AN INVESTIGATION INTO THE REMOVAL OF SEDIMENT FROM WATER INTAKES ON RIVERS BY MEANS OF JET-TYPE DREDGE PUMPS
Report No 1187/1/02
Excessive sedimentation at water intakes on rivers causes interruption in water supply and serious abrasion of pumps with consequent high operating costs.
The problem of sedimentation at water intakes on rivers can largely be minimised by appropriate design of the intake structures. Well established sound engineering practices have been developed to ensure sound engineering design of river intake structures. As background to this study, the causes of sedimentation problems and principles of effective design of river intake structures are therefore included in this report.
Sedimentation problems are relatively common in South Africa, especially in the case of smaller1 intakes on sediment laden rivers. The reason for this is the highly variable flow regime of relatively small rivers on which the sediment prone intakes are located (flow rates in relatively small rivers, usually vary between very low and very high (flash floods) in a season). The consequence of the variable flow regime is significant variation of the river morphology (variable flow channels and sandbank configurations in the river bed). Even the application of sound engineering design practice cannot prevent all sedimentation problems of such smaller intakes for all flow regimes.
1 Smaller intakes refer to intakes which do not warrant expensive civil works due to financial constraints such as in the case of small local authorities and riparian farmers.
In the case of water intakes on rivers which are prone to sedimentation problems in South Africa, there is a need for the removal of sediment from the intakes to ensure continuous water supply from these intakes and to protect the pumps and purification works against damage and sediment clogging, respectively.
With sediment problems being experienced in cases of poorly designed intake structures, temporary emergency intakes as well as intakes operated by farmers, it was considered necessary to launch this research project with the main objective to contribute towards the alleviation of sedimentation problems experienced by intakes on South African rivers and thus improve the assurance of water supplies, as well as to protect intake pumps and purification works against damage and sediment clogging, respectively.
Work which was carried out successfully on the South African coastline effecting the small scale removal of sediment by means of mobile jet type dredge pumps at small craft harbours and at the Koeberg Nuclear Power Station Cooling Water Intake, prompted the employment/customisation of the jet type dredge pump on rivers.
The objective of this research project was therefore to develop a reliable, simple to operate and. low maintenance jet type dredge pump sediment removal system for application in the following two main water intake cases on rivers:
Results and Conclusions
A national survey of intakes on rivers which experience sedimentation problems revealed the following:
Based on the above outcome of the survey it was decided to design and assemble a fixed system (dedicated to one intake installation) as well as a mobile (trailer mounted) jet type dredge pump system both of which could be used as experimental units on existing problem cases with the object to evaluate their performances and appropriateness in alleviating the sedimentation problems being experienced.
The fixed experimental unit was designed to be used at one of the problem cases (Sand River intake) where a tap-off from the intake's raw water discharge pipe was used to drive the jet pump system. The 2001 capital cost of this fixed unit was approximately R30 000.
The mobile experimental unit was designed as a self-contained diesel engine driven unit including pipes and accessories to be able to operate at any remote location. The 2001 capital cost of the mobile trailer unit was approximately R160 000. Both the fixed and mobile units had a pumping capacity of approximately 15 m3 per hour of sand (measured in bulk).
The Department of Water Affairs and Forestry (DW AF) significantly contributed to the investigation by initiating a 12 month contract with a contractor to operate the fixed and mobile experimental units on sediment prone intakes on rivers in Mpumalanga and the Northern Province.
Both the fixed and mobile units were operated successfully during the 12 month evaluation period. Approximately 80 cleaning operations were performed during this period with the mobile unit maintaining approximately 8 sites.
The operating cost (2001) of the mobile unit (including labour, fuel transport and maintenance of the unit) was approximately R3 000 per day in cases where divers were not required and approximately R6 000 per day in cases where divers were required.
One or two days were required to complete a cleaning operation per intake site.
These sediment removal costs are to be compared with the costs at a typical intake to replace or repair a pump which has been damaged by excessive sedimentation (2001):
|Capital cost of one raw water intake pump:||Rl80 000 to R200 000|
|Repair cost of one raw water intake pump damaged by sand intake:||R60 000 to R70 000|
|Time taken for repair (causing interruption in water supply):||approximately 2 months|
The fixed experimental unit (installed at the Sand River intake) was successful in creating a crater at the raw water pumps intakes. Since the fixed unit used only about 15% of the raw water pump station's discharge and only intermittently when the removal of sediment was required, the operating cost of the fixed system was minimal. Because the main channel of the Sand River at the intake, shifted during both minor and major floods, the crater created by the fixed unit was frequently cut off from the main river channel. The mobile unit was then employed successfully to dredge a connecting channel between the crater and the main river channel to ensure continuous water supply to the intake. [Although not attempted in this study it is considered that at the Sand river intake, two or three additional permanently installed jet pumps between the crater at the intake pipes and the main river channel could maintain a connecting channel to ensure continuous water supply to the intake.]
Although more tests work on sediment removal during the 12 month evaluation period were performed with the mobile unit (mainly due to the large number of existing sediment prone water intake structures), it is considered that the more appropriate application of the jet pump is in a permanently installed fixed mode (this mode does not require divers to operate the jet pump). This implies that the jet pump(s) are dedicated i.e. can be switched on whenever required to prevent sediment ingress and consequent damage to the water abstraction pumps at a river intake.
Based on the performance results obtained during the twelve month evaluation period of the fixed and mobile experimental units it was concluded that the main objectives of the research project were successfully achieved.
The experience of DWAF with the experimental mobile and fixed jet pump systems during the 12 month evaluation period of this research project was sufficiently positive for them to decide to implement a further 2 years contract for the removal of sediment from existing sediment prone intakes in Mpumalanga and Northern Province. During 2002 DW AF also incorporated a permanently installed jet pump system in their design of a new water intake structure at Hoxani on the Sabie River.
Based on the successful conclusion of the investigation it is considered that the project has contributed towards the alleviation of sediment problems at water intake structures on rivers.
Since the capital and operation costs of a permanently (dedicated) jet-type pump system (which is considered the most appropriate method of application) is significantly less than that of a mobile system, it implies that a fixed system will have a larger cost benefit than a mobile system. The capability of a jet-type pump system (either fixed or mobile) to clear an intake from sand has the additional, perhaps more important, benefit of maintaining an intake in an operational condition and therefore ensuring continuous water supply. This benefit is, however difficult to quantify.
It is considered that a large proportion of intakes on rivers with sedimentation problems could be effectively serviced by jet type dredge systems with the same capacity as that of the experimental units used in this investigation (details of which are presented in this report). It should however be stated that, specifically in cases of fixed jet type dredge systems, custom design is advisable to ensure effective/optimal performance.
As jet pumps could also be used on cohesive sediments it is recommended that further research be performed on the jet type pump's disintegration nozzle configuration to improve its performance in cohesive (not free flowing) sediments such as clay/silt mixtures.
The investigation, besides revealing a need for a sediment removal system, also revealed the need for guidelines on the effective design of intake structures on rivers. It is recommended that consideration be given to transfer the relevant technology in both the above fields to those who are involved in the abstraction of water from sediment laden rivers. The technology transfer could be accomplished by means of workshops or courses.
In all cases of sediment removal from and discharge back into rivers, all relevant legislation must be taken into consideration. The first step towards fulfilling the Department of Environmental and Tourism's legally prescribed Environmental Impact Assessment (EIA) procedure, would be to approach DW AF for a permit to allow the planned operations. Based on the degree of environmental sensitivity of each case, DW AF would be able to indicate if an environmental impact assessment is required before a permit can be issued.