1. INTRODUCTION
1.1 OBJECTIVE
The objective of this evaluation is to support License Application Design Selection (LADS) by assessing the potential of surface modification features that could eliminate or reduce net infiltration of surface water into the unsaturated zone and ultimately into the repository. Two design options have been identified that may accomplish this objective. These include:1.2 SCOPE
The scope of this evaluation included conceptual design, cost estimation, infiltration modeling, and evaluation of total system performance. All work was directed at comparison of the feature options against the Viability Assessment (VA) Reference Design. Specific tasks that were carried out in the evaluation include the following: #23a Alluvium Option -2. QUALITY ASSURANCE
This evaluation was prepared, reviewed, and approved in accordance with QAP-3-5, Development of Technical Documents, as detailed in the Technical Document Preparation Plan (TDPP) for Design Feature Evaluation #23 and #24, Surface Modification – Alluvium and Drainage (CRWMS M&O 1998a). The subject of this analysis does not include any permanent engineering items classified under QAP-2-3, Classification of Permanent Items, and is not included in the Q-List (YMP 1998). The subject of this evaluation has been assessed for its classification (CRWMS M&O 1998b) under QAP-2-0 Conduct of Activities, and determined to be quality affecting and subject to the Quality Assurance Requirements and Description (QARD) document (DOE 1998). This conclusion was based on the determination that it may affect items that are on the Q-List and will provide data or information that can be used to assess the dispersion of radioactive materials.3. USE OF COMPUTER SOFTWARE
This evaluation utilized the Lynx Geoscience Modeling Software (LYNX), Version 4.5, to develop the engineering design of the alluvium option and to calculate the volumes associated with it. This software is a three-dimensional, volume-based geology and engineering computer modeling system developed by Lynx Geosystems, Inc. of Vancouver, B.C., Canada and operates on a SGI Unix-based workstation (CRWMS M&O Equipment Tag #115721). LYNX was qualified in 1997 and carries the Computer Software Configuration Item (CSCI) number 30016 V4.5 (CRWMS M&O 1997a). This software was originally obtained to specifically perform this type of work and was qualified with that intent in mind. The software is appropriate for the application used in this evaluation, was not used outside the range of validation, and was obtained from Software Configuration Management (SCM) according to procedures. The files developed in this evaluation are listed in Appendix H and are contained on an archive tape (CRWMS M&O 1999b).4. INPUTS
The design features are being considered as conceptual and preliminary designs, which will not be used to support construction, fabrication, or procurement, and will not be used as part of a verified design package to be released to another organization for use in other design work. Therefore, existing data and inputs requiring confirmation will be identified as TBV, but the provisions of NLP-3-15, To Be Verified (TBV) and To Be Determined (TBD) Monitoring System, requiring an identifier to track the status of TBV/TBD information is not required.4.1 DESIGN PARAMETERS
4.1.1 Topography
The topographic data used in this design analysis were developed for the LYNX Design Model YMP.MO3 (CRWMS M&O 1997b, p 6, 9 and 21). The development of this data for use in the model is explained in detail on page 21 of that document. The topography data is non-qualified and was used in the alluvium option (#23a) to develop the engineering design (see Section 5.3). [TBV]4.1.2 Estimated Volume of Alluvium
For both of these feature options, an estimate of the volume of alluvium present on the hill tops and hill slopes had to be developed. For the alluvium option (Section 5), it represented the total amount of alluvium that would be removed in the pre-strip operation. For the drainage option (Section 6), it represented the amount of thin alluvium (less than 0.5 m thick) that would be removed to improve the surface water runoff. The 0.5 m definition for thin alluvium comes from the fact that the mean thickness of alluvium on the sideslopes and ridge tops is 0.5 m (Flint and Flint 1995, p 15). The estimate of the total volume of thin alluvium that was used in this evaluation was developed during the net infiltration simulation (CRWMS M&O 1999e, Item 1, pp 1-2). The estimate was based on the conceptual and numerical model of infiltration for the Yucca Mountain area (Flint, Hevesi, and Flint 1996). According to the calculation, the total alluvium volumes, and average alluvium depths are as follows [TBV]:4.1.3 Net Infiltration Simulation
The net infiltration simulation was performed as a response to a Design Input Request (CRWMS M&O 1998f). The simulations were performed for each of the two options using the Current Climate 1980-1995 Simulation Model and the Long Term Average Future Climate Simulation Model (LTA2) for 100- and 50-year. The non-qualified results of these simulations were received as a Design Input Transmittal (CRWMS M&O 1999d, Item 1, pp 14-20) and are summarized in this report in Sections 5.5 for the alluvium option (#23a) and 6.5 for the drainage option (#23b). The specific data input to this evaluation from the net infiltration simulation include average net infiltration, maximum net infiltration, and average runoff. These inputs were used to evaluate the effects of the features on net infiltration. [TBV]4.1.4 Performance Assessment Simulation
The performance assessment simulation was performed as a response to a Design Input Request (CRWMS M&O 1998e). These simulations used the output from the net infiltration simulation runs to assess the performance of the features. The non-qualified results of these simulations were received as a Design Input Transmittal (CRWMS M&O 1999c, Item 1, pp 10-11) and are summarized in this report in Sections 5.6 for the alluvium option (#23a) and 6.6 for the drainage option (#23b). The specific data input to this evaluation from the performance assessment simulation include Figure of Merit (FOM), peak dose and peak dose time for less than 10,000 years, and peak dose and peak dose time for 10,000 to 1,000,000 years. Because the drainage option was shown to have a negative impact on the net infiltration (see Section 6.5), the performance assessment simulation was not done. [TBV]4.1.5 Cost Estimates
Conceptual cost estimates (±50 percent) for both surface modification designs were developed and were transmitted as an unsolicited design input transmittal (CRWMS M&O 1999f, Item 1). The results are contained in Sections 5.7 for the alluvium option (#23a) and 6.7 for the drainage option (#23b), and a spreadsheet summarizing the estimates is contained in Appendix F. [TBV]4.2 CRITERIA
No criteria were used as input to this document.4.3 ASSUMPTIONS
4.3.1 Unconsolidated Material Thickness
For the alluvium option, the minimum thickness for the unconsolidated material (alluvium) that will be applied to the recontoured surface is assumed to be 2.5 m [TBV]. This is based on 2 to 3 m depth of water penetration identified by Flint and Flint (1995, p. 11) and the nominal 2 m maximum root depth (Flint, Hevesi, and Flint 1996, pp 45-46, Table 5). The additional half-meter of thickness below the maximum root depth will allow for some settling and erosion. This assumption applies only to the alluvium option and is further discussed in Sections 5.2.2 and 5.3.2. The unconsolidated material will be compacted to a percent determined in future study, but would probably be similar to existing alluvium. No specific compaction of the alluvium was included in this evaluation, so the placed volume of alluvium was equal to the swelled volume of alluvium.4.3.2 Swell of Excavated Rock and Alluvium
