Characterization of Peak Streamflows and Flooding in Select Areas of Pennsylvania from the Remnants of Hurricane Ida, September 1–2, 2021

Scientific Investigations Report 2023-5086
Prepared in cooperation with the Federal Emergency Management Agency
By: , and 

Links

Acknowledgments

The authors would like to thank the private homeowners and businesses who allowed us to access their property to identify, flag, and survey high-water marks after the flooding from Hurricane Ida. The authors would also like to thank the following U.S. Geological Survey field personnel who assisted with high-water mark documentation: Pat Anzman, Dylan Baumbach, Monica Conlin, Victor Cortes, Chad Coverstone, Dave Galeone, Nick Grim, Matt Gyves, Peter Kester, Jody McNerney, Aaron Menick, Dan Morrison, Mark Roland, Kurt Schmidt, Scott Sorber, Connor Souchek, Tony Spehar, Aaron Stephens, and Linda Zarr. In addition, the authors want to thank the Pennsylvania Water Science Center (WSC) Hydrologic Networks Program staff for their compilation and meticulous review of streamgage data that were used for our computation of flood-frequency estimates. Special thanks are extended to staff from the Pennsylvania, New England, and Ohio-Kentucky-Indiana WSCs who assisted with indirect streamflow measurements at 12 streamgages. The authors also thank our colleagues Victor Cortes and Mark Roland of the Pennsylvania WSC and Samuel Austin of the Virginia-West Virginia WSC who reviewed our manuscript and provided valuable feedback.

Abstract

Pennsylvania experienced heavy rainfall on September 1 and 2, 2021, as the remnants of Hurricane Ida swept over parts of the State. Much of eastern and south-central Pennsylvania received 5 to 10 inches of rain, and most of the rainfall fell within little more than 6 hours. Southeastern Pennsylvania experienced widespread, substantial flooding, and the city of Philadelphia and surrounding areas were particularly affected by the flooding. U.S. Geological Survey (USGS) streamgages registered peak streamflows of record at 19 locations, and 52 locations experienced top 5 peak streamflows for the period of record and an annual exceedance probability estimate of at least 10 percent. During this September 2021 flood event, USGS personnel made over 60 streamflow measurements at streamgages in Pennsylvania using direct and indirect methods. Many of those streamflow measurements were made to verify or improve the accuracy, extent, or development of new stage-streamflow relations at streamgages operated by the USGS. After the floodwaters receded, USGS personnel identified and documented a total of 338 high-water marks in Pennsylvania, noting such things as their general description, location, height above land surface, and quality. Many of these high-water marks were used to create five flood-documentation maps for selected communities in southeastern Pennsylvania that experienced substantial flooding because of the remnants of Hurricane Ida. Digital datasets of the inundated areas, mapped boundaries, and water depth are available (Stuckey and Conlon, 2023).

Introduction

The widespread, intense rainfall resulting from the remnants of Hurricane Ida caused historic flooding in eastern and south-central Pennsylvania on September 1–2, 2021. Much of this region received 5 to 10 inches of rain, and most rain fell in little more than 6 hours. Rain from the remnants of Hurricane Ida exceeded the rainfall characteristics of a 100-year (yr) storm at many locations in Pennsylvania—a 100-yr storm has a 1-percent chance of occurring in any given year (National Centers for Environmental Information, 2023a). (National Centers for Environmental Information, 2023a). This rainfall caused widespread, substantial flooding, and many streams and rivers experienced major flooding as defined by the National Weather Service (NWS) (National Weather Service, 2019). Some streamgages recorded their highest peaks on record, including two streamgages with over 100 years of record. The State of Pennsylvania declared a disaster emergency for the entire State before the storm, and the Federal Emergency Management Agency (FEMA) announced “Major Disaster Declarations” in eight Pennsylvania counties (Bedford, Bucks, Chester, Delaware, Montgomery, Northampton, Philadelphia, and York) (Federal Emergency Management Agency, 2023).

Flooding came in phases over the 2 days. Hours of torrential rainfall near and just west of the storm center’s track led to catastrophic flooding over parts of eastern Pennsylvania. Many smaller creeks overflowed their banks during this initial phase, and roads flooded because of overwhelmed drainage systems. The remnants of Hurricane Ida moved out of the region by the evening of September 1; however, the flooding was just beginning in the slower responding larger drainage area, main-stem rivers, which continued to rise during the second phase of flooding (National Weather Service, 2023b). This second phase led to additional flooding of homes and roads into the following day (September 2), and in some places, flooding continued for several days (fig. 1). Many roads were impassable, including parts of major highways such as the Vine Street Expressway running through downtown Philadelphia.

Figure 1. In first photograph, flood waters surround a building, vehicles, and railroad
                     tracks and in second photograph, the lower levels of buildings are submersed in floodwater.
Figure 1.

Photographs showing A, flooded railroad tracks and B, flood damage in Bridgeport, Pennsylvania, from the remnants of Hurricane Ida, September 2, 2021. Photographs by Andrew Reif, U.S. Geological Survey.

The U.S. Geological Survey (USGS) collects streamflow data from a network of more than 8,000 streamgages nationwide in cooperation with local, State, and Federal agencies. This network includes more than 350 streamgages currently (2022) operating in Pennsylvania. Streamflow data are critical for efforts to warn the population about flooding, forecast the extent and potential effects of flooding, and optimize the allocation of emergency management resources to the most severely affected areas. Long-term systematic streamflow data are essential for engineers conducting studies to mitigate the effects of flooding by designing or repairing roads, bridges, reservoirs, pipelines, houses, and other infrastructure. In the aftermath of the flooding caused by the remnants of Hurricane Ida, the USGS and FEMA initiated two cooperative studies to flag, survey, and formally document high-water marks (HWMs) and evaluate the flood’s magnitude, extent, and probability of occurrence at select USGS streamgages.

Purpose and Scope

The purpose of this report is to document HWM data, flood-peak magnitudes, and flood-mapping products generated by the USGS in support of the FEMA response and recovery operations after the September 1–2, 2021, flooding of eastern and south-central Pennsylvania caused by the remnants of Hurricane Ida. This report includes (1) a description of the atmospheric conditions during the remnants of Hurricane Ida and the temporal and spatial patterns of rainfall that triggered the flooding, (2) a narrative of the flood and its effects, (3) a description of how the 338 HWMs were documented, (4) an analysis of flood-peak magnitudes and their statistical probabilities at 52 USGS streamgages, and (5) a geographic information system (GIS) analysis of HWM locations and elevations to produce flood-documentation maps (areal extent of the maximum depth of flooding) for 5 heavily flooded areas in southeastern Pennsylvania. Data from this study are in tables and a separate data release (Stuckey and Conlon, 2023). The wording and presentation of the material in this report are based on previous USGS reports (Watson and others, 2017, 2018; Austin and others, 2018); the contents of each section are modified from these previous reports based on the specific flooding event.

Description of Study Area

The study area encompasses eastern and south-central Pennsylvania (fig. 2). Parts of 5 of the 6 physiographic provinces in Pennsylvania were flooded by the remnants of Hurricane Ida: the Appalachia Plateau, the Atlantic Coastal Plain, New England, the Piedmont, and Ridge and Valley (Pennsylvania Department of Conservation and Natural Resources, 2023); the Piedmont experienced most of the flooding across multiple watersheds during the event. The Delaware River Basin flooded, from north to south, in the Appalachian Plateau, Ridge and Valley, New England, Piedmont, and Atlantic Coastal Plain provinces. The northern part of the basin consists of rounded hills and valleys, and flat upper terrace surfaces cut by shallow valleys in the southern part with a mix of rolling lowlands and broad to shallow valleys in between. Land-surface-elevations within the Delaware River Basin range from 0 feet (ft) in the Atlantic Coastal Plain province to 2,690 ft in the Appalachian Plateau province, referenced to the North American Vertical Datum of 1988 (NAVD 88). The Lower Susquehanna and Potomac River Basins flooded in the north and west in the Appalachian Plateau and Ridge and Valley provinces and the south and east in the Piedmont province. Land-surface-elevations within the Lower Susquehanna and Potomac River Basins range in elevation from 20 ft in the Piedmont province to 3,210 ft in the Appalachian Plateau province, referenced to NAVD 88. In Pennsylvania, the 30-year normal annual rainfall (1991–2020) varied from 36.38 inches in Tioga County at Tioga Hammond Dam to 55.93 inches in Somerset County at Hidden Valley (National Centers for Environmental Information, 2023b).

