This section presents the results of the flood impact assessment on a comprehensive case study and discusses these results. In section 3.1, state-wide analysis results utilizing flood inundation data are given and discussed, while section 3.2 covers the result of the analysis of most impacted three counties using primarily flood depth data.
3.1. State-wide Analysis Results
Table 1 summarizes the rail components that were flooded in the state of Iowa during the 100- and 500-year flood scenarios. Our analysis reveals that up to 15% of Iowa railroads are at risk of flooding. This ratio is approximately the same at the intersection of public roads and railroads, and facilities such as rail warehouses. Considering the railway bridges, the result shows that they can be exposed to flooding up to 60%, but this rate is only according to the extent of the flood. According to the county-based flood depth analysis that we conducted in the next stage, this rate decreases, but still, flooding will cut off the accessibility of the bridges to a great extent. The ratios mentioned are calculated by dividing the exposed elements in every county by the ratio of the total element counts. This means that equal importance is given to all of them, regardless of whether they are small or overpopulated counties.
Table 1
Summary of rail elements exposed to flooding in Iowa State
Flood
Scenario
|
Railroad Length (km)
|
# of Railroad Crossings
|
# of Rail Bridges
|
# of Rail Facilities
|
Baseline
|
11,927
|
5,503
|
2,652
|
384
|
100-yr flood
|
1,040
|
437
|
1,551
|
23
|
500-yr flood
|
1,952
|
793
|
1,622
|
51
|
Figure 4 shows the percentage of impacted railway length per county during the 100- and 500-year floods. The railroad length was evaluated instead of the number of railroad segments in order to analyze the direct effect of flooding on the railroad. Considering the length of the railway that will be affected is an essential factor in estimating reconstruction or repair costs. Briefly, we calculated the inundated railroad length within each county and extracted the county damage percentage (Fig. 4). Most counties have damage levels of up to 5%. Moreover, under the 100-year flood event, Fremont County, located in the lower-left corner of the Iowa boundary, would be experiencing major inundated railways exceeding 50% of its railway length, while under the 500-year flood, four more counties (Pottawatomie, Harrison, Linn, and Allamakee) are impacted by that percentage. In general, it has been concluded that some counties, such as Pottawattamie, Mills, Plymouth, and Harrison, which are located along the Missouri River, as well as every other county along the Mississippi River, except Louisa, will suffer remarkable rail damage (5–50%). On the other hand, there are a few counties that have zero percent damage. This means that either the railway network does not pass through those counties, or the railways do not overlap with the flood extent in both scenarios.
Table 2
Top 15 counties based on impacted railroad length (km) sorted by 500-year.
County
|
Crossover
|
Main
|
Siding
|
Spur
|
Turnout
|
Yard
|
Total
|
100
|
500
|
100
|
500
|
100
|
500
|
100
|
500
|
100
|
500
|
100
|
500
|
100
|
500
|
All
|
Pottawattamie
|
0.6
|
0.6
|
68.7
|
108.0
|
9.3
|
10.4
|
28.2
|
33.7
|
1.4
|
1.5
|
101.4
|
144.5
|
209.6
|
298.7
|
445.0
|
Linn
|
-
|
0.8
|
24.3
|
56.3
|
1.1
|
2.3
|
12.1
|
23.4
|
0.6
|
1.4
|
34.1
|
83.3
|
72.5
|
167.5
|
335.6
|
Harrison
|
-
|
-
|
63.4
|
98.3
|
0.9
|
1.9
|
-
|
-
|
1.9
|
5.2
|
5.9
|
10.8
|
72.4
|
116.6
|
231.7
|
Scott
|
-
|
-
|
23.1
|
42.6
|
2.4
|
8.2
|
1.6
|
2.2
|
-
|
-
|
25.5
|
30.2
|
52.6
|
83.1
|
177.3
|
Lee
|
-
