Other Features
HEC-RAS
Jon FrippNDCSMC2016
Module 8: OtherFeatures
Horizontal variation in Manning’s nVertical variation in Manning’s nFloodway encroachmentsGraphicalcross section editingDrop StructuresChannelmodifications
….awareness level….
….in the 1-D world…
Horizontal variation in Manning’s n
Useful where a cross section bisects a stand of trees or a large parking lot.
.035
.08
.025
.06
.09
.05
.06
Horizontal variation in Manning’sn -Isthat a real concern?
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Horizontal variation in Manning’s n
Under “Cross SectionData”Clickon “Options”Then“Horizontal Variation in n Values”
Horizontal variation in Manning’s n
An extra column will appear.A roughness value must be entered for the first ground point.HEC-RAS will apply this value for all points having a higher ground station until another is entered.Enter the roughness value for each different area.
Horizontal variation in Manning’s n
Always a good idea to examine the section plot and check the entries
Can be used to reflect hydraulic changes that vary by either elevation or flowAllows the user to vary Manning’s n both vertically as well as horizontally
Note: Calibration is more difficult
Under “Cross Section Data”,Clickon “Options”Then“Vertical Variation in n Values”
Vertical variation in Manning’s n
A table will be opened that allows the user to enter Manning’s n values for stations/flows or station/ elevation combinations
The program will interpolate Manning’s n values whenever the actual water surface is between entered elevations.If the water surface is below the first elevation/flow, the program will use the first one.
Floodway Encroachments
FEMA defines a floodway as”…a channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water-surface elevation by more than the designated height.”Normally the base flood is the 100-year event and the designated height is 1 foot.The floodway is usually determined by encroachment analysis
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Floodway Encroachments
MainChannel
Floodway
FloodwayFringe
FloodwayFringe
Natural Water Surface
Encroached Water surface
The floodway fringe removed by the encroachment is assumed to be completely blocked.
Under “Steady Flow Analysis”click on “Options” then “Encroachments”
Floodway Encroachments
Floodway Encroachments
A screen will appear that will allow the user to enter the relevant data by River station.Five Methods:Method 1: right/left encroachment stationsMethod 2: top widthMethod 3: percent reduction in conveyanceMethod 4: target water surface increaseMethod 5: target water surface increase and max change in energy
Note: Usually methods 4 or 5 are used to get a first cut, final run is usually made with method 1
Floodway Encroachments
Method 1:A very direct method. The user enters estimates of the right and left encroachment stations. Typically used for a final assessment.Method 2:The user specifies a top width and HEC-RAS computes the left and right banks such that 1/2 of the specified top width is to either side of the channel.Method 3:HEC-RAS determines the encroachment stations such that a user specified percent reduction in the original conveyance is achieved. For example: is the user enters 30, the program will place the encroachment stations such that the conveyance is 70 percent of original.Method 4:Widely used. HEC-RAS will determine the encroachment station such that the conveyance in the encroached section is equal to the conveyance of the natural section at the originaloriginalwater surface. The computed surface may be less than or greater than the specified target increase.Method 5:Conceptually identical to method 4. The user specifies a target water surface increase and maximum change in energy. Uses a different optimization scheme. Possibly more accurate where there are large distances between stations and where the overbank is steep. Used for super critical flows (EGL vs HGL).
The goal is to determine the limits of encroachment that will cause a specified change
Floodway Encroachments
The initial runs may provide changes that are greater or smaller than target changes.Check sectionsIn this case, the user should go to another method to refine the estimate (method 1?)
Note: HEC-RAS assumes roughness along the interface unlike ineffective areas where it assumes a water interface along the wetted perimeter
Will not go within channel
Floodway Encroachments
Other Checks:Check local development plansLimits should be smooth
Graphical Cross Section Editing
Available under “Tools” menu from Geometric Data Editor. Allows user to:
Add, move, and delete pointsAdd normal ineffective flow areasAdd blocked ineffective flow areasAdd normal blocked obstructionsAdd multiple blocked obstructionsAdd leveesAssign bank station graphically
Graphical Cross Section Editing
Move objectsDelete objectsBank stationsIneffective areasBlocked obstructionManning's n
Remember that it is changing the geometryBe careful with naming the file
yc
yb
3 or 4yb
yb= 0.715 yc
Rectangular channel
stilling basin:used to dissipate energy
subcritical
subcritical
Drop Structures - 2options in HECRAS
1. Modeling a Drop Structure as an Inline Weir.2. Modeling a drop structure with cross-sections through the drop.
The standard weir equation is used:where: C = 2.6 - 4.0 (dependent on shape)L = Length of weirH= Upstream Energy Head
Modeling a Drop Structure as anInline Weir
Q = CLH3/2
Agoodapproach whereinterest is primarily in water surface profiles upstream and downstream of structure.
