The New Jersey Department of Transportation (NJDOT) initiated a statewide scour evaluation program for highway bridges in 1990 in response to increased national attention to the potential risk and catastrophic results of bridge scour. The program consists of a two-stage evaluation process:

• Stage I. Screening and Prioritization
• Stage II. In-Depth Bridge Scour Evaluation.

Our Princeton office has been involved in this program since its inception. We have also conducted scour evaluations for several counties in New Jersey, and the Pennsylvania Turnpike Commission. This work included Stage II evaluations for three of the County of Cape May's movable bridges on County Route 619 (Ocean Drive), which is a toll road along the coastline connecting several major shore towns.

The three bridges had been identified as being potentially vulnerable to scour damage and assigned high priority as a result of Stage I evaluations. These structures are located over tidal waterways along New Jersey's southern Barrier Islands, approximately 1.6 km (1 mile) off the mainland. All three waterways are in very dynamic hydraulic environments. The bridges are described in the boxes on the following page.

Evaluation Process

The primary objectives of our Stage II evaluations were to:

• Estimate the potential long-term scour depth
• Calculate the contraction scour and local scour depths for discharge events associated with Category 1 and Category 2 hurricanes (Saffir/Simpson system).

After we determined she scour depths, we made conceptual recommendations for scour counter measures and/or remedial actions. Descriptions of the steps taken follow.

Soil Parameters. We took available soil boring information from each bridge's construction plans and obtained four to six soil bulk samples from each channel bed. We performed median grain size analysis on these samples to determine the approximate soil parameters necessary for calculating the potential scour depths.

Hydraulics. There were no current hydraulic or hydrologic studies available for any of these inlets,
so we used:

• Computer programs Storm Surge Elevation (SSEL) and DYNLET 2.01 to perform the hydrologic and hydraulic analysis for our in-depth evaluation
• The Federal Highway Authority's (FHWA)'s recommended procedures for developing the storm stage hydrographs
• The 1990 Delaware Hurricane Evacuation Study for the required parameters
• Maximum storm surges of 2.6 m (8.6 feet), NGVD and 3.4 m (11.3 feet), NGVD for Category 1 and Category 2 hurricanes.

Table 1: Ocean Highway (C.R. 619) over Corson Inlet

Table 2: Ocean Highway (C.R. 619) over Townsend Inlet

Table 3: Ocean Highway (C.R. 619) over Grassy Sound

Variables. We obtained channel cross section information from National Oceanic and Atmospheric Administration (NOAA) navigation charts and United States Geological Survey (USGS) quadrangle maps. This information was supplemented with data from a fathometric survey at each bridge site. This step was consistent with the scope of scour analyses, which required a more site-specific hydraulic analysis. The information was then used for the computer model in order to determine the variables needed to perform the scour depth calculations.

Scour Depth. The scour depth calculations were performed in accordance with HEC-18, assuming that each scour component developed independently. The HEC-18 equations are based on laboratory data and are conservative, so the resulting estimates were used in conjunction with engineering judgment to determine the potential depth of scour. The calculated scour depths are summarized in Tables 1, 2 and 3.

General Approach at Each Structure

Once the hydraulic analysis was performed for each inlet bay system, we were able to estimate long-term scour and calculate contraction and local scour. Long-term scour estimates were based on the comparison between channel soundings taken periodically from 1986 to 1996 and the channel bed profiles shown on each bridge's original construction plans. We used Laursen's equation for live bed contraction scour, as presented in HEC-18, to estimate the depth of contraction scour. Clearwater scour equations produced very high values for contraction scour, which were considered unrealistic.

We used Froehlich's equation for live bed scour to estimate the potential depth of local scour at the abutments and the Colorado State University (CSU) equation to estimate the depth of local scour at the piers. [Ed. note: Dave Froehlich's equation for live scour was discussed in "Abutment Scour Prediction," a presentation he gave to the Transportation Research Board in Washington D.C. in 1989. Dave is also the column editor of PB Network's "PB Technotes," which frequently features articles on scour and other hydraulic topics.]

Site-Specific Findings

The bridges were considered to be scour critical because the total estimated scour depth was estimated to extend well below the pile caps at the abutments and main piers and expose a significant amount of the piles at some of the pile bents. Underwater inspections suggested severe scour at some of the piers.

Corson Inlet Bridge. Both the May 1984 underwater inspection report and the January 1987 in-depth inspection report stated that the eastern caissons at piers 28 and 30 were undermined at their bases. The underwater inspection of 1989 noted undermining of both eastern and western caissons of pier 28 only. In 1994, both the east and west caissons of piers 28 and 30 were noted to be undermined.

