The April 1996 Rockland Landslide

Discussion of Possible Courses of Action

Many courses of action are possible for dealing with the landslide hazard of northern Rockland Harbor. Some of these possibilities are listed here with brief comments. A more extensive analysis of alternatives should be considered prior to taking any action. For example, some of these options may require further geologic or engineering study of specific sites to better characterize their landslide risk. In addition to geologic factors, these options must be considered in terms of cost of the action compared with the value of properties to be protected or remediated. And since the risk of catastrophic landslides depends on a combination of several contributing factors, a combination of options should be considered if engineering solutions are pursued to reduce risk.

(1) Do nothing. This option certainly costs the least up front, but may eventually have severe and costly consequences as properties are destroyed and lives are jeopardized. If Mrs. Smalley had not been evacuated from her home, she could have been killed. To take no action may seem a relatively safe gamble, since it has been more than 20 years since the last major landslide in the area. However, as with hurricanes, the recurrence time and location of such events in the area is unpredictable. Yet the similarity of the 1973 and 1996 events shows that the 1996 event was not a unique occurrence, and recurrence is to be expected (Plate 1).

(2) Move structures from the highest risk areas of the shore. This option would require finding new building sites and moving existing buildings, which may not be economically feasible in all cases. The benefits of this option are that properties and lives would be removed from the areas of highest risk.

(3) Reduce slope and rip-rap base of the bluff at high risk properties, with appropriate surface drainage. This is expensive but most likely less costly than option 2. A reduction in slope would reduce load and lateral stress on underlying materials, which is the fundamental cause of catastrophic landslides. Enough room would be required between the current slope top and existing buildings to adequately reduce the slope. Rip-rap, if properly installed and maintained, would reduce or eliminate wave erosion at the base of the bluff slope and maintain the shape of the slope. However, installation of it might actually lead to increased erosion at the toe of the rip-rap if placed in the intertidal zone, and also may be illegal under the Natural Resources Protection Act under some circumstances. Properties so remediated would still be subject to landslide risk, although probably reduced. Engineering studies would be necessary to determine the factor of safety for proposed designs.

If slope reduction is considered, various engineered designs may be available depending on the site. One simple example is the engineered waterfront slope behind the Littlefield Memorial Baptist Church, at the west end of Waldo Avenue, about 1500 feet west of the 1973 landslide site (Figure 13). Because there was enough space behind the buildings, it was possible to grade the bluff to a long, gradual slope. Rip-rap was placed at the base to reduce wave erosion, and vegetation on the slope reduces gullying and erosion by surface water runoff. The cost of this project in 1979 was about $25,000. Other, more sophisticated designs may be neccessary for structural stabilization of higher, steeper slopes with less upland behind existing buildings. Such construction has been done on the Pacific Coast and has resulted in esthetically pleasing, structurally sound slopes, but at significant cost.

(4) Ground-water management. A design of French drains and/or pumping wells could be developed to artificially lower the water table in the areas of highest risk. Such a system would be expensive and would require maintenance. Furthermore, if pursued as the sole option, its effectiveness in reducing risk is uncertain because the level of the water table is only one of the contributing factors to catastrophic landslides, and it may not be the most significant factor.

(5) Infrastructure improvements. Minor contributing factors to landslide risk could include improper road drainage and leakage of water supply and sewer lines. These systems could be improved, but their effectiveness at significantly reducing risk, while uncertain, is probably low. Also, it may be worthwhile to consider relocation of some of these critical services outside of the areas of highest risk.

(6) Discourage inappropriate development in high-risk areas. In this way the future cost of reparations or remediation could be avoided. The buildings destroyed by the 1996 landslide were well within current setback and zoning regulations. Better education or new policies could be considered to avert the construction of additional structures at risk.

(7) Prepare for the next catastrophic event. Local emergency personnel, including the city manager, police, fire, and public works personnel were available and responded well to the 1996 catastrophic landslide. However, when Maine Geological Survey geologists visited the site on April 16, few of the local people on the scene knew about the 1973 event that had occurred in just the next cove. It would seem prudent to develop emergency plans and response policies for destructive landslides in northern Rockland Harbor and in geologically similar areas. Most homeowner insurance policies do not cover landslide losses. And with a total pricetag approaching $1 million, the 1996 event has been expensive for the city of Rockland as well.

Contents Introduction Description Other slides Factors Potential Action Summary References Appendix A Appendix B Appendix C Plate 1

Last updated on October 6, 2005