Due date Name Can Plate Tectonics Help Us Get Rid of Nuclear Waste? INTRODUCTION What would you do if you had to get rid of thousands of tons (1 metric ton = 1000 kg = 2200 lbs.) of used nuclear fuel, called high level radioactive waste? Where could you find a safe place to put it, a place where it could never hurt anyone? This is not an easy question to answer. The nuclear waste could not explode, but it is highly radioactive, and will remain dangerous because of that for hundreds of thousands of years. In a million years or so, the waste will be no more radioactive than the ore it was originally mined from. That doesn’t mean it’s safe — you wouldn’t want dust contaminated with uranium ore in the air you breathe! Many engineers and scientists all over the world have been trying to figure out a safe way of storing or disposing of this waste for a long time. Listed below are five of the ideas that scientists have come up with to get rid of radioactive waste. 1. Put it in rockets and shoot it into the sun. 2. Drop it into the mud in the middle of ocean basins. 3. Store it in deep mines (especially salt mines) or caves. 4. Place cans of the material on Antarctica, and let the atomic heat melt its way down into the ice. 5. Dump it into deep ocean trenches. Each of these ideas has both good and bad points. In this activity we are going to consider dumping the radioactive waste into ocean trenches to see if that would work out well or not. OBJECTIVES After you have completed this activity, you should be able to: 1. Plot the movement of a descending ocean floor plate on a graph, and discuss that movement. 2. Explain why it would be good or bad to dump radioactive waste into ocean trenches. 3. Explain some of the effects that subducting plates have on the edges of continents. 4. Tell the reasons why disposing of radioactive waste material is not an easy problem to solve. PROCEDURE First, read about subduction (section 2.5, p.44 48) and accretionary wedges (p. 276) in your book. An ocean trench is a long, narrow depression with steep sides, located on the deep-sea floor. Ocean trenches are located where the edge of an ocean plate is going down under a continent. The scientists who suggest that radioactive waste be dumped into ocean trenches say that it will sink into the sediment at the bottom of the trench and will be carried down and away forever by plate motions. Let’s see what would happen if this plan were carried out in the Japan Trench. The descendiEffectively Bioremediate Oil Field Pits. Oil & Gas Journal. Stilwell, C.T. 1991. Area Waste-Management Plans for Drilling and Production Operations. Journal of Petroleum Technology 67-71. Utah Oil and Gas Conservation Act, 40-6-1 et seq. UCA 1953, as amended 1995. Utah Oil and Gas Conservation General Rules, The. R649-1 et seq., amended June 2, 1998. 29 Chapter Reclaiming 2 Waterways n Drainage Drainage Reclamation Reclamation n Streambank Streambank Bioengineering Bioengineering Drainage Reclamation To achieve success in reclaimed mine sites, it is imperative to consider reclamation during the operational planning stage. Considering reclamation after the operational layout is designed results in difficulties in establishing appropriate landform and stream channel reclamation techniques. Considering reclamation during planning allows the operational layout to be built in a manner that will save time and money. This chapter reviews many of the concepts presented in Stream Corridor Restoration Principles, Processes, and Practices, 1998, which can be viewed at http:// www.usda.gov/stream_restoration. Begin initial reclamation planning with a channel layout. The overall channel grade is equal to the upstream and downstream elevation change where the upstream and downstream drainage ties into the existing drainage. Changes in the channel bed slope between these points should be designed based on the drainage characteristics that include the: n n n n n discharge characteristics pre-existing channel geometry, geomorphology, and gradient post mining substrate or fill type and amount of bedload and sediment transported throughout the system postmining adjacent area topography It is important to remember that the stream channel or drainage system has the potential to move both laterally and vertically. The existing and created drainage characteristics will effect the rate and extent that lateral and vertical erosion or deposition occurs. Figure 2.1: A road on the left and a railroad on the right of this channel dictated the channel design. Sunnyside Mine. Note: The channel elevation determines the lowest point on the regraded mine site, affecting the adjacent area configuration and elevation. Where appropriate, it can be advantageous to maintain or reconstruct the predisturbed channel characteristics. In areas where it is necessary to reconfigure the drainage, existing natural gradient controls such as competent bedrock outcrops and the upstream and downstream channels should be utilized in determining the channel bed elevations. A person familiar with fluvial processes should design perennial and intermittent channel reclamation, in addition to channels adjacent to important or sensitive water resources. Familiarity with geomorphology, channel and meander geometry, and the natural tendencies for channel adjustment toward stability is needed to predict the most effective design for long-term stability and function. 31 In ephemeral systems it is important to re-establish drainage density. Drainage density includes minor water pathways and swales that concentrate water and flow into the channels. This will minimize the erosion that would occur to establish a drainage density, which is in equilibrium with the reclaimed system. Drainage Characteristics An inventory using a stream classification system is useful in describing and defining the reclaimed channel configuration (Harrelson et. al. 1994). Drainage characteristics that must be considered prior to designing the site include discharge characteristics, channel geometry, and stream morphology. Discharge characteristics include the duration, frequency and magnitude of flow. These characteristics can change with season and climate. Consider designing drainages with a low and high flow channel or consider passing the flow across the adjacent flood plain under high flow situations. Note: Channel designs that allow the channel to respond to a wide flow range will increase long-term stability. Consider both bankfull flow, and extreme event flows when designing a channel to be stable. It does not matter whether the flow is ephemeral, perennial or intermittent. The channel will form according to the type and frequency of flows it receives. One must also recognize that designing for the range of flows the channel receives will result in the most stable design. Therefore, it is best to mimic the proper functioning characteristics observed