October 5, 2017

For Flood Hazard Mitigation Task Force, All Options Are on the Table

The City Engineer’s post-Harvey analysis largely confirms collective expectations based on our experience and observations during the storm:  the singular importance of Project Brays, the continuing validity of our approach to local drainage improvements, the obstacles impeding overland flow, and the success of our current building code elevation regulations.  It stops short, however, of actually making any specific recommendations.  That’s because at this point all options are on the table for thorough review and consideration.

Following and in conjunction with the City Engineer’s presentation, the City Council adopted an amending resolution to expand the charge of our Flood Hazard Mitigation Task Force.  By sheer coincidence Council had previously adopted the first resolution less than a week before Harvey.  It established the Task Force as a more or less routine planning activity required for our continued participation in the Community Rating System of the National Flood Insurance Program, to be composed of appointed citizens including some with certain subject matter expertise.

Post-Harvey, our thorough reexamination of every aspect of flood control is anything but routine.  The amending resolution, therefore, additionally charges the Task Force “to develop actionable local, regulatory and regional policy recommendations for the prevention of future flooding.”  It will do so in close consultation with the City Engineer, building upon his findings from this and prior work to come up with solutions for implementation.

Our three-pronged policy approach to flooding—local, regulatory and regional—is confirmed by the report.  It begins with an overview of just how extreme a rainfall event Harvey was.  Playing off the title of a June 2002 report on Tropical Storm Allison, Off the Charts, the City Engineer remarked of Harvey that it “broke the charts.”  The comparison between the two storms is natural, given the then-unprecedented rainfall amounts from Allison back in 2001.  Of course, Harvey shattered those records.

Isolating the two days during Harvey when Brays Bayou was at flood stage, the City Engineer cited the above estimate by the Harris County Flood Control District that the rainfall during that time period represented a 2,500- to 6,000-year event.  Note the HCFCD’s staggering numbers for all four days of Harvey, as much as a 20,000-year event overall.

The report explains the difference between shallow floodplain flooding and ponding/overland flow flooding.  The former is caused by Bellaire’s proximity to Brays Bayou and our situation within its shallow floodplain; when the Bayou is out of its banks it floods surrounding areas of low elevation.  The latter type of flooding occurs when localized rainfall overwhelms our storm sewer system and, with nowhere to go, the water rises.

Unfortunately, Harvey was an extreme combination of both types of flooding events.  At the same time the shallow floodplain was flooded with the Bayou out of its banks, we got more than 2-1/2 feet of additional rainfall that had nowhere to go.  (This is the two-day period referenced above as a 2,500- to 6,000-year event.)  Hence the convergence of both types of flooding.

Local.  Our local drainage infrastructure program targets the ponding/overland flow problem.  As originally recommended in the City Engineer’s September 2016 drainage study, two primary methods are employed.  First, in the areas of town with major localized drainage problems, storm sewers will be upsized from the typical 2-year capacity to a 100-year capacity, to provide underground detention storage in addition to their conveyance function.

Second, we are working on implementing backflow prevention devices in certain locations, to keep storm water already in the Bayou from backing up into our local drainage system during heavy rain events.  This will preserve our underground capacity for when we need it to hold localized rainfall.  In addition to those two principal types of improvements, we continue to look for opportunities to partner with other entities on mutually beneficial projects within Bellaire, like our 2-to-1 deal with TxDOT for additional, upsized drainage at the 610/59 interchange.

Regulatory.  Also within the City’s control to protect residents against flooding are our building code elevation regulations.  The City Engineer counted 2,318 structures flooded by Harvey, including those in which only the garage took on water.  The number of homes with damage to the living area is estimated to be 1,936.  Nearly 87% of those are in the southeast (1,143) and south central (538) parts of the City, both close to Brays Bayou and at low elevations within its shallow floodplain.

A preliminary review of data from elevation certificates and a breakdown of flooded structures by age shows that our current standard requiring that new homes be built at least one foot above base flood elevation (100-year) is working.  Of the 1,936 homes that got water in their living areas, only about 2% were built at the current standard that has been in effect since 2008.

As an aside, this also illustrates the policy objective of the substantial damage (50%) rule.  Over time it brings properties, especially repetitive loss properties, into compliance with updated building code standards, for the protection of both current and future owners against repeat flooding.  It also conserves public resources and reduces the number of times our first responders are put at risk in rescuing people from homes that frequently flood.

Regional.  The biggest takeaway from the report is that it reinforces the importance of Project Brays to Bellaire, in addressing the shallow floodplain type of flooding.  Significantly, it is anticipated to take most of Bellaire out of the 100-year floodplain by 2021.  That does not mean we’ll never flood again, but by widening the Bayou to keep it within its banks and modifying bridges that have in past storms created bottlenecks, it will surely increase our level of protection against more extreme rainfall events.

It’s hard to know just how much Project Brays would have helped in Harvey, but it’s fair to say that those who flooded by several feet probably still would have, while those who got only a few inches probably wouldn’t.  Harvey aside, since it really was a storm of astonishing magnitude—it “broke the charts”—Project Brays brings a great measure of confidence in our level of protection for the severity of events we’re more likely to face in the future, like the Memorial Day 2015 and Tax Day 2016 floods.

Another regional issue identified in the report is that overland flow was impeded by the elevated section of the IH-610 West Loop and the railroad berm at the eastern boundary of the City.  With our storm sewers already beyond capacity, this only contributed further to the extensive flooding in the southeast and south central areas of Bellaire.  The report suggests these overland flow obstacles might be mitigated with help from outside entities.

As many had anticipated, the City Engineer concluded that there’s nothing the City of Bellaire can do on its own to solve the flooding conditions we experienced in Harvey.  We absolutely need to continue working with our area partners, and the state and federal governments, on expediting Project Brays and other regional improvements.  We should also explore with them the possibility of addressing the overland flow problems created by the West Loop and railroad structures.

Harvey has been a call to action, and we’re answering that call.  The City Engineer’s report provides meaningful analysis specific to Bellaire, and forms the basis of the work yet to come.  Empowering the Task Force with its expanded charge is a strong statement of what your local government is doing in response to Harvey.  All of our flood hazard mitigation policies—local, regulatory and regional—are fair game for thorough reassessment.  The Task Force is expected to make its recommendations to Council by around March 2018.