Variation of Manhole Minor Head Loss Coefficient in Sub-surface Drainage Network and its Influence on Water Surface Profile

Manholes are commonly used in subsurface drainage networks while the hydraulic characteristics of
this structure for design purposes and for determining the discharge capacity of the network have not
investigated yet. In this research a prototype scale experimental setup was prepared using drainage
corrugated pipes in order to determine the minor head loss coefficient of the manhole and to
investigate its influence on the water surface profile. Data was gathered for a wide range of pipe
longitudinal slopes and main pipe to lateral pipe discharge ratios. The data indicated that when the
discharge ratio tends to unity the head loss coefficient decreases to a constant value. AccordinglyI an
average value was proposed for practical design purposes. The impact of applying such average value
on the water surface profile directly upstream of the manhole was investigated and reported herein.
The results indicated that applying such average value predicts the water surface elevation within a
maximum error of ±15%. Higher errors belong to lower discharge values which are not of practical
importance. ThereforeI the application of proposed average head loss coefficient is recommended for
design procedures.