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Published byAllie Chevis Modified over 2 years ago

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Flood Profile Modeling with Split Flows and Weirs Objective: To become familiar with RAS split flow and weir options.

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Split Flows General Modeling Guidelines Flow Around an Island Divided Flow Approach

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Weirs Inline Spillways and Weirs Entering Inline Spillway/Weir Data Lateral Spillways and Weirs Entering Lateral Spillway/Weir Data Controlling Inline and Lateral Gate Settings Inline and Lateral Spillway/Weir Output

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Flow Around an Island

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Separate River Reaches

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Splitting the Cross Sections

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Split Flow Optimization RAS can optimize flow splits at: Lateral weirs Hydraulic connections Stream junctions 1.Program calculates a water surface profile with flows given in flow data file 2.Using computed profiles new flows are determined and profile is re-run 3.Process continues until flows match

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Turning on the Split Flow Optimizer From Steady Flow Analysis screen, select Options, Flow Optimizations…

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What is saved by RAS Flow data are not changed after optimization You should input computed flows into the flow data editor and run the program again to see if the answer changes

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Modeling Weirs in HEC-RAS Two types of weirs: Inline weirs Lateral gated spillways Can be used to model the following: Large Dams Run of the river structures Drop Structures or natural drops Lateral storage facilities Natural overflows to ponding areas Levee breaching or dambreaks

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Inline Weirs and Gated Spillways

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Cross Section Locations 2 3 4 EXPANSION REACH CONTRACTION Gated Spillways Overflow Weir FLOW Inline Weir River Stationing is 2.5

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Cross Section 2 Cross section 2 is located a short distance downstream from the structure. The computed water surface at this cross section will represent the tailwater elevation of the weir and the gated spillways. This cross section should not include any of the structure or embankment, but represents the physical shape of the channel just downstream of the structure.

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Cross Section 3 Located a short distance upstream of the embankment Represents the physical configuration of the upstream channel. The structure and the roadway embankment are described by combination of the deck/road embankment data, cross section 3, and the gated spillway data

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Sluice and Radial Gates Up to 10 gate groups can be used at any one river crossing. Each gate group can have up to 25 identical gate openings Either gate type can be used with an ogee crest shape or a broad crested weir

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Setting the Gate Opening

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Inline Spillways and Weirs Radial gates (often called Tainter gates) Vertical lift gates (Sluice gates) Ogee or Broad Crested Weir shapes for both gated spillways and overflow weirs Gate equations can handle low flow, normal gate flow (upstream submerged) or fully submerged gate flow (both ends submerged) Up to 10 gate groups Up to 25 identical gates per group.

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Radial Gates Where: H = Z U - Z sp C = Discharge Coefficient, typically 0.6 – 0.8

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Submerged Radial Gates When the downstream tailwater increases to the point at which the gate is no longer flowing freely (downstream submergence is causing a greater upstream headwater for a given flow), the program switches to the Submerged Form of the equation: Where: H = Z U - Z D

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Fully Submerged Gate Flow Orifice Equation Where: H = Z U – Z D C = Discharge coefficient, typically 0.8 A = Area of gate opening

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Sluice Gates Where:H = Upstream energy head (Z U – Z sp ) C = Coefficient of discharge (0.5 to 0.7) W = Width B = Vertical opening

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Submerged Sluice Gates Where:H = Z U - Z D Transitions to fully submerged orifice flow at 0.8 submergence : Submergence begins when the tailwater depth above the spillway divided by the headwater energy above the spillway, is greater than 0.67

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Low Flow Through Gates Z sp U D Z Z H Upstream water surface is equal to or less than the top of the gate opening

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Uncontrolled Over-Flow Weirs Can represent emergency spillway or flow over entire embankment. Uses standard weir equation Can have Ogee or Broad Crested weir shape Weir flow submergence is calculated

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Entering Inline Spillway/Weir Data

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Weir and Embankment Profile

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Gated Spillway Data

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Lateral Spillways and Weirs Radial and sluice gates Uncontrolled overflow weir Same hydraulic equations for gates as with inline spillways Lateral weir can handle a sloping water surface as well as irregular weir profile. Includes culvert flap gates (flow limited to one direction)

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Cross Section Layout

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Lateral Spillway and Weir Hydraulics

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Lateral Weir Calculations See notes on next slide for definition of the constants

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Lateral Weir Calculation Equation The equation for a sloping line representing the water surface and the weir segment are shown on previous slide Constants a ws and a w represent the slope of the water surface and the weir segment, respectively Variables C ws and C w are constants representing the initial elevations The standard weir equation assumes that the weir is parallel with the water surface. The above general equation is derived for a sloping weir and water surface by integrating the standard weir equation

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Entering Lateral Spillway/Weir Data

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Lateral Weir Embankment Data

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Lateral Weir Gate Data

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Lateral Weir Culvert Data

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Lateral Diversion Rating Curve Use for modeling irregular structures

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Controlling Inline and Lateral Gates For steady flow simulations specify number of gates open and gate opening height

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Controlling Inline and Lateral Gates For unsteady flow simulations the following gate controls are available from the HEC-RAS unsteady flow data editor: Time Series of Gate Openings Elevation Controlled Gates

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Time Series of Gate Openings

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Elevation Controlled Gates

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Inline and Lateral Spillway/Weir Output The following types of output are available for inline and lateral spillways/weirs: Stage and Flow Hydrograph Plots Time Series Tables Profile Plots Cross Section Plots (Inline Spillways/weirs only) Detailed Output Tables Profile Summary Tables

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Example Profile Plot With a Lateral Spillway/Weir

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Cross Section Plot of Inline Spillway/Weir

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Detailed Output Table For a Lateral Weir

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Profile Output Table for Lateral Weir

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