But-For Causation and Expert Proof in Catastrophic Flooding: Commentary on Tenaris Bay City Inc. v. Ellisor

But-For Causation and Expert Proof in Catastrophic Flooding: Commentary on Tenaris Bay City Inc. v. Ellisor

I. Introduction

A. Background and Parties

Tenaris Bay City Inc. v. Ellisor arises out of Hurricane Harvey, a historically destructive storm that struck the Texas Gulf Coast in August 2017. The respondents are thirty homeowners in the Van Vleck and Bay City areas of Matagorda County whose homes flooded during the storm. The petitioner, Tenaris Bay City Inc. (“Tenaris”), operates a large pipe fabrication facility built on land formerly used as a sod farm.

Because the Tenaris facility dramatically increased impervious cover relative to the prior sod farm, the company retained Fluor Enterprises to design a drainage system. That system used detention ponds and an earthen berm, with a principal discharge point labeled “Outflow 1.” Jones & Carter reviewed the design and the local drainage district approved it.

After Hurricane Harvey flooded their homes, plaintiffs sued Tenaris, Fluor, and Jones & Carter, alleging that defects in Tenaris’s drainage design and maintenance diverted and concentrated stormwater in a way that caused or worsened their flooding. The claims sounded in:

  • Negligence
  • Gross negligence (as to Tenaris)
  • Negligence per se under Texas Water Code § 11.086
  • Negligent nuisance

B. Procedural History

The litigation proceeded in stages:

  • Jones & Carter settled before trial; Fluor settled after trial.
  • The district court directed a verdict for Tenaris on gross negligence but submitted negligence, negligence per se, and nuisance to the jury on a liability-only basis, with plaintiffs’ properties divided into three geographic “zones.”
  • The jury found Tenaris liable on all three theories across all zones. The parties then stipulated to $2.8 million in total damages.
  • The trial court rendered judgment for plaintiffs. The Fourteenth Court of Appeals (Houston) affirmed, holding the evidence legally sufficient on causation, emphasizing that Tenaris’s conduct was a “substantial factor” in the flooding. 704 S.W.3d 37 (Tex. App.—Houston [14th Dist.] 2023).
  • Tenaris sought and obtained review in the Supreme Court of Texas.

C. Central Legal Issues

The Supreme Court framed the case around a core negligence element: causation. The key questions were:

  1. Did plaintiffs present legally sufficient evidence that Tenaris’s alleged negligence was a but-for cause of the flooding at each of the thirty individual properties?
  2. Did plaintiffs’ expert, civil engineer Gabriel Novak, provide reliable expert testimony establishing cause in fact—both “substantial factor” and but-for—rather than simply showing that Tenaris added “some additional flooding” to the area?
  3. Do the same causation requirements (including but-for causation and, where necessary, expert proof) apply to:
    • Ordinary negligence,
    • Negligent nuisance, and
    • Negligence per se under Texas Water Code § 11.086?
  4. Can plaintiffs in a catastrophic natural-disaster scenario rely on lay testimony or generalized expert opinions about increased flooding, or must they offer more targeted expert analysis showing that, absent the defendant’s conduct, their specific properties would not have flooded?

The Court’s answer to all of these questions favors a strict, traditional approach to causation. It reverses the court of appeals and renders judgment for Tenaris.

II. Summary of the Opinion

Chief Justice Blacklock, writing for the Court, holds:

