Should We Place Any Value in the 100-Year Storm?

by Michael Igo, Affil. ASLA, PE, D.WRE, LEED AP, CID

Impacts of floodwaters
ASLA 2020 Professional Analysis and Planning Honor Award. Lumberton Community Floodprint: Strategies for Repurposing Vulnerable Landscapes After Disaster, Lumberton, North Carolina. NC State University Coastal Dynamics Design Lab. / image: Lee Stevens

In the age of awareness of climate change, we often hear the terms “100-year storm,” “500-year tides,” or “25-year drought” thrown around. Intuitively, we tend to think that a 100-year storm occurs once every 100 years. However, this is only partly true, as there are key phrases missing from this notion: a 100-year storm will occur once every 100 years on average and based on past data.

The use of an X-Year event derives from what is known in mathematics as the exceedance probability, or the likelihood of an event being greater than a predefined parameter in a given timeframe. Statistics of past data (storms, tides, earthquakes, etc.) are used to create charts based on size and the probability of occurring.

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Areas in Landscape Design Impacted by Water Conservation

by Michael Igo, PE, LEED AP, Affiliate ASLA

Irrigation
image: Aqueous Consultants, LLC

Water conservation is an important topic in landscape architecture, as its professionals are stewards of the built and natural environment for society. Without a balanced water supply, drinking water, sanitation, ecological balance, and safety cannot be secured for our existence. Thus, when we speak of water conservation, what we really mean is freshwater or domestic water conservation.

Furthermore, some of the areas of water conservation discussed below are not only about water quantity conservation, but also water quality conservation in our natural surroundings. The adage “an ounce of prevention is worth a pound of cure” is true in water conservation in the sense that not only does saving and using less water in landscapes reduce the quantity of water consumed, but it also prevents poor quality water for reuse or filtering site water from being reintroduced into the environment and conserves existing precious clean freshwater for domestic uses and for habitats.

Below are some of the major areas of concern that the ASLA Water Conservation Professional Practice Network (PPN) and ASLA members should keep in mind in their practice.

Stormwater

When a site is altered from its natural or previously disturbed state, the patterns of rainfall runoff and infiltration are also altered. Natural areas that were once reliant on ample infiltration can be deprived of recharge from paved and developed sites by sealing off underground soils from the atmosphere or just by increasing the velocity of water movement across a site such that runoff does not have time to infiltrate. High velocity, unmitigated stormwater flow can cause erosion of valuable land areas, transport sediment that could fill in and cause eutrophication of natural water bodies, cause damage to sites and structures downstream, and serve as a massive heat exchanging liquid as rain falls on, passes across, and carries away the latent heat trapped in urban pavement—polluting otherwise ecologically-balanced freshwater supplies and habitats.

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