2

SURFACE CHARACTERISTICS AND EROSION POTENTIAL

This chapter provides a review of the technical basis report (TBR) sections related to surficial geology; stream (alluvial) deposits and the potential for stream erosion; and the ages, stability, and erosion of hillslope deposits at Yucca Mountain. Discussions of these issues appear in several different sections of the TBR, as noted in Table 1.1 of this report. The committee has chosen to group and discuss these topics in a single chapter, so that the surficial geology of the site—the product of erosion—can be related directly to erosion rates and processes.

REVIEW OF SURFICIAL GEOLOGY

Sections 2.1 to 2.4 of the TBR discuss the surficial geology of the Yucca Mountain site. The focus of this discussion is on the distribution and relative ages of surficial deposits at the site. A later section of the chapter reviews the erosion potential of these deposits.



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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion 2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL This chapter provides a review of the technical basis report (TBR) sections related to surficial geology; stream (alluvial) deposits and the potential for stream erosion; and the ages, stability, and erosion of hillslope deposits at Yucca Mountain. Discussions of these issues appear in several different sections of the TBR, as noted in Table 1.1 of this report. The committee has chosen to group and discuss these topics in a single chapter, so that the surficial geology of the site—the product of erosion—can be related directly to erosion rates and processes. REVIEW OF SURFICIAL GEOLOGY Sections 2.1 to 2.4 of the TBR discuss the surficial geology of the Yucca Mountain site. The focus of this discussion is on the distribution and relative ages of surficial deposits at the site. A later section of the chapter reviews the erosion potential of these deposits.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion Adequacy of Data Collection and Analysis The TBR presents information on the distribution and ages of surficial deposits and the potential for hillslope and stream (fluvial) erosion, with an emphasis on the Fortymile Wash drainage basin (Figure 1.1 and Figure 1.2). Various geological maps constructed for the Yucca Mountain region identify surficial deposits (Frizzell and Shulters, 1990;1 Wesling et al., 1992; Faulds et al., 1994; Lundstrom et al., 19952). The TBR makes reference to some of these maps but does not reproduce any of them. Although a variety of surficial deposits are identified, the TBR addresses the age and stability of only two types of deposits: alluvial deposits and coarse (boulder) hillslope deposits. Maps published by Department of Energy (DOE) scientists and collaborators primarily cover the eastern side of Yucca Mountain (Wesling et al., 1992; Lundstrom et al., 1995). Maps published by the State of Nevada cover the western side of Yucca Mountain (Faulds et al., 1994). Identification of surficial geologic units on these maps is based on the following criteria: landform morphology, relative geomorphic position, relative degree of preservation of surface morphology, relative soil development, characteristics of vegetation on the geomorphic surfaces, and drainage network patterns (Wesling et al., 1992; Faulds et al., 1994; Lundstrom et al., 1995). The procedures used to define map units are consistent with practices used by most scientists for surface mapping. The map constructed by Lundstrom et al. (1995) for the eastern side of Yucca Mountain is largely compatible with the map 1   References cited in the TBR are denoted by boldface type throughout this report. 2   Listed as in press in the TBR.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion published by the state of Nevada for the western side of Yucca Mountain (Faulds et al., 1994). Differences between mapped units appear to be relatively minor and are due to differences in interpretation of some of the surfaces owing to relatively poor age control. The production of a surficial geological map requires that both the spatial distribution of mappable units and their ages be determined. The ages of most of the mapped surficial deposits at Yucca Mountain are not known. As noted previously, research reported in the TBR focuses on dating two types of deposits, alluvial and hillslope deposits. The ages of the hillslope deposits were estimated with cation ratio techniques. A review of this work is presented later in this chapter. The alluvial deposits have been dated by using radiocarbon techniques, U-trend techniques, and soil characteristics (Taylor, 1986; Hoover, 1989). More recent age determinations employ thermoluminescence, tephrochronology, and U-series disequilibrium techniques. These recent data are not included in the TBR but were presented to the committee during the field excursion. These recent determinations provide age estimates of 40-100 ka3 for the youngest terrace deposit adjacent to Fortymile Wash (S.C. Lundstrom, U.S. Geological Survey, personal communication). These estimates are considerably younger than the 150-ka estimate of this same terrace presented in Section 4.4.3.1 of the TBR. This difference is significant because it affects the assessment of the potential for fluvial erosion at the Yucca Mountain site. 3   The unit ka denotes thousands of years before the present, where the “present” is defined relative to some reference year, usually A.D. 1950 for radiocarbon dates.