  • But-for causation is indispensable in Texas negligence law. Cause in fact has two sub-elements—(1) but-for causation and (2) substantial-factor causation—and both must be proven. Plaintiffs must show that their harm would not have occurred in the absence of the defendant’s negligent conduct.
  • In flooding cases arising from a historic storm, the “default” cause is the storm itself. To shift responsibility to a neighbor, plaintiffs must produce reliable evidence that “their flood damage would not have occurred if not for their neighbor's tortious actions.”
  • Expert testimony was required in this case to establish causation, given the complexity of hydrology on relatively flat Gulf Coast terrain and the extraordinary rainfall. Lay testimony plus a general “more water” critique was insufficient.
  • Plaintiffs’ expert, Novak, did not do the necessary hydrologic work and expressly declined to opine on but-for causation for any specific property. He admitted he:
    • Did not perform a detailed drainage analysis,
    • Did not create hydrologic models, and
    • Could not say whether any plaintiff’s home would have remained dry “but for” the Tenaris facility.
  • Because there was no evidence of but-for causation for any property, the evidence is legally insufficient on all claims—negligence, nuisance, and negligence per se—and the Court renders judgment for Tenaris.
  • The same proximate-cause standard, including but-for causation, applies to:
    • Ordinary negligence,
    • Negligent nuisance, and
    • Negligence per se under Water Code § 11.086.
  • The relaxed causation approach in asbestos cases like Bostic v. Georgia-Pacific Corp. does not apply. It is reserved for rare situations where it is literally impossible to trace the specific causal contribution of each defendant; hydrologic modeling in a flood case does not fit that category.
  • Gragg and City of Keller are distinguished but applied consistently:
    • Gragg actually required but-for causation and did not dispense with it.
    • City of Keller confirms that complex drainage/flooding causation generally requires expert testimony.

In short, the Supreme Court characterizes Hurricane Harvey as an “act of God” whose effects cannot be lightly shifted to human actors without rigorous, property-specific proof of but-for causation via reliable expert analysis.

III. Detailed Analysis

A. Precedents and Authorities Cited

1. Core Texas Proximate-Cause Doctrine

The Court situates Tenaris firmly within longstanding Texas negligence jurisprudence.

  • Rodriguez-Escobar v. Goss, 392 S.W.3d 109 (Tex. 2013) and IHS Cedars Treatment Ctr. v. Mason, 143 S.W.3d 794 (Tex. 2004) are cited for the basic negligence elements: duty, breach, and damages proximately caused by the breach. Proximate cause comprises:
    • Cause in fact (including both but-for and substantial factor), and
    • Foreseeability.
  • Pediatrics Cool Care v. Thompson, 649 S.W.3d 152 (Tex. 2022) is quoted for the dual nature of cause in fact, requiring:
    • But-for causation: the harm would not have occurred without the defendant’s act or omission, and
    • Substantial-factor causation: the conduct must be a substantial factor in bringing about the injury.
  • Transcontinental Ins. Co. v. Crump, 330 S.W.3d 211 (Tex. 2010) and Ford Motor Co. v. Ledesma, 242 S.W.3d 32 (Tex. 2007) underscore that both components are necessary; dropping the but-for requirement renders the definition “incomplete.”
  • Rogers v. Zanetti, 518 S.W.3d 394 (Tex. 2017) reinforces that a cause-in-fact definition omitting but-for causation is legally deficient.
  • Gunn v. McCoy, 554 S.W.3d 645 (Tex. 2018) is referenced for the standard definition of but-for cause: an act is a cause in fact if “without the act or omission, the harm would not have occurred.”

These precedents combine to reaffirm a bright-line rule: Texas law does not recognize substantial-factor causation by itself as sufficient to prove cause in fact. Plaintiffs must carry both prongs.

2. Expert Causation and the Need to Exclude Other Plausible Causes

The Court also draws on cases emphasizing the reliability of expert causation testimony:

  • Helena Chem. Co. v. Cox, 664 S.W.3d 66 (Tex. 2023)
  • JLG Trucking, LLC v. Garza, 466 S.W.3d 157 (Tex. 2015)
  • Transcontinental v. Crump (again)

These cases establish that expert testimony on causation is fundamentally unreliable if the expert does not make a reasonable effort to:

  • Identify and consider other plausible causes, and
  • Explain, with reasonable certainty, why the defendant’s conduct is the actual cause (or at least a necessary but-for cause) of the injury.

