Learn how to perform a detailed analysis to determine if, how, and when coastal stormwater systems will be compromised by flooding.
Familiarity with stormwater engineering concepts are needed.
The data obtained from this site’s Quick Flood Assessment Tool are helpful for completing the detailed analysis.
Data from this analysis will allow the user to move to the next task, which is determining next steps using the “Take Action” section of this site.
Before starting the analysis, several decisions are required. These considerations are summarized here, followed by a more detailed explanation for each topic.
How good is good enough?
This is a judgement call regarding how seriously a community views flooding issues and how much flooding a community is prepared to tolerate as floods become more frequent and impact the local economy. The answer to this question will help you determine what level of analysis is needed (Basic, Intermediate, or Advanced).
What’s the scenario?
What risk level is assumed? Before diving into an analysis, decide how many and which design scenarios are needed. Design scenarios are used to help estimate the range of conditions that might occur in a community. The scenarios help pinpoint when and where the stormwater system is at risk of being compromised.
How Good is Good Enough?
The challenge before starting the analysis boils down to this: how good is good enough? The level of analysis that you need will depend on several factors, including the frequency of flooding, capacity of the stormwater system, depth of flooding across the community, time needed to recover from the flooding, and impact of rising sea level in the future. But socioeconomics plays an important role too. How seriously a community views flooding issues, and how much flooding the community is prepared to tolerate as floods become more frequent must be evaluated, as well as the impact on the local economy (direct and indirect damages).
The level of analysis needed to evaluate existing stormwater infrastructure and design new infrastructure to be resilient to current and future floods will vary according to these and other factors:
Criticality of components of the overall stormwater system (outlets, pipes, inlets)
Vulnerability of the stormwater system components
Expertise needed to analyze impacts
Current and future socioeconomic impacts
Looking across the array of available data, models, and methods, we have identified three broad levels of analysis: Basic, Intermediate, and Advanced.
While the detail, degree of complexity, and accuracy increase with each level (as does the effort, of course), the degree of uncertainty in the results decreases. A basic-level analysis may be sufficient if you’re starting a broad-scale planning effort and need a general sense of current or future problems, while an advanced analysis using highly detailed, state-of-the art modeling may be essential for the design of critical pump stations or other facilities essential to public safety.
This level uses existing data, tools, and resources (NOAA water level data, existing inundation maps, existing FEMA flood studies, published threshold and sea level scenarios, simple calculators, etc.) to answer questions about a stormwater system.
Further consideration may help you determine that the basic approach is not applicable. The basic approach should be used for a screening-level analysis only. Do not make decisions about exact cost and design based on the basic analysis alone.
This level uses existing published model data, basic GIS data and GIS expertise, simple modeling capability (such as NOAA Sea, Lake and Overland Surges from Hurricanes (SLOSH) model output, FEMA flood study model output, GIS data of stormwater system), and elevation data.
This level incorporates original modeling of coastal and inland total water levels for specified storm and climate scenarios, lidar-based digital elevation models, robust GIS or CAD data of the stormwater system, and the assistance of engineering and geospatial modeling experts.
What’s the Scenario?
Another critical decision to make before diving into analyzing impacts is to decide how many and which design scenarios you need. Design scenarios help you estimate the range of conditions that might occur in your community. You’ll use the scenarios to pinpoint when and where the stormwater system is at risk of being compromised.
The design scenarios you choose should incorporate the combined effects of coastal flooding and precipitation, and should mirror the range of combined flooding conditions likely to occur. Design scenarios range from current conditions to possible future conditions. Ideally, they will help you assess the most important contributors to flood risk in your community.
Design scenarios should include the entire spectrum of current flood risk (low to high, defined by the probability of occurrence and the amount of impact or damage), while also taking into account changing future conditions like sea level and lake level rise, increases in extreme precipitation events, and changing population and land use. As risk increases, more impact and damage will most likely occur but the probability decreases. Extreme flooding has a relatively low probability of occurring but will have major consequences if it does.
Current highest tides + minor precipitation event +/- (with or without) sea level or lake level change
Minor storm surge + moderate precipitation event +/- sea level or lake level change
Moderate storm surge + major precipitation event +/- sea level or lake level change
Major storm surge and/or extreme precipitation event +/- sea level or lake level change
Stormwater systems are not designed to handle extreme risk, extreme coastal water levels, such as storm surge, and extreme rainfall events, such as a 100-year rainfall. In fact, many of today’s current systems may only be designed for a 10- to 25-year rainfall event. We must assume, then, that combined extreme storm surge and precipitation events will overwhelm these systems now and in the future. Designing and building systems to handle extreme risk events is cost prohibitive.
