Our Beach

November 21, 2009

As reported in the Eastbourne Herald (Nov) the DBRA Committee recently held its first meeting since the AGM when a motion was passed to have a Special Meeting to discuss the beach issues & options.The date of the Special Meeting has yet to be decided but will be in the New Year, allowing residents to research & learn more about the changing Bay and offer their views. Below is some more information regarding official reports about the beach with links to download these, plus a summary of the 2 latest reports this year. We encourage concerned residents of the Bay to do their own research & discuss these issues.

A recently published book Castles in the Sand (Craig Potton Publishing) tells the story of the New Zealand coast–what it means to New Zealanders and what we need to do to care for it for future generations. It describes the natural processes that have sculpted the coast, the flora and fauna that inhabit it, and the ways it was utilised by early Maori and European settlers . . . Author Raewyn Peart, a senior policy analyst for the NZ Environmental Defence Society, sets out the social, political and economic factors that have fuelled the development of the coastline . . . . She concludes that it is not too late to change our management of the coast to ensure continued access for all New Zealanders, protection of our natural heritage, and responsible, sustainable development.

The DBRA welcomes all comments, to this Blog or directly to info@daysbay.org.nz or any of the DBRA Committee members.

Blair Treadwell has written a synopsis of the perceived problems, outlining 2 separate issues – Gravel Accretion & Sand Erosion –

There has been misunderstanding regarding the proposals of the Residents Committee in relation to the beach at Days Bay.

There are two quite separate problems,

1. Erosion of the beach and road verge.

2. The accretion of gravel.

Following repeated representation to the HCC, a seawall has been built to protect the road, with suitable planting between wall and road. No action has as yet been taken concerning the build up of gravel, despite frequent requests of the HCC.

However, as a result of widespread concern two scientific reports have been obtained, one from the department of geography of Victoria University, and one from Jim Dahm, Eco Nomos Ltd, Thames. Both reports were concerned primarily with the beach erosion. Neither tackled the problem of gravel accretion. The seawall north of the wharf has for the present, dealt with erosion, but the scientific opinion is that the benefit will probably be temporary. It is thought that the seawall is likely to be undermined and become redundant. Both reports advocate the re-establishment of dunes destroyed when the road was built, as the only long-term measure of erosion protection.

To achieve this, the road would have to be moved. It is this opinion which has lead to the Committee’s proposals concerning re-routing the road north of the wharf. Clearly there are various possible routes, which might or might not affect the Wellesley College driveway and / or the Pavilion. Some of the Committee, and presumably others, think that the Pavilion should be replaced anyway, but it could be altered.

Considerable work has been done on the reasons for gravel accretion on the beach. This is a sequel of massive rock falls in the Orongorongo area caused by the 1842 earthquake. A “pulse” of water-worn gravel has formed, comprising some 200,000 cubic metres, which has gradually crept around the southern coast and into the harbour. For many years it has been clearly apparent south of Burdan’s gate, and more recently has affected the southern end of Eastbourne, covering the sand and obstructing storm water drainage. It has now reached Days Bay. The movement of the pulse has been assessed with brick markers introduced at Pencarrow, now apparent in Days Bay. Considering the volume of the pulse, it is conjectured that it is likely to cover the whole beach and fill the Bay to the end of the wharf.

Suggestions advanced by committee members to relieve gravel accretion include removing the beach gravel annually, depositing sand over it (expensive and likely to wash seawards) and building a groyne.

It should be clear from the foregoing, why the Committee has been concerned. Indeed all residents of Days Bay should be concerned, as well as residents of Eastbourne as a whole, and the thousands from elsewhere who regularly use the Bay. As the Committee has stressed to the HCC, Days Bay is the premier beach in the harbour area, and a major recreation facility for the greater Wellington region.

I hope these comments will clarify the reasons behind the Committee’s proposals. We are trying to save a major Wellington regional feature, especially for the residents of Days Bay. Constructive ideas and comments would be welcome.

Blair Treadwell, September 2009

The Reports

Two recent reports were published this year. A short summary of relevant parts of both reports follows. Hutt City contracted a coastal expert, Jim Dahm, Eco Nomos Ltd. of Thames, to consult & write a report pertaining to the Petone & Eastbourne beaches. The HCC Coastal Care group had various public meetings over the past 18 months, when Jim presented & discussed his work.  At the same time David Olson & Dr. David Kennedy of Victoria University’s  School of Geography & Earth Sciences wrote a report for HCC on the Estuarine Sediment Monitoring Programme for Days Bay. These full reports may be downloaded at http://www.southlight.co.nz/DBreports2009.zip

A summary of some relevant findings from these two reports follows:

Report Prepared for Hutt City Council, Estuarine Sediment Monitoring Programme for Days Bay, Wellington Harbour

Prepared by April 2009 by David Olson and Dr. David Kennedy, School of Geography, Environment & Earth Sciences, Victoria University

(extracted pages 28 & 29 of the 30 page report)

6. Conclusions and Recommendations

(1) Days Bay currently represents the northward limit of the littoral drift system that transports gravel along the entire Eastbourne coastline, from the harbour mouth. This gravel front has been migrating progressively northward since the 1855 earthquake and has now only just reached the Days Bay embayment.

