2002-10-25_REVISION - M1986015 (5)

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Page 2 Pueblo East Pit adequacy responses M-1986-015 probably much closer to 4:1, this approach is adequate to create stability. If the slopes were steeper then actual compaction to some degree would be more important. Item 3 -Rule 3.1.5 - (summary) Question retarding stability of slopes when subjected to wave action from the lake. Response: Transit Mix of Pueblo, and her sister companies Transit Mix and Castle Concrete, have had experience with this matter. At the Pueblo West Pit, the natural groundwater lakes created in the reclamation have had some problems with wave cut banks on the downwind side of the lakes. The problems are not major and do not create a hazard, but there often is a low and steep cut along the water line where wave action is pronounced. These are usually on the east and south sides of the lakes. Although such banks can develop on the other shore lines', they are much smaller. At Caste Concrete's former Great Blue Heron Pit, two lakes were created. One was a groundwater lake similar to the Pueblo West Lakes. The other was- a recharge lake with a fluctuating water line that varied as much as eight feet over the course of a year. Wave cut action on the groundwater lake was, as expected, evident on the downwind side of the lake and very minor on the more protected shore lines'. But in the recharge lake wave cut banks did not develop. Instead, the shoreline developed a Blighty rippled texture with many horizontal lines. Each line represented a shore line at some point during tie water cycle. The next year, the previous year's shorelines were obliterated and new ones created. The slopes here remained quite stable. However, the recharge lake was only about 8 acres and therefore strong wave action did not have sufficient room to develop. Wave formation is a function of wind velocity and the distance across the water fiat the wind and the body of water in[eract. The bigger the lake the bigger the waves if wind velocity is kept constant. The distance of wave travel also affects the frequency of the waves• and, to some extent, the shape of the wave energy. Thus waves in the Pacific Ocean are generally larger than in the Mantic Ocean and the frequency (distance between waves) is greater on Pacific shores than Mantic shores. Considering that the reservoir being created at the Pueblo East Pit will be rather large, signiFicant wave action could develop. Although the fluctuating water level may help to even out the cut banks, there is an additional concern which is derived from ocean beach dynanics. The tme cut bank along a beach usually develops beyond the low tide line. 1'he true cur bank is where the greatest amount of erosional action occurs rather than at the back of the beach. The back of a beach is where headward erosion occurs but that is actually under control of the beach dynamics near the interface between the water and the land. The location of the tme cut bank is a function of wave size. This pattern is also present in lakes, but the waves are so small in a lake the tme cut bank is not readily evident except by close examination. The actual cut bank at any one time forms at a point that is approximately at the maximum depth of reach of the wave energy. Thus, during calmer seas tie tme cut bank can move dangerously close to the normal water line creating a hazard for swimmers and can contribute to the development of rip currents. Such features also develop ut lakes, but again, they are very small. During large storms the ocean beach cut bank can push far away from the beach and out to sea because the zone of high energy wave action extends much deeper into the water. These zones and the resulting cut banks similarly fluctuate in lakes, but it amounts to a variation of a few inches to a few feet. In the ocean that variation can range from a few hundred feet to many miles where the grade of the sea bottom is gradual as in the GuIE of Mexico. During this dynamic process which constancy changes as wave dimensions• change, large amounts of material are moved about. Beach material is washed out to sea during large wave events to create a more gradual and wider zone -extension of the tme cut bank. This zone then recedes• toward the beach during smaller wave periods. The material at the outer zone deposited during a large wave event is washed out to sea by currents flowing from the beach during smaller wave periods. In the ocean, the lost material on the beach is usually, but not always, replaced with long shore currents, but those currents are very weak and often reverse directions in lakes. Replacement of beach material in lakes usually occurs by erosion of the back of the beach by turbulence from breaking waves. Thus, on an ocean beach, the beach zone widens very slowly because the beach is rebuilt by migration of material along the length of tie beach. But in a lake, the shore line expands due to erosion of the back of the beach zone. The beach usually stays about the same width in a lake, but it slowly expands outward until something controls the erosion at the back of the beach. At that point the system tends to stabilize and eventually