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Farrow Drop Adds Stability In Sugar Creek

Farrow Drop Adds Stability In Sugar Creek

Caddo County, Okla.

 Representatives from construction companies attended a pre-bid meeting at the NRCS Anadarko Field Service Center to ask questions about the Farrow Drop project.  The group went out to the location to see the erosion first hand.  To the left of the creek a gully is forming through the middle of the landowner’s wheat field.

Representatives from construction companies attended a pre-bid meeting at the NRCS Anadarko Field Service Center to ask questions about the Farrow Drop project. The group went out to the location to see the erosion first hand. To the left of the creek a gully is forming through the middle of the landowner’s wheat field.

 

(L) A picture taken August 5, 2010, shows green grass and a fence. Under the fence, the bank of Sugar Creek has given away.  (R) October 5, 2010, shows engineers looking over the creek.  The photographer stood at the fence post seen in the picture (r) to take the picture (l).  The dirt that fell into the creek is now sediment. The dead grey grass shows where the next sections will likely erode with future rains.

(L) A picture taken August 5, 2010, shows green grass and a fence. Under the fence, the bank of Sugar Creek has given away. (R) October 5, 2010, shows engineers looking over the creek. The photographer stood at the fence post seen in the picture (r) to take the picture (l). The dirt that fell into the creek is now sediment. The dead grey grass shows where the next sections will likely erode with future rains.

Tiny carpenter ants gnawing away at the sunflower precariously located at the edge of the creek’s eroded banks and grass hoppers devastating the last of the wheat crop were not the biggest problem on the farmland adjacent to Sugar Creek in 2010.

On October 5, 2010, twenty contractor representatives from at least five states and Oklahoma, sat quietly around the conference table in the NRCS Anadarko Field Service Center. They were listening as NRCS Civil Engineer Marques Hunter detailed the expectations of the construction of the Farrow Drop project. The pre-bid meeting was a chance for any interested parties to ask questions of the design staff, though the room was very quiet. Later in the day the group traveled to the project site and got an eyeful of the expanse of erosion they would bid to help control with a rock drop structure.

“The erosion occurring in Sugar Creek is very dynamic,” said Oklahoma NRCS State Conservation Engineer Chris Stoner. “There are areas of degradation (where the water digs and picks up soil) and areas of aggradation (where the soil is dropped off). Depending on the storm event, these areas move up and down the stream channel. An area that was degrading six months ago, may be aggrading today and degrading again in another six months. It is all a function of the stream trying to find a stable slope so that it remains in equilibrium. The deepening and widening of Sugar Creek is due to the extremely erosive sandy soils and the instability of the channel bottom.”

A drop structure has a rock-lined, steep sloping “drop” section, then a rock pool that absorbs energy at the bottom, then another drop and another rock pool. It looks like a controlled waterfall. The drop structures change significant elevation differences at various points in the creek in a controlled manner. The process makes the channel between structures more stable.

John Mueller, former Oklahoma NRCS assistant state conservation engineer, recalled the idea for the design of a rock drop came from training in the Rosgen Stream Classification System. Most of Rosgen’s designs mimic natural processes – in Colorado. Mueller adapted Rosgen’s ideas to the unique problems of instability in the Sugar Creek watershed in Oklahoma.

Mueller worked with Hydraulic Engineer Kerry Robinson at the Agricultural Research Service Hydraulic Engineering Research Unit in Stillwater, Okla. Their team modified research already conducted studying rock chutes and introduced the waterfall-like aspect. Robinson said, “This is a good example of ARS and NRCS working together to solve a problem (and it) happens to work.”

Each structure in the Sugar Creek watershed is different and builds on what engineers have learned from previous designs. Previous structures on the main channel had a bridge or roadway that helped the structure. The existing structures have withstood 100 and 500 year flood events with very little, if any, damage. Since the Farrow Drop will be in the middle of a field where there is no bridge or road, engineers made changes they are confident will protect the drop structure throughout its life. Chris Stoner said, “By installing a series of these drops, we hope to be able to come up with a stable channel bottom.”

An advantage of the design of this structure, if erosion downstream of the structure travels up to the drop, another section of the drop can be added to help further stabilize the channel. This process is much cheaper than other solutions used in the past and it seems to work better as well. Kerry Robinson, who now works in the NRCS East Region National Technical Support Center, has seen hundreds of rock chutes used to stabilize streams across the nation but says the drop design with riffle pools is unique to Oklahoma.

Engineers started planning the drop in 2006. A year later flood waters from tropical storm Erin caused damage to the watershed. Stoner said the flood changed part of the original plan, “We had to do some more survey and modify the design somewhat. We added a grade stabilization structure into this drop structure because we had some erosion that was traveling, not up the stream channel, but into a man’s field.”

The plan sat on the shelf until funding was approved to begin the work. Remedial projects in the area that were funded by grants through Federal Emergency Management Agency (FEMA) and American Recovery and Reinvestment Act (ARRA) grants made it possible for the Farrow Drop to receive regular funding. This $2.2 million project is a partnership project between the NRCS, the landowners, the Oklahoma Conservation Commission, the North and South Caddo County Conservation Districts and various other partners. The structure will be maintained by the South Caddo County Conservation District.

The project was awarded to an Oklahoma City company. Work will begin January 17, 2011. The bid includes two structures: the rehabilitation of an existing concrete structure and placing the Farrow Drop. The company will have 238 days to complete both parts of the project.

By Public Affairs Specialist Crystal Young
NRCS January 2011

Last Modified: 04/21/2011

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