It is assumed that the swell of the excavated rock and alluvium will be about 30 percent. This is a reasonable factor to use for conceptual estimating of this type of material and is based entirely on engineering judgement. The swell is used in both options to calculate total excavated and placed volumes (Sections 5.3 and 6.3) and in estimating the costs for implementing these feature options (Sections 5.7 and 6.7). When the rock and alluvium is placed, it will be compacted to a percent determined in future study. No specific compaction of the rock and alluvium was included in this evaluation.4.4 CODES AND STANDARDS
No codes or standards were used as input to this document.5. ALLUVIUM SURFACE MODIFICATION FEATURE DESCRIPTION (#23a)
5.1 BACKGROUND
Winograd (1980, p 22) first proposed that in the arid environments of the Great Basin, where the thick unsaturated zone lies beneath valley fill, the fill acts as a buffer to deep percolation of surface water. In this proposed scenario, the porous valley fill, or alluvium, stores the bulk of the annual precipitation near the surface and returns it to the atmosphere by evaporation from the ground surface and transpiration from the established plants. Collectively, these processes are referred to as evapotranspiration. The alluvium, with its large storage capacity, the plants with their deep root structure, and the arid climate with the hot, sunny days all work together as a system to protect the unsaturated zone from surface water infiltration. Data from 99 neutron boreholes recorded over the ten-year period from1984 through 1993 (Flint and Flint 1995) helped define in more detail the infiltration processes taking place over the repository area of Yucca Mountain. These neutron boreholes were drilled at various topographic positions including channels, terraces, side slopes, and ridge tops (Figure 1). The moisture profiles in channel and terrace boreholes, where the alluvium was 3 meters or more in thickness, showed that the surface water penetrated in most cases to only 2 to 3 m. Most of the water was held close to the surface in the alluvium, which has a large water storage capacity (Flint and Flint 1995, p. 11). There, evapotranspiration processes remove the water to the depth of the root zone. In channels with very thick alluvium, deeper penetration of water to depths of greater than 5 m was observed after episodic runoff events (Flint and Flint 1995, p. 15). This depth is below the root zone so the water that penetrates this far adds to the shallow infiltration and may or may not travel downwards into the unsaturated zone to add to the net infiltration. The moisture profiles for side slope (Figure 1) boreholes that have thin alluvium with a mean thickness of 0.5 meter, exhibit deeper penetration of the water into the underlying fractured bedrock (Flint and Flint 1995, p. 15). Because the thin alluvium has limited storage capacity, it quickly becomes saturated during a precipitation event, with the excess water penetrating the bedrock or otherwise being removed as runoff. The steep slope of this topographic position creates conditions for rapid runoff so that very little water in relation to the runoff volume is actually added to the net infiltration (Flint and Flint 1995, p. 15). Evapotranspiration plays a minor roll in removing the small amount of water stored in the thin alluvium on the side slopes. The moisture profile for the ridge top (Figure 1) boreholes, regardless of alluvium cover, showed greater infiltration than for all the other topographic positions (Flint and Flint 1995, p. 15). The alluviums on the ridge tops were developed in place and typically contain more clay that tends to trap the water and reduce evaporation. The flat to gently sloping topographic surface, which encourages ponding of the surface water and increased precipitation due to higher elevation all contribute to deeper penetration of the water, which adds considerably to the net infiltration (Flint and Flint 1995, p. 15).5.2 DESIGN CONCEPTS
The premise for the alluvium surface modification design feature is that if the topographic surface above the repository could be covered with a blanket of alluvium, or unconsolidated material that has the same plant media and water storage characteristics as alluvium, then net infiltration could be reduced significantly. As in the natural system, the unconsolidated material would act like a sponge to hold the surface water coming from precipitation events. The stored water would then be slowly released back to the atmosphere by evapotranspiration, consisting of evaporation from the surface of the ground and transpiration through the plants. In terms of general water balance at Yucca Mountain, the water equivalent of available energy for evapotranspiration (876 mm per year) far exceeds the 170-mm annual precipitation (Flint, Hevesi and Flint 1996, pp 19-20). But, because precipitation occurs over short periods of time, it can easily exceed the water equivalent of available energy for evapotranspiration during single precipitation events. The storage capacity of the alluvium tends to even out the surge of surface water from a precipitation event and allows the transpiration process to remove the water from storage over a period of time. If the precipitation event exceeds the storage capacity of the alluvium, then the excess goes to runoff and infiltration. The field storage capacity of the natural surface material on Yucca Mountain ranges from zero for exposed bedrock to 1.2 m for thick alluviums (Flint, Hevesi and Flint 1996, p. 59). The key components of this system are:5.2.1 Ground Surface
One of the important components of the design is to form a surface upon which the blanket of unconsolidated material can be easily applied and maintained, and will not be subjected to adverse erosion that would cause failure of the system and down gradient detrimental effects. To accomplish this, the engineered system needs to be constructed to reduce the slopes so that the unconsolidated material will stay in place, yet not disrupt the drainage system so that erosion is increased on the site and down gradient. Blanketing the ground surface with a thickness of unconsolidated material is a fairly simple task in concept until the ruggedness of the topography of Yucca Mountain is considered. The eastern flank of Yucca Crest, which overlies most of the repository footprint, consists of long, east-trending, steep ridges separated by deep washes (Figure 2). The ridge slopes are too steep to hold any applied cover of unconsolidated material, and if placed on this surface would soon wash away. The same problem exists on the western flank of Yucca Crest where the steep, westward-trending slope extends down from the crest to the bottom of Solitario Canyon. This slope, which exceeds 40 percent and has a vertical drop of over 200m (CRWMS M&O 1997b, p.22, Figure 2), is also too steep to hold any applied unconsolidated material for any length of time. The design, therefore, must take into account the existing topographic relief (Design Parameter 4.1.1), and recontour it to a surface that is more receptive to applying and maintaining the blanket of unconsolidated material.5.2.2 Unconsolidated Material
