Standard step method: Difference between revisions

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{{Short description|Computational technique}}

The '''standard step method (STM)''' is a computational technique utilized to estimate one-dimensional surface water profiles in open channels with gradually varied flow under steady state conditions. It uses a combination of the energy, momentum, and continuity equations to determine water depth with a given a friction slope <math>(S_f)</math>, channel slope <math>(S_0)</math>, channel geometry, and also a given flow rate. In practice, this technique is widely used through the computer program [[HEC-RAS]], developed by the US Army Corps of Engineers Hydrologic Engineering Center (HEC).<ref>{{cite web|last=USACE|title=HEC-RAS Version 4.1 User's Manual|publisher=Hydrologic Engineering Center, Davis, CA}}</ref>

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[[File:Open Channel Flow Energy Lines.jpg|thumb|'''Figure 1.''' Conceptual figure used to define terms in the energy equation.<ref>{{cite book|last=Chaudhry|first=M.H.|title=Open-Channel Flow|year=2008|publisher=Springer|___location=New York}}</ref>]]

[[File:E-y Diagram.jpg|thumb|'''Figure 2.''' A diagram showing the relationship for flow depth (y) and total Energy (E) for a given flow (Q). Note the ___location of cricitalcritical flow, subcritical flow, and supercritical flow.]]

The energy equation used for [[open channel flow]] computations is a simplification of the Bernoulli Equation (See [[Bernoulli Principle]]), which takes into account pressure head, elevation head, and velocity head. (Note, energy and head are synonymous in Fluid Dynamics. See [[Pressure head]] for more details.) In open channels, it is assumed that changes in atmospheric pressure are negligible, therefore the “pressure head” term used in Bernoulli’s Equation is eliminated. The resulting energy equation is shown below:

:<math>H = z+y+\frac{v^2}{2g}</math> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <big>'''Equation 1'''</big>

For a given flow rate and channel geometry, there is a relationship between flow depth and total energy. This is illustrated below in the plot of energy vs. flow depth, widely known as an E-y diagram. In this plot, the depth where the minimum energy occurs is known as the critical depth. Consequently, this depth corresponds to a [[Froude Number]] <math>(F_n)</math> of 1. Depths greater than critical depth are considered “subcritical” and have a Froude Number less than 1, while depths less than critical depth are considered supercritical and have Froude Numbers greater than 1. (For more information, see [[Dimensionless Specific Energy Diagrams for Open Channel Flow]].)

:<math>F_n=\frac{v}{(g\frac{A}{B})^{0.5}}</math>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <big>'''Equation 2'''</big>

Under steady state flow conditions (e.g. no flood wave), open channel flow can be subdivided into three types of flow: uniform flow, gradually varying flow, and rapidly varying flow. Uniform flow describes a situation where flow depth does not change with distance along the channel. This can only occur in a smooth channel that does not experience any changes in flow, channel geometry, roughness or channel slope. During uniform flow, the flow depth is known as normal depth (yn). This depth is analogous to the terminal velocity of an object in free fall, where gravity and frictional forces are in balance (Moglen, 2013).<ref>{{cite web|last=Moglen|first=G.|title=Lecture Notes from CEE 4324/5894: Open Channel Flow, Virginia Tech|url=http://filebox.vt.edu/users/moglen/ocf/index.html|accessdate=April 24, 2013|url-status=dead|archiveurl=https://web.archive.org/web/20121105134341/http://filebox.vt.edu/users/moglen/ocf/index.html|archivedate=November 5, 2012}}</ref> Typically, this depth is calculated using the [[Manning formula]]. Gradually varied flow occurs when the change in flow depth per change in flow distance is very small. In this case, hydrostatic relationships developed for uniform flow still apply. Examples of this include the backwater behind an in-stream structure (e.g. dam, sluice gate, weir, etc.), when there is a constriction in the channel, and when there is a minor change in channel slope. Rapidly varied flow occurs when the change in flow depth per change in flow distance is significant. In this case, hydrostatics relationships are not appropriate for analytical solutions, and continuity of momentum must be employed. Examples of this include large changes in slope like a spillway, abrupt constriction/expansion of flow, or a hydraulic jump.

== Water surface profiles (gradually varied flow) ==

Typically, the STM is used to develop “surface water profiles,” or longitudinal representations of channel depth, for channels experiencing gradually varied flow. These transitions can be classified based on reach condition (mild or steep), and also the type of transition being made. Mild reaches occur where normal depth is subcritical (yn > yc) while steep reaches occur where normal depth is supercritical (yn<yc). The transitions are classified by zone. (See figure 3.)

[[File:Surface Water Profiles.jpg|Surface Water Profiles]]

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Using Figure 3 and knowledge of the upstream and downstream conditions and the depth values on either side of the gate, a general estimate of the profiles upstream and downstream of the gate can be generated. Upstream, the water surface must rise from a normal depth of 0.97 m to 9.21 m at the gate. The only way to do this on a mild reach is to follow an M1 profile. The same logic applies downstream to determine that the water surface follows an M3 profile from the gate until the depth reaches the [[conjugate depth]] of the normal depth at which point a hydraulic jump forms to raise the water surface to the normal depth.

'''Step 4:''' Use the Newton Raphson Method to solve the M1 and M3 surface water profiles. The upstream and downstream portions must be modeled separately with an initial depth of 9.21 m for the upstream portion, and 0.15 m for the downstream portion. The downstream depth should only be modeled until it reaches the conjugate depth of the normal depth, at which point a hydraulic jump will form. The solution presented explains how to solve the problem in a spreadsheet, showing the calculations column by column. Within Excel, the goal seek function can be used to set column 15 to 0 by changing the depth estimate in column 2 instead of iterating manually.