No-Rise Certification, How do I review a HEC-RAS Model? - Ben ...

© Amec Foster Wheeler 2015.

No-Rise Certification: How do I review a HEC-RAS Model?2016 TN AFPM Annual Conference – August 24 2016

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HEC-RAS Overview


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� Go to USACE website http://www.hec.usace.army.mil/� You are able to download older versions of HEC-RAS� http://www.hec.usace.army.mil/software/hec-ras/

Downloading HEC-RAS


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Project (input) files

Project (*.prj)

Plan (*.p01, p02, ...)

Geometry (*.g01, g02, ...)

Steady Flow (*.f01, f02, ...)

Output files

Run (*.r01, r02, ...)

Output (*.o01, o02, ...) – binary

File Management


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► Cross Section geometry

► Manning’s n values

► Reach lengths

► Network connectivity

► Ineffective Area

► Blocked Obstructions

► Expansion/Contraction Coefficients

► Bridge and Culvert Geometry

Geometry Files (*.g01)


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► Profile Discharges and River Stations

► Entered from upstream to downstream

► Different From HEC-2

► Boundary Conditions

Steady Flow Files (*.f01)


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► Geometry and Flow Files Used

► Sub- Super- or Mixed Calculations

► Floodway Encroachments

► Friction Slope Calculation Method

► Conveyance Calculation Method

► Calculation Tolerances

► Flow Distribution Calculations

Plan Files (*.p01)


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► Project Title

► Default Parameters

► Available Plan, Geometry, Flow etc. files

► Active Plan

Project File (*.prj)


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► Open Project and Steady Flow Analysis

► Can go back and forth between plans (model scenarios)

Verify Which Models Have Been Received


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► Right click on “.pXX” (plan file) extension

► Click “Select the Program from a list”, Chose Notepad or similar

► The version will display on top right of document

Verify Version of HEC-RAS Used


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► Make plan active that you want to run

► Open Steady Flow Analysis

► Click “Compute” (verify there are no error messages)

Running HEC-RAS


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► Click on “Run” and select “Run Multiple Plans”

► Click on “Check All Plans” to run all plans

► Then click “Run All Checked Plans”

► Model will run and you should get a “HEC-RAS Finished Computations” screen

Run Multiple Plans


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► Questions about Program – See Users Manual

► Questions about Engineering – See Hydraulic Reference (can click question mark at some locations)

Getting Help


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► Each HEC-RAS model outputs a lot of data

► Can view data in Graphical and Tabular Output

Viewing Results


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Graphical Output


Cross

Sections

Water

Surface

Profiles

General

ProfilesRating

Curves3D Perspective

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Cross Section Viewer

Graphical Output Examples


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Water Surface Profiles

Graphical Output Examples


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Tabular Output


Individual

Cross

Sections

Summary

Tables

Errors and

Warnings

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Individual Cross Section


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Summary Tables


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Errors and Warnings


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Detailed Hydraulics Review


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Typically Submitted HEC-RAS Models/Plans

► Effective Hydraulic Model

► The hydraulic analysis used in the effective FIS

► Obtain from FEMA library

► Duplicate Effective Hydraulic Model

► A copy of the effective model reproduced on the requestors computer

► Should not be modified – unless required to allow model to run

► Adding distance from upstream XS to bridge

Hydraulics


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Typically Submitted HEC-RAS Models/Plans

► Corrected Effective Model

► Corrects errors in the duplicate effective

► Adds cross sections

► More detailed topography

► Must NOT reflect man-made changes since the date of the effective model

► Existing Conditions Model (Pre-Project Conditions)

► Modified version of the Duplicate or Corrected Effective model

► Includes any modifications since the date of the Effective

► If no modifications since the effective:

► The Duplicate Effective or Corrected Effective becomes the Existing Conditions

► Proposed Conditions Model

► Modified version of the Existing Conditions model

► Includes modifications to reflect the project

Hydraulics


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CHECK THAT MODEL INPUTS ARE CONSISTENT BETWEEN EXISTING AND PROPOSED CONDITIONS PLANS!!!!!

Detailed Hydraulics Review


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► Check flows in HEC-RAS (select “Edit/Enter steady flow data”)

► Verify flows are the same for 1% annual chance and floodway

► If new hydrology is NOT submitted, flows should match effective model

Verifying Flows


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► Normal slope

► Estimated from channel invert at

downstream end of the reach

► Known water surface elevation

Starting Boundary Condition


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► Measuring Normal Slope

Starting Boundary Condition


HEC-RAS Hydraulic

Reference Manual

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► XS’s should be based upon topography

► Perpendicular to flow

► Placed at pinch points and immediately downstream of pinch points

► At breaks in ground profile

► XS’s should be placed at changes in Manning’s “n”

► XS’s should pick up large obstructive buildings

► General rule on XS placement is 2 or 3 times the width of the floodplain

► XS placement can vary depending on floodplain width and conditions of stream

► Ensure there is not a large stream section without a XS

► Keep in mind, effective model has been approved

Cross Section Placement


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► Ensure cross sections don’t cross stream centerline more than once



This XS is close to

crossing twice,

but it is

acceptable

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► Ensure cross sections do NOT intersect each other