Applying this framework, the Court criticizes Novak’s testimony because he:

  • Made no attempt to analyze or rule out Hurricane Harvey itself and regional environmental factors as independent causes of plaintiffs’ flooding.
  • Admitted he did no flooding analysis “whatsoever” in the area, despite acknowledging that such an analysis was within his expertise.

In other words, his testimony showed some additional flooding attributable to Tenaris but did not bridge the critical gap to show that Tenaris’s contribution was the difference between a flooded and a non-flooded home.

3. Nuisance Doctrine

The plaintiffs pursued a negligent nuisance theory. The Court relies on Crosstex North Texas Pipeline, L.P. v. Gardiner, 505 S.W.3d 580 (Tex. 2016), which clarifies that nuisance is a type of legal injury, not a stand-alone tort. Liability for a nuisance may arise from:

  • Intentional conduct,
  • Negligence, or
  • Strict liability (for certain ultra-hazardous activities).

Where the nuisance is based on negligence—as in this case—Crosstex instructs that ordinary negligence principles govern, including:

  • Duty
  • Breach
  • Proximate cause (foreseeability and cause in fact, including but-for causation)

Thus, the Court holds that plaintiffs’ nuisance claim rises or falls with their proof of proximate cause, defined in the jury charge to require that Tenaris’s conduct be:

“a substantial factor in bringing about an injury, and without which cause such injury would not have occurred.”

Because plaintiffs did not prove but-for causation for any property, the nuisance claim fails along with ordinary negligence.

4. Negligence Per Se and Water Code § 11.086

Negligence per se is described, following Missouri Pacific R.R. v. American Statesman, 552 S.W.2d 99 (Tex. 1977), as:

  • A negligence theory in which the duty and breach elements are supplied by a statute or regulation.

Here, the statute is Texas Water Code § 11.086, which provides:

(a) No person may divert or impound the natural flow of surface waters in this state, or permit a diversion or impounding by him to continue, in a manner that damages the property of another by the overflow of the water diverted or impounded.

(b) A person whose property is injured by an overflow of water caused by an unlawful diversion or impounding has remedies at law and in equity and may recover damages occasioned by the overflow.

The Court emphasizes that § 11.086 contains its own causation language (“caused by,” “occasioned by”), and that under both the statute and general negligence-per-se doctrine:

  • Proximate cause remains a required element.
  • The statute does not create strict liability untethered to cause in fact.

The opinion favorably cites Contreras v. Bennett, 361 S.W.3d 174 (Tex. App.—El Paso 2011, no pet.), which held that a plaintiff under § 11.086 must prove that damage “would not have resulted but for such unlawful diversion,” as well as Benavides v. Gonzalez, 396 S.W.2d 512 (Tex. App.—San Antonio 1965, no writ), interpreting a predecessor statute similarly.

Accordingly, the negligence-per-se claim is subject to the same but-for causation requirement. Because plaintiffs cannot establish that their homes would not have flooded “but for” Tenaris’s unlawful diversion or impoundment, the negligence-per-se theory also fails.

5. Flooding and Hydrology Cases: City of Keller and Gragg

Two prior flooding decisions frame the Court’s analysis of causation and evidence:

  • City of Keller v. Wilson, 168 S.W.3d 802 (Tex. 2005):
    • Although best known for its comprehensive articulation of legal-sufficiency review, City of Keller also dealt substantively with flooding allegations arising from a residential development.
    • The Court held that proving whether detention ponds and grassy ditches increased downstream flooding required expert testimony and hydrologic modeling.
    • Here, the Court analogizes: just as in City of Keller, calculating the effect of Tenaris’s detention ponds, berm height, and outflow on specific downstream residences required expert hydrological analysis—not lay inference.
  • Tarrant Regional Water District v. Gragg, 151 S.W.3d 546 (Tex. 2004):
    • Gragg involved claims that the operation of a dam increased downstream flooding on a ranch.
    • The Court there stated explicitly that the plaintiff had to prove that “the same damaging floods would not have occurred under the same heavy rainfall conditions had the dam not been constructed.”
    • In Tenaris, plaintiffs relied heavily on Gragg to suggest that less stringent proof might suffice, but the Supreme Court rejects that reading, emphasizing that Gragg actually insisted on a robust but-for standard.
    • The Court also notes that Gragg dealt with a single dam, a single floodgate, a single river, and a single property—making causation relatively more tractable than in a sprawling, flat, multi-property Harvey scenario.

