Flooding. General C-1. Flood Probability Summary. Source: FEMA

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1 Flooding General Flooding is the leading cause of death among all types of natural disasters throughout the United States. Typically the result of heavy precipitation, snowmelts, and ice jams, major flood events can last several days or weeks. Unfortunately, many homeowners fail to realize the average insurance policy does not cover flooding. For this reason, floods are a costly and dangerous hazard. A property s vulnerability to a flood depends on its location in the floodplain. Properties that reside along the banks of a waterway are the most vulnerable. A property within the floodplain is broken into sections, depending on its distance from the waterway. The 10-year flood zone is the area that has a 10 percent chance of being flooded every year. However, this label does not mean this area cannot flood more than once every 10 years. It simply designates the probability of a flood of this magnitude every year. Further away from this area is the 50- Flood Probability Summary year floodplain. This area includes the entire 10-year floodplain, plus additional property. The probability of a flood of this magnitude occurring during a one-year period is two percent. A summary of flood probability is shown here. Flood Recurrence Intervals 10 year 50 year 100 year 500 year Source: FEMA Chance of Occurrence 10.00% 2.00% 1.00% 0.20% In the past, most of Snyder County s flood problems resulted from seasonal change. With winter thaws and spring rain, the County has experienced the most flooding in late winter and early spring months (February-April). During winter thaws, pieces of ice can often back up and jam waterways, causing flooding. Flooding poses the biggest threat to those who reside or conduct business in the floodplain. The most significant hazard exists for businesses in the floodplain that process, use, and/or store hazardous materials. A flood could potentially allow for hazardous materials to leak out of these areas. As the water recedes, it could spread the hazardous materials throughout the area. Also threatened are the agricultural areas in the floodplain. Snyder County is mostly rural, and flooding could result in significant agricultural losses. Most flood damage to property and structures located in the floodplain is caused by water exposure to the interior, high velocity water, and debris flow. Snyder County is prone to two types of floods: riverine flood, which occurs in the floodplain of a river or stream when the amount of water and the rate at which it is moving increases; and flash flood, a type of riverine flood that occurs after a heavy storm, when the ground cannot absorb C-1

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4 Flooding Municipal Hazard Analysis History Snyder County will experience flooding during any given year. Typically, this flooding results from heavy rain or spring thaws. Snyder County Event History Location Date Type Deaths Eastern PA 11/23/1993 Flood/Flash Flood 0 Snyder County 10/21/1995 Flood/Flash Flood 0 Snyder County 1/19/1996 Flash Flood 1 Snyder County 9/6/1996 Flash Flood 0 Middleburg 9/13/1996 Flash Flood 0 Snyder County 12/1/1996 Flash Flood 0 Snyder County 12/13/1996 Flash Flood 0 Western Snyder County 9/11/1997 Flash Flood 0 Snyder County 1/8/1998 Flash Flood 0 Snyder County 3/19/1998 Flash Flood 0 Snyder County 4/19/1998 Flash Flood 0 Middleburg 8/20/1999 Flash Flood 0 Eastern Snyder County 9/6/1999 Flash Flood 0 Snyder County 9/16/1999 Flash Flood 0 Snyder County 9/30/1999 Flash Flood 0 Middleburg 9/23/2003 Flash Flood 0 Snyder County 9/23/2003 Flood 0 Snyder County 9/23/2003 Flood 0 Snyder County 12/11/2003 Flood 0 Snyder County 9/17/2004 Flood 2 Snyder County 9/17/2004 Flood 0 Snyder County 3/29/2005 Flood 0 Snyder County 3/29/2005 Flood 0 Source: National Climatic Data Center (NCDC) C-4

