G E O S C I E N C E A N D M I N E S B R A N C H Provincial Park,NovaScotia and CausewayDeteriorationatGravesIsland Geotechnical Aspects ofShorefaceErosion Open FileReportME2016-003 P. W.Finck,Geo. March 2016 Scotia Halifax, Nova

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Geotechnical Aspects of Shoreface Erosion and Causeway Deterioration at Graves Island Provincial Park,

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 (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

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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 immediately to the east, by 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.

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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.

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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. 3 and 4). It is not known if salt-tolerant bushes were planted as part of the shoreface stabilization or if they were already present, but they appear to be an important part of the successful protection of that length of shoreline. It is clear that this method of ‘armouring’ is successful in this type of low-energy environment. However, it is equally clear that the integrity of the stone-filled bags is presently compromised (Fig. 4). The bags are falling apart along the lines of closure (i.e. in the areas of the bags that are open during the filling process and later closed with a system of wire ties). Failure of the bags will be followed by shoreface erosion. Given that the stone- filled bags are successfully preventing coastal erosion here, then they are a candidate for more extensive use elsewhere on Graves Island.

It should be noted that similar wire bags are typically used as a retaining wall system, but without the plastic coating. In that application, the bags must be able to withstand the back pressure of the material being stabilized. At Graves Island, the force on the back of the bags is minimal. The force affecting the bags is on the front (waves hitting the front), on the top (waves crashing down on the bags), and a much smaller force on the back of the bags as water moves seaward as the waves recede. The issue is maintaining the bag integrity (Figs. 4 and 5). This really isn’t a geological or geotechnical issue as any fisherman could easily point out both the problem and remedy. The wire bags are analogous to fishing nets with rectangular-shaped meshes. The closures on the bags are too few and depend on the total integrity of one strand of wire. Fishermen would never ‘hang’ their nets or sew pieces of net together in this manner. Failure of one wire at any single point allows the wire bag to fall open (Fig. 5.1). The bags need to be mended shut with an appropriate piece of plastic-coated wire. When completing each mesh, the bag should be drawn shut and tied with an appropriate over and under knot, the same used when joining two pieces of net together (Fig. 5.2). This should be carried out at all bag openings. In doing so the bag is securely closed, and the tie wire can fail at multiple points without compromising the structural integrity of the stone-filled bag. In addition, each separate bag should be mended to the next bag with double hitches, whether placed end to end, or when stacked. This would result is a far stronger retaining system because of its enhanced structural integrity.

The third type of erosion control is apparent immediately west of the island-end of the causeway (Fig. 6). In this case, a rounded, sloping berm has been constructed using armour stone. The author’s observations are limited to the outside of the berm. It is referred to as a berm rather than a wall or revetment because of its very low height, extremely low slope and its shape, which closely mimics that of a natural berm found on beaches. Overall, without being able to see the interior of the berm, it appears to be well constructed from a geotechnical perspective. The size of stones used in the construction is appropriate for the very low energy conditions at that site. In addition, it is obvious that the stones were carefully placed so as to reduce the size of voids between the stones. However, there could be an issue if there is an unusually heavy, thick build-up of shore ice that, when it broke-up, dislodged some of the stones along the base and face of the berm.

Without being able to see inside the berm, it is unknown if appropriate riprap was placed under (inside) the berm. In addition, there was no evidence of the presence of a geotextile. When the site was visited in early and late spring, there was no evidence of any vegetation planting on the landward side of the berm, though there was some natural remnant growth at the western end (Figs. 6 and 7). In this type of structure, placing soil between the stones and planting bushes in the voids is a common practice. The presence of appropriate riprap within the berm and use of a geotextile theoretically allows the soil to be stable enough to allow bushes to become established. Carrying the bushes landward of the berm helps to prevent washout between the back of the berm and the shore (Fig 4). Some shredding of the geotextile

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Figure 4. Stone-filled wire-bag retaining system along with bushes are preventing erosion. However, the bags are failing due to inadequate closures.

Figure 5. Mending or knitting shut the openings in the gabions forms a longer lasting and more secure closure because the joining wire can fail at multiple points without loss of structural integrity of the gabion.

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Figure 6. Significant effort and expense expended in shoreface armouring immediately west of the causeway on Graves Island proper. buried in the soil behind the wall (landward of the red arrows in Fig. 7) will allow the roots of bushes to tangle more easily with the geotextile. My concern in this case is the commencement of washout as described above and shown in Figure 7. This washout, if it progresses or isn’t controlled, could easily result in failure of the structure. When I say failure of the structure, I am not referring to its structural failure, but rather erosion simply bypassing or ‘jumping over’ the structure and continuing landward of the berm (e.g. Fig. 2).

