Storms, Speculation, and But‑For Causation: The Texas Supreme Court’s Strict Causation Standard in Tenaris Bay City Inc. v. Ellisor

I. Introduction

In Tenaris Bay City Inc. v. Ellisor, the Supreme Court of Texas delivered a significant opinion on causation in negligence, nuisance, and Water Code flooding claims arising from catastrophic weather events. The case stems from Hurricane Harvey’s devastation in 2017 and a group of homeowners’ attempt to attribute their property flooding to an allegedly defective drainage system at a nearby industrial facility.

The Court reversed a $2.8 million judgment for thirty homeowners against Tenaris Bay City Inc., the operator of a pipe fabrication plant in Matagorda County. Despite evidence that Tenaris’s facility likely contributed to additional localized flooding, the Court held that the plaintiffs failed to prove a foundational element of their claims: that their homes would not have flooded “but for” Tenaris’s alleged negligence.

Beyond its immediate result, the opinion lays down and sharpens several important principles:

• Texas’s proximate cause requirement—including the “but‑for” component—applies with full force even in large‑scale flooding cases arising from historic storms.
• “Substantial factor” causation is not a stand‑alone standard; it must be coupled with but‑for causation.
• In complex flooding scenarios involving widespread catastrophic rainfall, expert hydrological testimony is required to prove causation; lay testimony alone is insufficient.
• The same strict causation requirement applies equally to negligence, negligent nuisance, and negligence per se claims under Texas Water Code § 11.086.
• The Court resists any broad reading of earlier flooding and toxic‑tort precedents (such as Gragg and Bostic) that might relax but‑for causation outside their narrow factual contexts.

The decision thus becomes a leading authority on causation in disaster‑related property damage litigation in Texas, especially where plaintiffs seek to attribute their losses to nearby developments or industrial facilities rather than to the storm itself.

II. Background and Procedural History

A. Factual Setting

The plaintiffs are homeowners in Van Vleck and Bay City, both located in Matagorda County on the Texas Gulf Coast. Tenaris Bay City Inc. operates a pipe fabrication plant on land formerly used as a sod farm. The transition from permeable sod to largely impervious industrial surfaces raised concerns about increased runoff and flooding.

To address this, Tenaris hired Fluor Enterprises (an engineering and construction firm) to design and build a stormwater drainage system, including detention ponds and a surrounding earthen berm intended to control runoff. Jones & Carter, another engineering firm, reviewed and recommended approval of the plan, which the local drainage district then approved.

Hurricane Harvey struck in August 2017. Evidence at trial characterized Harvey as the most significant tropical cyclone rainfall event in U.S. history in terms of scope and peak rainfall since reliable records began in the 1880s. Matagorda County received approximately 21.6 inches of rain over four days and sustained over $500 million in property damage.

The thirty plaintiff homes were among the many flooded properties. The plaintiffs alleged that design flaws and construction/maintenance failures at the Tenaris facility increased runoff and caused their homes to flood.

B. Claims and Trial

The plaintiffs sued:

• Tenaris Bay City Inc. (facility owner/operator)
• Fluor Enterprises (design–build contractor)
• Jones & Carter (engineering firm that reviewed and recommended approval)

They asserted causes of action for:

• Negligence
• Gross negligence
• Negligence per se (based on Texas Water Code § 11.086)
• Negligent nuisance

Jones & Carter settled pre‑trial, and Fluor settled post‑trial. The remaining dispute at the appellate stage was between the homeowners and Tenaris.

Before trial, the parties agreed to divide the thirty properties into three geographic “zones” and try liability first. On causation, the plaintiffs relied heavily on the testimony of their expert, civil engineer Gabriel Novak.

Key trial rulings:

• The district court directed a verdict in Tenaris’s favor on gross negligence.
• The negligence, negligence per se, and nuisance theories went to the jury.
• The jury found Tenaris liable on all three theories for all three zones.
• Tenaris and the plaintiffs had stipulated total damages of $2.8 million, which the trial court awarded with interest.

