Pipes in water distribution systems may change as they age. The accumulation of corrosion byproducts and suspended particles on the inside wall of aged pipes can increase pipe roughness and reduce pipe diameter. To quantify the hydraulic effects of irregular accumulation on the pipe walls, eleven aged pipes ranging in diameter from 0.020-m (0.75-in) to 0.100-m (4-in) and with varying degrees of turberculation were located and subjected to laboratory testing. The laboratory test results were used to determine a relationship between pipe diameter reduction and Hazen-Williams C. This relationship, combined with a manipulation of the Hazen-Williams equation, provided a simple and direct method for correcting the diameters of aged pipes in distribution models. Using EPANET 2, the importance of correcting pipe diameters when modeling water distribution systems containing aged pipes was investigated. Correcting the pipe diameters in the sample network reduced the modeled water age by up to 10% and changed the pattern of fluctuating water age that occurred as waters with different sources moved through the pipe network. In addition, two of the aforementioned aged pipes with diameters of 0.025-m (1-in) and 0.050-m (2-in) were modeled using Reynolds-Averaged Navier-Stokes (RANS) turbulence modeling. Flow was computed at Reynolds numbers ranging from 6700 to 31,000 using three turbulence models including a 4-equation v2-f model, and 2-equation realizable k-e; and k-ω models. In comparing the RANS results to the laboratory testing, the v2-f model was found to be most accurate, producing Darcy-Weisbach friction factors from 5% higher to 15% lower than laboratory-obtained values. The capability of RANS modeling to provide a detailed characterization of the flow in aged pipes was demonstrated. Large eddy simulation (LES) was also performed on a single 0.050-m (2-in) pipe at a Reynolds number of 6800. The Darcy-Weisbach friction factor calculated using LES was 20% less than obtained from experimental tests. Roughness elements smaller than the grid scale and deficiencies in the subgrid-scale model at modeling the complex three-dimensional flow structures due to the irregular pipe boundary were identified as likely sources of error. Even so, the utility of LES for describing complex flows was established.