Research ArticleGEOLOGY

Anthropocene rockfalls travel farther than prehistoric predecessors

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Science Advances  16 Sep 2016:
Vol. 2, no. 9, e1600969
DOI: 10.1126/sciadv.1600969
  • Fig. 1 Anthropogenic deforestation of Banks Peninsula.

    Removal of native forest (yellow) occurred rapidly in Banks Peninsula with the arrival of Polynesians (ca. 1280 CE) then Europeans (ca. 1830 CE). (A) Before arrival of the Polynesians (Māori), extensive native forest was present throughout Banks Peninsula. (B) Before European settlement, minor to moderate removal of indigenous forest by Māori had occurred in Banks Peninsula. Burning was the primary tool for clearance. (C) By 1920, Europeans had removed >98% of the native forest in Banks Peninsula, leaving slopes barren and low-lying areas vulnerable to slope hazards. (D) Minor reestablishment of old-growth native forest has occurred, but slopes in Banks Peninsula and the Port Hills (including Rapaki) remain largely unvegetated [data from previous studies (1416)].

  • Fig. 2 Spatial distribution and size comparison of modern and prehistoric rockfall at Rapaki.

    (A) Spatial distribution for mapped modern rockfall (n = 285) generated during the 2011 Christchurch earthquakes (22 February and 13 June events). Twenty-six modern boulders affected the Rapaki village and caused severe damage to residential properties. Maximum runout distance (map length) for modern boulders is ~770 m. (B) Spatial distribution for mapped paleoboulders (n = 1049). No evidence for prehistoric boulders in area now occupied by the Rapaki village. Maximum travel distance for prehistoric boulders is ~560 m. (C) Modern and prehistoric rockfall runout distance plotted as function of elevation. Travel distance for modern rockfalls exceeds limit of prehistoric predecessors. (D) Comparison of boulder size distribution for modern and prehistoric boulders indicates strong similarity. (E) The frequency-volume distributions for prehistoric and modern rockfall at Rapaki show a similar power-law trend.

  • Fig. 3 Modern boulder at Rapaki study site.

    Photo of large modern boulder (~28 m3) detached from Mount Rapaki and emplaced in the Rapaki village during the 22 February 2011 earthquake (photo courtesy of D. J. A. Barrell, GNS Science). The boulder traveled through the center of the residential home located in background (center). Boulder runout distance from source was ~700 m. Runout distance for furthest traveled modern boulders is significantly greater (~150 to 175 m) than travel distance for prehistoric boulders.

  • Fig. 4 Prehistoric boulder at Rapaki study site.

    Photo of exploratory trench excavated adjacent to Paleo-Boulder 3 (PB3), exposing hillslope sediments deposited before and after boulder emplacement. Locations for charcoal samples (yellow) Rap-CH01, Rap-CH03, and Rap-CH05 are shown. All samples were collected near the base of the most recent loess colluvial wedge sediments and yield similar conventional radiocarbon ages (203 ± 18, 197 ± 17, 222 ± 17 yr B.P., respectively). 2σ-Calibrated ages suggest a probable burning event occurred at Rapaki sometime between 1661 CE and 1950 CE, with highest 2σ subinterval probability between 1722 CE and 1810 CE. PB3 is deposited in footslope position, and volume is ~14.5 m3. Total travel distance for PB3 is ~560 m.

  • Fig. 5 RAMMS rockfall modeling.

    (A) Simulated rockfall assuming no hillslope vegetation (RAMMS 1). Source areas, rockfall trajectories, boulder velocities, and final resting positions are shown. (B) RAMMS 1 successfully predicts modern boulder distributions, highlighting effectiveness of RAMMS in replicating modern rockfall distribution. (C) Simulated rockfall assuming moderate to dense vegetation on the hillslope (RAMMS 2). Vegetation is modeled in RAMMS as forest drag, a resisting force that acts on the rock’s center of mass when located below the drag layer height. The forest is parameterized by the effective height of the vegetation layer (10 m) and a drag coefficient (moderate, 3000 kg/s; dense, 6000 kg/s). (D) Prehistoric and RAMMS 2 boulder distributions display strong correlation, suggesting that a moderate to dense forest likely existed on the Rapaki hillslope during prehistoric boulder deposition.

  • Fig. 6 Comparison of empirical and modeled rockfall spatial distributions.

    Empirical prehistoric and RAMMS 2 boulders (vegetation) display a strong similarity (P = 0.736) with the highest frequency (~62 to 64%) of boulders near the source rock (shadow angle 33°). Empirical modern and RAMMS 1 boulders (no vegetation) show an equivalently high correlation (P = 0.736). In contrast, mapped prehistoric and modern boulders display a poor fit (P = 0.012). ArcGIS has been used to determine percentage of boulders within each shadow angle.

  • Fig. 7 Rockfall and Anthropocene chronology for Rapaki study site.

