Dangers of Erosion when Developing Floodplains
A resident of The Commons on Lake Houston contacted me about some severe erosion in her community. I can only describe it as stunning. It destroyed trails owned by the Property Owners Association that people used for hiking, biking and horseback riding. The loss of these trails limits recreational opportunities and has physically divided large parts of the community.
Sadly, it didn’t have to be that way. Infrastructure and ditch maintenance did not keep pace with development.
As development crept closer to the East Fork of the San Jacinto over the years, the erosion worsened. In older neighborhoods on higher ground, a series of small check dams in a major drainage canal reduced erosion.
A check dam is a small dam constructed across a drainage ditch to counteract erosion by reducing water flow velocity.
Below Check Dams, Uncontrolled Erosion
The dams stop short of the East Fork. A tiny swale that residents used to step over has expanded into a steep-sided gully approximately 20 feet deep and 50-75 feet wide. Not even concrete can stop the erosion now.
Trails used to run alongside and across this ditch. Now they’ve been swallowed. Residents have nicknamed the ditch “The Grand Canyon.” They fear walking near the edge because of potential for cave-ins.
Causes of Erosion
Erosion can result from many things. Multiple factors played a role in the Commons.
As the developer built up land to elevate foundations, he increased the slope. That accelerated runoff.
Clearing land for a new subdivision along the ditch also accelerated erosion of soft, sandy soil.
Finally, concentration of runoff also played a major role. When runoff spreads out over over acres, it poses no threat. But concentrating it turns a thousand trickles into a firehose aimed at loose, sandy soil. The result: severe erosion every time it floods.
Residents of The Commons have already seen how that erosion can destroy recreational opportunities and infrastructure. They pray that their developer will fix the Grand Canyon before it starts eating homes.
Lessons for Kingwood
This Commons story contains timely lessons for the residents of Kingwood as we consider a potential high-rise development in the floodway and floodplain of the San Jacinto.
The Commons erosion reminded me of the Kingwood Rapids. Whitewater enthusiasts gave that name to the drainage ditch that runs between Kingwood and Forest Cove near Deer Ridge Park, just south of Walnut Lane (see below).
The proposed new high-rise development would use this ditch to drain hundreds of acres that they intend to pave with concrete.
High-Rise Concern: Erosion and Incision
As you can clearly see, the ditch can barely handle existing runoff during storms. It’s severely eroding.
Draining high-rise, high-density commercial space into these ditches will cause them to “incise.” Incise means “cut into.” Runoff will deepen and/or widen ditches. But ditch erosion already threatens nearby homes.
This same ditch runs through River Grove Park, which already cost Kingwood residents more than half a million dollars in repairs after major storms in 2015, 2016 and 2017. The soccer program at River Grove still has not fully recovered. The lacrosse league has abandoned its lease there. One shudders to think of the damage that the loss of River Grove to do to the entire community.
Impact on Water Quality
All this erosion also has a direct impact on water quality in several ways. First, the sediment flows into the lake. There, it reduces lake capacity. The sediment also increases turbidity, which increases water treatment costs and harms riparian vegetation. That vegetation helps stabilize banks, protect property and provide cover for fish which waterfowl and eagles feed on6
More food for thought as you compose your letters to the TCEQ and Army Corps.
Posted by Bob Rehak on 1/16/2019
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