Needham Flash Flooding: What the First 24 Hours After Fast Rain Look Like

What happened on August 8, 2023

Needham received an extraordinary amount of rain in a short period, and the flooding that followed was localized. About 220 reported homes and businesses were affected, and the town declared a state of emergency effective that day.

Using mapping, the Department of Public Works identified 8 neighborhoods where 4 or more homes were affected. Because the flooding was localized, looking at it neighborhood by neighborhood is the useful view.

The Public Works director named these places:

• The area around Junction at Chestnut and the Downtown
• Great Plain Avenue near Sportsman's Pond
• The areas around Alder Brook, the Needham Reservoir, Rosemary Pond and Rosemary Brook

Why fast rain floods a house so quickly

The Public Works director linked the flooding to a very small, intense, localized rain event that followed a very wet summer, particularly a very wet July.

Needham's 2020 hazard report describes the same pattern in general terms: large rain events cause flooding when stormwater drainage capacity is exceeded and when groundwater levels are high.

That is also why one street can look different from one house to the next. A low driveway, a garage floor below the street or a door at grade takes water that a house on a rise may never see.

Several things combine when a short burst arrives:

• A very wet July that left the ground full before the storm began
• Drainage capacity that a sudden burst can exceed
• High groundwater that leaves water nowhere to go
• Low spots, garages and door thresholds where water collects first

The first 24 hours, step by step

Events like this one fall under storm and flash flood cleanup, and the first day follows a fixed order.

Safety first

Standing water can hide debris and live wiring, and electrical equipment that is wet should be left alone.

• Keep children and pets out of the affected rooms
• Let an electrician or the utility turn off power to a flooded area
• Stay out of water that has risen through a drain

Record and lift

Photographs come before anything is moved. Valuables, documents and electronics go to upper floors or high shelves.

• The water height on walls and furniture
• Damaged contents, in place
• The floors and any flooring that has lifted
• Any water that rose through a drain

Extraction

The extraction crew pumps standing water first and then pulls loose water from floors and carpet. Water extraction in Needham comes before drying, because wet materials cannot dry while water still sits on them.

In a house on a slab, wet vacuums and truck-mounted units pull water from carpet and flooring, and the pad is cut out and removed. In a basement or a garage, submersible pumps do the first work and a wet vacuum finishes the thin film that is left.

Slab homes, garages and basements need different plans

After the August 8, 2023 flooding, the Public Works director said water reached basements, garages and single-level homes on slabs. Each of those holds water differently.

A single-level home on a slab has no lower level to hold the water, so the water spreads wide and thin and reaches every room at once. A basement fills from the bottom, and its depth sets the pumping plan.

A garage sits between the two, open to the outside and often attached to the living space.

• Slab homes: water spreads across the whole floor and into the base of every wall, so flooring is lifted and wall bases are opened to dry.
• Basements: water collects in one low pocket and is pumped out in stages while the ground outside is still wet, as described under basement flooding cleanup.
• Garages: water is pulled out quickly, and the bottom of any shared wall is checked for moisture.

When sewer water is part of the picture

The Public Works director also reported two sewer system overflows and flooded manholes during the August 8, 2023 flooding.

Water that comes up through a floor drain or a toilet is treated as Category 3 sewage, whatever the weather behind it. Protective gear is worn, and porous materials it touched are removed instead of dried.

Needham's storm drains and sanitary sewers are built as separate systems, yet the 2020 hazard report notes that infiltration and inflow into the sewer needs bypass pumping in some locations to avoid backups.

Floor drains in a basement or garage are usually where backed-up sewer water shows first, because they sit at the lowest point of the building.

Public buildings flooded too

Two Needham town buildings flooded that day: High Rock School, where water backed up through the parking lot, and the Department of Public Works building. Both have since been remediated.

The list of flooded places included public buildings as well as homes, which shows how widely a short, intense rain can spread.

After the water is out

Drying begins once the water is out and the surfaces are clean. Wet framing, subfloor and drywall can begin to grow mold in 24 to 48 hours, so equipment goes in early.

Needham has a warm-summer humid continental climate, with high humidity and precipitation year-round, so dehumidifiers matter as much as air movers. Readings at each visit show when each material reaches its dry goal.

Final readings, in writing, show that the structure is dry before walls and floors are closed up.

Everything the team does in town is listed under water damage restoration in Needham.