Mixerr Reviews: Exploring Headquarters Flat Sink/HQ Flat Sink. Another one of Austin’s lesser known sinkholes.

Headquarters Flat Sink/HQ Flat Sink is one of Austin’s lesser known sinkholes which not many people know about. This news article will explain the significance and environmental features of Headquarters Flat Sink/HQ Flat Sink.

Headquarters Flat Sink/HQ Flat Sink is a sinkhole which is a 46-acre internal drainage basin within the recharge area for the Edwards aquifer associated with Barton Springs. HQ Flat Sink is a research site centers on a 46-acre internal drainage sinkhole catchment. The fate of runoff in the basin is a sinkhole--runoff cannot escape the basin. (Ref: https://www.austintexas.gov/watershed_protection/publications/document.cfm?id=228332)

Below is the history of Headquarters Flat Sink/HQ Flat Sink.

Ira Yates was the previous landowner of Headquarters Flat Sink/HQ Flat Sink. He owned the land where Headquarters Flat Sink/HQ Flat Sink and Flint Ridge Cave is located on. Ira Yates reported that HQ Flat Sink “takes considerable water after rains”. (Ref: Russell, 2004)

In 1984, Bill Russell believed Headquarters Flat Sink/HQ Flat Sink was a significant recharge feature. So he cleared rocks and debris around the entrance to enhance recharge there. He described an estimated 170 m3 depressed area in the drainage leading to the sinkhole that had to fill before surface flow could continue to the cave. Russell estimated the catchment area drained “an extensive area to the south and west” of about 6.5 hectares (16 acres). Much of the known extent of the sinkhole had been investigated by cave explorer Mike Warton in the 1980’s. The cave was mapped to a depth of 10 m (32 feet).

A second hydrogeologic study conduced in 2000 describes surface drainage and karst development on the Headquarters Flat Sink/HQ Flat Sink on J17 tract. This report described surface flow on the J17 tract to be primarily sheet flow toward Danz Creek tributary of Slaughter Creek. It describes HQ Flat Sink as a “small sink” and noted “the recharge potential to the aquifer from the sink is probably small to medium since the surface opening is moderate in size but the drainage area affected by the sink is relatively small.” From the surface entrance the report believed the HQ Flat Sink “appears to pinch close at about 5 feet.” (Ref: Ford, 2000)

Over a single 2002 rain event, during which the ground was near saturation prior to precipitation, the amount of discrete internal runoff to HQ Flat Cave was 42% of the rainfall. Over a 1140-day interval under average precipitation conditions, discrete internal runoff to both HQ Flat Sink Cave and Flint Ridge Cave diminished to 3% of precipitation. (Ref: https://digital.lib.usf.edu/content/SF/S0/07/16/53/00001/K26-05342-JWARP_2014062616500532.pdf)

A report representing a 505-day water budget from April 2, 2004 to August 20, 2005 for HQ Flat Sink was studied. Measured evapotranspiration and internal runoff was subtracted from measured precipitation over a 505-day water balance from the South Austin based Headquarters Flat Sink/HQ Flat Sink research site in order to measure diffuse infiltration at a site scale. (Ref: https://www.austintexas.gov/watershed_protection/publications/document.cfm?id=228332)

From 50 to 85 percent of recharge to the aquifer is derived from streams originating on the contributing zone, located up gradient to the north and west of the recharge zone. Runoff reaching the HQ Flat Sink drain (3% of average precipitation) is comparable to runoff reaching the creeks outside of internal drainage basins. HQ Flat Sink research site water balance interval is 142% higher than average annual.(Ref: Wilcox, 2008)

The authors state "The results from the 505-day water balance at HQ Flat Sink are strikingly similar to results a similar water balance study reported by Dugas et al. (1998) where 65% of rainfall left as ET, 5% of precipitation became runoff, and 30% of precipitation recharged the Trinity Aquifer".

(Ref: https://www.researchgate.net/figure/Central-Texas-ET-Tower-Data-and-Annual-Precipitation_fig3_276045676)

The monitored water balance components of HQ Flat Sink that were measured for a year period (348 days), included the discrete recharge to the cave drain, evapotranspiration flux using an eddy covariance system mounted on a 15-meter tower, and soil moisture using insitu sensors and gravimetric analysis. Over the year test period, where rainfall totaled 106 cm, 58% left as evapotranspiration, 8% recharged directly through the cave drain, and 34% recharged diffusely across the internal microbasin of the cave site. The total recharge was measured to be 42% of the rainfall within the test basin. (Ref: https://www.austintexas.gov/watershed_protection/publications/document.cfm?id=186248)

The water balance for HQ Flat sink shows similar recharge and evapotranspiration percentages to other karst areas. Over a one year period, about 42% of the rainfall within the HQ Flat microbasin recharged the Barton Springs segment, with 33.5% infiltrating through the soils into the underlying bedrock and about 8% flowing into the cave drain at the lowest point of the sinkhole as discrete recharge. The rainfall measured over the reference year was 30% above the local average rainfall of 81 cm. Over the 30.5-month period of record for HQ Flat, the discrete recharge declined from 8% to 5.7% of the total rainfall. (Ref: https://www.austintexas.gov/watershed_protection/publications/document.cfm?id=186248)

The volume of discrete recharge to Flint Ridge over the year test period was 25% more than HQ Flat, although this is expected since Flint Ridge has a proportionally larger catchment area. The percentage of rainfall over the catchment area that entered Flint Ridge over the same year was 9.5%, which is surprisingly similar to the 8.4% of discrete recharge measured at HQ Flat despite their apparent differences, such as the large surface depression diverting flow from HQ Flat.

Typical soil thickness depths within the HQ Flat microbasin were found to be 60 cm or less on the northern half of the catchment basin, and about 0.2 cm or less on the southern portion. Assuming an average 30 cm soil depth, the estimated maximum change in soil moisture storage from a very wet period to a dry period (or vice versa) is 7 cm. The typical variation in soil moisture storage of 7 cm represents 7% of rainfall over the 12-month test period and declines to 3% over the 30 month test period.

In 2005, TxDOT commissioned a detailed site-specific investigation of soil conditions in the project area. Reports from those studies are attached. During the investigation, professional soil scientists and hydrogeologists conducted field studies within the FRC drainage basin and the Headquarter Flat Sink. (Ref: https://www.austinmonitor.com/wp-content/uploads/2014/12/Flint-Ridge-Letter.pdf)

An injection on May 5, 2005 in HQ Flat Sink, two miles southwest of Blowing Sink, likely did not pass through Blowing Sink Cave but instead flowed east. (Ref: https://www.austintexas.gov/watershed_protection/publications/document.cfm?id=196480)