EARTHQUAKE EFFFECTS

ON THE DAM

The most recent site earthquake study (1999) for Tuttle Creek Dam identified two earthquakes to be considered for the analysis of the dam as the maximum credible events:

- An earthquake with magnitude of 6.6 occurring 12 miles from the dam (at the Humboldt Fault "Hot Spot") and a depth of 6 miles

- An earthquake with magnitude of 5.6 occurring practically under the dam (less than one mile from the dam at a depth of 6 miles). The type of fault that could generate this earthquake is not considered a threat to the dam since the duration of such an event would be around 2 seconds which is too short to affect the dam.

There are no available records of strong earthquakes in the midwest United States. Therefore, the design earthquake motions necessary for the analysis of the dam were based on the record obtained during the San Fernando, California earthquake. This earthquake occurred in 1971 and had a magnitude 6.5. This is the earthquake that severely damaged the Lower San Fernando dam. However, the motions from this earthquake were increased about 11% for use at Tuttle Creek.

The analyses of Tuttle Creek Dam showed that magnitude 6.6 earthquake would cause the natural sands under the slopes of the dam to liquefy (literally become quick sand) and loose their ability to support the dam. The calculations show that if the sands beneath the dam liquefy, the dam would spread out, the top of the dam would drop, and it is likely that cracking and horizontal movement of the dam would significantly reduce the ability of the dam to hold back water. Although the top of the dam would not be expected to drop below the lake level, it is possible that the cracking and deformation could allow water to start seeping through the dam that could cause internal erosion of the dam and eventually uncontrolled release of the lake.

The 6.6 magnitude design earthquake has a very low probability of occurrence, on the order of once in 10,000 years or so. However, calculations were done to determine what size earthquake could shake the dam enough to cause significant damage. It was found that a magnitude 5.7 earthquake 12 miles from the dam is the smallest earthquake that could potentially cause significant damage. This earthquake is not expected to cause significant movement of the dam but it could damage relief wells that control the flow of water under the dam. Damage to the relief well system could eventually lead to loss of the dam. This "threshold" seismic event has an approximate probability of occurrence of once in 1800 years. Earthquakes between the 5.7 and 6.6 magnitudes cause increasing amounts of damage as the magnitude increases.

HYDROLOGIC RE-EVALUATION

Over the years, the Corps of Engineers has developed a number of sophisticated techniques for operating its system of flood control reservoirs. Generally these operations can be divided into three categories, day to day operations, operations during major floods (such as the 1993 flood) and operations for extreme floods where the safety and integrity of the dam is the paramount concern. This later category is now under evaluation at Tuttle Creek Lake.

The objective of the Hydrologic Re-Evaluation of Tuttle Creek Lake is to predict how the project would perform using the most up-to-date criteria for inflows resulting from a massive storm over the area that drains to the lake. The updated criteria was developed based on worldwide experience gained over the years since Tuttle Creek Lake was designed. It should be noted that the flows estimated by the new criteria are not much higher than the estimates used for the project’s design.

The first task was to estimate the total amount of rainfall. A storm known as the Probable Maximum Precipitation (PMP) was imposed on the basin, and positioned in such a manner as to result in the maximum runoff into the lake. This storm dropped 23.6 inches of rain in a three day period and is believed to be the worst storm that could ever occur in the area. Standard computer models were used to simulate the drainage running into the lake, flowing through the lake, and out the emergency spillway in accordance with the lake operating manual. Flow through the spillway approximately 10 times larger than the flow during of the 1993 flood would be made under these circumstances.

The items of special interest in this study are the highest lake level resulting from this storm, the maximum flow through the spillway, and the distance from the lake level to the top of the dam that would be necessary to keep wind-driven waves from splashing over the top of the dam. Water flowing over the top of an earthen dam such as Tuttle Creek can cause the downstream face of the dam to erode and could result in a breach of the dam.

These studies are not complete at this time. However, preliminary data indicates that the lake would not be expected to rise above the top of the dam during the flood, but wind-driven waves could splash over the dam.

SEDIMENTATION

When the Tuttle Creek Lake project was designed, a certain amount of storage was reserved in the lake for the accumulation of sediment carried into the lake from the Blue River watershed. The amount of storage allocated to sediment accumulation was 185,000 Acre Feet (AF) (one acre covered to a depth of one foot). This amounted to an annual accumulation of 3700 AF per year over a 50 year design life. As was the practice in 1952, that sediment accumulation was assumed to occur at the bottom of the lake. In this case, that meant below elevation 1061.

Surveys of the volume of storage available upstream of the dam have been conducted in 1962, 1972, 1973, 1983, 1987, 1993, 1996 and 2000. Using data from these surveys, it is possible to determine the amount of sediment that has accumulated in the lake. In the 38 year period from the dam closure (1962) to the 2000 survey, the lake accumulated about 145,000 AF of sediment. This total amounts to an annual average accumulation of about 3,800 AF per year, which is only slightly higher than the 3,700 AF per year rate used in the original design. At this rate, the 50 year accumulation will be 190,000 AF, slightly higher than the planned 185,000 AF.

Although the rate of sedimentation is as expected, instead of accumulating in the bottom of the lake, the sediment is building up as delta-like deposits. These deposits are present in the main channel of the Blue River upstream of the Highway 16 bridge, and in the Fancy Creek channel. There is very little accumulation in the bottom of the main body of the lake itself. This pattern has resulted in extensive shallow water-mud flat areas upstream of the Highway 16 bridge. There is a fairly steep drop off from these shallow water areas into the main body of the lake, which is the advancing front of the delta into the lake. While not consistent with the design assumptions of the 1950’s, this pattern of deposition has been observed at many other lakes. This pattern of deposition has not decreased the project’s overall flood control capabilities.

At the current rate of deposition, the sediment deposits will move down to the vicinity of the dam in the year 2076. The advance of the front of the delta is expected to slow down somewhat as it moves into the deeper waters of the lake.