Surface erosion assessment in the South-Canterbury downlands, New Zealand using 137Cs distribution

Soil Research ◽  
1995 ◽  
Vol 33 (5) ◽  
pp. 787 ◽  
Author(s):  
LR Basher ◽  
KM Matthews ◽  
L Zhi
Keyword(s):  
New Zealand ◽  
Mean Value ◽  
Slope Position ◽  
Surface Erosion ◽  
The South ◽  
Erosion And Deposition ◽  
Erosion Rates ◽  
Land Use Types ◽  
Radionuclide Tracer

Redistribution of the radionuclide tracer 137Cs was used to examine the pattern of erosion and deposition at two sites with contrasting long-term land uses (pasture and cropping) in the South Canterbury downlands, New Zealand. There were clear differences between the two land use types in variation in 137Cs concentrations and areal activity, erosion rates and topsoil depth variability. Erosion and deposition have resulted in greater variability and lower mean levels of 137Cs areal activity under cropping (46.3 mBq cm-2) than pasture (55.0 mBq cm-2). At the cropping site, erosion and deposition roughly balanced with the mean value over all sampling sites, suggesting no net soil loss, but considerable redistribution of soil within paddocks. At the pasture site results suggested slight net deposition. There was evidence for both sheet/rill and wind erosion being important in soil redistribution. While there was no difference in mean topsoil depth between pasture and cropping, there were significant differences with slope position. At the pasture site, there was little variation of topsoil depth with slope position, except for swales which tended to be deeper, whereas at the cropping site there was considerable variation in topsoil depth with slope position. Topsoil depth was a poor indicator of erosion status.

Effects of carrot growing on volcanic ash soils in the Ohakune area, New Zealand

Soil Research ◽  
2004 ◽  
Vol 42 (3) ◽  
pp. 259
Author(s):  
L. R. Basher ◽  
C. W. Ross ◽  
J. Dando
Keyword(s):  
New Zealand ◽  
Volcanic Ash ◽  
Water Movement ◽  
Reference Value ◽  
Primary Control ◽  
Erosion And Deposition ◽  
Deposition Rates ◽  
Erosion Rates ◽  
Wide Range

Effects of intensive carrot production on soils derived from young volcanic ash were determined at Ohakune, New Zealand. Erosion rates (derived from caesium-137) and key soil physical and chemical properties were determined in 3 fields with differing management history (6 and 16 years cropping) or topography (sloping and flat).Caesium-137 areal activity in cropped fields ranged from 90 to 2034 Bq/m2, compared with a reference value under long-term pasture of 602 Bq/m2. Mean areal activity was lower than the reference value in 2 sloping fields, but not in a flat field. Net erosion rates were low in both sloping fields (–16 and –5 t/ha.year), but within each field there was a wide range of erosion and deposition rates (–109 to +293 t/ha.year in Field 1 and –145 to +514 t/ha.year in Field 2). These very high rates imply total soil losses up to 238 mm and deposition up to 670 mm, consistent with observed elevation differences between the cropped fields and adjacent fields in long-term pasture and with topsoil depth variation from 145 to 1165 mm. Tillage erosion and deposition rates are high (up to c. 40 t/ha.year) but water erosion is the dominant mechanism of soil redistribution.Cropping has reduced organic matter and aggregate stability, increased bulk density in the lower part of the topsoil and subsoil, and increased aggregate size. Hydraulic conductivity was higher in the topsoil and lower in the subsoil under cropping than it was under pasture. However, it would not limit soil water movement as it was higher than typical rainfall intensities. Compacted wheel tracks were the primary control on runoff and erosion as they have low infiltration rates (4 mm/h) compared with carrot beds (853 mm/h).