6. The Limited Exception in Asbestos Cases: Bostic

The Court confronts Bostic v. Georgia-Pacific Corp., 439 S.W.3d 332 (Tex. 2014), where it adapted the but-for causation requirement to the unique context of asbestos disease:

  • In asbestos cases, fibers from multiple sources accumulate over decades, and medical science cannot pinpoint which fibers from which defendant physically caused the disease.
  • Recognizing this “is not humanly possible,” Bostic allowed proof that a defendant’s exposure was a substantial factor in producing the disease, without requiring proof that specific fibers caused it.

In Tenaris, the Court stresses that:

  • The Bostic relaxation is exceptional and rare.
  • It applies only where traditional but-for proof is literally impossible, not merely difficult or expensive.
  • Hydrologic analysis of surface-water flows in a localized area—while complex—is possible, as Novak himself admitted.

Thus, Bostic does not authorize any relaxation of causation in this flooding context.

7. When Is Expert Testimony Required? Mack Trucks

The Court applies Mack Trucks, Inc. v. Tamez, 206 S.W.3d 572 (Tex. 2006), which held that:

“Expert testimony is required when an issue involves matters beyond jurors’ common understanding.”

Here, several facts demonstrate that causation was beyond common understanding:

  • Novak testified that determining the cause of flooding at plaintiffs’ homes would require:
    • U.S. Geological Survey topographic data,
    • Detailed drainage analysis,
    • Computerized hydrologic modeling,
    • NRCS soil maps, and
    • A study of all relevant waterways.
  • The plaintiffs’ own counsel agreed pre-trial that expert evidence on causation was necessary and that lay opinion (“I think the water came from Tenaris”) would be insufficient.

On this record, the Court holds that lay testimony alone could not establish causation. Plaintiffs therefore live or die on their expert’s testimony—and that testimony did not address but-for causation.

B. The Court’s Legal Reasoning

1. The “Default Explanation”: The Storm Itself

A notable feature of the opinion is its framing of Hurricane Harvey as the “natural, default explanation” for the plaintiffs’ flood damage. The Court remarks:

“The natural, default explanation for flood damage during a historic and unpredictable rain event is the storm itself, not the actions of neighboring property owners.”

This framing accomplishes two things:

  1. It implicitly echoes the traditional “act of God” concept without formally invoking an affirmative defense—shifting the discussion to plaintiffs’ burden rather than Tenaris’s defense.
  2. It underscores that, when thousands of properties flood over a wide region, plaintiffs must produce evidence that their damage was not simply part of the storm’s baseline destruction.

In other words, in catastrophic natural disasters, the Court signals a strong presumption that the natural event is the primary cause, and plaintiffs must rebut that presumption with property-specific evidence of but-for causation.

2. Evaluating Novak’s Expert Testimony

The Court accepts, for purposes of review, that Novak’s critique of the Tenaris facility is valid:

  • He opined that it was a “fundamental” design flaw to route a “very large drainage basin” towards a single outfall (Outflow 1).
  • He concluded this design “caused additional flooding elsewhere in Van Vleck or Bay City.”

