The flooding that took place throughout the Susquehanna basin
on January 19th and 20th, 1996 was an unprecedented meteorological
and hydrological event. Much of the stage for the flooding was
actually set in the preceding months by above normal snowfalls
and below normal temperatures, a regime that had persisted basinwide
since the beginning of November 1995. Table 1 shows the snowfall
totals in inches for the season at four "first order"
National Weather Service stations in the basin. Table 2 shows
heating degree day data and days with the maximum daily
temperature below freezing for the same time period. These tables
indicate that by mid-January, the winter of 1995-96 was well on
its way to becoming a "record-setter". The most memorable
event prior to the flood was the "Blizzard of 1996"
which blanketed the southern two-thirds of the basin with a significant
snowfall that ranged from 18 to 24 inches. Adding to this total
was another storm several days later that added almost another
foot of snow basinwide on top of "The Blizzard". Mid-January
data indicated there was a snowpack throughout the basin that
contained a water equivalent of from 3.5 to over 5 inches. This
exceptionally deep snowpack, combined with the fact that there
were very few opportunities for the snow to melt throughout the
season because of the below normal temperatures, was certainly
one of the factors that contributed to the Flood of January 1996.
Actual | Normal | Actual | Normal | Actual | Normal | as of Jan. 19 | |
Harrisburg, PA | 8.1 | 2.1 | 17.4 | 7.3 | 38.9 | 10.8 | 64.4 |
Williamsport, PA | 13.8 | 2.9 | 25.0 | 8.1 | 32.7 | 21.8 | 69.1 |
Wilkes Barre, PA | 18.6 | 3.3 | 15.9 | 8.6 | 37.5 | 12.0 | 69.9 |
Binghamton, NY | 29.2 | 7.3 | 32.7 | 17.7 | 28.3 | 20.0 | 86.0 |
| ||
Harrisburg, PA | ||
Williamsport, PA | ||
Wilkes Barre, PA | ||
Binghamton, PA |
Another major contributor to the flooding situation was ice. The
air temperatures through most of December 1995 and January 1996
were below normal, with very few periods ever getting above freezing.
This caused the formation of ice on the river, up the mainstem
Susquehanna as well as on some major tributaries throughout the
basin. This ice cover would play a significant role in the flooding
problems later in the month, especially aggravating the situation
in the southern portions of the basin.
The region then experienced an unprecedented meteorological event
on January 18th and 19th. Air and dew point temperatures rose
from below freezing into the 30s and then into the 50s in a matter
of 12 to 24 hours. Temperatures stayed above freezing for about
48 hours. Strong southerly winds gusting as high as 50 mph accompanied
the rapid rise in temperatures. Sustained wind speeds varied,
but generally 20 to 30 mile per hour averages were common through
this period. These conditions caused a very deep snowpack "to
ripen" very quickly. This ripening is the process whereby
a deep fluffy absorbent blanket of snow becomes saturated as the
high winds and temperatures cause the snow temperatures to rise
to the 32 degree level and begin melting. This ripening precluded
the snowpack from absorbing and freezing any of the rainfall that
would follow and actually contributed an average of 1.5 inches
of runoff from snow melt. This warm air and high winds was associated
with a frontal system moving through the basin. This weather system
focused heavy rainfall across the region during this event as
well. Rainfall averaged 2.5 inches from this event, with most
of the rain falling in just three hours. Basin average rainfall
amounts were in excess of two inches throughout much the region,
with the heaviest hit areas approaching three inch averages (Attachment
1). These meteorological conditions produced rapid excessive runoff
throughout the region. The precipitation and associated runoff
on January 19th has been described as a Susquehanna River basin
wide flash flood event because of the intensity of the precipitation
and the speed of the runoff.
With between three and four inches of runoff occurring almost
simultaneously throughout the basin, stream levels rose quickly.
This rapid rise caused any ice cover that was present to break
apart. As the ice cover broke and was washed downstream, many
ice jams occurred at bridges and natural constrictions on both
the tributaries and mainstem. As more ice and water rose and backed
up behind these jams, they broke, often with catastrophic results
to those areas immediately downstream. Attachments 2, 3, 4, and
5 show the river stage hydrographs for the USGS mainstem gaging
stations at Sunbury, Harrisburg, Marietta, PA and Conowingo, MD
respectively. An examination of the Sunbury graph shows the "normal"
rapid rise of the river apparently unaffected by ice jam formation.