Figure 2. Streamgages are generally concentrated in the southeastern part of the State
                        in the Atlantic Coastal Plain, Piedmont, and Ridge and Valley physiographic provinces.
Figure 2.

Map showing the location of the U.S. Geological Survey streamgages used in study and physiographic provinces in Pennsylvania (Pennsylvania Bureau of Topographic and Geologic Survey, 1998).

Weather Conditions Before and During the Flood

Hurricane Ida made landfall on the Louisiana coast as a powerful Category 4 hurricane with sustained winds of 150 miles per hour on August 29, 2021. The storm weakened into a tropical depression as it moved inland in a northeast direction. As Ida entered the mid-Atlantic region on September 1, 2021, it restrengthened into a post-tropical cyclone when it interacted with a stalled frontal system, resulting in heavy rainfall and widespread historic flooding (National Weather Service, 2023b). Leading up to the event, many of the streams and rivers had daily streamflow greater than the 75th percentile because of multiple flooding events from a wet summer, including substantial rainfall from Tropical Storm Fred a few weeks before Hurricane Ida in August (National Weather Service, 2023b). Fed by remnant tropical moisture from Hurricane Ida, torrential rain fell near and just west of the storm center’s track (fig. 3), becoming one of the area’s worst observed natural disasters (National Weather Service, 2023b). The rainfall from the remnants of Hurricane Ida exceeded the amount of rainfall considered to be a 100-yr storm at many locations in Pennsylvania (National Centers for Environmental Information, 2023a). Much of eastern and southeastern Pennsylvania received 5 to 10 inches of rain (fig. 3), mostly within little more than 6 hours. At USGS streamgage 01480617 West Branch Brandywine Creek at Modena in Chester County, over 8 inches of rain (maximum intensity of over 2.5 inches in 1 hour) was recorded on September 1, 2021 (U.S. Geological Survey, 2023c). Highest rainfall totals per county in eastern Pennsylvania as reported by the National Weather Service (NWS) in Mount Holly, New Jersey, are shown in table 1. Downingtown, Chester County, had the greatest amount of rainfall at 10.10 inches followed by Hatfield, Montgomery County, at 8.41 inches and Newtown, Bucks County, at 8.36 inches (table 1).

Figure 3. Highest precipitation fell in the southeastern part of the State with a
                     maximum 24-hour precipitation of 7.01 to 10.0 inches.
Figure 3.

Map showing precipitation over a 24-hour period, ending at 1200 coordinated universal time, September 2, 2021 (National Weather Service, 2023a).

Table 1.    

Rainfall totals reported from meteorological stations in eastern Pennsylvania during the September 1–2, 2021, flood (Data source National Weather Service, 2023b).

[Time is given as military time. Dates are given as month/day/year. in., inch; EST, eastern standard time; CWOP, Citizen Weather Observer Program; COCORAHS, Community Collaborative Rain, Hail, and Snow Network; ASOS, Automated Surface Observing Systems]

Location County Rainfall amount (in.) Time (EST) Date Meteorological station operator
Huffs Church Berks 7.43 1200 9/2/2021 Trained Spotter
Newtown Bucks 8.36 1330 9/2/2021 CWOP
Lake Harmony 2.4 WNW Carbon 5.66 0700 9/2/2021 COCORAHS
Downingtown Chester 10.10 2140 9/1/2021 Trained Spotter
Thornton Delaware 4.00 1326 9/2/2021 CWOP
Center Valley Lehigh 6.99 1325 9/2/2021 CWOP
Mt. Pocono Monroe 5.56 0653 9/2/2021 ASOS
Hatfield Montgomery 8.41 0850 9/2/2021 CWOP
Bethlehem Northampton 7.50 1200 9/2/2021 Public
Philadelphia 4.7 NE Philadelphia 3.70 0530 9/2/2021 COCORAHS
Table 1.    Rainfall totals reported from meteorological stations in eastern Pennsylvania during the September 1–2, 2021, flood (Data source National Weather Service, 2023b).

Methods

The methods used to identify, document, and reference HWMs from the flooding caused by the remnants of Hurricane Ida and to create flood-documentation maps using these HWMs are discussed in this section. Also discussed are flood magnitude and frequency estimation methods using the annual peak streamflows of 52 streamgages operated by the USGS. All streamflow data used in this report may be accessed from the USGS National Water Information System (U.S. Geological Survey, 2023c).

Annual exceedance probability (AEP) flood for a streamgage is the probability of a streamflow being equaled or exceeded during a given year and is determined from the recorded annual peak streamflow for the streamgage. For example, an AEP of 0.01 means there is a 1-percent chance that a specific peak streamflow will occur at a given location in a given year. Stated another way, the odds are 1 in 100 that the indicated streamflow will be equaled or exceeded in a given year. AEP is directly related to the traditional concept of defining floods by recurrence interval, which is the average number of years between floods of a certain size. By definition, the recurrence interval (in years) is equal to one divided by the AEP. For example, the AEP of 0.01 (or 1 percent) flood corresponds to what has historically been termed “the 100-year flood” (a flood with a 100-year recurrence interval) (Holmes and Dinicola, 2010).

Collection of High-Water-Mark Data

High-water marks are the evidence of the maximum water levels observed during a flood, and they are valuable for efforts to understand flood events. The USGS followed the guidance provided in Koenig and others (2016) to identify and document HWMs. Small seeds or debris carried by flood waters that adhere to smooth surfaces or lodge in tree bark to form a distinct line are the best HWMs. Stain lines on buildings, fences, and other structures also provide excellent HWMs. HWMs are best identified immediately after the peak stage of a flood event because they may blow or wash away from the weather (wind, rain, or sun) or fade away with time.

Working with FEMA under the direction of a Mission Assignment immediately after the flood, USGS field crews identified 255 HWMs in Pennsylvania on stream and river reaches that were selected using input obtained from FEMA, the NWS, U.S. Army Corps of Engineers, Delaware and Susquehanna River Basin Commissions, and Pennsylvania Emergency Management Agency. Examples of the identified HWMs are shown in fig. 4. Of those 255 HWMs, 198 were surveyed to determine the elevation above land surface using NAVD 88. HWMs were identified and marked beginning September 5, 2021, and continuing through September 16, 2021. Written descriptions, sketches, photographs, and global positioning system horizontal measurements (obtained with a hand-held global positioning system unit) were made so these marks could be easily found later and surveyed to the standard vertical datum (NAVD 88). Descriptions and photographs of these HWMs are available for viewing or download from the USGS Flood Event Viewer (U.S. Geological Survey 2021). In addition, 83 HWMs were identified and surveyed by the USGS as part of indirect computations of peak flow at select streamgages, bringing the total number of HWMs documented from the flooding that occurred on September 1–2, 2021, to 338. Indirect computations of peak flows are sometimes needed when a direct measurement of streamflow cannot be made because of unsafe conditions, lack of access, or timing issues and are made after a flood. The number of identified and surveyed HWMs and associated water bodies are listed in table 2 and are individually shown in fig. 5. A full listing of all HWMs is in Stuckey and Conlon (2023).

Figure 4. High-water marks are several feet above ground surface.
Figure 4.

Photographs of select high-water marks identified after the flooding of the Schuylkill River and East Branch Perkiomen Creek on September 1–2, 2021. A, a mud line outside of a building; B, a seed line on a windowpane; C, a seed line on the back of a sign; and D, a seed line inside of a building. White dashed line represents high-water line. Photograph by the U.S. Geological Survey.

Table 2.    

Site descriptions and number of high-water marks identified and surveyed in Pennsylvania after the September 1–2, 2021, flood.

