Narooma Tidal Hydraulics

HOW TIDES AFFECT BOATING CONDITIONS ON NAROOMA BAR

Narooma Bar, at the entrance to Wagonga Inlet, has the reputation of being one of the most dangerous bars in New South Wales. During the period, 1992-2003, six bar boating fatalities occurred on Narooma Bar compared with a total of 13 on all of New South Wales bars over the same period. Fortunately, in recent years the number of fatalities has decreased considerably but Narooma Bar continues to be a danger to all boat crew.

This note provides a short explanation of the interaction between tidal flows into and out of Wagonga Inlet and the sea conditions on Narooma Bar.

Narooma Map

Wagonga Inlet (near the town of Narooma on the far south coast of New South Wales) has a water surface area of about 6.9 km² and is connected to the ocean by a constrained channel about 3.5 km long between the entrance to west of the bridge. Furthermore, the entrance is formed between rock-fill embankments restricting the flow width to about 60 m. As a result the tidal variations in Wagonga Inlet are significantly different from the tidal variations in the ocean. [Other inlets such as at Eden or Jervis Bay have very little tidal constriction and the tidal variations within those inlets are very similar to those in the ocean.]

L-naroomaentrance-2

At Narooma there are two tides each day, approximately 12.8 hours apart. Ocean tidal levels typically vary between 0.5 m below and 0.5 m above mean sea level (MSL). However, during spring tides this variation can increase to between 1.0 m below and 1.0 m above MSL. The tidal datum used is related to the lowest tide level, so (according to the Tide Tables) spring tides vary between about 0.0 m and 2.0 m. Because of the absence of any significant hydrographical features between Sydney and Narooma the tide levels and times at Narooma (in the ocean) can be taken as being approximately the same as published in the Tide Tables for Sydney.

Bar 3   Rough

Examples of “rough” and “very rough” conditions on Narooma Bar.

_BG_3302-A   _BG_3306-A

A typical, but fairly uncommon, misadventure on Narooma Bar.

At Narooma the constriction at the entrance and in the channel between the entrance to west of the bridge results in tidal level variations in lake (ie the water expanse to the west of the bridge) being only about half those in the ocean. So, during spring tides, lake levels vary between about 0.5 m and 1.5 m relative to the tidal datum. The entrance constriction and channel also result in a lag of about 2 hours in the tides in the lake compared with tides in the ocean. Level differences between the ocean and the lake can be up to 0.5 m and surface flow velocities at the entrance can be up to about 5 knots during spring tides. A typical variation of tide levels (in the ocean and lake) at Narooma is shown in the following diagram:

Graph 1

So what has this to do with Narooma Marine Rescue members, the boating community and visitors? Perhaps the most important point is the effect of tidal flows on the sea surface conditions at Narooma Bar (the relatively shallow water extending up to about 100 m outside of the entrance). As we all know, if there is only a light swell in the ocean (say less than 1 m) passage of vessels over Narooma Bar is generally not a problem. However, in larger swell conditions the bar conditions can become extremely dangerous, especially during an outgoing tide. On the other hand, an incoming tide can mitigate the sea conditions over Narooma Bar.

Therefore it is most important to understand when inflows and outflows occur at the entrance. These times are directly related to the ocean tide levels (as published in the Tide Tables). Outflow conditions (the bad type) occur between about 4.5 hours before and 2 hours after low tide. Similarly, inflow conditions (the good type) occur between about 4.5 hours before and 2 hours after high tide. This is illustrated in the following diagram:

Graph 2

Photographs and diagrams are worth a thousand words (so they say!), so here are four photographs to illustrate the effect of tidal flows at the entrance to Wagonga Inlet on swell conditions over Narooma Bar:

Bar 2

Near Low Tide when Outflow occurs.  Breaking swell occurs over the bar.

 

Bar 1

Near High Tide when Inflow occurs.  Note that swell is not breaking over the Bar, but it is elsewhere. This is a relatively safe time to cross the Bar.

 

Here are photos of the Narooma lifeboat crossing Narooma bar on 14 May 2004.

IMG_5404-B   IMG_5405_B

These two photographs were taken only five seconds apart and indicate clearly the rapid development of a breaking swell on Narooma Bar.

The entry was made 2.5 hours before the High tide – this is in the relatively “safe” time zone!  Despite almost broaching (first photo), the lifeboat managed to stay within the area of inflowing current (second photo) where the swell did not break.

This is not a recommended Bar entry, but is very illustrative of the effect of tidal flows on the Bar conditions.

 

The concluding message is: “If there is a significant swell in the ocean, you should avoid crossing Narooma Bar during the period of tidal outflow, between 4.5 hours before a Low tide and 2 hours after a Low tide.”

 

THE AUTHOR

This article was written by Brian Gunter of Narooma. In his previous life Brian was an engineering hydrologist involved over many years in the planning of water resources projects in Australia, Asia and Africa. Since 2000 he has been a member of the Narooma Unit of the Royal Volunteer Coastal Patrol and Marine Rescue NSW involved in marine radio work and has been a close observer of Narooma Bar over that period.

Brian Gunter

brigun@westnet.com.au

 

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