The unconsolidated material that will cover the area is an important component of the system. It provides for storage of the surface water during a precipitation event as well as a growth media for the plants. It may have characteristics similar to the naturally occurring alluvium or may be engineered to improve its performance in the system. The material must be compacted so that it is stable, yet porous enough to have the storage capacity needed to hold the water from a precipitation event. The thickness of the blanket of unconsolidated material is assumed to be 2.5 m thick (Assumption 4.3.1), which is dependent upon the rooting depth of the plants. Any water that travels to a level below this rooting depth is lost from evapotranspiration and may add to the net infiltration.5.2.3 Surface Stabilization
Erosion from runoff is an important consideration because it could remove the unconsolidated material in a short period of time and result in failure of the design feature. Adding a blanket of unconsolidated material to the re-contoured surface will greatly increase the potential for erosion, especially during the torrential rainstorms that are common in the area. Once the plants are established, they will provide some stabilization of the surface material, but they will not provide total control and will not provide any control until they are well established. The answer to this problem can be found by examining the natural surface of the desert landscape. Typically, the unconsolidated material on the desert surface is covered with rock fragments that are termed desert pavement. The rock fragments are what is left after the fine material has been removed by wind and water erosion. The desert pavement acts as a protective shield for the unconsolidated material and greatly reduces erosion on these surfaces. The re-contoured surface with the blanket of unconsolidated material can likewise have a protective barrier of broken rock, or artificial desert pavement, applied to act as a shield from erosion. The downside to this concept is that the desert pavement decreases the area for plant growth and reduces the effect of evaporation by shielding the alluvium from solar radiation. However, to offset these negative aspects, the increased moisture trapped below the artificial desert pavement promotes plant growth, which would remove this excess moisture by transpiration. If unconsolidated material were added to the recontoured surface so that the drainage base is raised, erosion on the slopes and active down cutting would naturally result. The artificial desert pavement would provide protection of the slopes, but the washes, which would experience concentrated runoff, would be subjected to increased erosion and down cutting of the channel because of the raised drainage base. On the other hand, if the current drainage profiles of the existing washes are preserved in the recontouring of the surface, then it is reasonable that the likelihood of increased erosion potential is likewise reduced.5.2.4 Evaporation
The arid environment of the site is characterized by high evaporation rates. However, evaporation rates vary considerably over the extent of the site. The steep, east-trending ridges on the eastern flank of Yucca Crest create considerable variation in evaporation rates. Different facing slopes receive different amounts of solar radiation, resulting in differences in temperature and water availablilty (Flint, Hevesi, and Flint 1996, p 45). South-facing slopes have the highest evaporation rates because of the direct solar radiation that is striking these slopes at a high angle. North-facing slopes generally have the lowest evaporation rates because the solar radiation is striking these slopes at a low angle. Likewise, east- and west-facing slopes have lower evaporation rates than south facing slopes because they receive less solar radiation. In addition to topographic control of evaporation rates, there is also considerable difference depending on the time of the year. During the summer months, evaporation is the greatest and in the winter months it is the smallest. Measured evaporation pan data (Flint, Hevesi, and Flint 1996, pp 59 – 60, Figure 29) shows a range from about 2 mm/day in the winter months to about 13 mm/day in the summer months with considerable variability. The most precipitation occurs in the fall and winter months when the evaporation rates are the lowest (CRWMS M&O 1998c, DTN: MO9808RIB00025.004, Table 1). During the summer months when the evaporation is the greatest, precipitation occurs in the form of brief, but intense thundershowers.5.2.5 Transpiration
The deep-rooted plants provide the important transpiration portion of the system. The transpiration process can remove water stored in the alluvium to the depth of the root zone. Water that travels to depths below the root zone are generally lost to infiltration. Native plants in the Yucca Mountain area include shrubs and grasses that have various rooting depths (Flint, Hevesi, and Flint 1996, pp 45-46, Table 5). Grasses generally affect only near surface water while the shrubs go to greater depths, such as the Creosote bush that sends its roots down to about 1.7 m. Other native shrubs have rooting depths of 0.5 to 1.2 m. The dominant native plant is Bersage, which accounts for almost half the cover, has a rooting depth of about 0.9 m (Flint, Hevesi, and Flint 1996, pp 45-46). The minimum xylem potential is a rough measure of relative transpiration potential by relating it to the sucking potential of the plant xylem (tissues of the vascular system in plants that carry fluids). For Yucca Mountain plants, the minimum xylem potential ranges from a high of –100 bars for Shadscale to a low of –37 bars for Cheese bush (Flint, Hevesi, and Flint 1996, pp 45-46, Table 5). The minimum xylem potential for Bersage is –50 bars. The native plant species also exhibit differential preference to north- or south-facing slopes, which reflect the different amount of solar radiation each slope orientation receives (Flint, Hevesi, and Flint 1996, p 45). The south-facing slopes receive the greatest amount of solar energy and are therefore hotter and dryer, whereas the north-facing slopes receive less solar radiation so are cooler and wetter. No attempt was made in this evaluation to select optimal plant species for planting. It identifies only that deep-rooted, high transpiration plants are desirable. Actual selection of appropriate plant species would need to follow extensive testing and evaluation of potential candidates.5.3 ENGINEERING DESIGN
The engineering design that was selected to address this design feature was based on the design concepts developed in Section 5.2 of this document. This section describes the engineering design and is subdivided into the separate components of the system. A cross section sketch illustrating the hydrologic processes acting on this feature is shown in Figure 1 (right side of illustration B).5.3.1 Ground Surface
