Protecting Coastal Areas from Flooding by Injecting Solids into the Subsurface

Subsidence and sea level rise conspire to increase the risk of flooding in coastal cities throughout the world, and these processes were key contributors to the devastation of New Orleans by hurricane Katrina. Constructing levees and placing fill to raise ground elevations are currently the main options for reducing flooding risks in coastal areas, and both of these options have drawbacks. We suggest that hydromechanical injection of solid compounds suspended in liquid can be used to lift the ground surface and thereby expand the options for protecting such coastal cities as New Orleans, Venice, and Shanghai from flooding. These techniques are broadly related to hydraulic fracturing and compensation grouting, where solid compounds are injected as slurries and cause upward displacements at the ground surface. The equipment and logistics required for hydromechanical solid injection and ground lifting are readily available from current geotechnical and petroleum operations. Hydraulic fractures are routinely created in the upper tens of meters of sediments, where they are filled with a wide range of different proppants for environmental applications. At shallow depths, many of these fractures are sub-parallel to the ground surface and lift their overburden by a few mm to cm, although lifting is not the objective of these fractures. Much larger, vertical displacements, of the order of several meters, could be created in low-cohesion sediments over areas as large as square kilometers. This would be achieved as a result of multiple injections. Injecting solid particulates provides the benefits of a permanent displacement supported by the solids. We have demonstrated that hydraulic fractures will lift the ground surface at shallow depths in Texas near the Sabine River, where the geological setting is generally similar to that of New Orleans (and where, incidentally, hurricane Rita landed in 2005). In these regions, the soft surficial sediments are underlain by relatively stiff Pleistocene deposits, which create in-situ stress conditions favorable for sub-horizontal orientation of hydraulic fractures. Based on the poroelastic effect, these conditions can further be improved by subsurface manipulations of pore fluid. Also, there are many geological examples of natural, sub- horizontal hydraulic fractures. These include multiple igneous sills (e.g., Henry Mountains, Utah) and sand- filled sills intruded into sedimentary formations (e.g., Shetland-Faroe Islands). Techniques that are currently used, or planned, for protecting coastal cities from flood are typically based on the concept of a barrier to the seawater (e.g., levees or water gates). However, the failure of any barrier to flood waters can be catastrophic when the city it protects is below sea level. Hydromechanical injection of solid compounds could permanently lift elevations above a Category 5 hurricane surge, so the risk of a catastrophic failure and subsequent flooding becomes insignificant. We envision that the hydromechanical method can be used in combination with other strategies. For example, in some areas it may be efficient to let most of a city retreat and only lift localized regions of particularly high value, such as airports, port facilities, refineries, historical areas, military bases, etc. In other cases, the protecting equipment itself may begin subsiding (e.g., massive, metal water gates on a soft-sediment foundation). Then, hydromechanical injections could be used to lift the region supporting this equipment.