Predicting Abnormally High-Water Events

On Christmas Eve, Kappas Green became Kappas Pond | post by Richard Pavek | photo by Larry Clinton

As described previously, tide tables are merely estimates calculated by averaging the highs and lows for the last ten years and projecting them forward for the coming year(s). During periods of rain, tide tables must be adjusted to higher levels than forecast. Here is a high water level forecasting tool that can help you determine if your parking lot, walkways or other areas are likely to flood.

How to use this Forecasting Tool: first, measure or otherwise determine the water level at which your parking lot starts to flood, i.e. the Flooding Level.

Check the expected high tide for the next day on your tide app or on  the interactive tide chart on the right side of the Floating Times. (There are two highs per day, so be sure you have the higher of the two.) Next get the rain forecast for the eight hours before the higher tide. If the hills are dry add about an inch for every expected inch of rain, but if the hills are super saturated, you’ll need to add up to eleven inches for every inch of expected rain, because the drainage area of the surrounding hills is 11 times the area of the Bay.

Examples:                                                             Dry Hills  (or)         Saturated Hills with heavy runoff

Forecast High Tide                                                          6′ 2″                                  6′ 2″
Forecast Rainwater                                                         1″                                       11″
Rainwater plus Tide =                                                  6′ 3″                                     7′ 3″
— Parking Lot Flooding Level                                  XX XX                                   XX XXm
= Amount of Flooding                                            __________                     __________

Copy this one to use in real time:

Dry Hills                                                                                              Super Saturated Hills

Forecast High Tide ____________                                                            ____________

Forecast Rainwater ___________                                                            ____________

Rainwater + Tide _____________                                                            ____________

– Parking Lot Flooding Level _________                                                ____________

= Amount of flooding __________                                                        ____________

The need for additional resurfacing of the parking lots to offset the increasing tide levels is accelerating. Recently the county cut a trench across our parking lot here at Yellow Ferry Harbor to lay a water line: the cut showed that our parking lot asphalt is 12 inches thick! There will be more over-paving to come!

Flash: Super high tides are due again January 10-12 and February 8-9.