Armouring can be effective in preventing shoreface erosion without the use of significant amounts of backing riprap or use of a geotextile. However, this is dependent on the nature of the eroding bank material, for example when the backing sediment is rich in boulders, cobbles and clasts. This is typical of some locally derived granite tills along the south shore of Nova Scotia. Generally speaking, when an armouring method is selected it is important to consider the geotechnical characteristics of the material that is being protected from erosion.

The fourth type of erosion control was noted during my survey of the eastern side of the island, specifically an area where coastal armouring was attempted using boulders to create a low abutment (Fig. 8). In this case, use of a geotextile (landscape cloth?) on the landward side of the armouring was apparent. The methods used at this site have several flaws. First, the stones in the wall should have been bigger and better placed to reduce gaps between the individual boulders. This could have been mitigated by the use of a suitable riprap backfill. The main problem with the stone wall is that it is not high enough to prevent overtopping (Fig. 8). In instances where an armour wall is overtopped, preventing erosion behind the wall will always be problematic. The improper use of the geotextile is also a significant problem compounded by the use of a clear (i.e. stone of all one size), small-sized, crushed stone

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Figure 7. Initial shoreface erosion due to overtopping and wave wash-through is occurring on the landward side of the armour stone wall. Vegetation on the landward side of the armour stone wall is also locally eroded. aggregate. It can be clearly seen (Fig. 8) where the geotextile is washed up into a roll within the seaweed. The purpose of a geotextile is to prevent wash-out of fine material through coarse riprap and fronting boulders. It is no longer effective for that purpose at this site. It is not apparent if the geotextile originally covered the top of the wall and in turn was covered by the crushed stone. Alternatively the geotextile may have been buried between the riprap and the face of the eroding soil. In either case the initial installation was inappropriate and faulty.

Where sediment being eroded from a shoreface is the source (or only source) of sediment to a beach, armouring may result in deflation of the beach. Armouring may also cause deflation of the shoreface immediately seaward of the structure. Since there are no beaches in the immediate vicinity of Graves Island, armouring isn’t a concern. Likewise, the intertidal zone is generally composed of coarse sand, pebbles, cobbles and boulders. Due the coarse nature of material in the intertidal zone armouring is unlikely to cause significant deflation.

Geotechnical Issues Related to Deterioration of the Causeway

The causeway at Graves Island is in an obvious state of disrepair. The ends of the causeway proper and the transition between the causeway and the land on both ends of the causeway are in need of significant

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Figure 8. Improper use of a geotextile (landscape fabric?) in the construction of a shoreface armoured abutment. repair. The upper parts of the stone walls on either side of the causeway are deteriorating. In addition, repairs to the stone walls are inadequate, and asphalt has been used to attempt repairs where I would suggest that the physical properties of asphalt are unsuitable for the application. I will discuss these various observations below. The reader should note that I do not know what the interior of the causeway is composed of.

Distinct coarse-grained, megacrystic granite was used to construct the base and lower walls of the causeway. It would appear that this stone represents the remaining part of the original causeway. This is evident by the quality of the construction (Fig. 9), compared to the rest of the stonework in the causeway. Most of the boulders exhibit flat faces and several sides that are clearly the result of splitting. I didn’t examine the boulders to determine whether the material is split fieldstone boulders or actual quarry stone. The lack of openings (gaps) between the boulders prevents waves from washing smaller stones and other material out from behind the boulders in the main wall. This maintained the walls integrity and strength over an extended period of time and also assisted in maintaining the integrity of the causeway surface. Where the original wall still exists today, it is very stable and is unlikely to fail in the foreseeable future. It is clear that the builders were skilled in stone placement.

The middle to upper part of the walls were built (likely rebuilt multiple times) by the addition of boulders with diverse shapes (Figs. 9 and 10). There are many different types of stone in the middle and upper parts of the walls. Thus the texture and colour of the stones are different even to untrained individuals. There are likely several different generations of repair represented in the construction of the upper parts of the walls. Large gaps are obvious between boulders in the upper, rebuilt parts of the causeway walls (Fig. 11). The large gaps allow waves to penetrate behind the fronting stones and erode the road base beneath the asphalt. This causes a loss of support, and the asphalt cracks and slumps into

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Figure 9. A well constructed wall along base and mid-level walls of causeway. New or repaired wall is shown above the older wall. The separation between ‘new’ and ‘old’ is only approximate.

