NCHRP Synthesis 396 – Monitoring Scour Critical Bridges - TSP2

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CHAPTER SIX
CASE STUDIES AND SITES WITH OBSERVED SCOUR DEPTHS
CASE STUDIES OF EXISTING SITES
The following case histories were selected for this report
because they cover a range of geographical locations and
types of fixed scour monitoring instrumentation.
Alaska
To better understand the scour process and to monitor bed
elevation at bridge piers, the USGS and the Alaska Department
of Transportation and Public Facilities operate a network
of streambed scour monitoring stations in Alaska (Conaway
2005, 2006a, b). To date they have instrumented 20 bridges
with sonar and river stage instrumentation (Figure 32). In
2008, 16 bridges remained in the monitoring program. A list
of the bridges and scour monitoring information can be found
in Appendix D. These stations provide state engineers with
near real-time bed elevation data to remotely assess scour at
bridge piers during high flows. The data also provide a nearly
continuous record of bed elevation in response to changes in
discharge and sediment supply. Seasonal changes as well as
shorter duration scour and fill have been recorded. In addition
to the near real-time data, channel bathymetry and velocity
profiles are collected at each site several times per year.
Each bridge is instrumented with a retractable, pier-mounted
sonar device. At locations with multiple scour critical piers,
sonar transducers have been mounted at each pier. The sonar
transducers were mounted either at an angle on the side of
the piers near the nose or on the pier nose to collect data just
upstream of the pier footing. Many of Alaska’s bridges are
situated in locations too remote for landline or cellular telephone
coverage. The scour monitoring instrumentation on
the remote bridges has incorporated ORBCOMM, a constellation
of low-earth-orbiting satellites. Data are sent from the
bridge to a passing satellite, which then relays it to an earth
station, which forwards the data to specified e-mail addresses.
The network of scour monitoring sites is dynamic, with locations
being added and removed annually based on monitoring
priority and the installation of scour countermeasures.
Instrumentation is subject to damage by high flows, debris,
and ice, and repairs at some sites can only be made during
low-flow conditions.
In 2002, one sonar scour monitor was installed at the
Old Glenn Highway Bridge over the Knik River near Palmer
(Figure 33). There are two bridges that cross the Knik River
at this location. The active bridge was built in 1975, is 154 m
(505 ft) in length, and is supported by two piers. The roadway
approaches to the active bridge significantly contract the
channel. Approximately 30 m (98 ft) upstream is the original
bridge, which is no longer open to vehicular traffic. Two
guide banks extend upstream of both bridges and route flow
through a riprap-lined bridge reach. The piers are approximately
aligned with the flow. The Knik River is a braided
sand and gravel channel that transports large quantities of
sediment from the Knik Glacier. The braided channel narrows
from approximately 4.8 km (3 mi) wide at the glacier mouth
to 0.12 km (0.07 mi) at the Old Glenn Highway Bridge, where
the channel is subject to a 4:1 contraction during summer
high flows.
The right-bank pier of the new bridge was instrumented
with a retractable, pier-mounted sonar monitor. This retractable
arm was designed to prevent ice and debris flows from
damaging the sonar bracket, as had occurred in other scour
monitoring installations in Alaska. Stage data were measured
by a nearby USGS stream gage. The sonar was mounted at an
angle on the side of the pier near the nose to collect data just
upstream of the pier footing. Data are collected every 30 minutes
and transmitted every 6 hours by means of satellite. When
bed elevation or stage thresholds are exceeded, data transmissions
increase in frequency.
The Knik River was the only bridge site within the monitoring
network that had large changes in bed elevation each
year. Annual scour ranged from 5.2 to 6.0 m (17.2 to 20 ft).
The near real-time data and historic cross-section data for the
Knik River and other bridge scour monitoring sites in Alaska
is available on the USGS website (http://ak.water.usgs.gov/
usgs_scour).
California
Caltrans has used a variety of scour monitoring techniques to
instrument its bridges. Six different types of fixed instrumentation
have been installed on 24 bridges including (1) stage
gage, (2) sliding rod (sounding rod), (3) sliding collar, (4) sonar,
(5) float-out devices, and (6) clinometer. More recent installations
generally use float-out devices and/or clinometers
(tilt meters). Caltrans notes that they fix or replace all scour
critical bridges. Monitoring is used as an interim measure.
The bridge scour monitoring devices in California all
include telemetry to allow remote access to the scour monitoring
data. The communications from the bridge sites go