Pneumatic Barriers to Reduce Salt Intrusion Through Locks
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No. 17 PNEUMATIC BARRIERS TO REDUCE SALT INTRUSION THROUGH LOCKS BY OR. IR. G. ABRAHAM IR. P. VAN DER aURGB IR. P. DE VOS (2 2032, 17 RIJKSWATERSTAAT COMMUNICATIONS PNEUMATIC BARRIERS Tü REDUCE SALT INTRUSION THROUGH LOCKS by DR. IR. G. ABRAHAM Head Density Currents Branch, Delft Hydraulics Laboratory IR. P. VAN DER BURGH Chief Engineer, Rijkswaterstaat, Deltadienst The Hague (up to 1-2-1972) IR. P. DE VOS Chief Engineer, Rijkswaterstaat, Study Division for the Northern Shallows and the Eemsestuary, Delfzijl Government Publishing Office - The Hague 1973 Any correspondence should be addressed fo RIJKSWATERSTAAT DIRECTIE WATERHUISHOUDING EN WATERBEWEGING THE HAGUE - NETHERLANDS The views in this article are the authors' own. ISBN 90 12001927 Contents page 7 1. Introduction 9 2. Salt intrusion through Jocks in the NetherJands 9 2.1 History and geography 9 2.2 Present position 11 3. Mechanism of salt intrusion through Jocks 11 3.1 Salt intrusion due to equalization of levels 11 3.2 Salt intrusion due to exchange of salt and fresh water 13 3.3 Effect of displacement of ships 14 4. Ways of reducing salt intrusion through Jocks 14 4.1 Flushing canal with fresh water 14 4.2 Pneumatic barriers 14 4.3 Water barriers 14 4.4 Selective withdrawal of salt water during intrusion 15 4.5 Selective withdrawal of salt water after intrusion 15 4.6 Equalization by pumping instead of gravity 15 4.7 Exchanging while gates are c10sed 17 5. SeJecting the method 17 5.1 For existing and projected locks 18 5.2 Current practice in the Netherlands 21 6. Hydrodynamics of pneumatic harriers 21 6.1 Pneumatic barrier in homogeneous water 21 6.1.1 Analogy with two-dimensional jet 22 6.1.2 Continuity equation 22 6.1.3 Momentum equation 25 6.1.4 Mathematical solution 28 6.1.5 Outcome of experiments 29 6.1.6 Theory and outcome of experiments compared 30 6.2 Effect of pneumatic barrier on salt intrusion 30 6.2.1 Introduction 31 6.2.2 Momentum equation 32 6.2.3 Continuity equation 3 page 33 6.2.4 Effect of volume of air on salt intrusion 35 6.2.5 Limitations of theoretical solution 37 6.2.6 Outcome of experiments 37 6.2.7 Theory and outcome of experiments compared 41 7. Design criteria for pneumatic barriers in locks 41 7.1 Ca1culating air supply 41 7.1.1 Extent to which water is exchanged in a loek chamber 44 7.1.2 Two pneumatic barriers in one loek 47 7.1.3 Twin pneumatic barriers in two or more locks serving one canal 47 7.1.4 Determining air supply for locks at IJmuiden 49 7.2 Aerodynamic features of pneumatic barrier plants 49 7.2.1 Designing air supply pipes 49 7.2.2 Designing perforated pipes 52 7.2.3 Size of holes in pipes 54 8. Upkeep of pneumatic barriers; experience gained at IJmuiden 54 8.1 Air compressors 56 8.2 Number of holes 59 8.3 Materials used for pipes 62 8.4 Effects on loek-gate maintenance 62 8.4.1 Northern Loek 62 8.4.2 Little Loek, Southern Loek and Middle Loek 63 9. Conclusions 64 Notations 67 Literature 4 Pholograph I. Nonhern Loek. Ijmuiden; scaward galC; pnel/malie barrier 5 Photograph 2. NOrlhern Lock, IJmuiden; ore-c-.arrier 'CETRA CARINA', 73.700 DWT, putS imo port and crosses pneumatic harrier near seaward gate 6 1. Introduction Extensive areas in the low-lying parts of the Netherlands are below average sea level. They are divided up into polders in which the water levels are controlled by pumping. The water that seeps upwards in the soil together with surplus rain water is pumped into canals encirc1ing the polders; the canals are linked and serve as collectors, the water level in most of which are a few decimetres belowaverage sea level. Formerly, the water in the 'collector' eanals was discharged into the sea through sluices which were opened for the purpose while the tide was running out. This natural method of getting rid of the surplus water has now been partly replaced by pumping. The 'collector' canals have always served more than one purpose. Their function as waterways is seeond only to that ofdisposing ofsurplus water. Constant improvement of the drainage side of the system has led to the inereased use of the eanals as water ways, which in turn has necessitated keeping the water in the canals up to a certain level; to achieve this, the polder water in the canals has to be supplemented from some other source. The increasing density of the population, more intensive use of the soil and industrialisation have been making water management in the 'collector' canal regions an inereasingly exacting task in the last few decades. One of