IPC2000-148 CASE HISTORIES OF PIPELINE EXPOSURES AT STREAM CROSSINGS IN

Craig D. Malcovish, Malaron Engineering Ltd. 13 Flint Crescent, St. Albert, Alberta, Canada T8N 1Y7 Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021 Phone: (780) 458-2710 Fax: (780) 419-2462 Email: [email protected]

Artur Janz, ATCO Pipelines 10035 - 105 Street, Edmonton, Alberta, Canada T5J 2V6 Phone: (780) 420-7536 Fax: (780) 420-7411 Email: [email protected]

Delton M. Gray, Gulf Midstream Services 1680 - 102 Avenue, Edmonton, Alberta, Canada T6P 1V7 Phone: (780) 464-9133 Fax: (780) 467-5046 Email: [email protected]

KEYWORDS: Pipeline, Stream Crossing, Case Studies

ABSTRACT • development of new or expanded overflow channels on floodplains This paper describes the history of channel changes and associated pipeline exposure problems at five and In contrast to scour, which refers to the localized and stream crossings in Alberta. Each case history reviews sometimes temporary lowering of bed levels in areas such the various hydrologic and geomorphic factors that as sharp bends, the term "degradation" implies an contributed to the erosion problem and describes the extensive and often progressive lowering of the bed corrective action that was taken. A number of the throughout long reaches. There are numerous man-made examples illustrate the inherent difficulties in identifying as well as natural causes of degradation and the process potential erosion problems at the project design stage. may proceed in either the upstream or downstream Others show that with systematic monitoring and direction. Experience has shown however that upstream inspection procedures in place, remedial action can be progressing degradation is generally the more common planned and implemented well before pipeline integrity problem at pipeline crossings in Alberta. The two has been compromised. principal causes, both flood-related, are:

• channel shortening by cutoffs across long meander INTRODUCTION loops, and • washout of downstream slope controls such as large In Alberta, most pipelines cross and creeks whose beaverdams or bouldery riffle sections channels are formed in erodible alluvial materials. The overall physical characteristics of these streams, the end In the early years of oil and gas development in Alberta, product of thousands of year of interaction between water, pipelines were built across long stretches of territory with sediment and the landforms through which they flow, are limited physiographic information and few environmental not usually altered significantly during a major flood. restrictions. Stream crossing sites were selected by Locally however, dramatic changes in channel veteran pipeliners whose primary focus was the viablility morphology can occur, including: of construction and cost, and little consideration was given to future channel behavior. Crossing designs were • extensive vertical lowering of the bed often based on arbitrarily established depths of cover • lateral migration of banks and/or shifting of meanders beneath the streambed as it existed at the time of • formation of cutoff channels across long meander construction. Sagbends were set at nominal distances into loops, and the existing natural channel banks and the final or "as- built" location of the pipe was seldom documented. Manv

Copyright © 2000 by ASME of the crossings built in the past have perfonned CASE 1 - satisfactorily; however there is also a histoiy of frequent CROSSING NEAR DRAYTON VALLEY pipeline exposure. In the Drayton Valley area, the North Saskatchewan River Experience has shown that scour and degradation are flows from southwest to northeast in a valley generally the main causes of pipeline exposure at approximately 60m deep and 1500 m to 3000 m wide at crossings of small streams and local drainage courses. crest level. The valley walls consist of near-vertical This is not the case however at major river crossings, bedrock cliffs in some places and densely wooded slopes