Figure 10. Reconstructed causeway walls are typified by the use of rounded, often pinkish-buff-coloured boulders with many obvious gaps.

Open File Report ME 2016-003 11 the underlying voids. (Fig. 12). The asphalt has been used to fill some of the voids near the road surface (Fig. 11). However, asphalt does not have the appropriate physical properties for this application since it is eroded by wave action, cracks easily due to frost heave and has a very low flexural strength. Further examination of the use of asphalt versus a salt water resistant concrete is an issue that should be looked at by an appropriately qualified individual. Figure 13 shows an area of the causeway facing west on the landward side. What is notable is the lack of an upper row (or more than one row) of wall stones. The road is actually narrower at this point because of the missing walling. Asphalt is built up in the area to compensate for the lack of stone wall, but someone with specific expertise in asphalt will likely confirm that asphalt does not have the necessary strength to support weight without the confining pressure/lateral support of a stone wall. This is one reason why asphalt cracks along the edges of a road next to the shoulder. This process is obvious in Figure 12. The east-facing side of the island end of the causeway shows exactly the same situation (Fig. 14).

Safety Guard Rails

At the highest point of the causeway where it spans open water, heavy timbers and metal guard rails are installed (Fig. 9). Elsewhere on the causeway the safety rails are simple, round (hollow or solid?) metal posts interconnected by one length of wire (Figs. 13 and 14). At high tide there is sufficient water depth to drown the occupants of a car should a motor vehicle inadvertently drive off the side of the causeway. At low tide there is also probably sufficient depth in the centre of the causeway. The individual metal posts are inserted into holes drilled into rocks that form the top course of the walls. Though the posts are securely fastened to the boulders, the boulders are too small, in my opinion, to resist being simply pushed off the edge of the wall. It is also clear that the posts would simply bend over and offer little resistance to a vehicle that hit them with any significant force. If the causeway is part of a public road then there may be guidelines or specifications for guard rails and it is necessary that the present rails meet these specifications. If the causeway is not a public road then the present railing is unsafe and does not pass the test for due diligence for public safety and must meet safe construction guidelines. In either case, the organization responsible for the causeway needs to be made aware of this safety concern.

The Impact of Present and Future Sea-level Rise on the Causeway

The geotechnical implications of rising sea level should be considered when examining issues surrounding coastal erosion and associated infrastructure and property.

During the 1800s and early 1900s sea level in Mahone Bay rose at a linearly extrapolated rate of approximately 16 cm/century (Fig. 15). This was approximately equal to the present crustal subsidence rate of 15.7 cm/century measured at Halifax (Forbes, et al., 2009). This suggests that the locally observed rise in sea level in the Mahone Bay area during the 1800s (based on dating proxy submerged tree stumps and roots) and early 1900s (proxy and direct tide-gauge measurements) was caused by crustal subsidence, not an increase in the volume of the local ocean near the coast. Readers should note that this statement does not preclude significant acceleration and deceleration of the rate of sea-level rise in Nova Scotia on decade to century time scales during the Holocene, that is the last 10,000 years.

However, during the early 20th century, and within the resolution of the data, the long-term rate of sea-level rise increased. Within the interval 1900 to 1920 there is a clear change in slope in the trend of the long-term data, corresponding to an increase (acceleration) in the rate of sea-level rise equal to an additional 16 cm/century of sea-level rise compared to the 1800s. An acceleration in the rate of sea-level

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Figure 11. The picture illustrates the improper placement of stones and asphalt being used as ‘gap filler’ in an off-specification manner.

Figure 12. The lack of vertical and lateral support of the causeway along its edges is apparent in this photograph.

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Figure 13. Asphalt is placed without the presence of a laterally reinforcing stone wall.

Figure 14. Wave action is washing away the wall of the causeway.

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Figure 15. This figure is re-drafted from Gehrels et al. (2005). It is a sea-level reconstruction for the years from 1800 to 2000 (red circles are based on foraminifera) compared with the tide-gauge record of Halifax (green circles are annual mean sea level). rise around the end of the 1800s and the early 1900s is consistent with other studies and authors, for example Gehrels, et al., 2005. Thus, sea-level rise based on a linearly extrapolated rate over approximately 90 years is approximately 32 cm/100 years for most of the 20th century and early 21st century. There has been no further local acceleration in the rate of sea-level rise during the latter part of the 20th century or during the 21st century when examined on multi-decadal time scales.

