International Dredging Review

International Dredging Review

Hamilton Pool is a naturally occurring pool on Hamilton Creek near Austin Texas. The Hamilton Pool Preserve is operated by Travis County Parks.

The natural feature has been a landmark for the Texas Hill Country and Austin area for centuries. In late 2007 the discharge of materials from an upstream residential development in Hays and Travis Counties into the Hamilton Creek watershed polluted and impacted the water quality of Hamilton and Davis Creeks and Hamilton Pool.

It is estimated that 7000 to 15000 cubic yards left the development site and deposited in the creek system and pool. In August 2007 Espey Consultants Inc. (EC) was retained by Travis County to assess the condition of Hamilton Creek and Pool and to develop a remediation and restoration plan.

EC was then challenged with the task for the direct removal of silt and invasive plant vegetation from six miles of Davis Creek and Hamilton Creek. The majority of site clean-up was conducted using high pressure water hoses (via water pumps) to wash the silt deposits down to controlled collection areas or to areas where light weight dams were pre-installed. Where accessible by rubber-track heavy machinery sediment was collected and removed off site.Hand brooms shovels and buckets were used to remove sediment from more environmentally sensitive and remote areas.

With Davis and Hamilton Creeks now clean the next phase would be removing the sediment that had been deposited into Hamilton Pool. EC teamed up with American Underwater Services Inc. (AUS) and Del Tank & Filtration Systems (DEL) to handle the extraction and filtration.

AUS Inc. used a commercial diver-assisted hand-dredge barge with a Deutz 100 HP turbo diesel driving a 6×6 Wemco materials pump with a six inch intake and discharge line. Due to limited access to the pool a 90-ton crane was used to lower the barge down a 50-foot cliff into the water. The barge would act as the staging area for the divers doing the underwater pumping. The dive teams working eight-hour daily shifts dove to the bottom and vacuumed the sediment and debris from the pool using a six inch EPDM suction hose with a nozzle.

The slurry was pumped from the barge straight up the 50 foot cliff where a booster pump pushed it the rest of the 3100 ft to the DEL Dewatering System. The DEL system comprises six 400 bbl mix tanks three 3.5 cubic yard filter presses three fast-fill pumps a three-panel DELineator shaker a 200 bbl pre-coat tank and a filtrate return tank.

The feed from the barge which averaged between 800 and 1200 GPM is first screened on the three-panel DELineator shaker equipped with 20-mesh screens to remove the rocks pebbles and larger debris. The slurry then enters a series of six 400 bbl mix tanks where it is tested and Perlite (Harbolite 635) is added according to filterability. The slurry now dosed with Perlite is pumped via the three fast-fill centrifugal pumps to the filter presses. (Prior to each new cycle the filter presses are pre-coated with a pure Perlite slurry that is mixed in the 200 bbl three-compartment pre-coat tank) The result was drinking-water-clear effluent.

Mike Kulbeth Project Manager for DEL: “This was our second project of this nature working with both Espey Consultants and American Underwater Services Inc. We learned a lot on a similar project (described below) about the type of sediment we would be dealing with at Hamilton Pool. We found that the sediment did not filtrate well without the use of a filter aid such as Perlite or DE (diatomaceous earth). So we knew going in that we would need to use a pre-coat method as well as constant body feed of the Perlite into the slurry to ensure that the flow going through the filter presses stayed as high as possible thus maximizing pumping time. The use of the Perlite as well as the carefully timed filter press and mix tank cycles were the key to keeping the divers in the water pumping sediment for the full eight hous without having to shut down to drop filter cake.

“The process ran smoothly from start to finish with few hiccups along the way. In the end up to nine feet of silt and sediment was removed from the bottom of the pool and clarity of the water improved from less than a meter to more than four meters in less than a week post sediment removal.”

A Similar Project Nearby

DEL used its Total Clean System in conjunction with two 100-cubic-foot mobile filter presses to remove sediment from a similar natural pool near Dripping Springs.
The Total Clean was equipped with a scalping shaker and one desand/desilt unit set up with 400-mesh Screens.

The project was in a deep cavernous pool that had been muddied due to an inflow of sediment from upstream construction during torrential rains. The constant flow of the waterfall into the pool continued to re-suspend the powder-like sediment turning the pool milky green. American Underwater Services (AUS) was contracted to provide the diver/dredging operations and DEL was contracted to remove the sediment from the dredged slurry and return sparkling clear water to the pool.

AUS’s divers had the difficult task of removing the powder-like sediment without disturbing the natural bottom of the pool. AUS pumped 600 to 800 gpm of milky slurry to DEL who then removed the minimal amount of sand and gravel that was present with the Total Clean. The rest of the suspended solids (-400 mesh) were removed by the two filter presses.

The clarity of the pool prior to the clean-up was less than 1.5 feet and by the time the dredging and filtration operations were completed the clarity was greater than nine feet.