**CR 14 over Minnesota River near Lac qui Parle**

SITE DESCRIPTION:

CR 14 over Minnesota River is located in a rural area. Satellite images
show four oxbow lakes in the vicinity of the bridge, however, the
current channel is relatively straight having a sinuosity of 1.07. The
flood plane to the left is wide as indicated by the oxbow lakes. The
highway on the left floodplain is overtopped at the 25-year flood
(17,500 cfs). The approach from the right is much steeper with a much
narrower flood plain. The floodplains are primarily forest and with some
pasture.

In 1994 a relief bridge in the left approach was apparently removed and
the main channel bridge widened.

DESCRIPTION OF REFERENCE POINTS:

All elevations and stages are referenced to mean sea level, based on the
elevation of the finished bridge deck, take from the plans dated
5-4-1990.

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:

No bed material data are available. There were not lithologic logs on
the bridge plans. The discharge measurement at low flow indicated the
bed was soft and uneven. Chippewa county highway engineer thought the
bed material would be a stiff clay.

DESCRIPTION FOR THIS BRIDGE:

The bridge was originally designed and constructed in the mid 1940s. The
original bridge had two twin-column piers located at the toe of the bank
slopes. The columns were hexagonal shaped, giving them a sharp upstream
and downstream nose. The foundations were poured footings with untreated
timber piles beneath the footings. The abutments had 45-degree wing
walls both upstream and downstream. In the early 1990s, the bridge was
widened. To accommodate the wider roadway a 20-inch round
concrete-filled steel pile was placed upstream and downstream of each of
the existing columns. The upstream left column was reenforced with
6-inches of reenforced concrete from its base up to a level determined
by the contractor in the field. The abuments were widened and reinforced
and new wing walls constructed. The spill slopes at the abutments were
graded to 2:1 on the left and 3.4 to 1 on the right. The spill slopes
were protected with grout to the waters edge at the time of
construction, then large riprap was placed on the slope below the
waterline.

PIER SCOUR COMMENTS:

none

ABUTMENT SCOUR COMMENTS:

The cross section at the bridge is deeper on the left side, which could
indicate 1-2 ft of abutment scour, but the location of the pier also
complicates the scour pattern and makes separating scour components
difficult. All scour was classified as contraction scour since the site
was both horizontally and vertically contracted.

CONTRACTION SCOUR COMMENTS:

Scour at CR14 was measured on 4-4-97 and 4-5-97. The left approach was
overtopped, with significant flow. Due to weather and site conditions
the overflow could not be measured. The bridge substructure was
partially submerged on 4-4-97, but the upper low-chord was still above
the water surface. The fall through the bridge on 4-4-97 was 0.42 ft. On
4-5-97 the entire substructure was submerged both upstream and
downstream and the water surface upstream and downstream was essentially
the same. Discharge through the bridge, but not over the roadway, was
measured on 4-5-97 and thus only scour data from 4-5-97 are reported.

The cross sections collected on 4-4-97, 4-5-97, and 7-16-97 were
compared. The downstream cross sections showed good agreement among all
the measurements. Upstream the 4-4-97 and 4-5-97 measurements again
showed reasonable agreement. However, the measurements made on 7-16-97
were more difficult to interpret. The left side of the upstream cross
sections always reflected a location near the bridge because access to
the upstream left bank was hindered by the oxbow lake. Thus the cross
sections at 75 and 100 ft upstream collected on 7-16-97 are actually
only about 25 ft upstream on the left bank. The left side of these cross
sections displays scour. It could not be determined from the data if
this was a remnant of the old channel or if the scour that developed
under the bridge during the flood, extended some ways upstream. Due to
the uncertainty in the upstream cross sections and the consistency of
the downstream cross sections, the cross section approximately 90 ft
downstream collected on 4-5-97 was used as the reference surface for
determining the depth of scour.

The average depth of contraction scour (3.9 ft) was determined from the
difference in average elevation of the active channel bottom using the
cross sections 90 ft downstream and along the upstream edge of the
bridge collected on 4-5-97. Nearly 5 ft of elevation difference was
observed from the downstream section to the deepest portion of the cross
section collected along the upstream edge of the bridge. Due to the lack
of access under the bridge, it is not know if the measured scour is the
maximum scour at the site.

