The Bridge Scour Assessment Work Group was formed to address technical and coordination issues specific to the scour assessment studies. The work group was initiated in September 1991 by the Office of Surface Water. District work group members are the scour assessment project chiefs. The work group did substantial work by correspondence and met March 9-12, 1992; in Atlanta, Georgia, to complete tasks and document findings. The work group appreciates the support of the Districts and the Office of Surface Water, and hopes that its efforts improve the quality of the scour assessment studies. This report summarizes the accomplishments of the work group and transmits its products.

Work Group Members

Bradley Bryan, Tennessee; Ed Vaill, Jr., Colorado; Bernie Helinski, Maryland; Noel Hurley, Jr., South Carolina; Ron Thompson, Indiana; Gene Parker, Massachusetts; Ed Fischer, Iowa; Bob Hejl, Jr., Texas; Mark Landers, OSW, Virginia

Original Work Group Tasks

Literature Review

Literature addressing scour assessment was sent to each member of the work group in September 1991. Two of the articles from the Transportation Research Board's March 1991 Bridge Engineering Conference describe scour assessment procedures different from those being used in the U.S. Geological Survey (USGS) studies. Work group members reviewed the literature and circulated their comments by EDOC. The following literature was reviewed:

Richardson, E.V. and Huber, F.W., 1991, Evaluation of bridge vulnerability to hydraulic forces, stream instability, and scour: Transportation Research Record 1290, Vol. 1, p. 25-38, Transportation Research Board, Washington, D.C.

Schall, J.D. and Lagasse, P.F., 1991, Stepwise procedure for evaluating stream stability: Transportation Research Record 1290, Vol. 2, p. 254-267, Transportation Research Board, Washington, D.C.

Shirole, A.M. and Holt, R.C., 1991, Planning for a comprehensive bridge safety assurance program: Transportation Research Record 1290, Vol. 1, p. 39-50, Transportation Research Board, Washington, D.C.

Simon, Andrew and Outlaw, G.S., 1989, Evaluation, modeling, and mapping of potential bridge-scour, west Tennessee: Proceedings of the Bridge Scour Symposium, P 112-139, FHWA-RD-90-035.

Purvis, R.L., 1991, Bridge Safety Inspection Quality Assurance: Transportation Research Record 1290, Vol. 1, p. 1-8, Transportation Research Board, Washington, D.C.

Molinas, Albert, 1991, Expert system for stream classification: from BRI-STARS User's Manual, National Cooperative Highway Research Program Project No. HR15-11. [USED WITH PERMISSION FROM NCHRP AND DR. MOLINAS]

Description of inspection and review (QA) procedures, and explanations of observed and potential scour indices prepared for the Tennessee District by Bradley Bryan.

Memorandum from Chief, Office of Surface Water, dated January 14, 1991 addressing several issues related to scour assessment studies.

After reviewing alternative scour assessment procedures, the work group concluded that the Tennessee form is more comprehensive and adaptable to regional conditions, and its results more meaningful than the other forms evaluated. However, this finding is not based on field comparisons and we did not determine a way to verify the "accuracy" of any of the forms.

Field Comparison of Scour Assessment Forms

All projects are using the scour assessment form developed by the Tennessee District with regional variations and unique information required by the cooperating agencies. The group discussed comparing inspection forms developed by Richardson and Huber, and Schall and Lagasse with the Tennessee form. From the literature review, the work group concluded that the Tennessee form is more comprehensive and adaptable to regional conditions, and the results are more meaningful than the other forms evaluated. Ed Vaill did use a version of the HEC-20 assessment procedure, which is very similar to that described by Richardson and Huber. Ed Vaill compared the two inspection forms and determined that the Tennessee form describes Colorado Streams more accurately.

Improve Regional and Temporal Transferability of the Data Bases

Ways to improve the regional and temporal transferability were reviewed. The benefit of obtaining more repeatable, quantitative measurements, such as bed-material or cross sections, was recognized but was not accepted as feasible within the time and cost constraints of these studies. The work group participants attempted to improve transferability by standardizing the interpretation and coding of the scour assessment variables. This facilitates analysis of assessment data bases across regional study boundaries, and will make it easier to use the data bases in the future after the studies are completed. Additionally, standard quality-assurance/quality-control procedures will improve transferability.