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion Ages of other types of hillslope deposits were determined as part of a research program designed to evaluate recent faulting (Paces et al., 1994). Trenches were dug across faults near the bases of certain hillslopes, and colluvial deposits exposed in the trenches were dated by using U-series disequilibrium methods on pedogenic carbonates and thermoluminescence techniques on quartz grains. Surface ages at these sites range from modern to approximately 100 ka (Paces et al., 1994). These ages suggest that the colluvial deposits at trench locations near hillslope bases are extremely stable. However, the TBR does not report these data. Support for Technical Interpretations The ages of the surficial units reported in the TBR are not well determined, which makes it difficult to assess the validity of the interpretations. The committee views surficial mapping as a first step in a determination of the potential for landscape change at the site. Other components of this analysis should include (1) an identification of the major geomorphic processes that are responsible for these mapped geomorphic surfaces and deposits, including possible interactions between hillslope and channel erosion processes, and (2) the ages of these surficial deposits. Credible Alternative Interpretations As noted previously, there are differences in the interpretation of mapped units between Lundstrom et al. (1995) and Faulds et al. (1994). The TBR should have cited the Faulds et al. (1994) map and discussed these differences.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion The TBR emphasizes the east side of Yucca Mountain in both the surficial mapping and the evaluation of fluvial erosion (next section). It does not give the rationale for this emphasis, but presumably DOE scientists reason that erosion potential is higher on this side of Yucca Mountain. An alternate hypothesis would be that there is also a significant potential for erosion of Yucca Mountain from the western drainages (i.e., the Amargosa River drainage) and hillslopes. As shown in Figure 1.4, the western side of Yucca Mountain is significantly steeper than the eastern side and therefore may be more prone to erosion. Is erosion of this slope likely under present or future climate conditions? Note that such erosion could occur by either scarp retreat or stream head-cutting. What would the effects of climate change be on erosion of the western slope during the period of regulatory concern? Surficial investigations on the western side of Yucca Mountain could provide the information needed to evaluate these questions. Testing to Discriminate Among Alternative Interpretations Comparison of surficial units and their ages on both the east and the west sides of Yucca Mountain is needed to evaluate the results of DOE and State of Nevada mapping programs and to compare landforms and geomorphic processes. Better age determinations are needed on the mapped units on both sides of Yucca Mountain. Such ages would significantly reduce scientific uncertainties in hillslope and fluvial erosion histories.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion Summary and Conclusions The geomorphic surfaces and deposits at Yucca Mountain are of different ages and likely were formed by a variety of geomorphic processes. The identification of surficial units is a starting point in any exercise to determine the processes by which these geomorphic surfaces have formed, their ages, and their relative stability. Such data are important for assessing long-term erosion potential, because they attest to the spatial and temporal rates of erosional processes operating in the region. Based on information received by the committee during the field excursion, surficial mapping appears to be a relatively recent effort at the site, and it does not appear to be well coordinated with efforts to evaluate hillslope and stream erosion processes. The committee believes that the coordination of this work would significantly improve scientific understanding of erosion processes at the site and the effectiveness of the TBR in conveying this understanding. REVIEW OF EROSION POTENTIAL OF ALLUVIAL DEPOSITS Sections 2.3-2.5 and 4.4.3 of the TBR discuss alluvial deposits and their erosion potential. The focus of that discussion is on the following two issues: What is the thickness of alluvium in the major stream valleys at Yucca Mountain? What is the potential for stream and debris flow erosion at the site?

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion Adequacy of Data Collection and Analysis This review of the adequacy of data collection and analysis focuses on three aspects of the discussion in the TBR: (1) thicknesses and ages of alluvial deposits; (2) surface drainage, fluvial erosion, and Fortymile Wash evolution; and (3) debris flow deposits and debris flow potential. These topics are treated sequentially in the following sections. Thicknesses and Ages of Alluvial Deposits In Sections 2.3 and 4.4.3, respectively, the TBR provides data on the thicknesses and ages of alluvial deposits. It also presents interpretations of alluvial history and potential for erosion based largely on these data. The discussion of alluvial deposits focuses on Fortymile Wash and adjacent drainages (Figure 1.1); there is no discussion of alluvial deposits on the west side of Yucca Mountain. An implicit assumption was made in the TBR that Fortymile Wash is of greatest concern in terms of the erosion potential of Yucca Mountain. A rationale for this assumption is not given in the TBR, but the committee speculates that it is based on the following site characteristics: Alluvial deposits on the east side of Yucca Mountain are extensive, and the North Portal (Figure 1.2) is immediately adjacent to these deposits. The elevation of the upper block of the proposed repository is between about 3,450 and 3,650 feet (1,052-1, 113 m)4 above mean sea 4   The committee estimated the elevation of the proposed repository using Figure 4.1.1-1 of the TBR and a topographic map of the region. The elevation of the upper block of the proposed repository dips slightly toward the north. Its elevation is about 3,650 feet (1,113 m) at the south end and about 3,450 feet (1,052 m) at the north end.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion level, which is above the alluvium-bedrock contact in Fortymile Wash (e.g., Figure 2.1). Data on the thicknesses of alluvial deposits were derived from analysis of sediments from drillholes. The locations of the drillholes are shown in Figure 2.3-1 of the TBR. The TBR does not present these data effectively (i.e., the data are listed in the text but not tabulated or analyzed) or evaluate variations in depth of alluvium in various parts of the stream network and the relationship of alluvium to the underlying bedrock surface. Consequently, the committee had difficulty in reviewing these data, particularly with respect to spatial variations in sediment thicknesses in the valleys east and south of Yucca Mountain. Stream profiles showing the thicknesses of alluvial deposits and depth to bedrock along Fortymile Wash were presented to the committee on the field excursion (Figure 2.1). The TBR should have included such profiles for all significant drainages. The data presented in the TBR provide a general picture of thicker accumulations of alluvium in Fortymile Wash than in the smaller tributaries. Figure 2.1 shows that the alluvial fill in Fortymile Wash thickens significantly in the downstream direction. Section 2.4.1 of the TBR describes the alluvial deposits of Sever Wash and Midway Valley near the North Portal Site (see Figure 1.2). Sever Wash is described as an area characterized by alluvial fan aggradation and minor erosion in contrast to the area west of Midway Valley, where narrow, teep-sided valleys are incised into bedrock. The TBR does not provide any data or discussion to support the hypothesis that Sever Wash is currently an area of minor erosion.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion FIGURE 2.1 Longitudinal profile of Fortymile Wash (from S.C. Lundstrom, U.S. Geological Survey, distributed to the committee during the field excursion). The approximate location and elevation of the proposed upper block repository with respect to Fortymile Wash is projected onto the cross section for comparison purposes.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion There appears to be an implicit assumption made in the TBR that current erosional and depositional trends can be extrapolated into the future. That is, the TBR assumes that sites of current alluvial deposition have low potential for future erosion, whereas sites currently eroded to bedrock have greater potential for future erosion. This apparent assumption ignores temporal patterns of erosion and sedimentation common to semiarid and arid regions (e.g., Schumm, 1977; Wolman and Gerson, 1978). Erosion rates may decrease after channels are eroded to competent bedrock. Thick sequences of alluvial fill represent material that can be eroded easily under the proper hydrologic conditions. Such conditions might occur, for example, with a change in climate. Surface Drainage, Fluvial Erosion, and Fortymile Wash Evolution Fortymile Wash in the vicinity of Yucca Mountain is dry except during runoff-generating storm events. Continuous streamflow from Fortymile Wash into the Amargosa River (Figure 1.1) and extensive streamflow within the Fortymile Wash drainage system occurred during the 1969 and 1995 flood events (Beck and Glancy, 1995). Bank erosion and sediment deposition occurred during these events as well (Beck and Glancy, 1995). Fortymile Wash has been gauged since 1969, which is not a sufficient time interval to evaluate flood magnitude and frequency and associated erosion potential. (Chapter 3 of this report discusses the magnitudes of flood events that have occurred at Yucca Mountain.) The

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion TBR does not address the possible effects of climate changes that may alter the rainfall-runoff regime and thereby affect the stability of alluvial deposits in Fortymile Wash. Sections 2.5.1 and 4.4.3 of the TBR address the potential for stream erosion in Fortymile Wash and its tributaries. Section 2.5.1 presents the early history of the evolution of Fortymile Wash. It describes the present channel as being in dynamic equilibrium, with neither net aggradation nor net erosion occurring within the system (Huber, 1988). Data that support this hypothesis of dynamic equilibrium are not presented, nor is there a discussion of whether such dynamic equilibrium would be maintained under reasonable scenarios of climate change over the period of regulatory concern.5 5   The concept of dynamic equilibrium as applied to rivers in arid and semiarid systems is not well defined, as noted elsewhere in the text. The committee takes the term dynamic equilibrium to describe the condition of a river channel that is adjusted to its water and sediment loads such that no net aggradation or degradation occurs over periods of decades to centuries. Debris Flow Deposits and Debris Flow Potential Surficial mapping by Swadley et al. (1984) and by Lundstrom et al. (1995) identifies a few debris flow deposits at Yucca Mountain. Colluvium that could have been transported by debris flows is noted to underlie the North Portal Pad (Figure 1.2). The TBR does not discuss the ages of these deposits and the potential for future debris flows at this location. The TBR does note that debris flows have occurred within recent years at Yucca Mountain. Although it considers debris flows as relatively unimportant agents of erosion under the current climate regime, the

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion different calculation methods, as discussed in more detail in the next section. Cosmogenic isotopic methods have come into their own only in the past few years. They can yield ambiguous results unless they are applied carefully and are interpreted within the geological context of the region. The integration of this geochronological work with surficial mapping is essential to provide the necessary context and control for interpreting the surface ages. Summary and Conclusions The lack of critical discussion of the uncertainties and assumptions of CRD is a significant deficiency of the TBR. The focus of the TBR on the precision of CRD, when accuracy is more important for the overall conclusions, is also problematic. The erosion rates at Yucca Mountain appear to be extremely low under current climatic conditions on the basis of the data presented in the TBR. However, the TBR does not effectively make the case that these rates are representative of the region, because it relies on a single type of hillslope deposit and a single method for age determination. Estimates of erosion rates should be made by using a variety of approaches, as discussed above and in the next section. There are a number of ways of testing the interpretations and conclusions presented in the TBR. None of these were presented or discussed. Different dating techniques, such as 3He and 10Be, could be applied to the colluvial boulder deposits to check CRD results. Additional dating studies of other geomorphic surfaces, as well as other methods for the erosion rate calculations,

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion should be utilized in order to obtain estimates of the spatial variability of erosion rates at Yucca Mountain. REVIEW OF EROSION RATES Sections 4.1-4.2 and 4.4-4.7 of the TBR (see Table 1.1) provide a discussion of erosion. The focus of this discussion is on estimating long-term rates of erosion at Yucca Mountain. The TBR employs a numerical approach to characterize erosion. It calculates a numerical erosion rate for two types of erosional processes operating at selected sites at Yucca Mountain. The TBR then generalizes these numbers to infer the long-term average erosion rate applicable to the ground surface overlying the proposed repository. Adequacy of Data Collection and Analysis Calculation of erosion rates for Yucca Mountain is based on a standard geomorphic rate formula that is a generalization of Equation 2.1: Process Rate (R) = Process Magnitude (M)/Time of Process Operation (T) (2.3) The numerical values obtained from applying this formula to selected sites and processes are used to infer average erosion rates at Yucca Mountain. The TBR devotes considerable attention to the

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion determination of surface ages that represent parameter T in Equation 2.3, as discussed in the previous section. The TBR uses this rate formula without critical analysis. It is well documented that apparent geomorphic process rates (R) depend on the time scale (T) over which process operation is determined (Gardner et al., 1987). This dependence is such that rates for a given process determined over relatively short time intervals are generally much higher than rates determined over relatively long time intervals. Reasons for this variation have to do with the natural variability of processes over time and space. For example, many processes vary in rate through time. They are more rapid in certain time intervals, such as periods of active tectonic uplift, and less active in other intervals, such as periods of tectonic quiescence. Processes may also be cyclic in character, such as the cut and fill cycles of aggradation and degradation in drainage systems. Calculation of a single rate (R), over an arbitrary time interval (T), completely ignores the relevant time scales of these variations. The TBR's analysis of longer-term erosion rates (R) via Equation 2.3 focuses on two specific processes: (1) hillslope erosion associated with heavily varnished (and presumably ancient) colluvial boulder deposits, and (2) stream incision along the major drainage (Fortymile Wash), which presumably provides local base level control for drainage off the eastern side of Yucca Mountain. The heavily varnished boulder deposits are a common though not ubiquitous feature on the hillslopes of the region. They are particularly prominent along the escarpment at the western side of Yucca Mountain. Determination of process rates for hillslope erosion and stream incision would conceptually appear to bracket the potential for future erosion of Yucca Mountain, which might be envisioned

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion to involve (1) escarpment retreat from the west or (2) headward drainage extension from the east, dictated by the base level of erosion established by incision of Fortymile Wash. The understanding of these relationships by readers of the TBR would be facilitated by inclusion of a simple cross section showing elevations of the land surface of Crater Flat, Yucca Mountain, and Fortymile Wash; the proposed repository; the water table; and other pertinent features. Figure 1.4 is an example of the kind of graphic that the TBR should have included. Support for Technical Interpretations The analysis of erosion rates in the TBR was performed on colluvial boulder deposits at Yucca Mountain, Little Skull Mountain, Skull Mountain, and Buckboard Mesa (see TBR Table 4.4.2-1). In addition, an analysis of erosion rates was made on sequences of terraces along Fortymile Wash on which age dates have been obtained. The colluvial hillslope deposits are not the only kind of hillslope feature in the region, nor are they necessarily representative of hillslope erosion rates. They may well occur at local sites representing some of the lowest long-term erosion rates in the region. There may be other areas at Yucca Mountain that are eroding at much higher rates than these stable hillslopes, as discussed later in this section. The TBR reports incision rates for Fortymile Wash that are based on time intervals (T) determined through U-trend dating, which, as noted previously, is now considered highly problematic. New dating results presented to the committee during the information-gathering sessions need to be incorporated into the calculations made in the TBR. Furthermore, the TBR assumes that the terraces represent continuous time sequences; they could have

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion been deposited over relatively short time periods, with long intervening periods of no deposition. The TBR makes a useful comparison of Yucca Mountain erosion rates, presumed to be represented by the ancient colluvial boulder deposits, to erosion rates from other parts of the world. The hypothesis of exceedingly low long-term erosion rates at Yucca Mountain (approximately 2 mm/ky or 2 m/My; TBR, Table 4.4.2-1)14 is comparable to the extremely low erosion rates known from ancient cratonic landscapes of the Gondwanaland continents, for example, Australia and southern Africa (Young, 1983; Bishop, 1985). DOE's conclusion that erosion rates at Yucca Mountain, which is part of the tectonically active Basin and Range Province of western North America, are comparable to rates for the ancient landscapes of the Gondwanaland continents is a fascinating scientific hypothesis. The committee believes that this hypothesis needs to be examined with some care and corroborated with additional data. Tectonically active areas, regardless of climate, typically have long-term, average erosion rates of 1,000 mm/ky or 1,000 m/My (Summerfield, 1991)—although rates may vary locally by several orders of magnitude depending on tectonic setting and activity and the time scale over which such rates are measured (Ollier, 1981). The hypothesized long-term average erosion rates at Yucca Mountain are only a fraction of this rate according to the 14   The TBR reports erosion rates by unit designations that are unconventional. Erosion rates are reported in units of cm/ka. The unit “ka” means “thousand years before present” and thus applies to ages, not time intervals. The proper unit for time intervals is “ky,” which means “thousand years.” The convention in geomorphology is to report erosion rates in millimeters rather than centimeters. This designation would promote comparison to the published literature in which erosion rates are typically reported as mm/ky or m/My (where My = million years).

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion data presented in the TBR. Is Yucca Mountain a region of unusual long-term stability within a larger, tectonically active region characterized by more rapid denudation? Such stability is certainly possible, because extension in the Basin and Range province is nonuniform and erosion is quite variable. What other data, besides the limited number of site-specific erosion formula calculations, are consistent with this interpretation of long-term stability? Consideration of these questions is essential to demonstrate that Yucca Mountain is indeed a long-term stable block in this otherwise tectonically active region. Credible Alternative Interpretations Perhaps the most compelling alternative hypothesis, which is based on general understanding of erosion rates in western North America, is that long-term erosion rates at Yucca Mountain are much higher than those cited in the TBR. Another alternate hypothesis, which is applicable to landscapes of high apparent stability (Twidale, 1976, 1991), is generally termed the “hypothesis of unequal erosion. ” This hypothesis holds that major elements of a landscape, particularly plateau surfaces and adjacent hillslopes, may exhibit very low rates of erosion, while adjacent elements, usually incised stream valleys, exhibit extremely high rates of erosion. A corollary of this hypothesis is that the zones of high erosional rate may migrate—for example, by headward stream erosion—into the zone of stability. Does this hypothesis apply to Yucca Mountain? Can this hypothesis be falsified? Note that this hypothesis introduces a mode of erosion—headward cutting of the stream channels—that is potentially applicable to the eastern and western sides of Yucca Mountain but is not explicitly discussed in the TBR. Figure 1.4 shows that the elevation of the upper block of

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion the proposed repository is about 200 feet (approximately 60 m) above the bedrock-alluvium contact in Fortymile Wash. Removal of this alluvium and initiation of headward erosion on the east side of Yucca Mountain could potentially unroof the repository.15 It is therefore essential to understand the overall pattern of erosion in time and space, and credible scientific evidence needs to be offered that this understanding applies over the time scale of regulatory interest. Testing to Discriminate Among Alternative Interpretations The focus of the TBR on ancient colluvial hillslope deposits and on the incision of Fortymile Wash ignores a variety of alternative approaches for determining regional erosion rates. The hypothesis that such sites represent the long-term erosion rate for the entire region, and specifically for the ground surface overlying the repository, needs to be tested. Such tests might involve estimating erosion rates at a few other hillslope sites in the Yucca Mountain region that are not mantled with heavily varnished colluvial boulder deposits. Examples of such sites include incised gulleys and stream channels that drain Yucca Mountain and show indications of more rapid erosion. Comparison of erosion at such sites can serve as bounding calculations to test the interpretations and conclusions presented in the TBR. 15   The distance between Fortymile Wash and the eastern edge of the upper block of the repository is approximately 6.1 km or 3.8 miles (TBR Figures 2.3-1 and 4.1.1-1). Headward erosion from Fortymile Wash into the upper block of the repository could be accomplished by a stream having a gradient of approximately 10 m per kilometer (53 feet per mile). The present-day gradient of Fortymile Wash (calculated from Figure 2.1 of this report) ranges from about 7.6 m per kilometer (40 feet per mile) near Highway 95 to about 13 m per kilometer (68 feet per mile) east of Yucca Mountain (see Figure 1.1).

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion The committee believes that a much better understanding of erosion rates at Yucca Mountain could be obtained by applying a number of different methodologies and evaluating their sensitivities to reasonable ranges of geological parameters. In this approach, “valid” solutions are those that fit all of the methodologies, and failures in agreement among the methodologies can be just as informative as successes. The calculation of erosion at a variety of hillslope sites in order to obtain a regional and temporal context for erosion is only one of several methodologies that could be applied. Other methodologies that may be applicable are the following: (1) calculation of long-term sediment budgets for drainage basins on and near Yucca Mountain; (2) topographic reconstruction of the known pre-erosional landscape and determination of the amount of degradation necessary to explain its present form; and (3) simulation of long-term landscape evolution employing quantitative modeling of hillslope and channel processes deduced from first principles. The following material offers brief comments on these alternative approaches. The committee wishes to emphasize that these suggestions are merely illustrative of the kinds of tests needed to reduce uncertainties in the estimates of erosion rates presented in the TBR. Sediment Budgets. Sediment-budget studies focus on the accounting within natural drainage systems for sediment sources, sinks, and transfers through time. This accounting procedure balances rates of erosion against the time scales and volumes of sediment accumulation. Thus, it automatically tests the consistency of inferred erosion rates with the natural process of landscape evolution.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion Topographic Reconstruction. The discussion of drainage evolution (Section 2.5.1) and long-term channel erosion rates (Section 4.4.3.2) in the TBR suggests the potential for reconstructing the topography of ancient landscapes at Yucca Mountain. Well-dated volcanic sequences are known in the region. These include the Tiva Canyon Tuff (12.7 Ma16), the Thirsty Canyon Group (9.4 Ma), and the Buckboard Mesa basalt flows (2.8 Ma). From a digital topographic model (DTM) of the original landforms associated with these dated landscape elements, it should be possible, by subtracting the DTM for the modern landscape, to determine an erosion volume. Variations in the rates of process operation will have to be considered within these long time intervals. Simulation Modeling. Geomorphic process simulation modeling, used in combination with data-intensive studies, may also be helpful in characterizing long-term erosion. As pointed out in the report Rethinking High-Level Radioactive Waste Disposal (National Research Council, 1990, p. 4), such “. . . models are vital for two purposes: (1) to understand the history and present characteristics of the site; and (2) to predict its possible future behavior.” The simulation output will have to be carefully compared to measured erosion rates, sediment budgets, landscape reconstructions, and other relevant data. The simulations might function as “thought experiments” comparing alternative conceptualizations of the physical processes controlling regional landscape evolution and making predictions that can then be tested against observations. 16   Ma = million years before present.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion These alternative approaches will be especially useful for fitting the erosion rate estimates at Yucca Mountain into a regional context. The committee believes that this regional context is essential for demonstrating that Yucca Mountain is indeed an area of exceptional erosional stability. Summary and Conclusions The focus of the TBR is on temporally and spatially averaged erosion rate estimates for comparison against a regulatory standard. The TBR presents its conclusions with little discussion of whether all credible alternative interpretations have been considered and eliminated on the basis of appropriate scientific testing. The TBR relies on specific Quaternary geochronological tools that are not widely used (U-trend dating) or are highly problematic in the light of current scientific understanding (CRD), as noted previously. The TBR devotes too much attention to analysis of a single dating tool, cation ratio dating, and does not give enough consideration to the many alternative approaches possible for Quaternary geochronology. As noted earlier in this chapter, a combination of geochronological tools is essential for the comparisons necessary to evaluate age uncertainties and to establish scientific credibility. Whatever the limits on dating uncertainties, the measurement of geomorphic processes and their effects over a variety of time scales is necessary to distinguish cyclic phenomena, such as those associated with climatic change, from other patterns. The entire range of erosion processes operating on the landscape needs to be characterized in terms of local effectiveness and the spatial variability of that effectiveness. If there is spatial and temporal variability of erosion rates, then analysis must be made of the pat-

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion tern of this variability and how it evolves over the time scales of regulatory concern. By focusing on a single calculated value, presumed to represent some spatially and temporally averaged regional erosion rate, the TBR fails to establish credibility in the scientific basis for numerical characterization of erosion. The uncertainty in that characterization is not solely a matter of measurement error in laboratory elemental abundance determinations (for geochronological quantification of time intervals) or in incision depth measurements (for quantifying process magnitudes). The major scientific uncertainties derive from concerns that portions of the landscape may be eroding much faster than those cited in the TBR; that processes not considered by the TBR may be important; and that the causes, regional patterns, and temporal variability of erosion at Yucca Mountain may not be understood. It is perhaps appropriate to close this chapter with a comment to DOE program managers: The committee is not recommending that the DOE undertake an extensive or expensive program of research on erosion at Yucca Mountain. Indeed, based on the information received during its information-gathering sessions, the committee suspects that much of the needed work is already under way (e.g., cosmogenic isotopic dating of some hillslope deposits), or that the needed data are already at hand or available in the literature (e.g., data for estimating regional erosion rates). Some new data collection may be necessary to estimate erosion rates of younger hillslope or fluvial deposits. The committee believes that some additional effort is essential to make a scientifically credible case for the low erosion rates at Yucca Mountain.