7/28/24 – A 2020 academic study in the journal “Cities” quantifies the relative effectiveness of three different ways to mitigate flood losses. They include storage, conveyance and measures designed to promote infiltration.

Best Ways to Mitigate Flood Losses

The study, called “How Effective are Drainage Systems in Mitigating Flood Losses?” found that storage was the most effective of the three. The existence of retention/detention basins or wetlands can reduce 92% of flood damage, according to the study.

This research provides insight into preserving wetlands. It also encourages planners and policymakers to incentivize the implementation of storage basins in flood-prone areas.

Measures designed to increase conveyance ranked #2 among the three factors studied.

And increasing infiltration ranked a distant third. In fact, the last approach actually had a negative correlation with flood-loss reduction where/when rainfall exceeded water-retention capacity. “Infiltration-based drainage systems are more effective as a response to small, low-intensity storms,” states the conclusion.

Measures designed to promote infiltration include “hydraulic disconnectedness.” For instance, when runoff from rooftops goes straight into storm sewers via gutters and downspouts, the authors would classify that as “connected.”

However, when rooftop runoff goes into ponds, grassy areas or swales, it is said to be “disconnected” because that gives the runoff a chance to infiltrate.

But compaction during construction can cause a reduction in infiltration rates by as much as 70% to 99%, limiting the viability of this approach in highly urbanized areas.

The study examined 1608 properties in the Buffalo Bayou watershed in Houston between 2009 and 2012 “at a parcel level.” The authors point out that local factors, such as flat topography, clayey soils, high-density development, and climate may limit the application of the results to other regions.

Authors of this thought-provoking, insightful study on the best ways to mitigate flood losses include:

• Samuel D. Brody from the Texas A&M Department of Marine Sciences
• Wonmin Sohn, Jun-Hyun Kim and Ming-Han Li from the Michigan State School of Planning, Design and Construction.

I asked several local flood-mitigation professionals for their opinions on the most effective forms of flood mitigation. They largely agreed with the study.

One prioritized “conveyance improvements with mitigation detention.” He said, “Get the water to the Bay rather than in our homes. But do so in a way that doesn’t make it worse for anyone else.” 

Another listed “regional detention” and “channel improvement projects” separately, but as his top two ways to prevent flood damage.

Importance of Strong Development Regulations

Of course, flood-mitigation professionals have more tools at their disposal than storage, conveyance and infiltration. So I asked a number of them to list their most important tools.

They all emphasized the importance of strong development regulations. One phrased it as “not building where flood risks are high.” Another talked about “development with flood prevention in mind.”

A third said, “Development criteria have been front and center in a lot of conversations recently. Nuanced differences exist from one area to another. And politics always get in the way. But in this region, regardless of location, STRONG development criteria help prevent making things worse. No other way around it.”

Floodplain preservation falls under the heading of “strong development regs.”

“Some claim floodplain preservation and prairie restoration are important components to flood-risk mitigation.  They say we need these to make things better. But the fact is that preservation and restoration help keep things from getting worse.”

Buyout Strategies

Most of the topics above address the prevention of flooding. But additional tools come into play when correcting flooding.

For instance, another professional emphasized “buyouts.” He said, “Where no other solution can help, buy them out.”

Harris County Flood Control District often buys out clusters of repetitively flooded homes in an area. Compared to individual problem properties, clusters give the district an opportunity to build large regional detention detention projects such as the one that spans US59 at Halls Bayou. HCFCD had to buyout whole subdivisions in this area.

Other Important Strategies

Finally, my panel of experts also pointed to several other important flood-mitigation strategies.

• Consistent Funding – It’s impossible to build giant stormwater detention basins and widen channels without large sums of money. Nor can you attract state and federal dollars without local matching funds. So it’s important to fund flood control at consistently meaningful levels. Projects can take decades.
• Political support/strong leadership – Without the support of city, county and state leaders, projects won’t happen. And they have to be willing to go to taxpayers for the funding. That requires a…
• Well thought-out, justifiable plan – The 2018 flood bond passed because it was based on local input from citizens and leaders in every Harris County watershed. Decades of previously conducted flood studies suddenly became actionable with the bond money. The logic of it helped justify projects and build community momentum.
• Communication – is important to selling the plan, keeping it sold, and justifying changes along the way.

One could easily argue that without any one of those things, nothing else would happen. We would have no ways to mitigate flood losses.

Posted by Bob Rehak on 7/28/24

2525 Days since Hurricane Harvey