Example for Civil 3D & Hecras

Hand On Training

For

Bridge Hydraulic Analysis

Via CIVIL-3D-2009 & HEC-RAS

Example: BRIDGE AT STATION 65+870

(AN05107-Benguela-Lubango Road Project)

1. INTRODUCTION AND OBJECTIVES

1.1 GeneralHEC-RAS is the one of popular programs that is made for hydraulic analysis. It has been designed to perform one-dimensional hydraulic calculations for a full network of natural and constructed channels. In spite of many benefit of this program, some shortcomings are experienced while running HEC-RAS.

The most significant shortcoming is the geometric input data. HEC-RAS conventional data entry procedure requires lots of manual work and gives engineers a burden of manipulating. AutoCAD Civil 3D could be an excellent assistant to aid this time-consuming work. AutoCAD Civil 3D allows engineers to develop geometric data for import in HECRAS and view exported water surface profile result data. As a result, the engineer could concentrate on hydraulic principle, rather than time-consuming work, during model analysis.

1.2 Objectives

The example presented in hand is for a bridge crossing with an ephemeral wadi stream in Angola. The main goal of this example is designated to perform a complete hydraulic analysis of a wadi bridge by using the integrated package that includes the following programs: AutoCAD Civil 3D and HEC-RAS.

The objectives of this comprehensive “hand-on training” example are to train the workshop participants to be able to carry out the following tasks:- How to generate automated river/wadi cross-sections using AutoCAD Civil 3D.- How to import the generated cross-sections to HEC-RAS.- How to carryout Hydraulic analysis using HEC-RAS and how to get the corresponding water surface profile and velocity variations through a bridge.- How to carryout scour analysis using the HD module in HEC-RAS in order to estimate the contraction scour as well as the local abutment and piers scour.

1.3 Used SoftwaresThe following software should be available and installed in order to be able to carry out the hand on training example:

- HEC-RAS version 3.1 or up.- AutoCAD Civil 3D 2007 or up.

1.4 Site DataThe bridge is located in Benguela-Lubango , Angola. The catchment area equals 158.67 Km2. The stream length is estimated as about 22 km and the

average catchment slope is 0.01 m/m. The following data are used in the design:

a) Roadway data and site data survey:PGL at the bridge site = 514.524Roadway width = 15mNatural wadi bed level at bridge location (u/s side)= 509.68 Natural wadi bed level at bridge location (d/s side)= 509.60

b) Rainfall dataRainfall depth corresponding to the 50 yrs event (P50) = 142.5 mm Rainfall depth corresponding to the 100 yrs event (P100) = 167.3 mm

c) Catchment dataCatchment area = 158.67 km2Max. stream length = 22 kmAverage catchment slope = 0.01 m/mTime of concentration (Bransby William) = 495.32 min.

d) Design flow data50yrs Peak discharge (Q50) = 259.1 m3/sec100yrs Peak discharge (Q100) = 359.8 m3/sec

e) Initial bridge spanBased on the given data, the initial expected bridge span ranges from 3.2 to 4.7 √Q = 60m to 90m.Based on this simple analysis, the initial guess of the proposed bridge span will be taken as 60m with 2 sets of piers (interspacing = 20m).It should be noted that the aforementioned initial bridge span dimensions will be checked using the following HEC-RAS Hydraulic Analysis.

2. GENERATING CROSS-SECTION BY AUTOCAD CIVIL 3D:

2.1 Open AutoCAD Civil 3D

By clicking the icon shown in Fig.1 (Fig.1)

Select this item: _AutoCAD Civil 3D (Metric) NCS LDT.dwt

(Fig.2)

In this step, the trainee is asked to add the contour data survey of a sample project using the attached AutoCAD file: Contour _Bridge_st65.dwg

This file contains the contour data survey of a bridge location in Angola.

2.2 check Units of theProject .

General

Tool space

(Fig.3)

Choose Meters for Drawing Units

2.3 Create Surface .

After pressing O.K. select all contour lines in the file

After that we will notes that there is aline is added around the contour

In case of this line does not appear we will make it again.

2.4 Draw flow line .

User should draw the flow line as the same direction of the flow

User can draw polyline by using this icon

Or by press (PL) then press Enter

In our Example the flow direction will be in the

Same as shown Fig.

Then go to Alignments

Creat Alignment from polyline

Remove this right sign

Then user should select the polyline

2.5 Draw The Cross-Section .

User should draw the polylines cross section as prevoius

The cross-section will draw as follow

Then user should select the Alignment

Press O.K.

After select the polyline press Enter

Creat Multiple Section Views

After pressing O.K. Click at any point in the drawing

Now the cross-section will be appear.

