WATER INTAKES - SITING AND DESIGN APPROACHES

 

Adnan M. Alsaffar and Yifan Zheng

 

Bechtel Corporation 9801 Washingtonian Blvd.

Gaithersburg, MD 20878, , Maryland, USA

Tel: 301-417-3175 , Fax: 301-963-2878 , E-Mail: aalsaffa@bechtel.com

 

 

ABSTRACT

The function of a water supply intake is to extract and deliver water to the users. Therefore the design of water intakes require a series of hydraulic design consideration in order to arrive at a desirable concept that can obtain and deliver the water economically with an acceptably low impact on the environment. Due to variability of site conditions, the environmental hydraulic engineer is faced with several challenges when assessing water supply availability. The major factors that can affect the selection of a concept and design development for a water intake are: water availability, bathymetry, sediment transport, environmental regulations, climatic conditions, constructability, initial and maintenance dredging requirements, and operation and maintenance. The paper examines these factors and discusses their importance in selecting a suitable concept.

To demonstrate the approach in evaluating the various variables, case studies at four sites with differing conditions are presented. The rationale for selecting each concept is presented along with illustrations. Site conditions considered in the cases are; Intake on rivers with high water level fluctuation, intakes on tidal rivers, intakes in mountainous streams and offshore velocity cap intakes.

The paper stresses the importance of appropriate hydraulic design to provide acceptable flow conditions at the pumps.

 

Keywords: Water Intake, Site Hydrologic Conditions, Hydraulic Analysis, Bathymetry, Sediment, Constructability, Environmental Regulation, Tide, River, Offshore

 

INTRODUCTION

The function of a water supply intake is to extract and deliver water to the users. Therefore, the design of water supply intakes requires a series of design considerations in order to arrive at a desirable concept that can obtain and deliver the water economically with an acceptably low impact on the environment. Due to variability of site conditions, the environmental hydraulic engineer is faced with the challenges when assessing water supply availability. The major factors that can affect the selection and design of an intake are site hydrologic conditions, site access, ease of construction, and operation and maintenance. Without a careful and responsible evaluation of various design factors, an intake may be designed and constructed but may not be operable due to lack of adequate water supply or may be adversely impacted due to degraded environment.

This paper examines the major factors that can affect the design and presents examples of design concepts encountered at various sites.

 

DESIGN CONSIDERATIONS

The following factors are considered of primary importance in siting and designing an intake:

 

·        Water Availability

·        Bathymetry

·        Sediment Transport

·        Environmental Regulation

·        Climatic Conditions

·        Constructability

·        Initial and Maintenance Dredging

·        Operation and Maintenance

 

By far the most important of these factors is availability of water to meet the required demand without creating an environmentally and physically adverse effect on the water body. This is particularly important for fresh water supply. Therefore, detailed hydrologic studies including analysis of historic data must be performed. In areas where no historic data on stream flow are available, rainfall data should be analyzed to determine rainfall frequency. Hydrologic modeling can be used to estimate the runoff.

Locating and selecting the specific type of intake requires adequate knowledge of the bathymetric condition of the river, estuary or sea bottom in the vicinity of the intake. Without this information, no specific intake concept can be selected. Making assumptions could lead to erroneous cost and schedule estimates for the project.

The type of sediment can be either bed load or suspended load in a river, and littoral drift in a coastal environment. The existence of sediment affects the design concept and the suitability of the site for locating an intake.

Other important factors to consider are any water withdrawal limitations as well as the feasibility of dredging and disposal of dredge spoil. In some situations, water may be physically available, however, because of water rights, water required for aquatic habitats or waste assimilation may not be legally available. In addition, dredging and disposal in areas where there are endangered species or contaminated soil, could be harmful to the environment. These factors and others could affect the selection of a desired intake site and may affect the feasibility of a project.

Climatic conditions such as severe winter weather can affect the concept and details of the pump intake structure. A region with below freezing air temperature requires protection for traveling screens and trash racks against the formation of anchor and/or frazil ice. Such protection will affect the design concept and should be considered in the planning phase. For power plant intakes warm water recirculation into the intake is commonly used. However, if the intake is remote from the power plant or if the intake is for water supply, electrical heating elements will be required which will increase the power demand. Alternatively the design could be made to encapsulate the intake and prevent air circulation. However, this concept can not eliminate the need for protection against frazil ice.

Construction, maintenance and access are also important factors to be considered in selecting the intake location. Availability of access road, potential for local and riverine flooding and access to the intake equipment all year round should be considered.

 

DESIGN CONCEPTS OF INTAKE STRUCTURES

 

i.                    General:

Experience in the design and operation of various water supply intakes indicates that no single design concept is suitable for all locations. Therefore, any intake design must be based on site specific information. This may not be possible at the planning phase of the project due to the absence of specific site data. Therefore, the hydraulic engineer must develop design parameters from the limited data that may be available, and develop programs for the field data collection and analysis for use in detailed design.

Lack of site specific information generally occurs in remote areas of the world where no historic data, studies or maps are available to help in the planning and design. The most practical approach for work under these conditions is to make a site visit and obtain aerial photographs. An important aspect of this effort is the identification of river banks and shoreline conditions and the presence of erosion and deposition. Aerial photos can best be utilized in assessing the presence of shoreline changes and of river meanders.

 

ii. Case Studies:

The following section addresses the approach utilized in selecting types of intakes at various locations and with differing hydrologic conditions.

 

Intakes on Rivers with High Water Level Fluctuation:

This type of river can be found in regions where rainfall and runoff occur in a short duration during the year such as the monsoon season. Designing a conventional intake in this type of environment may not be technically or economically feasible. To overcome this condition, an intake structure was designed as a super structure with an access pier connecting the intake to the shoreline as shown on Figure -1. This structure was also used in a lake with large water level variation and can be used in a coastal area where an offshore intake with a buried pipe is not practical.