5 Many of these flood events have resulted in historic crest levels. As reported by the National Weather Service, historic crest levels seen at Penns Creek are shown in the following table: Penns Creek - Historic Crests Crest Level Date ft 6/23/ ft 1/19/ ft 9/16/ ft 2/26/ ft 9/18/ ft 3/18/ ft 2/15/ ft 1/25/ ft 3/31/ ft 8/24/1933 Penns Creek - Flood Categories Major Flood Stage Moderate Flood Stage Flood Stage Action Stage Source: National Weather Service 12 ft 10 ft 8 ft 6 ft Flood Vulnerability Summaries for Municipalities The following municipal summaries detail flood threats in Snyder County. Not all municipalities in the County are included, because they are not equally vulnerable. This analysis was performed by the Federal Emergency Management Agency (FEMA) Flood Insurance Study. C-5

6 Adams Township Adams Township is located in the north central portion of Snyder County, bordered to the north by Union County; to the south by Beaver Township; to the east by Centre and Franklin Townships; and to the west by Spring Township. The Township contains Walker Lake as one of its major geographical features. This area is home to 852 residents and sits approximately 60 miles northwest of the City of Harrisburg. Limited flood data is available for Adams Township. Known flooding has occurred along the North Branch of Middle Creek and its tributaries, Moyer s Mill Run, and Walker Lake. C-6

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8 Beaver Township Beaver Township is located in the west central portion of Snyder County. Beaver Township is bordered to the north by Adams Township; to the south by West Perry Township; to the east by Franklin Township; and to the west by Spring Township. Beaver Township surrounds Beavertown Borough. The Township contains Middle Creek as one of its major geographical features which runs generally west to east in the northern portion of the township. This area has a population of 527 residents and is approximately 55 miles northwest of the City of Harrisburg. Limited flood data is available for Beaver Township. Known flooding has occurred along Middle Creek, North Branch Middle Creek, and the smaller tributaries of Kern Run, Lupher s Run, and Wetzel Run. C-8

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10 Beavertown Borough Beavertown Borough is located in the west central portion of Beaver Township. The Borough is bordered to the north, south, east, and west by Beaver Township and extends outward from U.S. Route 522. Beavertown Borough is home to 870 residents and is approximately 55 miles northwest of the City of Harrisburg. Limited flood data is available for Beavertown Borough. Known flooding has occurred along Kern Run to the west, Lupher s Run, and Wetzel Run to the east. C-10

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12 Center Township Center Township is bordered by Adams Township to the west, Franklin Township to the south, Middlecreek and Jackson Townships to the east, and Limestone and Lewis Townships to the north. Center Township is in north central Snyder County and is home to 2,299 residents. Moderate flooding is experienced along Penns Creek in Center Township. C-12

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14 Chapman Township Incorporated in 1820, Chapman Township is located in southeastern Snyder County, approximately 25 miles north of the City of Harrisburg. Chapman Township is bordered by Union Township to the north, Washington Township to the northwest, Perry Township to the west, Susquehanna Township to the south, and Lower Mahanoy to the east and southeast. U.S. Route 104 and U.S. Route 11/15 are the primary roads in Chapman Township. U.S. Route 104 runs north to Middleburg Borough, and U.S. Route 11/15 runs north to Selinsgrove Borough. Chapman jurisdiction covers 11.8 square miles and is primarily an agriculture and woodlands community. Chapman s population of 1,439 is scattered throughout the Township. Historical data indicates that flooding can occur during any time of year in Chapman Township. Each year, this Township can anticipate an average of 41 thunderstorms (60 percent in the summer, 25 percent in the spring, 14 percent in the fall, and 1 percent in the winter). Larger storms, such as Tropical Storm Agnes of 1972, Tropical Storm Ivan of 2004, and Tropical Storm Dennis of 2005 have become more frequent. Heavy rain can inundate a large portion of the floodplain in Chapman Township. West Mahantango and North Branch Mahantango Creek can also be affected by storms of all magnitudes. Using stream gauges along the Susquehanna River in Sunbury, PA, Chapman Township has been involved in four major flood events in the past 100 years. These events, in order of decreasing magnitude, occurred on June 24, 1972, March 19,1936, May 29, 1946, and March 11, The June 1972 flood had a recurrence interval between years. Other large events included Tropical Storm Ivan of 2004 and Tropical Storm Dennis of No stream gages exist along the West Mahantango and North Branch Mahantango Creek. However, personal interviews conducted by FEMA and the Pennsylvania Department of Transportation District 3 indicate the June 1972 flood was a 50-year flood. C-14

