[Paper Review] A Safety Factor Approach to Designing Urban Infrastructure for Dynamic Conditions

Sanjib Sharma, Ben Seiyon Lee+2 more|arXiv (Cornell University)|Feb 8, 2021

Flood Risk Assessment and Management56 references22 citations

TL;DR

This paper proposes a safety factor approach to design urban stormwater infrastructure under deep uncertainties in climate change, impervious surfaces, and infrastructure lifetime. By quantifying nonstationary rainfall projections and uncertainty propagation, it shows that safety factors of 1.4–1.7 above standard design guidance are needed to achieve 1/100-year hydraulic reliability, significantly increasing upfront costs but improving resilience to climate and socioeconomic change.

ABSTRACT

Current approaches to design flood‐sensitive infrastructure typically assume a stationary rainfall distribution and neglect many uncertainties. These assumptions are inconsistent with observations that suggest intensifying extreme precipitation events and the uncertainties surrounding projections of the coupled natural‐human systems. Here we demonstrate a safety factor approach to designing urban infrastructure in a changing climate. Our results show that assuming climate stationarity and neglecting deep uncertainties can drastically underestimate flood risks and lead to poor infrastructure design choices. We find that climate uncertainty dominates the socioeconomic and engineering uncertainties that impact the hydraulic reliability in stormwater drainage systems. We quantify the upfront costs needed to achieve higher hydraulic reliability and robustness against the deep uncertainties surrounding projections of rainfall, surface runoff characteristics, and infrastructure lifetime. Depending on the location, we find that adding safety factors of 1.4–1.7 to the standard stormwater pipe design guidance produces robust performance to the considered deep uncertainties. The insights gained from this study highlights the need for updating traditional engineering design strategies to improve infrastructure reliability under socioeconomic and environmental changes.

Motivation & Objective

• To address the limitations of stationary climate assumptions in current stormwater infrastructure design guidelines.
• To quantify deep uncertainties in extreme rainfall projections, surface imperviousness, and infrastructure lifetime affecting hydraulic reliability.
• To evaluate the impact of neglecting these uncertainties on flood risk and design robustness.
• To determine the safety factor required to achieve 1/100-year hydraulic reliability under nonstationary conditions.
• To inform updated engineering design strategies for climate-resilient urban infrastructure.

Proposed method

• A cumulative uncertainty framework is used to quantify the relative contribution of each uncertainty source—climate, runoff characteristics, and service life—to overall reliability variance.
• The method computes marginal cumulative uncertainty at each stage using variance-based decomposition of reliability estimates across multiple models and scenarios.
• Reliability is modeled as a function of extreme rainfall projections from climate models, runoff coefficients, and pipe lifespan under nonstationary conditions.
• Safety factors are applied to standard pipe design guidance to achieve target reliability, with cost implications quantified.
• The approach uses variance-based uncertainty decomposition to avoid limitations of ANOVA, such as normality assumptions and sensitivity to outliers.
• The analysis is applied across multiple locations using NA-CORDEX climate model outputs and statistical downscaling.

Experimental results

Research questions

• RQ1 How do deep uncertainties in climate projections, surface runoff, and infrastructure lifetime affect the hydraulic reliability of stormwater drainage systems?
• RQ2 What is the relative contribution of climate uncertainty compared to socioeconomic and engineering uncertainties in driving design risk?
• RQ3 What safety factor magnitude is required to achieve 1/100-year hydraulic reliability under nonstationary climate conditions?
• RQ4 How do neglecting deep uncertainties lead to underestimation of flood risk and poor infrastructure design choices?
• RQ5 What are the cost implications of applying safety factors to achieve robust performance under deep uncertainty?

Key findings

• Climate uncertainty is the dominant driver of hydraulic reliability uncertainty in stormwater drainage systems, outweighing socioeconomic and engineering uncertainties.
• Applying safety factors between 1.4 and 1.7 to standard stormwater pipe design guidance achieves 1/100-year hydraulic reliability across all considered cases.
• Neglecting deep uncertainties can drastically underestimate flood risks, leading to inadequate infrastructure performance under future climate conditions.
• The required safety factors result in sizable additional upfront costs, reflecting the trade-off between resilience and economic efficiency.
• The cumulative uncertainty approach effectively quantifies uncertainty propagation and identifies key uncertainty sources without relying on restrictive assumptions like normality.
• The study demonstrates that traditional stationary design assumptions are insufficient for future-proof urban infrastructure under climate change.