Aransas County commissioners, at their regular meeting Monday, March 22, received an update from Mott MacDonald Engineering’s Luis Maristany and Aaron Horine about the engineering studies and solution options to improve circulation of Little Bay (utilizing the 404 FEMA Hazard Mitigation Grant program).
The purpose of the grant is to improve the overall circulation of Little Bay, and to prevent future damage like the damage caused by Hurricane Harvey. The Texas Department of Emergency Management is administering the grant for FEMA.
Maristany noted the county secured the grant from FEMA to mitigate storm surge in the future, addressing issues found due to Hurricane Harvey.
Little Bay at one time had several natural outlets before being enclosed by Rockport Beach and Key Allegro.
“The south end was an open sand flat originally,” said Maristany. “We didn’t see issues (back then) because (the area) easily drained.
“Today, development (beach and Key Allegro) has closed off Little Bay, leaving only two channels to drain all the water (from storm surge, and rain) into Aransas Bay.
Mott MacDonald studied the effect of storm surge and rain after Harvey.
“In terms of velocity, we found rainfall was the primary factor,” said Maristany. “Water drained naturally toward Little Bay, and when you constrict drainage, you increase velocity.”
He said there wasn’t enough room in Little Bay to handle the drainage after the storm. That increased the velocity of outflow through Leggett and Blevin’s channels, which caused damage in those areas.
“To lower the velocity (of the draining water) you have to open up Little Bay (let it breath),” said Maristany.
He noted his firm is testing four alternatives, but nothing is set in stone. The alternatives are:
• Deepen Leggett Channel
• Widen Leggett Channel
• Add culverts in Key Allegro to provide additional outflow to Aransas Bay.
• Create a new inlet on the south end of Little Bay.
Maristany said deepening Leggett Channel is a limited option because it can only be dredged to a depth of about nine feet. Making it deeper will undermine the bulkheads on both sides of the channel.
Widening Leggett Channel, from 40’ to 80’, would provide more volume, as well, but would take away land on the south side of the channel.
He noted a 55-foot-wide channel at the entrance to Rockport Beach is another option, which would provide additional water exchange and improve hydraulic conveyance.
Maristany said high volumes of water, at high velocities, passed through Leggett and Blevin’s channels after the storm, causing the undermining of Key Allegro Bridge, and damaging the breakwater in Aransas Bay.
He said models show widening Leggett and Blevin’s channels will significantly decrease the water velocity, as will adding culverts in Key Allegro.
Adding a new inlet at the entrance to the beach will lower water velocity, as well
“This tells us we’re on the right track,” said Maristany, adding a combination of the four alternatives might be the answer.
From a hydraulic standpoint, the culverts in Key Allegro will reduce the pressure on Leggett Channel by almost half.
Maristany said all four alternatives reduce the flow through Blevin’s Channel. He pointed out it is the smaller channel of the two, and options are limited due to infrastructure on both sides of the channel.
Comparing the options based on general circulation, he said, “We want to make sure alternatives don’t hinder circulation.”
He said water exchange, or dilution, in the areas near the two current channels, is pretty quick based on modeling, but concentrations (water that isn’t circulated) stay stagnant on the south side of Little Bay, and in the channels of Harbor Oaks and Key Allegro.
“The model shows that after 41 days (the water in the south end of Little Bay hasn’t circulated),” said Maristany.
“A new inlet (at the entrance to Rockport Beach) into Aransas Bay shows the best results,” he said.
He noted modeling shows when only deepening Leggett Channel, the transfer of water between it and Blevin’s Channel increases, but it also increases the time needed to (flush out) the entirety of Little Bay.
Maristany said the modeling shows a new inlet at the entrance to the beach, followed by culverts in Key Allegro, widening Leggett, and deepening Leggett are the top alternatives, in order, for increasing circulation.
“To improve circulation, some combination (of the alternatives) is needed,” he said. “Now, we have to see what we can afford.”
The next step in the study is to receive input from the stakeholders, refine alternatives, and come up with cost estimates.
Commissioner Bubba Casterline asked if the grant allows the county to manipulate the flow into Little Bay.
Horine said the primary goal (as provided in the grant) is to address storm surge.
“Can you use data accumulated in this study to help in future studies to address what’s going into Little Bay?” asked Casterline.
Horine said this study will provide a lot of good baseline information that can be used in the future at any time.
He said the study, with solid alternatives, complete with cost estimates, should be completed by the end of the year.