IPC2000-148 CASE HISTORIES OF PIPELINE EXPOSURES AT STREAM CROSSINGS IN ALBERTA 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 river 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 rivers 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 - NORTH SASKATCHEWAN RIVER 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.