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EWP Protects Structures Following Granite County Flood

Very high water on Flint Creek in Granite County during flooed event on June 13th, 2017.A rain-on-snow natural disaster event occurred in Granite County on June 12th and 13th, 2017. Rainfall during the approximately 24-hour event recorded between 3.1 to 3.7 inches at various locations within the upper portion of the Flint Creek watershed. According to the National Oceanic and Atmospheric Administration (NOAA) publications, a comparable 100-year rainfall event records between 3.0 to 3.4 inches. With approximately two feet of snow still residing at the top of the watershed divide, peak flows of Flint Creek and Boulder Creek were recorded at values several times what the United States Geological Survey (USGS) determined the 100-year flowrate to be.

An initial rapid assessment of the area indicated a need for debris removal and protection of an irrigation diversion structure. With existing snowpack in the upper watershed, and the potential for more rain events, the Montana Natural Resource Conservation Service (NRCS) opted to request Emergency Watershed Protection (EWP) exigency status funding. The Montana NRCS conducted approximately eight preliminary site investigations for program eligibility within three days of the flooding event. Sites that qualified for EWP Exigency Funding met the following criteria:

  • Damage was incurred as a result of the June 2017 flood event.
  • The site was at risk for future damage/washout with an event as small as bankfull.
  • Infrastructure was still viable (ie., not destroyed).
  • Positive benefit to cost ratio existed.
  • Request for assistance had been submitted by a qualified local sponsor.

Debris build up along Flint Creek following flood event in June 2017.Repair and protective measures were implemented on two sites through EWP in partnership with the local sponsor, Granite County. Montana NRCS was responsible for providing reasonable solutions, design computations, drawings, specifications, and financial support for installation costs. The Montana NRCS reimbursed Granite County 75 percent of the construction cost up to an established maximum value. Granite County was responsible for a 25 percent match (at minimum), over-run costs, permits, land rights, easements, and construction contract administration.

Quick turn-around between the event and implementation was integral to a successful response. It was questionable whether the irrigation diversion structure could withstand another high flow event without being completely washed out. While the structure was still in place, sufficient irrigation water volume could not be diverted nor could the canal carry the needed capacity without repair. Timeliness was important as requests for 3,000 acres of irrigation water would be occurring within two to three weeks of this flood event. The debris jam was also considered exigent since the next bankfull type flow could dislodge debris pieces. Debris was of the size that getting hung up on the nearby, downstream, State Highway 1 bridge was likely.

All parties involved worked well together to accomplish the tight timeframes established for EWP Exigency Funding. The EWP construction work was completed within 14 days, between June 29, 2017 and July 12, 2017.

Flood event removed rock protection on bank, backfill material between wing walls, and rock diversion material at Allendale irrigation diversion structure. Rock and earthfill replaced around Allendale Ditch diversion to stabilize structure and function.
Pre-Condition – June 15th, 2017. Allendale Irrigation Diversion Structure. Flood event removed rock protection on bank, backfill material between wing walls, and rock diversion material. Post Condition – July 10th, 2017. Rock and earthfill replaced to stabilize irrigation diversion structure and function.
Flood event removed fill and rock riprap protecting Allendale Ditch canal bank and access to diversion structure. Earthfill and rock placed on Allendale Ditch to stabilize the canal bank and access to diversion structure.
Pre-Condition - June 15th, 2017. Allendale Irrigation Diversion site (looking downstream). Flood event removed fill and rock riprap protecting canal bank and access to structure. Post Condition – July 10th, 2017. Earthfill and rock placed to stabilize canal bank and access to structure.
Canal breached as a result of flood event. Access to diversion structure is impaired. Canal breach repaired to facilitate continued use of the irrigation system and access to the irrigation diversion structure.
Pre-Condition – June 15th, 2017. Canal breached as a result of flood event. Access to diversion structure is impaired. Post Condition – July 10th, 2017. Canal breach repaired to facilitate continued use of the irrigation system and access to the irrigation diversion structure.
Debris jam on Flint Creek has been removed and threat to bridge downstream is eliminated. Debris jam on Flint Creek formed as a result of runoff from the 100-year precipitation event on in June 2017.
Pre-Condition – Debris jam formed as a result of runoff from the 100-year precipitation event on June 12 and 13th, 2017. State Highway 1 bridge is located within ¼ mile downstream. Debris from this jam will easily get hung up on the bridge and threaten its stability if dislodged. Post Condition – Debris jam has been removed and threat to bridge is eliminated.