However, the Court deems his testimony legally insufficient on causation for several reasons:

  1. No property-specific analysis:
    • Novak admitted he did not conduct a “detailed analysis” for “any of the specific plaintiffs’ homes.”
    • He did not perform “any flooding analysis whatsoever” for Van Vleck or Bay City.
  2. Express refusal to opine on but-for causation:
    • When asked if he could answer the question “what caused these poor folks to have their homes flooded,” he unequivocally responded: “No, I cannot answer that question” for the specific homes.
    • He conceded he could not give even a “general conclusion” because he “hadn’t done the work” required.
  3. Ambiguous “factor” vs. “substantial factor” testimony:
    • When asked if Tenaris’s design was a “substantial factor” in the flooding of plaintiffs’ properties, he answered that it was “a factor” but conspicuously did not confirm it was a “substantial” factor.
    • He never testified that Tenaris’s conduct was a “but-for” cause of any property’s flooding.
  4. Failure to rule out other plausible causes:
    • He made “no attempt at all” to exclude Hurricane Harvey itself and other hydrologic, geologic, or land-use factors as independent causes of flooding.

Against the background of Helena Chem., Crump, and JLG Trucking, this lack of analytical rigor renders his causation opinions unreliable and, more importantly, no-evidence on but-for causation.

3. Application to Each Cause of Action

Because the jury charge for each theory incorporated the standard proximate-cause definition—including but-for causation—the Court’s holding on causation reaches every claim:

  • Negligence:
    • Even assuming breach (e.g., berm built at 43 feet instead of 44, poor maintenance), plaintiffs failed to prove that their homes would have stayed dry but for Tenaris’s negligence.
  • Negligent nuisance:
    • As Crosstex confirms, negligent nuisance is governed by ordinary negligence principles, including proximate cause.
    • The nuisance charge required a showing that Tenaris’s conduct was a substantial factor “and without which cause such injury would not have occurred.”
    • With no evidence of but-for causation, the nuisance verdict cannot stand.
  • Negligence per se under § 11.086:
    • The statute’s text (“caused by,” “occasioned by”) reinforces the need for causation in fact.
    • The jury was instructed that any diversion or impounding “must have caused damages to Plaintiffs’ properties and without which cause such injury would not have occurred.”
    • Again, plaintiffs failed to carry that burden.

Across the board, negligence without causation is not actionable.

4. The Court of Appeals and the “Substantial Factor” Misstep

The Fourteenth Court of Appeals upheld the verdict, finding legally sufficient evidence that Tenaris was a “substantial factor” in causing plaintiffs’ harm. The Supreme Court notes that:

  • It does not read the court of appeals as formally abandoning but-for causation.
  • But even if the lower court implicitly treated substantial-factor evidence as sufficient, that is contrary to Texas law, which treats but-for causation as an independent and essential component of cause in fact.

By insisting on both prongs of cause in fact, the Supreme Court re-centers Texas causation law and clarifies that any attempt by lower courts to relax the but-for requirement is doctrinally impermissible.

5. Lay Testimony and the Limits of “First-Time Flooding”

Plaintiffs argued—and the court of appeals suggested—that their lay testimony, especially that their properties had never flooded before, could itself support a causation finding. The Supreme Court rejects that position:

  • On the need for experts:
    • Given the complex interplay of rainfall intensity, topography, soil permeability, land subsidence, impervious cover, and multiple drainage channels, jurors cannot reliably infer causation purely from observation.
    • City of Keller and Mack Trucks underscore that hydrologic cause-and-effect is beyond common understanding.
  • On “never flooded before” evidence:
    • The fact that properties had not flooded in prior storms is consistent with many possible explanations: changing development patterns, land subsidence, incremental infrastructure changes, or simply that Harvey was an unprecedented storm.
    • It does not, by itself, prove that Tenaris’s facility tipped the scale.

The Court emphasizes that in Gulf Coast communities, where conditions evolve and rare extreme events occur, “there are many reasons why a gulf-coast neighborhood might flood for the first time,” and it is “not at all unexpected” that the first time coincided with Harvey.

6. The Normative Undertone: “Pinning the Consequences of an Act of God”

The Court’s rhetoric carries a notable cautionary tone. Statements like:

“Pinning the consequences of a historic act of God on your neighbor is no small thing.”

signal several policy concerns:

  • Flood litigation after large-scale disasters should not rest on generalized blame of nearby development without rigorous proof.
  • Courts must guard against hindsight bias—the temptation to assign liability whenever a defendant’s conduct seems negligent, even without clear proof that it changed the outcome.
  • Finite liability in the face of systemic risks: when thousands of discrete properties are damaged primarily by natural forces, liability needs to be anchored to truly causative human acts, not merely contributory background conditions.

