Avalon Focuses on Flooding

AVALON – Heavy rains flooded Avalon’s north end last July. The rainstorm, which Avalon’s Mott MacDonald Engineer Thomas Thornton termed a 50-to-100-year rain event, displayed the vulnerability of that section of the borough to flooding. 
Mott MacDonald recently completed a study of the north-end drainage systems and their ability to respond to intense rain events.
On May 23, Thornton presented a summary of that study to borough council.
Thornton said the study focused on the drainage systems that operate from 10th to 27th streets east of Avalon Avenue. He cited three major rain events in 2005, 2009, and July 2017 as examples of inundations to which the existing systems cannot adequately respond.
As Thornton explained, the drainage system in the area operates off two pump stations at 11th and 22nd streets. The collection system, a series of varying diameter pipes, collect and transport the water to the pump stations which then relay the water to outflow pipes.
The northern section of the drainage area is from 10th to 16th streets and is supported by the pump station at 11th Street.
The southern portion of the drainage area feeds the pump station at 22nd Street. Thornton noted that the pump stations have been at their current locations since at least the 1960s.
The study concluded that the current system is inadequate to handle a 10-year storm event. That means significant flooding when something as large as a 25-year or greater event occurs.
The terminology references a rain event that is to be expected every 25 years. Those terms should not allow for complacency given that the engineers have already identified three 50 to 100-year events in the past 12 years.
The study was focused on intense rain events and not the tidal flooding that may be caused by storm surges.
The system’s problems include a collection system in which numerous pipes need to be larger, pump stations in need of upgrades and additional pumps, and upgrades to outflow systems.
One issue is whether to continue to rely on a gravity-based system or move to a forced-main system for the 22nd Street pump station.
A challenge facing the borough is to identify a protection level for the engineers who will design a new drainage system.
If, for example, the borough wants a system capable of handling a 25-year storm event, that decision would impact the design in ways different from a decision to protect against a 100-year storm event.
One distinct difference caused by any decision on protection levels is cost.
Thornton presented rough estimates that ranged from $8.6 million for dealing with a 25-year event to $10.7 million for a 100-year event.   
Thornton emphasized that this was a conceptual study. The next stage, that of design, may allow for consideration of alternatives that could be more cost-effective. 
He also reminded council that the work could be phased over time, effectively spreading out the expense across multiple years.
One example was dealing with the pump station upgrades first and then turning attention to the network of pipes that constitute the collection system.
According to Business Administrator Scott Wahl, next steps include looking at the desired storm level protections that will impact any design, consideration of how a phased approach to construction might work given the need to preserve construction-free months for the summer season, and consideration of state and federal permit processes.
To contact Vince Conti, email vconti@cmcherald.com.