Standardize Interpretation and Coding of Assessment Variables

Based on the literature review by the work group, the Tennessee form was accepted as the basis or template for a set of standardized scour assessment variables. During the meeting, the work group discussed each variable used by each of the assessment work group projects. The interpretation of each variable was discussed at length, in the context of the widely ranging conditions being assessed by the different projects. Slides of selected sites from each study area and completed assessment forms also were reviewed. These discussions provided a significant training function in addition to standardizing interpretation of variables. Appropriate terminology also was addressed. Coding of the variables was standardized as much as possible. The resulting standardized list of assessment variables and the Tennessee Scour Critical Inspection Form is attached (attachment 1). Interpretive guidelines are provided where needed. This is not equivalent to a standardized form, as some studies are collecting additional information not integral to scour assessment, and formats of the forms differ. Also, the scour indices were not standardized. For example, South Carolina provides its cooperator with written comments along with the numerical data.# MIF code [0155] repeat [00]

Limitations of Methodology and Evaluation and Comparison of Potential and Observed Scour Index Computation Schemes

The inspection procedures were developed with the intent of collecting meaningful data during a short visit. The inspector's measurements are to be used by someone in an office setting to review and assess a site quickly. Although the inspections are done during periods of relatively low streamflow, data are collected from the perspective of bankfull flow conditions. These conditions can be envisioned without benefit of hydraulic or hydrologic analysis. However, the data collected do not describe site characteristics or scour potential for all streamflows. It must be fully appreciated that these methods are largely qualitative and will not reflect scour potential from sources that require quantitative analysis.

Work group participants have used the Tennessee District computation schemes as a guide to develop unique potential, and observed scour index algorithms. Each cooperator uses these indices as well as state-specific variables to determine scour-critical sites, and in some cases, to select sites for level II analysis. It was clearly stated that assessment of all of the factors could not be represented in a single number. For example, a site may be nearly scour critical due to only one or two parameters and have fairly good site conditions otherwise, therefore the site would not rank as a problem site. The data user is expected to look for high values of individual parameters and not just high ranks. Judgment would clearly indicate that some bridges are more or less at risk than the number indicated, according to these project chiefs. The cooperating agencies must be advised of the limitations of the potential and observed indices, and to be prepared to use additional information such as sub- and superstructure data and average daily traffic, as well as engineering judgement to classify sites as scour critical. Additionally, the cooperators should be advised that the observed scour index is incomplete if local scour has not been determined at all piers. Another important limitation is the length of time for which an assessment should be considered valid. Streams are dynamic. The group concluded that an assessment would be valid for roughly 3 years if large or long-duration floods do not occur at a site. Therefore, the data base should be updated or verified periodically to retain utility. Of course, large floods can rapidly change stream stability and would require that the data base be updated more frequently.

Data Quality Assurance and Quality Control

Quality assurance and quality control insure that where qualitative information forms the basis of decision, the information was generated by each group responsible using the same decision process, thus resulting in uniform data. Where judgement was warranted, it was used in a consistent manner. Purvis (1991) summed up the origin of quality data when he wrote, "The diligence and perseverance necessary to be a good bridge inspector is not present in every individual. Inspection involves looking at hundreds of details before finding a serious problem. Close-up inspection of all critical details is necessary. The work is physically demanding and access is difficult. Bridge inspectors often work at remote locations without senior supervision, and the accuracy of their work cannot be measured directly. How can the unit manager determine if an inspector is maintaining the proper level of intensity...." While Purvis specifies bridge inspectors, his quote applies fully to scour assessment inspections. For quality assurance and control to be a positive influence, they must be uniformly applied, and be consistent from place to place.

Quality Assurance

Quality assurance is a term used to describe programs and the sets of procedures, including quality-control procedures, which were used to assure data reliability. Quality assurance identifies problems and suggests corrections. Quality assurance comes from outside the project team in the form of quarterly within-agency reviews, reviews from Department of Transportation (DOT) and Federal Highway Administration (FHWA), and a special Scour Project Work Group comprised of scour-project chiefs within the Water Resources Division (WRD).


Training of project personnel is the first step in assuring uniformity of data collection and in quality assurance. Each person associated with data collection, review, and editing should be familiarized with the data collection procedures. This ensures that at each step of data processing, the maximum amount of oversight is achieved. Training should be treated as an ongoing process for this type project.

Initial training should be provided by someone who has participated at all levels in such a project. Classroom instruction is used to familiarize the participants with the data collection form, the concepts behind the variables, the data collection procedures and tools, the data base, and how best to approach a site assessment.

For field training, project personnel should be walked through the completion of the form at training sites. After the form is completed at each training site, the group reviews the form and discusses any problems encountered and differences in values. When it becomes apparent that data collection concepts are understood and are being followed, instruction by inspection by individuals should begin and should continue until it is clear the trainee has learned all the concepts. At that time, the trainees are considered fully qualified.

