The existing 72-inch diameter culvert creek crossing under Pit 5 Road has a documented history of becoming clogged with debris during periods of high-intensity, short-duration rainfall events, which are common in the area, typically with a frequency of 10 to 15 years. When this occurs, debris and water flow down the road causing severe erosion of the roadway shoulders and other improvements along the way, as it flows towards a powerhouse. The debris flow inundates the powerhouse, interferes with personnel’s access, and causes extensive damage to the equipment. An existing MSE wall on the downhill side of the crossing also shows signs of distress in the form of undermining and bulging, which may be related to the debris flow events. In addition to the loss of revenue while the powerhouse is out of operation, repairing damage and clean up are costly.
After a November 2012 event, the client, in conjunction with SAGE, performed some short- to mid-term repairs to the creek crossing to restore function and provide some additional protection measures until a permanent design could be evaluated. The repairs, completed in February 2013, included clearing the accumulated debris, removing the damaged portions of the culvert inlet, shaping and riprap armoring the inlet basin, and providing an overland flow bypass that directed water to a new basin and CMP arch culvert beneath the road.
During the preliminary engineering phase, SAGE performed a geotechnical/geologic investigation, a hydrologic and hydraulic analysis, and an alternatives analysis. The initial phase of the alternatives analysis evaluated five preliminary concept-level alternatives for creek crossing improvements, including: (1) do nothing; (2) replace the existing CMP culvert with a box or arch culvert; (3) replace the existing CMP culvert with a single bridge; (4) replace the existing CMP culvert with two bridges; and (5) construct a low water crossing downroad of the creek crossing to divert water and debris across the road and away from the powerhouse. As part of the alternatives analysis, SAGE established objectives and evaluation criteria, prepared opinions of probable construction cost, and performed a brief evaluation of each alternative.
The client and SAGE discussed the preliminary alternatives, site constraints, and key project requirements. Based on the discussions, the project team determined replacement of the existing CMP culvert with a buried arch structure would best meet the project constraints and objectives. Of particular interest was the ability to facilitate the complex site grading to maintain access to both the switchyards and powerhouse, as well as providing a supplemental parking area for powerhouse visitors. However, the final culvert material type, alignment and breadth of this option were still open for discussion and needed to be evaluated further in a detailed alternatives analysis.
Based on the results of our detailed alternatives analysis, the preferred solution for meeting the project objectives is to install a new, prefabricated multi-plate steel arch supported on concrete stem walls and a new MSE wall for outboard slope stabilization.
In 2017, concrete wing walls will be constructed at the inlet to direct water and debris flow materials through the culvert. The crossing and wall will accommodate AASHTO HL93 vehicle loading, restore the natural creek geometry and flow characteristics, and will allow flexibility in site grading for access to the powerhouse and switchyard, and for developing a parking area.