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Importing HEC-2 Data Into HEC-Importing HEC-2 Data Into HEC-RASRAS

What you need to know…

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Not all HEC-2 options have a parallel in HEC-RAS

Computation of Manning’s n values from high watermarks.

Archiving (AC) Free format (FR)

These options are ignored by the importer, so data sets which contain them can still be imported.

Some HEC-2 options are available, but are not imported

Split flow Vertical variation of Manning’s n values Storage outflow for HEC-HMS

What you should know first… What you should know first…

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All imported data should be reviewed carefully for accuracy and completeness, especially...

Special bridges (SB) and culverts (SC) Normal bridges (X2, BT) Encroachments and floodway determination (X3, ET) Ineffective flow areas (X3) Channel improvements and modifications (CI)

How the HEC-2 cross sections are Identified… By river mile or station? By an arbitrary numbering scheme? Are the section ID numbers increasing from downstream to

upstream? Are there duplicate section ID numbers?

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The Import Process – Step 1The Import Process – Step 1 Create a new project …

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Give your new project a title and a unique file name …

Confirm your title and file name…

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The Import Process – Step 2The Import Process – Step 2 After a new project has been opened, select

Import HEC-2 data… from the File menu.

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Select the HEC-2 file to be imported…

Tell HEC-RAS how to identify the cross sections as they are imported…

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Use the Geometry and Steady Flow Editors to review the imported data*.– The note below will appear whenever an HEC-2 data

set that contains bridge or culvert data is imported. This is a reminder that bridges and culverts are not handled in the same way in HEC-RAS as they are in HEC-2. You will need to check that the data has been interpreted correctly.

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Computational Differences Computational Differences between HEC2 & HEC-RASbetween HEC2 & HEC-RASCross Section Conveyance CalculationsCritical Depth CalculationsBridge Hydraulics

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• Cross Section Conveyance CalculationsCross Section Conveyance Calculations

– HEC-RAS Method: subdivisions at n-value break points.

n1 n2 n3 n4

A1 P1 A2 P2Ach Pch

A3 P3

Klob = K1 + K2

Kch

Krob = K3

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– HEC-2 Method: subdivisions at every ground point.

n1 n2 n3 n4

A2P2 A3 P3Ach Pch

A6 P6

Klob = K1 + K2 + K3 + K4

Kch

A4 P4 A5 P5A7 P7

A1 P1 A8 P8

Krob = K5 + K6 + K7 + K8

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– To choose the conveyance subdivision method to use, select Steady Flow Analysis… from the Simulate menu:

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– Select Conveyance Calculations… from the Options menu of the Steady Flow Analysis editor

– And select your method…

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• Critical Depth CalculationsCritical Depth Calculations

HEC-RAS– Parabolic Method

– Secant MethodUsed if parabolic

method… Does not converge. Finds a value at the

top of a levee or at an ineffective flow elevation.

HEC-2– Parabolic Method

Uses first local minimum it finds.

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– To choose, first select Critical Depth Computation Method… from the Options menu of the Steady Flow Analysis editor:

– Then select the method:

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• Bridge HydraulicsBridge Hydraulics

HEC-RAS– 4 Low Flow Methods

Energy Momentum Yarnell WSPRO

– 2 Independent High Flow Methods

Energy Pressure &/or Weir

HEC-2– 2 Low Flow Methods

Normal (Energy) Special (Yarnell)

– 2 High Flow Methods dependent upon Low Flow Method

Energy Pressure & Weir

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• HEC-2’s Special Bridge Method – Low FlowHEC-2’s Special Bridge Method – Low Flow

HEC-RAS– All methods (including

Yarnell’s) use actual bridge geometry to determine flow area

– Each pier is defined and located individually

– Bridge opening is determined by geometry.

HEC-2– Trapezoidal

approximation of bridge opening used to determine flow area

– Single equivalent width pier centered within trapezoid

– Net flow area input for pressure flow.

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• HEC-2’s Special Bridge Method – Pressure FlowHEC-2’s Special Bridge Method – Pressure Flow

HEC-RAS– Fully submerged or

upstream-only submerged.

– Bridge opening based on geometry.

– Weir profile is the union of the cross section and bridge deck.

HEC-2– Fully submerged

condition assumed

– Net flow area input for pressure flow.

– Weir profile is defined by BT or X2 data – not cross section.

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• HEC-2’s Normal Bridge MethodHEC-2’s Normal Bridge Method

HEC-RAS– Piers are defined

separate from the deck or ground.

– The deck data must have a spatial relationship to the cross section(s) but need not match ground points.

HEC-2– Piers were defined as

part of the cross section data or deck data.

– Each BT data point was required to match a cross section GR point.

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• Culvert HydraulicsCulvert Hydraulics

HEC-RAS– Wide variety of shapes

available: Box Pipe Arch

– Pipe– Low & High Profile– ConSpan

Ellipse– Vertical– horizontal

– Multiple culverts of different shapes and sizes can be placed at a single crossing.

HEC-2– 2 Shapes available:

Box Pipe

– Multiple barrels of only one shape and size can be used at a single crossing.

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• Floodway DeterminationFloodway Determination

HEC-RAS– Methods 1-3

– Method 4 0.01 ft. accuracy

– Method 5 Optimizes for Water

Surface or Energy or Both.

– Floodway determination is independent of blocked obstructions.

HEC-2– Methods 1-3

– Method 4 Parabolic Interpolation

– Method 5 Optimizes for a change

in Water Surface

– Method 6 Optimizes for a change

in Energy

– X3 encroachments override ET floodway determination.

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What can HEC-RAS do that What can HEC-RAS do that HEC-2 cannot?HEC-2 cannot? Mixed Flow Multiple Openings Momentum computation at bridges and junctions Complex dendritic streams and looped networks Blocked Ineffective Areas Normal Ineffective Areas at any station Blocked Obstructions Levees Inline and Lateral weirs and gated spillways Geometric cross section interpolation