Periodic Training

Project team members receive office and field technique review, to keep individuals from developing distorted concepts about variables. Frequent interaction between field inspectors and the project chief during data form reviews provides the most direct and most positive type of review and training. It is recommended that the project team participate in an update session at least quarterly. If an inspector has been away from field data collection for a month or more, the project chief should field review technique before certifying that person for field duty. Additionally, inspectors should be reviewed by personnel external to the project at least once a year.

Quality Control

Quality control is a term used to describe the routine procedures used to regulate measurements and produce data of satisfactory quality. This occurs within the project, through the project chief and inspectors monitoring quality of data collected by other inspectors.

Data Quality Control

The project chief assigns sites for inspection to maximize travel efficiency and draw on inspector experience. The inspection forms are completed on site, with sketches made and photographs taken as site documentation and for later use in data review.

Inspection forms are reviewed in the office by someone other than the original collector. Problems or unclear data are noted. The project chief reviews all inspection forms, prior to submitting data to the main data base, to determine if site revisits are needed and to determine whether conceptual discrepancies are developing.

After the data is confirmed by the project chief or his appointee, data are entered into a computer file. Before accepted as final, the preliminary data are verified through comparison between the original form and the computer file contents.

Field Review

Each inspector is field reviewed as part of the ongoing training process. There are several procedures that provide adequate information concerning the capability and work quality of the inspectors. The best method is to independently assess a site that the inspector will visit. The inspector is not informed that the site inspection is to be used for review purposes.

The project chief attends field inspections to observe field technique and discuss perceptions. This is a supplement to formal review and training, not a substitute for formal independent review. The project chief verifies that inspection equipment is in the best possible condition: clean and calibrated.


The Bridge Scour Assessment Work Group met in Atlanta, Ga., March 9-12, 1992, to discuss issues unique to scour assessment studies. The meeting was a culmination of work done by the project chiefs since the work group was formed in September 1991. From a literature review made by all work group participants, the inspection form developed by the Tennessee District was chosen as the template for creating a set of standardized scour assessment variables. By standardizing the variables, the regional and temporal transferability of the state-specific data bases will improve. The limitations of the data collection and the scour index computation procedures were documented. A detailed quality-assurance and quality-control plan was also developed by the work group. The work group members look forward to working with each other and the Office of Surface Water to develop the best product possible.


1. Scour Critical Inspection Form and explanation

Attachment 1.


(1) Intro: Date________ Stream______________________________ Vicinity_________________

Inspector__________ Land use____, 1=urban 2=row crop 3=pasture 4=forest 5=swamp 6=suburban 7=rangeland

(2) Location: Route_______ Cty_________ District No.______ Structure No.____________,

Seq. No.___ Lat_______ Long________ _____, ft. Max.span length_____, ft.

Original left edge of bridge to 1st channel pt., _____ ft., 2nd channel pt., _____ ft.

Current left edge of bridge to 1st channel pt., _____ ft., 2nd channel pt., _____ ft.

Physiographic reg. _____ 1=Blue Ridge 2=Piedmont 3=Inner C.P. 4=Lower C.P.

Number of overflow bridges: Left_____ Right_____.

(3) Flow conditions: Inspectable ____ 0=no, 1=yes. Flow regulated ____ 0=no, 1=yes

Gaging station at site ___ 0=no, 1-yes.

Orig. low steel to channel centerline bed depth _________ ft.

Low steel to channel centerline water surface _____ ft. Depth of flow _____ ft.

Tidal ___ 0=no, 1=yes; Wave action ___ 0=no, 1=yes; undercutting ___ 0=no, 1=yes

Deflected flow (debris) ___, 0=no 1=yes; Impact pt. ___ ft, ___ 1=LB 2=RB; +=US -=DS

Cause of deflection and effect on bridge crossing:



Capacity of bridge opening (qualitative), can the bridge handle all flow or is there some restriction for certain flow stages:

Capacity of channel (qualitative):



Observed High Water Marks (HWM) ____ ft. above/below __________ (reference pt.)

Road overflow risk (qualitative):



(4) Bank condition:

Height Woody

from bed Angle Veg. cover(%) Material Erosion

1 2 1 2 1 2 1 2 1 2


1 U/S ___ ___ ___ ___ ___ ___ ___ ___ ___ ___

2 D/S ___ ___ ___ ___ ___ ___ ___ ___ ___ ___

3 AT ___ ___

Note: Bank angle sketch with heights and angles.

Woody vegetation, approx age, species if recognized.

Material 1=ML/CL 2=sand 3=bedrock 4=gravel/cobble

Erosion 0=none 1=mass wasting 2=fluvial erosion

(5) Bed material characteristics: ____ 1=sand, 2=ml or cl, 3=gravel, 4=cobble/boulder, 5=bedrock, 6= alluvium (if can't tell others)

Resistant to scour ___ 0=no 1=yes

Estimated depth of gravel deposits ____ ft. (enter 999 if not observed).

