Sea Level Rise when the Ground is Sinking
Ground Deformation is a Hidden Key to the SLR puzzle
In the sea level rise (SLR) war we are all missing one of the most important data points under our feet. The ground is moving up, and down, all the time… in a trend that we have altered. The sea is raising, but in many coastal cities the ground is also sinking.
What are the impacts of fill compaction, aquifer depletion, and sea wall ageing?
More importantly, how can we monitor these changes accurately and timely to provide input to planning and prevent costly catastrophic change? What role do the various SAR processing techniques play in the flood equation side by side LiDAR?
Rapid Fill Compaction
The heading picture is the perfect example of filled land. Often our most valuable property is in a similar setting. A retention wall is built, material is dredged or trucked in and filled in behind the wall allowing property to be built. Often with values in the millions of dollars, these homes represent a huge risk to sea level rise. The trouble with the equation is the fill they are built on is often compacting more rapidly than estimated. Fill compaction changes with pore pressure and as there’s record rainfall and challenging drainage conditions pore pressure is not stable. This causes the property to sink, and not at a uniform rate, resulting in vulnerable foundations that during high stress events (flooding and hurricanes) may result in structural damage, or failure.
In Situ (on site) sensors such as piezometers are an industry standard for monitoring pore compaction rates. When these sensors are in place and networked with an IoT from SCATA platform such as sensemetrics and combined with SAR deformation data there’s an ability to more thoroughly understand what is happening in ground motion. Absent in situ sensor installations SAR time series analysis of structural and surface motion can enable a high level view of the stability or subsidence of the area.
In places such as California we are depleting our aquifers more rapidly than ever before. As water levels collapse, so too does the surface above the aquifer. This surface subsidence can result in widespread, rapid, subsidence. This is not isolated to California, but is a global challenge as surface water runoff no longer absorbs to replenish aquifer rates as naturally intended and abatement is relatively recent. A study performed by NASA JPL and the European Space Agency Sentinel 1 program (which Ovela is a supplier to) produced stunning results along the California aqueduct. Showing subsidence rates of nearly two feet in one year.
Seawalls were often built a generation, or two, before we come to monitor them regularly. Sadly, seawall monitoring is often still the work of a person in a rowboat, with a camera, and a measuring tape. We lacked any way to remotely monitor the deformation of the length of the defense structure. This meant that we could only intuit potential failure from a visual crack in the surface.
With SAR we can show where along the sea wall there is deformation that would indicate a potential failure condition in an extreme event. More than feeding into a maintenance and repair regime this can provide catastrophic risk modellers (cat models) with a more predictive indicator of flooding area than sea wall present or not. Where and how a sea wall fails can indicate the nature of the flooding event on the areas it protects.
Observing changes to all of the structures involved in responding and abating SLR with SAR involves first addressing the challenge of informing the community responding to SLR that the technique exists and is a valid method. Tide gauges and flood maps have long been the tools de jour of these experts. However, this long excludes the factors of ground motion and infrastructure ageing / failure. To adequately assess and design abatement measures the SLR teams need to have all of the information. This will often include uplift data as well, showing that some areas that were once in risk are exiting that risk. In a scenario where every inch of flooding can mean millions in damages the ability to monitor and predict those inches seems fundamental.
SAR can provide two key inputs to this equation: deformation monitoring and amplitude flood peak observations during storms. The first we have covered at a high level here, but the second is equally important. We often lack a precise understand of just how high the water reached along the extent of the predicted flood model. With amplitude monitoring SAR can penetrate the clouds and darkness to determine how high the water has reached and where. Together with even the most outdated flood map this can both be an important tool for first responders - but also, for understanding how accurate the terrain models have predicted the crest of the flood.
Sensor platform and temporal resolution (space craft and revisit frequency) have a direct impact on the efficacy of these techniques and use cases. Sentinel 1 revolutionised the SAR industry by providing a time series viable visit frequency of 6/12day revisits. ALOS, TerraSAR-X (TSX), Cosmos-Skymed, and RadarSat 1/2 could not reach that level of revisit frequency. However, TSX could be called upon in a flooding emergency to provide data in realtime - something that with amplitude processing could provide the valuable insight we need. As small/nano satellite constellations, such as the newly launched RCM-constellation RadarSat3, become more prevalent in the data market Ovela, and its user community, will have more sensor source options to pull from.
SLR - The greatest engineering fight in our time
Sea level rise is changing the shape of life for humanity, it will take the combined engineering talent and toolkits of all of us to define the future we want for ourselves and our children - SAR is here to rise to the challenge.