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API RP 1133-2017 pdf free download

API RP 1133-2017 pdf free download.Managing Hydrotechnical Hazards for Pipelines Located Onshore or Within Coastal Zone Areas.
4 Overview of Hydrology and Hydrotechnical Hazards at Onshore and Coastal Zone Pipelines Pipelines are static features within a dynamic environment with rivers, floodplains and coastal zones representing some of the most active areas within a landscape. Rivers can change course, migrate, deepen, and widen slowly over time or suddenly during large flood events. Coastlines and shorelines can retreat during hurricanes and, coastal land loss, particularly throughout coastal Louisiana, can cause pipeline exposure and suspension of previously buried pipeline segments. These hydrologic and geomorphic effects can put pipelines at risk for damage or failure. Understanding how rivers and coastal zones change and alter the landscape provides a framework for realizing the potential toll that hydrotechnical hazards can have on pipeline infrastructure and can increase the effectiveness in protecting the environment during the design, construction, operation, and integrity management of pipelines at river crossings, and throughout coastal zones. A brief description of riverine and coastal hydrology and hydrotechnical hazards is presented below. 4.1 Riverine Hydrology and Fluvial Geomorphology The magnitude of hydrotechnical hazards and potential for damage to pipeline integrity is greatest at pipeline water crossings during flood events. Water depth is deepest and water velocity is typically fastest during floods. As a result, floods have the potential to carry large debris, impart significant bending stresses and vortex-induced vibrations on unsupported pipeline spans, and potentially remove sediment from river bottoms and soils from banks, thereby exposing previously buried pipelines to lateral water forces and impact from waterborne debris. Understanding a river’s hydrology and geomorphology can help assist pipeline operating companies to better assess a river’s potential for scour, channel migration, avulsion, etc. at pipeline water crossings.
4.1.1 Overview—Flood Timing, Duration, Magnitude, and Frequency The hydrology of a particular river or stream is defined by the timing, duration, and quantity of flowing water. The observed rise and fall of the water levels or discharge at a given point can be graphed over time; such a graph is called a hydrograph. The portion of a hydrograph where the water levels increase from base flow to the flood event’s peak is termed the “rising limb”. The portion of the hydrograph once the peak has passed and water levels return to base flow is defined as the “falling limb”. The timing of flooding describes when flood events typically occur throughout the year. For example, rivers draining the Rocky Mountains tend to have predictable springtime flooding in response to snowmelt. Conversely, rivers within the arid American southwest and mid-west tend to flood in response to large rainfall events which can potentially occur throughout the year. Timing of flood events is more predictable within areas dominated by snowmelt, and consequently, flood preparation and response planning is easier than in areas where flooding is dominated by extreme rainfall events. The duration of typical flood events can vary across North America and understanding expected flood durations is important in flood preparation and planning. Snowmelt floods tend to build up slowly to a peak and then gradually decrease over time, often taking weeks to rise and fall.
The magnitude of a particular flood event is often described in terms of a flood frequency or return period, that is, an “X year return period flood event”. The return period (“X” number of years) is the “recurrence interval” which represents the average interval of time expected before the next flood event of equal or greater magnitude can be expected to occur over the long-term and should not be viewed as the number or years between floods of a given size. The exceedance probability is another term used to describe the size of a flood event and is the mathematical inverse of the recurrence interval and equals the probability that a given flood of a given size will occur per year. Determining the flood frequency for a given location within a river system requires a statistical analysis using the recorded history of annual peak flood events from a particular location. Further details regarding flood frequency calculation is provided in Annex C. 4.1.2 Overview—Fluvial Geomorphology The shape and size of a river and the extent and shape of a river’s floodplain is a key component in determining the depth of water and velocity during flooding, and therefore, the potential magnitude of channel bottom scour and erosion. Given similar flood flows, the larger a river’s cross section is, the lower the water velocity. During flood events that overtop a river’s channel banks, water can spill across the floodplain reducing the rate of increase in the water depth and velocity. Consequently, changes to a river’s floodplain can cause significant changes within the river itself. Construction of levees or roads across the floodplain can reduce the floodplain area and the amount of flood water that can flow across it, thereby increasing the depth and velocity of water within the channel during flood stage. The rate at which the channel bottom and banks are scoured and eroded is also related to the types of sediment that make up the channel and the vegetation type, and density lining the channel.

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