Deltares - R&D Highlights 2009

Contact: kees.sloff @deltares.nl +31 (0)88 335 8152 Modelling sand bars in the Grand Canyon Predicting bed topography changes in situations with complex 3D fl ow fi elds is still virgin territory in river engineering. The recent developments in the Delſt 3D soſt ware have supplied a powerful tool for simulating the relevant processes. Relevant potential applications of this tool are bed topography in eddy zones in groyne fi elds, and the morphological impacts of groyne adaptations for fl ood protection targets. The tool can also be used to study bar development caused by horizontal eddies and separation in sharp bends or off -takes (river bifurcations or side channels). However, the lack of experience with this relative ‘immature’ tool argues against operational use pending more thorough validation. Against this background, Deltares is working with the US Geological Survey (USGS) river groups in Flagstaff , Sacramento and Golden in the USA to model sand bars in the Colorado River in the Grand Canyon. Aſt er successful cooperation on coastal morphology studies, the USGS decided in 2007 to extend this cooperation to the river application in the Grand Canyon. The USGS needs process-based modelling tools for sand management in this river. The construction of the Glen Canyon dam in 1963 upstream of Grand Canyon National Park cut off the sand supply to the river. This is leading to the disappearance of the sand bars that serve as a habitat for endangered fi sh, as camping grounds for hikers and raſt ers, and as sand coverage for archaeological sites. These bars are mostly located in the complex eddies that develop downstream of riffl es. As part of a US environmental programme for sand-bar restoration, highdischarge releases from the dam are created during high sand infl ow from tributaries in order to get sand on the bars. To design optimal dam releases, it is necessary, among other things, to accurately predict the building-up of the bars. The 3D fl ow and morphology of Delſt 3D have been used for this purpose. 28

The focus of this study is the validation of the present features in Delſt 3D for 3D high-resolution modelling, and the implementation of improvements that do not require a full redesign of concepts or theory. In 2008, eddies and bar development at Mile 45 were monitored extensively during a high-fl ow release. The comparison of model simulations and survey data show that the 3D turbulent fl ow structure in the pools can be reproduced satisfactorily. The observed development of the bars is also reproduced qualitatively, but sensitivity to the components of transport models and sediment conditions, and some missing processes (such as slumping) make more indepth studies necessary. The study is being continued with more simulations and thorough analysis by the USGS Geomorphology and Sediment Transport Laboratory, and improvements to the concepts by Deltares. Further reading Sloff , C.J., S. Wright and M. Kaplinski (2009) High resolution three dimensional modeling of river eddy sandbars, Grand Canyon, U.S.A, 6th IAHR Symposium RCEM 2009 29

www.publitas.com, www.publitas.nl
Deltares - R&D Highlights 2009 main