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Water Monitoring Flume Replaced at the Rocky Flats, Colorado, Site

October 22, 2013 - 11:01am

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In August of this year the U.S. Department of Energy (DOE) Office of Legacy Management (LM) Rocky Flats, site took advantage of an existing water diversion structure that was no longer needed, to replace an aging water monitoring flume and avoid future repairs that could interrupt data collection.

The flume for Rocky Flats Legacy Management Agreement (RFLMA) Point of Evaluation (POE) monitoring location GS10 in South Walnut Creek was located at the bottom of fairly steep channel banks. The bank on the south side shows localized slumping and sliding toward the creek and GS10. Although the GS10 metal flume was operational, the aging structure and hillside movement to the south could compromise the quality of data collected in the future.

The GS10 flume was originally installed in 1993. DOE considered replacing the flume in 2000, when flumes for several other monitoring locations were replaced, but the project was a low priority in relation to other cleanup and closure work at the time, and due to the hillside slumping, replacement would have been costly and difficult.

RFLMA POE sampling location GS10 and the downstream diversion structure prior to beginning the replacement project.
RFLMA POE sampling location GS10 and the downstream diversion structure prior to
beginning the replacement project.

The GS10 flume was slightly upstream of a massive, deeply anchored, approximately 50-foot-wide concrete diversion structure in the stream channel. The diversion structure had openings to allow creek water that had already been monitored at GS10 to flow through to underground corrugated metal pipes. The pipe openings were fitted with gate valves, or “head gates,” to control the flow of the water. One head gate controlled flow through a pipe into former Pond B-1. The other two head gates controlled flow into a bypass line around former retention ponds B-1, B-2, and B-3 and then into B-4. Once the B-1 through B-4 dams were breached, the bypass was no longer required, since water was being routed through the former ponds. This allowed for an innovative and cost effective reuse of the diversion structure.

LM removed the old GS10 flume and installed a new flume on the downstream side of the diversion structure after cutting a notch to accept the new flume. The new flume is a fiberglass H-flume, better suited to measure the lower post-closure flow rate ranges in this portion of South Walnut Creek. After grouting the two pipes at the diversion structure, the creek channel upstream was filled and graded to raise the channel elevation to the elevation of the new flume. Creek water now flows through the diversion structure notch and into the new flume, instead of through the diversion structure via the pipes.

Construction contractors used heavy equipment to remove the sections of the diversion structure concrete that were sawn for the new notch.
Construction contractors used heavy equipment to remove the sections of the diversion structure
concrete that were sawn for the new notch.

The new flume was installed prior to removing the old flume then a temporary coffer dam was installed upstream to hold creek flow so it could be pumped to the new flume, ensuring continuous monitoring while the old flume structure was being removed.

The new flume is approximately 35 feet east of its former location. Because of the proximity of the new flume to the previous flume location, this monitoring location will continue to be identified as GS10 and no changes to the tables or figures in the regulatory documents that relate to GS10 are needed.

Excess soil generated by grading the area downstream of the diversion structure and clean, imported fill was used to raise the elevation of the area upstream of the diversion structure to above the current head gate elevation. Pieces of the current GS10 flume, pipes, head gates and associated hardware, and concrete that were removed are being properly managed as waste, or recycled if eligible for recycling.

The new GS10 surface water monitoring location with automated sampling equipment installed.
The coffer dam used to retain stream flows is visible to the left.

 

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