API RP 1133:2010 pdf download

API RP 1133:2010 pdf download.Managing Hydrotechnical Hazards for Pipelines Located Onshore or Within Coastal Zone Areas.
4.2.1 Vertical Channel Movement
Vertical channel movement, or scour, is often classified as either general or local scour and occurs in response to a given storm or flood event in contrast to lateral channel movement which typically occurs gradually over time. Both general and local scour can occur simultaneously within a particular cross section but are the result of different hydrologic and geomorphic processes. A variety of scour and erosion equations and models are available for predicting flood-related general and local scour (i.e., Shields 1936, Neill 1973, U.S. Bureau of Reclamation 1984, U.S. Army Corps of Engineers 1994, Melville and Coleman 2000, etc.). Equation and model complexity can vary greatly and the selection, application, and assessment of results should be done carefully. It should be noted that predictive models can often be conservative.
4.2.1.1 General Scour
With the rise in water level, or stage, during a flood there is an associated increase in water velocity, and subsequently the force of water acting on the channel bottom. As a result, the channel bed tends to scour during flood events. Buried pipelines can potentially become exposed or undermined while flood levels are increasing and then become re-buried when the flood levels recede. This occurs when water velocities slow down resulting in sediment carried from upstream are deposited back on the channel bottom (Figure 1). This phenomenon may not occur on every channel or during every flood and is dependent upon a variety of factors, including the flood duration and magnitude, the type of soils and sediment being carried, and how much sediment is supplied from upstream. For example, depending upon the depth and velocity of water during floods and the size of channel bottom sediments, erosion and scour may not occur until very large flood events. However, on other channels where the water velocity may be high, the sediments composed of smaller grains scour and erosion may occur during more frequent flood events. General scour can also be influenced, and typically increased, where the flow becomes confined as it passes through a narrow bridge opening or culvert which can increase water velocity and erosion potential. It is important to note that this increased scour potential occurs both at the confinement location and for some distance downstream from the confinement as the increased water velocity is maintained until the effect of the confinement dissipates as the channel returns to a wider and larger cross section shape.
4.2.1.2 Local Scour
Local scour results in the removal of sediment at a particular location in the channel induced by obstructions to the flow, such as around pipeline supports, bridge piers, abutments, rip-rap revetments, adjacent pipelines, and large woody debris. Local scour can also occur as the water accelerates around a channel bend or even at the confluence of two separate channels (Figure 3). Obstructions located upstream of a pipeline crossing can cause local scour that may extend downstream hundreds of feet and impact the crossing. Local scour depths are generally larger than general scour. Section 6.6.1 provides additional guidance on monitoring for features near pipeline water crossings that can cause local scour and increase risks to pipeline integrity.
4.2.1.3 Channel Degradation
Unlike temporary scour and fill that can occur during a particular flood event, channel degradation tends to occur over a longer period of time, over a longer reach of river, and tends to result in the permanent lowering of bed levels. Channel degradation typically results from an overall imbalance between the supply of water and sediment from a watershed. Such imbalances can occur with the construction of large flood control dams that trap a significant portion of the sediment. This loss of sediment results in an imbalance in the sediment supply, and the river will often respond by eroding and degrading its channel bed. Urbanization can both decrease the supply of sediment while also increasing the supply of water to a river system. As a result, rivers in urbanized settings will typically degrade their channel beds. Levee construction, removal of beaver dams, and channel straightening are common causes of channel degradation. Degradation is a common hydrotechnical hazard at pipeline water crossings.