Where did the river mouth go?
Shortcut URL: https://serc.carleton.edu/36032
Location
Continent: Australia
Country: Australia
State/Province:NSW
City/Town:
UTM coordinates and datum: none
Setting
Climate Setting:
Tectonic setting:
Type: Process
Figure 1. Georectified aerial photos of the study area (modified from Vila-Concejo et al., 2007). Details
Figure 2: Evolution of Winda Woppa spit between 1795 and 1941 (modified from Thom et al., 1992). Details
Figure 3: Photo from 2006 showing the evolution of the shoreline in Winda Woppa between 1951, 1986, 2001 and 2008. Details
Figure 4: Photos taken in April 2008 while measuring the study area using the RTK-GNSS. Details
Description
If you lived on Winda Woppa peninsula, in a house with a boat shed by the river bank, and only used your boat every few years, it is likely that you would have to travel further and further every time you wanted to put your boat in the water...
A sand spit is a linear accumulation of sand that attaches to the shore at one end. It is formed due to the alongshore transport of sediments caused by the oblique incidence of waves or by tidal currents. Port Stephens is a drowned river valley estuary that is located 230 km north of Sydney (NSW, Australia). Its northern shoreline is a highly dynamic environment that includes Jimmy's beach and Winda Woppa peninsula, both of which form a sand spit that deflects the mouth of the Myall River and forms its southern boundary. Figure 1 shows some aerial photos of the estuary where some of the most significant changes happening in the northern shoreline can be appreciated: (1) there is a shoreline attached sandwave that develops next to the entrance [if you want more information, there is another vignette by the same author]; and (2) the western end of Winda Woppa spit is extending westwards.
Before going any further we should introduce some of the forcing mechanisms that control the geomorphologic evolution of the area:
- Port Stephens is a tide-dominated estuary which is located in a wave dominated coast. It has an entrance that is 1.24 km wide with a deep, well defined channel (tide-dominated). Due to its location in a wave dominated coast, and to the fact that the entrance is quite wide, it is possible for the waves to propagate inside the estuary.
- The wave climate is moderate with mean significant wave heights (Hs) of 1.5 m and 8 s wave period (T). Waves from the NE dominate during summer and E-SE waves are prevalent for the rest of the year; although SE storms may occur at any time of the year, they are more frequent between April and September.
- Tides in the area are semidiurnal (two high-low tidal cycles per day) and microtidal (tidal range is less than 2 m) with mean tidal ranges of 1.6 m and 1.3 m for spring and neap tides respectively.
Port Stephens has a well developed flood-tide delta that is formed not only by the tidal currents but also by waves propagating into the estuary. The sand spit in the northern shoreline has been known to undergo periods of spit extension followed by breaching (Figure 2). It was reported by Thom et al. (1992) that a spit was initiated about 1820 and reached its maximum length about 1910; a storm in 1927 caused the breaching of the spit some 1300 m from its end. The same authors reported some westward extension of the spit occurring before the publication of their work in 1992.
We can use geo-rectified aerial photos to quantify the westward extension of Winda Woppa. Figure 3 shows the positions of the shorelines in 1951, 1986, 2001, 2006 and 2008; all shorelines except for 2008 were measured over georectified aerial photos using a Geographic Information System (GIS), the shoreline in 2008 was measured (Figure 4) in the field using a Real Time Kinematic Global Navigation Satellite System (RTK-GNSS). The sand spit underwent extension between 1951 and 1986; between 1986 and 2001 there were very few changes because dredging was undertaken to stop the extension; after dredging was abandoned, the spit extended again with an overall extension of some 800 m between 1951 and 2008.
There are several implications of such a sand spit extension, many of which require management interventions that can be delicate because they may affect the overall equilibrium of this dynamic area:
- Where does the sand come from? It is clear that the westward transport occurs through the combination of oblique waves that propagate through the entrance and tidal currents. Other parts of the peninsula are undergoing severe erosion and finding out whether the lost sediment ends up in Winda Woppa spit or not will greatly help the management of the area. Also, the western end of Winda Woppa spit is the updrift bank of a river mouth that, historically, has been navigable. Dredging of the mouth stopped due mainly to environmental reasons such as birds nesting areas, amongst others.
- We know from other authors of at least one example of spit elongation and breaching occurred between 1810 and 1927 (Thom et al., 1992). Without further data we may speculate that extension/breaching cycles take approximately 100 years and and that, unless management intervention is undertaken, we may expect another breaching in the spit to occur in the near future.
Several questions arise that challenge coastal managers:
A sand spit is a linear accumulation of sand that attaches to the shore at one end. It is formed due to the alongshore transport of sediments caused by the oblique incidence of waves or by tidal currents. Port Stephens is a drowned river valley estuary that is located 230 km north of Sydney (NSW, Australia). Its northern shoreline is a highly dynamic environment that includes Jimmy's beach and Winda Woppa peninsula, both of which form a sand spit that deflects the mouth of the Myall River and forms its southern boundary. Figure 1 shows some aerial photos of the estuary where some of the most significant changes happening in the northern shoreline can be appreciated: (1) there is a shoreline attached sandwave that develops next to the entrance [if you want more information, there is another vignette by the same author]; and (2) the western end of Winda Woppa spit is extending westwards.
