13 Split Flow and Weirs

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Flood Profile Modeling with Split Flows and Weirs

Objective: To become familiar with RAS split flow and weir options.

Split Flows

• General Modeling Guidelines

• Flow Around an Island

• Divided Flow Approach

• 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

Flow Around an Island

High Ground

Stagnation Point

Separate River Reaches

r u ce

Splitting the Cross Sections

0 20 40 60 80 100 120 140 16060

Divided Flow Plan: Divided Flow Opening River Mile 3

Station (ft)

Legend

EG PF#1

WS PF#1

Ground

Bank Sta

.035 .025 .035

0 20 40 60 80 100 120 140 16060

Divided Flow Plan: Divided Flow Opening River Mile 3

Station (ft)

Legend

EG PF#1

WS PF#1

Ground

Bank Sta

.035 .025 .035

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

Turning on the Split Flow Optimizer

From Steady Flow Analysis screen, select Options, Flow Optimizations…

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

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

Inline Weirs and Gated Spillways

0 200 400 600 800 1000-20

Inline Weir and Gated Spillay - Ex 12 Plan: Gated Spillway

Station (ft)

Legend

Ground

Bank Sta

Cross Section Locations

EXPANSIONREACH REACH

CONTRACTION

Gated Spillways

Overflow Weir

Inline Weir River Stationing is 2.5

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.

Ineffective Flow Area Stations and Elevations

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

Sluice and Radial Gates

Sluice Gate

Broad Crested Spillway

Radial Gate

Ogee Spillway Crest

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

Setting the Gate Opening

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.

Radial Gates

HEBETE HBTWgCQ 2

Where: H = ZU - Zsp

C = Discharge Coefficient, typically 0.6 – 0.8

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:

HEBETE HBTWgCQ )(2

Where: H = ZU - ZD

Fully Submerged Gate Flow

Orifice Equation

gHCAQ 2

Where: H = ZU – ZD

C = Discharge coefficient, typically 0.8

A = Area of gate opening

Sluice Gates

gHBWCQ 2

Where: H = Upstream energy head (ZU – Zsp)C = Coefficient of discharge (0.5 to 0.7)W = WidthB = Vertical opening

Submerged Sluice Gates

HgBWCQ 2

Where: H = ZU - ZD

gHCAQ 2

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

Low Flow Through Gates

2/3HLCQ

Upstream water surface is equal to or less than the top of the gate opening

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

Entering Inline Spillway/Weir Data

Weir and Embankment Profile

Gated Spillway Data

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)

Cross Section Layout

MainChannel

LateralWeir

Lateral Spillway and Weir Hydraulics

ChannelInvert

Main ChannelBank Elevation

OverflowWeir Gated

SpillwaysWaterSurface

Lateral Weir Calculations

Yws = awsX + Cws

Yw = awX + Cw

))()((5

2 2/5111

2/5121

CxaCxaa

See notes on next slide for definition of the constants

Lateral Weir Calculation Equation

• The equation for a sloping line representing the water surface and the weir segment are shown on previous slide

• Constants aws and aw represent the slope of the water

surface and the weir segment, respectively

• Variables Cws and Cw 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

Entering Lateral Spillway/Weir Data

Lateral Weir Embankment Data

Lateral Weir Gate Data

Lateral Weir Culvert Data

Lateral Diversion Rating Curve

Use for modeling irregular structures

Controlling Inline and Lateral Gates

For steady flow simulations specify number of gates open and gate opening height

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

Time Series of Gate Openings

Elevation Controlled Gates

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

Example Profile Plot With a Lateral Spillway/Weir

Cross Section Plot of Inline Spillway/Weir

Detailed Output Table For a Lateral Weir

Profile Output Table for Lateral Weir

13 Split Flow and Weirs

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