Connecting an Urban Drainage Network to a River Network - Coupling 1D models

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Connecting a 1D sewer network to a river model is a relatively simple operation, requiring you to attach links to common nodes so that their calculations are linked.

It is important to note that outfalls do not provide connectivity. So, if you have an outfall node that is located within a river reach, this will not transfer flows at that location. A break node—and cross section—is required at any point where you connect a conduit to a river.

A presentation slide suggesting that when connecting a sewer network to a river model, locations where a conduit is connected to a river should be planned ahead of time to ensure that there are break nodes where needed; as well as a couple of ways to make this connection.

IMPORTANT: If you are connecting existing models, there can be problems with misalignment of levels. This is usually a problem at outfall locations, where levels are not always available and are often assumed. For integrated models, the level is fundamental to understanding the hydraulic impact that the systems have on each other. Therefore, it is quite common to have to undertake additional outfall surveys when doing this work.

A presentation slide about misalignment during connection, stating that you may need to realign sewer networks and set an appropriate headloss at outfalls; and with an image of models being connected on the GeoPlan.

A multi-system model is going to have different peak timings. The challenge is trying to marry together several different systems and their interactions in a way that gives you the required output in the most efficient manner.

A presentation slide about multi-system models, stating that the response time of the watercourse to rainfall is critical when considering interactions, with a strong case for integration when the below and above ground systems have similar concentration times.

A level file is often applied to represent large rivers in sewer models. The justification is that the peak timing and duration of the river is a magnitude above that of the sewer system, so you are representing a worst-case scenario. However, the fact that the level is driven by a different system means that the chance of such an occurrence is much lower than with the rainfall probability alone.

A presentation slide stating that the use of level files significantly simplifies modeling of level files, with a graph comparing peak timing and duration of a river and sewer system, as well as graphs of rainfall intensity and inflow.

Note: When you have an urban watercourse, it may be a combination of rural and urban runoff that creates the peaks, or it may be that the peak timings are significantly different from the sewer, due to the nature of the systems—piped vs natural. This is when a fully integrated model is likely required.

A presentation slide stating that urban water courses may require a fully integrated model, and that it may be necessary to adjust input files to align peaks from both systems.

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