A look at Flea Market Sink. One of Austin’s lesser known sinkholes.

Flea Market Sink is one of Austin’s lesser known sinkholes which have not been as thoroughly explored. This news article will explain what Flea Market Sink is.

Tucked away near the Highway 290 right-of-way towards West William Cannon Drive is a small sinkhole called Flea Market Sink. Any signs indicating there is a sinkhole present are hidden away by the shrubbery of tall grass and oak trees. Vegetation hides the visibility of Flea Market Sink. Flea Market Sink is a significant sinkhole feature that is a significant recharge feature.

There is a black metal fence surrounding the perimeter of the sink area which is ironically not locked. Which is unusual because many land owners voluntarily protect known sensitive karst features such as sinkholes.

Not many people know this but Flea Market Sink is a sinkhole which is actually a closed depression. The sink area is approximately 35 feet in diameter, sloping to approximately 2 to 2.5 feet (2½ feet) in depth. Several limestone boulders approximately 1 foot in diameter are present. (Ref: https://www.oakhillparkway.com/upload/files/files/impact/OHP%20Final%20EIS_12.21.2018%20signed%20ROD%20cmpsd.pdf)

There is a vertical metal pipe in the center which connects to the bottom of Flea Market Sink which appears to be connected to the storm sewer system of the Industrial Oaks Pond. However there is no shaft feature present or extant. The vertical metal pipe is a water quality control feature which controls flows for recharge and discharge. The water quality control improves the quality of recharge to Flea Market Sink. (Ref: https://www.austintexas.gov/edims/document.cfm?id=313621)

The metal pipe extends to a depth of approximately 6 feet below ground level, where it intersects at an approximately 12-inch-diameter pipe that runs to the south toward the US 290 stormwater drain system. The metal pipe goes not further than 6 feet in length. (Ref: https://www.oakhillparkway.com/upload/files/ReEval/2019-Revised-Geologic-Assessment.pdf)

The metal pipe and storm sewer connection were constructed to alleviate ponding of stormwater in the feature and keep the ponded water from impacting the car lot east of and adjacent to the site. (Ref: https://www.oakhillparkway.com/upload/files/ReEval/2020-USFW-Attachments-b-sm.pdf)

Flea Market Sink is one of 6 known karst recharge features in the Oak Hill area. Several are immediately adjacent to proposed excavation. Flea Market Sink could be affected by excavation. Excavation is a threat and could pose a threat to Flea Market Sink because Flea Market Sink is a recharge feature in spite of connecting to the storm sewer system of the Industrial Oaks Pond. However Flea Market Sink does not connect to Edwards Aquifer. (Ref: https://savebartoncreek.org/page/13/?cat=-1)

Flea Market Sink is located less than a block west of 5510 US 290, Austin, Texas, US 78735.