Feature ArticleStudying Hydrodynamics Of Greenland Ice Sheets
By Steve Werblow
UCLA graduate student Lincoln Pitcher and University of Wyoming graduate student Brandon Overstreet measuring discharge in a supraglacial ice river on top of the Greenland Ice Sheet, immediately following record melting in July 2012. (Photo Credit: Brandon Overstreet and Dr. Carl Legleiter)
Much has been said about the contribution to rising sea levels of meltwater from the massive ice sheets and glaciers that cover Greenland, but little is known about the dynamics of how water is stored or moved across the ice. From beside fast-moving glacial streams on the Greenland Ice Sheet, a team funded by NASA is gathering data that will ultimately create the groundwork for a budget to fund tracking of the balance between ice and water under a variety of conditions.
The project, 'Towards Hydrologic Understanding of the Greenland Ice Sheet,' correlates data NASA has collected on the ice surface with reflectance data collected by satellite. A research team, led by Laurence C. Smith of the University of California, Los Angeles (UCLA) and coprincipal investigators Asa K. Rennermalm of Rutgers University and Carl Legleiter of the University of Wyoming, will provide vital supply-side data for sea level research and global climate change studies.
'We hope to use bathymetric data, water surface elevations and velocity measurements to produce hydraulic geometry relationships that can be used to estimate meltwater flux,' said Brandon Overstreet, a University of Wyoming Ph.D. student on the project.
'Due to the vastness of the Greenland Ice Sheet, our group decided to couple surface measurements of lake and stream bathymetry with WorldView-2 satellite imagery,' he added. 'Depth measurements from a drone boat, as well as an acoustic Doppler profiler are being used to create a relationship between water depth and surface reflectance measured by the satellite. This relationship can then be applied to entire image scenes acquired from the satellite to produce bathymetric maps for supraglacial lakes and streams, which store and convey large volumes of meltwater on the surface of the ice sheet.
'Similar hydraulic relationships currently exist for terrestrial rivers, but very little work has been done on supraglacial streams due to the difficulty of collecting these types of hydrologic measurements,' Overstreet noted.
'Difficult' hardly begins to describe the challenges of working on glacial streams. Nearly frozen water—0.5 to 1.0° C—flows fast over virtually frictionless beds of ice, reaching velocities up to 3 meters per second. Many of the streams are abruptly swallowed by vast crevasses, falling to unseen bottoms. As a result, even streams with low gradients can exhibit surprisingly powerful hydraulics.
Such unexpected forces acting on flow make ordinary assumptions about grade, volume and streambed geometry unreliable in predicting hydraulics on ice. On-site measurement allows the team to determine directly how much meltwater is conveyed through the hydrologic system of the Greenland Ice Sheet and how quickly it moves toward the ocean.
Most days started with a helicopter commute from the team base at the Kangerlussuaq International Science Support facility in southwest Greenland to sampling sites, often as far as 120 kilometers away.
The researchers outfitted a drone boat with an echosounder, GPS and spectroradiometer to measure the depth and optical characteristics of glacial lakes and slow-moving streams. Where the current was too fast for the drone boat, the group deployed a RiverSurveyor S5 acoustic Doppler profiler (ADP) by SonTek (San Diego, California). Using five acoustic beams and multiple frequencies, the ADP provided depth and velocity measurements. Built-in software also computed discharge. To continue this article please click here.
Steve Werblow is a freelance writer based in Ashland, Oregon. He covers agriculture, resource industries and water issues. He has written for Everything About Water, Industrial Environmental Technology, Brisbane Times, Australian Geographic, Process & Control Engineering, Hydraulics & Pneumatics, Industrial WaterWorld and Marine Scientist.