Recently, I flew up the West Fork of the San Jacinto in a helicopter and got some pictures of the new 91-acre Northpark Woods subdivision. It’s just north of Northpark, between Sorters Road and the West Fork. Where thousands of trees once stood, I saw a massive gash in the landscape. But when reviewing my photos, something else jumped out at me – the density of this subdivision and the percent of impervious cover it will have. Both the loss of trees and the higher percentage of impervious cover increase flood risk.
Inner-City Density Comes to Suburban MoCo
I previously wrote about Northpark Woods in a post called “Living on the Edge or the Death of Caution.”
As we saw Perry Homes do with Woodridge Village, this developer clear-cut the land. Trees often don’t survive the heavy equipment used in construction. And working around them consumes time. So builders find it simpler and cheaper to let homeowners replant them. However, regrowth can take decades, especially in heavily compacted soil which stunts tree growth.
But the tiny lots in this subdivision mean buyers may never even attempt to replant trees. There’s not much room for them.
More Impervious Cover Per Lot
Developments with tiny lots have more impervious cover (roofs, driveways, streets, sidewalks) as a percentage of the lot. The higher the percentage of impervious cover, the faster runoff accumulates, and the higher flood waters peak.
Graphically, it looks like this.
Aerial Images Taken on 10.2.19
Photographically, it looks like this. A civil engineer told me that homes like these can have 80% impervious cover.
In Shenandoah, TX, upstream from here, the City has allowed up to 10 lots per acre with 90% impervious cover!
Shifting Flood Plains
A Denver-based developer plans to sell these as starter homes. More experienced buyers ask tougher questions. When I bought a house in Dallas at a very young age, I didn’t even know what a flood plain was. Nor did I understand how inexact a science flood plain mapping can be. Or how quickly an upstream development can increase flooding downstream.
Within three years, I went from being two feet above the hundred year flood plain to ten feet below it.
The detention ponds on this site occupy the 100-year floodplain (aqua). About half the homes will be in the 500-year floodplain (brown). But keep in mind, the data on which this flood map is based has not been updated since the 1980s. So the real floodplains most likely cover far more area than shown here. Don’t be fooled by the 2014 date. The background image was updated then, not the height of the floods.
As long as less knowledgable people keep buying such homes in locations like these, developers will keep throwing them up.
Ironically, about three miles downstream, Harris County Flood Control is buying out another high density development near the West Fork. And Tammy Gunnels, only a quarter miles downstream – and in the 500-year flood plain – has now flooded 12 times in the last ten years.
Among the many other dangers of building in this location: river migration. The San Jacinto West Fork is migrating toward these homes at the rate of about 20 feet per year. When the river captures the abandoned sand pits next to these homes, it could migrate much faster.
The legal principle of “Caveat emptor” (Buyer beware) means that the buyer alone is responsible for checking the quality and suitability of goods before a purchase is made. Caveat emptor still applies when buying a house.
Most young people would not think to question when flood maps were last updated, especially if they see a date of 2014.
The data behind the flood map above is currently being updated as part of the San Jacinto River Basin Study by Harris County Flood Control, Montgomery County, SJRA and City of Houston. As of this writing, hydrologic and hydraulic models have been developed. Consultants are now calibrating those to known high water marks, such as those in this 2018 USGS study.
The USGS study shows that Hurricane Harvey at the Highway 99 gage (closest upstream gage) had an annual exceedance probability (AEP) of 2.4.
That means USGS classifies Harvey as a 42-year storm.
It could take years for that San Jacinto River Basin study to go through required public review and approval processes. Expect developers and other landowners to fight new maps every step of the way. More realistic flood maps will mean higher development costs for property that suddenly finds itself in a floodplain. And some properties could end up in floodways and be totally undevelopable. In the meantime, caveat emptor.
Posted by Bob Rehak on 10.22.2019
784 Days since Hurricane Harvey
The thoughts expressed in this post represent my opinions on matters of public policy and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.