Browsing by Author "Dong, Tian Yang"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Downstream elimination of gravel in multiple distributary channels on the Selenga River delta topset, Russia: a morphodynamics case study of fan delta at the Baikal Rift basin margin
    (2015-04-24) Dong, Tian Yang; Nittrouer, Jeffrey A.; Anderson, John B; Dickens, Gerald R
    The Selenga River delta, Lake Baikal, Russia, is approximately 600 km² in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge. This delta is positioned along the deep-water (~1600 m) margin of Lake Baikal, a half-graben styled rift basin, qualifying it as a modern analogue of shelf-edge system. This study provides a detailed field survey of channel bed sediment composition, channel geometry, and water discharge. Our data indicate that the delta exhibits downstream sediment fining, ranging from predominantly coarse gravel and sand near the delta apex to silt and sand at the delta-lake interface. An analytical framework is developed utilizing field data to evaluate the downstream elimination of gravel within the multiple distributary channels. Our major findings are: 1.) the Selenga River delta consists of at least eight orders of distributary channels, 2.) with increasing channel order, channel cross-sectional area, width-depth ratio, water discharge, boundary shear stress, and sediment flux all decrease downstream, 3.) the downstream elimination of gravel in distributary channels is caused by reducing boundary shear stress downstream where water discharge is partitioned among bifurcating channels, 4.) Gravel is trapped on the delta topset due to frequent basin subsidence, thus preventing coarse material from being delivered to the axis of the rift basin. The distribution of sediment in deltaic channels and the associated sediment transport processes that construct stratigraphy, combined with the active tectonic setting, allow sedimentary systems like the Selenga delta to be preserved in the long-term geologic record. Therefore, the location of the Selenga River delta along the active Baikal Rift margin renders the opportunity to explore the influence of tectonics and sedimentation on deltaic processes that produce basin stratigraphy.
  • Thumbnail Image
    Item
    Dynamics of delta building across different scales as informed by the Selenga River Delta, Russia
    (2020-04-24) Dong, Tian Yang; Nittrouer, Jeffrey A
    This dissertation examines dynamics of delta building across different spatial and temporal scales using field measurements and remotely-sensed data collected from the Selenga River delta, Russia. The research topics include evaluating the controls on hydraulic channel geometry for sand-bed rivers, developing a framework to better predict flow partitioning in delta channel networks, as well as deciphering the impacts of tectonic subsidence and variable basin depth on delta lobe building. Field measurements include detailed hydrological and sedimentological data collected within the distributary network of the Selenga Delta over four field campaigns, conducted 2013−2018. Remotely-sensed data include measurements of shoreline position, topset elevation, and basin depth since 1862. This research shows that, for the smallest of channel scales, bankfull width is directly, and depth is inversely, related to the characteristic median bank sediment size. Furthermore, new predicative relations of hydraulic channel geometry for sand-bed rivers that consider bank sediment size are derived, which improve upon previous work. On the channel network scale, channel geometry and planform variables, including cross-sectional area, depth, width, length, and sinuosity are determined to be the best parameters for predicting water and sediment partitioning in a delta system. The best predictive relations for water and sediment partitioning (channel width, length, and sinuosity) are incorporated into an exiting graph theory model, to yield a framework that accurately predicts flux distribution in delta networks. This method is tested by predicting flow partitioning in the Wax Lake delta, the Selenga River delta, and the Lena River delta. Predictions are in good agreement with field measurements. At the delta lobe scale, discrete tectonic subsidence events caused by the Baikal Rift are found to modify avulsion dynamics by varying receiving basin depth. These subsidence events create shallow embayments around the delta, by downdropping a portion of the topset below the average channel depth. As a result, flow is attracted to the partially subsided lobe, as the regional gradient is increased, due to a reduction in delta topset radius. Incisional terraces form as channels re-equalibrate to this new topset gradient. Theory predicts that these terraces reduce the frequency of lobe-scale avulsions by increasing the avulsion-setup threshold. Downstream of the terraces, smaller channel-scale avulsions still occur, and distribute water and sediment to topographic lows between the active channels. Predictive relations and models developed from the studies of this dissertation can be used standalone or incorporated into numerical models to better forecast the response of deltas to perturbations, such as the ongoing climate.