Tropical Storm Isaias and Hurricane Ida are storms that most of us will not soon forget. The severe flooding and damage wrought by these storms make us wonder about the next big storm – when will it come? Will it cause flooding? For the mid-Atlantic, a warming climate means more frequent and higher intensity storms. So, more flooding is more a matter of when, not if. Wissahickon Trails’ headquarters, the historic Four Mills Barn on Morris Rd, sits a stone-throw from the Wissahickon Creek and flooded during both of these recent storms. During storm events, we constantly monitor the Creek to prepare for potential floods – but we don’t do it the way you might think!
The U.S. Geological Survey has a stream monitoring station (#01473900) in the Wissahickon Creek where it runs through Fort Washington State Park. The monitoring station essentially measures stream height, also called ‘gage height’, as well as temperature, discharge, dissolved oxygen, and more. Visitors to the website can see real time information during a storm and even compare it to the previous year or even the previous storm.
‘Gage height’ is the measure we watch during high intensity rain storms. This measures how far above the baseline height the stream gage is, which provides a relative measure of stream height. The more water there is in the stream, the higher the gage height. For example, the graph below shows all of the gage height data (blue line) from mid-July 2020 through Mid-September 2021. During dry weather, the gage height is typically below 3 ft. The spikes represent increases (in feet) in gage height caused by precipitation (rain or snow). When the gage height reaches approximately 12 feet, the Wissahickon Creek will likely jump its banks, at 12.9 ft road overflow begins. Two storms during that time period exceeded 12 ft, Tropical Storm Isaias and Hurricane Ida. The maximum gauge height during Ida was 17.74 ft – almost reaching the maximum measurable gage height!
You can view the real-time gage height during storm events on the USGS website. This will give you a sense of the condition of the creek during the storm event and if you should expect flooding along the Wissahickon. You can even set-up notifications so you receive a text or an email when the gage height exceeds a threshold of your choice.
A few things to bear in mind:
Different parts of the watershed will flood at different ‘gage height’ thresholds, depending on local conditions and where you are along the Wissahickon Creek. You can start tracking gage height at the Fort Washington monitoring station during storms to give you a sense, after a few storms, of how the real-time monitoring station data relates to your local conditions.
Bear in mind that gage height generally continues to increase even after the rain has stopped, as water is still flowing into the creek from above and below ground.
Last but not least, due to the amount of development in our region, you can still encounter flooding even before the Creek exceeds its banks due to stormwater runoff. In fact, during Isiais, the Four Mills Barn flooding started from stormwater runoff, the creek didn’t overflow the banks until later.