User's guide of Proceessing Modflow 5.0

Processing Modflow 285
6.5.5 Compaction and Subsidence
Folder: \pm5\examples\geotechniques\geo5\
Overview of the Problem
Fig. 6.56 shows a plan view and a cross section through an aquifer, which consists of three
stratigraphic units of uniform thickness. The first unit of the aquifer is unconfined and the other
units are confined. The initial hydraulic head is 43 m everywhere. The areal extent of the aquifer
is assumed to be infinite large. Except a confining bed (clay) in the second unit, the sandy
sediments of the aquifer is homogeneuos, horizontally isotropic with an average horizontal
hydraulic conductivity of 0.0001 m/s and vertical hydraulic conductivity of 0.00001 m/s. The
specific yield of the first stratigraphic unit is 0.15. The specific stroage of the aquifer is assumed
to be 0.0001 [1/m]. The properties of the confining bed are:
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horizonal hydraulic conductivity = 1 × 10 [m/s],
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vertical hydraulic conductivity = 1 × 10 [m/s],
elastic specific storage = 0.002 [1/m] and
inelastic specific storage = 0.006 [1/m].
To construct a new building, an excavation pit with the size (200 m × 100 m) is required. The
bottom elevation of the pit is 40 m. The pit must be hold dry for one year. Your task is to
calculate the required withdrawal rate and the distribution of subsidence after one year.
Modeling Approach and Simulation Results
The aquifer is simulated using a grid of 3 layers, 36 columns and 36 rows. The extent of the
model grid is fairly large. Each model layer represents a stratigraphic unit. The layer type
3: confined/unconfined (Transmissvity varies) can be used for all layers, as layers of this type
switches between confined and unconfined automatically. In the Layer Options dialog box, the
Interbed storage flag for the second layer is checked. The pit is modelled as fixed-head
boundary with the hydraulic head h = 40 m. The compaction (and thus the land surface
subsidence) of the confining bed is modelled by the Interbed Storage package.
A transient flow simulation with one stress period and 30 time steps has been carried out.
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The length of the stress period is one year (= 3.1536 × 10 seconds). The required withdrawal
rate changes with time and can be calculated by using the water budget calculator (by assigning
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the zone number 1 to the pit). For the first time step, the required withdrawal rate is 0.0134 m /s
6.5.5 Compaction and Subsidence