DangerousRiverHydrologyResourceReport.pdf

12010918R0003, Amendment 03, Attachment 2

Hydrology Resource Report – Dangerous River Boat Ramp

On August 13, 2014, I joined Danielle Snyder, Teresa Hunt, Nate Catterson, dan Lambson, and Travis Bradshaw on the Dangerous River to investigate the location and details for designing an improved boat ramp. The Dangerous River drains Harlequin Lake and is located on the Yakutat Ranger District. State highway 10, crosses the Dangerous River just downstream of Harlequin Lake (Figure 1). Immediately downstream of the highway bridge is the primitive boat ramp which is proposed for improvement.

Figure 1. Dangerous River and Harlequin Lake. Notice highway 10 road/bridge crossing downstream of the lake.

The Dangerous River is a wide, fast moving, glacial river. The waters are grey and turbid transporting ice bergs year around. The river extends over 400 feet across at the bridge, drains a 250 mi2 watershed, and is approximately 100 feet above sea level. The location of the bridge, and boat ramp, is river right, along a long river right bend (Figure 2). It was designed in 1976 for a 50 year flood at an approximate 31,000 cfs (Figure 3). The existing boat ramp, and the proposed boat ramp, are located immediately downstream of the bridge. This location is optimal for the boat ramp due to the backwater effect of the inside curve of the natural meander bend. There was no evidence of recent channel migration and minimal bank erosion in this location, suggesting a relatively stable location.

Figure 2. The bridge crossing and location of the boat ramp along the Dangerous River.

The current boat ramp is a short, steep, gravel ramp from a driveway/parking area adjacent to, and downstream of the bridge. Relative to the river, the boat ramp is located in a swift bank eddy due to the river right bend and the rip rap for the bridge abutment. There is almost no bank erosion in this location, there are no evident eddies from the bridge abutment (upstream along the stream) and the water appears to have minimal velocity. For example, a boat located in the water, downstream of the boat ramp, was tied to the streambank at the time of our reconnaissance and there was no streamflow interruption resulting in bank erosion. The boat was almost still in this location suggesting relatively low channel velocities.

Figure 3. AKDOT sketch of the 1976 bridge design.

The thalweg appeared to be on the channel right approximately 75-100’out into the river, from the streambank at the boat ramp. The width of the slack/backwater in the location of the boatramp appeared to be about 20-40’ wide. Dowsntream of the boat ramp, there were two locations where icebergs were high centered. These may be rocks in the river, or locations where the channel is shallower.

Recommendations

Improvements to the current boat ramp would best be suited in the same location as it currently exists due to relatively stable channel conditions and low flow velocities. It would need to be downstream of the bridge far enough to limit any influence on the bridge abutment. Ensuring the integrity of the bridge abutment is imperative.

The distance that the ramp extends into the stream depends upon the channel depths at this location. It is advised to acquire channel depths in order to determine how far the ramp would extend. Because the backwater extends approximately 20-40’ from the streambank, it would not be recommended extending more than 15-20 feet. Because dewatering may be of concern, the ramp may need to be backed up into the riparian area (like an inlet). In this case, the streambank and ramp areas would need to be re-enforced to ensure stability. A smaller ‘inlet’ would result in less chance of recruiting icebergs and other deposition such as sediment.

Photo 1. Dangerous River upstream of the bridge

Photo 2. Dangerous River downstream of the bridge. Note the location of the boat ramp.

Photo 3. Dangerous River looking downstream from the bridge.

Notice the boat tied to the streambank and the extensive streambank vegetation.