Other Features HEC-RAS Jon Fripp NDCSMC 2016 Module 8: Other - PowerPoint Presentation

Other Features HEC-RAS Jon Fripp NDCSMC 2016

Module 8: Other Features Horizontal variation in Manning’s nVertical variation in Manning’s n Floodway encroachments Graphical cross section editing Drop StructuresChannel modifications ….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’s n - Is that a real concern?

5 Horizontal variation in Manning’s n Under “Cross Section Data”C lick on “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

Vertical variation in Manning’s n Can be used to reflect hydraulic changes that vary by either elevation or flow Allows the user to vary Manning’s n both vertically as well as horizontally Note: Calibration is more difficult

Vertical variation in Manning’s n Under “Cross Section Data”, Click on “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

Vertical variation in Manning’s n 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

13 Floodway Encroachments Main Channel Floodway Floodway Fringe Floodway Fringe 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 width Method 3: percent reduction in conveyance Method 4: target water surface increase Method 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 original original water 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 sections In 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 plans Limits should be smooth

Graphical Cross Section Editing Available under “Tools” menu from Geometric Data Editor. Allows user to: Add, move, and delete points Add normal ineffective flow areas Add blocked ineffective flow areas Add normal blocked obstructions Add multiple blocked obstructions Add levees Assign bank station graphically

Graphical Cross Section Editing Move objects Delete objectsBank stationsIneffective areas Blocked obstruction Manning's n Remember that it is changing the geometry Be careful with naming the file

y c y b 3 or 4 y b y b = 0.715 y c Rectangular channel stilling basin:used to dissipate energy subcritical subcritical Drop Structures - 2 options 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 weir H = Upstream Energy Head Modeling a Drop Structure as an Inline Weir Q = CLH 3/2 A good approach where interest 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 shown at right for a broad crested weir, the tailwater does 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 6 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 the tailwater and headwater elevations.

Modeling a Drop Structure as an Inline Weir Under the “Geometric Data ”Click on “Inline weir” A series of windows allow for entry of weir characteristics

Modeling a Drop Structure with a Series of Cross-Sections An appropriate approach where interest is in a profile through the drop When placing 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 ).

26 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. At transition 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).

29 Example - Modeled as an In-Line Weir Cross Sections Inline Weir

30 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 to impose a template or a series of trapezoidal cuts into the existing channel geometry Typically 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 Tools See two options 1 - Channel Design/ModificationNew & improved Simpler and streamlined 2 - Channel Modification (original) Classic A little cumbersome Channel Modification

1. Channel Design/Modification Template design Simple trapezoid option Options – New – give it a name Create a trapezoidal template Apply it to a single section or a range of sections Template shows up on existing cross-section

1. Channel Design/Modification Template design User defined table Options – New – give it a name Create a channel template Apply it to a single section or a range of sections Template shows up on existing cross-section

2. Channel Modification (original)Create a channel template (up to 3 trapezoidal cuts) Apply to selected range Can independently specify left and right side slopesCan change roughness Can set new reach lengths Apply it to a single section or a range of sections Template shows up on existing cross-section

Channel Design/Modification Things to keep in mind:Rename the geometry fileUse a different plan name Review cut/fill calculations (they are useful but approximate) Consider sedimentation and table channel design criterion

37 Any Questions?