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► Cross sections should be perpendicular to stream and floodplain

► If skew is greater than 30 degrees, a skew angle should be utilized in HEC-RAS



Avoid XS’s not perpendicular to stream More Ideal XS placement

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Two ways to view cross sections:

► Cross Section Data Viewer

► Graphical Cross Section Editor

Cross-Section Geometry


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► New cross-sections should be based on:

► Survey

► Recent Topography

► Spatial cross section

should correspond to

HEC-RAS cross section

► Effective cross section geometry can be used

Cross-Section Geometry


Survey

LiDAR

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► Typically at breaks in geometry

► Surround lowest effective point

► Consistent to survey and channel on aerial

► Surround stream centerline

► Consistent with adjacent banks unless warranted

► Check Bank Elevations via Profile

Banks


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► Checking Consistency

Banks


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► Manning’s ‘n’ is a roughness coefficient

► Channel and overbank values

► Horizontal Manning’s values

(see Table 3-1 in the HEC-RAS Reference Manual)

Manning’s ‘N” Values


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► Table 3-1 in the HEC-RAS Reference Manual

Recommended “N” Values


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► IFA typically placed for 3:1 expansion and 1:1 contraction

Ineffective Flow Area


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1

1

IFA

Flo

w D

irection

FP

XS

XS1

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► Typically 0.3 and 0.5 around structures

► 1 XS downstream

► 2 XS upstream

► Typically 0.1 and 0.3 for all XS’s not around structures

► Could be changed in parts of floodplain besides structures

► Drastic changes in floodplain width

Expansion/Contraction Coefficients


Old Railroad

embankment – not

input into the hydraulic

model

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► Obstructions should be used when a building/structure is obstructing flow

Obstructions


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► Distance from one cross section to another along the flow paths

► 3 flow paths

► Center flow path is equal to the stream centerline

► Left and right flow paths are theoretically

supposed to be drawn along the centroid of flow

between the banks and edge of the floodplain.

► Visually 1/3 and 1/2 the distance from the banks and

edge of floodplain

► Visually check on map

Reach Lengths\Flow Paths


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Reviewing Bridges & Culverts


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► Cross Section 2 and 3

► Off embankments

► Out of drainage ditches

► Cross Section 1 and 4

► “Fully Expanded”

► Typically, XS 4 should be

closer than XS 1

Cross Section Location atStructures


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Model should match existing and proposed construction drawings

► Bridge

► Opening height and width

► High and low chord elevations

► Sloping abutments

► Piers – width and shape

► Culverts

► Opening size

► Number of openings

► Length

► Invert elevations

► Manning’s ‘n’

Bridge/Culvert Data


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► Does conveyance opening agree with proposed plans?

► Compare measurements to Drawing

► Opening Height and Width

► Location of opening in respect to channel

► Railing and deck thickness

► Check bridge deck width and distance to US

XS

► Min. Weir Flow

► Typically okay to be blank

Checking Deck/Roadway Data


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► Quantity & location in HEC-RAS

► Width, height and detail

Checking Pier Data


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► Low Flow methods

► Always Energy

► Momentum - Piers

► “Highest Energy Answer”

► High Flow methods

► Pressure/Weir (typical)

► Energy – significantly overtopped

► Methodology can vary

► Often used to smooth profile or fix floodways.

► Need justification

Bridge Modeling Approach


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► More than one structure opening in the same cross section.

► Divides the area that contributes to each opening.

Multiple Openings


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► Across bridges/culverts, ensure that the IFA is consistent among profiles

► Approximately 1:1 ratio

► Outside opening(s)

► XS 3 – Upstream Face

► Elevation = Road Deck (typically)

► XS 2 – Downstream Face

► Elevation = between road deck and

top of opening (typically)

Ineffective Flow at Structures (IFA)


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► Ground surface profile

► Generally decreasing going downstream – unless based on survey

Ground and Water Surface Profiles


► Water surface profiles

► Profiles cannot cross

► Jumps and drawdowns

• Need to be corrected unless

warranted (ex. Jumps coming out of

a culvert)

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► Areas of critical depth should be investigated

► Need modeler justification

Critical Depth


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Reviewing Floodways


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► The regulatory floodway, adopted into the community’s floodplain management ordinance, is the stream channel plus that portion of the overbanks that must be kept free from encroachment in order to convey the 1% annual chance flood discharge without increasing flood levels by more than 1.0 foot.

Floodway Definition


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► Check surcharges

► Allowable range is normally

between 0.0 and 1.0

► Check encroachments

► Inside floodplain

► Within effective flow area

► Outside structure openings

► Equal to banks or wider

► Avoid drastic changes to floodway width from cross-

section to cross-section

A General Floodway Review


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Table for Reviewing Floodways


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CHECK-RAS


http://www.fema.gov/check-ras-hec-ras-validation-tool

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► Program Developed by FEMA to check HEC-RAS models

► Checks for compatibility with FEMA requirements.

► Checks for compatibility with basic assumptions

► It is not a substitute for engineering judgment

What is CHECK-RAS?


No-Rise Certification, How do I review a HEC-RAS Model? - Ben ...

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