(6) Channel profile: 1 U/S: 1=pool 2=riffle 3=smooth. Note: Code lake sites

2 D/S: 1=pool 2=riffle 3=smooth. as 3=smooth.

(7) Distance to confluences, diversions, road ditches, if any: 0=no 1=yes +=US, -=DS.


____ ft. 1= LB entry, 2= RB entry.

____ ft. 1= LB entry, 2= RB entry.

____ ft. 1= LB entry, 2= RB entry.

____ ft. 1= LB entry, 2= RB entry.

(8) Piers: To be listed from left to right. Stop at first flood plain pier past top bank.

Also inspect the two piers nearest the left and right abutments.


of 1 2 4 5 6 7 8 9 local Expos

elements shape skew depth (circle appropriate choice below) scour 0 1 2

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

______ _____ _____ _____ Loc: lfp, ltb, lb, mcl, mcm, mcr, rb, rtb, rfp. 0 1 2 N F P

Notes: Number of elements: Number of piles per bent or piers per set.

Shape is a standard: 1=squared 2=rounded 3=pointed

4=square piles 5=round piles 6=pointed piles 7=tower piles

Skew will be from upstream to downstream based on high flow

alignment: + = skew to right, - = skew to left.

Depth: distance from bridge deck to channel bed, ft.

Local scour: 0=none 1=observed 2=undefinable,

Piers: N=no exposure, F=footing exposed, P=piling exposed

Bents: N=default, F=moderate, P=severe

 exposed ftgs.

(9) Abutment: 1 2 or piles guide banks

1= left: skew_____ Loc: 0, +____ft, -____ft, sloping or vertical. 1=yes 0=no 1=yes 0=no

2=right: skew_____ Loc: 0, +____ft, -____ft, sloping or vertical. 1=yes 0=no 1=yes 0=no

Notes: Skew will be measured for high-flow conditions as difference

between normal flow and abutment; + = right skew, - = left skew.

Loc: + indicates the abut. is set back from the bank. - indicates the abut.

sits out into the stream. 0 indicates the abut. is even with the bank.


(10) Debris accumulation: % of channel opening blocked; horizontal _____ to _____%,

vertical _____ to _____%.

Type and size: _____ 1=brush 2=whole trees 3=trash 4=all of others

Potential for debris (qualitative):

Obstructions (describe):

Note: Horizontal: Left bank to right bank. 0 % = LB, 100 % = RB.

Vertical: Bed to low steel. 0% to 100%.

Take pictures, make notes.

(10a) Additional debris pile data:

Debris Bridging,

Pile Width, ft. Height,ft. Single pier, Solid Touching

Number begin end begin end or Free or Open Low Steel

1. _____ ___ _____ ___ __________ _______ _________

2. _____ ___ _____ ___ __________ _______ _________

3. _____ ___ _____ ___ __________ _______ _________

4. _____ ___ _____ ___ __________ _______ _________

5. _____ ___ _____ ___ __________ _______ _________

Notes: 1. Debris piles numbered from left to right.

2. Debris pile width referenced from left edge of bridge.

3. Height referenced from ground surface.

4. Is debris pile? 0=free standing, 1=lodged against a pier,

2= bridges two or more piers.

5. Is debris pile? 0=open, water passes through, 1=solid mass.

6. Does debris pile touch low steel? 0=no, 1=yes.


(11) Bank or channel protection on:

1= U/S rt bank: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=slumped.

2= U/S lf bank: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=slumped.

3= At rt bank: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=slumped.

4= At lf bank: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=slumped.

5= D/S rt bank: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=slumped.

6= D/S lf bank: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=slumped.

Type and size (qualitative):



If slumped, where and why:



7= bed: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=moved.

If moved, to what extent?


Type and size (qualitative)



8= At rt abut: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=slumped.

9= At lf abut: 0=absent 1=present, 2=good cond, 3=weathered to size smaller, 4=slumped.

Type and size (qualitative):



If slumped, where and why:



(12) Channel width, measured from tops of banks: U/S______, at______, D/S______

Blowhole ____ 0=no, 1=yes; _____ ft downstream, _____ ft wide, ____ ft long.

(13) Meander characteristics in vicinity of bridge (impact points):

1 Low flow 2 High flow

If straight, ft=999

1=Lb,2=RB ft. 1=Lb,2=RB ft.

U/S (ft) ____ ____ ____ ____

D/S (ft) ____ ____ ____ ____

Meander wavelength ____ ft. ____ ft.


Notes: Must impact opposite banks to calculate meander wavelength.

Entry will be LB or RB and distance from bridge. 0 = impact at bridge.

