Geology of the 1994 Flood

Introduction

The period of October 15-26, 1994, was a time of extreme events in the San Jacinto River Basin near Houston, Texas. Storms of a non-tropical nature dumped up to 76 cm (30 inches) of rain within the San Jacinto basin during October 15-20, 1994. An effective uniform depth (the depth of water if the total rainfall were equally distributed throughout the basin), was 44.5 cm (17.5 inches), the equivalent of 4.5 billion cubic meters (117 billion cubic feet). Lake Houston, a large man-made lake in the lower part of the basin, could have filled and emptied at least 10 times during the flood.

The peak discharge near the mouth of the basin, at Sheldon, TX, was measured by the USGS to be 10,200 m^3/s (360,000 cfs). The recurrence interval ranges from 100 years to 500 years depending on the tributary in the basin. The discharge at the mouth of the river is 1.6 times the computed 100-year discharge for that station.

The purpose of studying this river is that major changes can occur to a river system when disturbed by a high magnitude flood. Major floodplain erosion and deposition are characteristic effects of high magnitude floods. Although there have been many studies of large floods and their effects, none have been done on a low-lying coastal plain river like the San Jacinto. In addition, there have been many changes within the basin due to human exploitation of the basin. Subsidence on the order of 5-6 feet in the study area from groundwater withdrawal from underlying unconsolidated sediments, urbanization fo the basin, ssand mining of the floodplain, flow regulation and sediment starvation from two man-made lakes, and vegetation removal on the floodplain from the mining are just some of the influences man has had on the basin.

This flood is unique because a large amount of rainfall fell within the entire basin, a basin which is by no means small. Although the rain was not evenly distributed, much of the basin received extreme rainfall, in excess of 15 inches.

Erosion and deposition in the study area was extensive. Many homes were either damaged or destroyed not only by standing water, but by rushing water which scoured out the foundations of homes and caused them to either collapse or get taken downstream by the flood flow. Many large stands of trees were scoured out at their bases, taken downstream, and deposited over a wide area. Sand barges measuring 30 ft by 100 ft were transported downstream with two of them being deposited on landowners river front yards. But perhaps the most spectacular aspect of the flooding was a meander cutoff which isolated the Rio Villa point bar. The cutoff was approximately 1 km long and shortened the river's path by 5 km. By cutting through the floodplain, the river exhumed four petroleum products pipelines which were covered by a few feet of unconsolidated floodplain sediments. The pipelines ruptured either from their own unsupported weight or by obstacles flowing down the river. Due to the location of the cutoff and the fact it cut off a point bar which contains tens of homes, the cutoff was mended by several government and private agencies. At least half of the length of the cutoff was as wide as the river itself (300-350 feet) and the rest of the new channel was about 100-200 feet wide.

The meander cutoff was "fixed", many of the homes rebuilt, the pipelines fixed, scours filled and deposits bulldozed away, but the effects of the flood will remain. The study area is extremely vulnerable to another flood, should it come in the near, or not so near future. The location of the meander cutoff has not been completely filled, even if it were it is much more susceptible to erosion that it was even prior to the flood. It seems obvious that the river will eventually adopt this new shorter and steeper course.

Geographic Setting

The San Jacinto River Basin is the smallest of the major river basins that drain the state of Texas into the Gulf of Mexico. Geographically, the river lies between the larger Trinity River Basin to the north and east and the Brazos River Basin to the west (figure).

Geologic Setting

The river basin does not extend up into the heart of Texas as do the major rivers such as the Colorado, Brazos, or Trinity Rivers, thus the sediment supply is limited to a small area. The river extends approximately 140 km into the state as opposed to two or three times that distance for the other three mentioned rivers. The river is incised into the gently seaward sloping coastal sediments composed of sands, silts, minor gravels, and clays. The San Jacinto Basin is an uncharacteristically sandy basin when compared with the other major river basins of the Texas Gulf Coast region. The cause of this is due to the predominantly sand-rich stratigraphic units which comprise the basin. The Lower San Jacinto River is in an entrenched valley which is comprised of entrenched valley fill (mostly floodplain deposits, oxbow lakes, back swamp despoits, and point bar accretionary despoits). The entrenchment of the valley was caused by several Pleistocene sea level rises and falls associated with glacial and interglacial cycles.

