Emerging Issue Summary
COASTAL AND RIVERINE EROSION

Highlights:

  • The rates of North Slope coastal erosion are among the highest in the world and erosion is increasing.
  • Coastal and riverine erosion will most likely continue to increase due to sea-level rise, decreased amounts of shorefast ice and near shore pack ice, increased depth of the active layer, and more intense storms.
  • An accurate coastline and river corridor map, along with a program to remap selected targeted regions of the coastline every 3-5 years, would allow future estimates of North Slope erosion rates.
  • Accurate risk assessment (for loss of cultural sites, contaminant release, damage to infrastructure, and similar risks) will depend on development of a robust erosion model used in combination with accurate mapping of existing resources at risk.

Erosion
Coastal erosion fault from increased wave action. (Ken Dunton, University of Texas)

Overview and Management Relevance:

A recent study by USGS scientists and their partners noted that shoreline erosion rates along the Arctic coastline are among the highest in the world, with observed average erosion rates along the coast of the NPR-A in excess of 6 m/year.1 Although there was considerable variation from site to site, with some sites seeing deposition, 60 percent of the study’s sites saw an increasing rate of erosion. Another study reported a doubling of the rate of loss of coastal land area over the last 50 years.2 This erosion pattern has resulted in the loss of cultural and historic sites as well as the loss of modern infrastructure, and can be validly described as “a major landscape altering mechanism”1 of urgent concern to Arctic resource managers and native cultures. It was in this context that the list of concerns, questions, and needs were generated by agency staff. The following paragraphs organize some of the STAP’s thoughts on what kind of science can be brought to bear.

Coastal and riverine erosion will most likely increase on the North Slope due to sea level rise, decreased amounts of shorefast ice and near shore pack ice, increased depth of the active layer, and more intense storms. In some areas (such as portions of the Sagavanirktok River delta), accretion rates may increase in response to climate change and development. Recent increased human use of the coastal area has accelerated the importance of knowing the current rates of change as well as predicting future scenarios. Given this need a dedicated monitoring and modeling program is warranted.

Models that use sea ice extent, freeze-thaw cycles, soil variables, exposure, wind, wave, and extreme storm events should be generated to explain the observed 3-5 year erosion rates at selected test sites. Appropriate measurements (e.g., wind, wave, ice scouring, thermal processes) need to be collected to help develop and validate the models. Measurement sites should be selected based on the recommended baseline coastline survey and a range of expected erosion conditions, with input from local people familiar with the environment. These smaller sections can then provide a means to establish erosion rates that will to help determine the appropriate time scale for remeasuring the entire coastline.

The logical starting point for a coastal erosion monitoring program is related to an issue raised by the agency staff: “How and where is erosion being measured; is it adequate; and is the data accessible?” The answer to the adequacy and accessibility parts of this question appears to be “no,” or, at least “not yet.” NSSI-GINA is currently tasked with collecting the existing and historical shoreline data sets. To the extent that differences in methods and resolution allow, the most up-to-date data and any historical shoreline data that exist can then be compared within a geographic information system (GIS) to generate rates of erosion for the period between the respective coastline sections under investigation. This should be ‘step one’ and has already been accomplished to some extent for certain sections of the North Slope coastline, as noted above.

To determine the existing extent and rate of erosion across the North Slope, these types of studies will need to be expanded. The STAP recommends that a new remote sensing survey be undertaken. NSSI (through NOAA, USGS, industry, and/or the USACE) could request a coastal mapping mission using standard photography, satellite imagery, high resolution Synthetic Aperture Radar (SAR) data, or the Compact Hydrographic Airborne Rapid Total Survey (CHARTS) system that will also potentially provide water depth out to approximately 10 m depth or greater depending on water clarity. Due to turbidity concerns and weather and solar illumination constraints in Arctic Ocean waters, before mapping the entire coastline, a feasibility test should be conducted using satellite imagery, aerial photography, SAR data or the CHARTS system over a manageable test site to ascertain the most accurate and cost-effective method to map the entire coastline. For example, SAR data that can be acquired independent of cloud cover and solar illumination could be collected using the high-resolution mode on RADARSAT II as well as an airborne interferometric SAR system to evaluate whether the output product will satisfy the mapping accuracy requirements.

A multi-government center called the Joint Airborne LIDAR Bathymetry Technical Center of Expertise (JALBTCX) operates CHARTS and executes survey operations using the CHARTS system. CHARTS sensors include an Optech, Inc., SHOALS-3000 LIDAR instrument integrated with an Itres CASI-1500 hyperspectral imager capable of mapping surface and subsurface bottom types. The CHARTS system collects either 20 kHz topographic LIDAR data or 3 kHz bathymetric LIDAR data, each concurrent with digital RGB and hyperspectral imagery. Survey operations support the USACE National Coastal Mapping Program and NAVOCEANO nautical charting missions (see http://shoals.sam.usace.army.mil/).

