Where did all the sand come from?
Our San Jacinto River is clogged with sand that impedes the flow of water and contributes to flooding. Where did all the sand come from? When? Under what conditions? Are there ways to reduce the volume of sand coming downstream? As the U.S. Army Corps of Engineers prepares to dredge the San Jacinto for the first time, we should ask ourselves these questions.
The river has eight tributaries that affect the Lake Houston Area: Spring Creek, Cypress Creek, Lake Creek and the West Fork on the west; and Peach Creek, Caney Creek, Luce Bayou and the East Fork on the east. All produce sand naturally.
They send sand downstream at different rates at different times, depending on the location of rainfall within the watershed, the volume of flow, the speed of flow, and management of the flood gates at Lake Conroe.
Other factors include the percentage of sand content in soil and the health of vegetation along stream banks. Vegetation retains and slows runoff, reducing erosion.
The map above helps us understand why so many sand miners chose to locate along the West Fork – lots of sand. The West Fork is also sparsely populated compared to Spring Creek as you can see in the satellite image below. It shows the sand mines around the Humble/Kingwood area highlighted in red. One is located on Caney Creek (right); the rest are on the West Fork (left).
While sand has been coming down the river and streams for thousands of years, rapid sedimentation in the West Fork between Humble and Kingwood didn’t become an issue until the growth of sand mining on the West Fork in the late 1980s.
Notice how most of the areas in red above are filled with natural vegetation in the 1985 image below.
Today, mines expose approximately 20 square miles of loose sand on the West Fork alone between I-45 and US59.
Dikes around the mines are supposed to keep sand from being discharged into the river. However, Harvey inundated the mines.
An analysis of satellite images before, during and after Harvey shows massive loss of sand from stockpiles within many of these mines.
During floods like Harvey when the SJRA releases water from Lake Conroe, dikes are overtopped and broken. I suspect that sand then comes down the West Fork in tremendous volumes that dwarf Spring Creek’s contribution.
To test this hypothesis, I looked at USGS flow data for both tributaries. I also reviewed all my aerial photos and Google Earth’s historical images.
Under normal conditions, Spring Creek flows at 80 cubic feet per second (cfs) and the West Fork of the San Jacinto at 150 cfs. These are not sufficient flow rates to suspend sediment the size of sand. (For an excellent discussion of sedimentation, see Fundamentals of Sediment Transport at Fondriest.com.)
However, during Harvey, Sring Creek flowed at 78,200 cfs; and the West Fork at 55,000 cfs. Then the San Jacinto River Authority opened the gates at Lake Conroe. That flipped the ratio dramatically. With the flood gates open, Spring Creek still flowed at 78,200 cfs, but the West Fork increased to 130,000 cfs. Flow rates that high can (and did) move houses off their foundations.
Four hundred and fifty aerial photos in the gallery of this web site show a bright, white trail of sand between sand mines and the sand clogging the East and West Forks around Humble and Kingwood. Flood waters swept that sand from a to b. The giant sand deposits at River Grove Park and elsewhere grew exponentially during recent floods.
This tells me that when discussing the origins of the sand, we need to primarily evaluate the river during floods. More water is moving faster under greater pressure. That’s when erosion and deposition happen quickly. That’s when the river overtops and ruptures dikes. And that’s when twenty additional square miles of exposed sand surface on the West Fork make their major contribution to our sediment and flooding problems.
We can’t control sand coming down rivers naturally. However, with better sand mining practices, we may be able to reduce mankind’s contribution to our flooding problem, not to mention the related cleanup costs borne by taxpayers.
In upcoming posts, I will discuss my research into sand mining best practices.
Posted May 22, 2018 by Bob Rehak
266 Days since Hurricane Harvey