(2) The recent occurrence of significant quantity of gravel clasts on Days Beach beach is most likely directly related to this littoral drift finally reaching Days Bay

(3) Gravels are transported primarily in the intertidal zone, while sand can move subtidally, down to the limit of wave action on the sea bed. This means that sand-sized sediment was able to bypass the rocky headlands and therefore accumulate in Days Bay, while gravel-size material had to infill the various embayment to the south before it could be transported into the next bay. This likely explains the initial presence of sand in Days Bay, being deposited before the gravels.

(4) Localised erosion of material from the slopes above Days Bay and the rocky outcrops in the intertidal zone are also a source of sediment. This source is likely to have been significantly reduced as a result of extensive development that has occurred in the area. This development has cut-off sediment to the beach.

(5) Problems with erosion on the beach, noted in the past decades, indicates the system is highly susceptible to interruptions in sediment supply. Any interference with the sediment system (such as removal of material) may tip it back into an erosional phase. The current lack of erosion at the southern end of the beach can be attributed its recent connection to the littoral supply of gravel material from the south.

(6) If the system is left in its natural state, it is highly probably that a gravel beach will prograde seaward, similar to what has occurred in Eastbourne in the next decade or so.

(7) Any interruption to the sediment supply, such as groin construction to block the gravels or removal of the gravels from the beach, will likely return it to an erosional phase. Such activities will (i) directly starve the beach of sediment, and (ii) remove a source of sand, as gravel movement in the swash zone causes a breakdown of the clasts and is an important source of additional sand material to the beach.

(8) Proposed nourishment of the beach with sand would be very expensive and would be an ongoing expense. Sand is likely to be transported seaward, especially as the beach was suffering from erosion when it was sand dominated. In addition continued longshore movement of gravel will quickly intermix with any artificially placed sand material.

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(9) Applying “let the system be” management approach would be ideal for the beach. Days Bay is likely to become more gravel-dominated in the future, and this will provide protection for the infrastructure present near the beach, and likely extend the beaches width, though the precise change in dimensions is impossible to predict from the data this report is based on.

———————————————————————–

EASTBOURNE BEACHES – COASTAL PROCESSES SUMMARY REPORT prepared for Hutt City Council by Jim Dahm, Eco Nomos Ltd, Thames. May 2009

(extracted pages 19 & 20 of the 37 page report)

DAYS BAY

In its natural state, Days Bay Beach was backed by coastal lowlands up to 100 m wide (Carter and Gibb,1985), particularly behind the central and northern areas of the beach. The beach is mixed sand and gravel towards the southern end and predominantly sandy (with patches of gravel) towards the northern end; though locals advise that gravels are commonly transported to the northern end by southerly swell (Mr. John Butt, comments on draft report, June 2009). Photographs of Days Bay in the early 1900’s show 20-30 m of sand dune seaward of the (then narrow) foreshore road.The beach was already exceedingly popular at that time and uncontrolled use was already severely damaging the original dune vegetation

– with the dune only partially vegetated.

Over time, further seaward encroachment of human development associated variously with road straightening and widening, provision of parking, and footpath construction has gradually reduced and now largely eliminated the dunes at the northern end of the beach. Over the last 2-3 decades, the lack of a naturally vegetated dune in this area has resulted in windblown sand losses from the beach. Sand loss from this beach is undesirable given the limited sediment supply evident over the last century and the popularity of the beach. There is now insufficient beach width between high tide and the road to maintain a natural vegetated dune. Seaward encroachment of the road and parking over the former dune and onto the active beach has also led to the need for seawalls to be constructed in this area to protect the encroaching development. Dune vegetation has been planted in recent years but this is landscaping in clay soils rather than a functioning natural dune and the plantings are protected by the recently constructed wooden seawall. There has been community and professional concern with progressive loss of the dune in this area since at least the early 1980’s (Carter and Gibb, 1985).

Restoration of a naturally vegetated and functioning dune is highly desirable in this area as the present situation results in permanent windblown sand losses to landward areas. As noted in the earlier Petone report, a sandy beach without a dune is like a bucket with a hole in it. It is recommended that consideration be given to restoration of a natural dune along the central and northern areas of the beach. This would require either landward relocation/retreat of encroaching development or widening of the beach by nourishment.

Nourishment involves widening a beach by importing suitable sediment from outside of the beach system. Locals advise that suitable sediments exist offshore from the beach – but investigation would need to be conducted to ensure these sediments were not actually within the existing beach system. The seaward edge of the active beach at Days Bay, a location known as the closure depth, may be some distance offshore – possibly 3-5m depth below low tide. No long term benefits can be achieved simply by moving existing sediment reserves around within the beach system. If suitable sediments do not occur offshore beyond the existing active beach, others sources would need to be found. Nourishment should be investigated but as a rule it is expensive. It can also be difficult to match the sediment characteristics of the beach – especially with mixed sand and gravel beaches such as Days Bay.

Dune restoration by landward relocation of encroachment may be the more appropriate option at this site in the short to medium term. However, this would reduce existing parking opportunities, which are at a premium during peak use periods. Any works to restore a vegetated dune will probably also have to address parking and related matters.

Older reports by Carter & Lewis 1995 & Carter & Gibb 1985 can be downloaded at http://www.southlight.co.nz/DBreports1985&1995.zip

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