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16 Franklin Township Located in central Snyder County, Franklin Township is approximately 36 miles north of the City of Harrisburg. Center Township is located to the north of Franklin, while Adams and Beaver Townships are located to the west. West Perry and Perry Townships are located to the south, and Washington Township is located to the east. Incorporated in 1853, Franklin Township completely surrounds Middleburg Borough. More than 2,000 residents can be found in this primarily rural township that covers 28.2 square miles. Flooding can occur anytime in Franklin Township. On average, 41 thunderstorms (60 percent in the summer, 25 percent in the spring, 14 percent in the fall, and 1 percent in the winter) can be expected. Larger storms, such as Tropical Storm Agnes of 1972, Tropical Storm Ivan of 2004, and Tropical Storm Dennis of 2005 can severely impact Middle Creek. A large amount of floodplain land is affected by this creek as it travels through the Township. Additionally, TR 439 and LR 690 travel through the floodplain Middle Creek currently has no flood gauges on it. However, it is known that flooding occurred in 1889, 1933, 1936, 1942, 1952, 1956, 1972, 2004, and The June 1972 flood was estimated to be a 500-year flood on Middle Creek. The tributaries of Middle Creek experienced a 10-year flood during June This was based on personal interviews with residents and high-water marks from structures along Stumps Run. C-16

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18 Freeburg Borough Freeburg Borough is located in the east central portion of Snyder County, surrounded by Washington Township. This area is home to 584 residents and sits approximately 45 miles north of the City of Harrisburg. Limited flood data is available for Freeburg Borough. Known flooding has occurred along Dry Run and the Susquehanna Creek. C-18

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20 Jackson Township Residing in northeastern Snyder County, Jackson Township is home to 1,297 residents and is approximately 45 miles north of the City of Harrisburg. To the north, one can find New Berlin Borough and Limestone and Union Townships. Jackson Township also shares a border with Center Township to the west, Middle Creek and Penn Townships to the south, and Monroe Township to the east. Jackson Township was incorporated in 1854 and is a total of 14.8 square miles. Mostly rural and undeveloped, Jackson Township is situated on State Route 204 about half a mile south of Penns Creek. On average, Jackson Township gets inches of rain per year that primarily drains into Penns Creek. Penns Creek drainage area serves 310 square miles. Flooding can occur any time of year in Jackson Township. Large storms affect large amounts of floodplain land in the Township. U.S. Geological Survey (USGS) gauge number 5550, located upstream on Penns Creek, has been in operation since In June of 1972, 34,600 cubic feet per second (cfs) were discharged from Penns Creek, with a recurrence interval of 240 years. This is followed by 19,800 cfs in September 1934 (40-year recurrence), and 15,700 cfs in March 1936 (20-year recurrence). These occurrences were a result of either spring rainfall and snowmelts, or tropical storms that moved north along the Atlantic coastline. C-20

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22 McClure Borough McClure Borough is located in the southwestern tip of Snyder County. McClure Borough is bordered to the north, east, and west by West Beaver Township. The Snyder County line runs along the southern end of the Borough, with Mifflin County to the south. This area has a population of 975 residents and is approximately 55 miles northwest of the City of Harrisburg. Limited flood data is available for McClure Borough. Flooding is known to occur along the South Branch of Middle Creek. C-22