(14) Point bar location: ____, 0=absent 1=present

_____ to _____% (0% = LB, 100% = RB)

Distance U/S (+)_____ft or D/S (-)_____ft.

Width at mid bar ____ ft.

(15) Alluvial fan/delta in vicinity of bridge: 0=no 1=yes 2=questionable

If "questionable," then describe.


(16) Stage of reach evolution: 1=undisturbed, 2=constructed channel,

3=degradational bed, 4=degradation and bank failure, 5=aggradation or

stable, with bank failure, 6=fully recovered


(17) Culverts: No. of barrels _____

Underflow: ___ 0=no 1=yes; sidewall flow: ___ 0=no 1=yes; apron: ___ 0=no 1=yes

Overfall: ___ 0=no 1=yes, distance from invert to soil contact, _____, ft.

Cut-off wall exposure: ___ 0=no 1=yes, Depth exposed at entrance ____, ft.

Wingwall exposure: ___ 0=no exposure, 1=caused by high angle of approach,

2=caused by embankment runoff,


Depth exposed;


U/S ____ ft ____ ft

D/S ____ ft ____ ft


____ Pictures taken, frames ___ to ___ on roll ____.

Plan view sketch on back completed _____________________ (Date, time)

Place other sketches as needed on back.



The South Carolina District also adds comments for the following headings:

1. Introduction

2. Location

3. Baseflow

4. Tidal

5. High-flow angle

6. Debris deflection

7. Capacity of bridge opening

8. Capacity of channel

9. Road overflow risk

10. Bank condition

11. Bed material

12. Channel profile

13. Confluences

14. Piers

15. Abutments

16. Debris accumulation

17. Debris production

18. Obstructions

19. Riprap, bank

20. Riprap, bed

21. Riprap, abutments

22. Blowhole

23. Meander characteristics

24. Point bar location

25. General


Explanation of the "Scour Critical Information Form:

This explanation proceeds by numbered block on the form. As in any new endeavor, this data collection system is best tried in company with experienced personnel.

The word "crossing," as used here, refers to the channel reach from upstream to downstream limits of hydraulic influence at the bridge. The USGS convention for right or left bank is facing downstream.

1) Introduction:

Stream: Be consistent with names. Do not use multiple versions of the same name, because this can be a key sorting variable. Abbreviations: Cr = Creek, R = River, Br = Branch, Fk = Fork, and so forth.

Vicinity: Be sure the landmark is on readily available county or topographic maps. If the site is not near a real town or community, use some short description that locates the site in the county.

Inspector: Use initials with full last name. One person should be responsible for the site.

Land use: Refers to the general area around the site and is based on what the inspector knows about the area or what has been observed in approaching the site.

2) Location:

These items will be used in conjunction with Geographic Information System (GIS) for site location and for interfacing with other systems. Each DOT has a bridge inventory system that uses a codified bridge identifier. By using the DOT identifier, the USGS and DOT data bases will be directly comparable, not necessarily compatible. Many of the items listed in this section will be specific to the DOT in question.

Number of overflow bridges: Addresses the presence of relief openings. Normally, entries here indicate that bridges are in place specifically for overflow relief. Small bridges that span their own creek are not considered overflow bridges, even though a large river might use the same bridge at flood flow. Additionally, a bridge designated as an overflow bridge will not have overflow bridges.

3) Flow Conditions:

Inspectable: This refers to the ability of the inspector to see enough of the site (banks, vegetation, and so on) to properly fill out the form. It is important to know whether the stream is in flood stage or at some lower stage, because the inspector needs to be assured that he is getting the best, most extensive view of the channel possible. If the inspector can not see the channel banks, an effective inspection can not be made. If low-flow conditions are still bankfull, this is the best condition possible and should be used. This condition would be noted as a comment.

Depth of flow: This is depth in the thalweg at time of inspection. For culverts with floors, the depth is measured from the floor, not from the natural bed outside the culvert.

Underclearance at thalweg: Underclearance from low beam to water surface will be entered. An additional measurement for arched bridges may be made from the low point of the arch to the water surface. Potential for pressure flow and potential for lateral force on the bridge are considerations.

High-flow angle of approach: This refers to flow and bank alignment, not flow and bridge alignment. The inspector should imagine the site at approximately bank-full flow. For swamp type settings, the inspector should imagine the flow approaching the structure as it would when flowing through the flood plain or the wide area of swamp upstream. With this item, we are looking at how flow may affect the crossing and thereby affect the bridge. Determination at low water levels is made based on observations of debris piling, bank scouring, and any other at-site features available.

Additional site information includes: Gaging station at site, flow regulation, and tidal influence. Additionally, at lake and coastal sites the presence of wave action and bank undercutting caused by wave action is noted. Wave action undercutting can eventually undermine the bridge abutments and banks.