|
-
|
16.9
|
35.6
|
-
|
-
|
0.5
|
1.2
|
-
|
-
|
29.8
|
39.6
|
47.6
|
76.9
|
196.7
|
Polk
|
-
|
-
|
28.8
|
40.6
|
1.7
|
2.8
|
10.3
|
12.8
|
0.7
|
1.1
|
13.8
|
13.8
|
55.6
|
71.4
|
309.3
|
Plymouth
|
-
|
-
|
39.4
|
64.5
|
2.7
|
3.7
|
-
|
-
|
-
|
-
|
-
|
-
|
42.2
|
68.8
|
166.8
|
Allamakee
|
-
|
-
|
10.5
|
62.1
|
-
|
2.8
|
-
|
-
|
-
|
-
|
-
|
-
|
10.5
|
65.3
|
71.8
|
Clinton
|
-
|
-
|
20.2
|
41.5
|
1.9
|
4.0
|
-
|
3.3
|
-
|
-
|
2.0
|
14.0
|
24.3
|
62.8
|
238.0
|
Dubuque
|
-
|
-
|
9.1
|
34.2
|
0.6
|
1.3
|
-
|
7.4
|
-
|
-
|
-
|
19.0
|
10.0
|
62.0
|
145.2
|
Woodbury
|
-
|
-
|
1.5
|
16.7
|
-
|
0.7
|
-
|
13.2
|
-
|
1.0
|
-
|
28.8
|
1.8
|
60.8
|
193.0
|
Mills
|
-
|
-
|
40.2
|
45.1
|
3.0
|
3.1
|
-
|
-
|
4.1
|
4.1
|
3.3
|
3.3
|
50.9
|
55.9
|
122.4
|
Fremont
|
-
|
-
|
37.8
|
38.1
|
8.2
|
8.2
|
3.6
|
3.7
|
1.2
|
1.2
|
-
|
-
|
51.0
|
51.3
|
57.1
|
Clayton
|
-
|
-
|
7.9
|
35.8
|
1.3
|
3.0
|
0.8
|
0.8
|
0.8
|
0.8
|
0.5
|
5.0
|
11.3
|
45.4
|
108.2
|
Muscatine
|
-
|
-
|
9.8
|
21.8
|
3.1
|
4.9
|
2.6
|
8.0
|
-
|
-
|
-
|
4.4
|
15.5
|
39.1
|
156.1
|
According to the Iowa Rail Toolkit (Iowa DOT, 2023), a total of six railway class groups derived from the data can be briefly explained as follows:
Main: The primary railway route where uninterrupted train travel occurs between terminals and rail yards within the rail network.
Turnout: A configuration of tracks that allows a train to change its path from one track to another.
Crossover: A relatively brief section of track that redirects train traffic from one parallel line to another.
Siding: A brief additional track connected to the main track at both ends using turnouts, allowing trains to meet or pass. It runs parallel to the main route.
Spur (or stub in): A shorter, often dead-end portion of track constructed to provide specific facilities like loading and unloading ramps access to the main or secondary line. It can also serve as temporary storage.
Yard: A network of supplementary tracks is used for tasks such as organizing railroad cars based on their cargo or destination, assembling trains, storing cars, or conducting equipment repairs.
Although Fremont County has a high flood risk in both flood scenarios, looking at only percentages, Pottawattamie County ranks first when we look at the length of railroads that will be affected by flooding. Table 2 summarizes the information for the top 15 counties based on length of the railway that will be affected by the flood with different railroad type classification. At the same time, this table gives the total length of the railroad network for each of these impacted counties in the last column. The range of total impacted length during the 100- and 500-year flood events for the studied counties fluctuated from 10 to 209 km and 39 to 299 km, respectively. During a 500-year flood event, the total impacted length increased dramatically, often doubling or even exceeding the impacted length observed during 100-year flood events, as seen in areas such as Linn, Clinton, and Dubuque. Among the top 15 counties affected, the main class is the railway type that every county will be inundated with, with a range of 8–108 km. In contrast, the crossover class comes out as less vulnerable to flooding.
Additionally, we estimated the total length affected and the percentage of affected segments across the state by rail type (Table 3). The state of Iowa is overall threatened with losses of up to approximately 8% and 16%, respectively, in 100- and 500-year flood events, and the class most affected during these two floods is the "main" class in terms of road length, while the "yard" class has the highest share when we look at their percentages.
