The demand for temporary storage of sewage and storm water from urban areas has increased rapidly in recent years. The main reason for this is adoption of more stringent environmental protection standards. Rock tunnels have, in this connection, often been found suitable for creating the required storage volumes. In this paper three different principles for storage in tunnel systems will be discussed.


Measures aimed at environmental control in urban areas have increased in recent years and have been focused on the following sewerage problems:

  • Overflows of untreated water from combined sewer systems.

  • Shock loads on sewage treatment plants during periods of heavy rain.

  • Discharge of heavily polluted storm water to receiving water.

To meet the environmental protection standards it has become necessary to upgrade the existing sewerage systems by different means. One very important element in improving efficiency is to equalize peak flows in the system. In this respect utilization of the storage capacity of rock tunnels can often prove to be an economical solution. Rock tunnels for the conveyance of sewage and storm water have been used extensively in Sweden during the last forty years. This is especially the case in densely populated areas where the rock is suitable for tunnelling. A sewage tunnel is often deliberately given a capacity greater than the design flow. The reason for this is that the marginal cost for a larger tunnel cross-section is very small or is in some cases nil. The volume available in addition to the wet volume required for dry-weather flow is often considerable and could in certain conditions be used for storage of sewage and storm water.


With respect to function three main principles for the storage of sewage and storm water in a tunnel system can be distinguished, (Stahre, 1979). These are:

  • Tunnels for detention of peak flows.

  • Tunnels for temporary storage of sewage water from overflows in the sewer network.

  • Tunnels for sedimentation of storm water.

The conditions in which the different storage principles can be utilized depend on the purpose of the tunnel facility as well as local conditions.


Detention of peak flows in existing conveyance tunnels can easily be accomplished by installing some form of throttle regulator in the system. The regulator is designed in such a way that the flow to the sewage treatment plant can be restricted to a certain value. When this critical base flow is exceeded damming will take place in the tunnel system, see Fig. 1. Examples of regulator facilities for restriction of the peak flows are: fixed orifices, gate regulators, controlled pumping in the system and the screw regulator described by Janson, Bendixen and Harlaut (1976).

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