Erosion rates of the New Zealand Southern Alps reflect long-term tectonics and transient climate

2021 ◽  
Author(s):  
Duna Roda-Boluda ◽  
Taylor Schildgen ◽  
Hella Wittmann-Oelze ◽  
Stefanie Tofelde ◽  
Aaron Bufe ◽  
...  
Keyword(s):  
New Zealand ◽  
Strike Slip ◽  
Southern Alps ◽  
Fault System ◽  
Tectonic Deformation ◽  
Seismic Cycle ◽  
Erosion Rates ◽  
Alpine Fault ◽  
Oblique Convergence

<p>The Southern Alps of New Zealand are the expression of the oblique convergence between the Pacific and Australian plates, which move at a relative velocity of nearly 40 mm/yr. This convergence is accommodated by the range-bounding Alpine Fault, with a strike-slip component of ~30-40 mm/yr, and a shortening component normal to the fault of ~8-10 mm/yr. While strike-slip rates seem to be fairly constant along the Alpine Fault, throw rates appear to vary considerably, and whether the locus of maximum exhumation is located near the fault, at the main drainage divide, or part-way between, is still debated. These uncertainties stem from very limited data characterizing vertical deformation rates along and across the Southern Alps. Thermochronology has constrained the Southern Alps exhumation history since the Miocene, but Quaternary exhumation is hard to resolve precisely due to the very high exhumation rates. Likewise, GPS surveys estimate a vertical uplift of ~5 mm/yr, but integrate only over ~10 yr timescales and are restricted to one transect across the range.</p><p>To obtain insights into the Quaternary distribution and rates of exhumation of the western Southern Alps, we use new <sup>10</sup>Be catchment-averaged erosion rates from 20 catchments along the western side of the range. Catchment-averaged erosion rates span an order of magnitude, between ~0.8 and >10 mm/yr, but we find that erosion rates of >10 mm/yr, a value often quoted in the literature as representative for the entire range, are very localized. Moreover, erosion rates decrease sharply north of the intersection with the Marlborough Fault System, suggesting substantial slip partitioning. These <sup>10</sup>Be catchment-averaged erosion rates integrate, on average, over the last ~300 yrs. Considering that the last earthquake on the Alpine Fault was in 1717, these rates are representative of inter-seismic erosion. Lake sedimentation rates and coseismic landslide modelling suggest that long-term (~10<sup>3</sup> yrs) erosion rates over a full seismic cycle could be ~40% greater than our inter-seismic erosion rates. If we assume steady state topography, such a scaling of our <sup>10</sup>Be erosion rate estimates can be used to estimate rock uplift rates in the Southern Alps. Finally, we find that erosion, and hence potentially exhumation, does not seem to be localized at a particular distance from the fault, as some tectonic and provenance studies have suggested. Instead, we find that superimposed on the primary tectonic control, there is an elevation/temperature control on erosion rates, which is probably transient and related to frost-cracking and glacial retreat.</p><p>Our results highlight the potential for <sup>10</sup>Be catchment-averaged erosion rates to provide insights into the magnitude and distribution of tectonic deformation rates, and the limitations that arise from transient erosion controls related to the seismic cycle and climate-modulated surface processes.</p><p> </p><p> </p>

scholarly journals Denudation rates derived from spatially-averaged cosmogenic nuclide analysis in Nelson/Tasman catchments, South Island, New Zealand

2021 ◽  
Author(s):  
◽  
Abby Jade Burdis
Keyword(s):  
New Zealand ◽  
Dynamic Environment ◽  
Low Frequency ◽  
Short Term ◽  
Erosion Rates ◽  
Cosmogenic Nuclide ◽  
Denudation Rates ◽  
Conventional Methods

<p>New Zealand’s tectonically and climatically dynamic environment generates erosion rates that outstrip global averages by up to ten times in some locations. In order to assess recent changes in erosion rate, and also to predict future erosion dynamics, it is important to quantify long-term, background erosion. Current research on erosion in New Zealand predominantly covers short-term (100 yrs) erosion dynamics and Myr dynamics from thermochronological proxy data. Without competent medium-term denudation data for New Zealand, it is uncertain which variables (climate, anthropogenic disturbance of the landscape, tectonic uplift, lithological, or geomorphic characteristics) exert the dominant control on denudation in New Zealand. Spatially-averaged cosmogenic nuclide analysis can effectively offer this information by providing averaged rates of denudation on millennial timescales without the biases and limitations of short-term erosion methods.  Basin-averaged denudation rates were obtained in the Nelson/Tasman region, New Zealand, from analysis of concentrations of meteoric ¹⁰Be in silt and in-situ produced ¹⁰Be in quartz. The measured denudation rates integrate over ~2750 yrs (in-situ) and ~1200 yrs (meteoric). Not only do the ¹⁰Be records produce erosion rates that are remarkably consistent with each other, but they are also independent of topographic metrics. Denudation rates range from ~112 – 298 t km⁻² yr⁻¹, with the exception of one basin which is eroding at 600 - 800 t km⁻² yr⁻¹. The homogeneity of rates and absence of a significant correlation with geomorphic or lithological characteristics could indicate that the Nelson/Tasman landscape is in (or approaching) a topographic steady state.  Millennial term (¹⁰Be-derived) denudation rates are more rapid than those inferred from other conventional methods in the same region (~50 – 200 t km⁻² yr⁻¹). This is likely the result of the significant contribution of low frequency, high magnitude erosive events to overall erosion of the region. Both in-situ and meteoric ¹⁰Be analyses have the potential to provide competent millennial term estimates of natural background rates of erosion. This will allow for the assessment of geomorphic-scale impacts such as topography, tectonics, climate, and lithology on rates of denudation for the country where many conventional methods do not. Cosmogenic nuclides offer the ability to understand the response of the landscape to these factors in order to make confident erosion predictions for the future.</p>

