Science of Water : Concepts and Applications

30 The Science of Water: Concepts and Applications
natural storage sites (such as lakes, muskeg, and tundra ponds) or in man-made storage areas such
as dams. Man-made dams are either masonry or embankment dams. If embankment dams are used,
they are typically constructed of local materials with an impermeable clay core.
SURFACE WATER INTAKES
Withdrawing water from a river, lake, or reservoir so that it may be conveyed to the fi rst unit of
treatment process requires an intake structure. Intakes have no standard design and range from a
simple-pump suction pipe sticking out into the lake or stream to expensive structures costing several
thousands of dollars. Typical intakes include submerged intakes, fl oating intakes, infi ltration galleries,
spring boxes, and roof catchments. Their primary functions are to supply the highest quality
water from the source and to protect piping and pumps from clogging as a result of wave action,
ice formation, fl ooding, and submerged debris. A poorly conceived or constructed intake can cause
many problems. Failure of the intake could result in water-system failure.
On a small stream, the most common intake structures used are small gravity dams placed
across the stream or a submerged intake. In the gravity dam type, a gravity line or pumps can
remove water behind the dam. In the submerged intake type, water is collected in a diversion and
carried away by gravity or pumped from a caisson.
Another common intake used on small and large streams is an end-suction centrifugal pump
or submersible pump placed on a fl oat. The fl oat is secured to the bank and the water is pumped to
a storage area.
Often, the intake structure placed in a stream is an infi ltration gallery. The most common infi ltration
galleries are built by placing well screens or perforated pipe into the streambed. The pipe is
covered with clean, graded gravel. When water passes through the gravel, coarse fi ltration removes
a portion of the turbidity and organic material. The water collected by the perforated pipe then fl ows
to a caisson placed next to the stream and is removed from the caisson by gravity or pumping.
Intakes used in springs are normally implanted into the water-bearing strata, then covered with
clean, washed rock and sealed, usually with clay. The outlet is piped into a spring box.
In some locations, rainwater is a primary source of water. Rainwater is collected from the roofs
of buildings with a device called a roof catchment.
After determining that a water source provides a suitable quality and quantity of raw water,
choosing an intake location includes determining the following:
1. Best quality water location
2. Dangerous currents
3. Sandbar formation
4. Wave action
5. Ice storm factors
6. Flood factors
7. Navigation channel avoidance
8. Intake accessibility
9. Power availability
10. Floating or moving object damage factors
11. Distance from pumping station
12. Upstream uses that may affect water quality
SURFACE-WATER SCREENS
Generally, screening devices are installed to protect intake pumps, valves, and piping. A coarse
screen of vertical steel bars, with openings of 1–3 in., placed in a near-vertical position excludes
large objects. It may be equipped with a trash truck rack rake to remove accumulated debris.