The Lehigh Valley and Floods

Floods are one of the most common natural hazards in the United States and are the most prevalent type of natural disaster occurring in Pennsylvania. Over 94% of the municipalities in the Commonwealth have designated flood-prone areas. Both seasonal and flash floods have been the cause of millions of dollars in annual property damages, loss of lives and disruption of economic activities [Pennsylvania Emergency Management Agency (PEMA) 2013].

Flooding is the most significant natural hazard in the Lehigh Valley. Riverine, flash, storm water and ice jam floods occur around rivers, streams and creeks found throughout the Lehigh Valley. Stormwater/urban flooding occurs in areas of ditches, storm sewers, retention ponds and other facilities constructed to store runoff. Within Lehigh and Northampton Counties, the State has designated 16 watersheds for the purposes of Stormwater management. The Lehigh Valley has ordinances in place for all 16 watersheds.

Two major rivers, the Lehigh and Delaware, are located within the Lehigh Valley, along with the tributaries of these two rivers. The Lehigh River flows through Lehigh Gap at the northern boundary of Lehigh and Northampton counties southbound to Allentown where it turns eastward. The Lehigh River essentially splits the Lehigh Valley in half. From Allentown, the Lehigh River flows eastward to its confluence with the Delaware River at Easton. Major tributary streams flowing into the Lehigh River are Coplay Creek, Little Lehigh Creek, Hokendauqua Creek, Jordan Creek, Monocacy Creek and Saucon Creek. The Delaware River flows along the eastern portion of Northampton County and eventually flows into the Atlantic Ocean. Bushkill Creek and Martins Creek flow directly into the Delaware. In Lehigh and Northampton counties, all municipalities have areas prone to flooding along streams and/or rivers.

Ice jams are common in the northeastern US, and the Lehigh Valley is not an exception. Ice jams act as a natural dam and restrict flow of a body of water and may build up to a thickness great enough to raise the water level and cause flooding. The Lehigh Valley has experienced ice jams in the past.

Flood hazard areas are identified on the FEMA Flood Insurance Rate Map (FIRM) and are identified as a Special Flood Hazard Area (SFHA). SFHAs are defined as the area that will be inundated by the flood event having a 1% chance of being equaled or exceeded in any given year. The 1% annual chance flood is also referred to as the base flood or 100-year flood. The FIRM also identifies areas of the 0.2% chance flood or 500-year floodplain. The SHFA is the area where the National Flood Insurance Program’s (NFIP) floodplain management regulations must be enforced, and the area where the mandatory purchase of flood insurance applies. A structure located within a 1% floodplain has a 26% chance of suffering flood damage during the term of a 30-year mortgage (FEMA 2018).

Locally (Lehigh and Northampton counties) based on the analysis, more than 37 square miles or about 5% of the Lehigh Valley area lies within a 1% annual chance floodplain, and about 43 square miles or about 6% lies within a 0.2% chance floodplain.