Question

Identify the significance of the egg shaped ovoid sewer design and discuss the development of sewer design and construction that led to the present day solutions?

Answer 1

Egg-shaped pipes appear as a suitable geometry for combined sewer sewage networks.

Egg-shaped conduits present higher resistance against traffic loads than conventional circular pipes.

In addition, this kind of pipe also shows a better hydraulic performance in normal operation dry

weather conditions of combined sewer systems, in which a high percentage of the time the flow

discharge is conveyed by the lower part of the section. In these conditions, egg-shaped pipes present

higher flow velocities due to their smaller wetted perimeter, reducing the sedimentation of particles

and the sewer cleaning operational costs.

The resuspension of sewer sediments during wet weather

flows is an important source of the pollution of Combined Sewer Overflows , and their control is

one of the main objectives of the integrated urban water management in urban systems.

In spite of the structural and hydraulic advantages, egg-shaped pipes are not commonly used

in the construction of small combined sewer systems because of their highest production costs.

Nevertheless, with the evolution of production techniques such as plastic injection or extrusion,

the fabrication costs of plastic egg-shaped pipes can be as competitive as circular plastic pipes. In this

work we present the first stage of the collaborative OvalPipe R&D project that aims to develop a new

functioning egg-shaped plastic pipe that is commercially viable and market competitive with the

300–400 mm diameter circular pipes.

The first steps of the process consisted in the geometric definition and in the hydraulic analysis of

the egg-shaped cross section. The egg-shaped geometry was designed with the objective of maximizing

the hydraulic radius under low flow conditions and the discharge capacity under full-depth or near

full-depth conditions. Once the cross-section was defined, a real-scale egg-shaped pipe was built at

a laboratory facility to study its hydraulic characteristics.Most of the open-channel pipe flow studies were performed in circular conduits. For instance, the

early studies of turbulence developed by Nezu and Nakagawa. proposed different formulations to

describe velocity profiles in circular cross-sections. Guo et al. developed new velocity distribution

formulas for circular, elliptic, parabolic, and hyperbolic open-channels (hereinafter named as conic

open-channels). Particle Image Velocimetry (PIV) technique was also developed to determine

velocity distribution in small circular pipes . Nevertheless, detailed hydrodynamic experiments for

egg-shaped pipes are missing.

In order to analyze the behavior of the circular and egg-shaped pipes, open-channel flow

experiments were conducted with ANSYS CFX Computational Fluid Dynamics (CFD) code. To simulate

the open-channel flow in closed conduits such as pipes, a two-phase flow model was developed to

solve the interactions between liquid (water) and gas (air) interface. The experimental velocity

profiles and shear stress values were compared with the numerical results, following the methodology

proposed in previous studies. Finally, numerical results from egg-shaped and circular pipe analysis

were also compared with the analytical open-channel flow Manning and Thormann-Franke equations.