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Environmental Geology Consultant
The San Jacinto River drains a Quaternary and Pliocene terrain ranging in elevation from 425 feet to sea level. The river basin forms the homesites for hundreds of HGS members in communities north and east of Houston, Texas. During three days in October, 1994, up to 30 inches of rain fell within the river basin. The river's flood plains were rapidly inundated. Homes and roads were destroyed or badly damaged. Many lives were lost. Up to seven feet of sand, in the form of huge sand waves, was deposited on the flood plain south of Lake Houston During the height of the flood, segments of several petroleum products pipelines crossing the flood plain were ruptured south of the Lake Houston spillway.
There were geologic reasons for the erosion and the pipeline ruptures that occurred during the flood. Water flowed over the Lake Houston spillway at ten miles per hour and down the 4.5 miles of sinuous river bed towards the Banana Bend area. The volume of water coming down the spillway and entering the river channel was 356,000 cubic feet of water per second. When this wall of water reached the large meanders in the Banana Bend and Rio Villa subdivision areas, the main path of the flood waters left the thalweg of the channel and flowed straight south across the river levees. Three documented avulsions occurred at the levee cuts across the sand-rich flood plain. Point bar sands from the main river channel were rapidly eroded and deposited as huge sand waves along the flood's path.
A major avulsion at the Rio Villa subdivision isolated the community and ruptured several pipelines. The leaking hydrocarbons floated southward for 2.2 miles to the Whites Lake area near I-10 where they concentrated in an eddy and ignited. The fire subsequently spread north along the plume of floating hydrocarbons to the source at the rupture.
There was not a single pipeline rupture along the normal meandering course of the San Jacinto River. Instead, the ruptures took place at a point where the river attempted to cut a new channel into the flood plain, following normal geologic processes of a meandering river in flood stage.
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