[Twp, township; --, not applicable]

Associated Waterbody Community or communities County or counties Number of high-water marks identified and marked1 Number of high-water marks surveyed1 Number of additional high-water marks identified and surveyed2
Brandywine Creek Basin
Brandywine Creek Birmingham Twp, Pocopson Twp, Pennsbury Twp, Chadds Ford Twp Chester, Delaware 32 22 --
West Branch Brandywine Creek Valley Twp, Coatesville City, Modena Twp, Modena Borough, East Fallowfield Twp, Newlin Twp Chester 14 11 35
East Branch Brandywine Creek Uwchlan Twp, Downingtown Borough, Caln Twp, East Caln Twp, West Bradford Twp, East Bradford Twp, East Brandywine Twp Chester 13 12 31
Lackawaxen River Basin
Lackawaxen River Hawley Borough, Palmyra Twp Wayne 5 0 --
Lehigh River Basin
Jordan Creek Allentown City Lehigh 12 12 --
Little Lehigh Creek Allentown City Lehigh 9 8 --
Lehigh River Easton City Northampton 15 13 --
Neshaminy Creek Basin
Neshaminy Creek Wrightstown Twp, Northampton Twp Bucks 4 3 --
Schuylkill River Basin
East Branch Perkiomen Creek East Rockhill Twp, Sellersville Borough, Lower Salford Twp, Perkasie Borough, Skippack Twp Montgomery 15 14 --
Perkiomen Creek Perkiomen Twp, Skippack Twp, Lower Providence Twp, Upper Providence Twp, Collegeville Borough Montgomery, Bucks 26 21 --
Pickering Creek Schuylkill Twp Chester 3 3 --
Schuylkill River West Norristown Twp, Norristown Borough, Bridgeport Borough, Whitemarsh Twp, Conshohocken Borough, West Conshohocken Borough, Lower Merion Twp, Philadelphia City Montgomery, Philadelphia 59 52 17
Codorus Creek Basin
Codorus Creek Spring Garden Twp, West Manchester Twp, York City, Manchester Twp, Springettsbury Twp York 8 7 --
Conestoga River Basin
Conestoga River Lancaster City, Manheim Twp, Lancaster Twp, East Lampeter Twp, West Lampeter Twp Lancaster 12 9 --
Juniata River Basin
Frankstown Branch Williamsburg Borough, Catharine Twp Blair 6 6 --
Raystown Branch Liberty Twp Bedford 6 5 --
Lackawanna River Basin
Lackawanna River Old Forge Borough, Scranton City Lackawanna 8 0 --
Swatara Creek Basin
Swatara Creek East Hanover Twp, North Annville Twp Lebanon 8 0 --
Table 2.    Site descriptions and number of high-water marks identified and surveyed in Pennsylvania after the September 1–2, 2021, flood.
1

Collected as part of a Federal Emergency Management Agency Mission Assignment.

2

Collected as part of U.S. Geological Survey indirect streamflow measurements, included in flood-documentation map development.

Figure 5. High-water marks are generally located in southeastern Pennsylvania. High-water
                        marks are also located in northeastern and central Pennsylvania.
Figure 5.

Map showing the location of identified high-water marks in Pennsylvania from the September 2021 flood.

Peak-Flow Data and Streamgage Selection

During the Hurricane Ida remnant storm that resulted in flooding across eastern and southcentral Pennsylvania on September 1–2, 2021, USGS personnel made over 50 streamflow measurements using direct methods at streamgages in the study area (Rantz, 1982; Turnipseed and Sauer, 2010). In addition, in the months after the storm, USGS personnel computed streamflow measurements using indirect methods (Benson and Dalrymple, 1967; Rantz, 1982) at 12 locations. Many of the streamflow measurements were made to verify the accuracy of the stage-streamflow rating curve or extend the stage-streamflow rating curve for a given streamgage (Rantz, 1982). The stage-streamflow rating curve for a given streamgage is used to produce instantaneous streamflow values for a given streamgage, and the maximum instantaneous value for a given year is termed the annual peak streamflow (U.S. Geological Survey, 2023c). Streamgages with a long, annual peak streamflow record (more than 40 years) are considered the most reliable for estimating AEPs.

Magnitude and frequency statistics for floods were determined from annual peak streamflows measured or recorded at 52 USGS streamgages (fig. 2). Streamgages with at least 10 years of continuously recorded data were included in this study if those datasets met at least one of the following criteria: (1) the instantaneous peak from the September 1–2, 2021, flood event was the largest peak recorded during the period of record (peak of record) or (2) the recurrence interval of the instantaneous peak of September 1–2, 2021, was estimated to be at least 10-percent AEP and 1 of the 5 largest peak streamflow magnitudes recorded at the site over the period of record. If streamflow was regulated by a flood control reservoir during the period of record at a site, only the period after regulation was included in the analysis. The periods of record for the selected 52 streamgages ranged from 10 to 107 years, with a mean of 45 years. Annual peak-flow data for these streamgages were retrieved from the USGS National Water Information System (NWIS) website and carefully reviewed for completeness of record and appropriateness of historic peaks and qualification codes (U.S. Geological Survey, 2023c).

The annual peak flows of the 52 streamgages were analyzed to determine whether trends existed in their respective periods of record using a Mann-Kendall statistical test. Trends in annual peak flows can affect a flood frequency analysis by violating the assumption that the annual peak-flow data are a reliable and representative time sample of random homogeneous events (England and others, 2018). The results of the Mann-Kendall test showed that 17 of the 52 streamgages (33 percent) exhibited significant trends at the 90-percent confidence interval (p<0.10) (fig. 6). Data were evaluated for autocorrelation to confirm there was no serial correlation in the annual time series that would affect the results of the trend test (England and others, 2018). All significant trends were positive, indicating annual peak flows were increasing over time. Of the remaining 35 streamgages without a significant trend, 27 had a positive trend and 8 had a negative (decreasing streamflow) trend. A comparison of streamgages that were evaluated for trends by Roland and Stuckey (2020) and this current study show five streamgages that had changes in trend significance (at the 90-percent confidence interval); four streamgages now have a significant positive trend and one streamgage no longer has a significant positive trend. These trends in annual peak flows may be characterized by several factors, including length of the period of record, variations in precipitation over time, land use changes, or single causal factors (for example, the construction of a dam). Because the cause and longevity of the significant trends are uncertain to continue in the future, no streamgages were excluded from this flood-frequency analysis because of significant trends.

Figure 6. Significant positive trends are generally at streamgages in southeastern
                        Pennsylvania. Inset graph shows increasing linear trend at Valley Creek 01473169.
Figure 6.

Map showing select U.S. Geological Survey streamgages within Pennsylvania and trends in annual peak flows.

Flood-Frequency Analysis

The magnitude and frequency of flood flows at the 52 streamgages were computed using the expected moments algorithm (EMA) methodology (England and others, 2018), which fits a log-Pearson Type III probability distribution curve to annual peak streamflow data. The Interagency Advisory Committee on Water Data provides standard methods for computing peak streamflow frequency in its “Bulletin 17C” (England and others, 2018). Although Bulletin 17C maintains the moments-based approach of the previous Bulletin 17B procedures (Interagency Advisory Committee on Water Data, 1982), the method outlined in Bulletin 17C uses the EMA procedure (Cohn and others, 1997), which incorporates a generalized version of the multiple Grubbs-Beck test for the detection of low outliers (Cohn and others, 2013). EMA can also incorporate interval peak-flow data, which simplifies the analysis of datasets containing censored observations, historical data, low outliers, and uncertain data points, while providing enhanced confidence intervals for the estimated streamflows (Veilleux and others, 2014).

The log-Pearson Type III analyses were performed using the USGS program PeakFQ version 7.4 (Veilleux and others, 2014), following methods outlined in Bulletin 17C (England and others, 2018). Estimates of the annual peak-flow probabilities were computed in PeakFQ by determining the mean, standard deviation, and skew of the log-transformed annual peak-flow data and inserting the three statistics of the frequency distribution into the following equation:

log Q p = X + K p S
where

Qp

is the flood magnitude at a selected exceedance probability p;

X

is the mean of the logarithms of the annual peak flows;

Kp

is a factor based on the skew coefficient and the selected percentage AEP; and

S

is the standard deviation of the logarithms of the annual peak flows.

