Swift County Road 22 over the Pomme De Terre River near Fairfield, MN

SITE DESCRIPTION:

Swift county road 22 over the Pomme De Terre River is three-span
structure supported by round concrete pile bents. The site is located in
a rural / agricultural area. During the flooding in April 1997, the USGS
visited this site four times. The cross sections show a progression of
scour at the right abutment. During all three visits the floodplain flow
was concentrated in the right floodplain. This concentration of flow in
the right floodplain is likely due to the channel alignment upstream of
the bridge. The field crew searched for but could not define a location
of flow reattachment along the right embankment. Flow was towards the
main channel along the entire length of the embankment. The flow
separated from the right embankment, nearly perpendicular to the main
channel flow, and joined the main flow just left of the rightmost pier.
During the 4-5-97 visit the flow from the right floodplain was so strong
that a standing wave formed upstream of the bridge where the floodplain
and main channel flow began mixing. The area from the rightmost pier to
the right abutment was primarily slack and reverse flow. A slump failure
in the right upstream highway embankment was observed during the last
visit on 4-9-97. In July 1997 it was observed that riprap was used to
fill scour at the right wingwall.

Cross-section data were collected using a chart-recording echo sounder
with the transducer mounted on a knee board. The charts were digitized
and scaled. Velocities were measured using standard discharge
measurement procedures and a Price AA cup meter.

DESCRIPTION OF REFERENCE POINTS:

Water-surface elevations were measured from the bridge deck. The
elevation of the bridge deck was determined from the bride plans. All
measurements were made between the leftmost pier and the left abutment.

Date Time Upstream Downstream

4-4-97 1040.13 1039.85

4-5-97 1430 1040.57 1040.27

4-9-97 1800 1041.2

7-15-97 1410 1032.75

A local right-hand coordinate system was established with the postive
y-axis in the upstream direction and the x-axis parallel to the upstream
face of the bridge. This resulted in x-coordinates increasing from right
to left. Since step backwater models typically us left to right
coordinates, stationing was added which increases from left to right.
The stationing on the two sections 500-ft upstream was adjusted so that
the main channel aligned with the main channel at the bridge.

CONTACT:

David Mueller

U.S. Geological Survey

9818 Bluegrass Parkway

Louisville, KY 40299

PUBLICATIONS:

Mueller, D.S., and Hitchcock, H.A., 1998, Scour measurements at
contracted highway crossings in Minnesota, 1997: ASCE, Water Resources
Engineering '98, Memphis, TN, p. 210-215.

BED MATERIAL COMMENTS:

The boring logs of the site have been included in the bridge plan
profile. Generally the logs indicate sand with some loam layers with
fine gravel in the subbottom. No samples were collected and processed.

DESCRIPTION FOR THIS BRIDGE:

The bridge is a relatively new bridge with wide shoulders and concrete
guardrails. The bridge is angled about 15 degrees to the low flow
channel. All cross sections collected during the flood were collected
approximately parallel to the bridge deck.

PIER DESCRIPTIONS:

The piers are pile bents consisting of 5, 16-inch diameter concrete
piles spaced 9 ft apart in a single line. The upstream and downstream
piles are battered at 2 on 12.

PIER SCOUR COMMENTS:

The rightmost pier may have had some influence on the depth of scour at
the right abutment. It is difficult to separate its effect from the
abutment. The abutment had the major effect and all scour is credited to
the abutment with no scour reported for the pier.

ABUTMENT SCOUR COMMENTS:

The reference surface used to determine the depth of abutment scour was
the concurrent ambient bed. Therefore, the depth of abutment scour
reported is additional local scour below the depth of contraction scour.
Based on the cross sections from the bridge plans there appeared to be
little contraction scour.

Elevation of reference surfaces used:

4-4-97 1030

97. 1029

4-9-97 1029

The rightmost pier may have had some influence on the depth of scour at
the right abutment. It is difficult to separate its effect from the
abutment. The abutment had the major effect and all scour is credited to
the abutment with no scour reported for the pier.

The velocity reported for "at the abutment" is the maximum velocity
observed in the area of the scour hole. Note that the velocity dropped
considerably at the right abutment as the scour hole depth increased
causing an increase in the flow area. The velocity at the left abutment
held steady as did the depth and shape of the scour hole.

CONTRACTION SCOUR COMMENTS:

Contraction scour was computed as the difference in average bed
elevation between uncontracted and contracted sections, adjusted for bed
slope.

Based on the elevation of the main channel between the abutment scour
holes there appears to be only 1 ft or less of contraction scour and
therefore a value of zero contraction scour is reported. No measurements
in the uncontracted sections could be made.

The average depth and velocity of the contracted section were computed
from the discharge measurements. The average depth included the abutment
scour holes.

OTHER PHOTOS / GRAPHICS

Pdt22-brgpln-profile.jpg - profile plot from bridge plan, includes bed
material information.

Planview.wmf -- is a file showing the bridge with a sketch of the
channel and the locations of the cross sections. Note the location of
the cross sections from the bridge plans located 500 ft upstream and
downstream are approximate.

Pdt22-pier-details.jpg -- scan of bridge plan pier details

pdt22-topo.jpg

pdt22-brgpln-profile.jpg

Photos taken on 7-15-97:

Pdt22-ds-bridge.jpg -- photo along downstream edge of bridge

Pdt22-ds-channel.jpg -- photo of main channel downstream

Pdt22-ds-lbnk.jpg -- photo of left bank downstream from bridge

Pdt22-ds-rbnk.jpg -- photo of right bank downstream from bridge

Pdt22-us-bridge.jpg -- photo along upstream edge of bridge

CR22PDT.XLS -- contains the following worksheets

cross sections are label by location upstream (us) or downstream (ds)

distance from bridge

date or source (bp is bridge plans)

See appropriate worksheet

us500_bp

us70_7-15

us50_7-15

us50_7-15(2)

usfv_bp

us0_4-4

us0_4-5

us0_4-9

us0Q_4-5

us0Q_4-9

us0Q_7-15

lsrtww_4-9 longitudinal section along the right wing wall

lsp1p2_7-15 longitudinal section between piers 1 and 2

ds0_4-4

ds0_4-5

ds0_7-15

dsfv_bp

ds10_4-9

ds15_4-5

ds20_4-9

ds25_4-4

ds40_4-5

ds50_4-4

ds50_4-9

ds50_7-15

ds80_4-5

ds80_4-5(2)

ds90_4-9

ds100_4-4

ds100_7-15

ds500_bp

Q4-5-97 -- velocities from discharge measurement on 4-5-97

Q4-9-97 -- velocities from discharge measurement on 4-9-97

Q7-15-97 -- velocities from discharge measurement on 7-15-97

Hydrograph -- hydrograph from nearest gage