Editor's note: The following is the second in a series of articles about the problems the city of Kenai faces as the bluff overlooking the Kenai River erodes at a pace of 3 feet per year.
The erosion of the Kenai bluff is a process that began before the city was established 200 years ago and will continue for generations to come.
The city would like to install a sea wall along a one-mile portion of the bluff between Pacific Star Seafoods on Bridge Access Road and the mouth of the river below Eric Hansen Scout Park to stop the process of erosion in that area. If it does, that area may be saved from eventually falling into the river. But the miles of bluff beyond the river will continue to erode as they have for years, due to the makeup of the bluff and the environmental conditions it is under.
The Kenai bluff is 55- to 70-feet high and consists of sand and a layer of hard silt and clay about halfway up the face of the bluff. Several factors combine to erode the bank in different ways.
Wind mainly affects the upper portion of the bluff and is not the most significant contributor to erosion. Gusts of wind from Cook Inlet hit the face of the bluff and blow sand away. A fine cloud of sand can be seen swirling off the top of the bluff in windy conditions.
Water erosion also mainly affects the upper portion of the bluff. Rainwater striking the face of the bluff can loosen and wash away material in streams. Rainwater also will gather and run off the top of the bluff, which erodes the edge of the bluff and the face as the streams run down it to the shore. Groundwater seeping out of the bluff will take sand and clay with it as well. Water erosion is more damaging to the bluff than wind.
"Rain, runoff and groundwater coming out of the bluff will cause surface erosion very fast when it runs off the top," said Keith Kornelis, public works manager for the city of Kenai.
An engineering firm hired by the city of Kenai to study bluff erosion along the mouth of the river found the most significant factor contributing to the erosion in that area is wave action against the foot of the bluff. The firm discovered that, while ground and surface water runoff can damage the upper portion of the bluff, far more substantial damage happens at the bottom. Waves crashing into the base, tide-induced currents and the steady seaward current from the river collaborate to pull material off the foot of the bluff and carry it downstream.
The waves and currents work together to undercut portions of the bluff. Eventually, the base is undercut to a point where it can't support the sand and clay above it.
"Our biggest problem comes when we have high tides and a storm," Kornelis said. "Waves crashing into the bluff undercut the bluff, and the bluff won't hold so it erodes down. It makes it steeper, then it just sloughs down. (Wind, water and wave erosion) are much more damaging when we have a steep slough."
The wave- and current-induced erosion phenomenon is clearly visible along the Mission Avenue area. That part of the bluff sticks out a bit beyond the neighboring bluff so the waves crash up against it, causing severe erosion, Kornelis said.
Another form of erosion has nothing to do with natural causes. It comes from people climbing up and down the bluff. Every time someone walks or climbs on it, they do damage.
High fall tides combine with a windstorm to drive waves into the bluff below Old Town Kenai in September 1996.
Photo by M. Scott Moon
"Each step on that bluff, a few grains of sand fall off and they're never getting back up there," said Bob Peters, a Kenai resident who has lived on the Kenai bluff for 23 years.
"Every bit that they knock off is something that is never coming back. We can't control the rain washing it away or the wind blowing or the tide, but we can certainly control what we knock off ourselves."
There are several options in controlling bluff erosion. Generally speaking, piling tires, old cars or just about any material in front of the face of the bluff would be one way to slow its erosion. However, the engineering firm hired by the city looked at several options that were more environmentally friendly and aesthetically pleasing when coming up with its design for the sea wall and Kenai coastal trail project.
The key to stabilizing the Kenai River bluff is protecting the base from undercutting. Sheet piling, large metal pilings set in the bluff, is one option. In considering this option, the city decided it was too costly, too difficult to install in areas with steep slopes and not very attractive, Kornelis said. Another option is using armor rock, large boulders placed along the foot of the bluff to protect it from waves and currents.
Armor rock was used in the construction of the Anchorage Coastal Trail. It is the city's preferred choice and the one called for in its sea wall and coastal trail design. Although, as with all the options, there is no 100 percent guarantee it will work.
Along with armor rock, the city would like to decrease the slope of the bank so it's not as steep as it is now. Once the slope is reduced, the city hopes to vegetate the bluff face to help protect it from wind and water erosion. A drainage ditch is planned to be installed between the bluff and the proposed trail to catch the material that erodes away before the vegetation takes hold.
Although there has been argument about what is the most economic and environmentally friendly way to do so, most of the response the city has gotten to its proposed project has been in favor of doing something to protect the Kenai River bluff from erosion.
"They should do something," said Gary Foster, a Kenai resident who has lived on the bluff for 12 years. "This country is practically surrounded by water and people fight back everywhere, all over the U.S., so it's definitely something that needs to be done."
Peninsula Clarion ©2014. All Rights Reserved.