They Clearcut 533 Acres, Then Got 17 Inches of Rain

Construction is always the riskiest period during a development project. Especially when you clearcut 533 acres and get 17 inches of rain … in a week. However, thanks to best practices and luck, most people surrounding the Sila development in Huffman narrowly escaped what could have become a major disaster.

As the people in Woodridge Village learned repeatedly on far less rain, best practices can make all the difference. They lived near a development where stormwater detention basins were not built before the rain; the people near Sila did.

The new 553-acre Sila Project in Huffman on the East Fork of the San Jacinto seems to have had considerably less impact on neighbors. This is a story of people caught between a rising river and sheet flow coming down a slope.

Sila’s Similarities, Differences with Woodridge

Several similarities with the Woodridge project exist. Sila is big. Built on an identical slope. And clearcut. But after 17 inches of rain last week, the outcome was dramatically different.

Instead of flooding hundreds of homes below the development as Woodridge did, most residents near Sila only got water in their yards. One fish pond was ruined. At least two vehicles flooded. And at least one garage flooded.

The big differences between Sila and Woodridge?

Sila had already built stormwater detention basins. When the rain hit, a series of berms and ditches helped funnel runoff to the basins and away from neighbors. Silt fences intercepted much of the runoff. And crews replaced the fences when silt started to spill over the top.

Before looking at pictures, first let’s look at some stats that put the rainfall in perspective.

5- To 25-Year Rainfall

After Sila weathered this year’s January rains, it got slammed again in the week between April 29 and May 5 with almost 17 inches of rainfall.

*Nearest official Harris County gage at FM1485 on East Fork San Jacinto showed* *16.68 inches of rain fell in the seven days from April 29 to May 5.*

That included 7 inches in one day. And half of that fell in one hour.

*Of the 17 inches, 7.12 inches fell on 5/2/24 alone, with 3.48 inches in one hour.*

NOAA defines a 7-day/17.1 inch rain in this area as a 25-year rainfall. Ditto for a one-hour 3.88 inch rain. Seven inches in one day, however, is only a 5-year event.

However, the East Fork peaked at FM1485 at a level equal to a 500-year flood according to Harris County Flood Control’s Flood Warning System. That’s because even heavier rain fell upstream in less time. It then reached FM1485 when heavy rains were falling there.

*East Fork peaked over 77 feet at FM1485 on May 5.*

*According to HCFCD, 77 feet is more than a 500-year rain.*

Also, according to HCFCD, to date, the area near FM1485 has received more than two thirds of a year’s average annual rainfall in about one third of the year – 34.72 inches!

Pictures Taken During Week of Heavy Rains

Northwood Country Estates resident Max Kidd provided many of the following photos taken at ground level during the flood.

They show mainly severe street flooding. Thankfully, Harris County Precinct 3 Commissioner Tom Ramsey’s crews had just finished cleaning roadside ditches. Otherwise these photos might have told a much worse story.

*Water poured out of Sila across Northwood Country Road on 5/2, the day before the big rain.*

On 5/3 and 5/4/24, Kidd took these photos.

Kidd believes the home above likely flooded from the East Fork. However, Sila runoff may have added to the flood depth.

Photos Taken After the Storm

Sila is so big that it’s hard to get it all in one shot. The aerial shots below were all taken on 5/6/24 after the rain ended.

*Looking E across the southern portion of Sila. St. Tropez, a separate development is at very top of frame* *across FM2100*. *But Sila drains to the East Fork, and St. Tropez drains to Luce Bayou.* *Luce later re-enters the East Fork near its mouth at the headwaters of Lake Houston.*

Sila had a swale behind the erosion to slow water running down a hill out of frame to the left. However, that swale filled in with sediment, according to Kidd. And water then flowed into a ditch behind the fence filling it with sediment, too.

*Still, some sediment flooded into the backyards of neighbors.*

*A series of detention basins caught and channeled runoff through the development*.

*Shortly after the rain ended, it appears that bulldozers graded perpendicular to slopes to help retain or slow down any additional rain that might fall.*

In the photo below, also note the forested corridors that break up Sila runoff. While large portions have been clear cut, those that haven’t help retain sediment.

*Detention basin was holding water and emptying it slowly, presumably at the pre-development runoff rate.*

*Still, some sediment escaped the development. Note roadside ditch filling in at peak of triangle.*

As I left after this photo session, I noticed workers replacing and reinforcing the silt fence above. But the photo below shows the volume of the sediment that escaped despite their efforts. A lot!

*Kidd’s fish pond is no longer habitable* *by fish* *due to Sila runoff that polluted it.*

*Lowest of the detention basins at southwestern edge of development was sending stormwater into a wetland mitigation bank before it reached the East Fork.*

*Contractors had put silt fence at the outfall of the detention basin to help retain sediment.*

*But then they pumped water toward the river, through a mass of muck, creating more erosion.* *(Can’t win ’em all.)*

*Along the way to the river, some of the dirt will get a chance to settle out in the wetland mitigation bank.*

*Regardless, the East Fork was running orange on 5/6/24*.

No doubt, several sources contributed to the discoloration. They include Sila, other upstream developments, sand mines and natural, river-bank erosion.

Few developments that I have observed go to as much trouble to control runoff as Sila. And few bother to leave trees these days. I wish more did. We might have less sediment clogging our rivers and contributing to downstream flooding.

Posted by Bob Rehak on 5/9/24

2445 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.

Sila’s Similarities, Differences with Woodridge

5- To 25-Year Rainfall

Pictures Taken During Week of Heavy Rains

Photos Taken After the Storm

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