Shoreline
Definition of Shoreline:
Shoreline: The physical transition between land and water, either instantaneous or defined for a specified water level and averaging period.
This is the common definition for Shoreline, other definitions can be discussed in the article
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Notes
The terms shoreline and coastline are often used indiscriminately as synonyms. However, it is generally preferable to define the shoreline as the physical transition between land and water and the coastline as a proxy of the shoreline position that designates the boundary between land and sea for use in shoreline management, see the article Coastline.
On many nautical charts the depicted shoreline approximately represents a high-water line.
Shoreline position must be defined through an observable indicator. Depending on the purpose and observation method, this may be the instantaneous sand-water interface, the intersection of the beach profile with a specified tidal datum, a high-water mark or another morphological feature. Shoreline positions obtained from different indicators are therefore not necessarily directly comparable.
Shoreline detection techniques
A large number of techniques can be used to determine the shoreline[1]. The appropriate technique depends mainly on the spatial extent to be covered, required accuracy, observation frequency and whether a waterline or an elevation-based shoreline is required. A brief overview of the main techniques is given below.
Aerial photography. Conventional aerial surveys and, increasingly, drones can provide high-resolution images from which shoreline indicators are extracted after georeferencing. Software is available for georectification of the images and extraction of the shoreline.
LIDAR aerial beach surveys. High-resolution DEMs of the subaerial and exposed intertidal beach can be obtained by airborne LiDAR; surveys around low water maximize coverage of the intertidal zone. High costs are an impediment for frequent surveys.
Video imaging. Fixed cameras installed at elevated locations can provide frequent observations of shoreline position, beach width and wave run-up. A coastal stretch up to about 2 km long can be covered. With appropriate processing, video imagery can also be used to infer beach morphology and nearshore bathymetry; see Argus applications.
Fixed scanning LIDARprogres. A land-based laser scanner can provide very frequent, high-resolution measurements of beach topography and, under suitable conditions, hydrodynamic features such as the moving waterline.
GPS surveys. Use of a kinematic differential GPS mounted on a four-wheel-drive vehicle, which is driven at a constant speed along the visibly discernible line of interest. This method is relatively rapid, low cost, and highly accurate.
Opportunistic and citizen-science imagery.
- Recreational surf cameras ('surfcams') existing in many popular surf destinations can be used for shoreline detection if properly calibrated, using advanced geo-referencing techniques.
- An application of smartphones (called 'Coastsnap') to obtain crowd-sourced photos of beaches supplied by the community via digital media platforms. The underlying concept of CoastSnap (https://www.facebook.com/coastsnap) is that a mounting bracket is installed at sites of interest to provide a fixed location and camera view angle, where community participants can then place their own phone and take a snapshot[2].
Satellite images. Publicly available optical satellite imagery has progressively improved in spatial resolution and revisit frequency: about 80 m and 18 days for Landsat 1–3 (1972–1983); 30 m and 16 days for Landsat 4–7 (1982 – 2025); 30 m and 16 days for Landsat 8 and 9 individually, with an effective 8-day revisit when both are combined (from September 2021); and 10 m and 5 days for Sentinel-2 (from June 2015).
Major advances in shoreline detection achieved with satellite imaging are large spatial coverage, dense time series, automated processing, and explicit correction/uncertainty assessment. A refined version of the sub-pixel resolution shoreline detection technique was developed by Liu et al. (2017) [3] and later augmented with the addition of automated image classification and edge detection methods. Sub-pixel techniques can locate the sand–water boundary with a horizontal RMSE smaller than the nominal pixel size under favorable conditions[4]. A more detailed description of shoreline extraction from satellite images is given in Satellite-derived shoreline and nearshore bathymetry#Shoreline detection.
For an instantaneous waterline, tidal correction requires the water level at the time of observation and an estimate of the local beach slope. Wave set-up may also be corrected if offshore wave conditions are known. The oscillatory swash component of wave run-up cannot generally be reconstructed for a single image and remains a source of uncertainty.
Several free available algorithms provide automated shoreline extraction. The CoastSat software directly incorporates global FES2022 tide predictions for tidal correction. Tide correction requires knowledge of the beach slope and the time at which the shoreline position was determined. The required beach-face slope may be obtained from topographic surveys or estimated from a time series of satellite-derived shoreline positions and tidal levels[5]. At energetic beaches, wave-run-up estimates can in some cases improve the conversion of satellite-derived waterlines to a reference shoreline position[6].
Shoreline change should preferably be evaluated from observations representing the same shoreline indicator; combining different indicators can introduce systematic offsets that may be mistaken for real shoreline displacement.
Related articles
- Coastline
- Satellite-derived shoreline and nearshore bathymetry
- Use of aerial photographs for shoreline position and mapping applications
References
- ↑ Boak, E.H. and Turner, I.L. 2005. Shoreline definition and detection: a review. J. Coast. Res. 214: 688–703
- ↑ Splinter, K.D., Harley, M.D. and Turner, I.L. 2018. Remote Sensing Is Changing Our View of the Coast: Insights from 40 Years of Monitoring at Narrabeen-Collaroy, Australia. Remote Sens. 10: 1744
- ↑ Liu, Q., Trinder, J. and Turner, I.L. 2017. Automatic super-resolution shoreline change monitoring using Landsat archival data: a case study at Narrabeen–Collaroy Beach, Australia. J. Appl. Remote Sens. 11: 016036
- ↑ Vos, K., Harley, M.D., Splinter, K.D., Simmons, J.A. and Turner, I.L. 2019. Sub-annual to multidecadal shoreline variability from publicly available satellite imagery. Coastal Engineering 150: 160–174
- ↑ Vos, K., Harley, M.D., Splinter, K.D., Walker, A. and Turner, I.L. 2020. Beach Slopes From Satellite-Derived Shorelines. Geophys. Res. Letters 47: e2020GL088365
- ↑ Castelle, B., Masselink, G., Scott, T., Stokes, C., Konstantinou, A., Marieu, V. and Bujan, S. 2021. Satellite-derived shoreline detection at a high-energy meso-macrotidal beach. Geomorphology 383: 107707
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