Safe Conveyance of Rare Storm Events

by Brian Wagner, PE

(This is a synopsis of a more detailed article that was published in the May issue of Stormwater magazine.)

On the evening of July 30, 2016, two people were swept downstream and died during a catastrophic flash flood that struck the historic mill town of Ellicott City, Maryland. The infrastructure designed to carry stream flow and storm runoff along Tiber Run, which runs through the historic district, was unable to meet the demands of the 1,000-year storm event that produced 6.6-inches of rainfall in a three-hour period, according to a local National Weather Service (NWS) rain gauge. The unprecedented storm event caused damage never seen in the area before. Roads were literally washed away and more than 200 vehicles were damaged.

Ellicott City Flood River
Photo Credit: David Byerly
Ellicott City Flood Sidewalk
Photo Credit: David Byerly

Storms of this magnitude are uncommon; however, when they do occur, the damage is extraordinary. The power of water is relentless, and high-velocity flow has the ability to convey enormous volumes of water in moments. The civil engineering community is deeply challenged with designing infrastructure capable of transporting typical design storm rates downstream. Often, these systems of pipes, culverts and channels become an afterthought to the general public until they cannot meet flood demands, yet engineers formulate what-if scenarios on a regular basis throughout the design process.

Small Pond Analysis

Designers and engineers routinely consider storm events up to the 100-year flood but rarely are required to consider more substantial events. Land development and improvements to existing sites generally require stormwater management controls, which include quantity management and water quality treatment. These controls are typically focused on the 10-year 24-hour duration storm event, which has a 10% chance of occurring in any given year. In most cases, this is the normal requirement; however, in areas of known flooding, including the Tiber watershed, quantity attenuation of the 100-year 24-hour duration storm is required. This approach theoretically reduces flooding by managing peak rates of runoff from development sites by storing the water within stormwater management facilities and controlling the release rate downstream.

Ellicott City Flood Tiber WatershedUtilizing the rainfall distribution and data obtained from the National Weather Service from the Ellicott City storm, the Natural Resources Conservation District’s (NRCS) winTR-20 was used to look at three different stormwater management facilities to see how they would handle a similar event falling within their specific watershed in the design setting. Results of the simulated Ellicott City storm were compared to both the 10-year and 100-year 24-hour design storm events.

Each of the facilities analyzed were designed to manage a 4.69-inch, 10-year, 24-hour storm event to “pre-developed” (meadow/woods) conditions and safely convey storms up to and including the 8.06-inch, 100-year, 24-hour storm event with at least one foot of design freeboard. In the design scenarios, each was analyzed utilizing winTR-20 to evaluate and determine peak discharge rates and attenuation depths within the facility. The study incorporated the 6.6-inch, 1,000-year storm that struck Ellicott City in just three hours.

Surprisingly, in all three cases, the Ellicott City 1,000-year, three-hour event essentially matched the 100-year, 24-hour event. While this is a small sample of facilities, it shows with consistency that while the duration and intensity exceed the 100-year design storm, the overall conveyance and storage depths nearly match the longer duration 100-year design event. Obviously, this does not constitute what will happen in every scenario; however, it does show that a design focused on safely conveying the 100-year 24-hour duration event through the facility will, with some confidence, safely convey some rare, high-intensity storm events.

Site 100-Year Discharge Rate 100-Year Water Surface Elevation 1,000-Year Discharge Rate 1,000-Year Water Surface Elevation
1 449.4 cfs 854.37 424.8 cfs 853.79
2 196.5 cfs 712.55 180.4 cfs 712.46
3 1026.7 cfs 691.85 904.8 cfs 691.50

This initial finding seems to be consistent with the 2014 Ellicott City flood study that indicated 100-year flood depths along Main Street to be one-to-two feet. If in fact, the 1,000-year, three-hour event would materialize with a similar peak rate to the 100-year, 24-hour duration, storm flow depths through the city would inherently be similar. Based on available photographs and videos from various news outlets, these flow depths seem to be relatively consistent.


It is unrealistic to design storm drain infrastructure to safely convey rare storm events that may never occur. The cost for this infrastructure would be astronomical; however, when site conditions permit, an engineer should consider the value of increasing segments of infrastructure. Although not intending to over design systems that add unnecessary cost to the developer or owner, minor design changes such as increasing pipe slope or upsizing pipe segments to provide better design flows can be beneficial to safely conveying occasional high rate flows and build resiliency into the system and site.

In coordination with state and federal regulations, stormwater management and drainage principles and practices continue to evolve. This evolution includes advanced treatment methods for improving water quality along with the goal of reducing runoff during storm events. It is unrealistic to believe that one analysis or design practice would provide an exact prediction of a rare event.

With a better understanding of unusual and infrequent storms, additional case study information, and a compassion for both human life and environmental impacts, I believe we will only get better at predicting and designing to reduce flash flooding events. Nevertheless, nature is powerful and relentless. No design is perfect, and no matter how unlikely a storm is, it is impossible to design and construct infrastructure to safely convey all storm event runoff safely.

Read the complete article including more detail on the storm, watershed and analyses that was published in the May issue of Stormwater magazine.