Tag Archive for: street drains

Mythbusters: Common Misperceptions about Flooding in Harris County

Since Hurricane Harvey, I’ve continually run into several widely held misperceptions about flooding in Harris County. As we head into another hurricane season, let’s set the record straight about the most common myths. Some of the facts below have been adapted from information provided by the Harris County Flood Control District.

MYTH: The Harris County Flood Control District is responsible for addressing all types of flooding.

FACT: The Harris County Flood Control District is responsible for bayous and many of their tributaries. However, the City of Houston, other municipalities, and Precincts – in unincorporated Harris County – handle storm sewers and roadside ditches.

street flooding

The Texas Department of Transportation handles drainage of highways and their feeder roads.

The moral of this story: make sure you call the right people when you see a problem developing.

MYTH: I’ve lived in my house for more than 30 years and I’ve never flooded. Therefore, I don’t need flood insurance.

FACT: Most Harris County residents live in homes vulnerable to flooding because:

  • Our topography is flat.
  • Many of us have impermeable clay soils that increase runoff.
  • Our subtropical climate can produce large amounts of rain in short periods of time.

Storm rainfall patterns may have spared your area since you have lived there. But that could change like the weather.

Remember. People thought Tropical Storm Allison was the worst. It caused all the flood maps to be revised. Then along came Harvey. Now, HCFCD and FEMA are revising the flood maps again.

During Harvey, more than 68 percent of the homes that flooded in Harris County were outside the 100-year flood plain. So, consult your insurance agent. Most homeowner insurance policies do not cover flooding. You need a separate policy for that.

MYTH: A 1-percent (100-year) flood occurs only once every 100 years.

FACT: A 1-percent (100-year) flood can occur multiple times throughout a century. A 100-year flood has a 1-percent chance of occurring in any given location in any given year. Doesn’t sound like a lot? Think of it this way: A home in a 1-percent (100-year) floodplain has at least a 26-percent chance of flooding during a 30-year period of time – the duration of many home mortgages. And remember, Harris County experienced four hundred-year events in four years (Tax Day, Memorial Day, Harvey, and Imelda).

MYTH: I only need to worry about flooding during hurricane season.

FACT: Flooding can happen any time of the year. Of the four storms mentioned above, two occurred outside of hurricane season.

Short, high intensity rainfalls can cause street flooding that invades vehicles and homes built close to street level or near developments with insufficient mitigation.

Hundreds of homes flooded in Elm Grove on May 7, 2019. The causes: 5.64″ of rain in about 12 hours. And a 270-acre tract upstream that had recently been clearcut with only 9% of the promised detention ponds constructed.

high water rescue truck
High water rescue truck on flooded Elm Grove Street, May 2019

MYTH: If I didn’t flood during Allison or Harvey, chances are I won’t ever flood.

FACT: The greatest rainfall brought by Tropical Storm Allison hit the northeast part of Houston and Harris County, dropping more than 28 inches of rain in 12 hours and 35 inches of rain in five days. However, some areas received fewer than 5 inches of rain. Had the damaging rains of Allison targeted other areas, they would have experienced similar, devastating flooding.

Harvey also hit and missed certain areas. But the differences were even more dramatic. While Friendswood received 56″ of rain, Willis in Montgomery County received only 5″ between August 25 through September 1, 2017. See USGS, Table 1, Page 3.

MYTH: I don’t need flood insurance because I don’t live in a mapped floodplain.

FACT: We are all at risk for flooding regardless of our proximity to a mapped floodplain. Flood Insurance Rate Maps (FIRMs or floodplain maps) published by the Federal Emergency Management Agency are good indicators of flooding risks from bayous and creeks overflowing their banks. However, they do not show flooding risks from storm sewers and roadside ditches exceeding their capacity, risks from unstudied bayous and creeks, or risks from storms greater than a 0.2 percent (500-year) flood — such as Tropical Storm Allison in 2001 or Hurricane Harvey in 2017.

MYTH: New land development causes flooding.

FACT: New development can accelerate the time of concentration of floodwaters, contributing to faster, higher flood peaks. That’s why cities and counties regulate development. But some see lax regulation and enforcement as a tool to attract new development. And even those with strict regulations may find that they aren’t strict enough to handle storms of increasing intensity.

HCFCD graph showing effect of development in Brays Bayou watershed. Insufficiently mitigated development over 85 years accelerated runoff, building flood peaks faster and higher.

Flooding can be inherited from areas developed before our understanding of flooding improved. So it would be safer to say that “Insufficiently mitigated development causes flooding.”

Regulations dating to the early 1980s in many areas require stormwater runoff after development to be no greater than runoff before development. Developers must detain any excess stormwater on site. However:

  • Development prior to the 1980s was not as regulated.
  • Our understanding of what constitutes a 100-year rainfall continues to evolve. So pre/post estimates may be off.
  • Loopholes exist in many jurisdictions that allow developers to avoid building detention ponds.

Today, we have a hodge-podge of regulations throughout the region. Learn regulations in your area and monitor new developments to ensure compliance.

MYTH: A storm surge from a tropical storm or hurricane will inhibit our bayou system’s ability to drain.

FACT: Most of our bayous and creeks are upland and drain by gravity. Because of their natural slope toward Galveston Bay, a storm surge caused by a tropical storm or a hurricane will not impede this process. Of the roughly 2,500 miles of bayous and creeks in Harris County, only a small portion near Galveston Bay will be influenced by storm surge for a short period of time.

Posted by Bob Rehak on 6/20/22 with thanks to the Harris County Flood Control District

1756 Days after Hurricane Harvey