Deflected flow (debris): This item is designed to point out any abnormal channel obstruction that may be affecting the crossing; cars, pipelines, large quantities of trash, and so forth. This type of obstruction probably is going to be temporary. It is usually upstream, but could possibly be downstream. "Impact point" refers to which bank the deflected flow is affecting.

There are qualitative entries on the form. They are there so the inspector can note unquantifiable details for the benefit of the DOT.

Capacity of bridge opening and channel: This is strictly qualitative and based on the inspector's knowledge of the area hydrology, and on what he has been seeing in the area. Field experience is probably a must for accurate form notation.

Observed High-Water Marks: The inspector must take into consideration the hydrology of the area and also the potential storm event effects on the crossing. It also may provide data for other DOT activities, such as bridge replacement design at that site or on the same stream.

Road overflow risk: A qualitative observation for the benefit for the DOT. This refers to road approach if that is the low spot or to the bridge if that is the low spot.

4) Bank Condition:

Bank characteristics upstream, at the bridge, and downstream of the bridge will have a large affect on the banks and bed under the structure. Stable banks upstream and downstream will indicate that problems at the crossing are site specific, and not related to system wide response.

Bank heights: High, steep banks are more likely to fail and are generally indicative of rapidly occurring bank forming processes.

Vegetation coverage: Indicator of the overall health or stability of channel banks. Percent cover is determined based on the amount of crown cover from woody vegetation. Herbaceous vegetation may cover the banks during the growing season but die back and leave the banks relatively unprotected during the winter months. Additionally, herbaceous vegetation usually has a less substantial root system and therefore does not provide as much bank protection as woody vegetation. Large numbers of small woody vegetation will have crown closure, and a high percent of vegetation coverage. Sparser distribution of older trees may have crown closure and also receive a high percent of vegetation coverage. Low amounts of woody plant coverage may indicate mass wasting, frequent scouring flows, or that the area is cleared often.

Bank material: Important in controlling what sort of erosion will take place; silt and clay banks are susceptible to either fluvial erosion or mass wasting; sand and gravel banks may be susceptible to rapid erosion and (or) deposition. The observed process is important in determining the overall character of the site.

5) Bed Material Characteristics:

Sand and gravel beds are very susceptible to local scour. Bedrock beds may not scour, but extreme flows in a bedrock channel may cause excessive channel widening and affect the bridge. There may be special regional materials that will need to be listed on the form, possibly as a comment. Modifying the form is easy. Modifying the data program is harder and should be done only when the existing program is inadequate.

Resistant to scour: The inspector makes a judgement in regard to resistance to bed material movement and therefore resistance to scour. If the material will not move during bankfull flow conditions, it is considered resistant. If it is obvious that most flood flows move the majority of the bed material, it is not resistant.

Estimated depth of gravel deposits: This entry is used only when a depth is observed. An observation will usually be relative to a bar and the bedrock bed. If the deposit (sand or gravel) is very deep, scour may proceed vertically. If the deposit is not too deep, scour may go to the bottom of the deposit and then begin to act horizontally. If the deposit is shallow over a silt/clay bed, bed degradation may occur.

6) Channel Profile:

Pool and riffle profile is a common combination with an energy gradeline indicative of scour and deposition. A smooth profile indicates a generally stable energy gradeline and thus a more uniform bed material transport capability.

7) Distance to confluences, diversions, road ditches:

Contributions of flow and sediment from a tributary in the vicinity of the bridge crossing, either upstream or downstream, may affect flow patterns and scour through the structure. Road drains and diversions may also affect the bridge and should therefore be noted. Be sure to note size or any other descriptors of the tributary.

8) Piers:

Local scour at piers has caused many bridge failures; for example, the Schoharie Creek bridge failure in New York. The data collected, within the scope of the study, at piers can provide valuable information regarding the existing stability and possibly indicate future potential problems. Some projects concentrate on main channel scour, but if flood-plain scour or degradation is of concern, the data collection process can be modified to collect this data.

Number of elements: The number of elements defines the number of piles in each bent or the number of columns in a pier.

Shape: Shape has an effect on turbulence. The less streamlined, the more turbulence. Piers and bents are differentiated by the coding variables. Our methodology and approach assume that piers have footings and bents do not. Bents consist of driven piles.

Skew: Refers to pier and flow angle, not skew to bridge deck. This again describes creation of turbulence.

Depth: The maximum distance from the low steel to the bed is measured. This can be compared to existing information to quantify local scour or be stored and used for future reference.

Loc (location): Indicates the general placement of the pier or bent in the channel referenced from the left end of the bridge.