in the drainage and to consider the historic flow regimes occurring at the site. Streamflow types describe the duration and frequency of the flow. n n n Figure 2.2: The channel was designed to allow water flow over the adjacent flood plain during high flows. Sunnyside Mine. Ephemeral streams are above the water table at all times. These streams carry water only during and immediately after precipitation or during snowmelt runoff. Intermittent streams flow for part of the year. The water table may be elevated during part of the year. Spring discharge or the ground water flow contributes water to the stream for part of the year. Perennial streams flow most of the year. Definitions between perennial and intermittent streams vary. The United States Geological Survey (USGS) defined perennial sections as those stream sections which flow all year 32 except during severe drought periods for purpose of developing the 1:24000 series maps. Channel-Forming Flow There is no method to directly measure the channel-forming flow. The most common methods used to describe channel-forming flows are: n n n bankfull discharge specific discharge recurrence interval effective discharge These methods describe channel-forming flows and are determined through measured discharge and field observations collected over a representative time period. More than one method should be used to verify the channel-forming flow. To determine the channel-forming flows in cases where flow data is not available other field measurements and indirect methods such as regional analyses are necessary. Note: When designing an ephemeral system or a system where flow data is not available, look at the existing channel form and mimic those sections that function well. Bankfull Discharge Bankfull flow or discharge is the discharge that fills a stable alluvial channel up to the elevation of the active flood plain. There is no standard definition of bankfull flow. Therefore, the indicators used to active flood plain: the area determine bankfull elevation (the active flood plain) must be where alluvial materials are described in the reclaimed channel design (Nixon 1959; actively transported and deposited. Wolman and Leopold 1957; Woodyer 1968; Pickup and Warner 1976; Schumm 1960; and Leopold 1994). Bankfull stage and bankfull discharge are two phrases used to describe channelforming flows and floodplain formation. A rating curve is sometimes used to determine the bankfull elevation or stage. The rating curve plots the water elevation (feet) in the channel against the discharge (cubic feet per second). Since discharges greater than bankfull spread across the floodplain, the elevation of the water will rise slower above bankfull than below bankfull. The rating curve will flatten at the point of water spillage above the bankfull channel. The field identification indicators used to determine the bankfull elevation are often subjective and difficult to identify and should be observed in stream reaches that are stable and alluvial (Knighton1984). When designing a channel through a reclaimed site make sure to re-establish the floodplain if one existed prior to mining. Not very channel will have a floodplain. An active floodplain provides temporary storage for floodwaters and sediment. A floodplain will see floodwaters an average of two out of three years. Floodplains are important to the 33 biological diversity and stability of the site and are essential for any riparian plants such as willows and cottonwoods. Discharge, depth of flow, and velocity along the channel are not only important for floodplain determination, but also for determining changes of channel bed, slope, shape, or roughness. To determine the stage discharge relationship by direct discharge measurements, use velocity meters, Continuity Equation, hydraulic resistance equations (Manning’s Equation) and standard backwater calculations. Review design criteria and applicable uses before applying these equations. Additionally, care must be taken with highly mobile streambeds, such as sand, to accurately represent the bed forms (roughness as described by Manning’s “n”) occurring during a specific event. Figure 2.3: Willow cuttings are planted at the bankfull elevation. Sunnyside Mine. Hydraulic Resistance Equations Continuity Equation Q = AV Discharge = (cross sectional area of flow)(average velocity) Manning’s Equation V= k /n R2/3S1/2 k= 1.486 English (1 metric) n= Manning’s roughness coefficient R= hydraulic radius S= surface water slope Energy Equation The energy equation is used to calculate changes in water surface elevations between two similar cross sections. Computer models such as HEC-2 are available for complex cross-sections and backwater situations. Recurrence Interval A common assumption is that the channel-forming flow has a recurrence interval of 1 to 3 years. The recurrence interval is the average number of years between when the channel-forming flow was exceeded. This method requires data analysis from a gauged station over a representative time period. Unfortunately, this data is often unavailable for stream channels undergoing reclamation. When data is unavailable adjacent gauged drainages discharge data can be applied to ungauged drainages under the following conditions: 34 n n n The watershed is hydrologically similar. The drainage is not dominated by high intensity storms. Drainages must have similar land use. Adjacent drainage reaches should be analyzed to determine if bankfull discharge is logical. The bankfull elevation (stage) calculated from the recurrence interval method should conform to field observations. This is especially true for highly modified streams such as in urban or mined areas, as well as ephemeral streams in arid and semiarid areas. Indirect methods for determining bankfull discharge, such as using regional analyses have been done for streams in Utah (Blakemore et. al. 1983) It is important to correctly define the active channel when using the regional analyses methods, which compare bankfull discharge with the drainage area. These methods have a wide confidence interval in the arid western portions of the state and may result in over designed or under designed streams. Effective Discharge Effective discharge is the increment of discharge that transports the largest fraction of the sediment load over a period of years. Effective discharge is a function of the magnitude of the event and frequency of occurrence. It represents the flow that is responsible for transporting the most sediment over a defined time period. To determine effective discharge, flow duration data and sediment load data are required (Wolman and Miller 1960). Channel Geometry and Geomorphology Inventory To determine the channel geometry and geomorphology, conduct a drainage inventory prior to site disturbance. If the channel was disturbed prior to
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