5-Year Rain Brings White Oak Creek Close to 100-Year Mark in Royal Pines
Between 1:00 and 3:00 PM on 1/24/23, approximately 3.6 inches of rain fell over Royal Pines in southeast Montgomery County. According to Atlas-14 rainfall tables, that qualifies as a 5-year rainfall event. But floodwaters from White Oak creek approached the edge of the 100-year floodplain. And missing silt fences let sediment escape into the wetlands that border the property.
A friend who lives a mile from Royal Pines recorded about 4″ on his rain gage for the full day. A check of nearby rain gages on the Harris County Flood Warning System, showed that the official gage at FM1485 and the San Jacinto East Fork recorded approximately 3.6 inches between 1 and 3 PM today.
Cross-referencing that rate with NOAA’s Atlas-14 rainfall probability estimates for this area, we can see that 3.6 inches in 2 hours equals a 5-year rain.
100-Year Flood Line
Now let’s look at how close that 5-year rain came to the 100-year flood line. In the construction diagram below, the developer shows the edge of the 100-year flood plain. It’s the dotted line between Zone AE and Zone X. I’ve circled the relevant portion in red.
If you were to project that line toward the lower right, it would roughly parallel the heavy black line that forms the eastern boundary of Country Colony, which you can see in the middle right of the photo below.
Floodwaters from Creek Overflow Royal Pines
The water comes almost to the edge of the floodplain shown in the construction diagram above.
That big area filled with water, is a part of White Oak Creek cutting across Royal Pines. Think it’s just standing water? Think again.
The closer shot below shows water streaming through the soon-to-be subdivision and filling the Country Colony drainage ditch to overflowing.
Notice also how the floodwaters approach what appears to be some sort of water treatment facility in the upper right.
These shots also document the absence of silt fence on the eastern side of Royal Pines.
All that silt will migrate down White Oak Creek and Caney Creek into the East Fork San Jacinto which the City of Houston just dredged at great public expense. The public also must foot the bill for increased water-purification costs.
More Missing Silt Fence in NW Corner of Royal Pines
The SE corner of Royal Pines wasn’t the only part of the development missing silt fence. The developer removed it from the NW corner – where a neighbor has now flooded three times in two months.
Those woods contain sensitive wetlands.
How Can a 5-Year Rain Reach Almost as Far as a 100-Year Floodplain?
We need an answer to that question before this development starts pouring concrete. There are several possible explanations.
- Clearcutting accelerated runoff.
- Bulldozers compacted soil, limiting the rate of infiltration.
- The developer hasn’t built any stormwater-detention-basin capacity to offset the increased runoff.
- Planners used old (lower) Montgomery County rainfall data to determine the extent of the floodplain in their plats and plans.
- Engineers didn’t count on the cumulative impact of insufficiently mitigated upstream development, some of which used beat-the-peak, hydrologic-timing surveys to avoid building detention basins.
- The developer altered the landscape.
- More rain fell upstream than at the gage shown above.
- Some or all of the above.
I took these photos within an hour of the end of the rain. So there wasn’t much time for water to work its way downstream very far.
Two floodplain experts I consulted pointed to the cumulative impact of upstream development as a possible culprit. Engineers are likely working with flood data acquired in the 1980s before Montgomery County became one of the fastest growing counties in the region. The data is simply too complex to adjust after each new development. So, it never gets revised and errors compound over time.
I’m sure the Montgomery County Engineers Office and TCEQ will want to get to the bottom of this before the developer starts building homes. If homebuyers flood on rains that are far less than 100-year rains, tremendous liabilities could result.
When Perry Homes attempted to build Woodridge Village – only one-half mile from Royal Pines – its engineers pretended no floodplains existed. That cost Perry Homes millions of dollars and its reputation.
If Royal Pines or Montgomery County would like to rebut the issues I’ve raised, I will be happy to publish their point of view. The public deserves to know what’s going on.
Posted by Bob Rehak on 1/25/23
1975 Days since Hurricane Harvey
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