**US 12 over Pomme De Terre River near Holloway, MN**

SITE DESCRIPTION:

Bridge 5359 is located 10.7 miles west of Danvers, Minnesota on U.S.
Route 12 over the Pomme De Terre River. The single-span steel-truss
structure was constructed in 1933 with a maximum span length of 88.3 ft.
The upstream floodplain consists of a mixture of open agricultural land
with scattered trees and brush. There is a park on the upstream left
bank. The area downstream is more heavily wooded and is classified on
the maps as a wetland area.

The bridge has vertical-wall abutments with wing walls. Each abutment
and wing wall rest on concrete footings supported on timber piling.
Neither abutment is riprapped nor do they have any other scour
protection measures. A field investigation conducted by BRW, Inc.
(during a scour evaluation) revealed no evidence of significant scour at
the abutment face.

Regarding bed material, BRW reported the following:

The stream bed material in the vicinity of the bridge generally consists
of fine grained organic silty sand. A sieve analysis of a field sample
indicated a mean diameter of 0.00049 ft. Based on the soil borings and
blow counts documented for the bridge construction, the river bed
materials appear to become harder and denser as depth increases. Since
the type of the soil was not recorded, it is difficult to ascertain the
makeup of the soils at depth.

BRW noted minimal debris during their field investigation and the USGS
note no debris during the flood measurements. There are trees locate on
the upstream floodplain so the potential for debris exists, but is
probably low.

The nearest gaging station is located at Appleton, MN approximately 15
miles downstream. Station No. 05293960 has over 60 years of record.
There is one small drainage noted between the site and Appleton on the
right floodplain.

Summary of flood frequencies from BRW's scour analysis:

Return Period -- Discharge (cfs)

10 yr -- 2,075

50 yr -- 3,925

100 yr -- 4,880

390 yr -- 7,130 overtopping flood

500 yr -- 7,530

A scour-monitoring plan had been implemented by MNDOT and was available
at the time of the flood. This bridge was scheduled to be replaced and
has now been replaced with a new structure.

DESCRIPTION OF REFERENCE POINTS:

Elevations are referenced to MSL based on values provided by MNDOT on
their scour monitoring plan. Plans for the new bridge developed by BRW
showed elevations 30 ft higher. The scour report from BRW agreed with
the MNDOT scour monitoring plan and thus, that elevation reference was
used. The top of curb near the east abutment was used and was to have an
elevation of 998.7 ft. The horizontal stationing of data collected from
the bridge deck was referenced to the left abutment then adjusted in
post-processing to be consistent with stationing used in the BRW WSPRO
model. Distance of ADCP data from the bridge was visually estimated.
Horizontal stationing for the ADCP is based on bottom tracking. The
stationing was visually adjusted to agree with the BRW WSPRO model.

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:

Bed-material size distribution reported are from information provided
the MnDOT. A review of the lithologic logs for the replacement bridge
show that the subsurface material is primarily sands, silts, with some
gravel.

DESCRIPTION FOR THIS BRIDGE:

The bridge is an old truss bridge. The bridge and its approach
embankments are perpendicular to the main channel. However, during the
1997 flood there was considerable skew as a significant amount of flow
was coming from the left floodplain. The flow through the bridge opening
in the center of the channel was skewed about 50 degrees.

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.
For this site, it appears that the scour holes may interact as there is
only one or two depth measurement between the holes that define the
ambient bed.

There was significant flow from the left upstream flood plain through
the bridge opening. This flow from the left flood plain significantly
skewed the flow through the bridge opening, about 50 degrees on the
average. Refer to maps and sketches included in Other Detailed Data.

Measurements numbers 1 and 4 were made during a discharge measurement
along the upstream face of the bridge. The depths were measured with a
sounding weight. All other measurements were made using an echo sounded
mounted on a knee-board. The board was floated from upstream to
downstream under the bridge. The measurements reflect the depths at the
upstream or downstream face of the bridge.