The inline weir option in HEC-RAS accounts for reduced flow over the weir due to submergence. This is accomplished by multiplying the weir coefficient by a submergence reduction factor. As shownat rightfor a broad crested weir, thetailwaterdoes not affect the weir until it is greater than 76 percent submerged.
Modeling a Drop Structure as an Inline Weir - Cross Section Layout
1
2
3
4
5
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Inline Weir Structure
Cross Sections
Model the floor blocks as blocked obstructions.
The user must place cross sections in the appropriate places to get an adequate estimate of thetailwaterand headwater elevations.
Modeling a Drop Structure as an Inline Weir
Under the “Geometric Data”Clickon “Inline weir”A series of windows allow for entry of weir characteristics
Modeling a Drop Structure with aSeries of Cross-Sections
An appropriate approachwhere interest is in a profile through the dropWhenplacing cross-sections near and through a drop structure, they need to be placed where the water surface and velocity are changing rapidly (this applies when using an inline weir also).
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Modeling a Drop Structure with a Series of Cross Sections
Cross Sections
Model the floor blocks as blocked obstructions.
For a vertical drop, as the one shown above, the downstream cross section locations are the same as if modeling the structure with the inline weir option.
Cross sections need to be closely spaced where the water surface and velocity is changing rapidly (i.e. just upstream and downstream of the drop).
Cross Section Layout for Ogee Shapes Drop Structure
Cross Sections
The energy equation assumes that the water maintains a hydrostatic pressure distribution. This assumption begins to break down for slopes greater than 10 percent. Therefore the results at the cross sections on the face of the drop will have some error. In general, the program will show a lower water surface than what might actually occur on the face of the drop.
If the drop structure is an Ogee shaped drop, then you may want to place several cross sections along the drop at very short intervals (2-5 feet). This will allow the program to compute the transition from subcritical to supercritical flow over the weir.
Remember: energy slope is closely related to cross section spacing requirements.Attransition locations where flow is going from subcritical to supercritical or vice-versa, cross-section spacing should be smaller to help define the jump or draw down curve.Hydraulic jump occurs between 2 cross-sections. In order to locate it more precisely, the user should decrease the cross-section spacing (consider interpolated cross-sections).
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Example - Modeled as an In-Line Weir
Cross Sections
Inline Weir
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Example - Modeled with a Series of Cross Sections
Cross Sections
The HEC-RAS model cannot predict how long of a distance it will take for the jump to occur, but it can predict where the jump will begin.
Channel Modification
Allows user toimpose a template or aseries of trapezoidal cuts into the existing channel geometryTypically done for planning level studies
Does not address stable channel design. The user must separately assess the potential for scour or aggradation to bed and banks
From the Geometry Window, Select ToolsSee two options1 - ChannelDesign/ModificationNew &improvedSimpler and streamlined2 - Channel Modification (original)ClassicA little cumbersome
Channel Modification
1.Channel Design/Modification
Template designSimple trapezoid optionOptions – New – give it a nameCreate a trapezoidal templateApply it to a single section or a range of sections
Template shows up on existing cross-section
1.Channel Design/Modification
Template designUser defined tableOptions – New – give it a nameCreate a channel templateApply it to a single section or a range of sections
Template shows up on existing cross-section
2.ChannelModification (original)
Create a channel template (up to 3 trapezoidal cuts)Apply to selected rangeCanindependently specify left and right sideslopesCanchangeroughnessCanset new reach lengthsApply it to a single section or a range of sections
Template shows up on existing cross-section
ChannelDesign/Modification
Things to keep in mind:Rename the geometry fileUse a different plan nameReview cut/fill calculations (they are useful but approximate)Consider sedimentation and table channel design criterion
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