The city of Corpus Christi is implementing a scour protection project that the City Council approved at a February meeting to armor the bridges and approaches of the Whitecap and Gypsy bridges on North Padre Island against erosion.
City officials announced the start of the project on July 9, beginning with the import of crushed concrete rock and armor stone of various sizes from Bridge Specialties, a company based in Rockport, to the project sites.
Since then, contractors have arrived with barges to install the scour protection material, known as riprap, beneath the bridges and at the bridge approaches. The armoring will be locked in place with concrete to protect the canal bottoms from erosion. Navigation will be restricted beneath both of the bridges throughout the construction phases.
According to a project update given last Friday by Chloe Rybalka, public information officer for the city, crews began working about two weeks ago, shutting down to secure for the possibility of Hurricane Beryl hitting the island, and were back to work the following week.
At a Feb. 20 meeting, the City Council appropriated $1.5 million in Tax Increment Reinvestment Zone No. 2 funds for the scour protection project.
TIRZ No. 2 funding was authorized at a Jan. 23 meeting to retrofit scour protection countermeasures to the bridges, according to a recent news release published by the city.
Addressing vulnerabilities on 1970s-era bridges
The bridge scour protection project is part of a larger effort to address structural vulnerabilities in the bridges and bulkheads on North Padre Island. Both bridges were constructed by the Padre Island Investment Corp. in the 1970s and were rebuilt after bridge approaches were washed out by Hurricane Allen in 1980.
Jeff Edmonds, the director of the city's Engineering Services Department, said that the canals have what engineers call cohesionless soil that moves very easily, and that if water flow reaches a certain velocity, it will start to move the soil that, at certain locations, particularly at flow contractions, can cause scour.
Drawings of the Whitecap and Gypsy bridges rendered by LJA Engineering, the city’s contractor on the rock installation project, show the existing bridge bulkheads are composed of 10-foot concrete sheet piles that are driven 6.5 feet deep behind the bridge abutments, placing them a foot above the 7.5-foot-deep mudline, or channel bottom, in some places.
The riprap protection would elevate the channel bottom from 7.5 to 4.5 feet once the rock and crushed concrete are placed to fill the voids.
“Both of those bridges were built with very shallow foundations, meaning they were vulnerable,” he said.
The opening of the canals beneath the new Don Patricio Bridge in April was the first of four phases in the Whitecap NPI development project which, once complete, will connect Corpus Christi Bay to the Intracoastal Waterway. But many have questioned the impact of flowing water and change in hydrodynamics through the interconnected canals as a result of opening them to the Gulf of Mexico.
Hydrodynamic model looks at flow of water through canals
Multiple studies have been conducted on the water flow rates through the canals, including a supplemental report prepared last August by LJA Engineering under contract with the city that focused on key locations such as the Whitecap and Gypsy bridges.
In January of this year, LJA sent a report to the city showing that peak flows through the Park Road 22 bridge will increase from 2.64 feet per second in Phase 1 to 3.79 feet per second at the end of Phase 2, when the Commodores Drive bridge and Aquarius Street culvert crossing are opened in the next phase of the Whitecap NPI.
Conversely, the study indicated lower flow speeds at the Whitecap and Gypsy bridges at the end of Phase 2, because the flow of water coming through the Park Road 22 bridge to the Laguna Madre, though higher, will split to the north and the south.
According to the city engineer, the city found that scour was evident at the bridge toes after hiring contractor crews in February who measured the depth of the water in the canals from the top of the bulkhead caps to the mudline at various locations on the bridges.
The city and consultants agreed that the bridges would experience high-enough water velocity to be erosive.
Recent storms test the durability of canal bulkheads
Storm surge from Tropical Storm Alberto damaged two waterfront residences after bulkheads lining the canal properties failed. Officials then decided to expedite the improvement project on the bridges.
While the city is responsible for the bridges and bridge bulkheads, the Padre Isles Property Owners Association owns and maintains more than 30 miles of straight concrete bulkheads, or seawalls, that function as a protective barrier by separating the land from the canals to prevent land loss from erosion, currents and waves, and maintain proper water depth.
Residents of the damaged properties attended the association's monthly meeting on June 5 to impel the organization to take action and make immediate repairs.
One island resident said there was more water than he’d ever seen before flowing in torrents past the house, causing bulkheads to fail and sand to pour out from beneath the house and pool. He noted that where the canal bottom used to be 5 feet deep, the current that came by during the storm and that comes by every day due to tidal flow has scoured the canal bottom to a depth of 10 feet, taking away the wall of dirt in front of it.
The association began making repairs to the two residences the first week of July after delays in starting the work.
The association has also performed a technical risk assessment, had its engineering contractor approve emergency actions and hired the required contractors to begin implementing a temporary solution to stabilize the areas around the failed bulkheads.
With the two properties are still at serious risk of further structural damage, the property owners association is engaging in a collaborative effort with the city to come up with a final resolution to protect the bulkheads, Executive Director Rick McGinley said.
“We are trying to figure out within the expected flow rates what is the best strategy for protecting the bulkheads,” he said. “We cannot return to a high flow rate in the canals and expect there to be no damage.”
In a separate improvement project, a private sector developer constructed a sand cofferdam that now spans the Packery Channel where it runs under the Don Patricio Bridge.
The dam will cut off the passage of water from the Packery Channel into the canals to allow crews to make repairs to bulkheads that were damaged on the Lake Padre side of the canal channel after a deep scour hole was found along the bulkheads after Tropical Storm Alberto.
Water will continue to flow in and out from the canals on the side of the Laguna Madre.
The property owners association approved the building of the canal during executive session at the meeting on June 25, McGinley said.
According to Tim Lange, the manager of the Lake Padre Development Co., the project specifications and design plan have yet to be determined. He did not comment on how long the cofferdam would be in place.
Unlike the bridges at Gypsy Street and Whitecap Boulevard, the Don Patricio bridge was built with armoring, 65-foot-deep concrete sheet pilings and scour protection countermeasures, along with a deep foundation, Edmonds said.
However, the location of the scour hole up against the bulkheads in a narrower section of the canal could undermine the bulkheads in that section.
A second scour hole found on the Whitecap side of the bridge was also found, though it is located farther away from the bulkheads in a wider portion of the canal.
More: Padre Island homeowners seek urgent repairs to canals damaged in tropical storm
More: City celebrates completion of Packery Channel Restoration Project
More: What's in the cards for Lake Padre development? Here's what to know
This article originally appeared on Corpus Christi Caller Times: Project underway to armor Gypsy and Whitecap bridges on North Padre Island
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