All hydraulic information is from the one discharge measurement.

The velocity at the abutments was much higher on 4/5/97:

Left - 5.1 ft /sec

Right - 5.9 ft/sec

Embankment lengths were determined by projecting the measured water
surface elevation onto the approach section from the BRW WSPRO model.

CONTRACTION SCOUR COMMENTS:

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

The appropriate reference surface was determined from an analysis of
cross sections collected by BRW on 6/5/95 and the USGS during the flood
on 4/5/97. Cross sections on these two dates collected approximately 300
ft upstream from the bridge show only about 0.5 difference the channel
bottom elevation. The flood section was the lower of the two. Downstream
from the bridge the cross section surveyed on 6/5/95 (approximately 75
ft downstream) and the cross section surveyed on 4/5/97 (approximately
200 ft downstream) are similar, with less than 1 ft in variation in the
channel bottom elevations. The 4/5/97 cross section 100 ft downstream
was about 1.5 below the 6/5/97 cross section at 75 ft downstream. It was
assumed that the 4/5/97 cross section could have been effected by the
scour at the bridge section. Thus, it was not considered in the setting
of the reference surface. The WSPRO bridge section surveyed by BRW on
6/5/95 showed from 1 to 2 ft of abutment scour in the cross-section.
However, the center of the channel at the bridge appears to be
representative of consistent channel slope from the upstream section to
the downstream section. Since little general scour was observed at the
upstream and downstream sections the mean elevation of the unscoured
portion of the WSPRO bridge section will be used as the contraction
scour reference surface, elevation 981.5 ft.

The contracted section on 4/5/97 was measured under the bridge from data
collected by an acoustic Doppler current profiler. The depths represent
a weighted average of the four beam depths. Because a weighted-average
was used it is possible that the local abutment scour was not detected.
The maximum lowering of the stream bed was actually 7.5 ft, however,
when the entire bed below the bridge was averaged the depth of
contraction scour was only 3.1 ft. The hydraulic data presented for
measurement number 1 were collected with the ADCP. The ADCP data showed
many missing ensembles that were estimated in the final processing.
There was not clear delineation of the channel banks in the approach
section, creating a degree of uncertainty in the approach discharge.
Overall it is expected that the approach discharge is +/- 20% and the
total discharge is +/- 10%.

Measurements number 2 was made during a discharge measurement along the
upstream face of the bridge. The depths were measured with a sounding
weight. Measurements 3 and 4 were made using an echo sounded mounted on
a knee-board. The board was floated from upstream to downstream under
the bridge. The measurements reflect the depths at the upstream or
downstream face of the bridge.

The cross sections measured on 4/9/97 all showed a similar pattern with
abutment scour holes on each side and a sharp mound in between the scour
holes but skewed towards the left bank. It appears that the abutment
scour holes may have overlapped. The highest elevation in the center of
the cross section was subtracted from the reference surface to obtain
the depth of contraction scour. No data in the approach section was
collected on 4/9/97.

OTHER PHOTOS

The following photos were scanned from a black and white copy of the
bridge scour evaluation report completed by BRW:

pdt12-scrrpt-ds-channel.jpg

pdt12-scrrpt-abuts.jpg

pdt12-scrrpt-bridge.jpg

pdt12-scrrpt-nwcorner-bridge.jpg

pdt12-scrrpt-us-channel.jpg

pdt12-scrrpt-us-dam.jpg

pdt12-brgpln-siteplan.jpg is a site plan scanned from the bridge plans
provided by MnDOT.

pdt12-flood-us-bridge.jpg is a photo taken during the flood, from the
right bank looking across the face of the bridge to the left floodplain.
Note the slump in the foreground.

pdt12-flowfield.jpg -- sketch of flow field observed on 4-9-97

pdt12-rwingwall -- photo of data collection along the right upstream
wingwall. Note the slump in the embankment.