Multiple Layers of Defence

There are parallels between the coordinated layers of perimeter security for manmade threats (bombs, blasts, break-ins) and the multiple layers that provide resistance against climate threats, specifically flooding and sea level rise.   The five “D”s of security are well-known – deter, detect, deny, delay, and defend – but how do they translate to climate hazards?

Security Boundaries

  1. Deter: A fence deters only the most opportunist attacker, but if the security objective is to establish a psychological deterrent, then a physical presence is a necessary component. Fencing, lighting, signage (“No Trespassing”), and highly visible surveillance equipment communicate a message to unauthorized people that may discourage them from testing the system, looking instead for an easier target.
  2. Detect: An open area designed for detection allows for security to monitor and identify possible threats. This is often known as “no man’s land” between physical barriers that requires time to cross – enough time so that guards have the capacity to respond. Traditionally, this expanse was patrolled by guards in watchtowers or security personnel with dogs, but modern installations may be monitored by aerial reconnaissance satellites, infrared or thermal sensors, or surveillance cameras with the ability to zoom in to the area where the intrusion is detected.
  3. Deny: Control of a secure area is usually executed by a gate or doorway which allows authorized entry. From a medieval castle’s drawbridge to technically-superior card access or biometric systems, entry is still susceptible to human factors or programming flaws, and is not considered an effective barrier on its own.
  4. Delay: A series of doors with controlled access, such as a vestibule, enclose a space that may slow down the forward momentum of a potential threat. Delaying spaces include elevators, lobbies, and other areas with limited ways out, and surveillance cameras for situational awareness. Mics and speakers can provide another level of communication and intimidation.
  5. Defend: This perimeter relies on personnel to apprehend or disable the intruder. This is commonly known as the final defence, but its actual position in the sequence of layers encountered by a potential threat changes, as it overlaps all other perimeters.

Resilient Boundaries

  1. Deter: Sea level rise becomes more dangerous with the addition of storm surge. Offshore and nearshore improvements such as reef-building, marsh restoration, mangroves, and beaches attenuate wave action through friction caused by vegetation and the surrounding landscape. The effectiveness of salt marshes as a natural flood defence reduces the height of large waves by 18% over a distance of 40 meters.[i] Living breakwaters, oyster reefs, and similar techniques do not form a barrier to water, but they reduce wave height and velocity; calm water encourages sedimentation and prevents shoreline erosion, mitigating the threat over time.
  2. Detect: Occupation of the floodplain is accompanied by known risks – risks which can be estimated by multiplying the probability of a disaster by the potential consequences. The baseline impact of sea level rise, storm surge, and riverine flooding upon communities has been mapped[ii] (albeit not at a precise, parcel-level or fine-grained scale), and is estimated at about +30” by 2080 in North America. This is the early detection of a threat that should be a call to action to safeguard lives and properties by modifying land use planning and investment districts.
  3. Deny: Manmade embankments have been in use for centuries, including networks of earthen dams built in the Bronze Age (about 3,000-600 BCE). A palisade of earth stabilized by rocks and rip-rap or supported by concrete and steel is known by many names – levee (from the French lever, to raise), dike, dam, rampart, barrage, earthwork, bulwark – but they all serve the same purpose: to control the course of water and prevent it from spilling over into cities or fields.
  4. Delay: Spaces where water may be held without undue threat are sacrificial landscapes that delay the tide of human retreat from the fray. Retreat – the managed re-planning of the floodplain to align with anticipated sea level rise – is inevitable as the climate warms. As emissions climb, ever-higher estimates indicate a shrinking period of delay; the postponement of action becomes freighted with higher consequences, and the amount of space required to detain the threat grows.
  5. Defend: The only options for defending a city against water are to floodproof or elevate (elevation being another method of retreat). Water exerts a greater force than wind or gravity, so the extent of floodproofing is limited to the structure’s capacity to withstand the water, usually about 3-4 feet in height. Flood gates at openings, waterproof membranes, backflow preventers at piping, and raised mechanical and electrical systems are necessary components of a floodproof base.  The alternative is to elevate inhabitable levels above the base flood elevation, which advances with every new assessment.

These poor defences cannot disable the threat of flooding, quite unlike the options available to manmade threats.  Water is insidious stuff – but these layers of protection are the only tools available.  The alternative is to mitigate the threat at the source: halting the burning of fossil fuels, dramatically reducing carbon emissions, and sequestering carbon through widespread replanting. I suppose we all need an enemy to spur us to innovation and action.


[i] 2014, “Salt Marsh plants key to reducing coastal erosion and flooding, University of Cambridge.)

[ii] NOAA Sea Level Rise Viewer: