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Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
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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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
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;1Wesling 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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
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:

  1. What is the thickness of alluvium in the major stream valleys at Yucca Mountain?

  2. What is the potential for stream and debris flow erosion at the site?

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

TBR presents no data to support this interpretation. The TBR also does not address the potential for landsliding in the region.

Support for Technical Interpretations

The TBR presents several conclusions concerning alluvial deposits, debris flows, and the potential for erosion: (1) streams are in dynamic equilibrium; (2) the potential for erosion of the alluvial fills is low; (3) rainfall rates and intensities are too low to accomplish significant fluvial erosion under current climatic conditions; and (4) debris flows are minor erosive agents on the landscape. As noted in the previous discussion, however, the TBR does not present data or analyses to support these interpretations.

The assumption of dynamic equilibrium is not well documented in the report and probably should not have been made for the evaluation of fluvial erosion potential. As noted previously, determination of the ages of the alluvial fills is work in progress, and these ages are poorly established at present. The TBR does not define the relationships among rainfall events, runoff production, and channel erosion. Consequently, erosion processes cannot be evaluated based on the data in the TBR for current or reasonable future climatic conditions.

Credible Alternative Interpretations

There are several alternative interpretations or hypotheses that should be addressed in the TBR. These include the following:

  1. Stream channels in Fortymile Wash are not in dynamic equilibrium. The concept of dynamic equilibrium is not well de

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

fined for semiarid channel systems. Research on the alluvial history of stream channels in semiarid regions indicates that they experience multiple episodes of scour and fill that can be related to relatively minor variations in climate or tectonic activity (e.g., Schumm, 1977; Bull, 1991). Even under present climatic conditions, rare and infrequent flood events have the potential to cause significant erosion. It would be useful to examine conditions that are likely to lead to erosive phases, and the probabilities that these events might take place. Such erosive phases could destabilize hillslopes or change local base levels for streams headed on Yucca Mountain, thereby accelerating erosion near or over the proposed repository.

  1. The potential for erosion of the alluvial fills is high. The alluvial fill represents unconsolidated material that could be eroded and transported relatively easily under some hydrological conditions. Because the hydrological conditions required for transport are largely unknown, it would be useful to evaluate the consequences of erosion of the channels to the depth of the alluvial fills. What impact would erosion of this magnitude have on the proposed repository?

  2. Climate change involving higher mean annual precipitation over the period of regulatory concern could lead to higher rates of hillslope and channel erosion at Yucca Mountain. The TBR does not evaluate the potential impacts of climate change on erosion at Yucca Mountain. Yet climate change is possible over the period of regulatory concern. For example, recent research on global climate change suggests that a shift to more intense storm events might be a consequence of global warming (Karl et al., 1995). What impact would climate change have on hillslope and channel stability at Yucca Mountain?

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
  1. Debris flows are significant erosive agents on the landscape. Debris flows are not considered to be major erosive agents under current climatic conditions, yet recent occurrences of debris flows are reported in the TBR. The TBR does not document the potential frequency and magnitude of these events, however, or report the ages of debris flow deposits. Debris flows can be an effective mechanism for hillslope erosion since such flows can leave depressions on hillslopes that can develop into stream channels. How effective are debris flows as erosive agents on the hillslopes at Yucca Mountain under current climate conditions? What effects would possible climate changes over the period of regulatory concern have on the frequency and magnitude of debris flows?

  2. The potential for erosion of the alluvial fills on the west side of Yucca Mountain is high. As noted previously, the TBR does not address the potential for erosion of the alluvial fills on the west side of Yucca Mountain (Figure 1.1). What impact, if any, would erosion of the alluvium in the Amargosa River Basin have on the proposed repository under present or reasonable future scenarios for climate change?

Testing to Discriminate Among Alternative Interpretations

It is probably not possible to demonstrate that the stream channels are in dynamic equilibrium and that alluvial valleys could not be eroded at some future time. Therefore, a bounding calculation6 of erosion of the alluvial fills in the valleys to the depth of

6  

In conventional scientific practice, bounding calculations are used to estimate likely upper or lower values of processes or phenomena (e.g., erosion rates) when observational data or theories are inadequate. Such calculations are usually made by using relevant information from other sources and professional judgment to define plausible upper and lower limits to the possible values of the process that is uncertain.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

the bedrock should be made. The effect of such fluvial erosion on hillslope stability should also be evaluated. Removal of the valley alluvium could significantly increase gradients at the bases of hillslopes, which could lead to accelerated erosion of gulleys headed on Yucca Mountain or to increased debris flow frequency and magnitude. Such tests would significantly reduce uncertainties in potential for erosion at the site, particularly under scenarios for future climate change.

