Dewatering Example Model Overview
Review this model, download it, and play with it to learn how to apply AnAqSim to dewatering problems.
This is a model of a complex dewatering system that includes a partially-penetrating circular sheet pile barrier, three shallow wells pumping inside the barrier, and a shallow recharge trench outside the barrier, where the discharge of the wells is put back into the aquifer. The aquifer is unconfined and sandy, with about 120 ft of saturated thickness. The system will allow excavation 10 ft below the ambient water table within the barrier to install a tank.
The figure at right shows a map of the system elements. The sheet pile wall is 63 ft in diameter and it penetrates 30 ft below the water table. The pumping wells are screened in the upper 20 ft of saturated thickness and are 6 inches in diameter. The recharge trench introduces water into the uppermost part of the saturated zone and is 116 ft long.
Near the system, there are 7 layers, with thinner layers near the bottom of the barrier where there are important vertical flows (see below). Far away from the system, there is only one layer, with an interdomain boundary linking the 7 layers within to the single layer without. At the interdomain boundary, the total normal components of discharge are matched over intervals and heads are matched at points. The topmost layers (1) are unconfined and layers 2-7 are confined (fixed transmissivity).
This hypothetical model aims to give guidance on questions that are typical for dewatering system design:
- What initial pumping rates for the wells will allow the water table inside the sheet pile barrier to be drawn down at least 10 ft within a period of 3 to 4 days?
- Once the desired drawdown is achieved by pumping at the initial rates, what schedule of reduced pumping rates will be needed to keep the heads drawn down at least 10 ft for a construction period of 7 days?
- What are the predicted heads at the well screens during pumping?
- What amount of mounding will occur at the recharge trench, assuming it injects all the water extracted at the pumping wells?
After building and adjusting the model, it was found that initial pumping rates of 1000 ft3/d (5.2 gpm) at each of the three wells will get heads within the barrier down at least 10 ft in the top layer after 3.5 days of pumping. The model had two time periods, the first one 3.5 days long for the initial fixed-discharge pumping. During a second 7-day period, the discharge-specified wells were replaced with head-specified wells, so AnAqSim could determine the discharges needed to maintain the heads at required levels. This figure shows contours of heads and velocity vectors in level 1 (water table) at the end of period 1. A heavy blue line (many contours) occurs at the barrier, where heads jump from near 0 on the outside to about -10.5 ft on the inside. The location “x” is where head is highest within the barrier.
With AnAqSim, it was fairly quick work to get insight into all the key design questions regarding anticipated discharges and heads. The entire modeling effort took about 4 hours to complete, from constructing the model through adjustments and creating outputs. Most of the modeling was done with a coarser model (3300 equations, 5 time steps), which took about 2 minutes to solve on an i5 computer. The final model used for the plots here had finer basis point spacing and more time steps (6600 equations, 10 time steps) and took about 12 minutes to solve. To alter the basis point spacing and number of time steps takes seconds, so it is easy to adjust the resolution of a model. Once a model is solved, the solution can be saved to disk and reloaded in seconds. To download the input files, right-click on the following links and select “Save link as…”.