Site Feature Measures to Control the Hydrologic Cycle

EDT

Thursday, July 2, 2020

Nearly flat land that is situated adjacent to a large water body and susceptible to floodwater inundation, during a rainfall event, is identified as a floodplain. The Federal Emergency Management Agency (FEMA; https://msc.fema.gov/portal/home) delineates calculated floodplains on Flood Insurance Rate Maps (FIRMs). Site features, such as a dwelling, constructed within a floodplain can be expected to experience flooding conditions.

Controls that are implemented during the design and construction process can have considerable impact on a site’s vulnerability to flooding. The design objective is to prevent and/or minimize the impacts of flooding. Stilts would constitute an example of a design implementation, whereas the stilts are employed to elevate structures above floodplains and prevent damage, as water accumulates around coastal dwellings and buildings. Stormwater management practices should also be incorporated into the design and construction processes to prevent water from entering premises. Even structures built away from floodplains can encounter localized flooding. High subsurface groundwater elevations, due to seasonal high-water tables, can result in flooding of structures that are not designed, or constructed, with a proper standard of care. Constructing the foundation above the seasonal high-water table prevents groundwater penetration from this source and subsequent moisture damage. Further, surface water management, such as the installation of subsurface storm sewer inlets and piping, can mitigate flooding impacts.

The following tale is a representative example of the aforementioned considerations: An amusement park was built along the United States’ east coast. An architect had delineated that a floodplain extends 150’ horizontally and 15’ vertically, beyond the level of oceanic high tide. Thus, the theme park was designed to be constructed in a location both away from and above the floodplain extents, or floodwater spread. With construction of the attractions and parking lot complete, the subsequent grand opening was a success. Six months post-opening, during hurricane season, the amusement park general manager (GM) discovered water intruding subsurface, or underground, storage and utility units, following a mild tropical storm. The aftermath was 6”-deep stormwater on concrete floor surfaces of the vast units.

Commercial sump pumps were mobilized to remove the water. Winter followed hurricane season, at which time the park was closed, and several snowstorms took place. As the snow melted, water entered the subsurface units again. Spring arrived, and the cycle of heavy rainfall, followed by ponded water and its removal continued. One day, while operating the sump pumps, the GM observed a pattern of non-linear cracks on subsurface storage unit concrete slab surfaces. Further, biological growth and powdery deposits were observed on separate slab sections absent flooding. Following the discoveries, the GM notified the theme park’s insurance carrier, citing the subsurface unit flooding, cracked slabs, and unidentified growth and deposits. A forensic and consulting engineer, credentialed with professional engineering licensure (i.e., P.E.), was engaged to analyze and determine the damage causation, scope, and value. Upon investigation, the engineer identified several deficiencies associated with the site that resulted in water intrusion-related problems.

First, a sizeable tributary area of the curbed and impervious parking lot, along with perimeter landscaping, lacked slopes and/or storm sewer drainage systems, to collect and convey stormwater away from the parking lot. Second, after research and review of soils data from the National Resources Conservation Service (NRCS), the park’s subsurface high-water table was found to be several inches higher than the slab elevation of the subsurface units. Third, the site was designed to accommodate a 25-year storm event, but unable to manage average recurrence interval storms of 50 and 100 years, or 2% and 1% probability events, or 50- and 100-year storm events, respectively.

Although the architect had ensured that the theme park metes and bounds were removed from the floodplain, the site lacked appropriate design considerations and consequential construction implementation. The flat parking lot and perimeter landscaping — with no means of water drainage — resulted in a “tailwater” effect, or the backing up and retaining of water, which created a localized flooding condition. The grading defect, combined with the NRCS-researched high-water table, resulted in the repeated water intrusion and necessity to pump out the subsurface storage units. Further, it was determined that during freezing temperatures, coupled with the high-water table, frost action took place within earthen features, resulting in upheaval and cracking of the subsurface unit concrete slab. Furthermore, the biological growth and powdery deposits, or efflorescence, were the result of moisture that migrated and wicked upward, through the concrete slab.

About the Author

David V. Donovan Jr., P.E., CFEI is a consulting engineer with our Mid-Atlantic Office. Mr. Donovan provides services in the areas of civil and structural analysis, including root cause, damage assessment, and value of loss for residential, industrial, and commercial property. EDT services include origin and cause, failure analysis, and damage assessment. You may contact Dave for your forensic engineering needs at dvdonovan@edtengineers.com or (856) 662-0070.

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