5 Breach performance measures

Culvert closure and bridge construction required a 404 permit from USACE and a corresponding state 401 Water Quality Certification with conditions from DWQ. The 401 Water Quality Certification included conditions to ensure protection of GSL water quality. These include requirements for monitoring causeway opening morphology, flow volume through the causeway, and salinity in the lake. Conditions also include performance measures for causeway opening morphology and salinity values in the lake to ensure that the new opening matches the water and salt exchange of the previous causeway culverts. Direct connectivity between Gilbert (south arm) and Gunnison Bays (north arm) via the new causeway bridge was restored in December 2016. Monitoring and evaluation of the bridge opening and water quality impacts commenced following the opening of the causeway bridge.

The intent of the causeway bridge and opening is to compensate for the impacts to GSL from the emergency closure of the previously active causeway culverts. Therefore, the performance standards target the duplication of conditions prior to culvert closure. The performance standards associated with the 401 Water Quality Certification and 404 Permit are laid out in a compensatory mitigation and monitoring plan (CMMP). The CMMP specifies conditions for both salinity in Gilbert Bay of GSL and the geometry of the causeway bridge and opening. As required in the 401 Water Quality Certification of this project, “UPRR will conduct the required monitoring until the results demonstrate that the Salinity Performance Standard Ranges are being met and trends indicate they will continue to be met into the future. The UPRR may request cessation of monitoring and adaptive management by submitting a Completion Report that includes no less than 5 years of monitoring results after the most recent causeway modification affecting water and salt transfer,” and, “6. Determination of compliance with the Causeway Opening Geometry Performance Standards will be made semi-annually for the first two years after bridge completion and then annually until cessation of monitoring is granted,” by DWQ.

Additional information regarding the permitting process and performance measures is available at deq.utah.gov/water-quality/railroad-causeway.

5.1 Geometry

Causeway geometry is evaluated twice per year for the first three years following the causeway breach, then annually. There are four geometry performance measures:

  1. Channel bottom width, measured under the bridge at the rail centerline
  2. Cross section area based on water surface elevation (WSE) of 4200 ft
  3. Average water depth at bridge based on WSE of 4200 ft
  4. Invert control berm width

Observed values for performance measures 1, 2, and 4 have been within 10 percent of as built conditions in all surveys to date (Figure 5.1). Water depth at the bridge has declined from 95 percent of as built (21 ft) to 86 percent of as built (19 ft) since the initial opening of the causeway breach (Figures 5.2 and 5.3). DWQ has agreed with a UPRR request to allow the review of 2019 measurements, and evaluate at that time, if there is a need to conduct any maintenance activities.

Figure 5.1: Causeway breach geometry performance measures. Geometry is considered within performance measures when within 10 percent of as built.

Figure 5.2: Change over time in average water depth under bridge. Average water depth is measured from a WSE of 4200 ft.

Figure 5.3: Change over time in average water depth under bridge (percent as built). Average water depth is measured from a WSE of 4200 ft.

5.2 Gilbert Bay salinity

Gilbert Bay salinity is currently evaluated four times per year. Minimum and maximum ranges were derived via a USGS causeway and salinity model. Three samples were taken during times when lake elevations were below the model calibration range (Figure 5.4). Salinity values are generally on the lower end of the performance range. If the lake level continues to fall outside of historic ranges (two consecutive quarters), UPRR will consult with DWQ and either (1) update and extend the 2012 UPRR/USGS Model or (2) with DWQ approval, use alternative methodology, such as extrapolation of historic data, to determine a salinity range at the new elevation, as agreed upon in the CMMP.

Figure 5.4: Causeway salinity performance measures. Data from UPRR monitoring reports.