The identification of debris flow deposits in cores and valley sediments could be used to address their frequency of occurrence, volume of material transported, and spatial distribution. Surficial mapping, subsurface sampling, and dating studies could address the effectiveness of debris flows as agents of erosion at Yucca Mountain.

Summary and Conclusions

A significant deficiency in the TBR is a lack of discussion of the relationships among surficial deposits and mapped surfaces, the processes that formed these surfaces, and the choice of surfaces for age dating. A conceptual model of the possible interactions between hillslope erosion and fluvial erosion should be presented. Could fluvial erosion destabilize hillslopes in the Yucca Mountain area? What is the potential for debris flows or landsliding at the site as evaluated from the spatial and temporal distributions of these deposits?

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

The TBR appears to evaluate fluvial erosion in terms of minimum estimates. It assumes that stream channels are in dynamic equilibrium and, thus, unlikely to erode through the valley fills. The potential for fluvial erosion at the site should be evaluated in terms of bounding scenarios. For example, what would happen if the alluvial valley fills were eroded to bedrock?

DOE has not conducted a general program of research on erosion processes and rates. Instead, efforts to evaluate erosion have focused on two topics: (1) an evaluation of the alluvial history and the potential for channel incision in Fortymile Wash and its tributaries, and (2) an evaluation of the ages of hillslope deposits on Yucca Mountain and Skull Mountain (discussed later in this chapter). The TBR offers no justification for the decision to evaluate the erosion potential of Fortymile Wash and its tributaries and to ignore the drainages on the western side of Yucca Mountain. Similarly, the rationale for examining the ages of heavily varnished hillslope deposits rather than a systematic study of the major hillslope units, including debris flow deposits, is not presented.

REVIEW OF QUATERNARY GEOCHRONOLOGY

Section 4.3 of the TBR (see Table 1.1) discusses Quaternary geology. The focus of this discussion is on the determination of ages of colluvial boulder deposits on the flanks of Yucca Mountain and Skull Mountain. The hypothesis presented in the TBR is that these boulder deposits have been relatively stable and that “channels” incised into the hillslopes adjacent to them (see Table 4.4.2-1 and Figure 4.3.2.1-2 in the TBR) can be used to approximate long-term erosion rates by means of the following formula:

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

Erosion Rate = (Depth of Hillslope Channel)/(Age of Colluvial Boulder Deposit) (2.1)

The TBR relies almost exclusively on cation ratio dating (CRD) to date the exposure ages of colluvial boulder deposits. This method involves measurement of the ratio

(Ca + K)/(Ti ± Ba) (2.2)

in the manganese- and iron-rich rock varnish formed on boulder surfaces, which has been shown to vary inversely as a function of age (Dorn, 1983; Harrington and Whitney, 1987).

Section 4.3 of the TBR is almost identical to a section from the DOE topical report on extreme erosion (DOE, 1993). The Nuclear Regulatory Commission reviewed this topical report (USNRC, 1994). Many of the issues discussed in the following sections are also addressed in the USNRC review.

Adequacy of Data Collection and Analysis

The committee's review of the adequacy of data collection and analysis focuses on the following three aspects of the discussion in the TBR: (1) criteria used to select CRD methods for surface exposure analysis; (2) the calibration, accuracy, and precision of the method; and (3) the types of deposits selected for CRD and erosion estimates. The following sections address these issues.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Criteria for Selecting CRD

The rationale for selecting CRD for geochronology (see p. 4-4 to 4-6 of the TBR) was that it could be applied to boulder deposits having a range of Quaternary ages. The committee understands that the decision to select the CRD method was made in the mid-1980s. At that time, a number of other techniques for surface exposure dating were being developed (notably, 36Cl, 10Be, 26Al, and 3He) but were not yet readily available for use. These methods have come into general use only in the past few years.

A number of statements made in the TBR concerning the selection criteria for CRD are not well justified. These include the following:

  1. Alternate methods to date soils (i.e., U-series dating of carbonates) were not utilized because they provide “a minimum limiting age for the hillslope surface” (TBR p. 4-5). However, U-series dating has been used extensively in surface mapping efforts (Paces et al., 1994; Lundstrom et al., 1995) and could be useful in providing estimates of erosion rates. Analysis of soil ages would have provided complementary erosion rate data that are not dependent on assumptions concerning the origin of the boulder deposits.

