247^th^ Street Bridge over the James River near Mitchell, SD

[Site Overview]{.underline}

The study site is located on the James River approximately 6 miles north
of the town of Mitchell, South Dakota and east of State Highway 37 on
247^th^ street. The site is approximately 21 miles downstream from the
USGS gaging station near Forestburg (06477000). The USGS National Bridge
Scour Team in cooperation with NCHRP and the University of Louisville
was deployed to the site to collect real-time bridge scour measurements
during the flood in April of 2001. Boat access at the site was
unavailable therefore all scour measurements were collected from the
bridge deck. 247^th^ street and the bridge were closed at the time of
the measurements due to overtopping of the roadway on the left
floodplain.

The site is located in a highly rural/agricultural landscape with very
little topographic relief, especially in the left floodplain. The bridge
is a concrete girder, three span structure supported by two groups of
cylindrical piers (2 in each group) which are both founded on timber
piles. The channel bed is comprised of a silty-clay with a narrow
horizontal clay wedge. The James River has a great deal of meander in
the vicinity of the bridge. The left floodplain is expansive at the
bridge and the right overbank consists of bluffs rising relatively
steeply from the edge of channel.

# Hydrologic Conditions

Greater than normal precipitation starting with late fall rains in 2000,
greater than normal snowfalls, a delayed snowmelt, and above average
rains in April, all contributed to the upper Midwestern flooding in the
spring of 2001. The James River basin received a surplus of 10 inches of
precipitation through the winter of 2000-2001 and the early part of
spring 2001. The temperatures in February, March and the first part of
April were 10-15 degrees below normal, which delayed the typical period
of snowmelt enough to coincide with a period of above average rainfall
associated with a series of cyclonic weather systems characteristic of
early spring.

# Data-Collection Methods

Scour measurements were collected with by floating an echo sounder
attached to a knee-board across the river while being controlled by a
hand line from the bridge. The board was allowed to float downstream and
streambed elevations were collected as far as 50 feet downstream from
the bridge. Data collected upstream of the bridge was restricted to the
upstream edge of the bridge deck. Additional bathymetry data was
collected in the floodplains and throughout the channel upstream and
downstream from the bridge after the flood during a low-water site visit
on October 25, 2001.

Water-surface elevations were collected at the upstream and downstream
bridge faces by taping down from reference marks which were later
surveyed and tied into bridge elevations.

# Discussion of Contracted Site

The bridge contraction was asymmetrical with a large majority of the
contracted flow coming from the left floodplain. The bridge did receive
some relief by a section of road overtopped by the flood flow
approximately 625 feet west (on the left floodplain) of the bridge at
the time of the scour measurements. There was evidence that the road
overflow extended approximately 100 feet further towards the bridge. The
overtopped section is shown in figure 1.

## ![](media/image1.jpeg){width="4.2in" height="3.1375in"}

## 

## **Figure 1. --** Looking east from bridge at road overflow on left floodplain.

## Although the flow in the left floodplain overtopped the roadway, the floodplain flow on the channel side of the roadway flow was moving towards the bridge opening. A strong area of reverse flow was observed between the left-most pier and the left abutment. The area immediately downstream of the left-most pier was active with boils and eddies, making measurements in this region difficult. The flow angle of attack at the left pier was 40 degrees and was affected by the eddy along the left abutment.

## **Bridge Info**

The structure (#18-153-030) is a 30 ft wide, 262 ft long pre-stressed
girder bridge supported by two piers, both located in the main channel
of the James River. Pier #1 is on the left, looking downstream, and
consists of 2 separate 3 ft diameter cylindrical piles. Pier 2 is on the
right and also consists of 2 separate 3 ft diameter cylindrical piles.
The bridge has a type III contracted opening, meaning it has sloping
embankments and sloping spillthrough abutments. The bridge has a
30-degree skew to the James River and a downhill grade in the eastbound
direction. The low-chord elevation is 1224.6 ft above sea level.

## **Geomorphic Setting** 

The geomorphic setting and channel alignment of the James River at the
247^th^ street bridge is depicted in figure 2 as well as a graphical
representation of the roadway overflow section. A plot of the channel
bathymetry collected during the April 2001 flood and subsequent low-flow
survey can be found in figure 3.