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24 Middleburg Borough Completely surrounded by Franklin Township, Middleburg Borough is located in central Snyder County, about 40 miles north of the City of Harrisburg. Composed of 1.2 square miles in area, Middleburg is about 27 percent residential, 9 percent industrial, 2 percent commercial, 52 percent open space, and 10 percent miscellaneous and civic uses. Older structures are typically found to the north of Middle Creek, while newer ones are found to the south. The 1,363 residents of Middleburg can expect about 41 inches of rain evenly distributed throughout the year. Middleburg Borough has experienced extensive flooding in the past, particularly in the French Flats. Located between the U.S. Route 522 bridge over an area referred to as The Gut, and the U.S. Route 522 bridge over Middle Creek, the French Flats have recorded at least 46 occasions of flooding between Backwaters can cause flooding at that confluence of Middle Creek and Stumps Run in Middleburg Borough. Middleburg, like most municipalities in Snyder County, will experience flooding during all seasons. On average, 41 thunderstorms occur annually: 60 percent in the summer; 25 percent in the spring; 14 percent in the fall; and 1 percent in the winter. Larger storms have the ability to cause severe flooding in Middleburg Borough. No stream gauges currently exist along Middle Creek. However, it is known that major flooding occurred in 1889, 1927, 1933, 1936, 1942, 1952, 1956, 1972, 2004, and During the 1972 flood, Middle Creek experienced water levels at the 500-year mark. During the same event, Stumps Run experienced water levels at the 10-year flood mark. C-24

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26 Middlecreek Township Located 36 miles to the north of the City of Harrisburg, Middlecreek Township is located in central Snyder County. Jackson Township is located to the north, Penn Township to the east, Washington Township to the south, and Center and Franklin Townships to the west. Middlecreek was incorporated in 1838, and was later subdivided to form Jackson Township. Middlecreek s 2,031 citizens are scattered along U.S. Route 522 in the south central portion of the Township s 13.6 square miles. Approximately inches of rain falls in Middlecreek Township annually. Seventy percent of this rainfall drains into the Middle Creek watershed. The remaining 30 percent flows into Penns Creek. The June 1972 flood caused extensive damage in the Township, particularly to the Wood-Mode Cabinet Company s showroom. A 1977 flood caused a high degree of ground erosion in certain areas of the Township. Other large events included Tropical Storm Ivan of 2004 and Tropical Storm Dennis of Flooding can and will occur in Middlecreek any time during the year. Thunderstorms are usually the culprit of stream flooding. On average, 41 thunderstorms occur in Middlecreek Township: 60 percent in the summer; 25 percent in the spring; 14 percent in the fall; and 1 percent in the winter. No stream gauges are located on Middlecreek. However, recorded events note that major flood events occurred in 1889, 1927, 1933, 1936, 1942, 1952, 1956, 1972, 2004, and Middle Creek experienced a 500-year flood during June 1972, while Tributary 1 was considered to have registered between the 10- to 100-year level. C-26

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28 Monroe Township Monroe Township was incorporated in 1856 and currently has a population of 4,064. The Susquehanna River forms the southeast boundary of the Township, while Penns Creek forms the southwest boundary. The floodplain areas along the Susquehanna River include undeveloped land, small cottages, commercial development separated by a highway, residential development, and a portion of a steam power generating plant. Susquehanna flood data in Monroe Township dates back to Other large floods have occurred in 1784, 1810, 1833, 1846, 1847, 1865, 1889, 1904, 1913, and Several hundreds of thousands of dollars in flood damage was caused by the Great Flood of This flood caused water levels on the Susquehanna River to rise to 34.8 feet in Snyder County. Tropical Storm Agnes caused the Susquehanna to rise to feet. Residents experienced river cresting at feet in September Road closures and evacuations are typical in flood-prone areas of Monroe Township during flood watches and warnings. Other large events included Tropical Storm Ivan of 2004 and Tropical Storm Dennis of C-28