Based on Tables 2 and 3, as well as the description of railway types given above, it can be interpreted that two railway tracks, which are essential in terms of being main and complementary tracks, will be interrupted in the event of a flood. In particular, a class of yard that has the function of repair and maintenance should normally be a railway line that should be least affected in an emergency situation. Therefore, when we examine these results, it can be concluded that there is a deficiency in this regard.
Table 3
Impacted railroad length (km) for different railroad types sorted by the total length.
Railroad Types
|
100-yr
|
Percentage
|
500-yr
|
Percentage
|
Total Length
|
Main
|
658.9
|
4.97%
|
1,289.0
|
9.73%
|
13,247.5
|
Yard
|
245.2
|
19.35%
|
431.2
|
34.04%
|
1,266.9
|
Siding
|
50.4
|
6.57%
|
82.4
|
10.75%
|
766.7
|
Spur
|
72.8
|
10.90%
|
127.7
|
19.12%
|
667.8
|
Turnout
|
11.7
|
14.64%
|
19.3
|
24.14%
|
79.9
|
Crossover
|
1.3
|
9.90%
|
2.4
|
18.24%
|
13.2
|
In Fig. 5, we have investigated the total inundated length over the entire state while considering the year of construction. Relying on the rail-built-year information provided by the Iowa DOT, railroads that will be inundated during 100- and 500-year flood scenarios are relatively older built railroads. In Iowa, the first railroad was built in 1855, and the railroad lengths that are expected to be most affected in both 100 and 500-year flood scenarios were built between 1865 and 1874 with an estimated length of 285 km and 557 km, respectively. That could potentially be attributed to the oversight of not factoring in flood scenarios during the construction of these older railways. We also noticed that the length of the railway impacted by a 500-year flood is almost twice the length impacted by a 100-year flood, for each construction period. On the other hand, the railroad lengths that are expected to be the least affected in both scenarios are the recent railways made in 1900 and 2000. From these findings, it can be stated that the railways built in recent years were built considering the flood risk.
The railway system is linked to land transportation. Disruption in one place on the railway will also affect the public-use road connected to that road. Figure 6 demonstrates the number of impacted crossings between the railroad and public road per county. According to results, while Polk and Pottawatomie counties have the highest crossings inundated due to the 100-year flood, Linn and Woodbury counties will share the same range between 50 and 128 crossings with others in the 500-year flood scenario. It means that crossings in most counties are out of the hazard area.
A railway bridge is a specially constructed structure to carry the rail traffic flow across an obstacle, including rivers (Pipinato, 2022). However, flooding can cut off access to bridges and create challenges to freight and passenger transport. Figure 7 gives information about the number of railway bridges within the 100 and 500-year flood zones. A total of 87 counties in Iowa have railroad bridges within the two flood extents. It has been found that the highest number of 66 (100-year) and 71 (500-year) railway bridges within the floodplain were in Plymouth County, and some western and eastern counties have a major share. Linn County, where there is a large city like Cedar Rapids, also has a high number of railway bridges in flood zones.
However, it should be noted that these results were created using only 100- and 500-year flood maps, so we cannot say that all these affected bridges will be inundated during the flood. Nonetheless, our analysis can provide a comprehensive examination of rail bridges, enabling the identification of counties that may be susceptible to the inundation of rail bridges. Additional data, such as flood depth, is needed for bridge closure information, and the analysis on this subject is explained in more detail in the next part.
When we compare the Iowa DOT's railway facility locations with the 100- and 500-year flood maps, it appears that some of the storage facilities would be affected by flood scenarios, and Table 4 summarizes that analysis. When we evaluate the percentage of facilities that will be inundated in general, it can be said that there will not be a major loss. Almost every flood probability scenario is below 30%, even in the 100-year scenario, it has been revealed that no warehouse will be affected. Yet, when we evaluate the facilities' capacity and their location information that will be affected by the flood, the losses will be high and this may seriously affect other cities, perhaps even other states. For instance, among the grain facilities, the city that will suffer the most damage storage capacity is Hamburg (Fremont County) with 10,238 bushels. The second most damaged city is Council Bluffs, Pottawattamie County with a total storage capacity of 8,210 bushels, and Davenport (Scott County) will be ranked third city with 4,795 bushels of storage capacity.