![](media/image2.jpeg){width="7.44375in" height="5.321527777777778in"}

**Figure 2.** -- Geomorphic setting and channel alignment for the James
River at 247^th^ street bridge near Mitchell, SD

### 

![](media/image3.wmf){width="5.995833333333334in"
height="4.1in"}**Figure 3.** -- Surveyed cross-sections in the 247^th^
street bridge reach collected during the April, 2001 flood and
subsequent low-flow survey in October, 2001.

### 

### Flood Photos

Additional photos taken during the flood in April 2001 are shown in
figures 4 -- 7.

![](media/image4.jpeg){width="3.1666666666666665in" height="2.3625in"}

Figure 4. -- Looking at upstream bridge face from left embankment

![](media/image5.jpeg){width="3.15in" height="2.3625in"}

Figure 5. -- Looking at downstream bridge face from left wingwall

![](media/image6.jpeg){width="3.283333333333333in" height="2.45in"}

Figure 6. -- Looking at upstream left floodplain from bridge deck

![](media/image7.jpeg){width="3.4166666666666665in" height="2.5625in"}

Figure 7. -- Looking downstream from bridge deck

![](media/image8.jpeg){width="3.3in" height="2.4875in"}

**Figure 8.** -- Looking upstream at left abutment from under bridge

### Measurement of Contraction Scour 

Without measurements of the uncontracted channel sections, it was
difficult to establish an ambient bed from which to estimate contraction
scour. The post-flood surveys were used to estimate the contraction
scour that occurred during the April 2001 flooding.

Analysis of the data revealed that remnants of an old bridge at the site
appear to be influencing the bathymetry and amount of scour through the
bridge section. The surveyed data shows the upstream bridge face section
to be 3-4 feet higher than the downstream bridge face section as well as
other sections collected further upstream of the bridge. It is believed
that the foundation or deck from the old bridge is providing a form of
scour protection at the upstream bridge face, which would also explain
the inability to collect bed material samples at the upstream bridge
face. The measured live-bed contraction scour was estimated to be 1 feet
with accuracy of +/- 1 foot (see figure 3).

### Measurement of Abutment Scour

No abutment scour was observed during the April 2001 flooding.

### [POST-FLOOD SURVEY]{.underline}

A post-flood survey of the site was conducted October 25-26, 2001 during
low-flow conditions to collect more detailed topography data in the
floodplains, bed material samples, and more refined Manning's n
estimates. The following photos (figures 9-13) reveal the channel
alignment relative to the bridge as well as the scour remnants of the
April, 2001 flood.

![](media/image9.jpeg){width="2.9166666666666665in" height="2.175in"}

**Figure 9.** -- Post-flood conditions, looking upstream from bridge
deck

![](media/image10.jpeg){width="2.933333333333333in" height="2.2125in"}

**Figure 10.** -- Post-flood conditions, downstream at bridge from
upstream left bank

![](media/image11.png){width="3.0833333333333335in" height="2.3in"}

**Figure 11.** -- Post-flood conditions, looking downstream from bridge
deck

![](media/image12.jpeg){width="3.4166666666666665in" height="2.5625in"}

**Figure 12.** -- Post-flood conditions, looking eastbound at downstream
face, left floodplain, and section of roadway overflow.

![](media/image13.jpeg){width="3.3666666666666667in" height="2.525in"}

**Figure 13.** -- Post-flood conditions, looking under bridge from left
abutment.

### Bed Material Data

Bed material samples were collected at three locations in the main
channel; 200 feet upstream of the bridge, adjacent to the bridge
opening, and 350 feet downstream of the bridge. The samples consisted
primarily of a mildly cohesive clayey-silt and had a D~50~ ranging from
.02 mm to .04 mm. No material collected under bridge, due to the
presence of rocks and/or concrete from the old bridge. Three samples
were collected from boat just downstream of the bridge along the left
bank at a depth \~ 4 ft. The grain size distributions for the three
sample locations are shown in figures 14-16.

![](media/image14.wmf){width="5.995833333333334in"
height="4.1in"}**Figure 14.** -- Grain size distribution for the bed
material sample collected upstream of the 247^th^ Street bridge

![](media/image15.wmf){width="5.990972222222222in"
height="3.5708333333333333in"}

**Figure 15.** -- Grain size distribution for the bed material sample
near the 247^th^ street bridge opening.

![](media/image16.wmf){width="5.995833333333334in"
height="4.1in"}**Figure 16.** -- Grain size distribution for the bed
material sample collected downstream of the 247^th^ street bridge.