The HEC-RAS Model Refresher

Minmin Shu P.E.Transportation Review UnitWater Resources Division

Michigan Department of Environmental Quality12-6-2018

What Does the HEC-RAS Mean

• RAS----River Analysis System• HEC----Hydrologic engineering Center of U.S. Army Corps of Engineers

What Kind Analysis the HEC-RAS Can Do

• Steady flow water surface profiles (one dimensional)• Unsteady flow simulation (one or two dimensional)• Sediment transport/movable boundary computations• Water quality/water temperature analysis modeling

Which Part of HEC-RAS Software We Use Most in the Hydraulic Analysis for Obtaining the Part 31 Permit

1. Steady flow analysis(one dimensional) ---- for most cases

2. Unsteady flow simulation (one dimensional)

Steady Flow Analysis (one dimensional)

• We use energy grade line elevations (EGLE) instead of water surface elevations in Part 31 review.

• We focus on the EGLEs comparison of existing and proposed condition to decide if the proposed condition has equal or greater hydraulic capacity and no harmful interference.

• The modeling that we required does not mainly focus to find the precise floodplain elevation, but if there is FEMA detail study near or at the proposed site, the FEMA floodplain elevation should be considered as the boundary condition of the flow file in your model.

• The other conditions.

Unsteady Flow Simulation (one dimensional)

If the flow wave is dynamic: • The discharge will vary in space and time, attenuating as it moves

Downstream.• The calculated discharge-stage ratings will not be unique.

In practice if there is a dam, retention/detention pond, lake or other storage area, unsteady flow simulation may be considered.

A Simple RAS model

• Project title - Project file (.prj)• Geometric data – Geometry file (.g01, …)• Design flow - Flow file (f01,…)

Create a Project and Save it---.prj

Geometry file

River ReachCross SectionBridge/CulvertInline StructureLateral Structure…..

Basic Rules for Geometry FileDirection

• Draw the river reach from upstream to downstream• Looking downstream, input the cross sections, road profile and

structures from left to right.

Length of the model (Reach Length)• Minimum length from upstream of contraction starting to downstream

of the expansion ending. If there is backwater…..?

Number of the Stream Cross Sections• 4 are the minimum numbers of the stream cross sections, 2 on

upstream and 2 on downstream of the road.

Ineffective Flow Area

• A part of the cross section area that contains water which is not actively being conveyed (velocity is near or equal to zero).

• Select Ineffective Mode

1. Normal ineffective flow area.

2. Multiple blocks without/with check Permanent option.

• Station & Elevation Setting

1. Station:Ratio of 1 to 1 on upstream and 2 to 1 on downstream; by program default or by site inspection result.

2. Elevation:Low point of the road if it is within the stream floodplain.

Please Remember---

When using ineffective flow areas, no additional wetted perimeter is added to the active flow area.Ineffective flow area vs. obstructionsObstruction– permanent blocked out a part of cross section area and add the wetted perimeter when the water comes in contact with it.

Contraction and Expansion Coefficient

Contraction and expansion coefficients are to evaluate the amount of energy loss that occurs because of a flow contraction or expansion

The difference of setting on

the energy only vs. Pressure and/or weir

If the Bridge Has Piers

Geometry File Issues1. Should they be included If there are

other structures within or near the studied river reach?

2. How to modify If the EGLEs of the proposed condition are higher than that of existing condition?

3. How to test if the last downstream cross section is far enough?

Flow file

• Flow values• Flow change locations• Flow boundary conditions

Flows & Locations

Q100 290 cfsfrom RS 2846 to upstream of RS 1596.

Flow starting at RS 1596 will cover the stream to end of model.

Flow Boundary Conditions

Select option of “set boundary of one profile at a time”, high light the “Downstream” of the target flow and make the change.

How to Establish a RAS Model Quick

• Design flow, site survey data and design of proposed condition on hand

• Import data electronically

Import Data Through the HEC-RAS

• From HEC-RAS main window, under File, select Import HEC-RAS Data, any HEC-RAS format file could be imported.

• From HEC-RAS Geometric Data window, under File, select Import Geometry Date, only geometry file can be imported, you have the options to select whole geometry file or some items in a geometry file.

Simulating Each Plan

• 3 flow regimes • Set flow distribution

Read the Results

• Stand table• User defined table

In above red circle, click on the left button or right button, different stand tables will open.

• In above red circle, click on the left button open the Cross Section Output, under Type there are many other stand tables

• Click on the right button, open the Profile Output Table.

• Under the Options there is a list of the stand tables.

• Under Optionsyou could select to include different information in profile table.

Still under Optionsyou could select Define Table, and create Table Headingand save it as a special table, it will be kept under User Tables, it can be used when you run other projects.

Read the Figures

• Profiles• Cross sections• others

Profile Plot

Cross Section

3-D View

Refine the Model Based on If the Results Show an Issue

• Read the Errors Warnings and Notes• Adjust the model settings• Adjust the proposed design

If You Still Have Unsolved Issues

We are here to assist, please email your model and support data to the Transportation Unit’s engineers:

• Linda Hansen P.E., email: HansenL6@michigan.gov• Ashely Roznowski P.E., email: RoznowskiA@michigan.gov• Minmin Shu P.E., email SHUM@michigan.gov• Jim Watling, P.E. Supervisor. Email WATLINGJ@michigan.gov