 

Figure -1 : Hydraulik design of a river wate intake with high water level fluctuations

 

Fish protection is accomplished by installing wedge wire screens with air back wash systems. The design of the intake caisson and the supporting piles for the pier must be based on geologic and geotechnical considerations.

 

Intakes on Tidal River:

Locating an intake in a tidal river requires extensive evaluation of the method of installation and dredging. In a river with a wide tidal flat, dredging and disposal of sediment could be the most controlling factor since it could create disturbance to tidal habitats, increase river turbidity and cause contamination of the river if the soil is contaminated. In certain rivers, the tidal flat can be very wide and dredging can be very costly if not impossible to achieve. This situation was encountered in a tidal river in the United States. The tidal flat extend approximately 500 m from the shore line and the range of the normal tidal water level fluctuation is 1.75 m. Because of the environmental concerns about dredging and disturbance of aquatic and birds habitats an innovative design technique was required. Extensive visits to the site and meetings with regulators were made. Several alternatives were evaluated and the concept that was finally selected included a pier supported jetty and locating the pump intake at its end. The pump motor, control and the pier deck were set above the 100-year flood level. To protect the pumps from floating debris and to provide mechanism for screening the water, a caisson was installed to house the pumps. Wedge wire screens were used with air back wash. This intake concept is shown on Figure -2.

 

Figure -2 : Hydraulik design of an intakeon a tidal river

 

Intakes in Mountainous Streams:

Intakes on mountainous streams require special designs to exclude or to separate the heavy sediment load that can be carried by the flow which occurs as a flash flood. Sand can form bars during the flood and cause extensive deposits which can block the flow path. During the low flow season these streams carry generally low flow which can affect water availability at the intake. Therefore an intake must be designed to abstract water under all conditions without excessive sediment load. In most cases, particularly when water is pumped, sand exclusion must be made before reaching the pumps.

These requirements were applied in the design of several intakes in Andes Mountain. The approach consisted of estimating the low flow and 100 -year flood flows and water levels. Sediment samples from the river beds were collected and analyzed for gradation. Based on these information and considering space availability at the various sites, some intakes were designed with settling basins, some with sediment exclusion and by-passing. The intake presented in this paper is located in a very narrow river channel and the river has a very low flow during the drought season. Therefore, the design is based on abstracting all the river low flow and by-passing the extra flow with the sediment. The intake consists of a diversion dam across the stream with inlet grating. Sand is by passed through a sluicing pipe to the stream. The de-sanded water flows over a weir, through a pipe before reaching the pump forebay. This concept is shown on Figure -3.

 

Figure -3 : Hydraulik design of a water intake on a mountainour streams

 

Offshore Intakes:

Offshore intake is a submerged structure for withdrawal of water by gravity from the sea, lakes and in some situations from rivers to a shoreline pump intake. This type of intake consists of a velocity cap connected to an offshore buried pipe that conveys the flow to the on shore pump intake structure. This type of intake is used in regions with shallow water depths, with considerable littoral drift, drift ice and where fish protection is of concerns. The velocity cap creates a horizontal flow path which was developed through experimental work to preclude fish entrapment( Reference-1). The general velocity of approach is in the range of 0.30 m/s to 0.45 m/s. In thermal power plants it has the advantage of creating a selective withdrawal and therefore minimizes warm water recirculation from the discharge into the intake. An important factor that must be considered in the design is wave induced forces on the structure. Various wave theories must be examined to determine the applicable condition for determining the forces. Buried offshore pipes must be protected against movement by waves and currents action. This protection is in the form of granular backfill covered with riprap. Riprap sizing is based on consideration for the wave induced currents in addition to the ambient current following several analytical procedures such as PIANC (Reference-2) and others.

This concept was used to supply cooling water to a thermal power plant in the Mediterranean Sea, as shown on Figure - 4. Three conditions were encountered, namely : littoral drift, flat bathymetry, and severe wave conditions. From the hydrographic and bathymetric survey it was concluded that a shoreline intake with a dredged channel will not be technically feasible. Waves are very active and range in a height of 2 to 3 m. An offshore intake with buried pipe were designed. The design considered all wave and current loading as well as other applicable loads.

 

Figure -4 : Hydraulik design of a offshore intake

 

HYDRAULIC DESIGN OF PUMP PIT

In all the cases discussed in this presentation as well as any other intake, the selected design concept must not affect the performance of the pumps. Uniform approach flow conditions, adequate pump submergence, and flow free from surface and sub-surface vortices must all be considered in selecting the pit geometry. The design criteria and dimension of an intake for a given flow can be found in various publications such as IAHR( Reference-3), BHRA (Reference-4) and others.

 

CONCLUSIONS

The above presentation leads to the following conclusions:

 

1. Locating and designing a water supply intake requires careful consideration of hydrologic , environmental, geotechnical and economic factors.

2. Several types of intakes should be considered to meet various site conditions and operational requirements.

3. Long term hydrologic data should be collected and analyzed to arrive at the most suitable and reliable concept.

4. Hydraulic analysis must be performed as an integral part of the intake design to provide flow free from objectionable conditions at the pumps.

 

REFERENCES

1.      Design of Water Intake Structures for Fish Protection, American Society of Civil Engineers, New York 1982.

2.      Guidelines for the Design and Construction of Flexible Revetments Incorporating Geotextiles in Marine Environment, PIANC, Bulletin 7879, 1991.

3.      Swirling Flow Problems at Intakes, Jost Knauss, Coordinating editor, IAHR, 1987.

4.      The Hydraulic Design of Pump Sumps and Intakes, BHRA, July 1977.