Question
How is the infiltration in a soakaway node calculated in MIKE+?
Answer
The numerical formulation of soakaway nodes is intended to mimic the behavior of a storage basin with enhanced infiltration capacities deployed in stormwater collection systems. These units analyze the infiltration of water into the ground and the unit effectiveness in controlling runoff.
In MIKE+, the soakaway is modelled as a regular basin unit, but it includes infiltration. The node can receive inflow from multiple upstream pipes and similarly can discharge into multiple downstream pipes or into an outlet drain line. Alternatively, the flow can be regulated or connected to a weir or orifice structure.
This type of node can be connected in a series configuration to enable modelling of constructed infiltration trenches.
The unit can be set to have no infiltration, a constant infiltration or an infiltration based on the surrounding soil hydraulic conductivity. The last option will allow setting a different hydraulic conductivity to the basin bottom.
The infiltration based on the hydraulic conductivity allows the modelling option of having the infiltration change depending on the water level in the soakaway unit. The unit infiltration rate is calculated based on the geometry and the water level in the unit (h) - see Fig. 1.
Fig. 1 - Schematic description of the flow through a soakaway basin unit
The water balance of the unit above can be described using the equation below, where Q is the porosity in the soakaway - see Fig. 2:
Fig. 2 - Description of the infiltration rate equation terms
The infiltration rate can be calculated with the following equation - see Fig. 3:
Fig. 3 - Infiltration rate flow expressed in terms of the geometric definition and hydraulic conductivity
Where Kfs is the field saturated hydraulic conductivity at the bottom and sides, As is the surface area and Ac is the unit cross-sectional area. Ac is dependent on the unit's width "w" and water depth "d" (also understood as height, "h") The porosity of the fill material is used to estimate the water level in the unit.
For more specific details of the numerical formulation please refer to the documentation here .
How would the unit behave under pressure?
Commonly, soakaway units work with an open surface condition in which the overflow runs out of the unit when the surface depth is overtopped, but some soakaway/basins can be closed to work under pressurized conditions.
In a hydraulic sense, a soakaway node is treated as an ordinary basin node. Any geometry above ground level is ignored. Instead, if the water level exceeds the ground level the model automatically includes a large fictive basin that emulates spreading of water on the surface.
When calculating the soakaway infiltration (at the unit sidewalls) at any water level above ground, the model assumes that the geometry at ground level extends above ground and it is cylindrically extrapolated. In this case, the surface area of the side walls is growing with the water level together with the exfiltration rate.
This implementation is not suitable for simulating infiltration with water levels above the ground. One alternative solution would be to keep the infiltration constant for any water level above ground. The use of soakaway nodes is commonly limited to studies where the water level is kept below ground level.
Please be mindful that the calculation of side walls area is an approximation, as from the specified basin surface-elevation table the numerical engine cannot "know" the actual shape of the basin and therefore cannot calculate the surface area of side walls exactly.
Conclusion
One can model a soakaway unit by defining the soil hydraulic conductivity, geometric definition of the unit and the rainfall intensity. This approach aids in better understanding the complex interactions between soil, water, and infrastructure, enhancing the efficiency and sustainability of stormwater management strategies.
Nonetheless, the numerical solution of a soakaway unit is still an approximation based on the unit's geometric definition and has its limitations such as the computation of exfiltration once the unit gets under pressurized conditions.
FURTHER INFORMATION & USEFUL LINKS
Manuals and User Guides
MIKE+ Collection System. User Guide.
Release Notes
MIKE+ Release Notes
Training options
MIKE+ – Introduction to hydrological and hydraulic modelling of urban drainage systems