There are three different methods of projecting the future rate of global sea-level rise; deterministic modelling, semi-empirical models, and empirical prediction. The first method, deterministic modelling, uses global circulation models that make projections of future temperature rise based on greenhouse gas emissions. The output of these models is then used to model the response of the oceans and the cryosphere, and hence changes in sea level. This method is used by the Intergovernmental Panel on Climate Change (see Gregory, 2013). Based on the output of these models plus the incorporation of local factors such as crustal subsidence, sea level in Nova Scotia in 2100 (based on the author’s calculations) is projected to be approximately 0.7 to 1.1 m above the level observed around the year 2000, an intermediate rise when compared to results from the other methods.

The second method, semi-empirical modelling, uses a simple relationship linking global sea-level variations, on time scales of decades to centuries, to global mean temperature (Vermeer, 2009). The relationship is then used to construct models that generate projections of future global sea-level rise. These are based on the assumption that CO2, global temperature and sea level are directly linked in a predictable manner. For future global temperature scenarios of the Intergovernmental Panel on Climate Change's Fourth Assessment Report, the relationship projects a (global) sea-level rise ranging from 75 to 190 cm for the period 1990–2100. Incorporating approximately 16 cm of crustal subsidence in Nova Scotia over the period 2000 – 2100, based on the author’s calculations, this yields an approximate sea-level rise of 0.9 to 2.1 m by 2100, the largest amount of sea-level rise of the three different methods.

The third method, empirical modelling, is based on the use of observed tide-gauge measurements to

Open File Report ME 2016-003 15 project rates and thus amounts of local sea-level rise in the future (Morner, 2015). This method is predicated on studies showing sea-level rise and fall over the last 3000 to 4000 years in the order of up to 1.5 m. These are coupled with paleo-rates of sea-level rise to apply physical frames to what is possible and what is outside the frames of reality. Long-term tide gauge records exist in Nova Scotia for North Sydney, Yarmouth and Halifax (Fisheries and Oceans Canada, 2014). For the Halifax tide gauge, rates of sea-level rise vary on a multi-year, decadal and multi-decadal basis. The multi-decadal rate of sea-level rise is approximately 32 cm/century in the Halifax area, including the component of crustal subsidence. Using empirical modelling, the rate of sea-level rise would be projected to be in the range of the long term rate of 32 cm/century. Beyond a twenty year time frame, uncertainty increases so that a longer projection is unwarranted.

Regardless of which method is used, the next several decades will see little change in the rate of coastal erosion and sea-level rise in the area of Graves Island. This is because regardless of the method (excluding the empirical modelling results) the majority of the increase will occur in the latter half of the 21st century, and using the empirical results, sea-level rise will continue at a rate similar to today, that is business as usual.

Coastal erosion, associated sea-level-rise and flooding will impact infrastructure differently, depending on the types of infrastructure under consideration. Residential infrastructure, compared to large commercial or publically owned infrastructure, has much shorter planning horizons, is not constructed to last a century, and if necessary can be moved due to land erosion or sea-level rise. Public and possibly commercial infrastructure that may be built to last for a century must accommodate the long-term effects (e.g. roads, public buildings, or possibly sewage treatment plants with modifications). Planning to accommodate coastal erosion and future long-term sea-level rise will depend on which model is used. In the author’s opinion, repairs to the causeway are unlikely to be undertaken with an anticipated design life in the plus 40 year range. Thus a design specification proportional to a sea-level rise of 32 cm/century over the next several decades should be considered.

The southern end of the causeway is presently submerged during periods of storm activity and high, high tides. This is consistent with the landward extent of the seaweed noted in Figure 2 and also the erosion and seaweed position shown in Figure 14. Thus if there is a desire to prevent overtopping of the causeway now and in the future, then additional height needs to be added above the allowance for future sea-level rise on a time scale of several decades.

Conclusions and Recommendations

The following conclusions and recommendations are made based on an evaluation of the field observations. Discussion of each point is covered in the report and the author is available for further discussion or assistance.

Conclusions

The overall energy conditions at Graves Island, relative to more exposed areas along the Atlantic coast, are low due to its location at the head of Mahone Bay and the presence of sheltering land to the west, north and south, and the presence of the several islands to the south. Due to the islands sheltered location and based on qualitative observations, the magnitude of shoreline erosion is not atypical for sheltered areas along the south-central part of Nova Scotia’s coast. Since there is little man-made erosion protection along the east side of Graves Island proximal to the causeway and the till is highly erodible, despite low-energy levels the shoreface exhibits signs of significant recent and longer term erosion.

