Mercer Stormwater Detention Basin

Construction Begins on Mercer Stormwater Detention Basin

Harris County Flood Control District (HCFCD) has begun construction of a 512 acre-foot stormwater detention basin upstream from Lake Houston on Cypress Creek near Mercer Arboretum. HCFCD hopes to complete the project in 2024.

The Mercer Project is located on the northeast corner of Hardy Tollroad and FM1960. It stretches north to Cypress Creek. A residential neighborhood borders it on the east.

Clearing the wooded site in Commissioner Rodney Ellis’ Precinct 1 began in January and continues. Meanwhile, it appears that excavation has begun in the portion of the site closest to Cypress Creek.

The projects includes two connected basins. Construction of the north basin appears further along than the south basin.

Mercer Stormwater Detention Basin

Photos of Construction Progress

I took the photos above on Sunday afternoon, 2/25/24. Surprisingly, contractors were hard at work.

Looking NW from over FM1960 toward Cypress Creek and Hardy Tollroad. North Basin will be on right. Start of South Basin is on left. Lower right area contains wetlands.
Mercer Stormwater Detention Basin
Reverse angle looking S toward downtown Houston on horizon. Cypress Creek in foreground. Wooded area on left is designated for floodplain preservation.
Closer shot shows beginning of excavation of north pond.
Contractor is burning downed trees. I did not notice any piles of mulch.
Looking N at more new clearing for South Basin.

Details and Funding for Mercer Project

Both basins will have dry bottoms. An underground equalizer pipe will balance the water level in the two basins, while avoiding the majority of jurisdictional wetlands and streams on the property.

A 54″ outfall pipe will move water back into the creek from the basins after a storm at a rate consistent with pre-construction runoff. A 30-foot-wide berm will accommodate maintenance and future recreational activities such as hike and bike trails.

This project is listed in the 2018 Bond Program as F-88. HCFCD estimated the cost at the time at $25 million, which included land acquisition, design and construction. Construction alone costs $14.8 million.

Funding comes both from the bond and a grant from the U.S. Department of Housing and Urban Development’s Community Development Block Grant Disaster Recovery program.


The Mercer Project will lower the water surface elevation in the area shown below during a 100-year flood by .35 feet.

Mercer Project location and benefit limit per HCFCD.

The Mercer project will remove the 100-year area of inundation from 30 structures and the 500-year area from another 17 structures. The homes benefitted extend from the Hardy downstream approximately to Cypresswood Drive.

Impact on Lake Houston Area

I applaud this project because every little bit counts. But three to four inches of flood-level reduction along Cypress Creek will have a much larger impact along Cypress Creek than in the Humble/Kingwood area. Here’s why.

Cypress Creek is just one of 11 watersheds that feed into Lake Houston. Cypress joins Spring Creek just upstream from the US59 bridge over the West Fork. In a 100-year storm evenly distributed across the region, the total runoff volume for each watershed upstream of Lake Houston is almost 2 million acre feet.

This project provides a hair more than 500 acre feet. So, by itself, it will retain only 0.025% of the water running off upstream of Lake Houston.

However, the Mercer Basin will retain 0.27% of the runoff in the Cypress Creek watershed. See table below provided by the SJRA.

Acre-feet of runoff from watersheds upstream of Lake Houston, assuming a 100-year rain uniformly distributed across the region.

A regional drainage study for the Cypress Creek watershed found that flooding along tributaries is predominately caused by stormwater from a rising Cypress Creek backing up into the tributaries. It’s not a lack of sufficient stormwater conveyance or drainage capacity on the tributaries themselves. HCFCD believes stormwater detention basins could reduce that backwater issue.

HCFCD’s Ultimate Recommendation for Cypress Creek

The regional drainage study described here recommends nearly 25,000 acre-feet of additional stormwater detention in the watershed – 50X more than this project provides. It’s unclear where all that land will come from in a watershed that’s already highly developed.

And that dramatizes the need to preserve land along creeks and bayous before it gets developed. That will be a far more cost-effective strategy in the long run.

Posted by Bob Rehak on 2/25/24

2371 Days since Hurricane Harvey