where the cost of repair or replacement is an order of of glacial lake sediments and till elsewhere. The bottom Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021 magnitude greater. Bank erosion and channel shifting are of the valley is generally between 1000 m and 2500 m by far the most common causes of exposure at these wide and consists of broad, densely wooded floodplain locations. Bank erosion is also a major cause of valley segments bounded by fragmentary low terraces. slope stability problems at many crossings but this is not necessarily confined to large rivers. The present river channel has bouldery-gravelly bed surface and floodplain banks composed of partly-cohesive In this paper, a number of selected case histories (Figure fine-grained alluvium (sandy silt/clay) overlying gravel. 1) are presented to illustrate the types of erosion processes At high stages, the channel pattern can be generally that need to be recognized and accounted for in the described as irregular, partly-confined meanders, with design, operation and maintenance of pipeline crossings. occasional splitting around small wooded islands. At low Each case history reviews the various hydrologic and stages, large areas of unvegetated gravelly bars are geomorphic factors that contributed to the erosion exposed and the river is characterized by alternating problem and describes the corrective action that was stretches of multiple subchannels and irregular, single- taken. channeled loops. Average D50 and Dw sizes of the gravel on the surface of exposed bars are about 60 mm and 130 Some of the examples also serve to illustrate the inherent mm, respectively. difficulties in identifying potential erosion problems at the project design stage. Others show that with systematic The crossings (2-406 mm O.D. high pressure natural gas monitoring and inspection procedures in place, remedial lines) were constructed in the winter of 1983 - 84 by action can be planned and implemented well before conventional open cut methods. Since then, a total of five pipeline integrity has been compromised. other pipeline crossings have been built immediately upstream and downstream. Three of these lines were installed by conventional open cut methods in the mid 1980's. The other two were directionally drilled in the early 1990's.

Throughout most of the 2000 m long reach immediately upstream of the pipeline corridor, the river flows between near-vertical bedrock cliffs to the west and mostly wooded upper level or intermediate floodplain segments to the east. The bedrock cliffs terminate approximately 200 m from the crossings. Below this point, the surface of the outcrop dips steeply downward to the north, becoming buried under glacial deposits. The bottom of the valley also widens significantly and the river is free to shift laterally in both directions.

In 1996, a study was undertaken to evaluate the adequacy of existing burial depths at the two 406 mm O.D. pipeline crossings. Examination of aerial photographs-indicated that since 1951, cross-valley channel shifts of over 700 m had occurred at two downstream locations and that in one area on the east side of the valley some 500 m to 1500 m from the crossings, meander development had been accompanied by up to 300 m of local floodplain erosion. Much of this erosion occurred during a 3 to 4 week period of moderately high river flows in late June and early July of 1965 (Figure 2). The aerial photos also showed that since the summer of consists of a continuous gravelly floodplain bounded by 1965, virtually all of the shifting in the immediate crossing fragmentary low terraces of sandy silt/clay overlying area had been westward and that the location of the main gravel. The terraces, which are also densely wooded, channel had changed by over 200 m. It was not possible typically rise about 2 m above active floodplain level. to quantify the extent of the shifting on a year-by-year basis. However, it appeared that the vast majority of this At high stages, the channel pattern can be generally erosion took place during major floods in June, 1972 and described as irregular, partly - confined meanders, with Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021 July, 1986. occasional splitting around wooded islands (remnants of former terraces). At low stages, large areas of Comparison of surveyed channel cross sections indicated unvegetated gravel bars are exposed and the river is that since the two 406 mm O.D. pipelines were installed in characterized by alternating stretches of multiple channels the winter of 1983-84, the width of the gravelly floodplain and irregular, single-channeled loops. The low flow segment between the main channel and the west valley channel is generally about 40 m to 50 m wide and up to wall had decreased from about 90 m to 40 m and that the 1.5 m deep and has a bed surface composed of gravel thalweg was now located over rising sections of both cobbles and small boulders. Average D50 and D90 sizes pipelines. Minimum depths of cover in the deep water appear to be in the order of 75 mm and 120 mm, zone were in the order of 1.0 m at the north crossing and respectively. 0.6 m at the south crossing. The pipeline (114.3 mm O.D. natural gas) crosses the The study concluded that although the main channel may valley in a general northwesterly - southeasterly direction ultimately shift back to the opposite or east side of the and at the time of its construction in the fall of 1970, the floodplain, the most likely short-term scenario was main river channel was located along the northern edge of continued lateral migration towards the west valley wall the active floodplain. Aerial photographs indicate that a because the east floodplain bank opposite the bedrock major flood in June, 1972 caused a shift of the main flow cliffs above the crossing was also eroding (up to 100m to an old meander loop on the opposite or south side of since 1951). The downstream angle of attack on the west the valley. During the next two decades, this loop bank was expected to become increasingly more severe as migrated downvalley towards the pipeline crossing, this erosion continues and as a result, any future westward eroding up to 150 m of the south terrace bank (Figure 4). shifting in the crossing area would likely accompanied by substantial lowering of the bed. Computations indicated It is estimated that roughly 40 m of terrace erosion that if this scouring did develop, both pipelines would be occurred in the immediate crossing area during another severely exposed. major flood in July, 1997 and as a result of this erosion, a 25± m long rising section of the pipeline was exposed. In In the spring of 1998, application was made to Alberta late October of 1997, a temporary gravel berm was Environment to install a new crossing of the North constructed on the immediate upstream side of the Saskatchewan River by directional drilling (Figure 3). exposed pipe for protection against possible ice damage During high flow conditions prior to the start of during freeze up. The consequence of failure for the construction, one of the existing 406 mm O.D. lines was pipeline was severe as it provides the sole natural gas ruptured. The rupture occurred immediately adjacent to supply to the town of Grande Cache. the eroding west bank. According to depth soundings made shortly after the rupture, bed levels in this area had In the fall of 1997, a geotechnical investigation was also been lowered by up to 1.5 m. carried out to determine the feasibility of installing a new directionally drilled pipeline crossing either within or in Construction of the new crossing began in the fall of 1998 close proximity to the existing right-of-way. However and was completed in January of 1999. Due to some results indicated that the valley bottom stratigraphy was problems maintaining the hole during the reaming phase not suitable for this type of construction (deep gravel). of the operation, this was a challenging and costly directional drill. In the later winter of 1998, the exposed section of pipe adjacent to the south terrace bank was lowered by conventional open cut methods. The lowering was CASE 2 - CROSSING NEAR complicated by the fact that the line could not be by- GRANDE CACHE passed. To prevent an outage and re-light condition in Grande Cache, the line had to be lowered "live" under gas In the Grande Cache area, the Smoky River flows pressure. northward through a broad mountain valley. The lower valley walls consist of rock cliffs in some places and There was little or no bank erosion in this area in 1998 but densely wooded slopes of mainly till elsewhere. The during high flows in the summer of 1999, a section of the bottom of the valley is generally about 1000 m wide and line in the bed of the main channel became exposed and Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021 Toe of Valley Wall