Variation in mussel and barnacle recruitment parallels a shift in intertidal community structure in the Cook Strait region of New Zealand

2011 ◽  
Vol 62 (10) ◽  
pp. 1221 ◽  
Author(s):  
Rahul Demello ◽  
Nicole E. Phillips
Keyword(s):  
New Zealand ◽  
Intertidal Zone ◽  
Spatial Scales ◽  
South Coast ◽  
The South ◽  
Sessile Invertebrates ◽  
High Zone ◽  
Species Specific ◽  
High Intertidal Zone

Recruitment influences populations and communities of marine organisms to varying degrees and across a range of spatial scales. We hypothesised that recruitment plays a role in maintaining different intertidal invertebrate assemblages between two nearby locations in New Zealand (Wellington Harbour and the south coast), long reported to have dramatically different communities (with greater cover of sessile invertebrates in the Harbour). Sites in Wellington Harbour were hypothesised to have higher monthly recruitment rates of mussels and barnacles and greater barnacle colonisation after 1 year. Surveys were conducted to quantify community differences. In Wellington Harbour, the mid-intertidal zone was dominated by the mussel Mytilus galloprovincialis and the barnacle Chamaesipho columna and the high intertidal zone by C. columna. In contrast, on the south coast mussels were almost completely absent from both tidal heights and barnacles (predominantly Chamaesipho brunnea) were sparse. In the high zone, monthly recruitment and long term colonisation (over 1 year) of barnacles was much greater in the Harbour; in the mid-intertidal zone, mussel recruitment was up to two orders of magnitude greater in the Harbour than the south coast. Species-specific recruitment patterns differed between the locations, however and were consistent with those of adult abundance.

Soil erosion rates under intensive vegetable production on clay loam, strongly structured soils at Pukekohe, New Zealand

Soil Research ◽  
2002 ◽  
Vol 40 (6) ◽  
pp. 947 ◽  
Author(s):  
L. R. Basher ◽  
C. W. Ross
Keyword(s):  
New Zealand ◽  
Vegetable Production ◽  
Clay Loam ◽  
Erosion And Deposition ◽  
Deposition Rates ◽  
Erosion Rates ◽  
Soil Redistribution ◽  
Structured Soils ◽  
Soil Erosion Rates ◽  
Tillage Erosion

Rates of soil redistribution by water and tillage erosion were determined in 3 fields under long-term continuous vegetable production on clay loam, strongly structured soils derived from volcanic ash at Pukekohe, South Auckland, New Zealand. Erosion and deposition rates were estimated using a mass balance model to convert variation in 137Cs areal activity within the fields to estimates of erosion and deposition. Caesium-137 areal activity in cropped fields ranged from 171 to 2144 Bq/m2, compared with a reference value under permanent pasture of 774 Bq/m2. There was a characteristic pattern of 137Cs distribution within each field, with differences between the fields related to variation in topography. Lowest values of 137Cs were found in the upper parts of each field and highest values towards the base of each field. In all 3 fields there was a net loss of 137Cs, ranging from 13 to 32%, with an average over the 3 fields of 18%. Net rates of soil loss from the fields were 30, 11, and 7 t/ha.year. However, within the fields there was a much wider range of both erosion (up to 92 t/ha.year) and deposition (up to 100 t/ha.year) rates. Most of the soil redistribution is caused by water erosion, with tillage erosion accounting for 10-20% of the soil redistribution. The soil redistribution rates were 2 orders of magnitude higher than sediment export measured at small catchment scale. Soil erosion rates are not reflected in variation in topsoil depth, because frequent tillage and incorporation of organic residues maintains a uniform topsoil depth, but soil deposition rates are closely related to topsoil depth.