C. Appellate History

The Fourteenth Court of Appeals (Houston) affirmed the judgment. It held that the evidence—particularly Novak’s expert testimony and supporting lay testimony—was legally sufficient to show that Tenaris’s design and drainage system were a “substantial factor” in bringing about the plaintiffs’ harm. (Ellisor in the court of appeals is reported at 704 S.W.3d 37.)

Tenaris petitioned for review to the Supreme Court of Texas, primarily contesting the causation element—focusing on both the proper legal standard (but‑for vs. substantial factor) and the sufficiency and reliability of the evidence.

III. Summary of the Supreme Court’s Opinion

Chief Justice Blacklock, writing for the Court (with two justices not participating), reversed the court of appeals and rendered judgment for Tenaris on all claims.

A. Core Holding

The Court held that:

• The plaintiffs failed to present legally sufficient evidence that Tenaris’s conduct was a but‑for cause of the flooding of their homes.
• The plaintiffs’ expert explicitly disclaimed the ability to opine on what caused flooding at any of the specific homes and admitted not having performed the hydrological analyses necessary to determine causation at that property‑specific level.
• Evidence that Tenaris’s facility contributed to “additional flooding” in the area is not, by itself, evidence that the specific thirty homes would have remained unflooded “but for” the Tenaris facility, especially given the magnitude of Hurricane Harvey.
• Because proximate cause (including but‑for causation) is required for negligence, negligent nuisance, and negligence per se under Water Code § 11.086, the failure of causation proof defeats all claims.

B. Key Legal Conclusions

1. Proximate cause includes both “substantial factor” and “but‑for” components. A cause‑in‑fact instruction that omits the but‑for component is incomplete, and plaintiffs must prove both.
2. Expert testimony was required on causation in this case. Given the complexity of flood behavior over flat terrain during a catastrophic storm, causation was beyond the common understanding of jurors and could not be established by lay testimony alone.
3. The plaintiffs’ expert testimony was legally insufficient and unreliable. Novak admitted he had not done the necessary property‑specific analysis, had performed no flooding analysis in the area, and had not attempted to exclude alternative plausible causes like Harvey itself.
4. But‑for causation applies equally to:
   • Ordinary negligence claims,
   • Negligent nuisance claims (governed by ordinary negligence principles), and
   • Negligence per se claims under Texas Water Code § 11.086.
5. Earlier cases (e.g., Gragg and Bostic) do not relax but‑for causation here. Gragg itself required but‑for proof, and Bostic’s relaxed standard is confined to rare toxic‑exposure scenarios where precise fiber‑by‑fiber causation is humanly impossible.
6. In catastrophic storm cases, the storm is the “natural, default explanation” for flooding. Plaintiffs seeking to attribute flooding to neighboring landowners must prove, with reliable evidence, that their damage would not have occurred absent the defendant’s tortious conduct.

Because the causation failure was dispositive, the Court did not reach or dwell on other elements such as duty or breach. It reversed and rendered, rather than remanding, concluding there was legally no evidence of but‑for causation sufficient to support the jury’s verdict.

IV. Detailed Analysis

A. The Core Legal Issue: Causation After a Historic Storm

The central question was framed starkly:

“The question presented is whether the plaintiffs failed to prove a basic element of their case—that the defendant, as opposed to merely the extraordinary rainfall combined with other environmental factors, caused their houses to flood.”

At bottom, the Court asks: when a historic storm floods thousands of properties, under what circumstances can one neighbor successfully claim that another neighbor’s land use or drainage system caused their particular flooding?

The Court’s answer is demanding:

• The mere fact that a facility added some volume or rate of water to an already overwhelmed drainage environment does not automatically translate into legal causation for any particular flooded property.
• To recover, plaintiffs must show that their specific property would have stayed dry (or materially less damaged) “but for” the defendant’s wrongful conduct.
• Especially where “thousands of properties across southeast Texas were damaged” by Harvey, causation demands property‑specific, scientifically grounded analysis rather than general assertions about increased flooding.

By emphasizing that pinning the consequences of an “act of God” on a neighbor is “no small thing,” the Court underscores an important normative theme: catastrophic natural events are the default explanation; liability is an exception that must be proved, not presumed.

