RAINBOW BAY FIELD REPORT

Transcription

1 RAINBOW BAY FIELD REPORT

2 Contents Page Abstract 1.0 Introduction Methodology 2.1 Limitations Statement of Findings. 3.1 History. 3.2 Data Collection Observations Management Strategies 4.0 Evaluation 4.1 Rock Groyne Sea wall Dune Vegetation. 5.0 Justification Bibliography.. Appendix

3 Abstract Coastal erosion is a widespread issue across Queensland s southern beaches, impacting social, economic, and environmental interests. Rainbow Bay, located at the southern end of Queensland, was observed and analysed for the use of manmade and natural processes to prevent and manage coastal erosion. This report will discuss the findings from a field excursion, evaluate the effectiveness of erosion management strategies, and propose alternative methods for maintaining a healthy beach profile.

4 1.0 Introduction The Tweed River Sand Bypassing Project [TRSBP] was launched in 1999 to overcome the interruption of longshore drift after the construction of training walls at the mouth of the Tweed River. The TRSBP is a sand transport system that nourishes the southern Gold Coast beaches using sand pumped from the updrift side of the Tweed River entrance. Other strategies are in place around the southern beaches to prevent further erosion and maintain safe beaches for people and the environment. As seen in Figure One (refer to Appendix 1), Rainbow Bay is one of the beaches directly affected by the TRSBP, and is largely unsheltered in comparison to beaches like Coolangatta, indicating coastal erosion is a significant issue.

5 2.0 Methodology During a field trip to Rainbow Bay on the 4 th of May 2017, observations were conducted to measure natural processes and landforms, management strategies, and coastal conditions. Observations were recorded as notes, tables, field sketches, and photographs on the same day. Measurement tools such as tapes and trundle wheels were used to measure beach widths and groyne lengths. 2.1 Limitations While every measure was taken to ensure validity and accuracy, some records from the day were inevitably flawed. For example, when measuring the width of the beach, human activity impeded the process and required shifting across the beach to take a final record. Also, estimations of wind direction were made without proper wind measuring devices, instead with compasses and eye observation. These strategies leave substantial room for human error, however, were necessary given the time and resource restrictions on the day.

6 3.0 Statement of Findings 3.1 History During his first visit to Australia, Captain Cook sailed past Rainbow Bay and named the adjacent headland Point Danger in reference to its apparent danger to ships navigating the area (Travel Promote Pty Ltd, 2017). The area came to be famous for its surfing conditions and natural beauty, bringing many locals and tourists to the bay for a range of leisure and family activities (refer to Appendix 2). After the construction of the Tweed River mouth training walls in 1891, the natural process of northern longshore drift around Snapper Rocks and onto Rainbow Bay was impeded, causing the beach to erode and shrink (REFERENCE). The construction of the TRSBP complimented longshore drift around the headland to return sand to the beach, which has caused seasonally dependent shorelines to develop. For example, when the wave direction is southerly, sand pumped from the TRSBP jetty causes Rainbow Bay to increase in width and sand builds up around Snapper Rocks. Alternatively, when wave direction is northerly, sand moves out of Rainbow Bay, creating a deeper bay and often a lagoon (No Author, 2017). 3.2 Data Collection 3.3 Observations Location: Rainbow Bay Date: Conditions Dry and windy Wind Direction SE Wave Direction SSE Tide High incoming Berm No evidence of berm Swash Zone 10m Living Fauna Seagulls, blue bottles, sand crabs Dune Vegetation Beach grass, pandanus trees, pigface Human Impact Foot traffic around beach entrance and on dunes

7 3.3 Management Strategies While at Rainbow Bay, three management strategies were observed. Rock Groyne The Rainbow Bay groyne is located on the northern end of the beach, and helps to manage erosion caused by foot traffic through the main beach entrance. It is constructed of large granite rocks and stands approximately 3 metres tall. The groyne was 100% visible. Refer to annotated image here (insert image in appendix) Sea Wall A rock wall was identified at the back of the beach, in line with the back dune on the northern side of the beach, near the main beach entrance. The wall was made of large granite rocks and cement, and was only 70% visible due to encroaching sand. Refer to annotated image here (insert image in appendix) Dune Vegetation Dune vegetation was identified on the back dunes of Rainbow Bay, which help to stabilise the dunes and protect against erosion. Spinifex beach grass and pandanus trees were present, covering approximately 50% of the dune surface. While vegetation was healthy in some areas, generally it was sparse and degraded by foot traffic and heavy winds. Refer to annotated image here (insert image in appendix)

8 4.0 Evaluation 4.1 Rock Groyne The rock groyne constructed at Rainbow Bay was under no threat of erosion and appears to maintain the damage caused by foot traffic through the main beach entrance. Although the groyne creates a risk for physical injury, adequate signage provides warning for visitors. Overall, the rock groyne is effective in maintaining a healthy beach profile at Rainbow Bay. Refer to annotated historical images and comparison here (insert image in appendix) 4.2 Sea Wall The sea wall is currently under threat of erosion due to strong winds and the process of sand pumping through the TRSBP. This indicates that the process may not be effective if severe weather conditions persist. However, as the shape of Rainbow Bay changes dramatically with different seasons, the sea wall would presumably be adequate under normal conditions. 4.3 Dune Vegetation The dune vegetation present in the back dunes of Rainbow Bay are sparse and currently under threat of erosion. This is due to the strong winds and sand pumping, however, could be managed more effectively with fencing.

9 5.0 Justification The management strategies currently employed at Rainbow Bay demonstrate some effectiveness in controlling coastal erosion. In order to improve this, fencing should be constructed along the back dunes to protect vegetation from human interruption and destruction from living fauna. The construction of mesh fences will encourage the deposition of sand and reduce trampling that causes erosion. Furthermore, vegetation growth will be encouraged, providing further stabilisation for the dune systems (Scottish Natural Heritage, 2017).

10 Bibliography Friends of Rainbow Bay. (2015). History of Rainbow Bay. com/index.php?option=com_content&view=article&id=20&itemid=366 No Author. (2017). Rainbow Bay Beach Behaviour. gov.au/articles-and-studies/interesting-items/rainbow-bay-beach-behaviour Scottish Natural Heritage. (2017). Dune Fencing. Travel Promote Pty Ltd. (2017). Rainbow Bay History. rainbowbay/history.html

11 Appendix Appendix One: Gold Coast southern beaches Appendix Two: Popular Rainbow Bay