This can be contrasted with the Harrisburg graph which shows a
definite effect of one or more ice jam formations on the rising
limb of the hydrograph. These ice jams, when broken, send a surge
of much higher flow downstream. This is what happened below Marietta
at 1 p.m. on Saturday, January 20th and culminated in the flooding
event later that evening in Port Deposit, MD. In addition to the
four mainstem hydrographs described above, Attachment 6 shows
discharge hydrographs for six stream gages located throughout
the basin. The one characteristic common to all of the hydrographs
is the very rapid rise from near normal conditions to flooding
conditions in a matter of hours. The magnitude of this event is
further emphasized by the tabulation in Attachment 7. This tabulation
shows, for various gaging station locations throughout New York
and Pennsylvania, the maximum average daily flow value recorded
as a result of the flood. This average daily value is then "ranked"
according to all the daily flows values ever recorded in January
at the respective gaging stations. Finally, the median daily flow
value for the third week in January is shown along with a ratio
of the January 96 maximum to the median. In the Susquehanna basin,
these ratios range from 12.7 to 36.3, with an average of 22.1.
The operators of both the Safe Harbor and Conowingo hydropower
facilities were faced with a very unusual event on the afternoon
of January 20. A large ice jam that had formed approximately five
miles upstream of the Safe Harbor Dam in the vicinity of Turkey
Hill broke at approximately 1 p.m. on January 20. The release
of water from this ice jam caused the level of the river to rise
very quickly behind the Safe Harbor dam. The operator of the dam
began releasing water through the Safe Harbor flood gates, simultaneously
notifying downstream utilities and government entities according
to a predetermined action plan. The dam operator has estimated
that the river flow below Safe Harbor was increased from 224,000
cubic feet of water per second (cfs) to 826,000 cfs in a two hour
period. It must be noted that these flow values, particularly
the higher estimates, have been made as a result of information
gathered after the event. The flow values that were estimated
during the event, based on the best information available
at the time, were lower. This increase of flow into Lake Aldred
and subsequently into the Conowingo pool happened very quickly,
necessitating Conowingo operators to quickly institute their emergency
operations plan. In order to control a constantly rising pool,
Conowingo operators had to open a total of 42 flood gates before
the situation was stabilized on the evening of January 20 (reference
Attachment 6).
As bad as the Flood of 1996 was, it could have been much worse. The U.S. Army Corps of Engineers estimates that its flood control reservoirs stored a total of 167 billion gallons of water that would have otherwise added to the problem. This flow reduction, coupled with a wide array of local flood protection projects such as channels and levees, is estimated to have prevented a total of 1.37 billion dollars of additional damage. A tabulation of estimated stage reductions along with a more complete breakdown of damages prevented is included as Attachment 8.
Chemung River | 237.9 |
Upper Susquehanna River | 112.3 |
West Branch Susquehanna River | 333.2 |
Juniata River | 70.3 |
Susquehanna River - Main Stem | 612.2 |
TOTAL | 1,365.9 |
Chemung River | Corning, NY | 33.18 | 25.93 | 7.25 | 29 |
Elmira, NY | 18.65 | 13.25 | 5.4 | 10 | |
Lindley, NY | 23.82 | 13.18 | 10.64 | 14 | |
Juniata River | Lewistown, PA | 39.33 | 31.53 | 7.8 | 23 |
Newport, PA | 30.36 | 24.69 | 5.67 | 22 | |
Main Stem | Danville, PA | 28.54 | 25.97 | 2.57 | 20 |
Susquehanna R. | Harrisburg, PA | 25.51 | 24.97 | 0.54 | 17 |
Sunbury, PA | 32.06 | 30.32 | 1.74 | 24 | |
Wilkes-Barre, PA | 36.31 | 34.22 | 2.09 | 22 | |
Upper | Chenango, NY | 14.73 | 12.49 | 2.24 | 10 |
Susquehanna R. | Owego, NY | 44.13 | 40.24 | 3.89 | - |
Vestal, NY | 30.23 | 27.8 | 2.43 | 18 | |
West Branch | Lewisburg, PA | 28.69 | 25.92 | 2.77 | 18 |
Susquehanna R. | Lock Haven, PA | 26.84 | 23.83 | 3.01 | 21 |
Williamsport, PA | 31.44 | 26.71 | 4.73 | 20 |