Summary of Selected Studies Conducted by the USGS in Cooperation with the MDOT

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Summary of Selected Studies Conducted by the USGS
in Cooperation with the MDOT

• K. Van Wilson
• U.S. Geological Survey, WRD
• Pearl, Mississippi

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MDOT USGS Cooperative Program

• USGS assists the MDOT by providing hydrologic
  and hydraulic data for highway bridges crossing
  Mississippi streams. USGS also uses an ADCP to
  collect velocity-vector and channel bathymetry
  data at selected sites.

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The use of reliable flood characteristics by the
MDOT will hopefully reduce the number of drainage
structures that will likely fail and cause loss
of life and property.
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This presentation will focus primarily on some of
the data that has been collected with the ADCP
and selected scour data that has been collected
during previous studies.
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ADCP discharge measurements
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Bi-directional flow measured with ADCP at Jourdan
River at I-10 (If we had used conventional
measurement methods, we would not have detected.)
Note Light blue in lower depths indicating
upstream (inland) flow White at about
zero velocity Yellow in upper depths
indicating downstream (gulfward) flow
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Stage-storage-volume relation upstream (inland)
of Jourdan River at I-10
100.0
100.0
Hurricane Camille
Hurricane Betsy
10.0
10.0
ELEV., in feet above sea level
5-ft contour
Based on September 19-20, 1996, stage and ADCP
discharge measurements
1.0
1.0
0.1
0.1
1,000,000
10,000,000
100,000,000
1,000,000,000
10,000,000,000
VOLUME, IN CUBIC FEET
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Jourdan River at Interstate 10 near Bay St. Louis
GPS ship-track data from processed/merged
ADCP file.
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Lateral Channel Movement
One example of this problem is at the
State Highway 25 (Lakeland Drive) crossing of
the Pearl River in Jackson. Work began in
1967 on the construction of two two-lane
bridges. The crossing had a slight channel
meander to the left with the cut bank on the
right. By 1991, nearly 200 feet of the right
bank had eroded and the thalweg was located under
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Lateral Channel Movement (cont.)
the 40-foot approach spans. In an effort
to stabilize the right channel bank, the
Mississippi Department of Transportation
constructed two jetties upstream of the crossing
in 1993.
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Pearl River at Lakeland Drive
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Pearl River at Lakeland Drive
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Downstream Jetty
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Upstream Jetty
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ADCP Measurements
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Pearl River at Lakeland Drive
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Pearl River at Lakeland Ship-Track Data
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Right (West) Bank on September 20, 1991
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Right (West) Bank on July 17, 2000
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Right (West) Bank on July 17, 2000
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Right (West) Bank on July 17, 2000
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Right (West) Bank on July 17, 2000
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Right (West) Bank on July 17, 2000
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Right (West) Bank on July 17, 2000
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Right (West) Bank on July 17, 2000
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PIER-SCOUR DEPTHS AFFECTED BY CLAY IN MISSISSIPPI
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Suspending a Columbus-type sounding weight from
the downstream side of a bridge with transducer
attached to bottom of weight
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A clay stratum overlain by sand or gravel was
thought to affect some of the measured
pier-scour depths.
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Characteristics of 29 pier-scour measurements
thought to be affected by clay

• Pier-scour depth -- 0.40 to 2.53 m (1.3 to 8.3
  ft)
• Scour-hole top width -- 2.4 to 30.5 m (8 to 100
  ft)
• Properties of bed material overlying clay
• Median size -- 0.36 to 4.30 mm (0.00118
  to 0.01411 ft)
• gradation coefficient -- 1.3 to 8.3

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Characteristics of approach flow for the 29
pier-scour measurements thought to be affected by
clay

• Depth -- 1.80 to 11.16 m (5.9 to 36.6 ft)
• Velocity -- 0.55 to 3.14 m/s (1.8 to 10.3
  ft/s)
• Froude no. -- 0.08 to 0.45
• Skew -- 0 to 28 degrees

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Characteristics of the measured net pier-scour
depths through clay (Yscl)

• Ranged from 0 to 2.04 m (6.7 ft) compared to
  total pier-scour depth (Ys), which ranged from
  0.40 to 2.53 m (1.3 to 8.3 ft)
• 10 of 29 measurements with Yscl 0 and
• 9 of 29 measurements with Yscl Ys (no
  overlying bed material).

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Estimated shear strength of the clay ranged from
26 to 235 kN/m2 (545 to 4,900 lb/ft2).
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Measured pier-scour depths affected by clay in
Mississippi were compared to the following
computed pier-scour depths

• HEC-18 equation presented by Richardson and Davis
  (1995), and
• envelope-curve equation developed by Wilson
  (1995) for the Mississippi onsite high-flow scour
  measurements.

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Neither the HEC-18 or the Mississippi
envelope-curve equation accounts directly for

• the shear strength or
• chemical properties of cohesive material.

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HEC-18

• Clay bonded together by cohesion will have scour
  depths as deep as sand-bed streams, but many
  floods may occur before the maximum scour depth
  is reached.

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Time-dependent hydrologic conditions of scour in
cohesive materials are very important.
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Kamphius(1990) noted that sand inan eroding
stream

• Decreases the critical shear stress required for
  erosion of the clay,
• Increases erosion volume and erosion rate of the
  clay, and
• Determines where erosion occurs in the clay.

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Pearl River at county road bridge at Rockport,
Mississippi,May 3, 1991(Meas. 79)
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Pearl River at county road bridge at Rockport,
Mississippi,May 3, 1991(Meas. 79)
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Pearl River at county road bridge at Rockport,
Mississippi,November 19, 1991
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1
0
-1
-2
-3
RESIDUAL
-4
Using Miss. envelope curve
-5
Using HEC-18
-6
-7
1
2
3
4
5
6
7
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ESTIMATED PIER-SCOUR DEPTH FOR ONLY THE SITES
THOUGHT TO BE AFFECTED BY CLAY
meters
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CONTRACTION-SCOUR DEPTHS AFFECTED BY CLAY IN
MISSISSIPPI
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Selected relief bridges are located in a
vegetated flood plain with little or no
bed-material transport in the flood plain (Case
3 - - HEC-18).
CLEAR-WATER SCOUR CONDITONS
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The April 1979 flood was slightly greater than
the 500-year flood at both of these sites.
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Measured scour depths obtained during the
discharge measurement on April 13, 1979, were
compared to the HEC-18 computed contraction-scour
depths.
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Pearl River at State Highway 16 at Edinburg,
Mississippi
Looking left (east) from main-channel bridge
(April 14, 1979)
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Measured scour depths obtained during the
discharge measurements on April 14 and 16, 1979,
were compared to the HEC-18 computed
contraction-scour depths.
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Conclusions

• USGS operates and maintains streamflow gages
  throughout the U.S. and is actively sounding in
  the proximity of a number of bridges to determine
  stage and discharge.
• Conventional discharge measurements are being
  supplemented with ADCP measurements so that more
  data (velocity, depth, etc.) can be more
  efficiently obtained.
• Maximum depths can be determined from the
  discharge measurements to obtain an understanding
  of the ranges of bed fluctuations (scour and
  fill) that have occurred at a site.

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