Page 134 - Water and wastewater engineering

INTAKE STRUCTURES 3-31

Slope. The gallery can be horizontal, but a slight slope of 500:1 will minimize air binding/
blockage (Kawamura, 2000).

Pipe Velocity. At design capacity, the velocity is limited to 1 m/s at the outlet of the gallery.

Backfilling. Because the Ranney well screens are bored into the natural aquifer material, there
is no backfilling required. The pipe laid in the gallery is backfilled with gravel and sand similar
to that used in filter bed gradation (see Chapter 11 for details).

Junction Wells. At junction points in the gallery where the pipe changes direction and at the
end of the gallery, a well is provided. The recommended minimum diameter is 1 m (Kawamura,
2000).

Regulatory Considerations. In addition to the withdrawal restrictions that may be imposed,
the water removed from the infiltration gallery may be considered under the influence of the sur-
face water and thus subject to water quality regulations that apply to surface water. Nonetheless,
the filtering action of the ground improves the quality of the water and makes it easier to treat.

3-4 OPERATIONAL CONSIDERATIONS
Grit
Abrasive materials such as silt, sand, and shells is called grit. This material is very abrasive and
will result in excessive wear to mechanical equipment as well as potentially settling out during
low-demand flows.
When the source water bottom is soft and subject to scour or when flood conditions result in
a high sediment load and turbidity, grit chambers are provided. The grit chamber is a horizontal
flow settling tank ( Figure 3-19 ). The theoretical design basis of the grit chamber is Stokes’ law
which is discussed in Chapter 10.
The design objective of the grit chamber is to remove sand or silt particles greater than 0.1 mm
in diameter. Generally, particles smaller than this do not pose a hazard to pumps and pipelines.
The design objectives for raw water grit chambers are significantly different from those used for
wastewater treatment. Thus, wastewater treatment plant designs should not be used for raw water
grit removal. Typical design criteria are listed in Table 3-9 .

Ice
In cold regions, ice in its various forms is of concern. Surface ice and ice floes create struc-
tural hazards to exposed intakes. Selection of submerged intake alternatives alleviates this
problem.
Frazil ice is small, disk-shaped ice crystals (Foellmi, 2005). It has also been described as
slush. Frazil ice adheres to surfaces such as intake screens and pipe walls. The reduction in area
lowers the flow rate into the intake structure and ultimately will plug it. Frazil ice forms when
turbulent water is supercooled.