Programs such as FEMA’s Community Rating System and state regulations have prescribed scenarios that must be used to determine the impacts of coastal flooding. For example to get Community Rating System credits, a coastal community should evaluate at least an intermediate-high sea level scenario on its drainage system and the resulting flooding if it takes no action to mitigate impacts. FEMA coastal flood insurance studies model the 100-year stillwater elevation (reflecting storm surge and wave setup ), local wave action to determine flood zones, and base flood elevations for Digital Flood Insurance Rate Maps. Flood insurance studies (including riverine studies) often include additional return periods (for example, 500-year, 50-year, 25-year, 10-year). The lower return periods may be helpful for inclusion in stormwater design scenarios. Many states have prescribed design scenarios, such as 10-year or 25-year, that must be evaluated to receive a stormwater permit before construction can begin.
In addition to describing different potential coastal total water level and precipitation conditions (leading to low, moderate, or high risk), design scenarios could include adaptive actions, such as backflow preventers, augmented seawalls, pumps, and green infrastructure . The service life (how long the system will be used) can also be changed to develop future scenarios. These additional considerations would allow you to assess the efficacy of different types of protective measures (see “Take Action”). As such measures are implemented, you should revisit your design scenarios and, if necessary, revise your analysis.
Design scenarios should include a combination of flooding sources and adaptive actions. An iterative process should be used to tweak the input flooding scenarios and adaptation actions to determine the best scenario that addresses the flood risk and is the most cost effective.
Flooding Source Examples
Tidal Flap Valve
Sea Level Rise
Highest Astronomical Tide
There is a lot to think about in terms of the trade-offs in approaches and what future design scenarios you need to consider. Sound complex? It is, but don’t worry. In the remainder of this section, we provide the steps and data needed for a basic-level analysis, with a real-world example of how to apply it using several design scenarios. We also provide information on what would be needed for intermediate- and advanced-level analyses.
Detailed Impact Analysis: The Methodology
To evaluate whether your stormwater system will achieve the desired level of service during a coastal flood event, you must first define your design scenarios. As an example, let’s examine the following design scenario:
Focus year is 2043 – 25 years in the future to account for sea level rise.
Assume the system doesn’t have backflow valves in place to prevent seawater from entering the stormwater system.
Assume a 10-year (10 percent chance in any given year) precipitation event, because the stormwater system was originally designed for this event.
To estimate the impacts of this scenario, you will follow five principal analytical steps, as shown in the figure below.
A summary of each of the analysis steps appears below. For each step, the modeling and data needs are outlined for the desired analysis level (Basic, Intermediate, Advanced):
Step 1: Compute Coastal Total Water Level at Coastline or Outfall ×
Step 1: Summary
In this step, you will calculate standard coastal water level components, such as tidal elevations and regional oceanographic conditions, with optional components corresponding to each design scenario (for example, the addition of king tides or projected future sea or lake levels). The output is a coastal total water level that can be compared against the elevation of the outfall and coastline (see figure below). If the coastal total water level exceeds the elevation of the outfall, and no tidal backflow valve is present, then the system can be assumed to be overwhelmed. If a tidal backflow value is present and functioning as designed, then the coastal total water level should be compared to the lowest coastline elevation to determine if overtopping would occur.
You can use these stormwater outfall elevations as thresholds for Quick Flood Assessment Tool to determine current and future vulnerability, and convert between datums.
Step 1: Basic Analysis
For our example, we need to consider the forecast sea level in 2043, which we can estimate using one of several available forecasts, including these:
If we want to look at present-day flooding, we can use base flood elevations shown on FEMA’s Digital Flood Insurance Rate Maps, which include 1-percent annual chance or 100-year stillwater flooding plus local wave action.
We’d also need to consider tidal effects. For design purposes, we’re interested in the highest tide we can expect over the next several decades. We can use current and historical tidal data to estimate this level by adding the following:
Mean Higher High Water: The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch.
