Figure 34. Locations of TDR bed-level monitors. TDR numbers and the respective cable lengths: TDR 1 (400 ft); TDR 2, 4, 5, and 6 (500 ft); and TDR 7 (700 ft)

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cable being snagged by debris and the unnecessary loading. Because of attenua-

tion and dispersion of the TDR signal as it propagates along the coax cable, the

signal's accuracy diminishes with increasing cable length. To minimize these

effects on the signal, the maximum length of cable used during this study was

700 ft.

Seven TDR probes were installed during December 1998 at the Culbertson

site, immediately upstream of the Route 16 bridge. The probes were jetted into

the riverbed using compressed air so that the tops of the probes just protruded

above the bed for situations of anticipated bed scour or at the maximum level of

anticipated sediment deposition. Bedrock was approximately 10 ft below the

surface; in some areas, though, the channel had scoured to bedrock. The depth to

bedrock presented a challenge in locating areas with sufficient sediment to later-

ally support the probes while the bed was being eroded. In some instances the

anchoring section was shortened to ensure that the sensing section of the probe

would be in an active erosion zone. The probes were installed on the banks of

both subchannels, at the downstream tip island, between two bridge piers, and

upstream of a midchannel bridge pier. The locations of the scour probes are

shown in Figure 34, a plan view of the site.

TDR 7

TDR 1

TDR 5

TDR 6

TDR 4

TDR 2

Shelter

TDR 3

Middle Pier

North Pier

South Pier

Figure 34. Locations of TDR bed-level monitors. TDR numbers and the

respective cable lengths: TDR 1 (400 ft); TDR 2, 4, 5, and 6 (500 ft); and TDR 7

(700 ft).

Index