Page 1

21 Pitt Street PO Box 6345, Auckland 1141, New Zealand T: +64 9 300 9000 // F: +64 9 300 9300 E: info@beca.com // www.beca.com

Weiti Development Ltd Partnership PO Box 105054 Auckland 1143

16 May 2017

Attention: Evan Williams

Dear Evan Stream discharge onto Karepiro Bay Beach This letter has been prepared in response to the following requirements as set out in Russell McVeagh letter of 6 May 2008. (a) Williams Land shall investigate whether the existence of the sandbar in Karepiro Bay is reliant on the stream and culvert located in the middle of the beach at Karepiro Bay. Subject to agreement from ARC, Williams Land agrees to retain the stream and culvert unless its investigations demonstrate, to KOGSI’s satisfaction, that the sandbar is not caused by or otherwise reliant on the stream and culvert.

Overview Description In preparing this appraisal I have relied on the following information. n

Aerial photographs dating back to 1939

n

Site visits since 2005, with the most recent being on 15 May 2017.

Karepiro Bay is a shallow intertidal pocket beach between Okura Estuary to the south and Weiti Estuary to the north. The sandy beach is about 500m long and is flanked by cliffed promontories/headlands at both ends. The bay and backshore area is probably a wave cut platform that has been infilled with fine grey sand. The beach face comprises this fine sand (≈0.2mm diameter) with a mixture of shell deposits. The intertidal area comprises predominantly fine grain sand. The spring tidal range is about 2.6m. The mean wave height is 0.33m and has a similar wave climate to Orewa Beach (NIWA, “Hauraki Gulf wave climate simulation”, 2004). On the beach face, there are shell and sandy cusps which are accretional features, suggesting that the planform alignment is in equilibrium with the more-or-less unidirectional east-nor-east wave climate. See attached photos 9 and 10. The backshore area (the “Hay Paddock”) is some 300m wide to where it interfaces with the steeper facing Waitemata series landform. This area also mainly comprises fine sands. It contains a number of streams (some of which could be man-made) which generally drain to the Karepiro Stream to the northern end of the beach. Throughout much of the year this backshore area has elevated groundwater levels close to the surface. Between the backshore area and the beach is a low level dune system. Previously pine trees covering the dunes. These were removed after 2005, and the area has since been replanted with low level shrubs. Karepiro Bay is part of the Long Bay-Okura Marine Reserve and is classified as Marine 1 under the Unitary Plan.

Our Ref: 3411516 NZ1-14099262-5 0.5


Page 2 16 May 2017

Specific Description The “stream and culvert located in the middle of the beach” (referred to as the Stream) is located about 300m from the northern end of the beach. A range of historical aerial photographs are attached. The 1961 area photo shows the stream channel and discharge location. This indicates that it has been a long term feature and may not be a man-made channel/overflow. It is noted, however, that none of the aerial photos show a “sand bar” in the middle of the beach nor a culvert associated with the steam discharge. The oblique aerial photos of 2014 (attached Photo 5, 6 and 7) clearly show a channel discharging onto the upper beach face. Also shown in Photo 7 is a planform indentation at the discharge location with the adjacent coastline extending further seaward. This is a classical coastline response to a stream discharge where the sediment on the beach is dispersed into the nearshore, and the stream current creates 2 counter eddies either side of the stream and redistributes the sediment onto the beach. The symmetrical layout of the coastline response is indicative of a beach system with low levels of littoral drift. This feature, however, is mainly caused by the stream flow (i.e. the discharge hydraulics) rather than a supply of sediment from the stream system. It is a redistribution of sand within the beach system. About 1 month ago the stream was blocked off by infilling about 50m of stream. The remnant discharge is still located within the dune system but appears to be at a higher level than the 2014 photos. See attached photo 8. It is evident from this photo that the discharge is not continuous but acts as an overflow. When the stream levels within the backshore area are elevated overflows would break through the dune and discharge onto the beach. Today there are no features on the beach that resemble a ‘sand bar’ which is fed by sediment from the stream discharge. Nor is there any tidal channel away from the discharge location. This is in marked contrast to the Karepiro Stream discharge in which the stream and intertidal channel is flanked by shell/sand banks. The issue appears to be whether the “sandbar” is caused by or reliant on the Stream. Any discharge of sediment through overflows from the stream will be episodic. These events could contribute some sediment from the dune as it breaks through and also any sediment from the backshore channels if they were eroded during an overflow event. Any sediment contribution from the stream is likely to be small, particularly in comparison the Karepiro Stream which effectively drains the Stream’s catchment on a day-to-day basis. Overall I conclude that the beach’s reliance on sediment from the Stream is negligible. If the Stream overflow discharge is permanently curtailed then I would expect the beach to respond by having a more planar/linear planform similar to the remainder of the beach.

Our Ref: 3411516 NZ1-14099262-5 0.5


Page 3 16 May 2017

Yours sincerely

Stephen Priestley Senior Technical Director on behalf of

Beca Limited Direct Dial: +64-9-300 9282 Email: stephen.priestley@beca.com

Our Ref: 3411516 NZ1-14099262-5 0.5


Page 4 16 May 2017

Photo 1: 1939 Aerial Photo

Photo 2: 1961 Aerial Photo (Reverse Image)

Our Ref: 3411516 NZ1-14099262-5 0.5


Page 5 16 May 2017

Photo 3: 2005 Oblique View

Photo 4: 2005 Oblique View

Our Ref: 3411516 NZ1-14099262-5 0.5


Page 6 16 May 2017

Photo 5: 2014 Oblique View

Photo 6: 2014 Oblique View

Our Ref: 3411516 NZ1-14099262-5 0.5


Page 7 16 May 2017

Photo 7: 2014 Oblique View

Photo 8: May 2017 Remnant Discharge Location

Our Ref: 3411516 NZ1-14099262-5 0.5


Page 8 16 May 2017

Photo 8: May 2017 Discharge location onto beach

Photo 9: Beach face from discharge looking south

Our Ref: 3411516 NZ1-14099262-5 0.5


Page 9 16 May 2017

Photo 10: Beach face from discharge looking north.

Our Ref: 3411516 NZ1-14099262-5 0.5

Becca report stream discharge  
Becca report stream discharge  
Advertisement