peak current estimate

Consider a channel leading from ocean into a bay. High tide at T1 and low tide at T2. Can one estimate if peak ebb current occurs closer to T1 or T2? Same question for a flood current.

The general rule is that change happens fastest mid-cycle. 1/12, 2/12, 3/12, 3/12, 2/12, 1/12. That is a rule-of-thumb fraction of tidal effect for each of the 6 hours in a normal tidal cycle.

When you start talking current speed, you have to change it from between low tide (water level) and high tide (water level), to the time between low tide slack current and high tide slack current. Midway between is max current speed. But you have to know, for any given location, exactly how tidal current corresponds to tide water level.

Just quickly, when the ocean reaches high tide at a given location next to some type of inland bay or waterway, the water keeps flowing inland, until all the inland water has risen to the level of the ocean. So if the ocean rises to +5’ at high, and many miles inland, the connected waterway is only at +2’, When the ocean falls to +4.5’, it is still higher than some of the inland water at, say +3’. So you still have a flooding current at your spot, even though the water level is dropping. All kinds of features will play with this. So understanding slack current in relation to tidal water level at your location is important.

In San Francisco Bay, our currents trail the tides by roughly 1 hour. So for example high tide could be at 12, but the current is still flooding. At 1, the tide will have dropped from high, but at that time will be high slack tide. The delay is not exactly an hour in all cases and varies day by day and between high and low, but 1 hour is a decent estimate for us.

In normal 2 tides a day systems, which most places are, the currents go through the 6 hour cycle which @CapeFear talked about. That rule of twelfths talks about how much water moves through an area. The currents can be estimated off of maximum current by a ratio of .5, .9, 1, 1, .9, .5. So if your max ebb current is 2 knots, at 1 hour after slack you have a current of 1 knot. 2 hours after is 1.8, 3 and 4 hours after are 2 knots. Then 1.8 knots and 1 knot. This all requires that you can look up what the estimated max current would be to calculate.

There is a program called Xtides, which many tide charts and apps are based off of, which provides this info. You could plug into a Google search the location you are at and “tides” or “currents” and see if you can find the basic info.

In Jacksonville the river slows faster with an incoming tide. Yet out going tides often continue to flow out two hours after scheduled high tide. The river can’t read tide tables.

There are locations which have the good fortune to have excellent resources on tidal flow, strength and lag from the tide points. Like Eldridge’s for Cape Cod. I couldn’t tell where you are from, but you may be able to find such a resource for your local area.

To add to Celia’s remarks, If there’s no electronic or printed tidal current information for the location (and in some areas this information is sparse), it’s usually WRONG to assume that slack water coincides with high or low tide. Getting local knowledge can be a great help – contact fishermen, fishing/kayak guides, scuba outfits, etc, in the area.