Before going any further we should introduce some of the forcing mechanisms that control the geomorphologic evolution of the area:
- Port Stephens is a tide-dominated estuary which is located in a wave dominated coast. It has an entrance that is 1.24 km wide with a deep, well defined channel (tide-dominated). Due to its location in a wave dominated coast, and to the fact that the entrance is quite wide, it is possible for the waves to propagate inside the estuary.
- The wave climate is moderate with mean significant wave heights (Hs) of 1.5 m and 8 s wave period (T). Waves from the NE dominate during summer and E-SE waves are prevalent for the rest of the year; although SE storms may occur at any time of the year, they are more frequent between April and September.
- Tides in the area are semidiurnal (two high-low tidal cycles per day) and microtidal (tidal range is less than 2 m) with mean tidal ranges of 1.6 m and 1.3 m for spring and neap tides respectively.
Port Stephens has a well developed flood-tide delta that is formed not only by the tidal currents but also by waves propagating into the estuary. The sand spit in the northern shoreline has been known to undergo periods of spit extension followed by breaching (Figure 2). It was reported by Thom et al. (1992) that a spit was initiated about 1820 and reached its maximum length about 1910; a storm in 1927 caused the breaching of the spit some 1300 m from its end. The same authors reported some westward extension of the spit occurring before the publication of their work in 1992.
We can use geo-rectified aerial photos to quantify the westward extension of Winda Woppa. Figure 3 shows the positions of the shorelines in 1951, 1986, 2001, 2006 and 2008; all shorelines except for 2008 were measured over georectified aerial photos using a Geographic Information System (GIS), the shoreline in 2008 was measured (Figure 4) in the field using a Real Time Kinematic Global Navigation Satellite System (RTK-GNSS). The sand spit underwent extension between 1951 and 1986; between 1986 and 2001 there were very few changes because dredging was undertaken to stop the extension; after dredging was abandoned, the spit extended again with an overall extension of some 800 m between 1951 and 2008.
There are several implications of such a sand spit extension, many of which require management interventions that can be delicate because they may affect the overall equilibrium of this dynamic area:
- Where does the sand come from? It is clear that the westward transport occurs through the combination of oblique waves that propagate through the entrance and tidal currents. Other parts of the peninsula are undergoing severe erosion and finding out whether the lost sediment ends up in Winda Woppa spit or not will greatly help the management of the area. Also, the western end of Winda Woppa spit is the updrift bank of a river mouth that, historically, has been navigable. Dredging of the mouth stopped due mainly to environmental reasons such as birds nesting areas, amongst others.
- We know from other authors of at least one example of spit elongation and breaching occurred between 1810 and 1927 (Thom et al., 1992). Without further data we may speculate that extension/breaching cycles take approximately 100 years and and that, unless management intervention is undertaken, we may expect another breaching in the spit to occur in the near future.
Several questions arise that challenge coastal managers:
- Do we want to artificially keep the spit as is by dredging in the westernmost tip and placing the sediment in the thinning areas where a breach may happen?
- Do we want to leave nature to follow its course and allow extension and breaching? (removing all real estate before hand)?
- Do we want to 'work with nature' and choose a breaching point where we can relocate the river mouth artificially?
Associated References
- Short, A.D., 1985. Rip-current type, spacing and persistence, Narrabeen Beach, Australia. Marine Geology, 65(1-2): 47-71.
- Short, A.D. and Trenaman, N.L., 1992. Wave climate of the Sydney region, an energetic and highly variable ocean wave regime. Australian Journal of Marine and Freshwater Research, 43: 765-791.
- Thom, B.G., Sheperd, M., Ly, C.K., Roy, P.S., Bowman, G.M. and Hesp, P.A., 1992. Coastal geomorphology and quaternary geology of the Port Stephens-Myall Lakes area. Dept. of Biogeography and Geomorphology, Australian National University, Canberra, 407 pp.
- Vila-Concejo, A., Ferreira, Ó., Morris, B.D., Matias, A. and Dias, J.M.A., 2004. Lessons from inlet relocation: examples from Southern Portugal. Coastal Engineering, 51(10): 967-990.
- Vila-Concejo, A., Short, A.D., Hughes, M.G. and Ranasinghe, R., 2007. Shoreline implications of flood-tide delta morphodynamics. The case of Port Stephens (SE Australia). Coastal Sediments. ASCE, New Orleans, pp. 1417-1430.
- Vila-Concejo, A., Short, A.D., Hughes, M.G. and Ranasinghe, R., 2009. Formation and evolution of a sandwave on an estuarine beach. Journal of Coastal Research, SI 56: 153-157. http://www.geosci.usyd.edu.au/research/re_portstephens.shtml