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The use of stone-filled wire bags (gabions) on the east side of the island is effective in halting erosion. However, due to improper installation they are degrading and unless repaired, they will progressively fail over the next several years. Stone armouring is likewise effective in reducing or halting erosion. Due to the low energy of the system small-scale armouring will suffice; however, it needs to be properly designed and constructed (Fig. 16). This means that the cost of proper armouring should be relatively low. Armouring, and thus reduced sediment supply to the coastal system, will have negligible effects on the surrounding environment since there are no sandy beaches in the immediate vicinity of the park.

The walls of the causeway and the top sides of the asphalt road leading to Graves Island are in significant states of disrepair. Present materials and methods used to repair and maintain the causeway are inappropriate and ineffective. In particular, repairs to the sides of the causeway use stone that is too small and poorly placed. In addition, asphalt has and is being used in off-specification applications, for example it is being used in structural applications rather than as a road surfacing. Common materials placed in an appropriate manner would be sufficient to repair the causeway so that it is durable, would require significantly reduced amounts of maintenance and in the long term reduce costs (Fig. 17).

Recommendations

 The two methods of shore face stabilization described in the report (stone-filled wire bags and/or standard armour stone walling) are sufficient to eliminate or control the shoreface erosion given that modifications to the methods recommended in the body of this report are adopted.  Short, intermediate, and long-term life costing for each method may be relevant given budget restrictions.

Figure 16. An illustration of a simple, generic, shoreface armour stone protection system (modified from Young, 2002).

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Figure 17. An example of armouring of the edge of a private driveway along the shore of Mahone Bay. Contrast this with Figures 12 and 13.

 It is not recommend that minor cosmetic repairs be made to the top sides and edges of the causeway as it will be subject to rapid failure.  Do not use asphalt for applications to which it is not suited. This is not just a durability issue but might also be a potential liability issue in the case of a failure of the road edges or surface due to off-specification use of asphalt as a cementaceous or structural product rather than as a road surfacing product.  Investigate the use of a proper sulphate-resistant concrete (particularly above the HHWL) in combination with proper walling to prevent washout under the roadbed. However, proper walling with appropriate behind wall fill can make this unnecessary.  Stabilize the upper edge of the stone walls along the sides of the causeway.  A solid seal between the walls of the causeway and the road surface is not desirable. During periods of overtopping and wave surging hydraulic jacking of the road surface or blow-out of material may be an issue.

References

Fisheries and Oceans Canada 2014: Canadian Station Inventory and Data Download; http://www.isdm- gdsi.gc.ca/isdm-gdsi/twl-mne/inventory-inventaire/index-eng.htm.

Forbes, D. L., Manson, G. K., Charles, J., Thompson, R. B. and Taylor, R. B. 2009: Halifax harbour extreme water levels in the context of climate change: scenarios for a 100- year planning horizon; Geological Survey of Canada, Open File 6346, 22 p.

Gehrels, W. R., Kirby, J. R., Prokoph, A., Newnham, R. M., Achterberg, E. P., Evans, H., Black, S. and Scott, D. B. 2005: Onset of rapid sea-level rise in the western Atlantic Ocean; Quaternary Science Reviews, v. 24, p. 2083–2100.

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Gregory, J. 2013: Sea-level change, chapter 13; in The Physical Science Basis, Working Group 1 Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, eds. T. Stocker, D. Qin, G. Plattner, M. Tignor, S. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. Midgley; Cambridge University Press, Cambridge and New York, p.1137–1216.

Morner, N. 2015: Sea-level changes as recorded in nature itself; International Journal of Engineering Research and Applications, v. 5, issue 1 (part 5), p. 1–6.

O’Brien, B., Barrette, P. D., Kenney, D. A., Gouthro, G. F., Palmer, S. E. 1985: Geological map of the Mahone Bay Area, Nova Scotia; Geological Survey of Canada, Open File 1373, 4 maps, 1:25 000.

Vermer, M. and Rahmstorf, S. 2009: Global sea level linked to global temperature; Proceedings of the National Academy of Science, v. 106, no. 51, p. 21527–21532.

Young, K. 2002: Handbook of Coastal and Ocean Engineering; 8 US Army Corps of Engineers, Engineering Manual 1110, 1163 p.