Examining erosion in New Zealand over millennial timescales using in-situ 10Be and 14C

2020 ◽  
Author(s):  
Duna Roda-Boluda ◽  
Taylor Schilgen ◽  
Maarten Lupker ◽  
Wittmann Hella ◽  
Prancevic Jeff ◽  
...  
Keyword(s):  
New Zealand ◽  
Erosion Rate ◽  
Sediment Flux ◽  
Short Term ◽  
Sediment Fluxes ◽  
Landslide Activity ◽  
Erosion Rates ◽  
Order Of Magnitude

&lt;p&gt;Landslides are the major erosional process in many orogens, and one of the most sensitive erosional process to tectonic and climatic perturbations. However, it remains extremely difficult to constrain long-term or past rates of landslide activity, and hence their contribution to long-term landscape evolution and catchment sediment fluxes, because the physical records of landsliding are often removed in &lt;10&lt;sup&gt;2&lt;/sup&gt; yrs. Here, we use the in-situ &lt;sup&gt;10&lt;/sup&gt;Be and in-situ &lt;sup&gt;14&lt;/sup&gt;C concentrations of recent landslide deposits and catchments from the Fiordland and the Southern Alps of New Zealand to: (a) estimate landslide frequencies over 10&lt;sup&gt;3&lt;/sup&gt;-10&lt;sup&gt;4&lt;/sup&gt; yr timescales, which we compare against landslide inventories mapped from air photos (&lt;10&lt;sup&gt;2&lt;/sup&gt; yrs) to estimate changes in landslide activity, (b) quantify catchment-averaged erosion rates, and landslide&amp;#8217;s contribution to those erosional fluxes, and (c) test whether paired &lt;sup&gt;14&lt;/sup&gt;C-&lt;sup&gt;10&lt;/sup&gt;Be measurements can be used to trace erosional depth-provenance and identify transient erosion rate changes. We show that &lt;sup&gt;10&lt;/sup&gt;Be concentrations on landslide deposits can be used to estimate landslide recurrence intervals and frequency over 10&lt;sup&gt;3&lt;/sup&gt; yr timescales, and that &lt;sup&gt;14&lt;/sup&gt;C/&lt;sup&gt;10&lt;/sup&gt;Be ratios reflect the depth-provenance of sediment, and possibly transient changes in erosion rates. The comparison of our &lt;sup&gt;10&lt;/sup&gt;Be-based long-term landslide frequencies with short-term published inventories suggests that landslide frequencies have increased towards the present by up to an order of magnitude. We compare sediment fluxes inferred from these long- and short-term landslide inventories with sediment flux estimates derived from &lt;sup&gt;10&lt;/sup&gt;Be catchment-averaged erosion rates, which allows us to examine fluctuations in erosion rate estimates from 10&lt;sup&gt;1&lt;/sup&gt; to 10&lt;sup&gt;3&lt;/sup&gt; yrs timescales.&amp;#160;&lt;/p&gt;

scholarly journals Analysis of a 39-year continuous atmospheric CO<sub>2</sub> record from Baring Head, New Zealand

2013 ◽  
Vol 10 (4) ◽  
pp. 2683-2697 ◽  
Author(s):  
B. B. Stephens ◽  
G. W. Brailsford ◽  
A. J. Gomez ◽  
K. Riedel ◽  
S. E. Mikaloff Fletcher ◽  
...  
Keyword(s):  
New Zealand ◽  
Southern Ocean ◽  
Seasonal Cycle ◽  
Southern Oscillation ◽  
Atmospheric Co2 ◽  
Co2 Uptake ◽  
South Pole ◽  
The South ◽  
Wide Range