Existing 406 mm yj Gas Pipelines

Bedrock Cliffs

SEPTEMBER 14,1951 MAY 17,1965 SEPTEMBER 17,1965 SEPTEMBER 09,1991

0 200 400 $00 800 1000m SCALE I I I I I I

FIGURE 2: CHANNEL SHIFT MAP, NORTH SASKATCHEWAN RIVER NEAR DRAYTON VALLEY

Sept 14,1951 Aug. 25,1995

FIGURE 3: LOCATION OF DIRECTIONALLY DRILLED PIPELINE CROSSING, NORTH SASKATCHEWAN RIVER NEAR DRAYTON VALLEY 400 Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021

APRIL 28,1970 JUNE 18,1974

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PSIs" ttslm OCTOBER 8,1979 APRIL 18,1983

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SEPTEMBER 17,1993 AUGUST 26,1997

FIGURE 4: HISTORICAL AERIAL PHOTOGRAPHS, SMOKY RIVER NEAR GRANDE CACHE 401 subsequently ruptured. Upon analysis of the failed llll* pipeline section it was determined that the pipeline failed because of fatigue cracking and subsequent overload. The fatigue crack initiation and propagation was attributed to cyclic displacement caused by turbulent water flow. •••I The Smoky River near Grande Cache contains a large Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021 number of fish species with overlapping spawning cycles. Due to lack of a suitable window for a conventional open cut crossing installation, and the high risk of failure of a satis directional drill, an alternative "trenchless" method of construction was proposed. This was to excavate a large bellhole on each side of the proposed crossing, install WM large pumps to control the flow of water into the hellholes, •j and punch a large diameter casing pipe across the river at HHlKM^ llil an elevation below the foreseeable scour depth of the •RmmIIMMINllillli K river. The concept behind this type of installation is that fiftiwiifw'' the casing pipe is punched across "open ended" from one side of the river to the other. The pipe fills with gravel and rock and then, at the push side of the crossing, is sealed by welding a plate on the inside before adding on sections of "clean" pipe. The new clean casing pipe is then punched FIGURE 5: LOW BR RKACH OF HELLS CREEK across the river, with the "rock filled" section of pipe being cut apart and abandoned as it comes through the other side. Once the plate makes it to the other side, the clean casing pipe for the new river crossing is now in i place. The pretested line pipe is then pulled through the IImMI %mmrjM casing and tied in to the other section of conventionally installed line pipe.