The existing topographic surface is too steep and rugged to receive a blanket of unconsolidated material with the expectation that it would remain in place for any length of time. It is therefore required that the ground surface above the repository be recontoured to develop a surface that is more suitable for placement of the unconsolidated material. Initially, recontouring to a slope similar to an alluvial fan (about five percent) was considered, but was rejected because it required cutting Yucca Crest down to a level that would be below the PTn thermal-mechanical unit. The PTn unit acts as a natural barrier to net infiltration of surface water to the unsaturated zone, so breaching this unit could result in increased infiltration to the repository level. A slope of about ten percent was then selected and found to be suitable. This slope more closely follows the drainage profile of the washes on the east flank of Yucca Crest, so there would be no steepening of the drainage gradient that could potentially increase the erosion rate. The total cut and fill area was determined based on the extent of disturbance needed to maintain a slope (over the cut and fill area) of close to ten percent and to cover the repository footprint area. A secondary objective was to place as much cut material in Solitario Canyon as possible to reduce the amount hauled to Midway Valley. The cut and fill volumes for this option were developed using input derived from the LYNX software and the net infiltration simulation. The developed files are contained in the archive tape (CRWMS M&O 1999b) and are listed in Appendix H of this document. The final cut and fill surface was developed in LYNX by drawing contours to define the final surface (file AlSrf1c3 in the overlays directory). This final contoured surface was then subtracted from the existing topographic surface (file AlSrf1t3 in the overlays directory), using the LYNX software, to identify the total volume of cut (file G.ALSRF1, unit CUT in the 3d directory) and fill (file G.ALSRF1, unit FILL in the 3d directory) required to develop the final surface. The cut and fill thickness contour maps (files AlSf1cut and AlSf1fl2, respectively, in the overlay directory) were then developed from these volumes. The available alluvium pre-strip volume within the cut area was estimated to be 7.1 million m3 (Design Parameter 4.1.2). Using this volume and the design thickness of the unconsolidated material layer, which is 2.5 m (Assumption 4.3.1), final cut and fill volumes and alluvium volumes were then calculated. The final design volumes are presented in Table 1. The design calls for cutting the crest of Yucca Mountain down by up to about 60 m and removing the ridges to the east by up to about 80 m (Figure 3). The cut surface would be removed down to a level below the final grade to allow for placement of the unconsolidated material. The final cut profile with the unconsolidated material applied would nearly coincide with the current drainage profiles of the washes with lowered ridgelines (Figure 4). For calculating volumes, the Yucca Mountain cut rock, alluvium barrow from a pit in Midway Valley, and the pre-strip alluvium are assumed to swell by 30% from in place volumes to loose volumes (Assumption 4.3.2). The cut and fill volumes were calculated using the LYNX software system described in Section 3. Total rock cut is estimated to be about 220.3 million m3 in place and 286.4 million m3 loose (Table 1). About 137.8 million m3 of loose rock cut from Yucca Mountain would be placed in Solitario Canyon to fill the canyon and maintain the same grade of the surface over the repository. The deepest fill in Solitario Canyon is about 80 m (Figure 3). The remaining loose cut rock (148.7 million m3) would be placed in a muck pile located at an undesignated site in Midway Valley (131.8 million m3) or as backfill in the borrow pit from which the unconsolidated alluvial material would come (16.9 million m3). About 7.1 million m3 of in place alluvium would be pre-stripped from the Yucca Mountain cut area and stored in the vicinity so that it can be easily applied to the completed cut and fill areas. The pre-strip alluvium would swell to an estimated 9.3 million m3 of loose alluvium, which would all be placed in the cut area. Additional alluvium would be excavated from a barrow pit in Midway Valley (16.9 million m3 in place, 22.0 million m3 loose). Of this, 11.6 million m3 of loose material would be placed in the Solitario Canyon fill area and 19.6 million m3 would be placed on the Yucca Mountain cut area.5.3.2 Unconsolidated Material
The unconsolidated material will be placed over the cut area of Yucca Mountain and the fill area in Solitario Canyon. It will consist of alluvium that will be pre-stripped from the Yucca Mountain cut area and a borrow pit in Midway Valley. It is anticipated that about 7.1 million m3 of in place alluvium would be pre-stripped from the Yucca Mountain cut area (Design Parameter 4.1.2) and 16.9 million m3 would be removed from a barrow pit in Midway Valley (Table 1). These volumes would swell to 9.3 and 22.0 million m3, based on a 30 percent swell factor (Assumption 4.3.2). The thickness of unconsolidated material to be added to the surface is 2.5 m (Assumption 4.3.1) and would be compacted. The percent of compaction would be determined in future study, but would probably be similar to natural alluvium. In the design, the blanket of unconsolidated material would be placed directly on the cut rock surface and the fill rock in Solitario Canyon. Over time, some of the fines in the unconsolidated material will migrate into the fractures and pores of the underlying porous bedrock and rock fill. This would naturally seal the voids in the rock to some extent so that potential infiltration into the fractured rock would be reduced over time. In the Solitario Canyon fill area, however, the rock fill, if not compacted sufficiently, may have a greater porosity, which may result in considerable settling of the alluvium cover. For this reason, the fill area may need to be first covered with a filter fabric to prevent migration of fines. The filter fabric was not included in the conceptual design and cost estimate.5.3.3 Surface Stabilization
The surface of the unconsolidated material will be stabilized by applying a thin veneer of riprap, or cut rock, to imitate naturally occurring desert pavement. The rock will come from the cut operation. In this conceptual study, it is assumed that the cut rock can be obtained by screening of the cut rock and will not require crushing. It is anticipated that the veneer would be only about five cm thick and will consist of one layer of rock.5.3.4 Evaporation
The evaporation component of the system would be improved over the current conditions in that the re-contoured surface would have no steep north-facing slopes to reduce average evaporation. The new surface would be predominately east facing over the repository at a ten percent slope.5.3.5 Transpiration
As the cut and fill operation is completed and the surface is covered with unconsolidated material capped with the riprap pavement, plants need to be established to begin the transpiration part of the system. High transpiration plants with deep root structures should be selected for re-vegetation of the surface. It will take several years for the plants to develop the maturity and root structure required for the functioning system; so, during the initial years, infiltration will be high and should decrease with time as the plants develop and the system begins to function fully. The initial years may also require irrigation to get the plants started and to reach a stage where they can survive the harsh desert environment.5.4 POTENTIAL SYSTEM FAILURES
This design feature is a very complex system that is dependent upon the successful interaction of engineered features with many natural processes that are not fully understood. The concept is fairly simple to understand and functions in equilibrium in the natural environment, but the design and construction of a man-made equivalence in the natural environment may introduce new variables that prevent the system from operating as intended.5.4.1 Affects of Construction