As recommended in Bulletin 17C, a weighted average of the station skew and regional skew was used to determine flood frequencies for most streamgages, unless noted below. A regional skew of 0.343 (mean standard error of 0.178) was determined for Pennsylvania by Roland and Stuckey (2020) and used in this analysis. Note, potential computational issues related to the weighted skew calculation in PeakFQ remain unresolved at the time of preparation for publication (February 2023) (U.S. Geological Survey, 2023b). In some cases, the regional skew may be given too much weight over the station skew, particularly if the regional and station skew have a large difference (greater than 0.50). In this study, large differences between the regional and station skew were observed at eight streamgages (01467042, 01472198, 01472199, 01472620, 01472810, 01473000, 01474000, 01572025; table 3. One of these eight streamgages (01473000) had over 40 years of record, and the station skew coefficient option was used to avoid the potential weighting issue; the PeakFQ output for the remaining seven streamgages was evaluated for reasonableness and included in the analysis. The station skew coefficient option was used for streamgages with flow regulated by an upstream flood control reservoir because the regional skew was developed using unregulated watersheds and is not appropriate for use in regulated watersheds. A regulated watershed has at least 10 percent of the area upstream of its streamgage controlled by a reservoir.

Table 3.    

Site descriptions, summary statistics, equivalent recurrence intervals, and exceedance probabilities, for select streamgages associated with September 2021 flooding in Pennsylvania.

[Dates shown as month/day/year. USGS, U.S. Geological Survey; mi2, square miles; AEP, annual exceedance probability; ft, foot; ft3/s, cubic feet per second; PA, Pennsylvania; %, percent; <, less than; >, greater than]

USGS station number USGS station name Latitude (decimal degrees) Longitude (decimal degrees) Drainage area (mi2) Number of annual peaks in analysis September 2021 flood data AEP for observed September 2021 flood
Date of peak streamflow Peak gage height (ft) Peak streamflow (ft3/s) Period of record rank for peak streamflow Estimate Equivalent recurrence interval (years) Upper 66.7-percent confidence interval1 Lower 66.7-percent confidence interval1
01431500 Lackawaxen River at Hawley, PA 41.476200 −75.172119 290 62 9/2/2021 14.31 17,300 4 3.9% 26 9.2% 3.5%
01432110 Lackawaxen River at Rowland, PA 41.475923 −75.036281 589 14 9/2/2021 13.39 25,100 1 3.2% 31 12.0% 1.3%
01449000 Lehigh River at Lehighton, PA 40.829259 −75.705189 591 40 9/2/2020 14.11 27,100 1 3.2% 32 4.4% 0.5%
01451000 Lehigh River at Walnutport, PA 40.757040 −75.602964 889 61 9/2/2021 13.19 45,300 3 3.2% 31 7.3% 2.3%
01451650 Little Lehigh Creek at Tenth Street Bridge at Allentown 40.596486 −75.474071 98.2 35 9/1/2021 9.6 7,610 5 8.4% 12 19.3% 8.4%
01451800 Jordan Creek near Schnecksville, PA 40.661762 −75.626854 53 55 9/1/2021 11.28 5,880 4 6.4% 16 10.3% 3.9%
01452000 Jordan Creek at Allentown, PA 40.623153 −75.482405 75.8 77 9/2/2021 10.71 13,300 4 1.7% 59 7.4% 2.8%
01453000 Lehigh River at Bethlehem, PA 40.615376 −75.378791 1,279 61 9/2/2021 19.53 56,000 5 3.8% 26 11.3% 4.8%
01454700 Lehigh River at Glendon, PA 40.669266 −75.236288 1,359 55 9/2/2021 24.11 58,100 3 3.7% 27 8.1% 2.6%
01459500 Tohickon Creek near Pipersville, PA 40.433715 −75.116561 97.4 48 9/1/2021 11.18 15,600 4 5.3% 19 11.8% 4.5%
01464645 North Branch Neshaminy Creek below Lake Galena near New Britain, PA 40.312328 −75.206563 16.2 36 9/1/2021 5.4 2,350 1 2.5% 40 4.9% 0.5%
01464720 North Branch Neshaminy Creek at Chalfont, PA 40.288162 −75.203785 31.5 31 9/1/2021 11.58 7,290 1 5.2% 19 5.6% 0.6%
01464750 Neshaminy Creek near Rushland, PA 40.260385 −75.034892 91 26 9/2/2021 21.83 13,300 1 2.8% 36 6.7% 0.7%
01464907 Little Neshaminy Creek at Valley Road nr Neshaminy PA 40.229275 −75.119616 26.8 23 9/1/2021 12.78 8,740 3 3.3% 30 18.7% 6.2%
01465200 Neshaminy Creek near Penns Park, PA 40.251842 −75.008708 157 20 9/2/2021 24.46 26,400 1 1.7% 59 8.6% 0.9%
01465500 Neshaminy Creek near Langhorne, PA 40.173998 −74.956834 210 88 9/2/2021 20.81 27,000 5 4.6% 22 7.9% 3.3%
01467042 Pennypack Creek at Pine Road, at Philadelphia, PA 40.089833 −75.069061 37.9 31 9/2/2021 14.6 14,600 1 0.4%2 245 5.6% 0.6%
01470500 Schuylkill River at Berne, PA 40.522593 −75.998268 355 75 9/2/2021 15.67 26,000 5 7.3% 14 9.2% 3.9%
01470755 Maiden Creek near Virginville, PA3 40.525372 −75.874925 157 33 9/2/2021 -- 14,400 3 3.8% 26 13.3% 4.3%
01470779 Tulpehocken Creek near Bernville, PA 40.413426 −76.171613 70.4 48 9/1/2021 10.37 5,360 5 8.9% 11 14.3% 6.1%
01471875 Manatawny Creek near Spangsville, PA 40.339538 −75.742136 56.9 28 9/1/2021 8.54 4,970 4 9.6% 10 19.8% 7.7%
01472157 French Creek near Phoenixville, PA 40.151491 −75.601305 59.1 53 9/1/2021 12.32 7,660 4 7.0% 14 10.7% 4.1%
01472198 Perkiomen Creek at East Greenville, PA 40.393988 −75.515459 38 40 9/1/2021 9.24 8,850 2 3.4% 30 7.9% 1.8%
01472199 West Branch Perkiomen Creek at Hillegass, PA 40.373988 −75.522403 23 40 9/1/2021 6.42 3,400 4 7.3% 14 14.1% 5.4%
01472620 East Branch Perkiomen Creek near Dublin, PA 40.403993 −75.234342 4.05 38 9/1/2021 10.66 2,890 1 2.2% 46 4.6% 0.5%
01472810 East Branch Perkiomen Creek near Schwenksville, PA 40.258714 −75.428787 58.7 30 9/2/2021 16.88 18,000 1 1.4% 72 5.8% 0.6%
01473000 Perkiomen Creek at Graterford, PA 40.229547 −75.451567 279 107 9/2/2021 24.23 64,400 1 <0.2%2 >500 1.7% 0.2%
01473169 Valley Creek at PA Turnpike Bridge near Valley Forge 40.079274 −75.460748 20.8 39 9/1/2021 16.44 7,930 1 0.7%2 141 4.5% 0.5%
01473500 Schuylkill River at Norristown, PA 40.111219 −75.346850 1,760 36 9/2/2021 26.88 119,000 1 1.2% 81 4.9% 0.5%
01473900 Wissahickon Creek at Fort Washington, PA 40.123999 −75.219899 40.8 41 9/1/2021 17.74 13,700 2 1.0%2 104 7.7% 1.8%
01474000 Wissahickon Creek at Mouth, Philadelphia, PA 40.015390 −75.206846 64 56 9/2/2021 11.05 17,800 2 0.9%2 117 5.7% 1.3%
01474500 Schuylkill River at Philadelphia, PA 39.967891 −75.188512 1,893 43 9/2/2021 16.35 125,000 1 0.9%2 106 4.1% 0.4%
01475850 Crum Creek near Newtown Square, PA 39.976499 −75.436586 15.8 45 9/1/2021 12.33 4,030 3 5.5% 18 9.8% 3.1%
01476480 Ridley Creek at Media, PA 39.916222 −75.403250 30.5 34 9/2/2021 11.61 4,940 5 9.4% 11 19.9% 8.7%
01478120 East Branch White Clay Creek at Avondale, PA 39.828443 −75.780773 11.3 14 9/1/2021 9.75 2,600 1 5.4% 19 12.0% 1.3%
01480500 West Branch Brandywine Creek at Coatesville, PA 39.985661 −75.827447 45.8 61 9/1/2021 9.62 7,120 4 4.9% 21 9.3% 3.5%
01480617 West Branch Brandywine Creek at Modena, PA 39.961773 −75.801334 55 52 9/1/2021 13.36 12,300 1 1.3% 75 3.4% 0.4%
01480700 East Branch Brandywine Creek near Downingtown, PA 40.034828 −75.708549 60.6 48 9/1/2021 13.6 16,100 1 <0.2%2 >500 3.7% 0.4%
01480870 East Branch Brandywine Creek below Downingtown, PA 39.968719 −75.673272 89.9 48 9/1/2021 19.31 21,000 1 0.2%2 432 3.7% 0.4%
01481000 Brandywine Creek at Chadds Ford, PA 39.869833 −75.593262 287 102 9/2/2021 21.04 49,000 1 <0.2%2 >500 1.7% 0.2%
01494850 East Branch Big Elk Creek at Forestville, PA 39.822331 −75.896888 9.69 14 unknown 15.26 3,030 1 6.2% 17 12.0% 1.3%
01560000 Dunning Creek at Belden, PA 40.071747 −78.492517 172 83 9/2/2021 13.51 14,800 4 2.4% 42 6.9% 2.6%
01564512 Aughwick Creek near Shirleysburg, PA 40.28202 −77.890553 301 32 9/2/2021 15.51 18,500 3 9.2% 11 13.7% 4.4%
01572025 Swatara Creek near Pine Grove, PA 40.53259 −76.402179 116 33 9/1/2021 15.31 11,300 4 6.3% 16 16.9% 6.5%
01572190 Swatara Creek near Inwood, PA 40.479256 −76.530797 167 33 9/2/2021 17.35 12,500 4 9.4% 11 16.9% 6.5%
01573695 Conewago Creek near Bellaire, PA 40.195278 −76.567778 20.5 10 9/1/2021 8.81 2,490 2 8.3% 12 28.9% 7.4%
01573710 Conewago Creek near Falmouth, PA 40.149306 −76.687750 47.5 11 9/1/2021 14.71 6,000 2 9.3% 11 26.6% 6.7%
01573825 West Conewago Creek at East Berlin, PA 39.941222 −76.990000 218 18 9/1/2021 17.96 13,500 3 8.6% 12 23.6% 7.9%
01576500 Conestoga River at Lancaster, PA 40.050097 −76.277180 324 93 9/2/2021 18.5 23,000 4 3.9% 25 6.2% 2.3%
01576754 Conestoga River at Conestoga, PA 39.946489 −76.367739 470 37 9/2/2021 16.27 22,300 2 7.6% 13 8.5% 2.0%
01601000 Wills Creek below Hyndman, PA 39.812029 −78.716407 146 54 9/1/2021 13.02 15,500 2 1.5% 67 5.9% 1.4%
01613050 Tonoloway Creek near Needmore, PA 39.898421 −78.132229 10.7 59 9/1/2021 8.53 1,130 4 8.1% 12 9.6% 3.6%
Table 3.    Site descriptions, summary statistics, equivalent recurrence intervals, and exceedance probabilities, for select streamgages associated with September 2021 flooding in Pennsylvania.
1