B. The Law of Proximate Cause and Its Components

The Court situates its analysis within long‑standing Texas causation doctrine.

1. Proximate Cause and Cause in Fact

Citing Rodriguez-Escobar v. Goss and IHS Cedars Treatment Center v. Mason, the Court reiterates that proximate cause has two components:

• Cause in fact, and
• Foreseeability.

Only cause in fact is at issue here; foreseeability is not contested.

Drawing on Pediatrics Cool Care v. Thompson, Transcontinental Insurance Co. v. Crump, Ford v. Ledesma, IHS Cedars, and Rogers v. Zanetti, the Court clarifies that cause in fact itself has two required elements:

• But‑for causation: the injury would not have occurred without the defendant’s act or omission; and
• Substantial factor causation: the defendant’s conduct must be a substantial (not trivial or merely theoretical) factor in bringing about the harm.

Critically, both must be proved:

“The cause-in-fact standard thus ‘requires not only that the act or omission be a substantial factor but also that it be a but-for cause of the injury or occurrence,’ and ‘a cause-in-fact definition that omits the but-for component [is] “incomplete.”’”

The jury charge in this case adhered to this structure, stating that proximate cause required:

• Foreseeability; and
• “Actual causation,” defined as a cause that is a substantial factor in bringing about the injury “and without which cause such injury would not have occurred.”

2. The Court’s Concern with “Substantial Factor Alone”

On appeal, there was some suggestion that the court of appeals might have treated “substantial factor” as an independent, sufficient basis for causation. The Supreme Court reads the intermediate court charitably but makes clear that any such approach would be legally incorrect. The plaintiffs must satisfy both prongs.

This insistence reflects a broader line in Texas jurisprudence against diluting cause‑in‑fact requirements, particularly in complex or multi‑cause cases. The Court’s use of Rogers v. Zanetti reinforces this corrective: omitting “but‑for” from the causation test is doctrinal error.

C. Evaluation of the Expert Testimony

The Court’s causation analysis turns heavily on the testimony of plaintiffs’ expert, civil engineer Gabriel Novak.

1. What Novak Did—and Did Not—Do

Novak’s “fundamental opinion” was that the Tenaris drainage design:

• Relied on a “very large drainage basin,”
• Shifted “a lot of flow” to a specific point called “Outflow 1,” and
• Caused “additional flooding elsewhere in Van Vleck or Bay City.”

From this, he concluded that Tenaris’s drainage system exacerbated flooding in the general area. The Supreme Court accepts that this constitutes some evidence that the facility contributed to additional area‑wide flooding.

However, the causation question in this case is not whether Tenaris’s design caused some additional flooding somewhere, but whether it caused flooding at the thirty particular homes that are plaintiffs in this suit.

On that critical property‑specific question, Novak candidly admitted:

• He could not say what caused flooding at any specific plaintiff’s home.
• He had not conducted a “detailed analysis” of any plaintiff’s home.
• He had not performed “any flooding analysis whatsoever” in the area of Outflow 1.
• He had not done “a general flooding analysis at all about what happened in Van Vleck.”

He further explained that to determine what caused flooding at the plaintiffs’ homes, he would need to:

1. Study U.S. Geological Survey topographic maps,
2. Perform a detailed drainage analysis,
3. Create computerized hydrological models,
4. Consult Natural Resources Conservation Service soil maps, and
5. Study all relevant waterways.

He acknowledged that he had the capability to perform such an analysis but had not done so.

2. The Insufficiency of “A Factor” Testimony

At one point, Novak stated that the Tenaris system’s failures were “a factor” in the flooding of the plaintiffs’ properties. But when counsel pressed him to opine that Tenaris was a substantial factor or the but‑for cause, he did not do so. The Court emphasizes:

• He expressly refrained from offering “a definitive conclusion” about what caused any plaintiffs’ flooding.
• He admitted he could not even give “a general conclusion,” because he had not done the necessary work.
• He never testified that plaintiffs’ homes would not have flooded but for the Tenaris facility.

