Page 105 - Water and wastewater engineering

3-2 WATER AND WASTEWATER ENGINEERING

3-1 INTRODUCTION

Intakes are structures constructed in or adjacent to lakes, reservoirs, or rivers for the purpose of
withdrawing water. In general, they consist of an opening with a grate or strainer through which
the water enters, and a conduit to conduct the water by gravity to a low-lift pumping station. *
The water is pumped from the low-lift pumping station to the water treatment facility. Schematic
diagrams of lake and river intake systems are shown in Figures 3-1 and 3-2 .
The key requirements of the intake structures are that they are:

• Reliable.
• Of adequate size to provide the required quantity of water.

• Located to obtain the best quality water.
• Protected from objects that may damage equipment.
• Easy to inspect and maintain.
• Designed to minimize damage to aquatic life.
• Located to minimize navigational hazards.

3-2 DESIGN ELEMENTS

Reliability
Reliability is an essential feature of intake structures. The water supply system ceases to function
when the intake system fails. Small systems with only one intake structure are particularly vul-
nerable. For larger systems, current design practice provides for duplicate intake structures that
include multiple inlet ports, screens, conduits, and pumping units.

Capacity
Because they are very difficult to expand to provide additional capacity, a design life of the
intake structures in the range of 20 to 40 years (minimum) should be considered (Foellmi,
2005). Although reliable intake systems are very expensive, perhaps as much as two to four
times the cost of a similar project built on dry land, the additional cost of increasing capacity
may be relatively small. In one case, increasing the capacity by more than 70 percent resulted
in a construction cost increase of less than 20 percent (ASCE, 1990).

Location
The major factors to be considered in locating the intake are listed in Table 3-1 on page 3-4.

*The pumping station is called “low-lift” or “low-service” because the function of the pumps is to raise the water from the
surface water supply to the treatment facility. “High-service” pumps that supply water to the distribution system are selected
with the objective of providing a high enough pressure to make water flow at a high rate through service connections at various
elevations throughout the distribution system.