Arbitrary floodplain boundary example

While researching actual damage to businesses during Hurricane Harvey in my local area of Harris County, I found a Houston Chronicle article entitled "Harvey floods damaged 100s of safe-deposit boxes".  The article was written by Paul Takahashi and is dated October 2, 2017.


Mentioned in the article is that the Clear Lake City Boulevard branch of Chase Bank was one of nine branches that were flooded during Harvey.  Driving by that area in the aftermath of Harvey I had noticed that the building was closed for repairs.  The nearby Frost Bank branch was also closed.  It is likely that if these two buildings were flooded then a number of other nearby structures in the area were also flooded.  I was able to identify an area of flooded homes south of the Chase branch bank building in the Bay Knoll subdivision.

One of the themes of this blog is comparison of actual damage from Harvey to predictions of flooding risk from flood hazard maps.  The FEMA National Flood Hazard Layer map (NFHL) shows the Frost Bank building in the 500-year floodplain however the Chase building and the Bay Knoll flooded homes are in Zone X - Minimal Flood Risk (see Map #1).

Particularly notable is that the edge of the 500-year floodplain in this area is defined by straight lines suggesting some non-natural factors have been used to draw the boundary.  Normally the floodplains generally mimic the topography and are shaped beyond that by modeling of runoff rates and drainage capacity.  These parameters are not likely to generate straight line boundaries.

Map #2 is the detailed topography in the area from Digital Elevation Models (DEM) maintained by the US Geological Survey (USGS).  The topography of this portion of Clear Lake City slopes from southwest to northeast interrupted by a low area associated with the Horsepen Bayou.  This model has enough detail to define very small elevation differences such as those between home lots and streets.  Although subtle these features can be critical in determining whether or not flooding will occur in one area versus another.


In the vicinity of the damaged Chase Bank building and the flooded Bay Knoll houses there are low areas that could be considered extensions of the 500-year floodplain.  If the floodplain map included these areas instead of having the arbitrary-looking straight edge boundary it could have provided a more realistic representation of the true flooding risk.

Readers - are you aware of the details of the floodplain map for your area, especially if your house is close to a boundary or has been flooded even though it is in Zone X?  The statement is often made that "the floodplain maps are wrong", but if so what are the reasons?


 

Comments

Post a Comment

Popular posts from this blog

Considerations for neighborhood protected by levee

Image
Residents and potential home buyers in areas where protection from flooding is provided by levee systems may need to consider carefully the actual risk, which may be higher than many assume.  Levees are designed to reduce (but not eliminate) flooding risk.  There may often be no requirement for residents to be provided with an assessment of the specific risk by developers. However, the presence of levees indicates that significant drainage issues exist in the area. The levee system (usually composed of earthen berms, flood gates, and pumps) must perform two functions successfully to prevent flooding: Hold back flood water from outside of the protected area Remove water accumulating inside the protected area Failure of the levee system can result from multiple factors such as: Berms must have sufficient height and strength to hold back impinging flood water Gates must operate properly and be opened/closed at the appropriate times  Pumps must operate properly ...
Read more

Check flood control and drainage improvement projects in your area

Image
Residents of flood prone areas are well advised to stay informed regarding current and planned flood control and drainage improvement projects.  These projects by design will result in significant reductions to flooding risk.  In some cases the flood hazard maps will be revised to reflect the reduced risk.  This could allow a decrease in flood insurance premiums for the home owner. In my area I noted a project underway in Nassau Bay, a waterfront community in southern Harris County, Texas that has a history of repetitive flooding.  The project is being conducted by Harris County Flood Control District (HCFCD).  It is located on a lot that I had noted previously as the site of a home buyout and demolition after Hurricane Ike, which caused heavy damage to many Nassau Bay homes in 2008. HCFCD maintains an interactive map viewer on which the details of all Maintenance and Capital Improvement projects can be viewed at the link: https://www.hcfcd.org/int...
Read more

Drainage features without mapped flood plains

Image
All drainage features whether natural or man-made have flood plains, which are simply the adjacent low areas that fill with water during periods of overflow.  Water overtopping the banks will move to all connected lower areas and remain until drained away by other features.  However, these areas may not be mapped as flood plains on Flood Insurance Rate Maps (FIRM).  As a result, residents and property owners may not realize that they are located in a flood zone. Although unmapped these areas will flood during times when the nearby drainage features overflow. This issue was featured in an April 1, 2018 front page article in the Houston Chronicle article entitled "Damage outside flood plain exposes flaws - 'We just need to start again,' says professor about maps designating flood-risk areas".  The article was written by David Hunn, Matt Dempsey, and Mihir Zaveri and can be viewed at the link below: https://www.houstonchronicle.com/news/article/In-Harvey-s-delu...
Read more