Summary of Climatological and Hydrological Records
for the Venice Island area on the Schuylkill River

Manayunk, Pennsylvania

February 2000

I. Introduction

Venice Island lies in the flood plain along the Schuylkill River downstream from the Norristown flood gauge and just upstream from the Philadelphia flood gauge. Flooding in this area along the Schuylkill River occurs for three reasons, which can occur independently or in conjunction with each other. During the months of January and February , ice jams along the river can cause rapid rise flooding conditions along the Schuylkill River. Heavy rains from winter storms and tropical storms can cause flood stages to be significantly exceeded in the area. Localized extremely heavy rainfall associated with thunderstorms or tropical systems can cause rapid rise flooding conditions along the Schuylkill River. The rate of rise to reach flood stage varies upon the event that occurs. It should be noted that Venice Island ground level is significantly below flood stage heights.

II. Summary

Crest relationships between the flood stages and time to reach the flood stage exist for both the Philadelphia and Norristown gauges. Floods of record are available for the two locations as well. The threat of flooding in the area can be broken into the more likely occurrences versus the rarer events, each having a time line for development and reaching peak flood stage. The most common event for the area is a flood resulting from excessive rainfall due to a winter storm or a tropical storm. Typically, in this situation, flood stage will be reached in 10 hours after the ending of the heavy rainfall event with record water heights in the area of 15 to 20 feet above ground level. The second, and more rare event for the area, is a flash flood resulting from localized excessive rainfall due to a intense thunderstorm or an area of focused tropical rainfall. In this situation, flood stage will be reached in 4-6 hours after the ending of the excessive rainfall event with record water heights in the area of 10 to 15 feet above ground level. A rare event would be an ice jam at or below the location on the Schuylkill River. In this situation, the back up of river waters around the jam would on occasion cause flood stage to be reached within 6 hours with an estimated record water height of 10 feet above ground level.

III. Conclusion

Venice Island has the least time to respond and complete necessary evacuation in a localized heavy rainfall event. This would cause up to 10 feet of water above island ground level in less than 6 hours of the ending of the excessive rainfall event. Meteorological examples of hurricane Floyd in 1999 and hurricane Agnes in 1972 are a good case in point of such a quick rise scenario. Crest relationships are for reaching flood stage and not as to when Venice Island goes under water. Water levels on the island in excess of 5 feet could be reached in 3 hours or less, and indications are that this occurred during Floyd and Agnes.

G. C. Henricksen, Jr.
Consulting Meteorologist