Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Civil Engineering

Committee Member

Dr. Nigel B. Kaye, Committee Co-Chair

Committee Member

Dr. Abdul A. Khan, Committee Co-Chair

Committee Member

Dr. Earl J. Hayter

Committee Member

Dr. Nadarajah Ravichandran


This dissertation presents results of a computational investigation into the discharge characteristics of two stormwater runoff management approach, i.e., a perforated pipe-aggregate underdrain system, a common setup used in various Low Impact Development (LID) strategies and Best Management Practices (BMPs), and a circular pipe free overfall. A three-dimensional model of a perforated pipe-aggregate underdrain system was developed and validated using previously published experimental results for saturated subsurface flow (flow where the water surface is above the top of the aggregate) for a 10.2 cm perforated pipe shrouded in loose laid aggregate. Results showed that for the saturated case, the orifice flow approximation was valid; for the unsaturated case (water surface level is below the top of the aggregate level), energy losses in the aggregate layer were significant and the orifice approximation was not valid. The effects of several controlling geometric parameters, i.e., aggregate depth over the pipe, trench width, total head, pipe length, pipe wall perforation area per unit length of pipe, and the area of individual perforations on discharge characteristics of pipe-aggregate system were also investigated. For any combinations of these geometric parameters, there was a finite length of pipe, after which discharge did not increase with increasing pipe length. That length was defined as the critical length and was found to be sensitive to changes in pipe geometry only. A non-dimensional equation was proposed for predicting the peak discharge coefficient for porous pavements and infiltration trenches that use perforated pipe underdrains. The discharge characteristics of a free overfall from a smooth, horizontal circular pipe was also investigated. A free overfall can be used as a simple discharge measuring approach and is also common as an outflow condition for storm sewers. Based on the characteristics of flow, two different flow regimes i.e. cavity outflow flow and bubble washout flow were investigated. A constant End Depth Ratio (EDR) was found for the cavity outflow regime but it varied linearly with dimensionless critical depth for the bubble washout flow. The limiting discharge for a pipe flowing full and the cavity outflow, and bubble washout regimes has been established. Several important parameters, i.e., Froude number, pressure and momentum coefficients at the upstream and brink sections, and the minimum slope of the water surface behaved differently in the two flow regimes. However, the non-dimensional pressure distribution at the brink section showed same trend for both flow regimes. An expression for predicting discharge in the bubble washout flow regime has been proposed incorporating appropriate pressure and momentum coefficients and shows very good agreement with the computational data and available experimental data. Possible reasons of transition between cavity outflow and bubble washout flow was also explained. Findings from this dissertation have practical applications in design and analysis of porous pipe underdrain-aggregate systems as well as in flow rate control and improving the design methods of urban drainage facilities.