During construction of the system, the ridges on the east flank of Yucca Crest will be excavated by drilling and blasting of the bedrock. This operation would open up fractures in the bedrock that would increase infiltration at least during the construction phase. Since construction would take place over a number of years, the period of exposure could significantly increase the net infiltration into the repository during this period. Even after the unconsolidated material is placed over the excavated surface, it is uncertain how the open fractures in the bedrock will act on the near surface hydrologic system. It is conceivable that for a period of time, net infiltration may actually increase due to the open fractures below the unconsolidated material. In the currently existing situation, most of these fractures in the bedrock immediately below the alluvium are sealed with calcium carbonate minerals down to depths of 3 to 15 m (Flint and Flint 1995, p. 1). This natural fracture filling resulted from repeated wetting and drying. Seasonal water pulses were identified to correspond approximately to the depth of the calcium carbonate mineral filling, which they identified as a conceptual zone of evapotranspiration below which net infiltration is likely to occur. With the plants missing during the construction phase, and the fact that the fractures are opened up during construction, any precipitation landing on the exposed bedrock that does not go to runoff would add to the net infiltration.5.4.2 Erosion
Erosion can be a cause of failure for this system. If the riprap stabilization provisions included in this design do not function properly, channeling of large flows during short, periodic precipitation events could cause the system to fail within a relatively short period of time. One potential cause for failure through increased erosion would be the result of a radical change in climate with a significant increase in precipitation. To assure continued functioning of the system and to protect it from the effects of extreme erosion, the system will likely require constant maintenance. The amount of maintenance required would be dependent upon the future climatic conditions and the successful functioning of the designed system. The duration for which maintenance would be required is dependent upon future study that would be directed at determining when and to what extent the system can be allowed to fail so that there would be minimal impact on the waste packages and expected dose rate to the environment. It is anticipated, however, that the maintenance requirement will be minor in comparison to the original construction effort. This is based on the consideration that the design maintains existing drainage gradients and the riprap protective layer would act as desert pavement to naturally protect the system. Erosion and compaction may over time may develop depressions in the surface that could hold standing water. These would result in increased infiltration if allowed to continue. Continued maintenance would need to remove any depressions that develop over time.5.4.3 Catastrophic Events
Catastrophic events such as fire, drought, and disease could destroy the plants and cause failure of the system. Even short periods of drought has been shown to significantly alter the plant associations and distribution, such as the dramatic changes that occurred after the drought years of 1987 to 1990 (Flint, Hevesi, and Flint 1996, p. 45). After such a catastrophic event, it would take many years to recover and reestablish the transpiration part of the system. In the mean time, infiltration would likely increase significantly because there would be no way to remove the large quantity of surface water stored in the unconsolidated material. Also, since the feature required major down cutting of the bedrock, the exposed fractures lying directly below the unconsolidated material are open and receptive to the incursion of water, thus potentially increasing net infiltration. Over some period of time, these fractures would become sealed naturally, but this would take some time.5.4.4 System Life Expectancy
Barring these modes of potential failure and assuming the continuation of the current climatic conditions, the system could be expected to endure and function for several thousand years. There is no scientific evidence that can place a reliable life expectancy for this feature, but some general life expectancy can be assigned based on logic and comparison with current surfaces in the area or other desert environment. The design of this feature calls for armoring of the surface with riprap. In the natural desert environment, surface deposits are protected by a natural riprap called desert pavement. It can be seen that the desert pavement effectively protects the alluvium from the climatic elements so that the erosion is mainly confined to the washes where the surface runoff from the precipitation event is concentrated. If the engineered riprap is applied properly and can be made to function as the natural desert pavement, it can be assumed that the two systems will function identically. The average erosion rates for hill slopes measured at Yucca Mountain are very small at approximately 0.19 cm per thousand years (YMP 1995, p. 4-23). If the engineered riprap functions the same as natural desert pavement, then similar or even smaller erosion rates could be expected since the slopes are considerably less than the hill slopes. It has been identified that the ages of the hill slopes at Yucca Mountain, range from 170 to 760 ka (YMP 1995, p 4-18, Table 4.4.2-1). Based on this reasoning, the engineered surface could last for tens of thousands of years or even longer, but very conservatively, it can be expected to last at least 1,000 years. The surface may last this long or longer, but the vegetation is likely to change over time as a result of short term events such as droughts, as discussed previously in Section 5.4.3. These short term changes could change the performance of the feature even though it may not cause failure of the total system.5.5 NET INFILTRATION SIMULATION
A hydrologic evaluation to simulate net infiltration was conducted for the alluvium option of surface modification. This evaluation was submitted under a design input transmittal (CRWMS M&O 1999d) and was identified in Section 4.1.3 as an input design parameter. A summary of the results is presented in this section and in Table 2. Three cases were run to evaluate various thickness of alluvium cover:5.5.1 Case #1 - Current Climate Simulation
For Case #1 (cut area alluvium depth = 2.5 m), the current climate simulation for 1980 – 1995, indicates a major reduction in net infiltration rates over the repository area and within the cut and fill areas which are shown in Figure 4. The average net infiltration rate for the repository area is reduced from 8.5 mm/year for the 1980 – 1995 base case to 0.1 mm/year for this simulation case (Table 2). The maximum net infiltration rate is likewise reduced from 131.5 mm/year to 82.6 mm/year. Generated runoff is reduced from 7.6 mm/year for the current climate base case to 0.0 mm/year for the simulation cases. Increases in net infiltration rates of up to 30 mm/year are indicated for channel locations in Drill Hole Wash, but these results are suspect because the uniform alluvium thickness of 2.5 m may not be a good representation of the actual conditions for these channel locations (CRWMS M&O 1999d, p 14).5.5.2 Case #1 – Long Term Average Future Climate Simulation
The Long Term Average (LTA2) future climate 100-year simulation for Case #1 (cut area alluvium depth = 2.5 m) indicates a reduction in net infiltration rates for the repository area from 21.0 mm/year for the base case to 0.4 mm/year for this simulation case (Table 2). The maximum net infiltration rate is likewise reduced from 336.9 mm/year to 221.8 mm/year. The average runoff is reduced from 21.8 mm/year for the base case to 0.1 mm/year for the simulation case. Maximum infiltration rates occur along sections of the main channels in Solitario Canyon and Drill Hole Wash (CRWMS M&O 1999d, p 14).5.5.3 Case #2 - Current Climate Simulation
