Model investigation into the hydraulic performance of mattress protected slopes of low head hydraulic structures

Abstract

The aim of this study was to provide some information about the
hydraulic performance of mattresses protecting slopes of low head
hydraulic structures. The flow resistance as well as stability of
slope were investigated.

The experimental programme was conducted in the Chilworth
Hydraulics Laboratory, Department of Civil Engineering. Two
models were constructed and tested. The inpermeable model (first)
had a slope of 1:7 (V;H) with mattress dimensions of 320x350mm
and a height of 40mm. The permeable model (second) was built with
the same slope except that the mattresses were protecting a sand
core.

The flow resistance of the mattresses was studied which included
Manning's roughness coefficient, Chezy's resistance factor and
Darcy-Weisbach friction factor. Velocity distribution above the
mattresses was evaluated and found well describing the flow.
The stability of the mattresses and their ability to resist
failure when exposed to higher discharges were also examined and
found stable under the discharges tried.

Detailed measurement of pressures were conducted in the permeable
model in order to study their effects on the stability of the
mattresses; such as; pressure distribution above the mattresses,
uplift pressures and seepage forces. Shear stresses due to
seepage forces were found very small and could be neglected.
Comparison with the results of smooth board simulating a
reinforced concrete structure, favours the mattresses as a means
to control erosion and dissipate energy efficiently.

Measures were suggested in terms of installation to reduce the
drag and uplift forces. A design procedure for a slope protected
by mattresses was set up and followed by a numerical example
implementing the results obtained in this study.
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