  2. The use of cosmogenic nuclides was excluded because they were not considered applicable to the relevant lithologies (quartz-rich tuffs and basalts; TBR point 3, p. 4-5). Both preliminary results presented to the committee on the field excursion and published information (e.g., Cerling, 1990; Poths and Crowe, 1992; Zreda et al., 1993; Crowe et al., 1994; Wells et al., 1995) suggest that cosmogenic nuclides are applicable in surface dating

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

studies at Yucca Mountain. Indeed, surface exposure dating is now applied widely in the study of Quaternary processes (Beck, 1994).

  1. CRD was selected to avoid the problems of “inheritance” from exposure prior to deposition (TBR point 6, p. 4-6), which would produce unrealistically old ages for cosmogenic nuclide methods. In the case of cosmogenic techniques, the importance of prior exposure can frequently be evaluated by sampling at depth in the deposit or by measuring several nuclides with different half-lives (see Lal, 1991, for a discussion of cosmogenic isotopes and inheritance). Inheritance problems are also possible with varnish formation. In fact, inheritance is a general problem with all surface dating techniques.

The TBR implies that CRD is the best technique for estimating surface ages in the vicinity of Yucca Mountain. The committee disagrees with the notion that any one technique could be chosen as the “best” in this area and believes that several different techniques should be applied.

Calibration, Accuracy, and Precision of CRD

Because the processes of varnish formation are complex, application of CRD requires the development of an empirical calibration curve. 7 The calibration curve used to calculate ages in the

7  

A calibration curve is obtained by plotting the cation ratios (Equation 2.2) from several surface deposits against the ages of those surfaces. The ages are determined by independent geochronologic methods. After this curve has been established, an unknown sample can be dated by measuring its cation ratio and plotting it on the calibration curve to obtain its age.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

TBR is based on U-trend8 and potassium-argon (K-Ar) dating of lava flows at Crater Flat and clasts of welded tuff on an alluvial surface at Fortymile Wash ( Harrington and Whitney, 1987).

The calibration curve can introduce uncertainties and systematic errors in the determination of ages of colluvial boulder deposits. The committee could not evaluate these uncertainties and errors because neither the TBR nor the published literature present a detailed discussion of the calibration data. Varnish begins to form on surfaces some time after eruption of lava flows or deposition of alluvium. The interval of time between eruption or deposition and varnish formation is unknown. The age data for the calibration curve are also difficult to evaluate because the youngest ages were obtained by U-trend dating, which is not well understood, as noted in footnote 8.

Peterson et al. (1995) recently published a new CRD calibration curve for the Yucca Mountain region.9 The calibration ages on this curve are significantly different from the ages for the curve used in the TBR (see Table 4.3.2.3-1 of the TBR). These calibration ages are given in Table 2.1 and are plotted in Figure 2.2. As shown in Figure 2.2, most of the calibration ages from Peterson et al. (1995) fall outside the range of uncertainties for the

8  

U-trend dating is an empirical method that relies on the open-system behavior of the daughter isotopes of uranium within soils. Because the mechanism of open-system behavior is unknown, this technique is not widely used. This method is distinct from U-series disequilibrium dating, which relies on the closed-system behavior of thorium and uranium and their radioactive daughters.

9  

The primary differences between the calibration curve of Peterson et al. (1995) and that used by Harrington and Whitney (1987) are the number of points and the methods used to determine the calibration ages. Peterson et al. (1995) used 14C methods to obtain calibration ages, whereas Harrington and Whitney (1987) used U-trend dating. Both sets of ages are from samples collected near Crater Flat.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

calibration ages used in the TBR. This suggests that the uncertainties presented in the TBR are underestimates.10

The TBR also does not distinguish between the precision and the accuracy of CRD data.11 The uncertainties in the age ranges presented in the TBR are based on the standard deviation of the measured cation ratios in varnish samples and reflect the analytical precision (i.e., reproducibility of measured cation ratios on duplicate samples from the same site) rather than the accuracy of the mean CRD (i.e., how close the measured age is to the actual age). There are many ways to obtain precise, but inaccurate, CRDs. Accuracy can be assessed by using multiple geochronologic techniques and bracketing CRD ages by using dates on related deposits.

Support for Technical Interpretations

The following discussion focuses primarily on the following two issues: (1) the cation ratio technique itself and (2) the selection of sites for dating.

10  

The error estimates given in the TBR do not account for uncertainties in the calibration ages used to construct the calibration curve. Had these been included, the uncertainties in the cation ratio ages would have been significantly greater. The committee understands that efforts are now under way to recalculate the uncertainties for the data published in Harrington and Whitney (1987) to account for this additional error.

