Urban flooding isn’t anything like an “average” flood. Urban flooding, also sometimes referred to as “urban inundation,” is a chronic water flow issue caused by increasingly urbanized/built-up landscapes that lack the capacity to absorb otherwise average rainfall. And its devastation rivals that of major flooding events in the communities that are affected.
What are the Characteristics of Urban Flooding?
Flooding caused by impeded water flow in cities is not just a regular flood. And while its cause is different, it is every bit as devastating as the major flood events you may see on the news, causing millions of dollars in damage.
Unlike natural disaster events, like hurricanes or major storms, even “normal” rainfall can cause urban flooding: flooding basements and backing up sanitary and storm sewers throughout a community.
The problem, as defined by the Federal Emergency Management Agency (FEMA) is that these chronic flood events are characterized by “the inundation of property in a built environment, particularly in more densely populated areas, caused by rain falling on increased amounts of impervious surfaces and overwhelming the capacity of drainage systems.”
“Impervious surfaces” refers to any pavement, building, and/or edifice that does not allow water to flow into/through it back into the ground. The planners of these cities built in storm drainage to accommodate the overflow, but as cities continue to sprawl, the amount of rainfall being routed into the storm sewer system can quickly overwhelm it.
Beyond sprawl, shifts in ground soil, pipe settlement, sag, and sediment inside storm sewer lines can change expected/recorded pipe inclinations to impede the flow of water, which can impact urban flood models. Verifying sewer line inclination is actually one of the simpler solutions available to wastewater managers, which will be covered in more detail below.
The Cost of Urban Flooding
Examples of urban flooding abound. Often hidden in data as “torrential rains” or mere floods (without a major named storm attached), “On average, 1.9 significant flood or torrential rain events occur each year, and $362 million of losses to insured property are incurred each year from such events… In 2016, the total paid losses (approximately $2.9 billion) were dominated by severe storms and flooding in Louisiana, which caused more than $2.2 billion in losses.”
It is important to note that in 2016, “Louisiana was spared from damage by any tropical storms or hurricanes for the fourth straight year [in a row],” and that the 15 named storms including seven hurricanes, made landfall in regions other than Louisiana. Yet, Louisiana accounted for $2.2 billion in losses due to torrential rainfall, according to insurance records.
A joint study and paper by the University of Maryland and Texas A&M University, Galveston set out to find and identify what caused urban inundation, define its extent and consequences, and look for mitigation solutions.
They found that “urban flooding is… a growing source of significant economic loss, social disruption, and housing inequality. Extensive suburban development that creates higher flood flows into urban areas, aging, and frequently undersized infrastructure in older centers of communities, an inability to maintain existing drainage systems, increases in intense rainfall events, and uncoordinated watershed management all contribute to these increases…”
The study also concluded that “Many of the urban wastewater and stormwater systems that provide the backbone of urban flood mitigation are in poor condition and – in some locations – are inadequate and in need of strong support.” And among its 10 conclusions, the paper stated that Data… are not easily available or shared with local decision-makers, researchers, and the residents themselves. More accessibility and availability of data is critical to effective response, recovery, and long-term mitigation of flood events.”
The greater cost, cited by the National Resources Defense Council, is the “cascade of effects on affected individuals and communities.” The authors of the joint Maryland and Texas study found that lower-income residents are the most affected, and that’s before accounting for the health impacts of chronic floods. The Center for Neighborhood Technology found that 84% of those who suffered with urban flooding in the Chicago area were under stress, with 13% citing their health or the health of someone in their household being affected. Asthma resulting from exposure to flood-caused molds is an increasing concern for those populations.
How Can You Plan for & Mitigate Urban Flooding?
New information published in Nature in 2024 suggests that water/wastewater planning utilizing an urban mean-flow theory, considering the layout of “urban forms,” which are described as a “complex system much akin to granular media, disordered porous solids, glassy systems, and complex fluids,” among others are the key to overcoming the impervious surface problem.
And, when it comes to predicting how urban floodwaters will behave, one-dimensional and two-dimensional (1D and 2D) models and statistical simulations are the go-to for municipal water and wastewater managers, urban planners, and insurers.
The Storm Water Management Model (SWMM) is one of the most common models for predicting flood drainage for municipalities, particularly in urban environments. However, some researchers posit that the 1D model, with its focus on pipe nodes, is incomplete, in no small part because it does not allow for how water will flow through an urban landscape.
They argue that a coupled 1D and 2D model, a “dynamic 1D-2D model” is more effective at mapping surface inundation.
What are 1D and 2D Hydraulic Models & What’s The Difference?
A 1D (one-dimensional) hydraulic model allows for water flowing along a specific path, in one direction, like a river. A 2D (two-dimensional) model deals with water than can flow throughout an entire area with multi-directional flow.
1D refers to the mathematical output from the model, which delivers as a single water level, a single velocity, and a single flow rate for each node (pipe node).
These models utilize “Saint-Venant” equations, also called shallow water equations, to calculate expected flow. 1D models are great for providing a cross-sectioned average of the surface elevation of the water and its velocity, but can only calculate flow either forward or backward. Engineers will utilize equations like continuity and momentum to factor variables, and computer models that are programmed with these and other equations make the computations much simpler.
2D models, by contrast, allow for the creation of a grid that spans the floodplain. It creates a series of connected spaces for hydraulic calculations, and when those calculations are combined, the grids allow for a more accurate flood map over any given plane. 2D models can calculate flow forwards, backwards, and left and right, indicating multiple potential flow directions. The finer the grid pattern, the more options for flow calculations, and the closer the model gets to the reality of urban flooding.
More and more urban planners and water and wastewater managers are opting for the combination of the 1D and 2D model approaches: modeling rivers, sewers, and other flowing water systems in 1D, and everything else in 2D, to create the most accurate and easy to read predictions for urban water flow. There are several software programs that are able to run these hybrid 1D and 2D flow models, which can be very helpful.
Storm & Sanitary Sewer Inclination – A Missing Piece of the Computational Puzzle
No matter the model type being used, it will be flawed unless the accurate existing pipe inclinations are used when calculating storm and sanitary sewer system flow rates. Sediment, settling, and other impediments can drastically affect flow rates, and even bring them to a standstill. And without an updated inclination report, the flow cannot be accurately predicted, which will throw off the effectiveness of any model.
What is an Inclination Report?
In general, sewer scope reports, like the highly detailed NASSCO video pipe inspection (VPI) reports by GPRS, do not track inclination in and of itself. Inclination reports, also known as pipe slope or grade reports, provide an important data set for any wastewater system. The inclination report will include information on:
- Pipe Alignment
- Sag
- Irregularities
- Settlement
These reports can find areas that are not seen on record drawings/as-builts because they have developed over time as the pipes and ground around them have moved, and sediment may have built up. All of the factors above can impact flow direction and rate and impact existing models.
The good news is that inclination reporting can be added onto a GPRS NASSCO VPI report from the start to provide municipal managers and insurers with more detailed and accurate information on flow direction and rates.
GPRS specializes in visualization services for water & wastewater managers, nationwide. We Intelligently Visualize The Built World® for customers across the U.S. What can we help you visualize?
Water & Sewer Damage Awareness Week
If you are a wastewater or municipal manager in need of tools to mitigate flood or contamination risk for your systems, GPRS offers complimentary safety training for your systems via Water & Sewer Damage Awareness Week (WSDAW).
This year, WSDAW takes place October 21-25, 2024. If you would like a GPRS wastewater and sewer expert to give your team tools and strategies to help mitigate inflow/infiltration and other water and wastewater issues, click here to register for your free WSDAW talk.