Abstract. We present an analysis of a 39-year record of continuous atmospheric CO2 observations made at Baring Head, New Zealand, filtered for steady background CO2 mole fractions during southerly wind conditions. We discuss relationships between variability in the filtered CO2 time series and regional to global carbon cycling. Baring Head is well situated to sample air that has been isolated from terrestrial influences over the Southern Ocean, and experiences extended episodes of strong southerly winds with low CO2 variability. The filtered Baring Head CO2 record reveals an average seasonal cycle with amplitude of 0.95 ppm that is 13% smaller and 3 weeks earlier in phase than that at the South Pole. Seasonal variations in a given year are sensitive to the timing and magnitude of the combined influences of Southern Ocean CO2 fluxes and terrestrial fluxes from both hemispheres. The amplitude of the seasonal cycle varies throughout the record, but we find no significant long-term seasonal changes with respect to the South Pole. Interannual variations in CO2 growth rate in the Baring Head record closely match the El Niño-Southern Oscillation, reflecting the global reach of CO2 mole fraction anomalies associated with this cycle. We use atmospheric transport model results to investigate contributions to seasonal and annual-mean components of the observed CO2 record. Long-term trends in mean gradients between Baring Head and other stations are predominately due to increases in Northern Hemisphere fossil-fuel burning and Southern Ocean CO2 uptake, for which there remains a wide range of future estimates. We find that the postulated recent reduction in the efficiency of Southern Ocean anthropogenic CO2 uptake, as a result of increased zonal winds, is too small to be detectable as significant differences in atmospheric CO2 between mid to high latitude Southern Hemisphere observing stations.

scholarly journals Analysis of a 39-yr continuous atmospheric CO<sub>2</sub> record from Baring Head, New Zealand

2012 ◽  
Vol 9 (10) ◽  
pp. 15237-15277 ◽  
Author(s):  
B. B. Stephens ◽  
G. W. Brailsford ◽  
A. J. Gomez ◽  
K. Riedel ◽  
S. E. Mikaloff Fletcher ◽  
...  
Keyword(s):  
New Zealand ◽  
Southern Ocean ◽  
Seasonal Cycle ◽  
Southern Oscillation ◽  
Atmospheric Co2 ◽  
Co2 Uptake ◽  
South Pole ◽  
The South ◽  
Wide Range

Abstract. We present an analysis of a 39-yr record of continuous atmospheric CO2 observations made at Baring Head, New Zealand, filtered for steady CO2 mole fractions during southerly wind conditions. We discuss relationships between variability in the filtered CO2 time series and regional to global carbon cycling. Baring Head is well situated to sample air that has been isolated from terrestrial influences over the Southern Ocean, and experiences extended periods of strong southerly winds with low CO2 variability. The filtered Baring Head CO2 record reveals an average seasonal cycle with amplitude of 0.95 ppm that is 13% smaller and 3 weeks earlier in phase than that at the South Pole. Seasonal variations in a given year are sensitive to the timing and magnitude of the combined influences of Southern Ocean CO2 fluxes and terrestrial fluxes from both hemispheres. The amplitude of the seasonal cycle varies throughout the record, but we find no significant long-term seasonal changes with respect to the South Pole. Interannual variations in CO2 growth rate in the Baring Head record closely match the El Niño/Southern Oscillation, reflecting the global reach of CO2 mole fraction anomalies associated with this cycle. We use atmospheric transport model results to investigate contributions to seasonal and annual-mean components of the observed CO2 record. Long-term trends in mean gradients between Baring Head and other stations are predominately due to increases in Northern-Hemisphere fossil-fuel burning and Southern Ocean CO2 uptake, for which there remains a wide range of future estimates. We find that the postulated recent reduction in the efficiency of Southern Ocean anthropogenic CO2 uptake as a result of increased zonal winds is too small to be detectable as significant differences in atmospheric CO2 between mid- to high-latitude Southern Hemisphere observing stations.

scholarly journals Histories and Mechanisms of Change in the Development of Shore Platforms at Kaikoura and Rodney New Zealand: Application of Cosmogenic Nuclides and Numerical Modelling on Exposed Coastal Surfaces

2021 ◽  
Author(s):  
◽  
Aidan Duart McLean
Keyword(s):  
New Zealand ◽  
Numerical Modelling ◽  
Sea Level ◽  
Late Holocene ◽  
Cosmogenic Nuclides ◽  
Erosion Rates ◽  
Cliff Erosion ◽  
Exposure Ages ◽  
Shore Platforms