11  

The term precision refers to the degree of agreement among repeated measurements of a quantity such as the cation ratio. The term accuracy refers to the degree of agreement of a measurement of a quantity with its actual or real value.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

TABLE 2.1 Comparison of Calibration Ages and Uncertainties from the TBR with Calibration Ages from Peterson et al. (1995)

Sample Number

Cation Ratio (Equation 2.2)

Age in TBR (ka)

Age Range in TBR (ka)

Recalculated Age (ka)a

YME-1

2.99

640

610-670b

568

YME-2

4.52

170

40-180

83

YMW-1

3.34

465

400-515

365

YMW-2

2.97

645

630-660

582

YMW-3

2.88

710

680-740

652

YMN-1

2.79

760

710-820

730

LSM-1

2.52

960

930-990

1,025

SKM-1

2.74

800

760-830

778

SKM-2

2.68

830

800-880

838

SKM-3

2.28

1,180

1,110-1,270

1,387

SKM-3A

2.49

990

960-1,030

1,065

BM-1

2.09

1,380

1,260-1,510

1,762

Note: ka = thousand years before the present.

a Based on the calibration curve of Peterson et al. (1995).

b This range is reported as 610-970 ka in the TBR, but as 610-670 ka in Whitney and Harrington (1993).

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

FIGURE 2.2 Graph comparing calibration ages and uncertainties from the TBR with the calibration ages of Peterson et al. (1995). The horizontal error bars indicate age uncertainties.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Cation Ratio Dating

There is considerable scientific disagreement with regard to all aspects of the CRD method—mechanisms of varnish formation, sample selection criteria, and analytical techniques for measuring cation ratios (e.g., Dorn and Krinsley, 1991; Reneau and Raymond, 1991; Bierman and Gillespie, 1994; Bierman and Gillespie, 1995; Dorn, 1995; Harrington and Whitney, 1995). A comprehensive discussion of CRD is beyond the scope of this review, but several issues merit comment.

The procedures used to obtain cation ratio dates introduce a systematic bias toward older ages, and the tabulated ages in the TBR probably represent maximum ages for the reasons noted below. The following procedures are of particular interest in this regard:

  1. Colluvial boulder deposits were selected for geochronologic analysis because they were considered the most stable and thus the oldest surfaces at Yucca Mountain.

  2. Field selection of samples for CRD analysis was based on the appearance of the varnish (i.e., color, thickness, and reflectivity; see Harrington and Whitney, 1987), which may preferentially select for the older clasts in a mixed population (Dorn, 1994).

  3. Cation ratios are measured on six spots from each sample by using a scanning electron microscope with an energy dispersive x-ray attachment. The highest cation ratio of the six is discarded on the assumption that many processes can contribute to the development of high cation ratios (i.e., younger ages),

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

whereas artifacts contributing to low cation ratios (i.e., older ages) are rare (Harrington and Whitney, 1987).

The TBR argues that the cation ratio dates represent minimum ages, which may not be correct for the reasons noted above.

There are other controls on varnish chemistry that can introduce both random and systematic errors in age determinations. The iron-manganese crusts on the boulders can incorporate air-borne dust, resulting in anomalous cation ratios. Substrate chemistry, microbial activity, vegetation, and local climatic conditions may also affect varnish chemistry (Krinsley et al., 1990; Reneau et al., 1991; Reneau et al., 1992; Krinsley et al., 1995). Indeed, the calibration curves for Yucca Mountain are distinct from those at Espanola Basin, possibly due to climatic differences (Harrington and Whitney, 1987). This suggests that a calibration curve must be generated for each locality. There is also evidence that the varnish layers are heterogeneous, perhaps as a result of past climatic changes during exposure (Krinsley et al., 1995). The TBR does not discuss or evaluate any of these issues. Considering the large number of published papers evaluating the strengths and weaknesses of CRD, the committee believes that these issues should receive more comprehensive discussion.

Choice of Sites for Cation Ratio Dating

According to the TBR, dating of heavily varnished colluvial boulder deposits on the hillslopes of Yucca Mountain and Skull Mountain is the most reliable means for estimating long-term erosion rates (see also Whitney and Harrington, 1993). However, some investigators have suggested that these boulder de-

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

posits were formed by deflation of the underlying surface over time (Dorn and Krinsley, 1994; Wells et al., 1995). If this interpretation is correct, the CRD data presented in the TBR represent maximum ages and minimum erosion rates. As noted earlier, sample selection and analysis protocols may also select for maximum ages.

The method for calculating erosion rates assumes that the channels adjacent to the colluvial boulder deposits were produced by hillslope erosion (see Figure 4.3.2.1-2 of the TBR). It also assumes that no erosion occurred on the boulder deposits. Post-depositional modification of the hillslope could minimize the relief adjacent to the colluvial deposits (e.g., by mass wasting), which would make the calculated erosion rates less than maximum values.