Thus, even in its most favorable light to plaintiffs, Novak’s testimony:

• Showed that Tenaris’s design contributed to more water in the area.
• Did not show how that contribution translated into flooding at specific addresses.
• Did not rule out Hurricane Harvey and other environmental factors as independent sufficient causes of plaintiffs’ flooding.

3. Excluding Alternative Causes: Reliability Requirements

The Court links this evidentiary gap to a line of cases (notably Helena Chemical Co. v. Cox, JLG Trucking, LLC v. Garza, and Crump) emphasizing that expert causation testimony is unreliable if the expert does not reasonably exclude other plausible causes.

“As we have stated before, expert testimony on causation is fundamentally unreliable if the expert fails to exclude other plausible causes with reasonable certainty.”

Here, Novak “made no attempt at all” to exclude:

• Harvey’s exceptional rainfall itself, or
• Other regional environmental factors (pre‑existing drainage, subsidence, soil conditions, other development, etc.)

as independent causes of plaintiffs’ flooding. That omission is fatal because the storm and background environmental conditions are, in the Court’s words, the “natural, default explanation” for flooding in this context.

When an expert admits, under oath, that he has not conducted the analyses he himself identifies as necessary to answer the core causation question, his opinion cannot supply “more than a scintilla” of evidence on that element. The Court therefore characterizes the verdict as resting on “speculation, not on evidence.”

D. Application to Each Cause of Action

1. Negligence

The elements of negligence are standard: duty, breach, and damages proximately caused by the breach. The dispute here is limited to proximate cause. The jury charge correctly defined proximate cause to include both foreseeability and actual causation, with an explicit but‑for component.

Because the Court finds legally no evidence of but‑for causation, the negligence claims fail outright.

2. Negligent Nuisance

Under Crosstex North Texas Pipeline, L.P. v. Gardiner, an actionable nuisance may be based on:

• Intentional conduct,
• Negligent conduct, or
• In some limited contexts, strict liability for ultrahazardous activities.

The plaintiffs’ nuisance theory was explicitly negligent in nature, so it is “governed by ordinary negligence principles,” including proximate cause.

The nuisance portion of the jury charge mirrored the negligence charge, requiring that the nuisance be a substantial factor “and without which cause such injury would not have occurred.”

Because the plaintiffs’ nuisance theory rests on the same causation framework and factual evidence as their negligence claims, the same failure of but‑for causation is “just as fatal” to nuisance as to negligence.

3. Negligence Per Se under Texas Water Code § 11.086

Section 11.086 of the Water Code 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.

“Negligence per se” replaces the ordinary duty/breach inquiry with the question whether a safety statute or regulation was violated. But Texas law—illustrated by Missouri Pacific Railroad v. American Statesman—generally preserves ordinary proximate‑cause requirements even in negligence per se claims.

The text of § 11.086 reinforces causation explicitly:

• Liability is tied to an “overflow of water caused by an unlawful diversion or impounding.”
• Recoverable damages are those “occasioned by the overflow.”

Intermediate appellate authority, such as Contreras v. Bennett and Benavides v. Gonzalez, has interpreted the statute to require proof that damages “would not have resulted but for” the unlawful diversion.

The jury charge in this case echoed that requirement, instructing that any diversion or impounding “must have caused damages to Plaintiffs’ properties and without which cause such injury would not have occurred.”

Accordingly, the same causation deficiency undermines the plaintiffs’ Water Code‑based negligence per se claim. Showing that Tenaris unlawfully diverted or impounded water is not enough; plaintiffs also had to prove that this diversion was a but‑for cause of the flooding at their specific homes. On the Court’s view of the record, they did not.

E. The Role of Lay Testimony and the Necessity of Experts

The plaintiffs and the court of appeals argued that causation could be established, at least in part, through lay testimony—for example, homeowners’ observations that their properties had never previously flooded, even during heavy storms, and their perception that Tenaris’s facility changed local drainage patterns.

The Supreme Court rejects that in this particular case.