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30 Penn Township Located in eastern Snyder County, Penn Township shares a border with Middlecreek Township to the west, Jackson Township to the north, Monroe Township to the northeast, Selinsgrove Borough and Lower Augusta Township to the east, Washington Township to the southwest, and Union Township to the south. Several of these borders are formed by the Susquehanna River, and Penns and Middle Creeks. Between the Susquehanna River and Penns Creek, one can find a flood-prone area known as the Isle of Que. Penn Township covers an area of 18.1 square miles and currently has 3,894 residents. Penn Township was incorporated in 1758 and was originally a part of Northumberland County. Annually, Penn Township gets inches of rainfall. Most flooding hazards in Penn Township are caused by the Susquehanna River and Penns Creek. Tropical Storm Agnes of 1972 brought with it a 620,000 cubic feet per second (cfs) and a 200-year flood along the Susquehanna River. Local streams and tributaries also experienced high flows. Penns Creek saw its cfs record nearly double at 34,600 cfs. The Isle of Que experienced the worst damage in June Homes, farm buildings, machinery, and crops were all damaged in this area. Many homes were also destroyed by the high waters of Penns Creek. A few residential and agricultural reports were noted from Middle Creek. However, these reports were minor when compared with those along Penns Creek and the Susquehanna River. Many roads and utilities were disrupted throughout the region. Other large events included Tropical Storm Ivan of 2004 and Tropical Storm Dennis of C-30

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32 Perry Township Perry Township is located in southeastern Snyder County. Susquehanna Township is located to the south, Monroe and West Perry Townships are located to the west, Franklin Township is located to the north, Washington Township is located to the northeast, and Chapman Township is located to the east. This area is home to 2,036 people and is approximately 28 miles north of the City of Harrisburg. Limited flood data is available for Perry Township. Known flooding has caused water damage to structures along the West Branch and North Branch at Mahantango Creek. C-32

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34 Selinsgrove Borough Selinsgrove Borough is home to several major north-south highways, including U.S. Route 11/15 and U.S. Route 522. This area is in the northeastern section of Snyder County and is the most densely populated borough within the County. Incorporated in 1827, Selinsgrove Borough is home to 5,374 residents. Roughly, 17.4 percent is residential, 3.5 percent industrial/ commercial, 13.4 percent public/semi-public/institutional, 16 percent utilities, and 43 percent is open space/recreation land uses. Annually, inches of rainfall are expected and flooding can occur during any season. Also, 41 thunderstorms occur on average: 60 percent in the summer; 25 percent in the spring; 14 percent in the fall; and 1 percent in the winter. Large storms typically inundate large areas in the floodplain, because of Selinsgrove s proximity to the Susquehanna River and Penns Creek. The USGS gauge at Sunbury, PA recorded four major floods on the Susquehanna River in the last 100 years. In order of decreasing magnitude, these floods occurred in June 1972, March 1936, May 1946, and March Substantial upstream reservoir and protective levee construction in the Susquehanna River basin has occurred in the past 100 years. It is estimated that the June 1972 flood had a recurrence of 170 years. Other large events included Tropical Storm Ivan of 2004 and Tropical Storm Dennis of The highest discharge levels on the Susquehanna River were gauged at 34,600 cubic feet per second (cfs) in June 1972, 19,800 in September 1934, and 15,700 cfs in March The recurrence intervals of these floods were 240 years, 40 years, and 20 years respectively. Typical flooding in Selinsgrove occurs from spring rainfall and snowmelt or tropical storms that travel along the Atlantic coastline, such as Tropical Storm Agnes of 1972, Tropical Storm Ivan of 2004, and Tropical Storm Dennis of The June 1972 flood generated by Tropical Storm Agnes did not reach the 10-year flood zone level. However, it is possible that flooding from this event could have reached higher levels if there had not been several alternative flow paths for diversion of discharge into other watersheds. Given the parallel flow of the Susquehanna River and Penns Creek, considerable flooding can occur along the Market Street business district corridor in Selinsgrove. The South Tributary also causes damage. This is particularly relevant to the Sasafras Avenue area upstream. C-34

