HECRAS BASED SCOUR DESIGN

A quick guide by Chris Chatfield, P.E.

How to Discuss Scour???#1: Scour Component Fundamentals

#2:Characteristics of Stream Stability

#3: Shaping HECRAS Inputs for Proper Calculation

TOTAL SCOUR COMPONENTS➔ ABUTMENT ➔ PIER

➔ CONTRACTION

ABUTMENTVARIETY OF METHODS DEVELOPED TO ESTIMATE EROSION OF BRIDGE ABUTMENT

➔ FROELICHBased on empirical data derived from laboratory experience. Dependent on a length of protrusion into the channel that over-exaggerates the depth of scour.

➔ HIREDerived from field data of scour at the end of spurs in the Mississippi River. Slightly better method than Froelich of estimating scour, but not realistic.

Spur: object placed in channel to encourage stream to shift in certain direction

PIEROCCURS DUE TO THE DOWNWARD VELOCITY OF WATER TO THE BOTTOM OF THE PIER. THERE ARE TWO POSSIBLE EQUATIONS OF USE:

➔ COLORADO STATE UNIVERSITY (CSU)Most commonly used equation in most circumstances.

➔ Froehlich Some empirical support, Not recommended by HEC 18.

PIER SCOUR VARIABLES➔ Pier Nose Shape (K1)➔ Angle Of Attack (K2)➔ River Bed Condition (K3)➔ Bed Material Armoring (K4)➔ Width of Pier (a)➔ Depth of Flow (y1)

CONTRACTION

OCCURS DUE TO THE CONTRACTION OF FLOW BETWEEN THE FLOODPLAIN UPSTREAM AND THE FLOW PASSING THROUGH THE BRIDGE

CALCULATIONS ARE DIVIDED INTO TWO MAJOR CATEGORIES

➔ LIVE BEDSoil material is transported from upstream of bridge into bridge section. Based on the soil size specified.

➔ CLEAR WATERSoil material is not transported into bridge section from upstream

CONTRACTION SCOUR VARIABLES

➔ Approach/Bridge Flow (Q1/Q2)

➔ Approach/Bridge Width (W1/W2)

➔ Diameter of Soil Particle (Dm)

➔ Depth of Flow (y1)

CLEAR WATER LIVE BED

THERE ARE HOWEVER, SOME FACTORS THAT INDIRECTLY CONTRIBUTE TO THE TOTAL SCOUR CALCULATIONS…. Tip

Never use the default output of a hydraulic program unless carefully considered.!!!

EFFECT OF DEBRI

➔ Increase of pier area exacerbates scour

➔ Research conducted but no comprehensive methodology.

➔ Best prevention is complete maintenance plan.

Think about the stream as a whole system...Signs of instability::

➔ Varied Channel Width

➔ OxBow Lakes

➔ Point Bars

➔ Cut Banks

➔ Historic Shifting of Channel

Point Bar

OxBow Lake

Looking at overall stream characteristics gives a BIGGER PICTURE of potential scour issues that may probably occur in the future

Data Sources You Can’t Live Without:1. Google Earth (Historical Imagery

Option)

2. Maintenance Records (Ex:To clarify if clear water/live bed scour at structure)

3. Any information on structures upstream/downstream of the one in question

4. Soil Data Information (In some cases soil maps...other cases, borings)

He loved soccer, but feared he had no way to talk to a coach or teammates.

TipNot all data is created equal. Be sure to know the data source if information obtained from a general database hub.

WHAT TO DO IF STREAM IS UNSTABLE???➔ Design bridge with abutments to be

‘shiftable’ to compensate for changing stream

➔ Consider counter-measures to ‘train’ the river. For example, place spurs or other devices in the stream to encourage the stream to stay within desired banks.

➔ Implement frequent maintenance plan to track how the stream is changing.

Bendway Weirs AlternatingSpurs

But Doesn’t HECRAS Automatically Spit Out Scour For You?????

Yes…..

BUT IT’S IMPORTANT NOT TO LET THE PROGRAM DETERMINE THE APPROPRIATE VARIABLES!HERE ARE A FEW POTENTIAL PITFALLS...

APPROPRIATE REFERENCE SECTION

THE CHOSEN APPROACH CROSS SECTION (FOR Y1, Q1, & W1 VARIABLES)

➔ Sediment EquilibriumThe approach cross section should have equal sediment coming into it as leaving it.

➔ Comparable Geometry The approach cross section should have similar bank and channel areas.

MISC. VARIABLESKeep in mind the following factors in adjusting HECRAS for scour calculations:

➔ Width (W1,W2)Defaults to top width of section. May need to specify bottom width instead if section is non-uniform

➔ Bank StationsEnsure that bank stations only encompass areas of sediment transport.

➔ Mannings ValuesFEMA models often over-estimate values for WSE calculations, Fine tune based on material present in location.

TO CONSIDER CONCERNING ABUTMENT SCOUR...The abutment scour calculation performed by HECRAS is unreasonable and should not be included in the scour report. The following measures are recommended for abutment scour reduction:

➔ Sloped AbutmentsUse of this type (standard in most DOTs) reduces scour potential (as opposed to vertical abutments).

➔ Clear Abutment from ChannelEnsure that the abutments do not protrude into the channel.

➔ Stone Revetment Protection Use of this abutment material allows maintenance crews to notice potential issues for remediation.

So...what is the timeline of scour analysis???????

FirstExamine historical imagery to know general characteristics of stream

SecondGather info (incl. soil data, maintenance records, site visit)

ThirdInput variables into HECRAS. If appropriate, adjust bridge design based on results.

FourthImplement maintenance plan.

SOURCEShttp://blogs.scientificamerican.com/but-not-simpler/scour-why-most-bridges-fail/http://hydraulics.at.ua/photo/2http://35wbridge.pbworks.com/w/page/900747/Schoharie%20Creek%20Thruway%20Bridge%20Collapsehttps://www.researchgate.net/figure/279282383_fig5_Fig-5-Views-of-scour-at-model-spill-through-abutments-images-by-Atsuhiro-Yorozuyahttps://www.fhwa.dot.gov/research/tfhrc/labs/hydraulics/images/pic013.jpghttp://water.usgs.gov/osw/images/2003_photo_contest_entries.htmlhttp://onlinecalc.sdsu.edu/onlinescourhec18description.htmlhttps://user.engineering.uiowa.edu/~cfd/gallery/fluid-str-inter.htmlhttp://floodready.vermont.gov/flood_protection/river_corridors_floodplains/river_corridorshttp://www.nhdfl.org/about-forests-and-lands/bureaus/natural-heritage-bureau/photo-index/cobble-sand-river-channel.aspxhttp://www.abc.net.au/news/2014-06-23/tch-willow-removal-disaster/5542492http://www.co.washington.or.us/CommunityDevelopment/Planning/2015-2020-consolidated-plan.cfmhttp://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2597&context=jtrphttp://www.mvr.usace.army.mil/Portals/48/docs/Environmental/EMP/HREP/EMP_Documents/Design%20Handbook%202012/07_Training_Structures_and_Channel_Modifications.pdfhttp://www.hec.usace.army.mil/software/hec-ras/documentation/HEC-RAS_4.1_Reference_Manual.pdfhttp://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif12003.pdf