Consequently, the Court holds firm on evidentiary requirements, even while acknowledging the plaintiffs’ hardship and the widespread devastation of Harvey.

C. Impact on Future Cases and Texas Law

1. Flooding and Natural-Disaster Litigation

Tenaris will likely become a leading case on causation in Texas flood litigation, especially where:

  • A major natural event (hurricane, extreme rainfall, or similar) is the dominant background cause, and
  • Plaintiffs seek to assign responsibility to specific upstream properties, developments, or public works.

Key practical implications:

  • High evidentiary bar:
    • Plaintiffs must produce property-specific hydrologic evidence showing that, absent the defendant’s conduct, their particular parcel would have remained dry.
    • General testimony that a project increased flooding “in the area” is insufficient.
  • Routine use of experts:
    • Hydrologists or civil engineers will be indispensable in most flood cases involving complex drainage and multiple potential causes.
    • Experts must perform and present modeling (e.g., HEC-HMS, HEC-RAS, or comparable tools)—not just high-level design critiques.
  • Defense strategies:
    • Defendants will cite Tenaris to support no-evidence motions for summary judgment and directed verdicts when plaintiffs’ experts offer only generalized causation opinions.
    • Arguments will emphasize Harvey-like events as superseding or at least primary causes absent robust expert allocation of incremental contribution.

2. Water Code § 11.086 and Nuisance Claims

For practitioners in water-diversion disputes:

  • § 11.086 is not strict liability: Plaintiffs must show that the diversion or impounding was both:
    • “Unlawful” (duty/breach), and
    • The but-for cause of the overflow damage.
  • Negligent nuisance aligns with negligence:
    • Framing a claim as “nuisance” does not circumvent proximate-cause requirements.
    • The jury charge in future cases should continue to tie nuisance liability to standard proximate cause, including but-for causation.

Litigants must therefore treat nuisance and § 11.086 claims as causation-intensive torts, not as fallback avenues when traditional negligence proof falters.

3. Expert Practice and Litigation Strategy

Tenaris sends a clear message to expert witnesses and the attorneys who retain them:

  • Scope and depth of analysis:
    • Experts must perform enough analysis to answer the legally dispositive question: “Would this plaintiff’s property have been damaged if the defendant had acted non-negligently (or not acted at all)?”
    • Merely identifying design flaws or incremental changes to water distribution is not enough; the analysis must trace those changes to the specific harms pled.
  • Deposition risks:
    • Admissions like Novak’s—conceding that necessary work was not done and refusing to opine on specific causation—can be fatal at the appellate stage.
    • Counsel must ensure that experts either:
      • Perform the full analysis, or
      • Explicitly limit opinions so the case strategy can adjust (including potential abandonment or narrowing of claims).
  • Resource allocation:
    • The decision highlights the cost of litigating high-stakes environmental and flooding cases: comprehensive hydrologic modeling can be expensive, and failure to invest may leave plaintiffs with no-evidence on causation.

4. Relationship with the “Act of God” Concept

While Tenaris does not formally adjudicate an “act of God” affirmative defense, its language will influence how such defenses are conceptualized:

  • Courts may increasingly view extreme weather as the presumptive cause of damage, especially where:
    • Rainfall is historic or unprecedented, and
    • Widespread regional flooding occurred.
  • Plaintiffs in future storms will face a tougher road in attributing damage to human actors without rigorous modeling showing that a defendant’s incremental contribution was necessary for the specific flooding observed.

