Geotechnical Aspects of Shoreface Erosion and Causeway Deterioration at Graves Island Provincial Park, Nova Scotia P. W. Finck, P. Geo. Open File Report ME 2016-003 Halifax, Nova Scotia March 2016 N C H A N D M I E S B R A G E O S C I N Open File Report ME 2016-003 1 Geotechnical Aspects of Shoreface Erosion and Causeway Deterioration at Graves Island Provincial Park, Nova Scotia P. W. Finck, P. Geo. Introduction In 2010, the Geological Services Division (GSD) of the Nova Scotia Department of Natural Resources (NSDNR) received a request from Harold Carroll, Director of Parks and Recreation (NSDNR) to undertake a geological assessment of Graves Island Provincial Park near Chester, Nova Scotia. This was one of the first of many requests from Parks and Recreation for geological assessments of coastal parks. Discussions with other NSDNR staff, including Sandra Johnstone (Regional Geologist, Western Region), Pat Murphy (Area Supervisor, Western Region) and Alan White (Forestry Technician, Western Region), identified specific problems related to shoreface erosion and deterioration of the causeway at Graves Island Provincial Park. It was determined that I would examine these issues from a geological and geotechnical perspective to identify issues or causes and to recommend possible solutions. Graves Island was visited on April 29 and June 8, 2011. The northwest end of Graves Island and the adjoining causeway were examined in detail (Fig. 1) as this is the area where erosion is the main concern to the Parks and Recreation Division. Significant coastal erosion was observed, and a variety of historical shoreface stabilization methods were examined. In addition, the external, exposed parts of the causeway leading from the mainland to the park were examined to identify any geotechnical issues that are exacerbating the causeway deterioration. The southwest-facing side and the southern tip of the island were also briefly examined by foot and by boat. The results of the site visits and recommendations aimed at reducing shoreface erosion and causeway deterioration are discussed in the following sections in this report. Though this report is specific to an individual site and was originally written in 2011, the Geological Services Division decided to publish it as an Open File Report since the information is relevant to other coastal environments and infrastructure along Nova Scotia’s coast. The report also meets one of the NSDNR’s Strategic Plan objectives of educating and informing the public, private sector and government on issues relating to sea-level rise, erosion and coastal infrastructure sustainability. Readers should note that it is five years since the field work was conducted at Graves Island. As such, shoreline and causeway conditions may have changed significantly since this report was originally written. General Setting of Graves Island Provincial Park Graves Island is located approximately 3 km east of the village of Chester at the northeast end of Mahone Bay (Fig. 1). The island is a typical drumlin, as are many of the small islands in the inner part of Mahone Bay. A sandy, clast-poor till is exposed in the eroded banks east of the causeway along the north shore of the island. Drumlins in Mahone Bay typically have an internal layer-cake structure; individual layers represent till deposited from glaciers of varying ages and different flow directions. Excavation would likely reveal reddish-brown, silt-rich till at depth. At the base of the drumlin, it is probable that a greyish, compact silt-till and/or weathered bedrock would be found. Bedrock does not crop out on the Open File Report ME 2016-003 2 Figure 1. Location map of study area. Red squares and corresponding numbers show locations of figures referenced in this report. island; however, O’Brian et al. (1985) indicate that the bedrock is likely limestone, gypsum or conglomerate, all of Carboniferous age. Graves Island is sheltered by the Aspotogan Peninsula immediately to the east, by Big Tancook Island and other smaller islands to the south, and by elevated land in the Chester area immediately to the west. The island is considered by the author to be well sheltered from open-ocean swells and large open-ocean waves, as well as being sheltered from tropical cyclones. The north-facing and northeast-facing sides of the island and causeway are affected in a minor way by refracted waves created by winds from the northeast, east and southeast. The west-facing and southwest-facing sides of the island and causeway are also affected in a minor way by refracted waves created by winds from the west to southeast. In both instances, the size of the waves is greatly reduced by the shallow water depths that create frictional drag on the wave bases and also early breaking of the