Gotta be freshet: on floods and the evolving high water mark


[THE FRESHET MARK]

Etchings and markings of nature’s force on the built form are not new.  Markings of river heights are equal parts hydrology, memorial and admonition.  Above is an etching on the south side of the westernmost pier of an ignored railroad truss bridge over the Schuylkill known at one time as the Arsenal Bridge.  The “freshet line” marks the highest the Schuylkill River has ever crested–17 feet–some 3.5 feet higher than the floods of Irene. The United States Geological Survey and FEMA still consider Oct. 4, 1869 a 100 year flood of record.  After this late sodden summer of swollen creeks and rivers, of hunkering down, of anxious portents of meteorological doom, formal etchings like these resonate even more.


[ARSENAL BRIDGE PIER, ON A CALMER DAY]

Today we are all spectators of modern storm systems.  From their birth off the coast of West Africa to their dissipation over the North Atlantic, they are media darlings: slow, predictable and never out of the reach of our surveillance, they spit out human interest stories like bands of rain.  In the nineteenth century, the somewhat regular behavior of hurricanes was only imperfectly known.  Even if passing ships happened to clip a major system, data from various sources were rarely aggregated for forecasting purposes.

This makes the forecast of the October 4-5th, 1869 storm all the more amazing.  In an era of miasma theory, phrenology and other false prognostications, someone actually got it right—a year in advance.  On December 25th, 1868 an amateur Royal Navy astronomer, Stephen Saxby drafted a warning to the London Standard of a future maritime calamity.  Saxby had calculated that during the period of October 4-5, 1869 “the moon will be at that part of her orbit which is nearest to the earth. Her attraction will, therefore, be at its maximum force.”  He reinforced his message with another monitory editorial on September 16th, 1869 entitled “Equinoctal Gales” stating that “…one is justified in expecting (to say the least) quite as great an atmospheric disturbance early in October as we have had since 6th inst.; and I am sorry to say the same may be expected with equal uncertainty and intensity on the 1st to 3rd November next. The warnings apply to all parts of the world; effects may be felt more in some places that in others….  Could I save one life, it would be very cheaply purchased in making better known certain laws of nature.”


[SAXBY’S TRAIL]

Even if Saxby was correct in believing that the phases of the moon controlled weather, it wouldn’t have made any difference in Philadelphia.  It’s unclear whether his warning ever made any impact on the city.  Rains intensified through the 4th and 5th, swelling the Schuylkill.  On its banks sat a jumble of lumber, marble, railcars, refined oils; soon the river made flotsam and jetsam of the sundry products of the workshop of the world.  A humbled Inquirer wrote: “Following close upon the drought comes a freshet of unprecedented violence, so great a one, in fact, that bridges are carried away, factories, dwelling houses, ice houses, &c., are submerged, boats are swamped, and the river is swollen to three times its usual size, and to such and extent that Fairmount, Flat Rock, and other dams, which a few days ago were high and dry, have been completely hidden by the Niagara of waters that dashed and surged over them with terrific violence.”

This narrative of a seemingly outlandish meteorological warning—backed by modern science—going unheeded should not be new to us.  Like Saxby, modern climatologists feel the sting of rebuke from the factually challenged, those who feel nothing can be known causally between the great gulf between now and then.  Saxby’s forecast did little to elevate astronomy in the service of meteorology beyond mere astrology.  Yet, as the region’s manufacturing districts hobbled to their feet, they began to see how their various “improvements” on the earth exposed them to greater danger during disasters, especially floods.  As early as 1864, the brilliant George Perkins Marsh had observed in his innovative Man and Nature: or Physical Geography as Modified by Human Action that “it appears that the overflow of river banks was much less frequent and destructive than an the present day, or, at least, that rivers rose and fell less suddenly before man had removed the natural checks to the too rapid drainage of the basins in which their tributaries originate….  Even now the trees come down almost to the water’s edge along the rivers, in the larger forests of the United States, and the surface of the streams seems liable to no great change in level or in rapidity of current.”

As forests were denuded and development further constricted the floodplain, other observers began to note that former records of floods were inadequate guides to future flood heights.  After the Saxby Gale, the Reading Daily Eagle chastised builders who believed in the authority of the former high water marks.  “So it will not do, in repairing the recent damages, to take the height of the late floods as the greatest height the water can ever attain, and rebuild bridge and other structures only so as to clear this height.  Millions of dollars in property might have escaped destruction after the late storm if the builders had known that floods rise higher and more rapidly as the country becomes improved.”

It’s a simple but elusive corollary: the more you build the higher the floods. But after 132 years we’re only now pinning down the variables that impact higher floodwaters.  Since 2007, Temple University’s Center for Sustainable Communities has analyzed flooding in the suburban Pennypack watershed and found that all the inputs informing flood probabilities weren’t up to date.  Temple’s more fine-grained maps at larger scale show dangers that FEMA’s flood insurance maps didn’t.  In addition, they’ve rejected the existing U.S. Weather Bureau precipitation data from 1961 as far too conservative.  More recent data shows higher rainfall by nearly 15.6% over the same period of time.  The new precipitation data, coupled with the more detailed floodplain maps and keen knowledge of floodplain modification means we’re getting closer to understanding the new high water mark—but don’t etch it in stone.

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