Geologic Hazard Report of Golden Area

Geologic Hazard Report of Golden Area

Shao Jun Liu

12/01/14

Lab section: Friday 1-4 p.m.

TA: Mr. Alvaro Puente

Golden is a small city located in Jefferson County with the latitude and longitude of 39° 45' 20" N and 105° 13' 14" W. There are four major geologic hazards in Golden area includes swelling soil, flooding, rockfall, and landslide.

Swelling soil is one of the most significant geologic hazards in Colorado. Swelling soil is the expanding and contracting of soil because of the adding and removing of water in the rock formations. Swelling soils are commonly found in rock formations that contains clay. Clay is an expansive soil that will expand when water is added and shrink when water is extracted. There are three type of clay, including Kaolinite, Illite, and Sinectites. There are two types of swelling environment. The first type of swelling environment occurs when the rock formations are laid horizontally. The removal of water will create a moisture zone on the rock formation. The second type of swelling environment occurs when the rock formations are tilled at an angle. The differential moisture of rock formations creates the heaving of bedrock. This type of swelling environment is commonly found in front Range of Colorado.

In Golden area, there are three rock formations that have high swelling potential. These rock formations are Benton Shale, Pierre Shale, and Denver formations. Benton Shale and Pierre Shale formations could be found along Highway 6 and highway 93. Denver formation locates on the Eastern side of Golden. For example, the bases of the Table Mountains are parts of Denver formation. Laramie, Rosten Creek and Morrison formation have moderate risk of swelling soil. Swelling soil can cause damage to the foundation if properties are built on these rock formations. The expanding and contracting of soil will destabilized the foundation and cause cracks on the buildings. Avoidance and not building properties in these areas are the most efficient way to avoid property damages. Excavating with engineer refill is one way to protect the foundation. The house foundation will not be in direct contact of the swelling bedrocks. Another method to protect the foundation is by drilling down to the non-swelling bedrocks and anchoring the foundation under the soil. Finally, maintaining a stable amount of water in the rock layers can eliminate swelling of soil.

Flooding is also a potential geologic hazard in Golden area since Clear Creek flows through Golden City. Flooding occur when the discharge of water in a stream increase rapidly and causing the water surface rise above the river bank. Flooding can occur during heavy rain season or sudden break of natural dams or human dams. The damages caused by flooding depend on the discharge of water and the speed of the flood. A flash flood or flood with high speed and high discharge will cause more damages than slowly rising water. The 100-year flood plain of Clear Creek is constructed in the flood hazard map. The area covered by the 100-year flood included the low altitude areas along the side of Clear Creek.

Channelizing the stream is a good option to clear the river bed and sediments. Clear Creek has been channelized once after the flood in 1965. A channelized river can hold a higher discharge than a braded stream, allowing water to flow smoothly. Building a more stable river bank and dike can prevent the overflow of flood water. Zoning is very critical in a flood plain. Emergency agencies should never be built on the flood plain since they play an important role during a flood event. Expensive buildings and equipment should not be built on a flood plain. Some of the best ways to utilize a flood plain include turning it into public parks, playgrounds, parking lots, or cultivation land. Also, the bridges and roads in flood plain area should be built strong enough to withstand in a 100-year flood.

Rockfall is a potential geological hazard in Golden area. Rockfall is the sudden break of rock bodies from the edge of mountains or cliff and roll downhill with a high speed. Rockfall can cause a severe damage to the objects on the path of its trajectory because of the high kinetic energy is converted from gravitational potential energy. Buildings and roads can be severely damaged by rockfall. Building a road on a high risk rockfall area will create a danger environment for cars on the roads. There are three major rockfall hazard areas in Golden area, including both Table Mountains and Clear Creek Cannon. There are basaltic lava flows on Table Mountains. The cooling of basaltic lava created cracks within the lava bodies and rocks can be falling from the edge of the igneous rock layers. Highway 6 runs along the side of Clear Creek Cannon. The constructing of the highway and digging of the mountains create a potential rockfall hazard along the side of the road.

There are several methods that can be used to eliminate the rockfall hazards. First, the use of rock nets helps stable the rocks. Rock nets are commonly found along the side of a mountain highway. Second, stabilizing the rocks by inserting rebar horizontally and vertically. Rebar or iron bar chain the surface rocks and bed rocks together. Third, create ditches along the edges of mountains to capture the falling rocks. The falling rocks will reduce speed as it drop into the ditch. Regular cleanup and maintenance of ditches are required so that the ditches remain functioning. Finally, scaling and cleaning mountains can be used to eliminate the risk of rockfall. Loose rocks can be removed by breaking it using hammers or explosives. Regular removal of loose materials is important to maintain a safe environment.

Landslide is a geologic hazard that commonly occurs in mountainous areas. Landslide describes the sliding, and flowing of a region of soil and rocks downward and outward from a slope. There are two types of landslides, including circular and translational landslides. Landslide can cause severe damage such as the burring of properties, roads, and dam up rivers. The occurrence of landslide depends on the rock types and water content. If the water content is high in soil, then it creates a higher risk for landslide. Also, if the bed rocks of an area contain mostly sedimentary rocks rather than igneous rocks, it will also create a higher risk of landslide. The slope of the bedrock is a key factor affecting the risk of landslide. In general, sedimentary bedrocks become unstable when the slope is greater than 30 degree and 60 degree for igneous bedrock. Most of the rock formations in Golden area are sedimentary. The major igneous rocks in this area include Table Mountain Basalt and Idaho Springs formations.

Areas around Tables Mountains and the Western side of Golden are places with high risk of landslides since areas consist of mostly sedimentary bedrocks. Landslide can occur when the bed rock become unstable. Possible mitigation methods include vegetation, creation of buffer zone, horizontal stabilization and maintaining toe. Specific vegetation and roots system is one of the effective ways to stabilize the bed rocks and decrease the water content. Creating a buffer zone on the downslope can help slowdown the flow of the landslide and decrease property damages. Horizontal stabilization can be commonly found along some highways and roads. Inserting rebar horizontally can help stabilizing a circular landslide. Finally, maintaining the toe of a potential landslide region is very important. Stabilizing the toe can stabilize the whole area since most landslides occur when their toe got weaken and unstable because of erosion.

Altogether, swelling soil, flooding, rockfall, and landslide are four major geological hazards in Golden area. Avoidance is one of the most effective ways to eliminate damages from geological hazards. Also, proper zoning, scaling, and regular maintenance are some of the methods to eliminate geological hazards.

Reference:

"Landslide Types and Processes." Landslide Types and Processes. U.S. Geological Survey, July 2004. Web. 01 Dec. 2014. .

"Landslides | Colorado Geological Survey." Landslides | Colorado Geological Survey. Colorado Geological Survey, 2012. Web. 01 Dec. 2014. .

Geologic Map of Golden Fault. By Jeremy McCreary. Digital Image Oct 22, 2004

https://www.360docs.net/doc/0d256973.html,/colorado/dakota_hogback/ (Accessed: 12/01/14)

"NGMDB Product Description Page." NGMDB Product Description Page. U.S. Geological Survey, 1 Jan. 1972. Web. 4 Dec. 2014.