    Native forest likely persisted during the Last Glacial Maximum and thrived during the Holocene in Banks Peninsula and at Rapaki. The penultimate paleorockfall event at Rapaki occurred approximately 8 to 6 ka [taken from the study of Mackey and Quigley (23)] when a dense variable forest cover would have existed on the hillslope. Radiocarbon dating of charcoal at Rapaki suggests a probable burning event occurred sometime between 1661 CE and 1950 CE, with highest subinterval probability between 1722 CE and 1810 CE. Slope deforestation by Māori and later Europeans allowed modern rockfall generated during the 2011 Christchurch earthquakes to travel further than their prehistoric predecessors and affected the Rapaki village.

  • Fig. 8 Comparison of spatial distribution for modern (CES) and prehistoric rockfall in Port Hills of southern Christchurch.

    (A) Mapped modern (red) and prehistoric (blue) “in situ” rockfall. Rockfall data shown have been provided by the Christchurch City Council and were mapped in the field by GNS Science. [Prehistoric rockfall was only partially mapped in certain areas, such as upslope of the planted treeline shown in (B). However, for areas adjacent to high-density residential development, both modern and prehistoric rockfalls appear well mapped, and comparison of boulder runout distances clearly highlights the increased travel distance for CES-generated boulders. Burial of preexisting boulders by colluvial and alluvial sediments is possible, particularly in the footslope position, and would limit the number of observable prehistoric rockfalls.] Slopes in the Port Hills have been stripped of native vegetation. (B) Comparison of CES-generated and prehistoric in situ rockfall in Heathcote Valley reveals longer runout distances (~150 to 250 m) for CES rockfall boulders. Modern rockfall affected numerous residential dwellings. The southern planted treeline was effective in capturing modern rockfall, highlighting the importance of slope vegetation in mitigating rockfall hazard. (C) Modern rockfall boulders on the northwestern and southeastern sides of Sumner Valley display further runout distances (~50 to 100 m) than in situ [we recognize the possibility that, in rare cases, isolated exposures of intact volcanic bedrock may be mapped as in situ (prehistoric) boulders] or preexisting rockfall.

  • Table 1 Results from radiocarbon dating of charcoal within loess colluvium sediments at Rapaki, New Zealand.

    NZA, Rafter Radiocarbon Laboratory; 14C yr B.P., radiocarbon years before the present.

    Sample IDExposure unitNZA laboratory
    number
    δ13CRadiocarbon
    age
    Calibrated age 2σProbability for
    each 2σ range
    Materials and significance
    (14C yr B.P.)(Calendar year CE)(%)
    Rap-CH01Loess
    colluvium
    56801−28.6 ± 0.2203 ± 181664–1698,
    1724–1809,
    1870–1876
    22.8, 70.4, 1.0Charcoal in colluvial wedge
    sediment. Dates probable
    burning event at Rapaki.
    Rap-CH03Loess
    colluvium
    56802−29.1 ± 0.2197 ± 171666–1700,
    1722–1810,
    1838–1845,
    1867–1878,
    1933–1938,
    1946–1950
    25.8, 63.8, 1.3, 2.4,
    0.6, 1.1
    Charcoal in colluvial wedge
    sediment. Dates probable
    burning event at Rapaki.
    Rap-CH05Loess
    colluvium
    56803−27.9 ± 0.2222 ± 171661–1680,
    1732–1802
    15.8, 79.0Charcoal in colluvial wedge
    sediment. Dates probable
    burning event at Rapaki.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/9/e1600969/DC1

    fig. S1. Radiocarbon calibration report for charcoal sample Rap-CH01.

    fig. S2. Radiocarbon calibration report for charcoal sample Rap-CH03.

    fig. S3. Radiocarbon calibration report for charcoal sample Rap-CH05.

    data S1. KS comparison test of empirical prehistoric and modern boulders.

    data S2. KS comparison test of frequency-volume distributions for modern and prehistoric boulders.

    data S3. KS comparison test of empirical modern and RAMMS 1 boulders.

    data S4. KS comparison test of empirical prehistoric and RAMMS 1 boulders.

    data S5. KS comparison test of empirical prehistoric and RAMMS 2 boulders.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Radiocarbon calibration report for charcoal sample Rap-CH01.
    • fig. S2. Radiocarbon calibration report for charcoal sample Rap-CH03.
    • fig. S3. Radiocarbon calibration report for charcoal sample Rap-CH05.
    • data S1. KS comparison test of empirical prehistoric and modern boulders.
    • data S2. KS comparison test of frequency-volume distributions for modern and prehistoric boulders.
    • data S3. KS comparison test of empirical modern and RAMMS 1 boulders.
    • data S4. KS comparison test of empirical prehistoric and RAMMS 1 boulders.
    • data S5. KS comparison test of empirical prehistoric and RAMMS 2 boulders.

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