How Sand Mines Increased Erosion Potential by 33X During Harvey
Yesterday, I posted about how major storms transport the vast majority of the sediment that flushes down rivers. That’s a major reason to have wide buffers between mines and rivers, and to get the mines out of the floodway. One major event, as we have seen, can alter a river and people’s lives forever. However, in Texas we allow sand mining right up to the edge of the river, increasing the potential for erosion.
Scalping 20 Square Miles of Forest Increased Erosion Potential
We all know from science classes that when you remove ground cover, you increase the potential for erosion. That’s exactly what the sand mines upstream from the Humble/Kingwood area have done. They have removed about 20 square miles of ground cover to expose sand. And because virtually all of the mines are in the floodway, they effectively increase the riverbed width during floods by an average of 33X. Here’s the basis for the calculation.
Exposed Sand in Floodwater is 33X Wider than Natural Riverbed
Between I-45 and I-69 on the West Fork, a distance of 20 miles, we have approximately 20 square miles of sand mines. So we have one square mile of mines per mile.
If you lined all mines up, end to end, you would have a swath exactly one mile wide.
But the river is, on average, only .03 miles wide. Thus, mines widen the effective riverbed width by average of 33X.
By removing all the surface vegetation, miners also increased the potential for erosion during extreme storms, such as Harvey, by 33X.
To put a mile-wide riverbed in perspective, that’s twice as wide as the muddy Mississippi … in the Delta region.
Sediment Dams Increase Flood Levels
That helps explain how so much sand piled up virtually overnight during Harvey. The flood was wide enough to inundate the mines. Afterwards, a nearly continuous trail of sand along both shores of the West Fork led directly to the Humble/Kingwood area as the pictures in my gallery show. In many places, this trail was more than 5 feet deep. Here’s what it looked like onshore in River Grove Park.
Plus there was far more sand in the river after the flood than before.
If the river AND both shores were ALL higher after the storm than before, then it stands to reason that most of that sand had to come from somewhere other the river itself.
Looking at a satellite image, the barren sand mines dominate the landscape like no other feature in the watershed. The obvious conclusion is that much of the sand came from mines like this …
… and wound up in places like this.
Or places like those below, where it formed sediment dams, which the Army Corps says contributed to flooding.
Huge Pre/Post Increases in Deposition Rates Since Sand Mining
In its final report on Hurricane Harvey, Harris County Flood Control District confirms the enormity of the deposition. It says, “Large amounts of debris and heavy sedimentation upwards of 4.0-8.0 ft in some locations have been noted especially along the West Fork of the San Jacinto River.” (See Page 7.) But how does this compare to the deposition rate before Harvey?
Sedimentation Rate Much Higher than Before Sand Mining
In 1983, Turner Collie & Braden, an engineering consulting firm, estimated the loss of lake capacity due to sedimentation at 311 acre-feet per year. (See page 9.)
Large-scale sand mining on the West Fork began shortly after that and has grown ever since. On the East Fork, sand mining began in the early 2000s.
The Texas Water Development Board created the difference map below by comparing data collected in 2011 with data collected from March through June of 2018. Remember, three or four of those years were drought years when very little sediment came down river. Virtually all of what you see below happened during the last three years. We can also see from satellite photos that most of that happened during Harvey.
The map above shows we gained 1.0 to 5.5+ feet of sediment in most of the 3400 acre area between the mouth bar and FM1960.
Assuming we gained on average about 3 feet per acre, that means the City lost approximately 10,000 acre-feet of storage in this ONE SECTION of the lake in only three years.
Current Rate Estimated 22X Higher
That’s about an 11X increase per year compared to the Turner Collie & Braden study which measured the ENTIRE lake. However, we can also roughly adjust for the difference in lake and sample sizes shown above. Page 9 of the Brown & Root report in 2000 says that the area shown above collects a little less than half (42%) of the sediment flowing into the lake. So we can assume that 11X at least doubles to 22X. (Because there’s more in the bottom portion of the lake than the top.)
22X is less than 33X, but consistent with earlier observations when you consider that much of the sand was deposited on shores, as you saw in the River Grove photo.
Caused by Mining or Nature?
Some of this sand also came from urban runoff. And some undoubtedly came from other tributaries, such as Spring and Cypress Creeks, which have fewer mines. Some also came from the East Fork watershed, where there is a huge active sand mine on Caney Creek. Regardless, 131,000 cfs cut across that statistical mile-wide swath of sand on the West Fork during Harvey.
Analysis of satellite and aerial photos leads me to believe that the river carried millions of cubic yards of sand and sediment downstream from the mines, including their stockpiles. That sand, I believe, helped to create the blockages shown above, which contributed to flooding throughout the heavily populated Humble and Kingwood areas.
Let’s Get Sand Mines Out of the Floodway
Miners claim that the currents in Harvey were not strong enough to carry sand out of their mines. Several world-leading hydrologists that I have talked to claim the opposite. As one said, “Of course it could.”
That’s why we need to pass legislation moving mines back from the river. We can’t reduce natural sedimentation, but we can reduce man-made sedimentation by putting sand mines out of the reach of rushing floodwaters.
As always, these are my opinions on matters of public policy. They are protected by the First Amendment of the United States Constitution and the Anti-SLAPP statute of the Great State of Texas.
Posted by Bob Rehak on December 17, 2018
475 Days since Hurricane Harvey