1. When Expert Testimony Is Required

Relying on Mack Trucks, Inc. v. Tamez, the Court reiterates:

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

Here, both counsel and the trial court agreed pre‑trial that expert testimony was required to prove causation. Novak’s own explanation of the analyses he would need to perform underscored that point: topographic analysis, computer modeling, soil mapping, and hydrologic study are not within ordinary juror experience.

The Court distinguishes situations where lay testimony alone might suffice, such as:

• Simple car accidents, where the mechanism of impact and resulting harm is obvious.
• Certain straightforward flooding cases where the path of water and the cause of overflow are evident (e.g., a single blocked culvert immediately adjacent to a plaintiff’s property).

In contrast, this case involved:

• Thousands of flooded properties across a wide region,
• Relatively flat coastal terrain,
• Complex water movement dictated by small elevation differences, channels, and varied soil permeability, and
• The overlay of a historic, unprecedented storm.

Given this complexity, the Court concludes that no reasonable factfinder could reliably determine but‑for causation for specific homes without expert hydrological analysis.

2. Consistency with City of Keller v. Wilson

The Court analogizes to City of Keller v. Wilson, a precedent also involving alleged increased flooding due to real estate development. In Keller, the Court held that determining whether detention ponds and a long grassy drainage ditch altered flooding patterns required expert testimony, because it involved hydrological formulas, computer models, and complex calculations.

Just as in Keller, the Court here views expert analysis as essential to causation. Novak’s own testimony about the analyses he did not conduct effectively confirms that the issue is beyond lay comprehension.

3. Distinguishing Gragg

The plaintiffs leaned heavily on Tarrant Regional Water District v. Gragg, another flooding case in which a ranch owner claimed that dam operations downstream caused damaging floods.

They advanced two arguments:

1. Gragg allegedly permitted causation to be proved without strict but‑for analysis.
2. Gragg could be read to allow causation to be shown largely through lay testimony.

The Court rejects both readings:

• On but‑for causation: Gragg explicitly required the plaintiff to prove “that the same damaging floods would not have occurred under the same heavy rainfall conditions had the dam not been constructed.” Gragg thus supports, rather than weakens, the but‑for requirement.
• On the role of experts: Even if lay testimony played a significant role in Gragg, that case involved a relatively simple hydraulic path (release from a single floodgate down a single river channel to a specific ranch). By contrast, the hydrology here is multivariate and diffuse. The Court therefore sees no inconsistency: what may be accessible to lay inference in one setting is not necessarily so in another.

F. Bostic and the Limits of Relaxed But‑For Causation

The Court also addresses Bostic v. Georgia-Pacific Corp., a leading Texas toxic‑tort case involving asbestos exposure. In Bostic, the Court recognized the difficulty of proving which specific asbestos fibers from which source caused a plaintiff’s disease and adopted a more flexible approach to but‑for causation in that narrow context.

In Bostic, the Court held that plaintiffs do not have to prove exactly which fibers from which defendant caused the disease when doing so is “not humanly possible,” but they must meet rigorous dosage and exposure standards under a careful causation framework.

In Tenaris, the Court emphasizes:

• Bostic is an exception, not a new default rule.
• It applies only in “rare cases” where pinpointing causation is literally impossible given the science and the nature of the injury.
• This case is not such a situation. By Novak’s own admission, a property‑specific hydrologic causation analysis was possible—just not undertaken.

Accordingly, the Court declines to relax but‑for causation. If expert methods exist to trace and quantify the contribution of a facility’s runoff to particular properties, the plaintiff must use them or live with the consequences of failing to do so.

G. Negligence vs. Causation: Evidence of Fault Is Not Evidence of But‑For Cause

The plaintiffs emphasized, and the record apparently contained, substantial evidence that:

• A portion of the berm was constructed one foot lower than the design specified (43 feet instead of 44 feet).
• Tenaris performed inadequate maintenance on components of its drainage system.

The Court is clear that even “vivid proof” of negligence is not proof of causation:

“Proof that the defendant was negligent—even vivid proof that the defendant was woefully negligent—is not proof that the plaintiff's injury would not have occurred but for the defendant's negligence.”

