Flooding Denver Streets!! Setting your Finished Floor Elevation.
Updated: Oct 21, 2020
In land development design, one of the first things you want to do is set your building's Finished Floor Elevation (FFE). In the City of Denver, the default required elevation is 24" above adjacent street curb line. It's vitally important during the concept phase of your project to set your FFE correctly. Failing to do so will create delays, surprises, and negative building impacts. It will even anger your architect.
City of Denver has flooding streets (AHHH!) in major storm events. If you live in the City, you've probably noticed it during a storm. As you drive around, full intersections will flood and low lying streets will become rivers. For example, I have a vivid memory of watching a cocky Esclade driver get stuck in a street "river" as he tried to breeze by all the smart drivers in the flooded inside lane of N Federal Blvd. That's not typical in many cities around the United States so... what's up with that?!
Lets do a quick history lesson here. When City of Denver's storm infrastructure was initially constructed many years ago (some as old as 1890), it wasn't design to handle the modern understanding of storm events. It was undersized and this means that when a major storm rolls through Denver. We get excessive street flooding, which many times over-tops the curbs/sidewalks. The City knows this and has a plan in place to up-size storm pipes in the City (eventually). This is a called the "Storm Drainage Masterplan" and includes all of the projects required to modernize the City's stormwater systems. In the interim, the burden of ensure that "street flooding" doesn't turn to "building flooding" is the burden of the Civil Engineer.
Lets go to how your civil engineer can set your finished floor elevation in the City of Denver. I'll assume for the purposes of this article that you just received your concept comments from the City of Denver and you have some questions.
What's a Flo-2D?
The City of Denver hired an engineering firm several years ago to prepare a detailed flood model on the most problematic portions of the City. This model is called a 'Flo-2D'. This model's results are not public information but you can obtain a copy of the data in your project's area by asking your City Wastewater reviewer. If a Flo-2D exists adjacent to your project, it will drive the elevation required for your building. You will take the '2 times 100 year' elevation and set your finished floor elevation above that.
Can I just set the FFE of my building 24" above the curb flowline?
If there's no Flo-2D, YES. Worth mentioning, each street facing door will need to be set 24" or higher than the curb flowline adjacent to that door. Also, this can make ADA accessibility tricky with those doors. Many times you will need a ramp on your property to get up to those door heights.
My project is heavily burdened if I need to set the door elevations that high. What can I do?
Your Civil Engineer will need to prepare a Flood Study analyzing the 2 times 100-year flood depth in the street adjacent to your project. That study will analyze upstream the watershed and determine your flood depth in the 2 times 100 year storm event. It's not uncommon to see 500+ CFS analyzed in this scenario. Once you've set that elevation, you can set your building's elevation above that number. You can also set your building FFE 12" above the determined 100 year flood elevation (vs. doubling the flow). That said, typically the 'double option' is more advantageous for projects.
Raptor Civil Engineering has completed dozens of these studies and has experience with the City's wastewater staff. We are always happy to provide help during your concept phase to ensure your design is completed correctly and no surprises arise during permit.