No hydraulic measurements are available in the approach or exit sections
of the channel.

According the hydraulic analysis provided by Chippewa County the mean
velocity through bridge at the overtopping flood of 17,500 cfs would be
6.3 ft/sec. On 4-5-97 only 11,800 cfs was measured going through the
bridge with a mean velocity of 3.6 ft/sec and a maximum point velocity
of 5 ft/sec.

HYDROGRAPH:

No gage nearby.

SUMMARY OF FILES:

CR14MR.XLS -- Excel 97 workbook containing the following worksheets:

Summary -- summary of basic site, bridge, and scour data

q-4597 -- discharge measurement notes from 4-5-97

q-71697 -- discharge measurement notes from 7-16-97

US0-71697 -- cross section along upstream edge of bridge collected on
7-16-97

US75-71697 -- cross section 75 ft upstream of bridge collected on
7-16-97

US100-71697 -- cross section 100 ft upstream of bridge collected on
7-16-97

DS0-71697 -- cross section along downstream edge of bridge collected on
7-16-97

DS25-71697 -- cross section 25 ft downstream of bridge collected on
7-16-97

DS50-71697 -- cross section 50 ft downstream of bridge collected on
7-16-97

DS100-71697 -- cross section 100 ft downstream of bridge collected on
7-16-97

US0-4597 -- cross section along upstream edge of bridge collected on
4-5-97

DS0-4597 -- cross section along downstream edge of bridge collected on
4-5-97

DS25-4597 -- cross section 25 ft downstream of bridge collected on
4-5-97

DS50-4597 -- cross section 50 ft downstream of bridge collected on
4-5-97

DS90-4597 -- cross section 90 ft downstream of bridge collected on
4-5-97

US0-4497 -- cross section along upstream edge of bridge collected on
4-4-97

DS0-4497 -- cross section along downstream edge of bridge collected on
4-4-97

DS50-4497 -- cross section 50 ft downstream of bridge collected on
4-4-97

DS80-4497 -- cross section 80 ft downstream of bridge collected on
4-4-97

DS-BRG-R.jpg -- photo looking from the right bank across the downstream
edge of the bridge on 4-4-97

DS-BRG-L.jpg -- photo looking from the left bank across the downstream
edge of the bridge on 4-5-97

DS-CHL-797.jpg -- photo looking downstream on 7-16-97

DS-CHL-2-797.jpg -- photo looking downstream from the right bank on
7-16-97

DS-RB-BE.jpg -- photo looking at bank erosion on the downstream right
bank on 7-16-97

EMB-FP-L.jpg -- photo of the road overflow in the left floodplain on
4-4-97

Loc-map.jpg -- location map

Satellite_image.jpg -- satellite image of area from TerraServer

US-BRG-L.jpg -- photo looking from the left bank across the upstream
face of the bridge on 4-5-97

US-BRG-R.jpg -- photo looking from the right bank across the upstream
face of the bridge on 4-4-97

US-CHL-797.jpg -- photo of the upstream channel on 7-16-97

US-FP-L-797.jpg -- photo from the left approach looking upstream towards
the main channel on 7-16-97

US-FP-OX-797.jpg -- photo from bridge looking upstream into the oxbow
lake on the left floodplain on 7-16-97

US-FP-497 -- photo of upstream left floodplain in April 1997

US-LB-BE1.jpg -- photo of bank erosion on left upstream bank on 7-16-97

US-LB-BE2.jpg -- photo of bank erosion on left upstream bank on 7-16-97

TOPO.jpg -- scan of USGS topographic map

PIER-1990.jpg -- pier details from 1990 bridge plans

ABUT-L-1990.jpg -- left abutment details from 1990 bridge plans

ABUT-R-1990.jpg -- right abutment details from 1990 bridge plans

BRG-PLN-1990.jpg -- bridge plan overview, 1990

BRG-PLN-1946.jpg -- bridge plan overview, 1946

HYD_ANAL1.jpg -- page 1 of the hydraulic analysis

HYD_ANAL2.jpg -- page 2 of the hydraulic analysis