The Seven Mile Beach spit is located approximately 20 kilometres east of central Hobart. The spit lies at the head of Frederick Henry Bay and also the entrance to Pitt Water (which is the mouth of the drowned Coal River Valley). To your north you will observe Pitt Water which has become semi-enclosed as a result of the growth of the mid-bay spit of Seven Mile Beach. The length of the entire spit and beach is greater than 10 km, with a width of 2.5 km at its widest point. The Hobart Airport lies adjacent to the west of the area. To reach the end of the 7 Mile Beach Spit from near the end of the airport runway would mean over a 10 Km return walk along the beach or along sand tracks. To complete this cache, we have taken you less than 1 Km away (across the mouth of the Coal River) giving you a good view of the end of the 7 Mile Beach area but with much easier “access”.
In geomorphology, spits are elongated depositional landforms that extend from the mainland into a body of water, such as a sea, lake or river mouth. They are highly dynamic features, often changing shape over short geological timeframes. Natural and man-made changes near to, and on, the spit can have significant effect on the change of shape and even the influence the disappearance of the spit.
Core Formation Processes
- Longshore Drift: This is the primary mechanism, where waves approach the beach at an angle, carrying material up the beach with the swash and returning it perpendicular to the shore with the backwash.
- Coastal Realignment: As longshore drift moves sediment, it encounters a bend in the coastline, a headland, or an estuary, causing the waves to lose energy and deposit the transported material.
- Sediment Accumulation: The continuous deposition of sediment at these "re-entrant" points allows the spit to gradually build up, growing outwards into the water over time.
- Wind and Waves. The prevailing wind direction influences the angle of the waves and thus the direction of longshore drift.
- Presence of an estuary. The deposition of sediment can be particularly significant where a river or estuary meets the sea, with the river's discharge providing a counteracting pressure that can stabilize the deposited material.
The Spit is an area of deposition, opposite you would expect to find an area of erosion.
The 7 Mile Beach Spit.
The formation of the 7 Mile Beach spit follows the natural functions indicated above. However, it has also been significantly influenced by human activities. The introduction of long rooted marram grass in the early 1900s (for the purposes of dune stabilisation) has had significant impacts on many components of the coastal dune ecosystem. The area has been classified as an extensive and significant Spit land system by Davies, (1988) as part of a resource classification survey of land systems of southeast Tasmania.
Pine plantations have been a feature on the peninsula since 1927. Until 2023 there were 600 hectares of existing and redundant pine plantations across the spit. These plantations added to the stability of the dunes across the spit. Sand mining on the Spit has long been considered a potential source for the region's concrete sand needs, but significant environmental and socio-economic issues have impeded its development and operation, preventing large-scale extraction from the main beach and dune areas.
From the mid-20th Century horse riding (social and professional), walking, bird watching, off-leash dog walking and orienteering (some of Australia's most complex maps) backed by significant work by Department of Parks and Wildlife Rangers have been common recreational functions in the dune and spit areas.
Since 2023 an 18 Hole "International Standard" Golf Course has been developed across a considerable portion of the Spit with the aim of increasing to two courses, clubhouse and accommodation facilities and ultimately - to paraphrase the marketing blurb - “an attractive experience for local, interstate and international golfing”.
To Log this cache please send answers to the following questions to the CO.
- Look to the south-west and describe the end of the spit at the mouth of the Coal River. How has this spit formed? With reasons, explain if the area you have described is stable or unstable. How would stability be improved?
- Looking at the GZ side of the river, below you and to the south, what is happening here and how is this shore different to the shore of the Spit? Give reasons for the differences you have shown.
- Looking to the north-west section of the spit you will see an area of almost cleared vegetation (this is the Golf Course development mentioned above). How do you believe that this development will influence the stability of the Spit into the future?
- Attach a photo which indicates the conditions on the day you visited. If a photo is not possible describe those conditions
If you wish to preserve the sequence of your logging you may log immediately, however, if appropriate answers are not received within 14 days logs will be deleted.