Introducing: the MOUSE - MIKE SHE Coupling
Modeling of the two way interaction between pipes in the ground and the surrounding aquifer is now possible using a combination of MOUSE and MIKE SHE.
This allows for studies of the side-effects of infiltration prevention - and for the design of efficient infiltration prevention systems based on drains rather than (or as a supplement to) pipe rehabilitation techniques.
To illustrate the potential side-effects of pipe rehabilitation, see the this figure:
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The figure shows the effects of succesful prevention of infiltration into a sewer system in a small town in Sweden. The left hand side shows the situation before sewer rehabilitation. The right hand half shows the situation after a completely succesful rehabilitation. Dark blue indicates water on the surface. The red dots are houses.
It can of course be quite nice with lakes in the city, but probably not for those inhabitants, who have houses located in the middle of the lakes...
Fortunately, the actual rehabilitation was not quite that succesful, and fortunately the model was used to design a new drainage scheme aiming at reducing the high inflow to the small local treatment plant, caused by groundwater infiltration into the sewer network.
The new drainage scheme has been operational for some years now. The figure below shows flows to the local wastewater treatment plant before and after implementation of the infiltration prevention scheme.
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The blue curve (1988) shows inflow to the WWTP before the construction of the new drainage system. The green curve (1997) shows the inflow during a year after the construction. Observe the rapid increase in the inflow to the plant during the brief stop of the drain pump early in 1997.
The Link between MOUSE and MIKE SHE
MOUSE and MIKE SHE are coupled at three physical elements:
- The two way interaction between pipes and the aquifer. The interaction equation is based on the temporal and spatial variation of the water pressure in the pipes and the surrounding aquifer, the pipe surface to which the ground water is exposed to (or the waste water is exposed to if ex-filtration), and a leakage coefficient. The equation is:
Qpipe = (Hgw - Hpipe) x Ppipe x Cpipe,
where Hgw and Hpipe are level of groundwater and water level in sewer respectively, Ppipe is the surface of pipe in contact with groundwater, and Cpipe is a pipe leakage coefficient.
- The one way transport of flow from foundation drains, service lines and smaller pipe systems (not described as pipes in MOUSE) into a certain manhole in the MOUSE sewer network. The drainage equation is based on the ground water level, the drainage level (typical foundation level or similar), and a drainage coefficient. The equation is:
Qdrain = (Hgw - Hdrain) x Agrid x Cdrain,
where Hgw and Hdrain are groundwater level and drainage level respectively, Agrid is the horizontal area of a computational grid cell, and Cdrain is a drainage coefficient.
- The abortion of areas in MIKE SHE being impermeable and consequently described with the surface runoff model inside MOUSE. The areas aborted in MIKE SHE might be a fraction of a full grid square.
Alternatively, the surface runoff to manholes can be described by MIKE SHE, leaving only the sewer pipe hydraulics for MOUSE. The advantage of this is that not just the runoff from impervious areas can be described with a more physical approach, but also surface runoff from pervious and semi-pervious areas connected to manholes - or open channels and rivers.
In all other respects, the two models are built and executed as if they were two stand-alone models. The principle is illustrated in the figure below.
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The new MOUSE - MIKE SHE coupling has been extensively validated in several urban areas in Sweden. The results have been excellent and have been applied in the design of drainage schemes, which have been successfully implemented.
A more detailed description of the MOUSE - MIKE SHE coupling will be given at the DHI Software Conference in Prague in April 2003 as well as the DHI Software Conference in San Francisco, November 2003. You are also most welcome to email DHI and ask for an web-based live presentation by one of the developers.