Q. What caused the flood of the San Jacinto River and tributary streams?
A. The rain that produced the record floods began on Saturday and Sunday, October 15 & 16 with one to two inch rains that soaked the ground. Heavy rains in the 10-to 15-inch range followed late Sunday night and all day Monday. An additional 6 to 10 inches fell on Tuesday This resulted in unusual high runoff and high flow rates.
Q. What are the main streams flowing to Lake Houston?
A. The two main tributaries to Lake Houston and the San Jacinto River are the East Fork and West Fork. Major tributaries to the West Fork are Cypress Creek, Spring Creek and Lake Creek. Major tributaries to the East Fork are Peach Creek, Caney Creek and Luce Bayou.
Q. How big is the San Jacinto River Watershed above Lake Houston?
A. Total area contributing to Lake Houston is 2,828 square miles, with about 1,800 square miles from West Fork and 1,000 square miles from the East Fork.
Q. How does the 1994 flood compare to historical floods and to the 100-year flood?
A. Water levels during a flood are the primary concern for most people. However, flood magnitude is measured by flow rate (cubic feet per second); usually a high flow rate means a higher flood level. However, changes in channel capacity to channel improvement, bridges and vegetation can change the elevation-discharge relationship.
Listed below are selected sites showing a preliminary comparison of the 1994 flood to the historical record and to the flood plain maps. The 1994 flood data at some of the sites in Harris County have been adjusted for subsidence to 1973 which is the datum adjustment used for the Flood Insurance Rate Maps.
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130,000 |
157 |
35,000 | |||||
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112.7 |
84,800 |
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30,800 |
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Jacinto River at Cleveland |
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near Splendora |
147.0 |
45,000 |
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Jacinto River at Humble |
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70.5 |
307,000 |
230,000 | |||
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Record Since |
Insurance Study (1973 datum adjusted) 100 Year 500 Year |
subsidence) | |||||||
(cfs) |
(cfs) |
datum) (ft) |
(ft) |
(cfs) | |||||
nearcrosby Tex. |
1989 |
56.3 |
57.0 |
10,000 |
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1942 |
27.2 |
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* |
* |
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*To be determined
Q. What is the 100-year flood?
A. The "100-year flood" is a common term that refers to a stream discharge which has been statistically determined to have a 1 percent chance of occurring in any given year. This flood can occur several times in a 100-year period or even twice within a single year, but is statistically unlikely. A stream can go for more than 100 years and never experience a 100-year flood.
Q. Why did a lot of homes flood that are not in the 100-year flood plain?
A. As shown on the table above, the flood exceeded the 100-year flood levels. Flooding on Spring Creek was near the 500-year level. Many of the 100-year flood plain maps were based on synthetic data and need to be revised to incorporate data recorded from recent floods. The flood plain maps need not be revised to match the recent flood if the magnitude exceeded the 100-year event. However, home owners can and possibly should purchase flood insurance even if their home is not located in the 100-year flood plain.
Q. How is the 100-year flood determined?
A. On major rivers (such as the San Jacinto) a statistical analysis is made of the annual peak discharges. Based on the resulting synthetic 100-year discharge, a mathematical model computes the water level. This level is also verified by the stream rating curves developed by the US Geological Survey. The 100-year flood discharge for smaller streams, are based on models that utilize the statistical 100-year synthetic rainfall.
Q. How can this be a 100-year flood when the rainfall never exceeded the 100-year storm of 12-inches in 24 hours?
A. One method of evaluating floods is by describing the rainfall depth over a certain time period. For example, a 100-year rain storm can be 4.5 inches in one hour or 13 inches in 24 hours. The 4 day 100-year rainfall depth is 16.1 inches. Floods are generally caused by rain which does not always correspond to the same rainfall frequency. Floods on big watersheds can be caused by the 3-day rainfall depth. The Saturday through Tuesday (4 days) total across the watershed ranged from 18 to 30 inches. Soil moisture prior to a storm will affect the runoff from a given rainfall amount. The ground in the San Jacinto River watershed was saturated from the Saturday and Sunday rains prior to the heavy rains of Sunday night, Monday, and Tuesday. Since very little rain could be absorbed by the ground, most of the rain that fell after Sunday afternoon contributed directly to the flood waters in the river.
Q. Who is responsible for collecting this historical flood data?
A. The US Geological Survey, an agency of the Interior Department, began systematic stream flow monitoring before the turn of the century. That agency currently operates about 500 stream data collection sites within the state, usually at the request of and partially funded by such local entities as Harris County Flood Control District, City of Houston, San Jacinto River Authority and the State of Texas.
Q. Why did the San Jacinto River Authority release the high flow rates from Lake Conroe and Lake Houston?
A. These two dams were constructed for water supply and do not contain flood storage capacity. The San Jacinto River Authority must keep the dams full at all times, so the storage volume is available for an extended drought. Lake Conroe is gated. The San Jacinto River Authority releases water as it flows into the lake. They operate according to a manual that provides instruction for opening the gates based on rate of change in the lake level and rainfall. This operation should result in flows below the Lake Conroe that would have existed if the dam or lake did not exist. As shown on the table above, the flow from Lake Conroe contributed 35,000 cfs of the total flow of 115,000 cfs at I45. The remainder came from Lake Creek which has a drainage area slightly smaller than the area draining to Lake Conroe. Lake Conroe was about 1 foot from the top at the beginning of the storm. This small volume plus some increase in the flood storage above the normal level probably resulted in less flow than if the lake had not existed.
Lake Houston is operated by the City of Houston. The lake has an uncontrolled spillway with two small gates to release water.
Q. Why can't Lake Conroe and Lake Houston be increased to store flood waters?
A. The storage volume required to control even the 100-year storm on Lake Conroe would require raising the dam approximately 8 feet and purchasing about 10,000 acres of land, much of which is developed. Even if flood flows from Lake Conroe could be controlled, that would be for only 445 square miles of the total area draining to Lake Houston.
The existing volume of Lake Houston is 150,000 acre-feet. If Lake Houston could have been drained prior to the 1994 Flood, it would have been full by Tuesday morning.
After the 1994 Flood subsides and Lake Houston returns to normal levels, about 2 million acre-feet of water will have flowed over the Lake Houston Dam from this event.
Q. I heard one live helicopter news report say the water was flowing at 45 miles per hour. Is that true?
A. No. There was a little sensationalism in that report. Water velocities in most streams during the flood were in the 4 to 8 feet per second range (2.5 to 5.5 miles per hour). The maximum at a few locations may have approached 12 feet per second or 8 miles per hour.
Q. If a medium size watershed like the San Jacinto River can produce the above flow rates: what kind of flow rates do the Brazos River or Trinity River experience during a flood?
A. Strange as it may seem, the 1994 and 1991 Floods on these two Rivers were caused by flows in the 100,000 to 150,000 cubic feet per second range.
Q. How can that be?
A. The Brazos and Trinity rivers have flood control reservoirs. However, even in their unregulated state, the historical flows were less than the San Jacinto River flows. Those rivers have wide flood plains and very mild slopes. In addition, the water has greater distances to travel. The San Jacinto River on the other hand has steeper slopes and is relatively short compared to the other rivers. Furthermore, all of the tributaries to Lake Houston drain to the lake at the same time.
