Dec 05, 2019

Post-Flood Storm Water Planning

This article originally appeared in Storm Water Solutions December 2019 issue as "Master Modeling"

storm water planning

The morning after June 30th, 2014,  officials of the city of Cedar Rapids were inundated with phone calls from residents about the previous night's rainfall. Within the first hour of the work day, the sewer superintendent shared that the rain gauge on the roof of a city building had registered a rainfall intensity of 4.28 in. per hour in a 15 minute period between 9:45 p.m. and 10 p.m. that night.

This event became a catalyst for the city to evaluate its approach of addressing localized flooding across the city. The account below details how the city's thinking fundamentally changed. Future storm water infrastructure projects are now more focused, resulting in a more effective flood mitigation program.

The 2014 Storm 

Cedar Rapids is no stranger to the flooding phenomenon. 2008 saw the largest flood in recorded history. The flood of record was a mere 20 ft seen in 1851 and 1929. 2008 saw a flood stage of 31.12 ft. The city then had its second largest flood just eight years later in 2016 where the gauge topped out at 21.95 ft. Usually, Cedar Rapids gets at least a few days notice – usually 48 to 72 hours to prepare.

However, in 2014, the city dealt with flash flooding localized in many seemingly unpredictable pockets. That Monday morning, large areas were ponding that usually do not. Residential basement walls caved in where they once supported homes that sat safely elevated by naturally high topography. Around town, people were sharing grainy pictures on their phones of the night before – wet mail in 4 ft high mailboxes and rushing water flowing through drainage easements. Basement floor drain backups and seepage through basement floor cracks. Washed out trails and highly eroded ravines. Many residents described “rivers” in drainage ditches and channels that came up fast and were empty again in less than an hour. 

It did not take city officials long to realize that they had seen a sizeable event that night. It was clear that the usual approach to solving storm water issues would not be sufficient moving forward. Officials learned they needed to move away from the anecdotal response to analytical and strategic planning. 

The city would have considered its previous approach to drainage problems somewhat objective. It would rain 3/4 in., and the engineering staff would receive a call from a property owner who had seen a basement backup, retaining wall failure, erosion, safety concern or other general flooding complaint. Staff would investigate, write a short report on its findings and ultimately, assuming the city had an obligation to address the issue, add it to a list of pending projects. Based on other priorities already in line, it may take weeks, months or even years to actually see a project to fruition, as a result of the initial complaint. A resident may check in to see where “their project” was on the list or just give up asking if it took too long to address in their mind.

What became apparent in 2014, however, were two clear facts:

The city did not have nearly enough resources to address all complaints from that storm alone in a timely manner, if each issue was addressed separately.

Many issues reported were actually somewhat related, especially those in the same drainage basin or watershed.

Storm Water Planning Moving Forward

As staff strategized next steps, engineers shared that a storm water master planning exercise had not been conducted since 1998. The city realized that instead of seeing each complaint as a single issue to be corrected with an individual project, staff needed to step back and analyze the city’s system as a whole. It was necessary to model the existing system and compare the results to current design standards. Staff intended for the 5-year runoff to be conveyed through the pipes, and the 100-year runoff to be conveyed overland without damaging real property nor creating a safety hazard. 

After internal discussions, engineering staff got to work and hired HDR Inc. to help. They had a limited budget, so modeling efforts would need to be incremental. They settled on performing two models – each at a different scale. The first was a one-dimensional “macro” model, a high-level model. This would analyze pipes larger than 18 in. and drainage ditches. Bottlenecks identified from the macro model would then help staff prioritize the order in which to analyze the city’s drainage basins in greater detail. These detailed “basin-level” models allowed them to pinpoint specific hotspots around town that would occur during events between 5-year and 100-year return periods. It also allowed them to understand how high the water would pond in specific rain events. 

Staff has completed the fourth year of its five year master planning effort and has learned a lot. Some of the most interesting workshops were when the modeling team had city staff ground-truth the results they were seeing. There were a few areas where further detailed study was needed in particular locations because either the model did not pick up known hotspots well or hotspots were identified but not experienced in reality. However, generally speaking, the model results aligned well with the experience of sewer maintenance staff and some tenured engineers, who had been on many of those drainage complaint driven calls in the past. 

Planning Priorities 

After looking at the first two drainage basins, staff was then able to generate a list of projects, a few of which had been identified in the previous master plan, but many that were not. This raw list of projects allowed them to:

Consolidate solutions. One project correctly placed could solve multiple issues. When staff believed they had a solution, they would go back and model that solution, to determine if there was a substantive difference in the result.

Weed out projects that were not as effective as thought. The results of implementing a project virtually were not always obvious. Some proposed solutions were much more effective than originally suspected and some were not.

Accurately estimate the total cost of projects that should be considered for future funding.

The next step in the planning process was to prioritize the list of viable projects. There are a lot of them, even after whittling out the ineffective ones. Before even attempting to move projects up and down the list, staff realized that the list would need to be both defensible to the public and flexible to change. So the team came together once again and debated the criteria that should be considered when selecting a project for funding. They landed on the following categories, which were then weighted: health and safety, cost-benefit, current design capacity of the existing infrastructure, asset functionality, water quality benefits of the project, sanitary sewer inflow susceptibility and future growth potential. Finally, staff took the weighted scoring matrix and applied it in a group setting to each of the projects identified. Each project was given a score, which allowed the team to create a master list of projects in prioritized order. The team convenes each year to re-score projects and review the list based on changing conditions.

In addition to these grey infrastructure projects, the city also is tackling storm water issues with green infrastructure. Specifically:

Instituted a BMP cost share program, with a 50:50 cost share. Both residents and commercial property owners are taking advantage of the program.

The storm water utility fee is now based on a property’s individual impervious areas and what a property owner is doing to mitigate runoff.

The city installed numerous green infrastructure pilot projects to help guide a “Green Infrastructure Master Plan” for infiltration practices in the right-of-way all over town.

Now, after four years of modeling and three years of project implementation, staff is starting to see some projects get completed. 

Master planning efforts also have allowed staff to see the big picture in two other areas. 

They have spent time analyzing and quantifying the cost of growth in areas where growth is anticipated. This will allow staff to have more intelligent and educated conversations with developers on how to appropriately pay for the storm water infrastructure necessary for future growth.

Staff has looked closer at the idea of planning for and implementing regional detention basins around the city.

As a result of performing the master plan, the city now optimizes projects to be as effective as possible at minimum cost and can eliminate the projects that are minimally effective. 

About the author

Sandy Pumphrey is assistant development services manager for the city of Cedar Rapids. She can be reached at [email protected]