Local scour and Expos(ure): Are used in the scour indices and can also be supplied to DOT as an immediate benefit. For Local Scour, if 2 = undefined is chosen, be sure that an appropriate warning is appended to any use made of that data, whether as data or as part of a computation.

For Expos(ure), bents are ranked as 0 = none, 1 = some scour, 2 = apparently serious scour depending on how much of the piling has been exposed. The inspector generally will not know how much of thee piling was left exposed at the end of the construction project. Relative size of scour hole is a clue. Also, assume that if the channel has degraded moderately, there is moderate piling exposure (1 = some scour), and if the channel has severely degraded there is severe piling exposure (2 = severe or serious exposure). Piers are ranked as 0 = no exposure, 1 = pier footing exposed, and 2 = pilings beneath the pier footing are exposed.

9) Abutment:

Skew: Refers to skew to flow, not skew to bridge deck.

Loc (location): In regard to channel bank, it gives DOT and USGS some estimate for how soon bank erosion may affect the abutment. Distance is measured to the toe of the abutment slope, unless the abutment forms the channel bank. In this case, distance equals 0.

Exposed footings or piles: Used in developing the observed scour index and is of immediate benefit to the DOT. Footing exposure may be due to construction on bedrock or be an indicator of scour.

Guide banks: Also known as spurdikes or other names. They guide the flow around the abutment slope and usually extend some distance upstream and downstream.

10) Debris Accumulation:

By specifying percent of horizontal and vertical blockage, the total blockage can be computed and location of blockage can be specified. The computation programs in use now do not identify separate debris stacks, but data on separate stacks of debris may be collected using section (10a). The percent blocked is an integration of all debris in the channel at the bridge. Percent blocked is based on the conveying channel opening and the low beam of the bridge.

Debris type and size: Often indicative of channel forming processes ongoing upstream. Many trees or parts thereof may indicate extensive mass wasting upstream, and also that debris accumulations may be expected to occur during all significant rises. That is, the normal in-channel debris accumulation and flushing process no longer is in effect.

Potential for debris production: Refers to how much debris the site might generate in the near future. The debris in question is composed of brush and trunks and is generated by bank erosion and sometimes agricultural practices (clearing, plowing up to the bank, and so forth). At this time, the entry is qualitative. A good entry will be based on field experience, an understanding of erosion processes, the normal appearance of vegetation at streamside, and the condition of the banks at the site as described on the form. A rating of high, medium, or low is recorded, along with a very short rationale. One word descriptors without narrative are useless, and considered an error on the part of the inspector.

Obstructions: Are objects at the site causing problems? They may or may not be part of what is causing deflected flow. Pipelines and abandoned automobiles are two common obstructions.

10a) Additional debris data:

Additional data on separate debris piles can be collected. The data include: A unique debris pile identifier, the width of the pile referenced to the corresponding station of the bridge (left edge of bridge equals 0); the height of the debris pile; if the pile is free standing, lodged against a pier, or bridges two or more piers; if the debris pile is solid or open; and finally, if the pile touches the low steel.

11) Channel protection:

It is strongly suggested that the project chief confer with the DOT to get information on channel protection normally used by that DOT, and how they officially describe it. For example, channel protection is material that has been placed on purpose or by default (old bridge deck left where it fell). Bedrock and boulder bank material are not man placed channel protection but can be noted as channel protection. The Tennessee scour study did not consider bedrock or boulder banks to be channel protection, but the South Carolina scour study did. This point is open for debate.

When considering location of the channel protection, the limit of the hydraulic influence of the bridge is a good general boundary. If the material goes beyond this limit of influence, one could consider the channel protected upstream or downstream, respectively.. "At" refers to the bank at the bridge. Material wrapping an abutment is still "At" the bridge. Upstream and downstream refer to distance away from the bridge, not which side of the bridge the material is observed. Guide banks set back into the flood plain are not channel protection, nor are they abutment protection, they are guide banks.

The size and type of material is important, because it indicates to the inspector the scale of flow energy needed to move it. If the banks are protected and the bed is not, a structure may be open not only to local scour, but also system response type degradation. Downcutting is the first response of increased energy in the fluvial system. Increased energy may come from downstream channel modification or upstream altered hydrology (increased impervious area; dam construction and resultant clear-water scour; clearing and snagging, and so on). Additionally, if the bed is protected, but not the bank, the crossing is left open to lateral energy dissipation. That excess energy WILL be used.

12) Channel Width:

Crossings with contracted openings create overwidened sections where flow expands just downstream of the opening. This "blowhole" can expand enough to threaten the abutments. Additionally, increased hydraulic head will lead to greater potential for bed material export from the crossing, potentially leading to excessive scour. Channel width upstream and downstream are the "normal" channel width, not the "abnormal." Channel width at the structure is measured under the structure. For the case of box culverts, the width is the active channel in the culvert or the combined width of the barrels if all barrels form the channel.