the criteria by which the quality of the water in the canals is judged is its salt content. The eanal water is in danger of becoming saline because the salt water seeping up through the polder soil is pumped into the canals so that it may run along them into the sea. Salt water also reaehes the canals from the sea; it eomes in through the locks as ships enter. Any sea water that gets into a canal ean spread over a wide area because of the difference between the density of sea water and that of the water pumped from the polders into the canal. The means by which this intrusion of sea water through locks can be controlled and their efficacy are dealt with in this issue. The 'pneumatic barrier' is the chief expedient described. The pneumatic barrier is the outcome of several sets of experiments carried out in locks. The first set was conducted in the middle Loek at IJmuiden in April 1961. The next was carried out in a loek in the Zuyder Zee Barrier Dam at Kornwerder zand in September 1961. Lastly there were experiments in the Southern Loek at IJmuiden, in the middle Loek at IJmuiden a second time and in the Western Loek at Terneuzen. The experiments were most useful in that they provided material for treatises on the hydrodynamic aspects of pneumatic barriers for locks (see Chapter 6) and on the design criteria (see Chapter 7). The experience gained with pneumatic barriers at IJmuiden over a period of years is described in Chapter 8. 7 Photograph 3. Kornwcrderzand: seaward gate: experimental pneumatic barrier PhOlograph 4. Compressor batte!'"y supplying air ror pneumatic barrier experiments 8 2. Salt intrusion through locks in the Netherlands 2.1. History and geography As early as 1667 Hendrie Stevin drew attention to the salinization of the surface water in the low-lying parts of the Netherlands [18]. Stevin regarded the locks between the salt water in the sea and the fresh water inland as the main sourees of salt and described how the quantity of salt water intruding during the locking process could be reduced by pumping the water out of the loek chamber. Stevin also proposed that the Zuyder Zee be closed off and that the sea arms in the south-west be dammed off to create large reservoirs containing fresh water. It was not until this century that his ideas were put into effect. Various steps have been taken down the centuries, however, to reduce as much as possible the salt bugbear attaching to locks. Salt-intru sion and wastage of fresh water due to Ieaks were lessened by taking increasing care to seal the walls of locks and associated structures. Sets of sluices were built next to locks to facilitate discharging the intrusive salt water back into the sea before it could affect the water in the 'collector' canals. Ships became bigger and the canal locks had to be enlarged to accommodate them. In many instances larger locks were built beside the ones that had become toa small so that in many piaces there are now three or four locks of varying dimensions side by side. The increased size of ships, locks and canals has aggravated the salination problem. It is gradually becoming impossible to draw fresh water for various purposes from the 'collector' canals in many places, whereas the demand for fresh water is constantly increasing. The problem has prompted the search in the last few decade~ for more effective ways of reducing the salinification hazard at locks and improving the quality of the water in the 'collector' canals. 2.2. Present position Figure 1 shows the average salinity of the canals in the low-lying parts of the Nether lands at the present time. Although the quantitiesof salt indicated spring from both the salt seepage pumped up from the polders and the water intruding through the locks, the figure shows clearly that it is near the locks that salinification is greatest. It is aften noticeable dozens of miles away from the locks and prevents the canal water from being utilised for a number of purposes over a wide area. 9 NETHERLANDS seAlE ~.~·=·È=="·===':;·'~ '"LOllIGE CONtENT 0' ""'_1[111 IN StOIlAGE USI,..S "'NO 'COLLECTOR' ("NAlS IN A YUIl WITH ....tR..GE MEfEOAOt.OGICAL (ONOmONS ( IN "'g. PER 1.1 = < 300 O"UEM = JOD - 500 = 500 -1.000 '" = 1,000 -2.000 = 1.000 -5.,000 '" ... 5,llOO LOCI( (5) ~ , " -l' SLUICf; (S) " ~ .... I OElFZ~L LAUW[RSlCE 2 lOU1Ul J HARLINGEN " ' , lUYOERZEE BARRIER DAM ' .. 5 ZUYOERZEE BARRIER DAM 5 OEN HELOE:R 7 'JMUIOEN , NEW WATER WAY - NEW MEUSE HARTELCANAL BRIHSE MEER 10• HARINGVLIET ."' 11 YOLKERAK 12 C"HAL ACROSS WALCHEREN SCHELDT - RHINE CANAL 13 "CROSS ZUID BEVElANO " '''NAL " TERNEUZEN Figure 1.