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36 Shamokin Dam Borough Shamokin Dam Borough is about 1.8 square miles of land along the Susquehanna River in eastern Snyder County. This Borough is home to 1,473 residents and is approximately 45 miles north of the City of Harrisburg. Flood problems in Shamokin Dam are much like those in Monroe Township. Monroe Township completely surrounds the north, west, and south side of Shamokin Dam. The east side of the Borough is bordered by the Susquehanna River. Susquehanna flood data in Shamokin Dam dates to Other large floods have occurred in 1784, 1810, 1833, 1846, 1847, 1865, 1889, 1904, 1913, and Several hundreds of thousands of dollars in flood damage was caused by the Great Flood of This flood caused water levels on the Susquehanna River to rise to 34.8 feet in Snyder County. Tropical Storm Agnes caused the Susquehanna to rise to feet. Residents experienced river cresting at feet in September of Road closures and evacuations are typical in flood-prone areas of Shamokin Dam during flood watches and warnings. Other large events included Tropical Storm Ivan of 2004 and Tropical Storm Dennis of C-36

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38 Spring Township Approximately 40 miles northwest of the City of Harrisburg, Spring Township is located in southwestern Snyder County. Hartley Township is located to the north, while West Perry Township is located to the south. Adams Township is located to the northeast, Beaver Township to the southeast, and West Beaver Township to the west. Spring Township is home to 1,574 citizens. Minor flooding will occur along Middle Creek, South Branch Middle Creek, Beaver Creek, and Stony and Swift Runs. C-38

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40 Union Township Union Township was incorporated in 1869 in southeastern Snyder County. This area is approximately 30 miles to the north of the City of Harrisburg. Lower Augusta and Jackson Townships are located to the east, while Penn Township is located to the north. Washington Township is to the west, and Chapman and Lower Mahanoy Townships are to the south. Union Township consists of 13.9 square miles, is primarily rural and undeveloped, with only 1,537 residents. Visitors can access Union Township using U.S. Route 11/15. Union Township experiences about 41 inches of rainfall per year. This rain can have an adverse affect on the eastern part of the Township near the Susquehanna River and Penns Creek. Middle Creek impacts the northern area of the Township, while Silver Creek impacts the southern area. Overall, the banks of these waterways are undeveloped. However, pockets of development can be found along the Susquehanna River and Silver Creek. Flooding can occur during any season in Union Township. On average, 41 storms impact the Township: 60 percent in the summer; 25 percent in the spring; 14 percent in the fall; and 1 percent in the winter. Flooding will typically occur on the smaller streams with localized storms. This increase in water flow can cause flooding along the Susquehanna River. Within the last 100 years, four major flood events have occurred along the Susquehanna River. In order of decreasing magnitude, these floods occurred in June 1972, March 1936, May 1946, and March The Susquehanna River experienced water levels that were comparable to a 100- to 500-year flood during the June 1972 flood. Other large events included Tropical Storm Ivan of 2004 and Tropical Storm Dennis of Middle Creek does not have a gauge system; however, it is known that major flooding occurred in 1889, 1927, 1933, 1936, 1942, 1952, 1956, 1972, 2004, and Middle Creek experienced a recurrence interval of 500 years during June Silver Creek, however, was only listed between the 100- to 500-year level during the flood of June C-40

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42 Washington Township Located in the eastern portion of Snyder County, Washington Township is bordered by Union and Chapman Townships to the southeast, Penn Township to the northeast, Middlecreek Township to the north, Franklin Borough to the northwest, and Perry Township to the southwest. Currently, 1,590 residents live in Washington Township. No flood data is currently available for Washington Township. C-42

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44 West Beaver Township West Beaver Township is located in the southwestern corner of Snyder County. The county boarder runs along the south and west edges of West Beaver Township with McClure Borough to the south, Mifflin County to the south and west, Spring Township to the east, and West Perry Township carving in along the southern boarder. West Beaver Township is home to 1,124 residents and is approximately 55 miles northwest of the City of Harrisburg. Limited flood data is available for West Beaver Township. Flooding can occur along the South Branch of Middle Creek. 44

