Photo Essay on Role of Riparian Vegetation in Reducing Erosion
Riparian means “of or relating to the banks of a river.” To see the role of riparian vegetation in reducing erosion, one need only compare the two forks of the San Jacinto River. They provide a stark contrast. But the real story is the role of sand mining in reducing riparian vegetation.
After years of sand mining on the West Fork, much of the shoreline vegetation has been lost and the resulting erosion is staggering. Between I-45 and US59, sand miners have stripped vegetation from approximately 20 square miles of floodplain and floodway (the main channel of a river during a flood).
Sand Mines on West Fork of the San Jacinto form an almost continuous line from I-45 to US59. They have stripped approximately 20 square miles of ground cover.
On the East Fork above the Caney Creek confluence, however, there are no sand mines. The vegetation is lush and the erosion is negligible. Let’s start there for a look at how nature protects us.
Forests come down to the river’s edge. Grasses and cattails abound, protecting the banks.
Dense forest anchors the land. Grasses, forced to compete for sunlight, thrive along the river’s edge, protecting banks.
A perfect time and place for reflection. A nice place just to “be.”
All images so far were taken on the East Fork of the San Jacinto River above where it merges with Caney Creek. Image courtesy of Google Earth.
Red lines on left measure width of East Fork on 3/3/16, before the Tax Day Storm. They are in a separate layer. Switching the background image to 10/28/17 shows that the river is virtually unchanged, thanks in large part to the lush riparian vegetation.
Now, A Trip up the West Fork
Now, let’s look at the West Fork. It’s vastly different.
Townhomes on Marina Drive in Forest Cove. Concrete, steel and wooden walls on the West Fork were less effective at preventing erosion than blades of grass on the East Fork.
Same area. Note steepness of banks where vegetation can no longer take hold, perpetuating cycles of erosion.
Remnants of concrete retaining wall.
Site of a breach in sand mine dike on the West Fork. The mine discharged sediment directly into the river.
Two weeks after Harvey. Just north of US 59 bridge.
West Fork Sand mine complex. Note one of many dike breaches in various mines that allowed sand and sediment to pour downstream. All helicopter images taken two weeks after Harvey on 9/14/17.
Mining a point bar after Harvey. Miners are supposed to work within their dikes to avoid disrupting vegetation along the river. Photo taken 9/14/17.
Note more repairs to dikes.
The next three images form a series.
River is migrating toward pit in background at the rate of 12 feet per year, in part, due to lack of vegetation protecting banks. See next two images before for overhead views.
This is what the area above looked like in a 1995 USGS aerial photo on Google Earth. Compare the location of the red line in this image with the location in the next image. The GPS coordinates of the line are identical. But the river has migrated.
In just 23 years, the West Fork migrated 258 feet toward the dike on the right and now threatens it. The river has eaten away at the dike an average of 12.4 feet per year. The dike is now only 38 feet wide.
A bright white trail of sand leads all the way from the mines to the mouth bar which helped back water up into the highly populated Humble/Kingwood area. Fresh sand is several feet deep. Note absence of grasses. Many of the trees will also soon die.
Nearing the US59 bridge
Confluence of West Fork and Spring Creek, which also contributed sand to this event.
The next two images form a before/after pair.
West Fork of the San Jacinto over the US 59 Bridge before the Tax Day flood in 2016. River was 330 feet wide. Image courtesy of Google Earth.
GPS coordinates of the red line have not changed; the river has. After Harvey (in a little more than two years), the West Fork widened to 489 feet and shifted north by 113 feet. In part, this was due to excessive sediment that killed vegetation along the banks and accelerated erosion. Dead trees swept downriver were trapped by the bridge pilings, forming a dam that helped flood Humble businesses south of this photo. The southbound lanes of the bridge had to be replaced by TexDoT at a cost of approximately $20 million because of erosion.
Union Pacific railroad traffic was disrupted for months.
Mountains of sand may kill the remaining trees in this area, exposing it to even more erosion during the next storm.
Sand, in part, from the mines, has almost totally blocked the West Fork where it meets Lake Houston. Before/after measurements show that as much as ten feet was deposited in this area during Harvey (approximately five feet below water and five above). This forms a dam behind the dam, that backs water up into the Humble/Kingwood corridor during storms. Unless this sediment is removed, a storm smaller than Harvey could create Harvey-scale flooding.
Tree Loss in East End Park Has Already Started
Acres of trees in Kingwood’s East End Park have already started to die back as a result of being buried in dunes 10-15 high. I believe that sand, in large part, from the 750-acre mine upstream on Caney Creek is causing this. Piling as little as six inches of sediment around the base of a tree can kill it.
Trees dying in Kingwood’s East End Park because of massive sediment build up around their trunks.
The website SF Gate describes how this die-back happens. “Soil added around a tree reduces the amount of oxygen available to the roots and slows the rate of gas exchange in and around the roots. There may be less moisture and nutrients available to the roots or too much moisture may remain around the tree’s roots. Inadequate oxygen reaching the roots or microorganisms in the soil around the roots can lead to an accumulation of chemicals that can injure tree roots. The tree’s bark may decay where soil is newly in contact with it. Damage or injury to the tree because of the added soil may not become apparent for several months or years and generally appears as a slow decline followed by death.” The same thing can happen with grasses and smaller trees along riverbanks. Once they die back and there is nothing left to bind the soil…
“Sediment is the primary pollutant expected from quarry operations.”
The Texas Commission on Environmental Quality says, “Sediment is the primary pollutant expected from quarry operations.” See page 22 of this 2012 report from the Texas Commission on environmental quality about the John Graves Scenic Riverway District on the Brazos. The TCEQ also conducted experiments showing that certain types of revegetation can reduce sediment discharge from mines by 98 percent.
These findings are consistent with Louisiana Best Management Practice Guidelines for Sand Mines. They state that grasses can reduce erosion by 99%.
Conclusion
In the upcoming legislative session, the Lake Houston area needs to push for the creation of a river preservation district like the John Graves. The Graves District excludes sand mines from the 100-year flood plain and floodway where most erosion happens.
All Lake-Houston-area mines are in the FLOODWAY with the exception of one. A floodway is defined as the main channel of the river during a flood. This makes the mines more susceptible to river capture and massive erosion, which can create a downward spiral as we have seen above. Eventually it can lead to loss of property.
Our preservation district would stretch from Lake Conroe to Lake Houston, the primary sources of water for two million people.
The lives, health, homes, and businesses of two million people are certainly worth as much as protecting some scenery.
Posted by Bob Rehak on 12/1/18
449 Days after Hurricane Harvey