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Hydrology expert testifies HMS model shows Wixom Lake rose but remained more than a foot below breach level; disputes defense and IFT methods
Summary
In trial testimony, plaintiffs’ hydrology expert Dr. David T. Williams said his HEC HMS modeling estimated peak inflow into Wixom Lake at about 28,474 cfs and ~94,683 acre‑ft of volume for the May 17–19, 2020 event; he said those results calibrated to USGS gauges and that alternative, higher runoff numbers proposed by the defense were not consistent with land‑cover, soils, or measured streamflow.
Dr. David T. Williams, a hydrologist retained by plaintiffs in the Edenville litigation, told the Court that he used the U.S. Army Corps of Engineers’ HEC HMS hydrologic model to reconstruct how water entered and behaved in Wixom Lake during the May 17–19, 2020 storm.
Williams said he and his team delineated the watershed with WMS (Watershed Management System), subdivided it into nine representative subbasins, used radar‑based rainfall input supplied by a meteorologist, and applied NRCS (TR‑55) runoff curve numbers (RCNs) adjusted for antecedent moisture conditions. He testified that his calibrated HMS simulation produced a peak inflow into the lake of about 28,474 cubic feet per second and an inflow volume of roughly 94,683 acre‑feet for the event.
Why it matters: Williams said he validated his choices by checking that modeled runoff matched two USGS streamgages on the Tobacco River where they exist, and then applied a standard similarity analysis to the neighboring Tittabawassee watershed where gauges were absent. He told the Court that calibration to measured gauge data is a routine industry standard and that, in his view, it demonstrated his inputs were defensible.
Williams challenged the defense’s higher runoff numbers, saying they failed two basic tests: they were not physically representative of the watershed’s land‑use/soil mix and they did not reproduce the USGS measured streamflows when applied to the same subbasins. "My numbers are correct," he said when pressed about that disagreement. He concluded that the defense’s RCN choices would produce peak inflows (about 51,837 cfs) and volumes (~138,097 acre‑ft) that could not be reconciled with observed streamgage data.
Outflow and gates: Williams told the Court he required time‑stamped gate‑position logs to model reservoir outflows definitively but that contemporaneous gate position logs were not available. In their absence he used a documented surrogate (emulator) method: he adjusted a surrogate weir/orifice configuration (he testified to using an effective discharge coefficient of about 2.6 for the surrogate) until the model reproduced measured Wixom Lake water‑surface elevations. He said that surrogate approach is an accepted fallback in practice when reliable gate logs or SCADA records do not exist.
Comparison with other studies: Williams compared his results with the Independent Forensic Team (IFT) report and with the defense expert’s calculations. He said his peak inflow (28,474 cfs) was higher than the IFT’s 24,527 cfs but that his computed volume (94,683 acre‑ft) was similar to the IFT’s 96,081 acre‑ft. He said the differences with IFT could not be fully evaluated because IFT had not provided underlying model files; he further testified that the particular way IFT combined HMS and RAS in its report was not described in sufficient detail for an independent scientific review.
Sensitivity and outcomes: Williams reported two run‑of‑river starting cases he ran: with a lower starting elevation (667.8 ft) his model’s maximum elevation was about 679.9 ft; with a 670.0 ft starting elevation (the scenario IFT used) his model reached about 680.1 ft. The actual failure elevation of the dam reported in the trial record was 681.3 ft; Williams said his runs were more than a foot below that failure elevation in those scenarios.
Court ruling and procedure: Counsel moved to admit Williams as an expert in hydrology and hydraulics, and the Court granted that motion. The witness was cross‑examined at length about the absence of gate logs, the choice of surrogate parameters, and the sensitivity of final elevations to outflow assumptions. Williams said a replacement run using an alternate spillway curve (opponent expert’s spillway shape) reduced his model's peak by about 0.4 ft, illustrating sensitivity to outflow parameterization.
Next steps in the case: Williams said he would not opine about whether the dam would have failed at a specific modeled elevation; his testimony focused on modeled inflows, volumes, reservoir elevations, and the methods used to obtain them.