Highest Astronomical Tide (highest tide): highest tide predicted in the current tidal epoch
Regional Oceanographic: effects on tide brought about through phenomena such as El Nino and time of year
Finally, we need to consider the effect of wave runup, which we can estimate using an engineering rule-of-thumb assumption of 10 percent of the maximum stillwater level.
Though our scenario does not include it, storm surge might also be a factor impacting coastal total water level. It can be calculated using a statistical approach or by using published results (for example, coastal stillwater elevations published in a FEMA flood study), which yield a surge height for a specific return period.
For a more detailed example of this analysis, please refer to our basic example calculation for the City of Charleston.
Step 1: Intermediate Analysis
An intermediate-level analysis will focus more on the stormwater system itself and the impacts that coastal flooding will have on it. Coastal total water level calculations for an intermediate analysis are the same as for the basic analysis. For a more detailed example of this analysis, please refer to our intermediate example calculation for the City of Charleston.
Step 1: Advanced Analysis
An advanced analysis will replace the statistically based approach to evaluating coastal total water level with an explicit numerical model that seeks to simulate tides, regional and seasonal effects, storm surge, and wave runup. Sea level rise is added to this model as a boundary condition. The advantage of this approach is that it gives coastal total water level on a minute-by-minute basis and at each point along the coast, a forecast that when coupled with an equally detailed model of the stormwater system and coastal land surface, provides great detail of when, where, and how severe flooding will occur.
For Step 1, the table below outlines some potential modeling approaches and the corresponding data needs for each level of analysis.
If the result of Step 1 indicates that the coastal total water level would exceed the elevation of the coastline and overtop the structure, then complete this step to determine the expected flow rate.
Step 2: Basic Analysis
Most coastal communities have a coastal barrier at the ocean and land interface. This can be a natural barrier such as a sand dune, or a man-made barrier such as a seawall. If overtopping does occur, then seawater will flow into the community and eventually be drained by the community’s stormwater system. This adds additional demand on the stormwater system, which in the case of the scenario we defined, is already required to drain the 10-year rainstorm.
In a basic calculation, it is important to estimate the volume and flow rate of overtopping water from ocean to land. A simple way to do this is to assume the ocean and land interface is a weir , and that water flowing over it follows a weir-flow relationship. The figure below illustrates the weir-flow concept. In weir flow, the important factors are the height of the overtopped water, the length of the barrier, and a coefficient of discharge, which varies with factors such as roughness of the material the barrier is made from.
Most stormwater models will typically partition your community into stormwater service areas. Similar to watersheds, these service areas are defined by the fact that each drains stormwater to a single point, such as a manhole, or a line at the edge of the area, such as the shore. The model typically estimates water flow over land using approaches such as the curve number method or rational equation, where flow rate is assumed to be a function of the precipitation and intensity of the storm event, the extent of the service area, and the roughness of the land over which the water is flowing.
Once the water makes it to the drainage point of the service area, it is then routed through the stormwater system, which is a network of connected channels, pipes, and manholes.
With this type of model, you can simulate how your system will handle a variety of design scenarios and get estimates of flow rates through the system at each manhole, and at each minute of the design event. Further, you can estimate important metrics like the maximum depth of flooding in each of your service areas and the number of hours that each service area will be flooded during your design scenario.
In terms of overtopping flow, you can replace the single estimate of maximum flow rate with a time series that follows the tide during your design scenario. By doing this, the model not only evaluates the maximum flow rate, but also combines the flooding from both the overtopping and the rain storm and estimates the minute-by-minute rate of overtopping, as well as the locations in your system that overtopping will impact the most. If you decide to add adaptation actions to your scenario, the intermediate model can evaluate how effective those actions will be at the service area scale.
In the example analysis for the City of Charleston, we present some results of an analysis using the SWMM model. Please refer to the example for more detail.
Step 2: Advanced Analysis
An advanced analysis increases the level of detail of the stormwater model even further, including these additions:
2D Overtopping time series along the coast
the ocean-side model provides an estimate of flow into your community at each foot along the coastline, capturing both the timing and location of overtopping.
2D surface-flow modeling
the advanced model replaces the simplified rational equation approach for estimating overland flow with a gridded or mesh model that covers the land surface, and evaluates in a foot-by-foot scale what the volume and direction of flow would be across the landscape.
