Classic Pass Through Nodes

Pass through nodes remain fixed throughout the sediment simulation. Sediment that enters the cross- section control volume associated with that cross section leaves that control volume. Capacity for the node is set equal to supply. The cross section will not deposit or erode. This feature was designed for channel bends where multi-dimensional channel dynamics keep sediment from depositing but the one-dimensional transport approach computes deposition. To use this option the user simply selects the river station locations in which they would like all sediment to pass through (i.e. no deposition or erosion).

Modeling Note: Using Pass Through Nodes to Troubleshoot Models

Pass through nodes can stabilize an unsteady sediment model, focusing bed change on the region of interest, running the rest of the model as flow only. They can also help add complexity incrementally, getting a stable hydraulic model first, then bringing sediment cross sections (upstream to downstream) online incrementally. Additionally, pass through nodes only limit vertical bed change. They do not prevent BSTEM bank failure or toe scour. Therefore, users sometimes use them to isolate bank processes from bed processes.

Pool Pass Through Nodes

Pools are one of the primary applications of pass through nodes. Pools are fundamentally three-dimensional features. One-dimensional morphodynamic models do not simulate helical currents and mobility inversions that maintain pools.*  Because 1D models do not simulate these multidimensional pool maintenance processes, cross-section averaged models convert the larger cross-section area of pool cross sections to low velocity transport nodes. Therefore, 1D models tend to deposit in pools. These deposits become artificial, numerical, sediment sinks, removing sediment load from the model, and inducing erosion in other cross sections.

Pass-through nodes are one way to mitigate artificial pool deposition in HEC-RAS. Making pools pass-through nodes keeps them "open." Pass through nodes do not erode or deposit. Turning bed change off can seem like an aggressive solution to a numerical artifact, but in most cases, forcing a pool to no-change leads to a more realistic local- and reach-scale model than depositing with the standard morphodynamic equations.

However, if the base-level change of the reach is changing over time, making pools pass-through will fix their elevation, while the rest of the reach rises or falls together. HEC developed the Pool Pass Through Node option as an intermediate tool, that allows the pools to adjust with the neighboring cross sections, without over-depositing based on the 1D assumptions.

The Pool Pass Through Node applies the transport capacity of the bounding cross section(s) to the specified node. If the modeler selects multiple, consecutive, Pool Pass Through Nodes, HEC-RAS will assign the transport capacity of the non-pass through cross section(s) next to the pool pass through cluster. Assigning pool nodes the transport capacity from a "representative" bounding cross section, will allow the pool nodes to erode or deposit at a similar rate as the upstream run or crossing reach, without artificially depositing in a pool that the river keeps open.

Define Pool Pass Through Node(s) by selecting the Options Menu, and then selecting Set *Pass Through NodesPool{*}. This will launch a cross section selection dialogue. Select designated Pool Pass Through Cross Sections. The HEC-RAS Sediment Data interface will highlight standard and pool pass through nodes in blue and green respectively to help modelers keep track.

Because HEC-RAS computes sediment transport from upstream-to-downstream and transport capacity is dynamic based on bed gradation, the Pool Pass Through Node option cannot average the upstream and downstream transport capacities during the same time step. So the 1D Methods under the *OptionsTransport Methods* menu offers two different approaches to compute the capacity of Pool Pass Through Nodes.

Method 1: Upstream Capacity – The Pool Pass Through Node(s) adopt the transport capacity of the next upstream cross section that is not a pass through node (from the same time step). See the schematic of this approach below.** 

Method 2: US and Lagged DS – HEC-RAS averages the transport capacity of the upstream cross section (from the same times step) and the capacity of the downstream cross section at the previous time step. The Pool Pass Through Node(s) adopt the averaged of the synchronous upstream capacity and the lagged downstream capacity.

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*Incidentally, 2D models do not simulate some of these processes either. Three-dimensional modelers in the USACE have found that even 3D models tend to under-represent these process. At this point, most numerical models require some form of empiricism or simplification to simulate transport through pools. 

**This is one of the large bluff-associated pools (#3 in Figures 5 and 8) on the Madeira river documented in Gibson et al (2019) "Two pool-to-pool spacing periods on large sand-bed rivers: Mega-pools on the Madeira and Mississippi," Geomorphology, 328, 196-210.