This distinction is crucial in disaster‑related cases:

• It prevents factfinders from equating “bad behavior” with “responsibility for this particular harm” without a causal bridge.
• It signals that liability must be grounded in evidence that the negligence made the difference between harm and no harm, not merely that it increased risk in the abstract.

The Court gives the example of car crashes, where the same evidence often establishes both breach and causation. But flooding in the context of a historic storm is markedly different; many homes would have flooded regardless of any particular actor’s negligence. That makes the causal bridge far more demanding.

H. Prior Non‑Flooding as Insufficient Causation Evidence

The plaintiffs testified that their properties had never flooded before, even during heavy rains comparable (in their experience) to Harvey. The Court acknowledges this evidence but deems it insufficient to establish causation:

• Coastal flood risk changes over time due to incremental development, changes in impervious cover, soil compaction/subsidence, and climatic variation.
• It is neither surprising nor legally conclusive that a neighborhood’s “first flood” might occur during an extraordinary storm like Harvey.
• Without a scientific causation analysis, the fact of “no prior flooding” is at best suggestive and cannot answer the but‑for question in a legally reliable way.

Thus, a plaintiff cannot bootstrap from “never flooded before” to “this neighbor caused my first flood,” especially in the face of a widely destructive natural event.

V. Impact and Implications

A. Heightened Causation Burden in Catastrophic Flood Cases

The opinion draws a bright line: in large‑scale storm events, the default explanation for flooding is the storm itself. Plaintiffs who wish to attribute their losses to a particular landowner, developer, or industrial facility must:

• Produce reliable expert hydrological evidence.
• Demonstrate that, absent the challenged facility or design, their specific property would not have flooded—or at least would have suffered materially less damage.
• Address and reasonably rule out the storm and other environmental causes.

This places a substantial—and expensive—burden on future plaintiffs. It effectively requires:

• Retaining qualified civil or hydrological engineers.
• Performing detailed site‑specific modeling, including elevation, flow, and soil data.
• Tracing the path and volume of water attributable to each defendant’s property or facility.

Without such analysis, plaintiffs risk having their claims dismissed on no‑evidence grounds even after jury verdicts in their favor.

B. Effects on Negligence, Nuisance, and Water Code § 11.086 Litigation

The decision has unified implications across three related theories:

1. Negligence: Plaintiffs must meet full but‑for causation standards in flood and stormwater disputes, not merely show increased flood risk or area‑wide impacts.
2. Nuisance (negligent): Because negligent nuisance is governed by ordinary negligence principles, nuisance claims cannot be used as a doctrinal “work‑around” to evade rigorous causation requirements.
3. Negligence per se under § 11.086: Alleging an unlawful diversion or impoundment of water does not soften the causation standard; plaintiffs must still prove that the unlawful activity caused water to overflow onto their property in a but‑for sense.

This integrated approach limits the possibility of plaintiffs relabeling claims in order to avoid strict causation proof.

C. Litigation Strategy for Plaintiffs and Defendants

For plaintiffs:

• Early and thorough expert engagement is essential. Plaintiffs must demand, and experts must perform, property‑specific analyses tying the defendant’s runoff to the plaintiff’s damage.
• Experts must be prepared to testify not only that the defendant’s conduct contributed to additional flooding but that it was a substantial + but‑for cause of the plaintiff’s specific damage.
• Any admission that the expert has not performed necessary analyses (and thus cannot opine on property‑specific causation) will likely be outcome‑determinative.

For defendants:

• They will focus heavily on expert depositions, pressing whether plaintiffs’ experts have done site‑specific modeling and whether they can rule out the storm as a sufficient cause.
• They will cite Tenaris to argue that generalized opinions about “increased flooding” or “a factor” are inadequate.
• They may seek summary judgment or judgment as a matter of law early if plaintiffs’ experts concede lack of property‑level analysis.

