Author ORCID Identifier

https://orcid.org/0000-0001-6778-5779

Date Available

12-11-2025

Year of Publication

2025

Document Type

Master's Thesis

Degree Name

Master of Civil Engineering (MCE)

College

Engineering

Department/School/Program

Civil Engineering

Faculty

Lindell Ormsbee

Faculty

Mei Chen

Abstract

The Soil Conservation Service (SCS) Curve Number (CN) method is widely used to estimate direct runoff, and its TR-55 urban-adapted Curve Numbers are commonly applied in stormwater design. However, the underlying assumptions of the method have been the subject of contemporary review, and concerns remain that these values underestimate runoff in actual urban watersheds. This study evaluates the performance of TR-55 CNs using rainfall–runoff data from twenty real-world urban watersheds in the National Stormwater Quality Database.

Empirical Curve Numbers were derived using two calibration techniques, the asymptotic CN method (CN∞) and a nonlinear least-squares optimization method (CNLS), and then compared to CNs derived from the tables provided in the TR-55 methodology. Runoff predicted using each CN was statistically compared to observed runoff.

Results show that TR-55 CNs are consistently lower than empirically derived CNs and systematically underpredict runoff. CNLS produced runoff estimates that closely matched observations and outperformed both TR-55 tabular CNs and CN∞. An exploratory effort to disaggregate composite CNs into pervious and impervious components was attempted, but proved unstable, indicating such decomposition is not feasible for the studied watersheds.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2025.581

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