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46 West Perry Township Sharing a border with Perry Township to the east, Franklin Township to the northeast, Beaver and Spring Townships to the north, West Beaver Township to the northwest, Fayette Township to the southwest, and Monroe Township to the south, West Perry Township is located in southern Snyder County. Approximately 1,060 citizens reside in West Perry Township. Limited flooding will affect structures along the West Branch Mahantango Creek. 46

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48 Probability Flooding is a frequent annual event that affects every municipality in Snyder County. There is a high probability that the County will experience low- to high-impact flooding late in the winter and much of the spring seasons. The potential also exists for periodic flooding along lakes, streams, rivers, and tributaries throughout the remainder of the calendar year. Maximum Threat Potential flooding impacts range from very low to catastrophic depending on the type and location of the flooding. The maximum threat to the County lies along the floodplains of the Susquehanna River and Penn s Creek with the impact flooding in these areas would have to the County s social and economic vitality. The eastern tier municipalities of Monroe, Penn, Union, and Chapman Townships, and the Boroughs of Shamokin Dam and Selinsgrove experience the maximum threat of flooding from the Susquehanna River. Center, Jackson, Middlecreek, Union, and Penn Townships are at maximum threat of flooding from Penn s Creek. The maximum threat for Snyder County exists at the confluence of the Susquehanna River and Penn s Creek, along the U.S. Route 11/15 transportation corridor in the vicinity of Selinsgrove and Shamokin Dam. Consequently, these dense population centers and surrounding residential communities would also experience the greatest impact. The potential for loss of life and injuries to occur in these areas is high. Additionally, the longterm impact severe flooding could have to the health and safety of citizens is high. Depending on the scope and magnitude of the flooding, the likelihood of long-term economic disruption is possible. Flooding may have a moderate impact on property, facilities, and infrastructure with varying levels of damage to structures in the affected area. Mobile homes are especially threatened. Basic life support systems may experience moderate impact, as disruptions for short periods of time could occur. Government operations are expected to continue without disruption. The environmental impact should be minimal, unless hazardous materials are released as a result of the flooding. Secondary Effect Power failure is the most likely secondary effect that results from flooding. Power failures, combined with a shortage of critical services and supplies, could cause a public health emergency. Disruption in traffic patterns could place a strain on the County s transportation networks, increase the frequency of traffic accidents, and lead to a deterioration in emergency service coverage for both affected and unaffected areas. Flooding also has the potential to cause landslides along steep gradient areas and can introduce hazardous materials into the environment. The latter can occur as industrial, commercial, and public infrastructure facilities become inundated with floodwater. The most dangerous secondary effect could be the failure of dams in the County. Although unlikely, the possibility exists of the Susquehanna River forcing a total or partial failure of the Adam T. Bower Memorial Dam. More likely, however, is C-48

49 the failure of smaller dams, such as Walker and Faylor Lake Dams along Middle Creek and Kern Run Dam. Additionally, small earthen and man-made dams along Penn s Creek and Middle Creek are susceptible to failure, though their impact would be minimal. Severe flooding may have long-term secondary effects that affect the population, economy, and infrastructure of the County. Escalating costs of damages to private structures and the frequency of flooding can cause permanent population displacement. Small businesses that contribute to the local economy may close if unable to recover from damages. Disruptions to commerce and/or transportation modes can have an adverse effect on municipal economies in affected areas. Critical infrastructure, such as sewage and water treatment facilities, can be irreparably damaged. C-49

