Muriwai Coastal Geographic Environment A Study of Natural Processes

Muriwai INTRODUCTION We define the Muriwai Coastal Geographic Environment as being south from the stack (Motutara Island) to the river (Okiritoto Stream) 2.5 km north. Offshore to the outer limits of surf and approximately 500 m inland to the road east of the golf course and giving access to the beach beside Okiritoto Stream. The interacting natural processes operating at Muriwai are: Coastal erosion including wave erosion and refraction and sand saltation. Coastal transportation including longshore/beach drift, longshore currents and littoral drift. Coastal deposition including dune formation, spit and bar formation, beach formation.

Muriwai The erosional features found at Muriwai Coastal Geographic Environment include: - The stack (Motutara Island) Shore Platform (Fisherman’s Rock) The Blowhole/s The Cave The depositional features found at Muriwai Coastal Geographic Environment include:- The beach and associated sand spit (Rangitira Beach) The dunes The River Delta The Cliffs The Offshore sand bar

Muriwai On a large scale the West Coast of New Zealand is subjected to prevailing S.W. winds which are strongest during the winter season (about June to September). The longshore current that occurs around New Zealand tends to move in a northerly direction on both coasts due to the polar sea from Antarctica meeting the warmer Tasman Sea and Pacific Ocean. This is called the sub-antarctic convergence. Waves tend to be a westerly swell that varies in height and therefore energy seasonally. They approach Muriwai from a southwesterly direction as the beach faces toward this direction (S.W.) This means that longshore (or beach) drift tends to move in a northerly direction. The combination of both longshore currents and beach drift is called littoral drift.

Muriwai What are the elements and interactions involved in each process? At Muriwai Coastal Geographic environment, (MCGE) there are three main processes operating. Coastal erosion, coastal transportation, and coastal depostition. Each of these also has various sub-processes. The elements and interactions within the process of Coastal erosion are quite varied. The main elements however are things such as the wind, the waves, the sand, the rock type, the vegetation, sun, humans, landforms and tides. However the interactions that occur within these elements are the important thing. One of the biggest interactions that occurs is the interaction between the landforms and the tides and tidal processes.

Muriwai What causes coastal erosion? We know that periods of erosion coincide with storms and large waves, so the obvious answer to the question is BIG WAVES

Muriwai Waves At Muriwai Waves at the Shore Platform Waves at the Beach

Muriwai Types of Waves Breaking Wave Types Beach Slope Wave Steepness Spilling Shallow Steep Plunging Shallow to intermediate Less steep Collapsing Intermediate to steep Intermediate Surging Steep Shallow

Muriwai CHARACTERISTICS OF WAVES The character of any beach is determined by the type of waves that break on the beach, however the wave types themselves are determined by the shape of the sea bed.At any one time beaches are either in a CONSTRUCTIVE period or a DESTRUCTIVE period, either moving material away from the backshore to form a LONG SHORE BAR or moving material on to the backshore to form a BERM. TASK:- Using the following labels construct a table with two columns . One describing CONSTRUCTIVE and the other describing DESTRUCTIVE features. Include a diagram. FLAT GENTLE WAVES SHORTER WAVELENGTH MORE FREQUENT WAVE STRONG SWASH PERIOD LOW ENERGY WAVES OCCUR ON FLAT BEACHES (6-8WAVES PER MIN) LOW WAVE HEIGHT MAINLY WINTER WAVES STEEP WAVES CONSTRUCTIVE WAVES WEAK SWASH STRONG BACKWASH LONGSHORE BAR FORMS BERM FORMS OCCUR ON STEEP BEACHES (10-14WAVES PER MIN) (HEIGHT >1METRE) MAINLY SUMMER WAVES STRONG BACKWASH DESTRUCTIVE WAVES LONGER WAVELENGTH HIGH ENERGY WAVES LESS FREQUENT PERIOD ENERGY SPREAD OVER ENERGY CONCENTRATED A LARGE AREA ON SMALL BEACH AREA

Muriwai Description "The shoreline is an asymmetrically concave line which at first is more or less straight for 32 km and then bulges seawards, later to be recurved to the east in the sand hook at Kaipara South Head. This cuspate shape is the result of contrasting processes even now affecting the shoreline-retrogradation along the straight portion and progradation at the bulge. The straight shoreline trends thirty degrees west of north and is therefore not at right angles to a wind blowing from the south-west. As a result, transverse dunes behind the beach lie at an angle of ten to fourteen degrees with the shoreline and the sand moves along the coast to a small degree as well as inland.

Muriwai A foredune backing the steep wide beach is continuous from Muriwai township along the entire length of the shoreline rarely being more than 10 m high and 20 m wide, and is broken only by the mouth of Okiritoto Stream. Many large blowouts have been developed by the removal of loose sand from the seaward face of the foredune, producing large marram grass hummocks with short tail dunes. Immediately to leeward of the foredune and parallel to the beach is a well-defined shallow eddy hollow, downwind from which the loose sand rises again as long tongues gaining height inland. "Much of the detritus forming the dunes was brought to the West Coast by the Waikato River from an ultimate source in the Central Volcanic Plateau.Fluctuations in the quantity of material carried to the coast by the Waikato River must have affected the rate of progradation in the dune area north of Muriwai. However, it is doubtful if such fluctuations in the supply of sediment were sufficient to induce alternating periods of extensive deposition and erosion.

Muriwai An added impetus to progradation of the coast would have been supplied by the recession of sea level that is known to have occurred in Recent time. Progradation and sand dune formation concurrent with this path of sea level undoubtedly added great quantities of sand to the Muriwai foreland, and therefore could account for the development of any one of the dune belts. Nevertheless such a sequence of events fails to explain the intervention of two erosion periods. It seems most probable that the main course of dune destruction and in part, of dune formation, has been the climate cycle. Periods of calm climatic conditions many years in length are known to alternate with similar periods of generally stormy weather.