The greatest uncertainty associated with the erosion rate calculation in the TBR is the assumption that a single type of deposit can yield representative estimates of long-term erosion rates at Yucca Mountain. Erosion may occur over different time and spatial scales, and proper characterization of past erosion requires consideration of a variety of possible mechanisms, as discussed in the next section.

Credible Alternative Interpretations

The TBR should address several important alternative hypotheses that emerge from the foregoing discussion:

  1. The ages determined for the heavily varnished boulder deposits do not reflect the duration of stability of the hillslopes. It is possible that the colluvial boulder deposits are relatively recent

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

features and have relatively old CRD ages because the varnish formed, or began forming, prior to their “deposition” in their present positions on the hillslope (e.g., Dorn and Krinsley, 1994; Wells et al., 1995).

  1. The erosion rates calculated from the hillslope deposits (if one assumes, for the moment, that the CRD calibration curve is correct and no sampling bias has been introduced) are not representative of hillslope erosion rates at Yucca Mountain. The calculation of erosion rates assumes that the small amount of relief adjacent to the colluvial boulder deposits represents the total erosion on the hillslope (e.g., Figure 4.3.2.1-2 in the TBR). However, the original configuration of the hillslopes is unknown. The boulder deposits could have been deposited in small channels, and subsequent erosion could have inverted hillslope topography.

  2. Erosion rates at Yucca Mountain are spatially variable. The approach used in the TBR assumes that erosion rates estimated from the colluvial boulder deposits can be used to characterize an “average” erosion rate for the entire region. However, it is well known from studies of other regions that erosion occurs at different rates in different areas (e.g., stable hillslopes versus faulted hillslopes versus stream drainages) and that erosion may be episodic rather than constant through time.

Testing to Discriminate Among Alternative Interpretations

A number of important tests could be performed to evaluate the alternative hypotheses:

  1. Given the importance of CRD to the TBR and the uncertainties involved in the technique, additional geochronologi-

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

cal techniques should be applied to validate the CRD method and obtain independent age estimates. A number of independent dating methods are applicable to the boulder deposits, including 10Be and 3He methods. Both methods should apply over the age range in question. The shorter-lived cosmogenic nuclides 14C, 26Al, and 36Cl may not be suitable, but could be used if the 10Be and 3He ages are significantly younger than indicated by the CRD method. The committee notes that 3He and 36Cl methods have been used to estimate exposure ages of lava flows at Lathrop Wells Volcanic Center (Crowe et al., 1992; Poths and Crowe, 1992; Zreda et al., 1993; Crowe et al., 1994) and appear to provide consistent results.12 On the other hand, comparisons of CRD on Buckboard Mesa with 36Cl dates (TBR section 4.3.3) differ by a factor of approximately two to four.13 Clearly, more comparative work is needed to check the CRD results.

  1. It should be possible to determine the spatial variability of erosion rates by applying other geochronological methods to a variety of hillslope and alluvial surfaces. For example, cosmogenic nuclide measurements from the top of Yucca Mountain could be used to calculate local erosion rates, independent of geomorphological assumptions.

  2. The hypothesis that an erosion rate calculation obtained from a single type of deposit (colluvial boulders) is sufficient to characterize the regional erosion rate could be tested by applying

12  

Exposure ages of the lava flows at Lathrop Wells are estimated to be 48-73 ka for 3He dating with an absolute uncertainty of about 30% (Poths and Crowe, 1992) and 75-85 ka for 36Cl dating (Zreda et al, 1993).

13  

The estimated mean CRD age of Buckboard Mesa is given as 1,380 ka in Table 4.3.2.3-1 of the TBR, versus 310-600 ka for the 36Cl ages given in Section 4.3.3. However, the TBR notes that the latter ages are somewhat problematical owing to near-saturation of 36Cl in some of the rocks.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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,

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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).

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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).

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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.

  1. 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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
  1. 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.

  2. 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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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-

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×

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.

Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
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Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 28
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 29
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 30
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 31
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 32
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 33
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 34
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 35
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 36
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 37
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 38
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 39
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 40
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 41
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 42
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 43
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 44
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 45
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 46
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 47
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 48
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 49
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 50
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 51
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 52
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 53
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 54
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 55
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 56
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 57
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 58
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 59
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 60
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 61
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 62
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 63
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 64
Suggested Citation:"2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL." National Research Council. 1995. Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion. Washington, DC: The National Academies Press. doi: 10.17226/9233.
×
Page 65
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