50 The Wyoming Valley Levee Raising Project Flood Warning and Response System 1 The Flood Warning and Response System (FWRS) is a component of the Wyoming Valley Levee Raising Project. It is an automated flood plain mapping system designed to be used with Geographic Information Systems (GIS) technology as a planning and warning tool for a portion of the Susquehanna River. The National Weather Service (NWS) routinely predicts expected river levels at specific points in the study area. The FWRS will allow the system user to enter NWS predictions into the system and will create an extent-of-flooding map based on those predictions. It will then be able to determine levels of impact a particular flood event will have on buildings and infrastructure in terms of depth of flooding, and in the case of buildings, expected damages. The user can use this information to begin the warning notification process, plan evacuation routes as needed, estimate damages before flooding occurs, and evaluate many other flood planning scenarios. The FWRS is currently in the design stage as the large volume of base information and modeling required for such a system is being developed and compiled in GIS. The U.S. Army Corps of Engineers is leading a cooperative effort with other federal, state, and local agencies to inform all interested parties of the details of the FWRS as it is designed. Key partners in this project include the Susquehanna River Basin Commission, Luzerne County Flood Protection Authority, NWS, U.S. Geological Survey, and the Federal and Pennsylvania Emergency Management Agencies. The Philadelphia District of the Army Corps of Engineers is developing the FWRS and the entire project is funded by the COE Baltimore District, LCFPA, and PA DEP. Digital Topography One of the first FWRS ingredients was the Digital Terrain Model (DTM), completed in early The DTM incorporates ground elevation (accurate to a two-foot contour interval), layout of streets, railroads, and bridges, location, size of all structures (about 84,000), and all water courses in the area. It covers an area from the southern boundary of Selinsgrove, Snyder County, to the northern boundary of Luzerne County about 110 river miles and includes elevations up to the approximate 500-year flood level. The DTM was compiled using aerial photography, and is complemented by digital orthophotographs (photos corrected for geometric distortions, such as curvature of the earth and viewing perspective). The topography is accurate to a two-foot contour interval and the model includes building footprints for approximately 84,000 structures in 1 March 22, 2006 C-50

51 the study area. The footprints also contain point elevations at all corners of each building. All transportation features and corporate boundaries are included as part of the DTM. Data that could not be collected by aerial photographic methods, such as bridge opening geometry and below the water surface riverbed geometry, has been collected in the field, verified by existing information and used to supplement the DTM. Structure Inventory Structural inventory and data collection efforts are currently underway. Structures in the 100- year floodplain will be inventoried as part of this effort. Information to be collected includes the name of property owner, building address, phone number, digital photos of each structure, first floor elevation, type of building use, and some additional information related to hazard mitigation planning efforts (possible building retrofit, relocation, and flood-proofing recommendations). For each building inventoried, additional data for a FEMA elevation certificate will also be gathered. Assessments will be made to associate structure and content damage curves with each applicable structure. The damage curves will be used with the depth of flooding to calculate damage estimates for each building for a specific flood event. These estimates can then be processed and reported in different ways according to county, community, or individual structure. Hydrology and Hydraulics The U.S. Army Corps Hydrologic Engineering Center (HEC) is currently developing an updated hydraulic model for the FWRS effort using the DTM with riverbed geometry. Once this model is complete, a series of flood profiles will be run and corresponding flood layers developed for flood events up to and including the 500-year flood discharge. The flood layers will be separated into a minimum of one-foot increments and rendered as flood inundation maps for each increment. A separate model will be developed that will account for the effects of backwater due to debris and ice blockages along the study area. System Use and Interface The system is set up to use predicted river stages at the four existing stream gauges in the study area as critical input data. Once a flood stage is predicted by the NWS at each stream gauge, the system user will input the prediction and have the FWRS automatically generate the depth-of-flooding layer that corresponds to that stage prediction. At this point, the user will have multiple output and viewing options accessible through a graphical user interface (GUI). The GUI will run on an ArcView-type platform and will make the system easy to use for any user. Any number of additional options will be available, including the generation of depth of flooding maps in digital and hard copy form, notification lists, and lists of impacted transportation features. The system interface will be designed and tested by the Corps Philadelphia District and HEC. Local officials will be consulted to make the system as user-friendly as possible. In C-51

52 addition, the final phase of the project will be the District s assistance with local implementation of the system, as needed. C-52