2.6 Export Data to Hec-Ras:

Select toolbox

Select HEC RAS Export

Export (Browse for save data as .Geo file)

Angola Bridge STA.65+870.prj

3. IMPORTING CROSS-SECTION DATA FROM GEO-RAS TO HEC-RAS

3.1 OPEN Hec-RAS

By double click on HEC-RAS icon, the following window will appear:

Fig. 25 Make sure that the

SI (metric) units is active

3.2 Go to File

If not go to options

New Project

Unit system

Select “system international”

(Metric system)

Fig. 26

Write the title: Angola Bridge and File Name: STA. 65+870(1)

(1) Note that user must remove the star from the file name

3.3 Edit

Geometric Data

Fig. 27

Our stream reach with its sections will appear as follow:

Fig. 28

3.4 Save data as follow

Enter the name of geometric data (say Natural)

Fig. 29 then, browses to save data

Sec. 408.7355

Sec. 352.5538355

Sec. 328.4692

3.5 Check Data It is recommended to check the geometric data to make sure that no errors have been taken place.

Cross section

Fig. 30

3.6 Adjusting Local Losses for the nearby bridge sections

The default values of contraction expansion Coeff. are 0.1 and 0.3 respectively However ,as flow approaches the bridge these values should be raised to account for the bridge local losses

The common practices is to set the contraction coeff. To 0.3 (increased by 0.2)&the expansion coeff. To 0.5 (i.e. increased by 0.2)

There will be done for the following sections

Scroll to go through all the imported X-section Geometries

3.7 Inserting Manning (n) coefficient

Data will be shown as follow

Insert “n” value =0.033

4. IMPORTING BRIDGE DATA

4.1 Click Edit and/or Create bridges and culvert

Fig. 31

4.2 Insert Bridge at station (340.5114)

Fig. 32

Upstream cross-section

Downstream cross-section

Now the bridge is inserted at the aforementioned station and Fig.33 shows the upstream & downstream cross sections of the bridge

Fig. 33

4.3 Click Deck /Roadway to insert the roadway or the bridge deck.

Fig. 32

In this step, the geometry of the roadway embankment is entered by entering the lateral station and elevations (high chord, low chord levels) of the shown points in Figure 33. Following that, the roadway data window will be as in the Figure below.

Click copy US to DS since they are similar.

5. EDITING PIER DATA In this task, we will enter the pier data. In this example we have two sets of piers.

Click pier

Fig.33

(1) (2) (4)(3) (5)

(6)

Pier No. 1

Fig.34

Copy the pier #1 data by clicking the copy button shown in Figure 34 to transfer the common data to the second pier. Change the centerline station in the u/s and d/s as shown in Figure 35.

Fig.35

Pier No. 2

6. ENTERING FLOW DATA AND BOUNDARY CONDITIONS

EDIT/ENTER STEADY FLOW DATA

This two profiles means we have two

Alternatives:

Design flow of 50 year =259.1 m3/sec

& 100 year flow of 359.8 m3/sec

Select Normal Depth type boundary and

Insert Upstream &Downstream slopes=0.01

it should be mentioned that in other examples

the u/s slope might be different than the d/s slope

Fig.35

No. of cases (we choose 2 cases: 100Yr, 50Yr)

Before adding the bridge & piers user can make the following :

1- After adding the flow (Fig.35) user can run the data by pressing

Note:

Use mixed flow regime If it is expected to have subcritical and super critical flow in the same reach/river.

Mixed

2- Export data to Civil 3D :

In the main HEC-RAS window, click File

Export GIS Data

In the GIS Export dialog box:

- Under Results Export options, select export Water Surfaces.- Select any profiles you may have created during the analysis.- Click export data. an SDF file is created .

3- Open Civil 3D file (which included the first surface and the alignment and the cross section)

In the Import SDF dialog box:

- Select the site for reach alignment.- Select the style for reach alignment.- Select the alignment label style set.- Select the profile style.

After that user will find that the section locations and flood elevation profiles are imported into the drawing.

Browse for HEC-RAS File

7. ADJUSTING/INSERTING OF THE INEFFECTIVE FLOW AREA IN UP&DOWN STREAM CROSS-SECTION BRIDGE

In this step, user could adjust the ineffective flow areas that have been added already via GeoRas (Item 4-step 9)

Fig. 36

Fig. 37

Click to adjust/enter the ineffective area for the u/s Bridge cross section

Click to adjust/enter the ineffective area for the d/s Bridge cross section

8.INSERTING ABUTMENT DATA

Upstream (Bridge) Downstream (Bridge)

Fig. 38

In this step, user will add the abutment data (if the abutment type is not vertical),in many cases ,the abutment type is spill-through accordingly it is essential to present the slopes of the abutment faces so as to have the correct local abutment scour.

It should be mentioned that the failure of not representing the slopes of the abutment might lead to significant under estimation of the abutment local scour.

The data for each abutment consist of a skew angle (this is optional) and the station and elevation information. The station and elevation information represents the high chord information of the abutment. The low chord information of the abutment is assumed to be below the ground, and it is therefore not necessary to enter it. The geometric information for each abutment can vary from upstream to downstream. If this information is the same

(1)

(2)

(1)

(2)

(1)

(2)(2)

(1)

Abutment No. 2

Abutment No. 1Abutment No. 1 Abutment No. 2

Abutment No. 2

Abutment No. 1

9. FLOW DISTRIBUTION LOCATIONIt is important for scour analysis to use the local velocities (not the average values) near the abutment toe and bridge piers. HecRas has a built in module that could be used to calculate the local velocities from the average cross-section velocities. This can be done by activating the flow distribution module as follow.