The Case #2 (cut area alluvium thickness = 0 m) current climate simulation for 1980 – 1995 indicate significant increases in net infiltration and represents the situation where all the alluvium is removed by erosion. For the repository area, the average net infiltration rate is increased from 8.5 mm/year for the base case to 10.9 mm/year for this case (Table 2). The maximum net infiltration rate is increased from 131.5 mm/year to 2,114.6 mm/year, with the greatest increases in channel locations in Drill Hole Wash, Split Wash, WT-2 Wash, and Dune Wash (CRWMS M&O 1999d, p 15). These significant increases are due to large runon volumes generated as runoff upstream. Generated runoff within the repository area is increased from an average rate of 7.6 mm/year for the base case to 167.5 mm/year for this case. Large net infiltration increases within the cut area are suspect because the model utilized a uniform alluvium thickness of zero meters, including areas of deep alluvium in the washes.5.5.4 Case #3 – Current Climate Simulation
The Case #3 (cut area alluvium thickness = 1.0 m) current climate simulation for 1980 – 1995 indicates a general reduction in net infiltration rates relative to the base case. This simulation shows that even with one meter of alluvium left in place, the system still provides some advantage over the base case. For the repository area, the average net infiltration rate decreased from 8.5 mm/year for the base case to 1.4 mm/year for this case (Table 2). The maximum net infiltration rate decreased from 131.5 mm/year to 78.7 mm/year. The average runoff decreased from 7.6 mm/year to 0.0 mm/year. Within the cut area, maximum net infiltration rates of about 5 mm/year occur along Yucca Crest and east-trending ridge tops (CRWMS M&O 1999d, p16). Higher net infiltration rates of about 50 to 80 mm/year are located at a few channel locations in Drill Hole Wash and upper Solitario Canyon.5.5.5 Net Infiltration Conclusions
Implementing the alluvium option of surface modification design feature would clearly decrease the average net infiltration at the repository site. With the 2.5 m designed thickness of the alluvial cover and in the current climate, average net infiltration decreased from the VA Reference Design by 99 percent to a rate of near zero (Section 5.5.1, Table 2). For the long term average future climate, the average net infiltration had a similar decrease of 98 percent (Section 5.5.2, Table 2). Maximum net infiltration likewise had significant decreases of 37 percent and 34 percent, respectively, for the current climate and average future climate cases (Sections 5.5.1 and 5.5.2, Table 2). Average runoff was also significantly decreased. For the current climate and the long term average future climate, the decrease was 100 percent. The additional cases run indicated that even if the alluvial cover erodes to an average thickness of only one meter, there is still a reasonable decrease in average net infiltration, maximum net infiltration, and average runoff. Decreases from the VA Reference Design for these three parameters are 84 percent, 40 percent, and 100 percent, respectively (Section 5.5.4, Table 2). If the alluvial cover is totally removed by erosion, net infiltration dramatically increases by 28 percent over the base case (Section 5.5.4, Table 2). The maximum net infiltration and the runoff dramatically increase from the base case by over 1500 and 2100 percent, respectively (Section 5.5.4, Table 2).5.6 PERFORMANCE ASSESSMENT SIMULATION
A total system performance assessment simulation was conducted for the alluvium option of surface modification. This evaluation was submitted as a design input transmittal (CRWMS M&O 1999c) and was identified in Section 4.1.4 as an input design parameter. The performance assessment was carried out on the net infiltration results from the Case #1 current climate simulation and long term average future climate simulation (Section 5.5.1 and 5.5.2 of this document), and assumed that the 100-year infiltration effects continued for the 10,000-year period.5.6.1 Performance Assessment Simulation Results
The peak dose rates to an individual of a critical group at a distance of 20 km from the repository site were determined for two periods – less than 10,000 years and between 10,000 and 1,000,000 years. The results are summarized in Table 3. The 10,000-year case showed an extremely low peak dose rate of < 1e-10 mrem/year. This is significantly smaller than the VA Reference Design base case of 0.04218 mrem/year. The 1,000,000-year case showed a peak dose increase from 300.88 mrem/year for the base case to 338.00 mrem/year. The Figure of Merit (FOM) for the alluvium option is 25.24, compared with 25.02 for the base case (CRWMS M&O 1999c, Item 1, p 11). For the 10,000 year case, the dose source is from one juvenile failure (premature failure of waste package), the same as assumed in the base case. But, since the alluvium options of surface modification is very effective in reducing the net infiltration, the travel time through the unsaturated zone is greatly increased, thus decreasing the dose rate. With reduced water seepage into the drift, waste package corrosion is also reduced. The corrosion failures due to dripping water would therefore be negligible and waste package degrading would be very slow and result from humid air corrosion.5.6.2 Performance Assessment Conclusions
The performance assessment simulation shows a significant improvement over the VA Reference Design in terms of peak dose for the less than 10,000-year case. The peak dose of <1e-10 mrem/year is extremely low in comparison to the base case of 0.04218 mrem/year. This decrease reflects the improvement in net infiltration provided by this feature option. With significantly reduced infiltration, travel time through the unsaturated zone is increased so that very little moisture from the repository reaches the accessible environment. With reduced moisture, waste package corrosion is also reduced to near zero. For the long term simulation (10,000 to 1,000,000 years), the peak dose increases. This reflects the eventual erosion of the alluvial cover and failure of the system. The overall Figure of Merit (FOM) for the option increases slightly from 25.02 for the VA Reference Design to 25.24 for the surface modification alluvium option.5.7 COST ESTIMATE
A conceptual cost estimate was developed for this design feature based on the design described above in Section 5.3. This cost estimate is not included in the activity evaluation (CRWMS M&O 1998b) and is non-Q. The estimate is conceptual (±50 percent) and is expressed in 1999 dollars. A summary spreadsheet and scope document for this cost estimate is presented in Appendix F (CRWMS M&O 1999f, Item 1) and was identified in Section 4.1.5 as an input design parameter. The estimated cost for implementing this feature is approximately $3.1 billion. These costs are in addition to the VA Reference Design estimated costs and are not qualified or subject to QARD requirements. The major cost for implementing this feature is the construction cost, which is presented in the estimate. Other costs, such as maintenance, are not included in the estimate, but is considered to be minor in comparison to the construction costs. The costs are based on an estimated 25-year construction schedule.5.8 LADS CRITERIA
The technical analysis of the alluvium surface modification feature addresses the eight categories of information requested by the LADS group (CRWMS M&O 1998d). The criteria are presented in Appendix A of this document. The results of the evaluation are presented in table format in Appendix B. The confidence assessments for the LADS criteria are included in the Appendix C tables and are detailed in Appendix G.6. DRAINAGE SURFACE MODIFICATION FEATURE DESCRIPTION (#23b)
