Nuisance Flooding

By: Tiffany Cousins, M.S.

Nuisance flooding is a poorly understood growing problem as urbanization increases and climate change unfolds. Due to scale and scope, nuisance flooding like other small-scale disasters, do not receive major attention from federal agencies in regard to recovery (Allaire, Moftakhari, Matthew, AghaKouchak, & Sanders, 2018; Cowell & Desouza, 2018). It is not a federally declared disaster and therefore there is a lack of federal assistance ((Allaire et al., 2018)). FEMA IA and SBA loans require a declared disaster for funds to be allocated. Additionally, pluvial flooding (rainfall-driven flooding) is not within the terms of US National Flood Insurance Program and therefore excluded from insurance coverage, flood hazard mapping, and consideration in flood management projects ((Rosenzweig et al., 2018) There is currently no proven or widely agreed upon methodology to systematically monitor nuisance flooding. Therefore, there is a lack of available data.

Nuisance flooding can be defined quantitatively, by its impact, and by hazard type. In literature, nuisance flooding is defined as a flood that is 3-10cm (~1-4 inches) with a velocity of up to 3m/s, causes no significant damage, and is cumulative. Unlike extreme infrequent events, nuisance flooding is not large enough to cause significant property damage or significantly threaten public safety. However, it is large enough to cause public inconveniences, economic loss, infrastructure stress, real estate devaluation, and heightened public health risk (Allaire, Moftakhari, Matthew, AghaKouchak, & Sanders, 2018; Moftakhari, Aghakouchak, Sanders, & Matthew, 2017; Rosenzweig et al., 2018).

The impact on transportation, property, and public safety also drives how nuisance flooding is defined. At the height of 13cm, passengers cannot safely open their car door, and at velocities of 10-30cm, cars begin to lose their stability and could be swept away (Allaire et al., 2018). For buildings constructed with the entrance at ground level or residential property constructed at grade, the building contents are vulnerable to flood damage if flood levels pass the top of curbs (Allaire et al., 2018). Therefore, nuisance flooding should be limited to 10-20cm as that is the height above the crown of roadways (Allaire et al., 2018). Lastly, any depth of water can cause public health and safety concerns. Ponded waters can house mosquitos and other disease vectors and transmit electric shock, while floods from surcharged sewers can hold bacteria and toxic contaminants (Allaire et al., 2018). Therefore, the definition of nuisance flooding is an upper limit of 10cm and a lower limit of 3cm. The lower limit of 3cm is established to account for pedestrian inconveniences (wet shoes, mitigating having every rainfall event be a nuisance flood, and (Allaire et al., 2018).

This definition does not limit the context of flooding to only high tide flooding but includes fluvial and pluvial flooding. Nuisance flooding is also known as Minor flooding in fluvial contexts and is known as high-tide, clear-sky, or sunny day flooding in coastal contexts (Allaire et al., 2018).

Lastly, nuisance flooding is defined as a cumulative hazard. A cumulative hazard is a small-magnitude frequent hazard whose cumulative cost exceeds the costs of infrequent extreme hazard (Moftakhari et al., 2017). By this definition, nuisance flooding is similar to repetitive flooding. Properties that are defined as repetitive loss are properties that have 2 or more losses greater than $1000 within a 10-year span. However, this is for properties insured by the NFIP and within the floodplain. However, due to increased urbanization, flooding happens outside the floodplain ((Haz & Plan, 2011)).

Given the increased need to prioritize funding for at-risk areas and the growing risk of flood hazards, the lack of flood mapping data is problematic because we can expect there to be populations in high-risk areas that will not have the necessary resources available to be resilient. If we do not know the flood risk of localities, we cannot accurately allocate necessary funding and therefore these communities will not be able to recover when another event comes. However, existing research suggests that citizens in these unmapped localities actually have the local spatial knowledge needed to inform us of their flood risks. Thus, the problem may not be a lack of knowledge at all; instead, it might just be that we have not yet been able to bridge the gap between the people with the local knowledge about flooding and the people who make decisions about how we plan for resilience to flooding at the local level.