Typical structure's calculated scour depth show graphically.

Townsends Inlet Bridge

Townsends Inlet Bridge. The channel has migrated laterally southwards over the years, as evidenced by changing conditions reported in inspections over the years. Repair plans of 1977 had specified that the repair jackets extend below the channel bed, however:

• The underwater inspection of May 1984 stated that the repair jacket on the north pile on the eastern side at bent #4 terminated above the channel bed.
• The in-depth inspection report dated December 1986 documented scour problems that caused a significant loss of pile embedment and required supplemental supports to be installed at some of the pile bent piers.
• The underwater inspection of 1989 noted undermining of repair jackets at both north and south piles on the eastern side of bent #2.
• In 1994, the south pile of the eastern cluster at bent #4 and east pile of the eastern cluster at bent #5 were both noted to have repair jackets at 0.6 m to 1.2 m (2 feet to 4 feet) above the channel bed.

Grassy Sound Bridge. The channel has migrated laterally northwards. The in-depth inspection report dated October 1986, documents scour problems at the northern end that are probably due to channel migration to the north, causing a loss of fill within the bin type abutment and undermining a portion of the abutment walls. The underwater inspection of May 1989 noted local scour at the western shaft of pier 4. The underwater inspection of 1994 noted scour holes at piers 1, 3 and 4.

Countermeasure Recommendations

Based upon the physical conditions present at each structure and the scour calculations performed, we recommended installation of stone riprap to preserve the structural integrity of the bridges from scour damage. We recommended that the riprap extend partial length or full length under all the spans (as appropriate) across the channel at a width of 12 m (40 feet) and 3 m (10 feet) beyond the bridge limits on each side. Similar countermeasures were recommended at the abutments for a length of 8 m (25 feet) and width of 9 m (30 feet).

The dumped riprap was estimated to be 0.5 to 0.9 meters (1.5 to 3 feet) maximum, an approximation derived from the calculated depth of the contraction scour plus four times the estimated median stone size (D50) for the computed peak velocities.

Future Monitoring and Countermeasures

Because riprap at the piers is considered to be only a temporary countermeasure, routine monitoring will be required. We recommended that diving inspections be performed during each biennial bridge inspection and after any major storm or flood, due to the current scour conditions.

NJDOT is preparing to solicit design firms to perform detailed design and prepare contract documents for the installation of scour countermeasures for many of the scour critical structures throughout the state. Please contact us for additional information about the scour evaluation program in New Jersey or to discuss other issues regarding movable bridges.

Three Cape May Bridges for Stage II In-Depth Scour Evaluations

• North abutment-most piers generally well aligned
• South abutment-a few southern pile bent piers skewed due to existing shoreline.
  

• Approach span pile bents are precast concrete driven piles.
• Main piers are supported on steel bearing piles.
• Abutments are supported on creosote treated timber piles.

• Estimated lengths of all piles, location of the bottom of pile caps relative to the original channel bed
• Actual lengths of substructure piles was unknown because plans may not be "as-built" drawings.

Ocean Highway Bridge Over Townsends Inlet

• Supplemental piles at some pile bents in 1962
• Riprap in 1962 and 1972
• Jacketing some of the piles in 1977
• Strengthening steel members in 1990.

• Eastern (seaward) side-northern and southern shore lines are skewed with respect to the bridge
• Western (inland) side-shore lines generally in alignment with the bridge.

• Detailed information available on original "as-built" construction plans of 1939/1940
• Pier driven pile lengths supplied
• Abutment pile lengths not available, assumed to be similar to another bridge of similar construction built about the same time.

Ocean Highway Bridge Over Grassy Sound

Type: 21-span viaduct, single leaf bascule bridge.

Location: Middle Township. Spans Grassy Sound Channel to the west, Hereford Inlet and the Atlantic Ocean to the east.

Construction Date: 1940. Reconstruction includes:

• Riprap in 1962 and 1976
• Jacketing some piles in 1977
• Strengthening steel members in 1990.

Length Between Abutments: Approximately 325 m (1,067 feet).

Bascule Span: 20 m (65 feet).

Waterway opening: Full length of the structure.

Alignment to normal flow: Abutments and piers are well aligned.

Construction: Same as for Corson Inlet Bridge.

Design plans:

• Detailed information available on the original "as-built" construction plans of 1938/1940
• Pier driven pile lengths supplied
• Abutment pile lengths not available, assumed to be similar to another bridge of similar construction built about the same time.