DESIGN OF APRONS
For the downstream concrete block apron with open joints filled with
spawls,length1.5d 2 9m.Use cast in situconcrete blocks of 1 m1.5 m1 m deep, laid open jointed
over a graded inverted filter construction (design details later).
For the downstream flexible apron of broken stone,length2.5d 2 15 m.Volume contents per metre length2.63d 2 2.63 6 15.78 m^3 m^1.
Therefore the thickness of the apron15.78/151.05 m. The minimum
stone size is given by (US Army Waterways Experimental Station, 1959)dkV^2 /2g∆,whereVis the mean velocity, ∆
(^) s
(1.65) and k1 for calm flow
and 1.4 for highly turbulent flow, or (Peterka – see Bos, 1976)
d0.032)1/2V9/4.
The mean velocity, V1800/(2006)1.5 m s^1 , and therefore the stone
size,d0.1 0.15 m.
Select stones that are almost cubical in shape with a size of around
150 mm or more (not flat slabs) for the downstream flexible apron.
For the upstream impervious concrete block apron,
length1.5d 1 5m.
Use cast in situconcrete blocks 1 m1.5 m0.6 m deep.
UPSTREAM FLEXIBLE APRON
For this apron,
length 2 d 1 6m.
The volume contents per metre length2.25d 1 2.25 3 6.75 m^3 m^1.
Therefore, the thickness of the apron6.75/61.15 m. Adopt the same
graded stone as recommended for the downstream apron.
For the inverted filter design, the layers in a filter should be such that
the filter is as follows (see also Section 2.6.3):