This horizontally punched crossing of the main channel was successfully completed in the fall of 1999. As it is possible that the river may eventually shift to the opposite or north side of the floodplain, the entire existing 400± m long section of pipe in this area was replaced with Jilt rockguard "gunnite" coated pipe. The new floodplain •MMT ill section was installed by conventional open cut methods, glHiiJII with depths of cover in excess of 4 m. tiH

CASE 3 - HELLS CREEK CROSSING NEAR GRANDE CACHE

Hells Creek originates on the eastward - facing slopes of FK..I Rl. !i: I IMM'R Ul U ll OF Ur.l I.S ( Kl.I- k Mount Hamell, approximately 7 km northwest of the town located about 450 m upstream of Highway 40 and has a of Grande Cache. The channel emerges from a mountain drainage area of around 2.0 km2. Because of the canyon at about the 1050 m level and for the remainder of extremely rugged basin topography (Figure 6), the creek its length, flows across a large forested alluvial fan in the is subject to very large and rapid increases in discharge Smoky River valley. The materials comprising the fan are during periods of heavy summer rainfall. Flood recession essentially mudflow deposits: cobbles and boulders is also very rapid as there is insufficient surface water embedded in a matrix of sand and silt. The total drop in storage in the basin to sustain prolonged high flows. Dry elevation from the apex of the fan to the Smoky River is weather flow appears to be either very low or nonexistent. more than 120 m. The average overall slope of the fan surface is about 10.5 percent. At the time the first pipeline crossing was built in 1969- 70, Hells Creek was essentially a low-banked meandering Highway 40 crosses Hells Creek at a point approximately stream with a tendency for lateral channel shifting during 150 m upstream of the Smoky River confluence (Figure high flow periods. Exceptionally heavy rainfall struck the 5). The pipeline crossing (114.3 mm O.D. natural gas) is Grande Cache area in June, 1972 and it appears that the channel widening after complete degradation has bed of the creek began to degrade shortly thereafter. By occurred. 1984 almost the entire length of the channel upstream of Highway 40 was entrenched some 3 m to 4 m below the The creek has not been surveyed since 1994 but on the fan surface. The only exception was the pipeline right-of- basis of site visits in 1997 and 1999, it would appear that way area where intermittent backfilling was carried out by the channel may now be slowly aggrading rather than pipeline operations/maintenance personnel in an effort to degrading (Figure 10). There are also indications of some Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021 maintain the original creekbed level. minor channel widening, particularly in the immediate crossing area. Backfilling operations ceased when the original crossing was replaced in the winter of 1984-85, and by 1991, bed elevations within the pipeline right-of-way had dropped CASE 4 - MODESTE CREEK CROSSING by approximately 4 m (Figure 7). Another 2 m of bed NEAR BRETON lowering occurred in this area between 1991 and the summer of 1994, re-exposing the pipeline (Figures 8 and In the Breton area, Modeste Creek follows an irregular, 9). partly meandering northerly course through a valley with narrow, discontinuous floodplain segments and occasional In the fall of 1994, a third crossing was installed by terraces. The channel is approximately 25 m wide at low conventional open cut methods. Minimum recommended flow stages and has an average slope of about 0.0015. y depth of instream cover for this crossing was 5.5 m (below The bed surface contains gravel cobbles and small 1994 bed level). This was based on a "worst case" boulders and is underlain at relatively shallow depths by degradation scenario, namely washout of the existing hardpan or poorly consolidated bedrock. The banks of the Highway 40 culvert crossing or eventual replacement of creek rise about 4 m above the bed and are composed the culvert with a bridge. The sagpoints were set far primarily of sandy silt. enough back from the banks to allow for possible future