The AEP 66.7-percent confidence interval is a nonparametric estimate based on a flood of a given rank within a specified period of record. These confidence intervals are not centered about the interpolated AEP estimate. Consequently, the range of the confidence interval may exclude the AEP estimate.

2

Streamgages with AEP estimates less than 1-percent, also highlighted in gray.

3

Peaks include those from sites 01470756 (not listed) and 01470755.

The resulting flood frequencies for unregulated streamgages were weighted using the variance of independent estimates to reduce uncertainty as described in Bulletin 17C (England and others, 2018). Observed at-site flood-frequency estimates and predicted regression estimates from Roland and Stuckey (2020) are weighted in inverse proportion to their respective independent variances. Flood frequencies for regulated streamgages are not weighted because regression equations are not applicable. Flood frequencies associated with the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent AEPs for the 52 streamgages in Pennsylvania are available in Stuckey and Conlon (2023).

Flood-Probabilities of Peak Streamflow

After a substantial flood, emergency managers and water resource engineers often need to know the expected frequency of peak streamflow for the observed streamflow to provide information on the rarity of the event. The September 1–2, 2021, peak streamflows were analyzed using PeakFQ software version 7.4 (Veilleux and others, 2014) following guidance provided in the USGS Office of Surface Water Technical Memorandum 2013.01 (U.S. Geological Survey, 2012). The AEP for the flood event was estimated through the log-linear interpolation of flood frequencies determined from PeakFQ using the following equation:

A E P O b s = A E P 1 + A E P 2 A E P 1 × log 10 Q O b s log 10 Q 1 log 10 Q 2 log 10 Q 1  
where

AEPObs

is the AEP of the flood event;

AEP1

is the AEP value associated with the lower flow Q1 bracketing the flood event;

AEP2

is the AEP value associated with the upper flow Q2 bracketing the flood event;

QObs

is the flow from the flood event;

Q1

is the lower flow bracketing the flood event; and

Q2

is the upper flow bracketing the flood event.

Assigning an AEP to an observed flood results in greater uncertainty than determining traditional flood frequencies because the lack of reference data for the extreme flood being assessed. Limits based on the 66.7-percent confidence level were computed for each estimated AEP to convey this uncertainty.

Flood-Documentation Mapping

Flood-documentation maps were created using a geographic information system (GIS) approach to show the extent of inundation from the September 1–2, 2021, flooding along five selected river reaches in southeastern Pennsylvania that contained HWMs in sufficient number and density (fig. 7). These flood-documentation maps are intended to be estimates of the areal extent and the maximum depth of flooding that correspond to the HWMs identified and surveyed by USGS field crews after the flood event. The community, county, waterbodies, reach lengths, and number of HWMs used to generate the flood-documentation maps are listed in table 4.

Figure 7. Locations of flood-documentation maps are in southeastern Pennsylvania.
Figure 7.

Map showing the locations of the flood-documentation maps created to show the extent of flooding caused by the remnants of Hurricane Ida on September 1–2, 2021.

Table 4.    

Site descriptions of high-water marks in southeastern Pennsylvania used to generate flood-documentation maps for the September 1–2, 2021, flood.

Community or communities Associated Waterbody County or counties Approximate reach length (miles) Number of high-water marks used to generate flood-documentation map
East Bradford, East Brandywine, East Caln, Uwchlan, and West Bradford Townships and Downingtown Borough East Branch Brandywine Creek Chester 5 43
Modena Borough West Branch Brandywine Creek Chester 0.5 40
Birmingham, Chadds Ford, Pennsbury, and Pocopson Townships Brandywine Creek Chester, Delaware 9 22
Lower Providence, Lower Salford, Perkiomen, Skippack Townships, and Upper Providence Townships and Collegeville and Schwenksville Boroughs Perkiomen Creek Montgomery 8 23
Upper Merion and West Norriton Townships and Bridgeport and Morrisville Boroughs Schuylkill River Montgomery 4 26
Table 4.    Site descriptions of high-water marks in southeastern Pennsylvania used to generate flood-documentation maps for the September 1–2, 2021, flood.