This data will be like that

Fig. 39

10. ADDING INTERPOLATED SECTIONS

The accuracy of the predicted wsp depends on the distance between the x-sections.

Accordingly it is recommended to have the sections spacing not to big .User could add interpolated sections as follow

Fig. 40

No. of interpolated section

Click here to interpolate the sections

11. RUNNING STEADY FLOW ANALYSIS

Perform a steady flow simulation

Mixed

Compute

User might face one of the following cases:

Case 1: there is no error

Fig. 41

Use Mixed flow regime If it is expected to have subcritical and super critical flow in the same reach/river.

Case 2: there is an error:

Error Sample #1

Sometimes, error messages are pupped up and the following screen might appear:

Fig. 42

Station and elevation data contains duplicate points. This problem can be solved by using the cross section filter as follow:

Solution

Error statement

Fig. 43

By this step, the duplicated points problem has been resolved. Now, let us demonstrate another probable error that could be faced.

1-Click here to choose sections

2-Filter points on selected XS

Then press O.K

Solution Over than 500 point

Error Sample #2

Fig. 44

Error statement

The maximum number of station-elevation points, blocked obstructions ,and levees is 500 point.

This error means that the number of points in a section exceeded the maximum number (500 points). This problem could be resolved by decreasing the x-section points. This can be done by using the filter.

Change it to 0.2 or any value until no. of points decrease below 500 points, then press O.K and rerun the model again.

12. POST PROCESSING

12.1 Check Max. water level

*Press (Ctrl) to measure the distance between the max. Surface water level to the bridge

*If the available free board is not sufficient, some cross section reshaping might be a solution or longer bridge span might be required.

0.6 m min.

Min. 2.4 to 2.6 (set by Bridge)

In case of high debris, use higher values

12.2 Check Velocity

High velocity area just d/s of the bridge accordingly, protection is required

12.3 results (scour calculation)

Perform Hydraulic design Computation.

Click the Hydraulic Design icon to start scour calculations

12.3.1 Filling the missing data for Contraction Scour.

D50 of soil = 1mm for size particles (from sieve analysis

of bore holes)

Coefficient depends on Temperature

Check the discharge for return period 100 Yrs.

12.3.2 Filling the missing data for Pier Scour.

D95 of soil = 3mm for size particles (from sieve

analysis of bore holes)

Click K1 button

Temperature in the bridge zones button

D50 of soil = 1mm for size particles (from sieve

analysis of bore holes)

3.00

Insert the skew angle. We put it zero if the road

Perpendicular on bridge

12.3.3 Filling the missing data for Abutment Scour.

We choose the correction factor for abutment shape based on the following cases:

1- when the value of (LYa

) < 10 use K1 = 0.55 (Spill-through abutment)

(This data is for example only)

Insert the skew angle. We put it 90 if the road

Perpendicular on bridge

Choose Correction factor for abutment Shape

Choose the biggest of left and right abutment to

estimate L/ya

2- when the value of 10< (LYa

) > 25 use K1 = 0.82 (Vertical with wing walls)

(This data is for example only)

3- when the value of (LYa

) > 25 use K1 = 1.00 (Vertical abutment)

After this make Compute then Report.

Choose the biggest of left and right abutment

Choose the biggest of left and right abutment

12.3.4 Exploring scour results.1- Local Pier Scour

2- Local Abutment Scour

Local Pier Scour Depth

Local Abutment Scour Depth Choose the biggest from the left& right abutment scours

Contraction Scour Depth only

3- (General + Contraction) Scour

HEC-RAS estimates the local and contraction scours only. General scour should be estimated by other techniques such as Regime theory

For General Scour:

General scour could be calculated using Lacy equation or any other equations based on the regime approach. In this example, an in-house excel program has been used to calculate it (the Excel program is attached with the Example).

- Input data for “in house” Excel program (general/contraction scour solver)

Take average

Fill the excel sheet using the data from HEC-RAS as per the following Figures.

Where:

- Channel Slope

H1

V

H2

Q2W2

W1

Q1

D50

- Bridge Width- No. of Piers

- Pier Width

Bridge Width

No. of piers

Pier Width

- General/Contraction Excel Solver Results

Then select the biggest of the following to estimate the General+contraction scour depth:

1. The average value from Excel Solver.2. The Lacey value from Excel Solver+ contraction value from HEC-RAS

General + contractionGeneral only

Choose the Bigger

12.4 Export cross section

User can export u/s and d/s bridge cross sections as “dxf” File as follow:

Lines and symbols, user can change the lines shape and the symbols of the cross section as follow:

Browse to save cross section, then press o.k.