#OurWater

Discussion on Storm Events and Their Return Frequencies

Some Key Points from the Discussion

Although each rain event is unique, events are typically categorized by storm frequency. Events whose total rainfall (or rain intensity)
fall at or near a 5 year storm, would be categorized as an event with a 5 year return period.

It is important to note that the nomenclature, which refers to storm events, implies that the return period of a storm is representative of its
frequency. For example, a 10 year storm may imply that the storm will only occur once every ten years. This is not the case. Instead, the
10 year storm has a 10 percent chance of occurring in any given year.

Climate change appears to amplify the effects of extreme weather, thereby increasing the frequency of extreme storm events.

Stormwater management systems are designed to provide a certain level of service, such as the 10 year 24 hour storm. However, in
practice the capacity of these systems may be overwhelmed by larger, more intense, storm events. While these designs mitigate
significant risk, they do not entirely eliminate the risk of flooding.

Building infrastructure that addresses the most severe events would not
be feasible from a cost, siting, permitting, and operational perspective. Additionally, there are events that exceed the capacity of even
the most robust infrastructure – drainage systems designed for a 100 year storm cannot handle a 500 year storm event.

For More Information, please see committee memorandum.

Sunny Day Flooding and King Tides

Slides from the Presentation

Stormwater Disposal Overview

Slides from the Presentation

Discussion on How the Cities of Miami and Miami Beach Measure Flooding

Some Key Points from the Discussion

The City of Miami Beach owns and operates a stormwater utility that mitigates the impacts of rainfall flooding throughout the City.  The City’s Stormwater Operations Division frequently cleans and maintains stormwater infrastructure to ensure its sustained viability during rain events.  Moreover, in an effort to continuously improve the level of service, the City uses a data driven approach to measure the efficacy of the stormwater system, analyze flood events, and has adopted a prioritized list of neighborhood improvement projects.

Staff has presented to LUSC detailing the various equipment and procedures currently used by the City to measure and track flooding.  Data collection methods presented included precipitation, tide, and groundwater gauges, deployment of engineering teams citywide to record anticipated rain events, and resident flooding complaint logs collected in Cityworks, EnerGov, and calls to the Public Works Control Room.  Staff additionally presented how the PowerBI tool is used to integrate resident complaint logs with tidal data to analyze historical flooding data within the City. 

The City is able to examine types of flooding within different categories to analyze flood events when they are reported. Flooding is measured and recorded in the following format:

· Standing water: 0-3 inches of water usually accumulating on the lower side of the street.

· Significant standing water: 3-6 inches of water which may cover most of the roadway, but traffic can continue to travel through it.

· Stormwater flooding: 6-12 inches of water which could be a nuisance for the traveling public and could inhibit traffic from using the roadway.

·  Major flooding: 12 inches or more, at this point residents and property could be in danger.

Additionally, the City’s groundwater data has been compared to NOAA’s Virginia Key tidal readings. A direct correlation has been found between the tidal elevation and groundwater elevation, albeit with a minor delay – the groundwater level rise can be delayed by approximately 30 minutes.  

Public Works continuously reviews the data and uses it to inform long term design decisions, as well as decisions to implement short term relief measures, such as temporary pumps. Moreover, the data is used to keep the public informed, provide data for bond rating agencies, and plan for future projects and operational provisions, like the deployment of temporary pumps and assessment teams to different zones.

For More Information, please see committee memorandum.

Water and Wastewater Master Plans

Water, Wastewater, and Stormwater Best Management Practices

For More Information, please see committee memorandum.

What is Affecting Biscayne Bay & City of Miami Beach’s Stormwater Management

Slides from the Presentation