CRESCENT BEACH PROJECT
The purpose of our project is to establish a base for making community decisions on the long-term protection of the dunes and natural habitat, in the light of multi-use patterns at Crescent Beach.
This report is a summary of information generated together with Friends of Crescent Beach, Green Bay and Area between October 2001 and August 2002.
The Friends organization is focusing on protection of Crescent Beach. One approach is to view the beach dunes as mobile, variable, yet requiring assistance to rebuild between erosion events.
This report uses color coding: descriptions and conclusions are presented in black; answers to questions posed by Friends are shown in green; action items are blue.
2. GREEN BAY RESERVOIR AND ITS FLOWS OF SAND
2.1. The ultimate reservoir of sand for this beach is on the bottom of Green Bay. Available scientific evidence indicates this reservoir to be of limited extent. Sea- level rise can make this reservoir less accessible to the low energy waves which transport sand shoreward (Figure 2.1).
2.2. The rise in sea-level on the Atlantic coast of Nova Scotia, which has been recorded for decades at an average rate of 4 mm/yr, exhibited a reduced rate for the period 1975-1995. This may provide a partial explanation why Crescent Beach, though degraded, has survived since 1975 in spite of past scientific projections to the contrary.
2.3. The flow of sand from the Bay to the wet beach, en route to the dry beach and dunes, ultimately limits the potential for regeneration of the dunes. It seems appropriate now to revisit the information on the Green Bay sand reservoir and attempt to refine this assessment.
3. WET BEACH RESERVOIR AND ITS FLOWS OF SAND
3.1. The flow from Green Bay to the wet beach may be limited, as mentioned. Similarly, the flow from the wet beach to the dry beach, mostly by waves, may be limited.
3.2. The aerial photos show a change in curvature of the beach near the (west) end of the rock wall, indicating possible alongshore transport of sand by waves to the eastward. The question arises: Is sand being lost from Crescent Beach here?
3.3. Cars drive along the wet beach. This is seen by residents as having less adverse impact than if people parked along the road and climbed over the dunes to reach the beach. More signage explaining Friends’ initiatives would help to counter the impression created by the vehicle traffic that this beach is not valued, cared-for or protected.
4. DRY BEACH RESERVOIR AND ITS FLOWS OF SAND
4.1. Researchers report that the dry beach can be ‘clawed back’ to low levels even in July of some years. This is due to storms and storm waves, which may occur in any season. Therefore we would expect an irregular, event-driven variation in the height of the dry beach, rather than a predictable seasonal variation.
4.2. The height of the dry beach reservoir is being measured by Friends. The data to date are shown in Figure 4.1 and indicate levels in March, 2002, to be 30 to 45 centimetres (10 -18 inches) lower than the first measurements in October, 2001. However by July 30, 2002, levels in the east sector of the beach had practically recovered to October, 2001, values, while in the west sector of the beach levels remained near March, 2002, values. Since March, net build-up of sand has occurred in the eastern, but not in the western sector. Additional beach height measurement stations are planned for various features along the beach to be monitored at approximately monthly intervals. Measurements after storms will also be informative.
Figure 2.1 Reservoirs and flows of beach sand.
Figure 4.1 Distance to Sand Surface along Crescent Beach
5. DUNE RESERVOIR AND ITS FLOWS OF SAND
5.1. Even though dunes are expected to be partially sacrificed during severe storms, the ideal is that they regenerate during the period between storms, with reserves being built up and secured by deep-rooted vegetation.
5.2. Where sand supply is scarce, the dune reservoir will reflect this. Sea-level rise
raises the effective height of waves which attack the dunes, as well as rendering
sand in the Bay less accessible (§2).
5.3. With the cooperation of Dr. R. Taylor, Atlantic Geosciences Centre, Bedford Institute of Oceanography, Friends were able, on July 31, 2002, to re-survey beach profiles that researchers have surveyed repeatedly since the 1970’s, and most recently in May, 2000. The results for May 2000 and July 2002 are shown in Figures 5.1 to 5.5. Figures 5.1 to 5.3 are profiles for transects situated near the end of the rock wall in the east sector of the beach. Figures 5.4 and 5.5 are profiles for transects situated approximately 150 and 750 metres respectively along the beach from the west end. In response to the Friends question, Is the dune vertical profile changing or is it fairly stable, these July 2002 profiles indicate little deviation from the profiles of May, 2000.
5.4. The beach height measurements at the toe of the dunes and the ability to repeat the beach profiles annually enable us to track changes in the wet beach, dry beach and dunes. The information gained can be used to evaluate the longer-term feasibility of dune restoration work. The adequacy of the sand supply is critical here.
