Stormwater, Among Other Things

Matthew E. Buckner, P.E.

A floodplain, as mapped by the Federal Emergency Management Agency (FEMA), is flat or nearly flat land adjacent to a stream, river, or any other large body of water and is susceptible to being inundated by flood waters that develop during rainstorms. Structures built within or encroaching on a floodplain can be expected to experience flooding conditions. Features that are implemented during the design and/or construction process can have considerable impact on a structure’s vulnerability to flooding.

The design objective is to prevent and/or minimize the impacts of flooding. For buildings built along the coast, stilts can be employed to minimize damage as water spreads. Grading, stormwater management implementation, and water retention methods can be incorporated into the design and construction to prevent water from entering the premises.

Even structures built away from floodplains can encounter localized flooding. High groundwater elevations (due to a high seasonal water table) can result in the flooding of structures not designed or constructed with a proper standard of care. Constructing the foundation so as to be above the seasonal high water table can help prevent groundwater penetration and resulting moisture and structural damage. In addition, surface water management, such as installing storm sewer drains beneath lawns or along gutters of impervious parking lots, can negate flooding impacts.

The following case study is a good example of these considerations:

A church was built along the East Coast. The architect had noted that the floodplain extended 150 feet in the horizontal direction and 15 feet in the vertical direction beyond the level of high tide. Thus, the church was designed to be built in a location both away from and above the floodplain. With construction of the church and its parking lot complete, services were underway.

Six months later, during the peak of summer, the pastor observed water entering the church basement following significant storm events. The recurring result was a two-inch depth of water on the concrete floor. The pastor would use a sump pump to remove the water. Winter arrived, and several snowstorms took place. As the snow melted, water entered the basement again. Spring arrived, and the cycle of heavy rainfall, followed by ponded water and its removal continued. One day, while setting up the pump, the pastor observed a pattern of non-linear cracks in the concrete basement slab. Further, the concrete block foundation walls displayed the formation of unidentified biological growth and powdery deposits on the walls’ surfaces.

At this point, the pastor notified the church’s insurance company, citing the basement flooding, cracked concrete, and issues with the walls. A professional consulting engineer was engaged to analyze the damages to determine a cause. Upon investigation, the engineer identified several problems associated with the site that resulted in water intrusion-related problems. First, the curbed, impervious parking lot was paved without a slope or storm sewer system to convey stormwater away from the church. Second, the perimeter landscaping was graded without the ordinance-required slope to convey rainwater, as discharged from the downspouts, away from the church. Third, after research and examination, the high water table was found to be several inches above the surface elevation of the basement floor.

Although the architect had ensured that the church was removed from the floodplain, the site lacked appropriate design considerations and consequential construction implementation. The flat, curbed, and paved lot — with no means of water drainage — resulted in a “tailwater” effect (the backing up and retaining of water). Coupled with the lack of slope to the landscaping, the surface water was being retained, creating a flooding condition. It was the grading combined with the high water table that resulted in the repeated water intrusion and the need to pump out the basement.

But, that was not all.

It was also determined that, during freezing temperatures and due to the high water table, frost developed in the ground, resulting in the upheaval and cracking of the concrete basement floor. Furthermore, the biological growth and the powdery deposits (efflorescence) were the result of moisture intrusion through the foundation wall, a condition related to the combined groundwater and surface water conditions.

We see that in addition to flooding that would be expected to take place in floodplains, flooding conditions may be created in locations away from floodplains, as well. So, while the design objective in and around floodplains is to prevent or minimize the impact of flooding, in other locations, outside of floodplains, it is also necessary to take appropriate measures, through design considerations and construction implementation, to avoid creating flooding conditions.