DAM FAILURE

ACT US

8. DAM/LEVEE FAILURE

Throughout history, a variety of dams were built to store water for a variety of reasons. Some of these dams have the dubious distinction of failing and sometimes inflicting tremendous loss of life, as well as great damage to property. Dam failure has occurred to dams (1) built according to accepted engineering standards of design and construction at the time and also to (2) dams built without application of engineering principles. Regardless of the type of construction, when a dam fails or is subject to massive overtopping, huge quantities of water rush downstream with great destructive force.

Polk County has several earthen dams that were constructed without any application of engineering principles or regard to environmental impact. Several subdivisions were developed into retirement communities with hundreds of residential homes built along the creeks and man-made lakes. A great many of the Sub-divisions that were developed were not maintained and fell into disrepair, creating several potentially hazardous dam sites in danger of failure.

Most dams in the Polk County area are low risk. A dam failure would not cause a great deal of damage, however, many of these sites are the only means of ingress and egress leaving residents living on the other side of these structures strained with access to their homes. Polk County has over 301 subdivisions that were developed for retirement purposes, not all of these subdivisions are located on creeks with dams, however, and many of these developments would be affected by a dam failure on Lake Livingston.

The Dam located in Big Thicket Lake Estates is a primary example of an area being developed around an earthen dam. The Dam failed in October of 1994 of a dam failure that did not cause any damage downstream, but did isolate over 800 residents leaving them without access to their homes.

In 1972, Congress passed Public Law 92-367, The National Dam Inspection Act. Due to limited funding, the program began with the preparation of an inventory showing all dams in the United States. In Texas, the Texas Natural Resource Conservation Commission (TNRCC) contracted with the U. S. Army Corps of Engineers to accomplish the Texas inventory. The inventory lists contain a great amount of information on each of the over 7,212 dams in Texas.

The most significant piece of information is dams by hazard rating; high hazard, significant hazard and low hazard. The hazard rating has nothing to do with whether the dam is about to collapse immediately or will last forever. It does not relate to the condition or structure of the dam. Basically, it has to do with whether there are people living downstream in the floodplain area, which would be endangered in the event of dam failure. Every dam is rated in this manner in the inventory. The following explains what the hazard classification means:

a. Hazard Classification 1 -- High Hazard; Dam failure would probably result in loss of life and major damage to property.

b. Hazard Classification 2-- Significant Hazard; Dam failure could possibly cause some Loss of life and property damage.

c. Hazard Classification 3-- Low Hazard; Dam failure would be unlikely to cause loss of life or property damage.

The largest dam found in Polk County is the Livingston Dam located on the Trinity River, approximately 129 miles upstream from the mouth of the river in the Gulf of Mexico. The reservoir covers portions of Trinity, Walker, San Jacinto, and Polk Counties. The Trinity River downstream of the dam courses through Polk, San Jacinto, Liberty, and Chambers Counties. The location of the dam is shown in Attachment 8 – A

The dam is approximately 14,400 feet long and is generally between 45 and 60 feet high, reaching 90 feet at the old riverbed. The earth embankment has an impervious core and its section has slopes of 1 vertical to 2.5 horizontal. The crest of the dam is at elevation 145.0 ft. National Geodetic Vertical Data (NGVD), and is 24 feet wide. It accommodates a single-lane-paved road, which provides access to TRA personnel, law enforcement and emergency vehicles.

The dam is served by a concrete gravity spillway with an ogee crest at elevation 99.0 ft. NGVD. Releases are controlled by twelve 40-foot long by 32-foot high trainer gates. Concrete upstream and downstream aprons direct the water through the spillway and back to the original river channel.

The outlet works consist of a vertical inlet tower with five gates, a 550-foot long by 10-foot diameter conduit, a 170-foot long stilling basin, and a concrete broad-crested weir. A short channel downstream of the stilling basin directs flows back to the river.

Normal (conservation) pool level for the reservoir is 131.0 ft. NGVD, which encompasses a reservoir area of about 82,600 acres and storage of 1,750,000 acre-feet. Maximum pool design surcharge elevation is 134.0 ft. NGVD, which corresponds to a reservoir surface area of 88,900 acres and 2,045,000 acre-feet of storage. The drainage area above the reservoir is approximately 16,583 square miles and average flows are 7,440 CFS.

Livingston Dam is classified as a large, high-hazard structure with a recommended spillway design flood equal to the probable maximum flood (TDWR, 1978). According to the TDWR, the project will pass only 90% of the PMF before overtopping the dam.

The downstream hazard area lies in Polk, San Jacinto, Liberty, and Chambers Counties and includes the communities of Liberty, Romayor, Goodrich, Holiday Lakes, Ace, Hardin and various smaller communities along the river. See Attachment 8 – B.

Major and minor flooding has been common along the Trinity River, before and after the presence of the Livingston Dam. The dam is not a flood control structure, so flood flows are normally passed downstream through the dam.

Storms This area is subject to frequent intense rainfall events from thunderstorms and from extreme weather such as hurricanes. Normal rainfall per year is 48.22 inches.

Massive Landslides This type of dam-threatening event is typical of mountain streams and reservoirs. It is not applicable to this project.

Volcanic Eruptions There are no known volcanic activity within the project area or its drainage basin.

Fire There exists the possibility that a major fire could damage the outlet control structure and make difficult the operation of the spillway, leading to a dam failure.

Civil Disturbance Sabotage of the dam control structures or the dam itself is possible but unlikely. Dams are difficult to be seriously damaged in this form.

Polk County and the Trinity River Authority are aware of the hazards of a dam failure of the Lake Livingston dam. Extensive planning has been done for warning of residents downstream from the dam site.