A flood-elevation raster surface was created in GIS to generate the flood-documentation maps. GIS surfaces documenting the areal extent of the maximum depth of flooding were created independently for each area that was mapped using the point elevations of the HWMs and streamgages by using a GIS interpolation technique—the ArcGIS Topo to Raster tool (Esri, 2021) described by Musser and others (2016). A geographic areal boundary limit was placed on the generated surface because of the distribution of HWMs and an understanding of the natural hydrologic flow in each area that was mapped.

The flood-elevation raster surface that was created using GIS interpolation was then combined with 1-meter digital elevation model (DEM) data with a 10-meter cell size. The DEM was derived from light detection and ranging (lidar) data that had a 10-centimeter vertical root-mean-square-error and a nominal point spacing of 2 points per square meter or better (U.S. Geological Survey, 2023a). A DEM is a digital point representation of a surface; in this instance, the DEM is a topographical representation of the earth’s surface in the study area. An inundated area was depicted where the flood-elevation surface was higher than the DEM land surface. The maximum depth of flooding was calculated as the difference between the flood-elevation surface and the DEM land surface. Because of the many bridges in the mapped reaches (common in Pennsylvania), the inundation surfaces were not modified to show any bridges that were not inundated.

The flood-elevation interpolation method was used to develop the water-surface elevation layer from points represented by the HWM dataset and the USGS streamgage peak stage water-surface elevation values (where available). Interpolation was conducted between these elevations to generate the water-surface elevation using the ArcGIS Topo to Raster tool with the point interpolation procedure (Esri, 2021).

Uncertainties in the mapped extent and calculated depth of flooding within the maps result from the interpolation methods described above and the number and spatial distribution of HWMs within each mapped reach. Hydraulic models were not used to determine the extent or depth of flooding. Without hydraulic models, factors that can affect the extent or depth of flooding are not accounted for using this GIS interpolation methodology. Factors such as land-surface features, changes in flood plains, as well as the timing of the flooding event in the smaller inflow tributaries versus the larger main-stem tributaries and intermingling flows from adjacent streams, likely contribute to the extent and impact of flooding. In locations where HWMs are at a comparably greater distance of separation, spatial interpretations of the extent and depth of flooding may be less accurate. Within a given mapped area, some extrapolation was used beyond the most upstream and downstream HWMs. Often, the mapping boundary was extended to some anthropogenic structure, such as a road or bridge crossing.

The flood-documentation maps as described and shown in this report should not be used for navigation or regulatory, permitting, or other legal purposes. The USGS provides these maps “as is” for a quick reference, emergency planning tool but assumes no legal liability or responsibility resulting from the use of the maps. The maps shown in figs. 812 only depict the September 2021 remnant Hurricane Ida flood event on select stream reaches in southeastern Pennsylvania and may not be representative of other flooding conditions.

Figure 8. Near Downingtown, an extensive area bordering the creek flooded.
Figure 8.

Map showing the inundated area along the East Branch Brandywine Creek in Downingtown, Pennsylvania, September 1–2, 2021.

Figure 9. Near Modena, an extensive area bordering the creek flooded.
Figure 9.

Map showing the inundated area along the West Branch Brandywine Creek in Modena, Pennsylvania, September 1–2, 2021.

Figure 10. Near Chadds Ford, an area bordering the creek flooded.
Figure 10.

Map showing the inundated area along the Brandywine Creek near Chadds Ford, Pennsylvania, September 1–2, 2021.

Figure 11. Near Schwenksville, Graterford, and Collegeville, an area bordering the
                        creek flooded.
Figure 11.

Map showing the inundated area along the Perkiomen Creek near Graterford and Collegeville, Pennsylvania, September 1–2, 2021.

Figure 12. Near Norristown and Bridgeport, an extensive area outside the stream channel
                        flooded.
Figure 12.

Map showing the inundated area along the Schuylkill River near Norristown and Bridgeport, Pennsylvania, September 1–2, 2021.

Estimated Magnitudes and Exceedance Probabilities for Peak Streamflows

Peak gage-height data, streamflow data, and associated AEPs with 66.7-percent confidence intervals determined for the September 2021 flood event from the 52 USGS streamgages analyzed are listed in table 3. Streamgages had 10 to 107 years of peak streamflow records; and the mean was 45 years. New peak streamflows of record (peak of record) were observed at 19 of the 52 USGS streamgages (table 3; Stuckey and Conlon, 2023). Peak streamflows ranked second or third among the highest peaks of record at another 14 streamgages. At the 19 remaining streamgages, peak streamflows ranked fourth or fifth among the highest peaks of record measured at these stations. The AEP estimates for the analyzed streamgages during the September 2021 flood ranged from less than 0.2 to 10 percent (greater than a 500-year recurrence interval [RI] to a 10-year RI). The September 2021 AEP at nine streamgages is estimated to be less than or equal to the 1 percent (at least a 100-year RI); these nine streamgages are in Pennypack Creek, the Schuylkill River, and the Brandywine Creek Basins. Two streamgages with over 100 years of record have less than a 0.2-percent AEP or greater than a 500-year RI for the September 2021 flood (01473000 Perkiomen Creek at Graterford and 01481000 Brandywine Creek at Chadds Ford).

Flood frequencies associated with the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent AEPs were determined for 52 streamgages in Pennsylvania using EMA methods and are available in Stuckey and Conlon (2023). Comparisons of AEP results were made using these updated flood frequencies and flood frequencies using data through 2015 published by Roland and Stuckey (2020) to determine the percentage change since the flood frequencies were last published. Percentage changes in the 1-percent AEP flood frequencies (100-year RI) for 45 streamgages included in both analyses ranged from −2 percent to 57 percent, with a mean change of 9 percent (table 5). Of the 45 streamgages, 5 had a negative percentage change in the 1-percent AEP (100-year RI), indicating the 1-percent AEP estimate has decreased since last computed, and 36 had a positive percentage change, indicating the 1-percent AEP is larger than previously computed.

Table 5.    

Summary of percentage changes from previously published peak-flow annual exceedance probabilities for select streamgages associated with September 1–2, 2021, flooding in Pennsylvania.

[Previously published flood frequencies from Roland and Stuckey (2020) using data through 2015. Flood-frequency method determined from weighted estimates (WTD) or the at-site expected moments algorithm (EMA). USGS, U.S. Geological Survey; PA, Pennsylvania; %, percent; ---, not included in Roland and Stuckey (2020)]

