TxDOT Water Quality Pond

I continued the inspection of the TxDOT water quality pond on Loop 360 the 30th of December after the afternoon’s short rain event passed over the area to verify my previous assertions. The afternoon light revealed a minor accumulation of stormwater runoff in the sedimentation pond and two significant holes draining water into small collapsed areas at the base of the dam on either side of what appears to be the top of the culvert under a layer of clay.

Collapse zones (notice the apparent outline of one side of the culvert in the earth between the pools of water)

After waiting about twenty five minutes from first observing drainage through holes in the inner edge of the sedimentation pond and walking back around to the culvert outfall that drains the splitter box I could detect no discharge of untreated stormwater. Since it was after 5 PM, my camera was out of memory and I was soaked, I didn’t linger to see when or if a discharge around the wing walls of the culvert would occur.

Collapse zone…

and hole #1 (in circled area #1)

It occurs to me that a possible washout along or under the culvert could offer some storage capacity under the culvert and the dam. Large voids at the base of the dam could compromise the integrity of the earthen dam during periods of extended rainfall.

Hole #2 (in circled area 2)

Hole #2 (detail)

Conclusion:

There are a number of small sinks, holes and widened cracks in the sedimentation pond some of which discharge unfiltered highway stormwater runoff into the draw downstream. The water in the sedimentation pond flows directly over to discharge pipes into the sand filter with little dissipation of the sediment load. This could be corrected with a serpentine pilot channel in the sedimentation pond.

The sand filter is covered with a layer of algae and silt. There is a relief valve at the edge of the sand filter that I discovered to be half open. (Now closed) The rocks downstream of the filter system are coated with algae and apparent hydrocarbon residue.

This TxDOT BMP is in need of immediate maintenance. The collapse of the clay liner suggests that perhaps a plastic liner over the area of the culvert is in order to prevent future breaches. The discharge of silty water from the sand filter suggests that replacement of the filter mediums and the drain collection pipes may be required.

If the sand filter walls are to be allowed to be a “canvas” for graffiti artists there should be the consideration of stronger pipes and more secure filter and drainage structures under the sand where people walk.

Respectfully submitted,

Tim Jones