Introduction to Sediment Transport (ST)

Urban drainage networks are exposed to sediment loads carried by wastewater and/or by stormwater runoff. Such sediment loads may vary strongly both in space and time, as well as they may include a variety of particle types, ranging from large pieces of solid waste, street litter, sand and gravel from construction sites and unpaved areas, wind-borne dust, traffic debris, as well as small organic particles originating from wastewater. These sediments, if present in forms and quantities larger than the self-cleansing capacity of the sewer can handle, may create significant operational problems, such as reduced hydraulic capacity, increased overflows and full blockages of sewer pipes. These problems are typically associated with higher operational costs.

Modelling sediments in urban drainage networks does not belong to mainstream modelling work. However, the sediment modelling may be motivated by the wish to understand and eliminate existing or anticipated sediments-related problems, or by the need to document compliance with design criteria in terms of self-maintaining the sediment-free network.

In river contexts, modelling sediment transport can be used to assess long term morphology changes and alluvial resistance changes, considering both cohesive and non-cohesive sediments.

MIKE+ ST provides a modelling platform for such analyses. The ST module can be used in two main modes: a basic one, primarily designed to represent transport in pipe networks, and of a full-scale sediment transport model developed for river modelling applications. The ST module requires input from the hydrodynamic module, varying in time and space. When morphological changes are included in the simulation, the ST module also affects in return the hydrodynamic simulation. Scientific background for the implemented solutions can be found in the document “MIKE1D Reference Manual, Sediment Transport (ST)”.

Inevitably, keeping the modelling apparatus reasonably simple and practically applicable requires some conceptualization of the complex reality. In MIKE+ this is achieved by limiting the number of model parameters and computational options to those most important ones. The following are examples of simplifications and conceptualization in MIKE+:

• Various sediment fractions are distinguished by the median grain size (D50), relative density and the applied transport mechanism. Typically, the sewer sediments are represented by one coarse fraction (D50>=0.1 mm) and one fine fraction (D50\< 0.1 mm).
• Transport of the coarse fraction is preferably computed as non-cohesive transport, using one of the available sediment transport formulae.
• Transport of the fine fraction is preferably computed as cohesive transport, with advection-dispersion transport mechanism.

A detailed description of the ST editors and workflows is provided in the following paragraphs.