D. Public Policy Dimensions

The opinion implicitly reflects a policy judgment about allocating losses from catastrophic natural events:

• Absent compelling evidence, courts will not shift these losses from property owners (and their insurers) to neighboring landowners or industrial actors.
• Even in the face of serious engineering or maintenance failures, liability requires proof that those failures altered the “bottom line” outcome for the plaintiff in a storm that would otherwise have inundated many properties.

To some, this will seem protective of developers and industrial facilities, raising the evidentiary bar to a level that is difficult for individual homeowners to meet. To others, it prevents speculative and hindsight‑driven assignments of blame in the wake of disasters where nature is the primary driver.

Either way, Tenaris makes clear that, in Texas, robust causation analysis is non‑negotiable in disaster‑related tort litigation.

VI. Complex Concepts Simplified

A. Proximate Cause vs. Cause in Fact

• Cause in fact asks: Did the defendant’s conduct actually contribute to the injury in a legally meaningful way?
• Proximate cause = cause in fact + foreseeability.
• Foreseeability asks: Was the general type of harm reasonably predictable as a result of the defendant’s actions?

B. “But‑For” Causation

“But‑for” causation means:

“Without the defendant’s act or omission, the harm would not have occurred.”

It is a counterfactual test: imagine the world as it would have been if the defendant had acted properly. If the plaintiff would have suffered essentially the same harm anyway, then the defendant is not a but‑for cause.

C. “Substantial Factor” Causation

Even if the defendant is a but‑for cause, the law also requires that the conduct be a “substantial factor” in bringing about the harm. This prevents liability for trivial or remote contributions. In multiple‑cause situations, more than one party can be a substantial factor.

In Texas, cause in fact normally requires both:

• Defendant’s conduct was a substantial factor, and
• The harm would not have occurred but for that conduct.

D. Negligence Per Se

Negligence per se occurs when:

• A statute or regulation sets a standard of conduct.
• The defendant violates that statute.
• The statute is designed to protect the class of persons and type of harm involved.

The violation substitutes for proof of “breach of duty.” But plaintiffs still must prove:

• Duty (via the statute),
• Causation (including but‑for causation), and
• Damages.

E. Nuisance (Negligent)

An actionable nuisance is a condition that substantially interferes with the use and enjoyment of land (e.g., flooding, odors, noise). When based on negligence:

• The plaintiff must show that the defendant negligently created or maintained the condition.
• The same causation standards for negligence apply (proximate cause, including but‑for and substantial factor elements).

F. Expert vs. Lay Testimony

• Lay testimony consists of what ordinary witnesses perceive (what they saw, heard, experienced).
• Expert testimony involves specialized knowledge beyond average jurors’ understanding, such as engineering, medicine, or hydrology.

Courts require expert testimony when:

• The causal mechanism is scientifically complex.
• Understanding it requires technical data, modeling, or specialized training.

Flooding patterns across varied terrain during historic storms are typically in that category.

VII. Conclusion

Tenaris Bay City Inc. v. Ellisor is a major modern statement by the Supreme Court of Texas on the law of causation in disaster‑related property damage cases. It reaffirms and sharpens several key doctrines:

• Proximate cause demands both substantial‑factor and but‑for causation; substantial factor alone is legally insufficient.
• Catastrophic natural events like Hurricane Harvey are the “default explanation” for flooding; plaintiffs bear the burden of proving that a defendant’s wrongful conduct made the critical difference between flooding and no flooding at their specific property.
• In complex flooding cases, expert hydrological evidence is not optional. Plaintiffs cannot rely on lay impressions or generalized statements that a facility “added to flooding.”
• The rigorous causation requirement applies uniformly to negligence, negligent nuisance, and negligence per se under Texas Water Code § 11.086.
• Relaxed causation standards, such as those described in Bostic, are confined to rare contexts where conventional but‑for proof is scientifically impossible—not merely difficult or expensive.

By reversing a substantial jury verdict and rendering judgment for the defendant, the Court sends a clear message: in Texas tort law, even against the backdrop of extraordinary natural disasters, liability must rest on solid, scientifically grounded proof of causation—never on speculation or inference untethered from rigorous expert analysis. This opinion will serve as a central reference point in future litigation over flood damage, land development, and infrastructure design across the state.