Blowhole: A colloquialism describing the effect of flow expanding outside a constriction. Size and distance from the structure indicate potential to affect the structure.

13) Meanders:

Meanders are a different consideration than high-flow angle of approach (discussed in section 3). They can correspond, but do not always. Meanders are brought into the evaluation because they shift. It is important to appreciate the power and tenacity of the channel evolution process. Notes in this section may indicate that a meander impact point is moving into the crossing or, worse yet, that one already exists at the crossing. A meander impact point at a crossing will likely result in bank undercutting and either mass wasting or very rapid fluvial erosion and endangering of the bridge.

Wavelength as used in this project is a local phenomenon, not an overall consideration made after review of an extensive river reach. The stream must impact opposite banks for the inspector to be able to compute a wavelength. Meander wavelength will probably be different for high and low flow. It may also be that the low-flow pattern is having the more serious effect on the crossing. This evaluation needs to be made on site and so noted.

14) Bar Location:

This entry alerts the inspector to changing hydraulics in the vicinity of the crossing. As bars build, the thalweg shifts and bank undercutting or erosion proceeds. The location of a bar dictates how much impact it will have on flow and thereby the crossing and structure.

15) Alluvial fan:

The presence of an alluvial fan indicates the site is in a reach where bed material from the upper portions of the basin is being deposited. This large supply of bed material could be transported and deposited at the site, causing flow concentrations at piers or abutments. Additionally, if this supply of bed material is interrupted, downstream degradation may begin.

16) Stage of Reach Evolution:

The current bridge scour project approach evolved from 6 years of previous experience with channel evolution in west Tennessee. Andrew Simon and Cliff Hupp devised a method of ranking western Tennessee channels based on state of stability and thereby stage of evolution. This entry is based on almost all the information previously listed.

Even though stage of reach evolution was devised to rank modified channels, with a little imagination and an understanding of the complex data listed on this form, we are able to glean a good idea of site stability from this one entry.

The six stages are:

Stage 1. Undisturbed: The stream is acting and reacting naturally and shows only long term changes.

Stage 2. Constructed: The channel has been altered by man, either by straightening and (or) widening.

Stage 3. Degradational: The streambed is lowering, but the banks remain stable.

Stage 4. Degradational with bank failure: The streambed is lowering, and the banks are widening by mass wasting.

Stage 5. Aggradation or stable bed with bank failure: The streambed is stable, but the banks are still experiencing mass wasting.

Stage 6. Fully recovered: The stream has recovered and is acting as an undisturbed stream.

17) Culverts:

Some cooperators require information on culverts. Culverts are inspected in the same manner as bridge sites; however, problems unique to culverts are also inspected.

The number of barrels is noted.

The presence of underflow and (or) sidewall flow are two of the more serious conditions encountered, as they indicate that streamflow is passing under or around the culvert. This situation usually requires immediate evaluation by the cooperator, as flow under the culvert in particular can undermine the culvert and cause failure.

The existence of cutoff wall exposure at the culvert entrance or overfall at the exit are evidence of streambed lowering and can eventually lead to underflow. An entrance apron protects the culvert by transferring the point of bed scour from the culvert entrance to some point upstream of the culvert.

Wingwall ends are normally buried. If a wingwall is exposed, the depth of exposure and the cause of exposure should be noted. Wingwall exposure can indicate future underflow or sidewall flow.

18) Comments:

Qualitative comments are recorded on the forms and should be entered into the data base. South Carolina modified the input and output programs to accept and produce comments. South Carolina adds comments for the following headings:

1. Introduction

2. Location

3. Inspectable

4. Tidal (under Flow Conditions section)

5. High-flow angle

6. Deflected flow (caused by debris)

7. Capacity of bridge opening

8. Capacity of channel

9. Road overflow risk

10. Bank condition

11. Bed material

12. Channel profile

13. Confluences

14. Piers

15. Abutments

16. Debris accumulation

17. Debris production

18. Obstructions

19. Channel protection on banks

20. Channel protection on bed

21. Channel protection on abutments

22. Blowhole

23. Meander characteristics

24. Bar location

25. General

Two 80-column lines are provided for each comment heading. Comments should adequately describe the conditions, but be as brief as possible.

19) Photographs:

A minimum of four photographs should be taken at each site (upstream and downstream views of the bridge and upstream and downstream views from the bridge). Additional photographs of any hazardous or unusual conditions should be taken. These photographs aid in form checking and in describing any problems at the site.