Metric-Resolution 2D River Modeling at the Macroscale: Computational Methods and Applications in a Braided River

Abstract

Metric resolution digital terrain models (DTMs) of rivers now make it possible for multi-dimensional fluid mechanics models to be applied to characterize flow at fine scales that are relevant to studies of river hydraulic geometry (HG) and ecological habitat, or microscales. These developments are important for managing rivers because of the potential to better understand system dynamics, anthropogenic impacts, and the consequences of proposed interventions. However, the data volumes and computational demands of microscale river modeling have largely constrained applications to small multiples of the channel width, or the mesoscale. This report presents computational methods to extend a microscale river model beyond the mesoscale to the macroscale, defined as large multiples of the channel width. A method of automated unstructured grid generation is presented that automatically clusters fine resolution cells in areas of curvature (e.g., channel banks), and places relatively coarse cells in areas lacking topographic variability. This overcomes the need to manually generate breaklines to constrain the grid, which is painstaking at the mesoscale and virtually impossible at the macroscale. The method is applied to a braided river (BR) and shown to yield an efficient fine resolution model. The sensitivity of model output to grid design and resistance parameters is also examined based on analysis of hydrology, HG, and river hydraulic habitats and the findings reiterate the importance of model calibration and validation.

An intercomparison of remote sensing river discharge estimation algorithms from measurements of river height, width, and slope

“…The Seine is an example of a multichannel river; throughout the domain utilized here, the river is often two channels. The simplest solution of merging the top widths and averaging the water surface heights [Schubert et al, 2015]) led to adequate performance, here: AMHG, GaMo, and MetroMan had RRMSE of 33.9%, 22.5%, and 9.1%, respectively. Thus, the presence of multiple channels is not necessarily a cause of a great deal of algorithm performance degradation.…”

“…In the case of multiple channels, hydraulic quantities were summed across channels to derive a single value, which matches Schubert et al . 's [] “integrated” form for multiple channels. Reaches on the order of 5–10 km were defined based on inflection points in the water surface elevation data; methods to automatically identify optimal reach boundaries are needed, and are currently in development.…”

AirSWOT measurements of river water surface elevation and slope: Tanana River, AK

“…Additionally, AirSWOT's fine spatial resolution (<4 m) allows for better visibility in small river systems that play a nonnegligible role in the water and carbon cycles and are poorly gauged or not observable by satellites [ Raymond et al , ; Biancamaria et al , ]. AirSWOT measurements can be spatially averaged to get WSE and slope estimates in rivers with widths <50 m. AirSWOT measurements are also valuable for distributed hydrological analysis in highly multidimensional systems, such as braided rivers and deltas, where spatially distributed in situ observations are difficult to attain and tidal effects require fine temporal resolution measurements [ Wolski et al , ; Schubert et al , ].…”

The effects of spatial resolution and dimensionality on modeling regional‐scale hydraulics in a multichannel river

“…Simpler model formulations reduce computational burden, increase viable domain sizes, and improve the feasibility of ensemble modeling. Previous research has explored the effects of spatial resolution and model dimensionality independent of one another on both single‐thread and multichannel rivers [ Lane et al ., ; Horritt and Bates , ; Horritt et al ., ; Nicholas et al ., ; Schubert et al ., ; Javernick et al ., ]. To the best of our knowledge, however, no previous work has explored the effects of both model resolution and dimensionality on a multichannel river at the scale of ∼100 km or more.…”

“…To the best of our knowledge, however, no previous work has explored the effects of both model resolution and dimensionality on a multichannel river at the scale of ∼100 km or more. Fortunately, advances in algorithms, data availability, and computational resources now allow us to address this question, as we can build fine‐resolution (≤25 m) models of 100 km+ reaches that can resolve all river channels explicitly [ Schubert et al ., ]. These fine‐resolution models can act as benchmarks against which we assess how simplifications to the bifurcating and converging channel network affect modeling flood wave propagation, water level, and inundation extent in multichannel systems at regional to global scales.…”