Greens Bayou Mid-Reach Segment Reaches 10-Year Level-of-Service Goal. Now What?

In the Tuesday 8/23/22 Harris County Commissioners Court Meeting, Harris County Flood Control District (HCFCD) will deliver the results of a study on the Greens Bayou mid-reach area. The study shows that when the Aldine-Westfield Phase 2 and Lauder Phase 2 Basins are complete, the area will be protected from a 10-year flood (10% annual chance). But the study doesn’t stop there. It also recommends building another large detention basin and increasing channel conveyance to protect the area in a 25-year flood (4% annual chance).

10-Year Protection Achieved

Back in 2003, HCFCD started working on a plan to bring much needed flood reduction to the area between Veterans Memorial and JFK along Greens. That stretch covers 11 miles of Harris County Precincts 1 and 2.

Since then, four stormwater detention basins have been built along the bayou.

Also since 2003, NOAA has developed new Atlas-14 rainfall probability estimates.

The new estimates show that northern Harris County could experience 30-40% more rain than previously estimated in major events. With both rainfall and detention basin capacity increasing, the question became, “Where do things stand?”

This new engineering study shows that the four existing detention basins (Kuykendahl, Glen Forest, Aldine-Westfield and Lauder) should protect homes and businesses in a 10-year flood. But achieving greater protection will require something more.

Four Alternatives to Increase Capacity

Engineers looked at four different alternative combinations of stormwater detention capacity and/or channel conveyance improvements to provide more protection. Each alternative involved the proposed Hardy Stormwater Detention Basin shown in red on the map above.

• Alternative 1 – Building a 3,000 acre-foot basin
• Alternative 2 – Smaller 2,000 acre-foot basin with more room for recreation
• Alternative 3 – Same as #2 but also with channel conveyance improvements
• Alternative 4 – Only conveyance improvements; no additional storage capacity
• Alternative 5 – Build nothing else. Stop with existing basins.

The chart below summarizes what they found.

The engineers recommended Alternative #3 – given sufficient funding. It would achieve a 25-year system capacity in general. The area immediately downstream of I-45 would achieve closer to a 10-year system capacity. And the area immediately downstream of the Hardy Tollroad would be closer to a 50-year capacity.

The proposed and existing improvements would create enough storage to hold a foot of rain falling across 16.4 square miles!

Cost: A mere $196 million on top of the $126 million already invested in the other four basins. Compared to Alternative 5, #3 would protect another 25 structures in a 10-year flood, 173 in a 50-year, and 239 in a 100-year. HCFCD did not list a number for a 25-year event. But we can assume it’s somewhere south of 173.

This HCFCD presentation recommends a phased approach to implementation to accommodate annual funding levels.

• Phase 1: 3.6 miles of channel conveyance improvements
• Phase 2: 3.5 miles of channel conveyance improvements and 500 acre-feet of stormwater detention 
• Phase 3: 3.5 miles of channel conveyance improvements and 500 acre-feet of stormwater detention
• Phase 4: Final 1,000 acre-feet of stormwater detention

The slide below shows how much the existing and new improvements would shrink the 25-year floodplain. Mentally subtract the purple areas to see before and after.

For the full presentation, click here.

Posted by Bob Rehak on 8/21/2022 based on material from HCFCD

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