Internal Boundaries

In a one-dimensional network representation of a river (or conduit), reaches are separated by internal boundaries which may be control structures, losses, reservoirs or junctions (bifurcations or confluences). These boundary conditions impose a relationship between the stages and discharges at the nodes involved.

Control Structures

A wide variety of structures can be used to control flow in open channels each of which imposes a different relationship between flow and stage. In free mode, the general form of the equation is:

These relationships are semi-empirical in nature and may require calibration. In general most structures can be categorised as weir or orifice types.

Weirs may be natural sills but are usually man-made structures transverse to the flow direction. They may span the full width of the channel or only part of it. Flood Modeller has the capability of modelling the following types using empirical or semi empirical equations from the literature:

• Sharp Crested Weir
• Round Nosed Weir
• Crump Weir
• Spill (Jagged) Weir
• Notional Weir
• Triangular Profile Weir
• Gated Weir (time varying crest elevation)
• Labyrinth Weir
• Flat - V Weir

These structures may operate in dry mode (no flow), or free or drowned mode according to the modular limit.

Sluices operate in a variety of flow modes including weir equations when the gates are out of the water and the obvious orifice type flow for normal sluice operations. Flood Modeller considers many possible flow modes for the sluice types including no flow, free and drowned weir flow, free and drowned gate flow, free and drowned flow over the top of a sluice gate and combinations of flow both under and over gates.

It is possible to control automatically the opening of sluice gates during a run, such as:

• according to pre-specified times
• according to upstream or downstream water levels
• to relate gate openings to water level at a remote node to simulate actions initiated by a flood warning.

The types of sluice gates available within Flood Modeller are:

• Vertical Lift Sluice
• Radial Gated Sluice

You can also input a rating curve for a channel control that is not described by any of the standard structures listed above, such as a flume or a general channel constriction, by using a Flow-Head Control. This unit also has the option of free and drowned modes according to a specified modular limit.

Reservoirs

Reservoirs are defined as large (or sometimes small) storage areas with a flat water surface where the dynamic effects are negligible. This need not necessarily be a formal reservoir, but could be a depression on the flood plain. It is also very often acceptable to use a reservoir to model parts of the floodplain (or channel, in some instances) if dynamic effects are negligible.

The balance of inflow and outflow of the reservoir is related to the rate of change of the head as follows (in a simplified form):

The reservoir together with the Spill adds a great deal of flexibility into Flood Modeller for schematisation of complex natural open channel systems.

Losses

The discrete energy losses such as those caused by a sudden contraction or expansion in the channel can be represented by a Bernoulli Loss which relates the head loss to the upstream velocity head:

Bridges can be modelled explicitly using either the US Bureau of Public Roads method (US BPR Bridge) or the Arch bridge method devised by HR Wallingford (Arch Bridge).

Junctions (Bifurcations and Confluences)

In looped or branched systems junctions are an obvious requirement. In Flood Modeller junctions are represented by simply equating water levels at the nodes of the junction or energy for an Energy Junction and conserving mass by applying Kirchhoff's Law to the flows.

Storage effects are neglected so each junction node must be effectively at the same position. They should also be at the same bed elevation. It is possible to include storage effects by replacing a junction with a reservoir.

If the velocity is significant at any of the junction's nodes, then total heads should be investigated. Total head (Energy) junctions may be used as an alternative to the traditional junction. Alternatively, the Bernoulli Loss or other loss unit can used on some or all of the junction arms.