USGS station number USGS station name Number of peaks used in current analysis Flood-frequency method Change of peak-flow exceedance values from previously published values (percent)
Exceedance probability
0.5 0.2 0.1 0.04 0.02 0.01 0.005 0.002
01431500 Lackawaxen River at Hawley, PA 62 EMA 0% 0% 0% 0% 1% 1% --- ---
01432110 Lackawaxen River at Rowland, PA 14 --- --- --- --- --- --- --- --- ---
01449000 Lehigh River at Lehighton, PA 40 EMA −3% −2% 0% 1% 3% 5% --- ---
01451000 Lehigh River at Walnutport, PA 61 EMA −2% −1% 0% 1% 3% 4% --- ---
01451650 Little Lehigh Creek at Tenth Street Bridge at Allentown 35 WTD 12% 14% 14% 13% 12% 11% 11% 10%
01451800 Jordan Creek near Schnecksville, PA 55 WTD 3% 4% 4% 5% 5% 5% 5% 6%
01452000 Jordan Creek at Allentown, PA 77 EMA 2% 6% 8% 10% 12% 13% 14% 15%
01453000 Lehigh River at Bethlehem, PA 61 EMA 2% 2% 1% 1% 2% 1% --- ---
01454700 Lehigh River at Glendon, PA 55 EMA 2% 2% 2% 2% 2% 2% --- ---
01459500 Tohickon Creek near Pipersville, PA 48 EMA 0% 0% 1% 1% 2% 3% --- ---
01464645 North Branch Neshaminy Creek below Lake Galena near New Britain, PA 36 EMA −3% 0% 3% 7% 10% 14% --- ---
01464720 North Branch Neshaminy Creek at Chalfont, PA 31 EMA −9% −4% 3% 16% 27% 39% --- ---
01464750 Neshaminy Creek near Rushland, PA 26 WTD 2% 3% 4% 5% 5% 6% 6% 6%
01464907 Little Neshaminy Creek at Valley Road near Neshaminy PA 23 WTD −1% −1% 0% 1% 2% 1% 2% 2%
01465200 Neshaminy Creek near Penns Park, PA 20 WTD −2% 0% 1% 3% 4% 4% 5% 6%
01465500 Neshaminy Creek near Langhorne, PA 88 WTD −1% −1% −1% 0% 0% 0% 1% 1%
01467042 Pennypack Creek at Pine Road, at Philadelphia, PA 31 WTD 4% 8% 9% 11% 11% 11% 11% 12%
01470500 Schuylkill River at Berne, PA 75 WTD 1% 0% 0% −1% −1% −1% −2% −2%
01470755 Maiden Creek near Virginville, PA 33 --- --- --- --- --- --- --- --- ---
01470779 Tulpehocken Creek near Bernville, PA 48 WTD 5% 3% 2% 1% 0% 0% −1% −1%
01471875 Manatawny Creek near Spangsville, PA 28 WTD 2% 8% 13% 15% 17% 20% 22% 25%
01472157 French Creek near Phoenixville, PA 53 WTD 3% 2% 1% 0% 0% −1% −1% −2%
01472198 Perkiomen Creek at East Greenville, PA 40 WTD 8% 14% 16% 17% 17% 17% 16% 16%
01472199 West Branch Perkiomen Creek at Hillegass, PA 40 WTD 5% 14% 18% 21% 44% 24% 25% 26%
01472620 East Branch Perkiomen Creek near Dublin, PA 38 WTD 3% 5% 7% 7% 8% 8% 8% 8%
01472810 East Branch Perkiomen Creek near Schwenksville, PA 30 WTD 0% 4% 5% 7% 8% 8% 9% 9%
01473000 Perkiomen Creek at Graterford, PA 107 WTD 1% 7% 8% 8% 7% 5% 3% 1%
01473169 Valley Creek at PA Turnpike Bridge near Valley Forge, PA 39 WTD 8% 15% 17% 18% 18% 18% 17% 17%
01473500 Schuylkill River at Norristown, PA 36 --- --- --- --- --- --- --- --- ---
01473900 Wissahickon Creek at Fort Washington, PA 41 WTD 3% 5% 6% 7% 7% 7% −4% 7%
01474000 Wissahickon Creek at Mouth, Philadelphia, PA 56 WTD 2% 5% 6% 6% 6% 7% 7% 6%
01474500 Schuylkill River at Philadelphia, PA 43 EMA 2% 4% 6% 9% 11% 13% --- ---
01475850 Crum Creek near Newtown Square, PA 45 WTD 5% 8% 9% 9% 9% 8% 8% 8%
01476480 Ridley Creek at Media, PA 34 WTD 3% 2% 2% 1% 1% 1% 2% 1%
01478120 East Branch White Clay Creek at Avondale, PA 14 --- --- --- --- --- --- --- --- ---
01480500 West Branch Brandywine Creek at Coatesville, PA 61 WTD 4% 4% 4% 3% 2% 1% 0% 1%
01480617 West Branch Brandywine Creek at Modena, PA 52 WTD 3% 6% 7% 7% 8% 8% 8% 8%
01480700 East Branch Brandywine Creek near Downingtown, PA 48 EMA −1% 8% 18% 32% 44% 57% --- ---
01480870 East Branch Brandywine Creek below Downingtown, PA 48 EMA 0% 9% 18% 31% 42% 55% --- ---
01481000 Brandywine Creek at Chadds Ford, PA 102 WTD 0% 4% 6% 9% 11% 13% 15% 17%
01494850 East Branch Big Elk Creek at Forestville, PA 14 --- --- --- --- --- --- --- --- ---
01560000 Dunning Creek at Belden, PA 83 WTD −1% 1% 2% 3% 4% 5% 5% 6%
01564512 Aughwick Creek near Shirleysburg, PA 32 WTD −2% −3% −3% −2% −2% −2% −2% −1%
01572025 Swatara Creek near Pine Grove, PA 33 WTD 8% 6% 5% 4% 4% 3% 3% 2%
01572190 Swatara Creek near Inwood, PA 33 WTD 7% 4% 3% 1% 1% 0% 0% 0%
01573695 Conewago Creek near Bellaire, PA 10 --- --- --- --- --- --- --- --- ---
01573710 Conewago Creek near Falmouth, PA 11 --- --- --- --- --- --- --- --- ---
01573825 West Conewago Creek at East Berlin, PA 18 WTD −5% −3% −2% −2% −1% −1% −1% −1%
01576500 Conestoga River at Lancaster, PA 93 WTD 0% 1% 0% 1% 1% 1% 1% 1%
01576754 Conestoga River at Conestoga, PA 37 WTD −3% −1% 0% 0% 0% 0% 0% 0%
01601000 Wills Creek below Hyndman, PA 54 WTD 0% 2% 4% 6% 7% 7% 8% 9%
01613050 Tonoloway Creek near Needmore, PA 59 WTD −1% −1% −2% −1% −1% −1% −1% −1%
Table 5.    Summary of percentage changes from previously published peak-flow annual exceedance probabilities for select streamgages associated with September 1–2, 2021, flooding in Pennsylvania.

Flood-Documentation Maps

Five flood-documentation maps were created for communities in southeastern Pennsylvania (figs. 812). Each map shows the areal extent of the floodwaters along affected reaches. The HWMs used to create the flood-documentation maps and derive the associated information are provided in Stuckey and Conlon (2023) Digital datasets of the inundation area, modeling boundary, and water-depth are available for download at Stuckey and Conlon (2023). The areas covered by the specific flood-documentation maps are described in the following sections. All reported peak-stage data obtained from USGS streamgages (U.S. Geological Survey, 2023c) in this report represent final approved data unless explicitly stated as provisional data. All elevations are referenced to NAVD 88 unless otherwise noted.

Brandywine Creek

Brandywine Creek generally flows southeast through Chester County in southeastern Pennsylvania to its confluence with the Christina River in Delaware. Brandywine Creek’s two main branches, the East Branch Brandywine Creek and the West Branch Brandywine Creek, both begin in northern Chester County. Separate flood-documentation maps for the East Branch Brandywine Creek, West Branch Brandywine Creek, and the main stem Brandywine Creek were created to show the areal extent of flooding in the Brandywine Creek Basin (figs. 810). The map extent for the East Branch Brandywine Creek includes 5 miles (mi) through several municipalities, including the borough of Downingtown, Pennsylvania. (fig. 8). The map extent for the West Branch Brandywine Creek includes a short distance (about 0.5 mi) as it flows through the borough of Modena (fig. 9). The main-stem Brandywine Creek includes 9 mi as it flows through several municipalities, including Chadds Ford Township (fig. 10). A total of 105 HWMs surveyed along the Brandywine Creek were used to create flood-documentation maps: 43 in the East Branch Brandywine Creek, 40 in the West Branch Brandywine Creek, and 22 in the main stem Brandywine Creek. The measured height above ground surface at the HWMs ranged from 0 to 7.5 ft, and the peak water-surface elevations ranged from 47.8 to 84.5 ft above NAVD 88. The USGS operates two streamgages on the Brandywine Creek that were also used in the creation of the flood-documentation maps:

  1. 01480700 East Branch Brandywine Creek near Downingtown—a peak stage of 13.60 ft above streamgage datum (approximate water-surface elevation of 268 ft above NGVD 88) was recorded at 19:30 on September 1, 2021; and

  2. 01480617 West Branch Brandywine Creek at Modena—a peak stage of 13.36 ft above streamgage datum (water-surface elevation of 275.9 ft above NAVD 88) was recorded at 19:00 on September 1, 2021.

The USGS operates four precipitation gages in the Brandywine Creek Basin, only three of the four undergo review and approval of the data. Approved rainfall totals from the USGS precipitation gages ranged from 6.68 inches (01480399) to 8.18 inches (01480617) on September 1, 2021 (U.S. Geological Survey, 2023c). The NWS reported rainfall totals in Chester County to be from 10.1 inches in Downingtown to 1.68 inches in Octoraro Creek Nottingham during the event (National Weather Service, 2023b).

