Flood channels encountered in small urbanised and semi-urbanised catchments offer a number of problems not normally associated with river type flood routing computations.
The conduits are usually of short length and have very marked differences in roughness, shape and slope. Lateral inflow can also form a considerable input to the channel section. As some of the channels are extremely short, sometimes down to 100 m or less, acceptable algorithms not requiring infinitely small routing increments are required.
A review of routing procedures (Price, 1973) suggested the use of a method developed by Cunge based on the Muskingum method now commonly known as the Muskingum-Cunge procedure. The method overcomes most of the analytical problems associated with other methods in so much as it accepts lateral inflow and converges with realistic time increments of the same order as the major routing increment used for the XPRafts run. The routing solution provides similar response to the diffusion portion of the Saint-Venant momentum equation (De Saint-Venant, 1871). The partitioning of the momentum equation into its diffusion and kinematic components is described by Havlik (1996).
The Muskingum-Cunge procedure also allows for various factors such as channel roughness, shape and slope to be included in the deduction of suitable Muskingum routing parameters thus incorporating physical attributes that were previously not included in the Muskingum method.
The procedure described by Price was specifically oriented towards routing in British rivers and hence had to undergo minor amendment to meet the requirements of XPRafts to also include minor channels and streams.
As an alternative to channel routing where physical data is lacking, XPRafts allows a simple channel lagging procedure whereby the flood hydrograph is simply lagged by an appropriate time with zero attenuation. Lag times can be evaluated using Manning's equation to estimate a flow velocity.
For preliminary studies, particularly urban catchments with either steep or lined channels and pipes, it is usually sufficient to only run the lag routine. It only becomes necessary to run the Muskingum-Cunge module if the system has a significantly large channel storage component or has many branches with lateral inflow.