FIGURE 7: HELLS CREEK CROSSING FIGURE 8: HELLS CREEK CROSSING IN JULY 1991 IN JUNE. 1994

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FIGURE 9: HELL CREEK CROSSING FIGURE 10; HELL CREEK CROSSING IN AUGUST. 1994 IN OCTOBER, 1997 The three pipeline crossings (406 mm O.D. natural gas, After examining several possible options for repair of the 168.3 mm O.D. oil and 406 mm O.D. oil), are believed to three exposed pipelines, it was decided that the most cost have been constructed in the mid 1960's. As there were effective approach would be to rebuild a portion of the no indications of any active erosion at that time, the lines eroded east bank to its approximate pre-flood were installed with nominal setbacks from the existing configuration using locally available backfill materials, banks. and then protect the rebuilt portion of the bank with heavy rock riprap (Figure 12). The armoured berm was Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021 In mid-July of 1986, heavy rainfall produced extreme constructed in the late-fall of 1990 and has performed flood conditions throughout much of west-central Alberta. satisfactorily since then. Thus far, no maintenance has During this flood, a cutoff channel formed across the neck been required. of large U-shaped meander located approximately 150 m below the pipeline crossings, effectively shortening the creek by roughly 2 km (Figure 11). CASE 5 - FREEMAN RIVER CROSSING NEAR Normally, the immediate effect of the shortening and consequent local channel steepening caused by a cutoff is The pipeline crossings (two 114.3 mm O.D. natural gas) upstream progressing degradation. However, in this case, are located approximately 15 km southwest of the town of any bed lowering appears to have been relatively minor Swan Hills. In this area, the Freeman River follows a due to the presence of boulders and hardpan in the creek meandering southeasterly course through a broad, densely bed. wooded valley. The bottom of the valley is generally around 1000 m wide and consists of a continuous active During the exceptionally high flows that followed another floodplain bounded by fragmentary low terraces. major rainstorm in early July of 1990, the creek attacked the east bank, causing partial removal of a private access The present river channel is generally 40 m to 50 m wide road and exposure of some 12 to 15 m of pipe at two of and up to 2.5 m deep at bankfull stage (i.e. active the existing crossings. The extent of the bank erosion is floodplain floodplain level) and has an irregular partly- illustrated on Figure 12. confined meander pattern. The bed surface is mainly gravel and cobbles, with interstitial sand. The banks are composed of a variety of materials, depending on the landform intersected by the channel. Sandy silt overlying Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021

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IN MAY, 1991 IIS* MAY, 1991

FIGURE 12: MODESTE CREEK CROSSING NEAR BRETON gravel is found in most areas where the river is either Between 1950 and 1995, a number of long, U-shaped actively building its floodplain or cutting into previous meanders downstream of the crossings were abandoned as floodplain deposits. Materials ranging from gravel to clay a result of cutoffs. There were no cutoffs upstream of the are found in areas where the river flows against the valley crossing until a flood in mid-June of 1996, when virtually walls or terrace segments. all of the flow was diverted down a small existing secondary channel to the east, effectively abandoning a 3± The river has an average slope of about 0.0030 and is km long section of the river (Figure 13). characterized by almost continuous near-vertical cutbanks along the outsides of bends and large unvegetated gravelly Field observations indicate that adjustment of the river to point bars along the insides. At low stages, diagonal bars the shortening and consequent local steepening caused by and occasional mid-channel bars also become exposed the upstream cutoff has been mainly through accelerated and the flow drops through a fairly regular series of steep, lateral erosion (i.e. channel widening). However there are shallow riffle sections in crossovers between bends. also indications of increased meander activity and the