Perkiomen Creek

Perkiomen Creek generally flows southeast through Berks, Bucks, Lehigh, and Montgomery Counties in southeastern Pennsylvania to its confluence with the Schuylkill River. Perkiomen Creek consists of two main branches: the Perkiomen Creek, which begins in Berks County, and East Branch Perkiomen Creek, which begins in Bucks County. A flood-documentation map was created to show the areal extent of flooding in the Perkiomen Creek Basin near Graterford and Collegeville, Pennsylvania, which is downstream from the confluence of the two branches at Schwenksville, Pennsylvania (fig. 11). The map extent includes 8 mi of the Perkiomen Creek, flowing through the boroughs of Graterford and Collegeville, Pennsylvania. (fig.11). A total of 23 HWMs surveyed along the Perkiomen Creek were used to create the flood-documentation map. The measured height above ground surface at the HWMs ranged from 0 to 9.7 ft, and the peak water-surface elevations ranged from 35.6 to 49.7 ft above NAVD 88. The USGS operates two streamgages on the Perkiomen Creek that were also used to create the flood-documentation map:

  1. 01472810 East Branch Perkiomen Creek near Schwenksville—A peak stage of 16.88 ft above streamgage datum (approximate water-surface elevation of 167 ft above NGVD 29) was recorded at 01:30 on September 2, 2021.

  2. 01473000 Perkiomen Creek at Graterford—The equipment at the streamgage was inundated; therefore, an exact peak stage is unknown. An estimated peak stage of 24.23 ft above streamgage datum (water-surface elevation of 136.89 ft above NGVD 29) was determined for September 2, 2021.

The NWS reported rainfall totals in Montgomery County to be 8.4 inches in Hatfield to 3.48 inches in Glenside during the event (National Weather Service, 2023b).

Schuylkill River

The Schuylkill River generally flows southeast through the Schuylkill, Berks, and Montgomery Counties of southeastern Pennsylvania to its confluence with the Delaware River near Philadelphia, Pennsylvania. The Schuylkill River flows for 135 mi from the confluence of the East and West Branch Schuylkill River to Philadelphia and is joined by several major tributaries. One flood-documentation map was created to show the areal extent of flooding in the Schuylkill River Basin near Norristown and Bridgeport, Pennsylvania, which is downstream from the confluence of the Schuylkill River and Perkiomen Creek at Phoenixville, Pennsylvania (fig. 12). The reach includes 4 mi of the Schuylkill River, flowing through the boroughs of Norristown and Bridgeport, Pennsylvania (fig. 12). A total of 26 HWMs surveyed along the Schuylkill River were used to create flood-documentation maps. The measured height above ground surface at the HWMs ranged from 0 to 9.1 ft above ground, and the peak water-surface elevations ranged from 22.3 to 24.0 ft above NAVD 88. The USGS operates one streamflow-gaging station on the Schuylkill River that was used in the creation of the flood-documentation maps:

  1. 01473500 Schuylkill River at Norristown—A peak stage of 26.88 ft above streamgage datum (approximate water-surface elevation of 78 ft above NGVD 29) was recorded at 06:15 on September 2, 2021.

The USGS operates a precipitation gage at 01473500 Schuylkill River at Norristown. These precipitation data are not reviewed and approved and offered as provisional data for 120 days only (U.S. Geological Survey, 2023c). The provisional rainfall total recorded at 01473500 during the event was 7.00 inches. The NWS reported rainfall totals in Montgomery County to be 8.41 inches in Hatfield to 3.48 inches in Glenside during the event (National Weather Service, 2023b).

Flood Damages

The remnants of Hurricane Ida in September of 2021 that moved through the mid-Atlantic area caused historic flooding in many streams and rivers and the loss of many lives. Of the 96 fatalities caused by the storm along its entire path from Louisiana to New England, 22 were in the mid-Atlantic area (National Weather Service, 2023b). Many roads and bridges were damaged or destroyed. Estimates of monetary losses from damage to many homes, businesses, and infrastructure are estimated to be over 70 billion dollars; Pennsylvania alone had 117 million dollars’ worth of damage (National Centers for Environmental Information, 2023a).

Summary

In September 2021, rainfall from the remnants of Hurricane Ida caused flooding in many streams and rivers in eastern and south-central Pennsylvania. Rainfall totals of 5 to 10 inches were widespread throughout the area, and most of the rain fell in little more than 6 hours. At least 22 fatalities were reported and damages were estimated to be in the millions of dollars for the mid-Atlantic area. In the aftermath of the flooding, the U.S. Geological Survey (USGS) and the Federal Emergency Management Agency initiated a cooperative study to flag and survey high-water marks and evaluate the flood’s magnitude, extent, and probability of occurrence at select USGS streamgages in Pennsylvania.

Fifty-two USGS streamgages in Pennsylvania with at least 10 years of record had a 2021 annual peak streamflow related to Hurricane Ida that ranked in the top five of all annual peaks for a given station and a recurrence interval estimated to be at least 10-percent annual exceedance probability (AEP). The 52 analyzed streamgages had 10 to 107 years of peak streamflow data with a mean of 45 years of peak streamflow data. New peak streamflow records were recorded at 19 of the 52 USGS streamgages. The AEP estimates for the September 2021 event ranged from less than 0.2 to 10 percent (greater than a 500-year recurrence interval to a 10-year recurrence interval). Flood frequencies were also computed for the 52 streamgages using annual peak streamflow data through 2021 and compared with flood frequencies previously published in 2020 (using data through 2015). On average, the 1-percent AEP flood frequency statistic increased by 9 percent using the additional 6 years of data at streamgages used in both analyses.

USGS hydrographers identified and documented 338 high-water marks, 154 of which were used along with geographic information system methods to create 5 flood-documentation maps that document the depth and extent of flooding. Flood-documentation maps were created for communities along the West Branch Brandywine Creek near Modena, the East Branch Brandywine Creek near Downingtown, the main stem Brandywine Creek near Chadds Ford, the Perkiomen Creek near Graterford and Collegeville, and the main stem Schuylkill River near Norristown and Bridgeport. Digital datasets of the inundation area, modeling boundary, and water-depth are available for download.

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Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain
Length
inch (in.) 2.54 centimeter (cm)
inch (in.) 25.4 millimeter (mm)
foot (ft) 0.3048 meter (m)
mile (mi) 1.609 kilometer (km)
Area
square mile (mi2) 259.0 hectare (ha)
square mile (mi2) 2.590 square kilometer (km2)
Flow rate
cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations

AEP

annual exceedance probability

DEM

digital elevation model

EMA

expected moments algorithm

FEMA

Federal Emergency Management Agency

GIS

geographic information system

HWM

high-water mark

lidar

light detection and ranging

NWS

National Weather Service

RI

recurrence interval

USGS

U.S. Geological Survey

For more information about this report, contact:

Director, Pennsylvania Water Science Center

U.S. Geological Survey

215 Limekiln Road

New Cumberland, PA 17070

https://www.usgs.gov/centers/pennsylvania-water-science-center/connect

Publishing support provided by the Baltimore Publishing Service Center

Suggested Citation

Stuckey, M.H., Conlon, M.D., and Weaver, M.R., 2023, Characterization of peak streamflows and flooding in select areas of Pennsylvania from the remnants of Hurricane Ida, September 1–2, 2021 (ver. 1.1, September 28, 2023): U.S. Geological Survey Scientific Investigations Report 2023–5086, 28 p., https://doi.org/10.3133/sir20235086.

ISSN: 2328-0328 (online)

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Characterization of peak streamflows and flooding in select areas of Pennsylvania from the remnants of Hurricane Ida, September 1–2, 2021
Series title Scientific Investigations Report
Series number 2023-5086
DOI 10.3133/sir20235086
Edition Version 1.0: September 7, 2023; Version 1.1: September 28, 2023
Year Published 2023
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Pennsylvania Water Science Center
Description Report: vii, 28 p.; Data Release
Country United States
State Pennsylvania
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details