Table 4
Rail facilities within floodplain
|
Warehouse
|
Federal Grain
|
State Grain
|
Transload Facility
|
|
100yr
|
500yr
|
100yr
|
500yr
|
100yr
|
500yr
|
100yr
|
500yr
|
Inundated
|
0 (-%)
|
4 (29%)
|
13 (6%)
|
24 (12%)
|
5 (4%)
|
11 (9%)
|
5 (11%)
|
12 (26%)
|
Total
|
14
|
206
|
118
|
46
|
3.2. County-based Analysis Results
In this section, depending on statewide analyses, the top 3 vulnerable counties, namely Harrison, Linn, and Pottawattamie County, were selected to carry out a detailed analysis using the flood depth maps as additional data because the standard approach for estimating the cost of flood damage usually relies on flood depth as the primary measure of the extent of damage severity (de Moel, 2012; Gerl et al., 2016; Koket et al., 2004; Wagenaar et al., 2016; Martello et al., 2023).
The classification of rail lengths according to flood depth originated from Bubeck et al. (2019) study that used the RAIL method for the railway network in Europe. From those methodologies, flood damage classification begins with classification of the first damage class when floodwater levels reach up to 20 cm along the track portion. In the standard framework, the second damage class pertains to water heights ranging from 21 to 140 cm, while the third damage class is designated for water levels over 140 cm.
Table 5
Affected railway length (km) for the top three counties within the floodplain for different damage classes (cm).
|
100-yr Flood Scenario
|
500-yr Flood Scenario
|
County
|
< 20 cm
|
20–140 cm
|
> 140 cm
|
< 20 cm
|
20–140 cm
|
> 140 cm
|
Pottawattamie
|
66.5
|
135.0
|
8.1
|
6.9
|
175.2
|
118.2
|
Linn
|
18.7
|
47.3
|
6.5
|
22.2
|
108.2
|
37.1
|
Harrison
|
16.3
|
53.5
|
2.5
|
12.7
|
86.8
|
16.0
|
In Table 5, we assessed the various damage classes and their potential impact on railway lengths. Our analysis indicates that the second damage class predominates in both considered scenarios, representing the most significant share compared to other classes. Notably, while the low damage class ranks as the second most impactful in the 100-year scenario, it falls to third in the 500-year scenario. A closer examination of the second and third damage classes, which are more prone to severe impacts, reveals that Pottawatomie County exhibits the highest rate of rail length among the three counties analyzed. Consequently, given its leading position in terms of rail length, repair costs in Pottawattamie are anticipated to be substantial. These findings provide a realistic basis for estimating repair costs associated with damage. While quantifying the risk of damage is essential, identifying the precise locations of highest risk remains a challenge, and this aspect could not be meaningfully addressed in our study.
Table 6
Railway bridges in the top 3 impacted counties
|
|
# of Bridges in Floodplain
|
# of Inundated Bridges
|
County Name
|
# of Bridges
|
100-yr
|
500-yr
|
100-yr
|
500-yr
|
Pottawattamie
|
97
|
59 (61%)
|
64 (66%)
|
29 (30%)
|
37 (38%)
|
Linn
|
72
|
18 (25%)
|
36 (50%)
|
0 (0%)
|
12 (17%)
|
Harrison
|
82
|
56 (68%)
|
63 (77%)
|
27 (33%)
|
39 (48%)
|
According to one of the Iowa DOT (2008), some railway companies commonly adopt a practice where they place loaded train carriages on specific bridges when there is high water. This is for increasing the bridge's stability by adding extra dense materials such as rocks, ballast, scrap metal, or other heavy and non-reactive substances. In order to identify the location of these bridges, it is important to know which bridges will be inundated, and which will be open.
Table 6 shows how many railway bridges would be in floodplains and inundated based on 100- and 500-year scenarios for the top 3 impacted counties. Even though almost more than half of the bridges seem to be in the floodplain in both scenarios, the rate of bridges to be closed is less than 50% according to the method used in this research. Nonetheless, it should be noted that about half of the bridges in the flood zone will become non-functional. It has been noticed that the railway bridges in Pottawattamie and Harrison counties will be affected more than Linn County. Especially in Pottawattamie and Harrison, it was concluded that 30–50% of the rail bridges would be inundated in both flood scenarios.