updated on time. Based on observation satellites will need to increase the number of ground-based measurements and data from existing and future integrated hydrographic survey array.
Can we only consider SAR data?
No! We must consider the climatic and hydrology data as well! Floods are mainly from extreme bad weather such as typhoon or hurricane. So in that case, the climatic conditions must be considered. The combination of climatic data and SAR data is needed when we do an analysis of flood. Hydrology data is surely needed to be considered. Flood is a nature disaster about water, so absolutely we need to concern about hydrology.
Moreover, satellite remote sensing can be combined by GPS data in order to provide a high-precise accuracy of spatial deformation and reflection.
Figure 1 2010-2011 Queensland flood Source: au.gov
What method is used in dealing floods?
Interferometric synthetic aperture radar (InSAR) is the basic of current flood mapping techniques. Radar remote sensing techniques can provide all weather capabilities and visible images for flood areas. Due to the different types of flooding, it can be separated into forest areas flood mapping, non-forested areas flood mapping, and urban areas flood mapping.
A very high backscatter based on the double bouncing between the stems and water surface will happen. Also the canopy of forest needed to be concerned. It may cause several reflections.
Figure 2 Forest areas scattering mechanisms
Small backscatter will happen at this time because the water surface is smooth as the radar wavelengths can be regarded as specular reflections.
It is hard to do flood mapping with SAR because urban area has strong scattering due to its varieties of materials. The long repeat times and temporal decorrelations makes it unable to do great flood mapping through SAR images in urban areas.
Only SAR(InSAR) techniques?
Definitely not. it is essential to use combined techniques, not only remote sensing techniques, but geographic information system as well. Such as the data used before, GPS, Climatic data, Hydrology data are all needed to be combined during flood mapping process. The same meaning in techniques, RS techniques, GPS techniques, GIS techniques are also needed. There are only few applications use only one or two techniques!
What’s in the future?
Probably LiDAR or advanced SAR images!
From analysis to monitoring, flood mapping has changed into making contribution during floods, but not only analysis after floods. From optical to radar remote sensing techniques, it provides all weather conditions that can be used for flood mapping. From radar images to future advanced SAR images, maybe it can overcome the current issues of hard to monitoring in urban areas because of the varieties of material and scattering!
Also LiDAR techniques has been putted forward in some small fields. Hopefully it can overcome the SAR drawbacks and do a high precise flood mapping!