2D, time-dependent rainfall
the advanced model replaces a single estimate of total rainfall (basic analysis), or a single time series of rainfall assumed to fall on the whole community (intermediate analysis), with a spatially and temporally distributed storm forecast.
The changes described above give you great detail for your entire system and can help you find which individual buildings and pieces of infrastructure in your community are most at risk. Using these results, you can produce highly detailed management plans and engineering designs to improve your community’s resilience.
Many software packages are available to conduct this type of advanced analysis. Since the model covers both the ocean and land side, a key step is to couple models of each of these domains. This can be done with a combination of models such as ADCIRC, HEC-RAS 2D (Hydrologic Engineering Center's River Analysis System), and SWMM.
For Step 2, the table below outlines some potential modeling approaches and the corresponding data needs for each level of analysis
If the design scenario includes a modeled precipitation event coincident with a coastal flooding event, complete this step to determine the flow rate resulting from expected runoff.
Step 3: Basic Analysis
For the scenario we defined, we are using the stormwater system’s design storm: the 10-year, 24-hour storm. In a basic analysis, the rainfall is treated as a single flow of water that is assumed to fall over the entire community and at a constant rate. You can use NOAA’s Atlas 14 site to find the specific intensity of rain you should use in your community. This site incorporates decades of historic rainfall measurements across the nation to provide the statistically based estimates.
Once you have the rainfall estimate, you can then use a simple method to estimate how much of the rain runs off into the stormwater system. In the City of Charleston example, we used the rational approach, which assumes runoff is a function of the input rainfall amount, the area that is being rained on, in this case the community’s land area, and the roughness and perviousness of the land.
Of course, roughness and perviousness vary over the landscape. Often, you can use an average roughness and perviousness for a simple calculation. Alternately, you can use GIS software with map layers like land use and soil type to estimate these parameters more accurately.
The spatially distributed model described in Step 2 includes an ability to add time series of rain, so that you can vary the rain inputs across your community in both space and time. Often, a constant rainfall rate is still used for planning purposes, but a more realistic storm could be used as input. Further, the model will usually include an ability to bring in GIS layers like land use and soils and use these to estimate the runoff potential of the stormwater service areas.
Step 3: Advanced Analysis
The fully spatially distributed (both ocean and land) and coupled advanced model described in Step 2 increases the level of detail you can include, particularly for overland flow. Because the model includes a grid or mesh over the land surface, you will be able to determine roughness and perviousness foot by foot, and the modeling is impacted by this level of detail. Further, the model may include subsurface conditions, which allows you to understand the rates of infiltration and exfiltration of water into the groundwater system, often an important factor in coastal communities.
For Step 3, the table below outlines some potential modeling approaches and the corresponding data needs for each level of analysis.
Step 4: Compute Discharges from Other Appropriate Flooding Sources ×
Step 4: Summary
Communities often have complex hydraulic features. In New Orleans, a complicated system of pump stations and levees provides security from flooding from the surrounding ocean. In Savannah, a system of berms holds back the Savannah River to prevent flooding. Communities like these should explore design scenarios where features breach or malfunction, as they pose a flooding threat that can exacerbate the risk of other flood sources, like sea level rise. If the design scenario includes a breach event for one of these sources, use the following methods.
Step 4: Basic Analysis
Because of its complex dynamic impacts, we recommend that dam or levee breach be tackled only at the intermediate or advanced level of analysis.
Step 4: Intermediate and Advanced Analysis
There are many software packages available to simulate dam breach, such as the U.S. Army Corps of Engineer’s HEC-RAS 2D. These packages allow you to include the dam within your river model. Also, there are some empirical equations relating to the amount of storage of height to peak breach and outflow.
For Step 4, the table below outlines some potential modeling approaches and the corresponding data needs for each level of analysis.
Coupled overland flow and storm drain modeling (e.g., coupled FLO-2D and SWMM, XPSWMM)
Same as intermediate level data
Land use and land cover data
Storm drain network
Step 5: Model Stormwater System and Compare Total Flows to Capacity ×
Step 5: Summary
After the flow rates from Steps 2, 3, and 4 have been determined, compare them to the system capacity to determine if the system can handle the total water levels or if mitigation actions are needed to ensure the desired level of service.
