Why Flood Risk Increases During Construction
Pictures taken of a new Huffman development after a 3.75-inch, 2-year rain last week illustrate why flood risk increases during construction compared to other phases of a project.
Aerial images taken over Sila, west of the Huffman-Cleveland Road, show:
- Cleared land that accelerates runoff and decreases infiltration
- Partially constructed stormwater detention basins
- Trenches not yet consistently sloped
- Rampant erosion
- Workers pumping water out of one flooded area under construction only to flood another
Friendswood took more precautions with the 553-acre Sila than most developers take with their projects.
Regardless, despite their best intentions, a 2-year rain still overwhelmed them in places. Let’s look at examples of each point above.
Accelerating Runoff, Decreasing Infiltration
Natural ground cover slows the rate of runoff. Removing the ground cover lets stormwater flow faster.
Contractors also grade the land to create consistent slopes for roads and storms drains. That can eliminate little ponds, pockets of wetlands, and other natural catchments that slow water down.
I took these pictures just hours after the rain stopped. Notice how fast the runoff accumulated, even without storm drains.
Partially Constructed Mitigation Facilities
Stormwater detention basins collect accumulated runoff and later release it at slower pre-development rates. Thus post-development runoff rates equal pre-development rates. At least if everything is done correctly.
But what happens when a large rain slams you before you complete the detention basins?
Trenches Not Yet Consistently Sloped
During work in progress, like this, trenches may not be consistently sloped. In the picture below, water flowed downhill from left to right. A swale caught most of the runoff. And a silt fence backstopped it. Regardless…
Rampant Erosion
Backslope interceptor systems that reduce erosion on the sides of ditches and basins have not yet been built here. Neither has grass been planted on the side slopes of those ditches and basins. The result: rampant erosion that can clog ditches and streams, and reduce the capacity of basins.
Workers Saving One Area Only to Endanger Another
The first three shots below show the partially completed detention basin and workers scrambling to expel the rapidly accumulating runoff.
From there, water raced downslope toward the East Fork of the San Jacinto.
That turned the outfall for a neighboring ditch (right of greenish pond above) into a mass of muck. From there, the muck flowed toward the river.
Could Have Been Worse
I didn’t see any homes that flooded on this day. Muck did flow into a few yards and across roads. And the outfall to a neighborhood drainage ditch, which was just cleaned out, will have to be cleaned out again.
But no doubt, it could have been much worse. See below.
It’s also worth noting that runoff from Sila must go through more than a half mile of forested wetlands before reaching the East Fork. That will slow the water down, capture some of the erosion before it reaches the river, and keep the sediment from reducing the conveyance of the river channel.
Construction always entails flood risk, especially large scale construction that can take years. But with the right precautions, large rains don’t have to spell disaster.
Posted by Bob Rehak on 4/14/2024
2420 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.