Thanks Jack. What are safe flow levels? I understand that tidal flows can complicate it but is there a magic number or an unsafe flow level? 

It is worth looking at the "Practical Guidance" on the NC website home page for general description about the hydraulics of the river in the stretches we use.   Since we published that guidance there have been some better understanding of the impact of discharges because of increased availability of data on line. 

Innisacarra Dam delivers most but not all the water in the Lee as it enters Cork City at the Split weir near the Kingsley Hotel.  The split weir diverts the river towards the Gilabbey mills site at UCC - so river water in the north channel is limited to the volume that passes the sluice gap in the waterworks side of the split weir....until the split weir overspills.

Factor 1... The Split weir overspills when the discharges from the dam are at approx 40 cubic meters per second (CUMECs). 

If the discharges rise to 60 CUMECs, the half of the flow is already directed to the north channel coming down past the Mercy, and will pass under Grattan Bridge (North Gate), roughly estimated average flow of 3.5 kph. at low tide. Quite manageable by a competent crew in a 4 hander.  At a discharge of  80 CUMECs the flow is approaching 6 kph which is a struggle and dangerous to pass under the bridge.   However, at a full neap spring tide, of 3.5m height, the flow rate for 80 cumec discharge is estimated at 1.3 kph; no bother.

Factor 2.... A spring tide ebb contributes to flow

There are 3 pools to be drained during the ebb past Grattan Bridge, these are delineated by Sunday's Well Weir (Dries @3.3m) Distillery weir (Dries at 2m) and Grattan Bridge.  In the case of a 5m spring tide, the total hourly tidal contribution to flow above Grattan Bridge along the North Mall is 

After HW    CUMECs    Flow Rate kph

1st hour       9.4            0.2

2nd hr         20              0.55

3rd hour       30             1

4th hour       23.5           0.55     No tidal water coming from above Sundays Well

5th hour       10              0.6       No tidal water coming from above the Distillery weir 

6th hour         3              0.4

assuming the low water average depth along the north mall is 1m.  If the low water average depth is 0.5m such as a Popes Quay the low water flows need to be doubled. 

Factor 3......  River water floats on salt water

We can detect the reach of salt water by the presence or absence of fucus seaweed on bridge buttresses, and by leaves that float on the separation zone.   This means that it very unlikely to observe west flowing stream between Blackrock Castle and the Sundays well weir.  The hydrodynamics of river water and tidal water combined are too complicated for this exercise, but we know from observation that during the flooding tide the flow of river water during periods of high (> 90 CUMECs) discharge is greatly reduced, in particular after 2 hours from low water.  

So .... In my own case... for comfort pleasure and safety... 

Above 50 CUMECs Avoid Mercy pool  within 2 hrs of low water

Above 80 CUMECS Avoid Grattan br between 2hr before and 1 hr after LW.  Avoid upstream of Christy Ring Br in ebbing spring tide

Above 150 CUMECs  Limit rowing at north mall to flooding tide

Above 200 CUMECs Off to Blackrock with a good workout on the way home .

In addition to the online data, there is a phone service giving forecasted flows from the ESB... call 021 4871233 and select "Fishing information" from the options. 

On a related matter about tide heights

• Christy Ring Br is lowest on the north channel and is passable at a tide level of 3.6m
• Spring tides occur every 2 weeks and in Cork are normally of High Water between 5 & 8 am/pm  UTC   (6 and 9 in summer time)
• Height and time of actual HW and LW is impacted by air pressure/wind