6.1 BACKGROUND
It has been observed in data from the neutron boreholes, that the steep slopes create conditions for rapid runoff of surface water (Flint and Flint 1995, p. 15). Consequently, very little water in relation to the runoff volume actually infiltrates into the bedrock. Infiltration on these slopes occurs where the thin alluvium covering traps the water and allows it to enter into the fractures of the bedrock. On the ridge tops, a similar situation takes place where the alluvium, which contains a lot of clay minerals, traps the water and allows it to infiltrate into the fractured bedrock (Flint and Flint 1995, p. 15). Ponding of water on the ridge tops also adds to the surface water infiltration by prolonging the contact of the water with the bedrock. As almost an opposite of the alluvium surface modification design option, the drainage surface modification design option is based on the idea that if rapid runoff could be encouraged, then infiltration could be reduced significantly. To promote runoff, the thin alluvium could be removed from the ridge tops and slopes. This would prevent the water from ponding on the ridge tops, or otherwise being trapped by thin alluvium on the ridge tops and side slopes, so that very little water, if any, would have time to infiltrate into the bedrock.6.2 DESIGN CONCEPTS
The basic premise behind this design feature is that if runoff could be improved so that surface water is removed rapidly, then infiltration would be greatly reduced. To improve the runoff, the thin alluvium covering on the ridge tops and side slopes would be removed. Other obstacles that would hinder the rapid runoff would also be removed. On the ridge tops (Figure 1) where the slopes are gentler, ponding conditions are common. These would need to be removed and the drainage improved to encourage rapid runoff. Since the terrace and channel areas (Figure 1) that make up the washes are functioning as a transpiration system like the alluvium surface modification design feature, these areas will be left intact.6.3 ENGINEERING DESIGN
Basically, the engineering plan for this design feature involves the removal of the thin alluvium covering and other obstacles from the ridge tops and side slopes, and the removal of ponding conditions on the ridge tops. This would promote the rapid runoff of surface water from the ridge tops down to the washes. Since the alluvial fill and vegetation in the terrace and channel topographic positions are functioning as a transpiration system like the alluvium surface modification design feature, they will be left in place. A cross-sectional sketch illustrating the hydrologic processes acting on this feature is shown in Figure 1 (left side of illustration B). Removal of the thin alluvium would be accomplished with bulldozers, front-end loaders, and trucks to remove the bulk of the material. It would be hauled down to a site in Midway Valley were it would be placed in a storage pile. The length of the haul to the Midway Valley site is estimated to average about 15km. It is anticipated that about 2.1 million cu-m of alluvium of 0.5 m or less (Design Parameter 4.1.2) would be removed from the ground surface above the repository footprint and to a distance of about 300-m outside it (Table 4). These volumes were calculated using the LYNX software system described in Section 3. The area of disturbance, including the repository footprint and the 300-m buffer surrounding it are illustrated in Figure 5. Final cleaning of the surface would be done using water and fire hoses to wash down the bedrock surface. On the ridge tops, features that would develop ponding would need to be removed and drainage developed to drain the features and prevent future ponding. During the process of alluvium removal, some damage could be done to the bedrock surface to open up fractures, thus allowing the water to enter the bedrock and add to the net infiltration. The alluvium material would need to be bulldozed downhill to the wash bottoms where it would be loaded onto trucks. Some damage could be done to the evapotranspiration system in the washes where the loading process would require roads and loading areas to be cleared. These would all need to be reclaimed so that the washes can function as a evapotranspiration system.6.4 POTENTIAL SYSTEM FAILURES
Like the alluvium surface modification design option, probably the greatest potential for system failure is that the system will not perform as designed because it depends upon performance of poorly understood natural processes. By altering one part of a system that is in equilibrium, other parts can be radically changed as a result.6.4.1 Affects of Construction
The plan for the removal of the thin alluvium specifies the use of bulldozers followed by washing down the surface with water hoses. The use of bulldozers may result in some damage of the bedrock surface. The heavy bulldozers may fracture the bedrock and open existing fractures, resulting in increased infiltration.6.4.2 Erosion
Over a period of time, chemical and mechanical erosion will again begin to create alluvium covering for the cleared area. Although it will likely take hundreds or thousands of years to accumulate sufficient alluvium to affect the infiltration rates, it will eventually happen.6.4.3 Surface Hydrology
By increasing the speed of drainage on the ridge tops and side slopes, the channels may be overloaded due to the increased runoff surge, possibly resulting in increased erosion and infiltration in the terrace and channel topographic positions. Increasing the runoff speed could possibly cause failure to the terrace and channel evapotranspiration system.6.4.4 System Life Expectancy
There is no scientific evidence that can predict the expected life expectancy of this feature. In comparison to the alluvium surface modification design feature, it is expected that this feature would not last as long. Over a period of time, fine silt and sand would be blown in by the wind and eventually, alluviums would again develop as a result of chemical and mechanical erosion of the bedrock. Without continued maintenance, it is anticipated that the system may last only 500 years. With continued maintenance, the feature could last much longer.6.5 NET INFILTRATION SIMULATION
A hydrologic evaluation to simulate net infiltration was conducted for the drainage option of surface modification. This evaluation was received as a design input transmittal (CRWMS M&O 1999d) and was identified in Section 4.1.4 as an input design parameter. A summary of the results is presented in this section and is summarized in Table 5. Two cases were run to evaluate the variations of this option. These cases are described as follows:6.5.1 Case #1 – Current Climate Simulation