With an accurate coastline location, continually improving erosion rate estimates, and well-populated data layers in GINA, the agency questions pertaining to threats to cultural sites, communities/subsistence activities, and contaminant risk can be addressed. GIS software has the capability to generate threat maps to various land-based features based on erosion rates, topography, and soil material.

It will be important to try to improve temporal resolution of mapping efforts. Historical remote sensing data (both unclassified and restricted) could be used to help generate historical rates of both coastal and riverine erosion that can be compared to present rates to better predict future changes to coastal and riverine structures.

A number of the emerging issues raised by the agencies addressing mechanisms to adapt to or mitigate erosion will require a comprehensive modeling approach. For instance, addressing how erosion rates will vary with climate-induced wave, wind, sea level, ice cover, and habitat changes will require new models. For model predictions to be realistic, good inputs are needed that include not only the rate of erosion, but information on the wave, wind, and freeze-thaw cycle as well as sea ice climatology. However, there are limited wave and wind data for the near shore of the North Slope.

The importance of year-round wind and wave data of North Slope coastal conditions needs greater emphasis. Erosion models use wind and wave statistics as input. NOAA should be encouraged to implement a buoy system to provide the required data. Data from other sources, including industry, should be shared with researchers.

In addition, modeling efforts would benefit from detailed data on a limited area or areas that could be used in model development and testing. That is, if an area can be identified for which substantial historical data are available; a set of accurate coastline and river corridor maps (both present and historical) could be generated for use in model development. To some degree, data such as these may exist for the area around Prudhoe Bay and may be available from industry. A dedicated monitoring program with repeated data collection every 3-5 years could add to the value of this dataset. To increase the value of this exercise, the area selected should be an area subject to high risk of habitat or infrastructure loss.

The issues of increasing rates of coastal erosion on fresh and salt water redistribution, its relationship to changes in the active layer, and the impacts to the water quality of freshwater and near shore environments, need to be addressed through a monitoring program. Until the cause-and-effect relationship between increased erosion, active layer thickness, coastal salinization, and water redistribution is quantified, the consequences cannot be addressed. Thus, NSSI or an NSSI-affiliated partnership needs to start documenting the water parameter changes of the freshwater and near shore waters of the North Slope.

The use of remote sensing data (such as the PALSAR Polametric Synthetic Aperture RADAR (SAR) satellite data) to monitor changes in the active layer may warrant further investigation (see also Emerging Issue paper on Permafrost). Additionally, hyperspectral imagery can detect subtle changes in vegetation around lakes that may be a result of coastal salinization (see also Emerging Issue paper on Vegetation Change).

In respect to the agencies emerging issues on mitigation and remediation, there are generally two ways to handle coastal erosion: 1) move infrastructure of interest back from the water’s edge or 2) construct engineering structures to slow down the erosion (to protect infrastructure that cannot be moved or to protect habitat).

Recommendations:

  1. Inventory aerial and satellite imagery from all accessible sources.
  2. Collect and store current and historical shoreline data in GINA for a localized area that can be easily accessed and that has reasonably good historical aerial photographs or comparable information. This data should be used to generate a set of accurate coastline and river corridor maps (both present and historical). These maps will serve as the baseline for models of future erosion and will be useful in model development and testing. A dedicated periodic monitoring program focused on the same area with repeated data collection every 3-5 years would increase its value over time. To further increase the value of this exercise, the area selected should be an area subject to high risk of habitat or infrastructure loss or known sites of contaminant release risk.
  3. Generate a North Slope-wide accurate coastline and river corridor map as a baseline for generating rate of erosion information. Repeat data collection over selected test sites every 3-5 years to determine a monitoring rate for the entire coastline. Remap the entire coastline on a time interval determined by the modeling effort and the selected re-surveyed coastal areas.
  4. Instrument portions of the North Slope coastline with autonomous sensors that measure wind and waves.
  5. Undertake a modeling effort that considers sea level rise, shorefast ice, near shore pack ice, increasing active layer thickness, and more intense storms in conjunction with climate models to estimate future erosion rates. Use this data as input into adaptive management of the coast.
  6. Include local knowledge in the development and assessment of models and monitoring programs.

 

Click here to download the Coastal and Riverine Erosion Emerging Issue Summary in PDF format

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1 Jones, B.M., K.M. Hinkel, C.D. Arp, and W.R. Eisner. 2008. Modern erosion rates and loss of coastal features and sites, Beaufort Sea coastline, Alaska. Arctic 61(4): 361-372.
2 Mars, J.C. and D.W. Houseknecht. 2007. Quantitative remote sensing study indicates doubling of coastal erosion rate in the past 50 yr along a segment of the Arctic coast of Alaska. Geology 35(7): 583-586.