waves. Only the south-and south-east facing sides are exposed to larger waves generated within the inner part of Mahone Bay, that is that part of the bay north of the Tancook Islands. Open File Report ME 2016-003 3 Shoreline Erosion and Assessment of Previous Shoreline Stabilization Methods Readers of this report should note that the author has not observed the study area at high tide during storm conditions and with a significant storm surge. However, the author is very familiar with typical storm conditions along the coast elsewhere in Mahone Bay. In arriving at some conclusions and recommendations in this report, the author has relied on interpretations of what storm conditions would look like at Graves Island. For example, various Parks and Recreation employees noted that the southern part of the causeway is completely submerged under severe storm conditions. Such knowledge is invaluable. Finally, the readers should note that in undertaking this study the author did not examine erosion on the south end of the Park in detail, that is the southern exposed end of the drumlin. Information in this report should not be used for the purpose of erosion mitigation with respect to the southern end of the island without first consulting with the author. With respect to storm conditions, a very important observation is the presence of seaweed on the park lawn. The seaweed was transported several tens of metres inland from the shore edge by late fall and/or winter storms; however, inland transport was limited to the area where the elevation of the shoreface was less than approximately 1 m (Fig. 2, centre back). In addition, gravel and cobbles were not observed to have been transported inland in tandem with the seaweed. The presence of the seaweed, the lack of gravel and cobbles, and the height limitation indicates that small, low-energy waves were responsible for deposition of the seaweed. This is consistent with a sheltered coastline such as this one. The reader should note that these storm conditions do not represent a direct hurricane strike on the Figure 2. Shoreface erosion has ‘jumped’ the simple row of protective boulders. The red line shows the landward extent of seaweed and thus denotes the extent of recent flooding. Open File Report ME 2016-003 4 park at high tide with the eastern edge of the hurricane tracking up the western side of Mahone Bay; this would be a worst case scenario. The shoreline of Graves Island in the area of the causeway is typical of an eroding shoreline. The upper cliff edge is very steep and in places almost vertical. At the cliff-grass interface the sod hangs over the edge and lies as slumps at the base of the cliff (Figs. 2 and 3). The surface till has a silty sand matrix, which would suggest that it should readily slump. However, there is sufficient silt and clasts in the till to allow for formation of a sub-vertical erosion scarp. Past attempts at preventing erosion are evident from lines of boulders arranged sub-parallel to the present shoreface but located within the intertidal area of the beach (Fig. 2). It is noteworthy that there is minimal erosion along the north-, east- and southeast- facing side of the island, particularly along some of the highest areas, but also where trees and other vegetation are well established. The areas that are most susceptible to erosion seem to have elevations above highest high-water line (HHWL) in the order of 0.5–2 m. Erosion is limited along the low west side of the island (just above HHWL), but it is more vulnerable to flooding due to historical and future sea-level rise and land subsidence. It is important to keep in mind that even without a net rise in sea level, coasts will generally erode until they reach a point of being in equilibrium with incoming erosional wave action. Four main types of erosion control are apparent immediately to the west and east of the island end of the causeway. As mentioned above, the first type of erosion control is a failed boulder supratidal berm along the front of the shoreface (Fig. 2). The nature of the construction of the armouring cannot be determined; for example it may have been carefully placed stones with appropriate riprap back fill. However, it is my opinion that the armouring was simply dumped along the face of the eroding shoreline without any riprap and/or geotextile. Figure 3. Stone-filled wire bags prevent shoreface erosion. Open File Report ME 2016-003 5 The second type of erosion control is along a short length of shoreline east of the causeway. It is protected from erosion by stone-filled, rectangular-shaped, plastic-covered wire bags (Fig. 3). The more technical name for such bags are gabions. It is obvious that the stone-filled wire-bag system is successfully preventing erosion of the shoreface (Figs.