Hydrologic Setting

The San Jacinto River is the smallest of the major rivers in Texas (or the largest of the minor ones). It drains approximately 7700 km^2 mostly north and east of the fourth largest city in the U.S., Houston. Many Houstonians live within the San Jacinto River Basin. The basin can be divided into three major sections, the West Fork basin, the East Fork basin, and the Lower San Jacinto. The West Frok of the river drains roughly two thirds of the basin and the East Fork drains about one third. The two basins meet in the upper reaches of Lake Houston, a man-made lake constructed in 1954. The river flows out of Lake Houston into what I have called the Lower San Jacinto River. The river flows for about 30 km until it is joined by Buffalo Bayou, a river which has been heavily dredged to ctreate the Houston Ship Channel. The dredging continues into the San Jacinto as it flows in the the upper section of Galveston Bay. No major tributaries feed the Lower San Jacinto with the exception of Buffalo Bayou. Because of subsidence of the lower part of the basin (that which contains the Lower San Jacinto), the river is nearly unrecognizable as such much below the Rio Villa Meander Loop. Only small streches of the higher parts of the natural levee stick out above the water surface. Much of the lower valley is permanently inundated with brackish water from tidal flow from Galveston Bay.

Topography

Above Lake Conroe, a man-made lake in the upper part of the basin completed in 1973, the landscape is dissected by the many small streams in the region which create rolling hills at an altitude of 90-120 m (300-400 feet). From the northern reaches of the basin to just south-southeast of Conroe the average slope is .1% with locally steeper areas. The land is relatively steep in the northern areas as compared to the southern areas. South of the 33o50' line of latitude, the relief is quite flat, an average slope of .06% and remains so towards the mouth of the river.

Climate

The climate of the basin ranges from humid in the northeast to moist subhumid in the west part of the basin (Winker, p.7 and Thornthwaite). The average precipitation for the basin is shown in Figure 9. The northwest part of the basin receives about 102 cm (40 inches) annually and the southeast receives about 122 cm (48 inches) annually (Carr, 1967).

• Naturally influenced results

  • The flood was wholly and unequivocally due to heavy rainfall over a short time period throughout the entire basin
  • Rainfall in excess of 76 cm in some locations and basin-wide average of 44.5 cm would have produced a severe flood regardless of activities within the basin
  • Unconsolidated floodplain deposits including sands, silt, and clays were easily erodible in the study area because of the intensity of flow
  • Natural topography and slope also would have influenced the direction of flow

• Man influenced results

  • It is obvious that Lake Houston was not effective at controlling flooding (probably because it was not built and not designed to do so)
  • Vegetation removal on the floodplain lead to several distinct features
    • Extensive vegetation removal (that which left open land) resulted in a much smoother path through which a greater flow could move
    • Selective vegetation removal (that which left a lower density both in large trees and in underbrush) created a smoother path for flow and the local development of scours around the remaining trees
  • Many structures (mostly homes) were not built to prevent scouring around the base of the foundations, concrete driveway slabs, and piers on which the homes were built. The piers in fact initiated scouring around their bases in much the same way as trees did around the base of their trunks.

It seems obvious that better floodplain management needs to be implemented on the San Jacinto River. The Rio Villa subdivision is primarily built on the unconsolidated natural levee along an active point bar of the river; natural levees are built by the river by overbank flooding. Meandering rivers are active systems which are continually eroding their banks, depositing sediment (sometimes in large volumes), cutting off meanders, and flooding.

Meteorology of the Storm

On Saturday, October 15, 1994, the remnants of Pacific Hurricane Rosa moved up along the western coast of Mexico. Traversing northeast across Mexico, it flowed a path roughly along the south Texas coast. It then moved over the Houston area and brought moderate precipitation. This initiated very wet antecedent moisture conditions for the storm which would eventually set up over southeast Texas. On Sunday, October 16, a disorganized convection cell moved over the Houston area and became stationary. A stationary front was fed by a low level jet stream that pumped in tropical moisture from the Gulf of Mexico (figure). These conditions initiated the development of a supercell over the Houston area and the counties immediately to the north.

The rain caused by this stalled system began on October 15 and 16, was heaviest on Monday, October 17, and it continued through Tuesday, October 18. At times the rainfall intensity exceeded 76 mm (3 inches) per hour in parts of the basin (Schwertz, unpublished data). Much of the San Jacinto River Basin received total rainfall amounts in excess of 380 mm (15 inches) as seen in the accompanying figure (Schwertz, unpublished data). Rainfall from two gaging stations within 3 miles of the study area exceeded 230 and 410 mm (9 and 16 inches) respectively over the period of October 15-20 (Harris County Flood Control District, unpublished data). The effect of this rain was to cause a catastrophic flood on the San Jacinto River.