5.5. Have the cribwork and/or rocks stabilized the dunes? Located at the toe of the dune or where it once was, the cribwork is well placed to disorganize waves attacking the toe, and to reduce the wind speed somewhat so that wind-blown sand may settle.
5.6. Increasing deep-rooted vegetative cover on the dunes is the optimum strategy and observations made with Friends suggest that the cribwork can facilitate the establishment of vegetation. This happens especially in situations where the dune is close to the cribwork and is vegetated. In these cases, the dune can grow through and in front of the cribwork, and become vegetated there.
Where the dune is some distance, e.g., more than two metres, back from the cribwork, the cribwork seems less able to stimulate rapid dune-building. This appears to be a situation where conifers can be used to simulate the vegetated dune so that more wind-blown sand will settle.
Where there are armour rocks (aside from the rock wall), there is the question ‘Is it realistic to envision sand dunes built over the armour rock?’ Rocks comprise the defence in that vulnerable sector approximately three hundred metres west of the rock wall where the dune is severely degraded, wash-overs have occurred in the past and where wave transport of sand eastward may be strongest. If the sand supply was plentiful and the interval between severe storms was usually several years, then the answer to the question could be, ‘Yes’. There is a need to evaluate the sand supply and to establish the ‘return-period’ of waves capable of substantially dismantling the dunes.
The armour rock defence collects sand preferentially where the rocks are smaller – and negligibly where rocks are large. This suggests that conifers could be used to cover the rocks with simulated dune shapes and vegetated surfaces, if rejuvenating dunes over these rocks were judged to be a feasible project.
5.7. Some experiments are underway to capture sand, initially at the toe of the dunes, either at or in front of the cribwork or in front of armour rocks. Sand traps of conifers and washed-up seaweed were installed by Friends on July 30 and August 10, 2002. The intention is to create a structure close to the sand surface which retards but does not deflect the wind - leading to sand settling. Inspection on August 17th indicated that sand was accumulating at a rate of approximately 5 cm/week at the better performing sites.
5.8. Dr. R.B. Taylor made recommendations in a 1993 report, Recent Morphological Changes and Future Management of Crescent Beach, Lunenburg County, Nova Scotia in which he emphasized the vulnerability of Crescent Beach to natural forces due to the narrow width of the dune system, with lack of room to retreat, and the lack of new sand.
5.8.1. His recommendations for maintaining Crescent Beach in its present position for as long as possible centred around keeping sand loss to a minimum, improving the continuity of the dune system by keeping breaks through the duneline to a minimum, and keeping the dune crest height at an elevation which restricts large scale wave overwash.
5.8.2. Specifically Dr. Taylor recommended: that careful thought be given to access routes over the dunes. Walkways through dunes can become focal points for wave overwash and flooding, carrying sand to the backshore estuary where it is essentially lost to the beach system.
5.8.3. He recommended that the cribwork be maintained and possibly extended along the central part of the beach. The cribwork fence has been successful in trapping sand and securing the dune line by allowing a free exchange of wind-blown sand between the foreshore and the dunes and causing snow-drifting on its seaward side which protects the fence and dunes against storm waves.
5.8.4. He recommended that since vegetation is required to stabilize accumulated sand, if this does not happen naturally, it must be introduced through transplanting. Seaweed and other flotsam should be allowed to accumulate on the seaward side of the cribwork to trap sand and promote vegetation.
5.8.5. He finally recommended that the large quantities of sand blown onto the highway should be collected and distributed along the intertidal beach slope before it is lost to the beach system.
5.9. Combining information from past initiatives and researchers’ reports with updated information recently generated, two thrusts are suggested: the first of these addresses Taylor’s first, third and fourth recommendations
5.9.1. Minimize losses of sand from the dunes by capturing sand through use of conifers and transplanted beach grass, as well as repairing the cribwork.
5.9.2. Assess the supply of sand, and the return-period of waves which will dismantle the dunes.
Figure 5.1 Line 38, over the rock wall.
Figure 5.2. Line 38A, a few tens of metres west of Line 38.
Figure 5.3. Line 38B, a few tens of metres west of Line 38A.
Figure 5.4. Line M, ~750 metres from the west end of the beach. This diagram also shows,
on the right-hand scale, the volume of sand in a 1-metre wide swath from the wet beach
over the dunes to the road, some 70 cubic metres.
Figure 5.5 Line 3, ~150 metres from the west end of the beach.