Ōrongorongo River Flow Information
Many of us know of the Greater Wellington Regional council (GWRC) website where the Ōrongorongo river condition can be checked. If you didn’t know, have a look at: Truss Bridge flow.
A typical graph is shown below, but it is not always obvious how to interpret what is shown when assessing the safety of a planned river crossing. In particular, it is important to recognise significant increases like that seen on this graph on 23 September 2020.
Interpreting the graph
In Step 1, Select Site, there are many sites listed in the drop-down, but not all of them include river flow data. If you are going into the Ōrongorongo Valley, choosing ‘Ōrongorongo River at Truss Bridge’ does offer ‘Flow’ under Step 2, Select Measurement. Another measurement option is ‘Stage’, a term that hydrologists use for the water level with reference to a specific datum or survey point. Unless you know the details of the datum, a single value of ‘stage’ means little. The flow rate is probably more useful.
But where is the Truss Bridge? It is NOT, as some people have assumed, the Turere Stream bridge at the end of the Ōrongorongo Track. There is no environmental monitoring at the Turere Stream Bridge.
The Truss Bridge is approximately seven km upstream from Turere Bridge and one km downstream from the Wellington Water intake weir (that feeds the Wainuiomata Water Station). It is the left bridge shown on the excerpt below from topo map BQ32 Lower Hutt. (:nztopo ll=-41.297026,175.032678 zoom=14 pin=1 label="Truss Bridge" width=900 height=400:)
The two locations have very different channel morphologies. The upstream Truss Bridge site is in a steep-sided narrow valley and the Ōrongorongo River at Turere Bridge is wide and flat. There are many streams and creeks that add to the flow between the two locations. Localised storms downstream of the monitoring site or over any of the feeding tributaries would obviously add to the flow.
Therefore, the Truss Bridge data is a rough indicator only of conditions at the Turere Stream. Use caution when referring to values of flow rate or stage (river level) that are measured at a place that is remote from the point of interest. However, the measurement trend over time, when considered alongside your previous observations, can suggest what to expect further downstream.
Short-lived peaks in flow
In the graph above, the flow rate on 23 September climbed quickly at about 1300 hours from about 0.8 cumecs to 21.5 cumecs over the next five hours. That was very likely after rain, and could have made a crossing downstream hazardous. Small, short-lived peaks in the flow rate, of the order of a few cumecs only, are sometimes seen and they are insignificant compared to the increase seen after rain.
The GWRC team advised that the small peaks are caused by Wellington Water’s routine maintenance schedule at the weir upstream of the Truss Bridge. Wellington Water close the water intakes at the weir during heavy rainfall as their treatment station (in Wainuiomata) can’t process the dirty/silty water. As the water level recedes, and in preparation for opening the intakes, they undertake a ‘scouring’ of the weir. This entails opening a valve close to the intakes which allows water to flow beneath the weir. This sucks all the sediment and rocks away from the supply intake, leaving it clear. The scour valve is then closed and the treatment station can safely open the intake and start processing the city water supply again.
The short-lived peaks in the graphs follow the opening of the scour valve which drains the water stored behind the weir into the river downstream. The scour valve is then closed and the river level/flows drop as the weir starts to fill again. Once the weir has filled up, the water flows return to normal. The next graph illustrates the process clearly:
The increase in flow or level after scouring is most unlikely to be noticed downstream at the Turere Bridge. It is insignificant compared to the dangerous increase in level following heavy rain, as the next graph (showing water level (stage), not flow) clearly illustrates:
This article
This article is based on information provided by Dave Turner, a Senior Environmental Monitoring Officer with the GWRC Hydrology Team that collects the river level data for the Ōrongorongo river.
GWRC has asked for trampers to be reminded of the restricted access to the water collection area where the Truss Bridge river monitoring site and Wellington Water intakes are situated. Map 3 – Page 12 of the (extremely comprehensive) management plan shows the restricted area starts approximately one km upstream of the Papatahi Hut.