System Capacity Maps for Harris County Channels Reveal Widespread Problems
12/2/24 – System capacity maps for each of Harris County’s 23 watersheds show widespread problems and raise questions about whether the county’s most severe needs are confined to historically underserved areas.
Harris County Flood Control District (HCFCD) released the maps in response to a Freedom of Information Act (FOIA) request.
What “System Capacity” Means
System capacity reflects the ability of a stream or channel to handle rainfall of different intensities before coming out of its banks. The way system capacity is expressed also reflects the expected frequency of over-bank events.
A channel that can hold a:
- 100-year rainfall has a 1% system capacity
- 50-year rainfall has a 2% system capacity
- 25-year rainfall has a 4% system capacity, etc.
“1% system capacity” indicates that the infrastructure is designed to handle the peak flow or volume associated with a 1% event without causing overflow or flooding.
One Key Part of Comprehensive Flood-Risk Picture
System-capacity maps are one piece of information used in determining a comprehensive view of flood risk. They help you quickly spot areas that need closer examination.
Other key factors used to determine flood risk include: topography around the channel, the elevation of a structure, degree of development in an area, building codes in effect when an area was built, population density, and more.
Impact of Atlas 14
The maps released today rely on pre-Atlas 14 data. Harris County did not provide current maps. Nor do they reflect improvements made to channels recently. But the older maps are still instructive because they formed the starting point for spending billions of flood-mitigation dollars. However…
Because these maps are based on old rainfall standards, today’s true system capacity is actually lower than shown.
Said another way, the situation on the ground is worse than the maps show. That’s because the transition from pre-Atlas 14 to Atlas-14 data in Harris County resulted in significant increases in estimated rainfall depths for various storm events.
Notably, the 1% annual exceedance probability (AEP) 24-hour rainfall depth—commonly referred to as the “100-year” event—experienced substantial changes.
Prior to Atlas 14, Harris County was divided into three hydrologic regions with the following 100-year, 24-hour rainfall depths:
- Region 1: 12.4 inches
- Region 2: 12.8 inches
- Region 3: 13.5 inches
With the implementation of Atlas 14, these values increased to:
- Region 1: 16.3 inches
- Region 2: 16.9 inches
- Region 3: 18.0 inches
This represents increases of approximately 31%, 32%, and 33% for Regions 1, 2, and 3, respectively.
These updated figures reflect a more accurate understanding of rainfall patterns, incorporating additional years of data and improved analytical methods. Consequently, infrastructure design and floodplain management practices in Harris County have been adjusted to align with these revised estimates, enhancing resilience against flooding events.
Value of Maps Based on Old Rainfall Standards
Still, these maps have value. They are a starting point for the $2.5 billion 2018 Flood Bond. They also show that:
- All watersheds have problem areas
- Channel capacity in some areas is extremely low
- Some watersheds that are not “historically underserved” have more severe system-capacity issues than those that are underserved.
System Capacity Maps
Below are low-resolution maps for each of the 23 watersheds in Harris County. For high-resolution maps of all watersheds in one file, click here. Caution: [26-meg download.]
Correlation of Maps with Spending
It’s instructive to correlate HCFCD flood-mitigation spending with these maps. Below is where more than $2 billion has gone since Hurricane Harvey, in large part, to address the problems shown above.
Compare the maps with the spending. And use the contact form of this website to let me know if you feel your area is not getting its fair share of flood-mitigation funding.
Note on Next Update with Atlas 14 Data
I have tried to get accurate flood risk data for years. However, HCFCD says it does not routinely update these maps every time it completes a new construction project. The District says it may update them again as part of the MAAPnext project after FEMA approves new maps based on Atlas-14 data.
Posted by Bob Rehak on December 2, 2024
2652 Days since Hurricane Harvey
The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.