<p>Global sea level rise is contributing to the acceleration of cliff erosion rates in New Zealand, where it surpasses rates of uplift. A significant challenge facing scientists and managers is that we have no method for reliably extracting past rates of coastal erosion along harder rock cliffs over the time-scales that significant sea level change occurs (100s-1000s of years). This gap in knowledge is limiting efforts to model and understand the relationship between sea level rise and cliff erosion rates and what form of that relationship takes. Cosmogenic Beryllium-10 analysis has been applied on two low angle shore platforms in New Zealand to produce chronologies of sea cliff retreat during the late-Holocene. Surface exposure ages were attained on a tectonically active platform at Kaikoura, Canterbury and a tectonically quiescent platform at Cape Rodney, Auckland. This is the first application of cosmogenic nuclides to a shore platform study in New Zealand and adds two new data-sets to the very small group of global shore platform chronologies. Exposure ages show New Zealand platforms have developed in the late-Holocene. Long-term platform surface erosion rates at Kaikoura (0.4mm a-1), potentially due to uplift driven positive feedback such as altered sea level position, driving up weathering rates on the tidally inundated platform. Nuclide concentrations at Okakari Point, Rodney, reveal a significant role of recent sea level fall after ~4000yrs BP, driving surface denudation (0.1mm a-1). The long-term cliff back-wearing rate at Okakari point was found to be 24.66mm a-1. Patterns in cosmogenic nuclide concentrations in New Zealand’s shallow platforms differ from global examples recorded on steeper platforms. Exploratory numerical modelling was applied with the coupled Rocky Profile CRN model (RPM_CRN) to identify process relationships between key drivers within platform coastal systems and scenarios of sea level change and active tectonics. This combined geochemical and numerical modelling study has shown that shore platforms in New Zealand have complex histories, with different potential driving forces at Kaikoura and Okakari. This highlights the local variability in platform development and cliff retreat, suggesting that estimates of future shoreline erosion will need to take local contingencies into account.</p>

scholarly journals Histories and Mechanisms of Change in the Development of Shore Platforms at Kaikoura and Rodney New Zealand: Application of Cosmogenic Nuclides and Numerical Modelling on Exposed Coastal Surfaces

2021 ◽  
Author(s):  
◽  
Aidan Duart McLean
Keyword(s):  
New Zealand ◽  
Numerical Modelling ◽  
Sea Level ◽  
Late Holocene ◽  
Cosmogenic Nuclides ◽  
Erosion Rates ◽  
Cliff Erosion ◽  
Exposure Ages ◽  
Shore Platforms

<p>Global sea level rise is contributing to the acceleration of cliff erosion rates in New Zealand, where it surpasses rates of uplift. A significant challenge facing scientists and managers is that we have no method for reliably extracting past rates of coastal erosion along harder rock cliffs over the time-scales that significant sea level change occurs (100s-1000s of years). This gap in knowledge is limiting efforts to model and understand the relationship between sea level rise and cliff erosion rates and what form of that relationship takes. Cosmogenic Beryllium-10 analysis has been applied on two low angle shore platforms in New Zealand to produce chronologies of sea cliff retreat during the late-Holocene. Surface exposure ages were attained on a tectonically active platform at Kaikoura, Canterbury and a tectonically quiescent platform at Cape Rodney, Auckland. This is the first application of cosmogenic nuclides to a shore platform study in New Zealand and adds two new data-sets to the very small group of global shore platform chronologies. Exposure ages show New Zealand platforms have developed in the late-Holocene. Long-term platform surface erosion rates at Kaikoura (0.4mm a-1), potentially due to uplift driven positive feedback such as altered sea level position, driving up weathering rates on the tidally inundated platform. Nuclide concentrations at Okakari Point, Rodney, reveal a significant role of recent sea level fall after ~4000yrs BP, driving surface denudation (0.1mm a-1). The long-term cliff back-wearing rate at Okakari point was found to be 24.66mm a-1. Patterns in cosmogenic nuclide concentrations in New Zealand’s shallow platforms differ from global examples recorded on steeper platforms. Exploratory numerical modelling was applied with the coupled Rocky Profile CRN model (RPM_CRN) to identify process relationships between key drivers within platform coastal systems and scenarios of sea level change and active tectonics. This combined geochemical and numerical modelling study has shown that shore platforms in New Zealand have complex histories, with different potential driving forces at Kaikoura and Okakari. This highlights the local variability in platform development and cliff retreat, suggesting that estimates of future shoreline erosion will need to take local contingencies into account.</p>

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