Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion (1995) / Chapter Skim
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2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL
2 SURFACE CHARACTERISTICS AND EROSION POTENTIAL
Pages 27-65
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From page 27... ...
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.
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From page 28... ...
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 (Welling et al., 1992; Faulds et al., 1994; Lundstrom et al., 19951.
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From page 29... ...
These recent determinations provide age estimates of 40-100 ka3 for the youngest terrace deposit adjacent to Fortymile Wash (S.C. Lundstrom, U.S.
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From page 30... ...
an identification of the major geomorphic processes that are responsible for these mapped geomorphic surfaces and deposits, including possible interactions between hilisiope and channel erosion processes, and (2) the ages of these surficial deposits.
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From page 31... ...
Better age determinations are needed on the mapped units on both sides of Yucca Mountain. Such ages would significantly reduce scientific uncertainties in hilisiope and fluvial erosion histories.
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From page 32... ...
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 hillsiope and stream erosion processes.
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From page 33... ...
surface drainage, fluvial erosion, and Fortymile Wash evolution; and (3) debris flow deposits and debris flow potential.
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From page 34... ...
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.~. The TBR should have included such profiles for all significant drainages.
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From page 35... ...
35 (laAal leas ueaw aAoq~ s~a~aLu)
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From page 36... ...
This apparent assumption ignores temporal patterns of erosion and sedimentation common to semiarid and arid regions (e.g., Schumm, 1977; Wolman and Gerson, 19781. Erosion rates may decrease after channels are eroded to competent bedrock.
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From page 37... ...
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~.
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From page 38... ...
Support for Technical Interpretations The TBR presents several conclusions concerning alluvial deposits, debris flows, and the potential for erosion: (~) streams are in dynamic equilibrium; (2)
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From page 39... ...
Such erosive phases could destabilize hilIsIopes or change local base levels for streams headed on Yucca Mountain, thereby accelerating erosion near or over the proposed repository.
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From page 40... ...
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 calculat~on6 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)
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From page 41... ...
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.
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From page 42... ...
I-2 in the TBR) can be used to approximate long-term erosion rates by means of the following formula:
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From page 43... ...
to date the exposure ages of colluvial boulder deposits. This method involves measurement of the ratio (Ca + K)
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From page 44... ...
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.
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From page 45... ...
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 curved The calibration curve used to calculate ages in the 7 A calibration curve is obtained by plotting the cation ratios (Equation 2.2)
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From page 46... ...
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.
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From page 47... ...
I 47 The TBR also does not distinguish between the precision and the accuracy of CRD data. 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)
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From page 48... ...
(1995) Sample Cation Ratio Age in TBR Age Range Number (Equation 2.2)
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From page 50... ...
Colluvial boulder deposits were selected for geochronologic analysis because they were considered the most stable and thus the oldest surfaces at Yucca Mountain.
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From page 51... ...
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 longtenn erosion rates (see also Whitney and Harrington, 1993~.
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From page 52... ...
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 hilIsIope erosion (see Figure 4.3.2.~-2 of the TBR)
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From page 53... ...
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 hilisIopes versus faulted hilisiopes versus stream drainages)
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From page 54... ...
For example, cosmogonic nuclide measurements from the top of Yucca Mountain could be used to calculate local erosion rates, independent of geomorphological assumptions.
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From page 55... ...
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 hilisiope deposit and a single method for age determination.
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From page 56... ...
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.
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From page 57... ...
The TBR's analysis of longer-term erosion rates (R) via Equation 2.3 focuses on two specific processes: (~)
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From page 58... ...
Figure I.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 4.4.2I)
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From page 59... ...
The hypothesis of exceedingly low long-tenn erosion rates at Yucca Mountain (approximately 2 mm/ky or 2 m/My; TBR, Table 4.4.2-~4 is comparable to the extremely low erosion rates known from ancient craton~c landscapes of the Gondwanaland continents, for example, Australia and southern Africa (Young, 1983; Bishop, 19851. 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.
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From page 60... ...
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.
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From page 61... ...
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 hilIsIope sites in the Yucca Mountain region that are not mantled with heavily varnished colluvial boulder deposits.
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From page 62... ...
Thus, it automatically tests the consistency of inferred erosion rates with the natural process of landscape evolution.
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From page 63... ...
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.
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From page 64... ...
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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From page 65... ...
The committee believes that some additional effort is essential to make a scientifically credible case for the low erosion rates at Yucca Mountain.
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