Reaches

A reach is a one-dimensional connection between two nodes in a network.

The reach has many synonyms in different areas of application. In sewer models it is often called a link or pipe, in river models a branch, stream, canal or channel, and in graph and network theory it is called a link or an edge. In this manual only the term “reach” is used.

A position along a reach is given as the distance from a reference “start” point on the reach. This distance is called a chainage.

The start of the reach is defined as the point in the reach with the lowest chainage, and the end of the reach is defined as the point with the highest chainage; the terms start and end of reach is thus defined based on low and high chainage values, which does not necessarily correlate with the direction of water flow in the reach. The chainage at the start of the reach need not to be zero.

A reach also has an upstream end and a downstream end. These are defined based on the direction of natural flow (part of the reach specification): If the flow is naturally in the direction of increasing chainages, the upstream matches the start of the reach and the downstream matches the end of the reach. If the flow is naturally in the direction of decreasing chainages, the upstream matches the end of the reach and the downstream matches the start of the reach.

A location is a position along a reach, and is defined by the reach name and a chainage value.

Reaches come in many different geometrical variations:

• A sewer reach is featured by a constant cross section and constant friction properties along its entire length. It can also assume a straight line layout between its nodes, giving a constant bottom slope.
• A river reach is featured by varying cross sections along its length, varying friction with the cross section shape and the current water level. The river reach tends to meander through the landscape.

Many variations exist between these two extremes.