5. Broader Tort-Law Implications

At a doctrinal level, Tenaris reinforces several themes:

  • Inflexibility of but-for causation: Outside narrow categories like asbestos (Bostic), Texas remains committed to traditional but-for analysis as a component of cause in fact.
  • Limited role of “substantial factor” alone: “Substantial factor” is not a free-standing causation test; it must always be paired with but-for analysis.
  • Guardrails on mass or systemic torts: Even when a defendant’s wrongdoing is real and seemingly connected to large-scale harm, liability remains tightly constrained by individualized proof of causation in fact.

IV. Complex Concepts Simplified

1. Proximate Cause

“Proximate cause” is a legal shorthand for two ideas:

  1. Cause in fact:
    • The defendant’s conduct must actually have produced the plaintiff’s injury.
    • This has two components:
      • But-for causation: The injury would not have happened without the defendant’s conduct.
      • Substantial-factor causation: The conduct was significant enough in the causal chain; it was not trivial or remote.
  2. Foreseeability:
    • A reasonable person, in the defendant’s position, could have anticipated that their conduct might create the type of injury suffered.

2. But-For vs. Substantial Factor

  • But-for causation asks: “If we imaginatively remove the defendant’s negligent act, would the harm still have occurred?” If yes, then the defendant is not a but-for cause.
  • Substantial factor asks: “Was the defendant’s conduct important enough in producing the harm that it is fair to hold them responsible?”

In Texas, plaintiffs must establish both aspects for cause in fact. Being a substantial factor alone is not sufficient if the injury would have occurred anyway.

3. Negligence Per Se

Negligence per se uses a statute or regulation to define what a reasonably prudent person must do. If:

  • The defendant violates a statute,
  • The statute was designed to protect against the type of harm that occurred, and
  • The plaintiff is within the class of persons the statute intends to protect,

then the statutory violation can substitute for the traditional “duty” and “breach” elements. However, proximate cause and damages must still be proven.

4. Nuisance (Negligent Nuisance)

“Nuisance” refers to a substantial and unreasonable interference with the use and enjoyment of land (e.g., noise, odor, flooding). But it is not always a separate tort concept:

  • If the interference arises from negligent conduct, then plaintiffs must prove:
    • Duty
    • Breach
    • Proximate cause (including but-for causation)
    • Damages

5. Expert Testimony Requirement

Courts require expert testimony when the issues are too technical for laypersons. Examples include:

  • Medical causation for complex diseases.
  • Hydrology and watershed modeling in multi-factor flood cases.
  • Structural engineering for building failures.

If expert testimony is required and the plaintiff’s expert does not connect the dots on causation, the case can fail as a matter of law.

V. Conclusion

Tenaris Bay City Inc. v. Ellisor is a significant reaffirmation and clarification of Texas causation law, especially in the context of catastrophic flooding and water-diversion disputes. Its key takeaways include:

  • But-for causation remains indispensable in Texas negligence cases, including negligence, negligent nuisance, and negligence per se under Water Code § 11.086.
  • Substantial-factor terminology does not relax the need for but-for causation; the two are jointly required elements of cause in fact.
  • Complex flooding cases typically require expert hydrologic testimony, grounded in detailed modeling and property-specific analysis, to establish that a defendant’s conduct was the difference between flooding and no flooding.
  • Catastrophic natural events are treated as the default cause of related damage; plaintiffs must bring strong evidence to show that human conduct, not just the storm, was the but-for cause of their particular loss.
  • Relaxed causation doctrines like those in Bostic are confined to rare, medically driven contexts and do not extend to hydrology or other fields where detailed analysis is possible, even if costly.

By reversing the court of appeals and rendering judgment for Tenaris, the Supreme Court of Texas sends a clear message: in the wake of historic natural disasters, liability against neighbors and upstream actors cannot rest on generalized fault or incremental contributions to flooding. It must rest on rigorous, reliable proof that, but for the defendant’s tortious conduct, the plaintiff’s property would have remained unharmed. That evidentiary discipline, the Court holds, is essential to maintaining the integrity and fairness of Texas tort law.

Case Details

Year: 2025
Court: Supreme Court of Texas

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