Average D50 and D90 sizes of the gravel on the surface of cutoff channel is in the initial stages of developing a long exposed bars appear to be about 80 mm and 120 mm, sweeping left-hand bend immediately upstream of the respectively. crossings. The bend is migrating southward and is expected to cause erosion problems at the crossings in the Aerial photographs suggest that the river is characterized future unless remedial action is undertaken (Figure 14). by relative high rates of gravel transport during high flow periods. The gravel tends to accumulate at the entrance of The two 114.3 mm O.D. gas line are located in the middle sharply curving bend sections, causing extensive bank of a 50 m wide cleared corridor running almost due north- erosion and much of the flow to cut across downstream south. Within this corridor, there are also 2 high voltage meander loops to form cutoffs. Typically the loops are cut power transmission lines, 2 operating pipelines to the off by chutes that break diagonally or directly across the west, and at least two other operating pipelines to the east. neck. Prior to the 1996 cutoff, erosion problems had developed along the north bank of the river at the pipeline crossings located in the eastern or downstream half of the corridor • redirect the flow down the presently abandoned series and some local bank armouring was installed. Because of of meander loops above the crossings by constructing the cutoff, the point of attack has now shifted to the a channel and dyke at the inlet to the existing cutoff unprotected existing south bank at these crossings. as shown on Figure 15.

The cutoff channel parallels the corridor for a The cost of the first option, crossing reconstruction, was considerable distance above the existing crossings and the expected to be very high given the fact that deep burial Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021 east bank of this reach is actively eroding, particularly would be required for almost the entire width of the active along the outside of a long, sweeping bend located floodplain. There were also concerns that directional roughly 300 m to the north. The present separation drilling would not be viable due to the possible presence distance between this bend and the corridor is very small of deep gravel in the bottom of the valley. (less than about 30 m). In addition, floodplain levels in this area are relatively low and it is possible that the river As the cost of flow redirection was an order of magnitude could breach the bank and flow eastward across the smaller than the cost of crossing reconstruction, corridor, threatening the transmission lines and all of the discussions were initiated with the regulatoiy authorities pipelines (Figure 14). in the fall of 1999 to determine whether or not this approach would be acceptable from an environmental For the two 114.3 mm O.D. gas lines, two basic options viewpoint. The project was approved and construction of were considered. the diversion channel and dyke were completed in March of2000. • rebuild both crossings to accommodate future channel shifting

JULY, 195® JULY, 1978

MAY, 1993 JULY, 1997

FIGURE 13: HISTORY AERIAL PHOTOGRAPHS, FREEMAN RIVER SWAN HILLS mMHJL rtmiKE BAJfK EHDSIOH EXISTING) CROSSINO ' ABANDONED OXBOW

FLOW Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC2000/40245/V001T03A010/2507311/v001t03a010-ipc2000-148.pdf by guest on 27 September 2021

PROPOSED ACTIVE EROSION CHANNEL

RECENT CUTOFF CHMKL NORTH: /RIGHT OF WAV •f m&imm CORRIDOR IPw* (5 LINES)

FIGURE 14: LOOKING SOUTH, FREEMAN RIVER NEAR SWAN HILLS

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FIGURE 15: LOOKING WEST, FREEMAN RIVER NEAR SWAN HILLS

CONCLUDING REMARKS A variety of solutions are available to mitigate situations which threaten the integrity of pipeline crossings. Anticipating the potential for and recognizing the Selection of the most appropriate remedial action plan is existence of channel instability is a critical aspect of the highly site specific and depends on a thorough assessment design and maintenance of river and stream crossings. A of a number of factors. Although the bottom line in the thorough analysis of the stability of a river or stream must selection process is usually cost, security of supply, include consideration of past channel changes as well as potential environmental impacts, maintenance geomorphic analysis to predict future changes. This will requirements, viability of construction, impacts on river enable the pipeline operator to focus, from an operation hydraulic characteristics, and the economic importance of and maintenance viewpoint, on the most critical areas of the crossing are also important considerations. concern and develop a plan for implementing any necessary remedial action before, rather than after, The case histories presented in this paper highlight the pipeline integrity has been compromised. need for proactive inspection. By utilizing appropriate mitigation measures, the maintenance of river and stream crossings can be successfully accomplished.