City of Englewood
In the summer of 2018, the City of Englewood experienced an extreme rainfall event cataloged by Urban Drainage & Flood Control District (the District, Urban Drainage, UDFCD) as a "major" rainfall event, meaning that in isolated areas, the rainfall rate experienced had a less than 1% chance of occuring in a given year.
The flood event resulted in one tragic death, caused wide-spread damage, was front-page-news, resulted in a sinkhole in a major roadway, and created a sense of urgency for stormwater planning throughout Englewood.
The City of Englewood Public Works Department immediately set about the task of prioritizing stormwater improvements through revitalized stormwater master planning.
The july 2018 storm
Understanding a "STorm Event"
What does it mean to experience an "X-Year Storm Event"?
Think of a 5-year storm event as a cup with four white marbles and one red marble. If you randomly pick one marble out of five, the odds of you picking out the red marble is one out of five. If you replace the red marble and pick again, the odds of picking the red marble are still one out of five. But, you could in fact pick the red marble twice in a row. This is the same with the 5-year storm event. The odds of experiencing the 5-year storm are one out of five in a given year, but it could happen twice in a row, even in the same week.
The same is true for a 25-year, 50-year, or 100-year storm event. A 100-year storm event is like having 99 white marbles and one red marble. The odds of picking a red marble are one out of 100, but you could draw that red marble at any time, even twice in a row.
According to Urban Drainage and Flood Control District:
On July 23, 2018, up to 1.3 inches in 10 minutes fell in this area of Englewood with as much as 2.7 inches in less than an hour.
On July 24, 2018, another 1 inch was recorded in 10 minutes with 1.7 inches in less than an hour.
Please see the UDFCD Rainfall Assessment below for more details on the storm of July 2018. Please note, all numbers and analysis from UDFCD has specifically been taken from UDFCD resources with little to no changes in wording.
This monsoon rainfall generated storm runoff that exceeded storm sewer capacities, expected street capacity, and resulted in significant flooding of homes and businesses.
According to Urban Drainage and Flood Control District and based on gridded gauge-adjusted radar rainfall estimates (GARR), on average, the storms of July 2018 were categorized as having a 1-2% Annual Exceedance Probability (AEP).
According to NOAA Atlas data, the precipitation frequency estimates show this level of rainfall equating to a 10-25 year event for 5-minute rain gauge data and 50-100 year event for the 10 to 30 minute rain gauge data.
The resulting runoff caused significant flooding in streets, damage to many properties, and tragically led to a flood-related death in the South Central Englewood Basin. Readers should note that the amount of rainfall, how the storm cell is distributed, and its direction of travel can all impact runoff at a given location, so categorizing this as a 50-100 year event does not relate to the exact depth of flooding.
Wright-McLaughlin Engineers. Project No. 9203 Harvard Gulch Flood Control $2,300,000 General Obligation Bond Issue. Sponsored by City and County of Denver Department of Public Works Engineering Division. Summer 1967.
Gingery Associates. Flood Hazard Delineation Harvard Gulch, West Harvard Gulch, & Dry Gulch. Sponsored by Urban Drainage and Flood Control District, City and County of Denver, and Arapahoe County. December 1979.
Summary of Previous floods and flood Studies
In 1971, large areas of Englewood neighborhoods reported localized flooding and ponding. It was determined in the 1971 Storm Drainage Plan for Englewood by Sellards and Grigg that this flooding was attributed to a 'lack of outfall.' In other words, urban development had spread into local creeks and rivers, blocking the water from being able to leave the area.
According to the 2017 FHAD (Flood Hazard Area Delineation) created by Matrix Design Group, another major flood occurred on July 8, 2001. No major damage was reported, but the storm system was at full capacity and caused damage and flooding to detention areas. In the same report, Matrix notes that other major flood events that exceeded pipe capacity and caused damage occurred on July 19, 2011, June 6, 2012, August 8, 2013, June 11, 2015, and June 24, 2015. During these events, water escaped the storm pipes via manholes, sinkholes, and inlets, and caused major street and residential flooding.
Most of the previous studies listed above concluded that the best course of action was to increase the size of the existing storm infrastructure or to add additional storm infrastructure to take floodwater underground and away from homes and buildings.
During the subject study, Calibre visited many homes in the study area and noted efforts by individual homeowners to floodproof their homes. This indicates that the 2018 flood was not the first to result in damage to homes. Many eye witnesses related past flooding of garages, basements, and the main floor levels. It is a common belief among homeowners that the frequency and severity of flooding has increased over time, although the comparison of modeling data from 1971 to today does not necessarily support that belief.