Step 5: Basic Analysis
For the basic analysis, Step 2 provides an estimate of the inflow of water from the ocean side at its maximum level. Step 3 provides an estimate of the flow through the system from rain sources. The system has a maximum flow rate, which is typically determined by evaluating the rainfall-runoff flow rate for a storm. For example, if the system in the scenario we’re evaluating was designed for the 25-year, 24-hour storm, then we could use the method outlined in Step 3 to evaluate the maximum flow rate the system can handle.
Once we know the maximum flow rate, we add the flow from overtopping and the flow from the design-scenario storm and compare the total to the maximum flow rate of the system to determine if the system is overwhelmed by the scenario.
An important note is the role of adaptation actions in this analysis. If the community decides to include tidal backflow valves, for example, the stormwater system might be able to manage the rainfall without flooding, even as high tide is occurring. Of course, in this case, the stormwater system would act as a storage system instead of a conveyance of stormwater; when the system becomes filled with rainwater, flooding will occur.
Step 5: Intermediate Analysis
The stormwater model described in Step 2 would include an ability to determine if flooding is occurring, including the times and the service areas in which it occurs. As such, events where the system becomes overwhelmed are an important part of the analysis. With the level of detail given, you can also determine the duration and depth of flooding in each service area, results that can help you plan detailed adaptation actions.
Step 5: Advanced Analysis
Similar to the intermediate analysis, the high level of detail in the advanced model (see Step 2) allows you to know when and where flooding occurs, and provides you with the ability to stress-test your community with different adaptation actions in place.
For Step 5, the table below outlines some potential modeling approaches and the corresponding data needs for each level of analysis.
Comparison of total flow entering the system (basin) with the capacity of the storm drain system
Coastal total water levels from Step 1
Coupled overland flow and storm drain modeling (e.g., coupled FLO-2D and SWMM, XPSWMM)
Inventory or geodatabase of all stormwater management structures
Storm drain network
Stormwater modeling (e.g., EPA SWMM, StormCAD)
Inventory or geodatabase of flow control structures
Impoundment water level data
Geometry data of impoundment and levee
Storm drain network
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Basic and Intermediate Example for Charleston, South Carolina
To see how to apply the steps outlined in this methodology, review this basic and intermediate analysis prepared for Charleston, South Carolina. This analysis considered the following design scenarios for a small watershed on the western side of the lower peninsula of the city.
This scenario represents the current condition of the stormwater system. It assumes present day sea level (2018), highest astronomical tide, seasonal variations in sea level, and no tidal backflow preventer to stop sea water from entering the stormwater system.
This scenario represents the original-design storm that the stormwater system was created to handle without flooding but includes 25 years (2043) of future projected sea level rise in the conditions to evaluate the impact of sea level rise over the course of the design life span of the stormwater system. It assumes a 25-year design life, 25 years of sea level rise (intermediate scenario), seasonal variations in sea level, a 10-year rainfall runoff event (based on the current stormwater design storm), and a tidal backflow preventer to stop sea water from entering the stormwater system but allowing rainfall runoff to drain.
This scenario represents the sunny-day condition of the system in 25 years, when sea level rise may exacerbate the current sunny-day flooding the city is experiencing. It assumes a 25-year design life, 25 years of sea level rise (intermediate scenario), highest astronomical tide, seasonal variations in sea level, and a tidal backflow preventer to stop seawater from entering the stormwater system.
This scenario represents a storm surge condition coupled with sea level rise and seasonally related high-tide conditions. The 25-year storm surge could result from a hurricane or a strong non-tropical storm. The height of the storm surge itself is a function of the strength of the storm and the direction and speed in which it approaches Charleston. It assumes a 25-year design life, 25-years of sea level rise (intermediate scenario), a 25-year storm surge, seasonal variations in sea level, a coincident 2-year rainfall runoff event, and a tidal backflow preventer to stop seawater from entering the stormwater system but allowing rainfall runoff to drain.
Calculation Example: Impacts of Coastal Flooding on Stormwater Infrastructure – City of Charleston, South Carolina
As you’ve learned, there’s a lot to consider in this section. Even the basic level of analysis will take some time to complete. However, the outcomes of this analysis will provide you with a better idea of how well your stormwater system would perform during different coastal flooding and precipitation events. What do you do if you determine that your system is inadequate? Many options are available, depending on the scope and time frame of potential impacts. The next section will introduce some of the different courses of action and resources that you can leverage to build the resilience of your stormwater system to coastal flooding.