By Hydrosimulatics INC  

SUMMARY: Those Dam Guys LLC was contracted to take on an controversial project - removing the Plankton Dam along an otherwise picturesque stretch of the Plankton River. While they were using a control structure to drawdown the water level in the reservoir, the system failed, causing the downstream river levels to rise by 4m. Several citizens sued Those Dam Guys, claiming their basements were damaged because of the rising groundwater levels caused by high river levels after the dam break. The consultants argued that the basement flooding due was to the inherent surface drainage issues in the clay rich floodplain area, and had nothing to do with river level rise after the dam break. You be the Those Dam Guys LLC responsible??

Background & Controversy

Those Dam Guys LLC was contracted to take on an controversial project - removing the Plankton Dam along an otherwise picturesque stretch of the Plankton River.  A committee appointed by nearby cities has determined that it is no longer economically feasible to produce power at the dam and that the aging infrastructure poses a hazard to the community. Environmental groups have shown their support for the dam removal because it would improve the quality of the river and the aquatic and riparian ecosystems it supports. 

But there was significant pushback from citizens living the floodplain areas downstream from Plankton Dam. They were concerned about downstream flooding after the dam removal and the adverse impact on properties and the local economy.  

The project moved ahead anyways. The Plankton Dam was a run-of-the-river hydroelectric plant, so it provided a limited amount of storage. Because of this, the committee thought that removing the dam wouldn't  cause serious flooding downstream. So Those Dam Guys LLC was hired on as consultants responsible for dismantling the dam and its supporting infrastructure. Before the final, complete take down of the dam, the consultants needed to drawdown the water level in the reservoir using a temporary control structure. But something unexpected happened: in the middle of the water level drawdown, the control structure failed due to improper design, causing significant, rapid river rise and flood damage to some properties along its banks. This only added fuel to the fire.

A number of lawsuits ensued. Although most were quickly settled, one lawsuit (involving multiple properties on one particular stretch of the river) dragged on. The arguments for these cases got quite scientific. The crux of the matter boiled down to: how does the aquifer water table respond to river level rise - by how much, and for how long?
Homeowners in the area claimed their basements were flooeed because of the rising groundwater levels caused by the dam break. "the rive level increased 4m after the dam removal break! Our basements are only 1.8m above the water table!" The consultants contended that the wet basements had nothing to do with the dam break event. Instead, it was "due to the inherent surface drainage issues in the clay rich area. An increase in river level does not mean an equal increase in the water table". Based on their modeling analysis  using site specific data, the consultants predicted that water table increases are  significantly dampened away from the river because of the tight soil/floodplain sediments. "in fact," they argued, "the influence distance (where there is a water table rise) due to the dam break is 200m, even under most conservative assumptions."


Site Graphics: (Left) regional hydrogeology and area of interest; (top-right) site map, including the locations of the monitoring well and plaintiff basement locations; and (bottom-right): cross-section of the river and monitoring wells.

Hydrograph: Before, during and after the storm event. 

Objectives & Deliverable

You are hired as an expert witness. Specifically, you are asked to address the following questions

  • Are the consultants liable? 
  • How permeable are the floodplain sediments ? K and S?
  • What is the influence range of dam break?
  • How do water levels increase as a function distance away from the river in response to the dam break?
  • What is the water level rise at the properties in questions?

Develop a MAGNET model to simulate and visualize the impact of a dam break on the aquifer water table, and perform a systematic sensitivity analysis. Utilize monitoring well water level data from a previous storm/flood event to calibrate the hydraulic conductivty (K), recharge (E) and aquifer storage (S). Then apply the calibrated model to simulate water levels at the plaintiffs' basement locations during the dam break event. Specifically, the modeling sequence is:

  1. Initial Condition: Model the steady-state initial condition just prior to the storm/flood event (t=0). Calibrate K and E1 (the pre-storm "average" recharge) to match the observed heads at the monitoring well at t=0
  2. Calibration Event: Model the storm/flood event with new river boundary condition (see River curve in hydrography). Calibrate K, S, and E2 (storm-event recharge) to match the monitoring well curve. Some iteration may be needed between the initial steady state and transient model calibration models. 
  3. Dam Break Event: Model the dam break event using the new river boundary condition and calibrated K and S parameters.  Assume no rain during the break event.

Prepare a 1-2 page report that summarizes your approach and findings. You should discuss your findings with regards to responsibility for the contamination. Include any detailed model results / graphics in support of your conclusions in an appendix.

Available Data/Evidence

The Plankton River stage was recorded at a stream gage near the plaintiff properties prior to and during the storm event, as well as shortly after the dam break event (see the site map cross-section and the hydrograph). During the dam break, the river rose from 244.15 to max 248.96 in one hour and receded back to the original during the following three days.  

The aquifer is in a direct hydraulic connection with the large perennial Plankton River.  For a flooding event, the fluctuations of the water table are recorded in a monitoring well 30 meters from the river and are shown in the hydrograph above. The hydrograph shows a quick rise in the river stage and a consequent rise in the aquifer water table.  

The hydraulic conductivity of the aquifer can be assumed be homogeneous and isotropic.  Within the site of interest, the average land elevation is 250m. The average elevation of the impermeable clay layer beneath the aquifer is 236m. 

Locations of three home owners at properties 1, 2 and 3  are 290m, 350m, 387m away from the river, respectively. The elevations of the basements are 247.4, 247.8, and 248.1m.  

MAGNET/Modeling Hints:

  • Use ‘Synthetic mode’ in MAGNET to create a model domain with the same dimensions as shown in the map
  • Overlay the provided SiteMap image file included in the problem description. Choose ‘Use Domain Extent’ to fit the image to the established domain size.
  • Conceptualize the model as 1-layer, unconfined aquifer, with the bottom of the model following the flat impermeable layer below. The aquifer top can be simpliefied as one single constant elevation
  • Assign a single hydraulic conductivity, storage, and recharge input value (which are calibrated following the process described above).
  • Conceptualize the River as time-varying prescribed head boundary conditions
  • Utilize no-flow lateral boundaries, assuming i) flow is directed predominantly toward the river (little flow across north and south boundaries); the river fluctuations do not significantly impact flow along the east (right) boundary.
  • Add monitoring wells at the plaintiff properties to observe and compare the simulated water levels to the basement elevations.