The results for Case #1 (affected area alluvium thickness = 0 m) current climate simulation for 1980 – 1995 is similar to the results for Case #2 of the alluvium surface modification (Section 5.5.3). Differences include higher net infiltration rates for channel locations in Solitario Canyon, lower maximum rates within the repository area along the west-facing slope of Solitario Canyon, and lower rates for channel locations on the east slope of Yucca Mountain (CRWMS M&O 1999d, p 16). Although removal of alluvium cover does succeed in reducing net infiltration along ridge tops and upper side slopes within the repository area, the large increase in runoff generated causes very high net infiltration (> 100 mm/year) for channel locations downstream of the affected area. For the repository area, the average net infiltration rate is increased from 8.5 mm/year for the base case to 10.7 mm/year for this simulation case (Table 5). The maximum net infiltration rate likewise increased from 131.5 mm/year to 185.7 mm/year. The average runoff increased from 7.6 mm/year to 173.7 mm/year. This significant increase would likely result in increased erosion of the alluvial fill in the washes. Because of this, it is not considered that the results would be significantly different if the modeling removed only the thin alluvium from the ridge tops and side slopes, as designed. Extreme increases in net infiltration of greater than 2,000 mm/year are indicated in channel locations in Solitario Canyon, Drill Hole Wash, Split Wash, and WT-2 Wash. A maximum increase of more than 9,500 mm/year occur for a location in Drill Hole Wash just outside the affected area. To some extent, these extreme values should be interpreted with caution because the uniform removal of all alluvium effectively removes evapotranspiration for locations where deep alluvium currently exist and would not be removed in the engineering design (CRWMS M&O 1999d, p 16).6.5.2 Case #1 – Long Term Average Future Climate Simulation
The 100-year simulation of Case #1 (affected area alluvium thickness = 0 m) for the long term average future climate indicates reductions in net infiltration rates along ridge tops and upper side slopes, but very large increases in rates for downstream channel locations. The average net infiltration rate for the repository area decreased from 21.0 mm/year for the base case to 15.7 mm/year for the case simulation (Table 5). The maximum net infiltration rate is reduced from 336.9 mm/year to 327.3 mm/year. Average runoff increased significantly from 21.8 mm/year to 307.7 mm/year. Maximum net infiltration rates exceeding 2,000 mm/year occur for channel locations in Solitario Canyon, Drill Hole Wash, Split Wash, and WT-2 Wash (CRWMS M&O 1999d, p 17).6.5.3 Case #2 – Current Climate Simulation
The Case #2 (affected area alluvium thickness = 0.1 m) simulation results for the 1980 – 1995 current climate indicates an increase in net infiltration rates relative to the base case. Increases in net infiltration occur for both ridge top locations within the repository area and channel locations outside the repository area (CRWMS M&O 1999d, p 17). Average net infiltration rate is increased from 8.5 mm/year for the base case to 14.5 mm/year for this case (Table 5). The maximum net infiltration rate increased only slightly from 131.5 to 132.8 mm/year. The average runoff decreased significantly from the case with no alluvium, but was still double the base case of 7.6 mm/year to 23.6 mm/year. Although runoff amounts are substantially reduced relative to the Case #1 simulation, surface water volumes are still 6 to 7 times higher than for the base case. At the same time, net infiltration along ridge tops is still higher than the base case because evapotranspiration rates are lower than the base case (CRWMS M&O 1999d, p 18).6.5.4 Net Infiltration Conclusions
In all the net infiltration simulations run, average net infiltration, maximum net infiltration, and average runoff all increased dramatically over the VA Reference Design base case (Section 6.5.1, Section 6.5.2, Section 6.5.3, Table 5). The average runoff was increased dramatically by over two thousand percent, but instead of reducing the net infiltration because of reduced exposure time, average net infiltration increased by 26 percent and the maximum increased by 41 percent due to overloading of the alluvial fill in the washes. If a thick alluvium is overloaded (saturated) and the water percolates to a depth below the root zone, it can no longer be removed by evapotranspiration processes. The dramatic increase in net infiltration is largely a result of this overloading. In addition to this, the increased runoff would increase the erosion of the alluvial fill in the washes.6.6 PERFORMANCE ASSESSMENT SIMULATION
A performance assessment simulation was not completed on the drainage option of surface modification because of the disappointing results from the net infiltration simulation. It was identified in the net infiltration simulation study (CRWMS M&O 1999d) that if the thin alluvium is removed from the surface, net infiltration is improved for the ridge tops and slopes, but substantially increased for the overall system. With increased runoff, the washes are overloaded and net infiltration increases dramatically. For these reasons, it was decided not to run a performance assessment simulation for this option.6.7 COST ESTIMATE
A conceptual cost estimate was developed for this design feature based on the design described above in Section 6.3. This cost estimate is not included in the activity evaluation (CRWMS M&O 1998b) and is non-Q. The estimate is conceptual (±50 percent) and is expressed in 1999 dollars. A summary spreadsheet and scope document for this cost estimate is presented in Appendix F (CRWMS M&O 1999f, Item 1) and was identified in Section 4.1.5 as an input design parameter. The estimated cost for implementing this feature is approximately $141 million. These costs are in addition to the VA Reference Design estimated costs and are not qualified or subject to QARD requirements. The major cost for implementing this feature is the construction cost, which is presented in the estimate. Other costs, such as maintenance, are not included in the estimate, but is considered to be minor in comparison to the construction costs. The costs are based on an estimated 9-year construction schedule.6.8 LADS CRITERIA
The technical analysis of the drainage surface modification feature addresses the eight categories of information requested by the LADS group (CRWMS M&O 1998d). The criteria are presented in Appendix A of this document. The results of the evaluation are presented in table format in Appendix C. The confidence assessments for the LADS criteria are included in the Appendix C tables and are detailed in Appendix G.7. CONCLUSIONS
The following conclusions are considered to be preliminary based on the non-qualified inputs. Out of the two surface modification options investigated in the document, the alluvium and drainage options, it is clear that the only option that may have some potential is the alluvium option. The alluvium option, which involves the recontouring of the surface above the repository and covering it with a layer of unconsolidated material, showed significant improvement in net infiltration over the VA Reference Design. With this feature, the net infiltration was reduced by 99 percent to a rate of near zero. Waste package performance would consequently be improved by removal of the dripping water case for corrosion of the waste packages. The drainage option, which involves removal of thin alluvium less than 0.5 m thick, improved net infiltration on the ridge tops and side slopes, but increased net infiltration in the washes. The improved drainage overloaded the washes and resulted in the increased net infiltration there. Also, it is likely that the increased runoff would seriously erode the washes and cause serious downstream problems. The two options were evaluated in terms of the LADS criteria (Appendix A, B and C) and are summarized below. The criteria used a scale and confidence rating for comparison of results. The scale ratings are identified in the LADS Evaluation Criteria (Appendix A) and the confidence ratings are identified in the Confidence Assessments (Appendix G). Surface Modification Feature, Alluvium Option (#23a)8. REFERENCES
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