Benefits of Flood Damage Reduction

As of 11/28/2017

What are Avoided Damages?

Avoided damages are flooding damages that WOULD HAVE OCCURRED for a particular structure at a particular location during a particular rain event, EXCEPT FOR either:

  • Previously completed structural flood damage reduction projects, such as widening or deepening a bayou or excavating a stormwater detention basin, or
  • Buyout programs in which flood-prone homes are purchased and demolished, and the homeowner assisted in moving to higher ground.

How does the Flood Control District Calculate Avoided Damages?

Structural Projects:

When estimating avoided losses due to structural projects, Flood Control District analysts FIRST estimate the extent of the flooding that occurred during a particular rain event.

Step One: The Flood Control District measures peak water surface elevations for an event – how high the stormwater rose – in two ways.

  • During an event, the Flood Control District’s county-wide network of gages record water surface elevations in the channels.
  • After an event, Flood Control District teams go into the field to identify and survey high water marks along channels where flooding occurred.

Step Two: Once we know how high the stormwater rose in flooded channels, we map the areas where the ground elevation (or ground elevation plus typical foundation slabs) is lower than those measured peak water surface elevations. This shows the area where flooding occurred during the particular rain event. (Ground elevation data is derived from 2008 LiDAR – light detection and ranging – which is a commercial technology that uses a laser mounted in an airplane to measure the elevation of the ground.)

The Flood Control District ALSO has to estimate the extent of the flooding that would have occurred in a particular rain event without a particular structural project.

Step One: We use scientific models that represent conditions before the structural project was built, and use them to estimate how high the stormwater would have risen without the structural project, given the rainfall from a particular event.

Step Two: Once we have that estimate, we map the areas where the ground elevation is lower than this estimated stormwater level (again taking into account the extra inches provided by foundation slabs). Those are the areas that would have flooded during the particular rain event if the structural project had not been built.

Thanks to the Harris County Appraisal District, we know the “footprint” of structures in Harris County. We can use this information to identify and count structures that ARE within the area that would have flooded without the structural project, but ARE NOT in the area that actually flooded during the event with the structural project completed. We say that those structures avoided damages from flooding during that particular rain event.

Multiplying the average flood insurance claim during the rain event…times the number of structures that avoided damages…provides a ballpark monetary figure for the avoided damages.

Note: A structure can flood repeatedly in successive rain events; structures can also avoid damages in successive rain events.

Home Buyouts:

When estimating avoided losses due to home buyouts, Flood Control District analysts first estimate the extent of flooding that occurred during a particular rain event.

Step One: The Flood Control District measures peak water surface elevations for an event – how high the stormwater rose – in two ways.

  • During an event, the Flood Control District’s county-wide network of gages record water surface elevations in the channels.
  • After an event, Flood Control District teams go into the field to identify and survey high water marks along channels where flooding occurred.

Step Two: Once we know how high the stormwater rose in the channels, we map the areas where the ground elevation (or ground elevation plus typical foundation slabs) is lower than those measured peak water surface elevations. This shows the area where flooding occurred during the particular rain event. (Ground elevation data is derived from 2008 LiDAR – light detection and ranging – which is a commercial technology that uses a laser mounted in an airplane to measure the elevation of the ground.)

Using records of the location of acquired buyout structures, the Flood Control District is able to identify which acquired-and-demolished buyout structures would have been within the flooded area. We say that those structures avoided damages from flooding during that particular rain event.

Multiplying the average flood insurance claim during the rain event…times the number of structures that avoided damages…provides a ballpark monetary figure for the avoided damages.

Note: A structure can flood repeatedly in successive rain events; structures can also avoid damages in successive rain events.