Riverine Flooding
Past and present development has dramatically increased runoff in the US Virgin Islands. Stormwater infrastructure prevents riverine flooding, but infrastructure can fail and cause flooding. Many people and important infrastructure exists in the 100-year floodplain. Much can be done to improve the resilience of the USVI to riverine flooding.
Riverine Flooding in the U.S. Virgin Islands
• Past and present development has dramatically increased runoff.
• Stormwater infrastructure prevents riverine flooding, but infrastructure can fail and cause flooding.
• Many people and important infrastructure exists in the 100-year floodplain.
• Much can be done to improve the resilience of the USVI to riverine flooding.
Flooding on the Waterfront, St. Thomas after heavy rains. Photo courtesy of Virgin Islands Source.
Riverine Flooding: Causes & Solutions
Floods are a natural part of the environment. Before development of our Territory, storms occasionally dropped large amounts of rain just as they do today. Some of this rainwater evaporated or soaked into the ground and became groundwater. The rest became surface runoff and quickly flowed downhill to the nearest gut and eventually to the sea. Especially intense rains sometimes caused water to pool in low-lying areas or overflow the banks of guts and flood surrounding areas. These floods were a natural fixture of the USVI.
The current situation is different. Development has changed what rainwater does when it hits the ground, usually by increasing the amount that becomes surface runoff. Forests turn into farms. Farms turn into residential areas. Residential areas urbanize. Each step of development takes away surfaces that let water soak into the ground and adds surfaces that don’t, especially roads, parking lots, and rooftops. In the past 30 years, development has followed a similar pattern in the USVI and has transformed the landscape (Figure 1).
Figure 1. Land cover change in the USVI between 1985 and 2018.
To illustrate how dramatic changes in land cover have caused dramatic changes in runoff, we used a computer model to simulate runoff from a moderately heavy rainstorm that dropped 2.54 inches of rain at the Cyril E. King International Airport in St. Thomas on 11/8/2018. The maximum rainfall rate of this storm was 1.0 inches per hour, which is high, but not exceptional for USVI rainstorms. Statistically speaking, both the rainfall amount and rainfall intensity are approximately the magnitude of a storm expected to occur once per year. We used the hourly rainfall from this storm as a model input and simulated runoff to one gut on each of the three main islands in the territory. Land cover was the only variable that was adjusted between the runoff simulations for 1985 and 2018.
Results show that runoff in 2018 is 143%, 397%, and 240% of what occurred in 1985 in the Queens Quarter, Bethlehem, and Guinea Guts, respectively (Figure 2). This suggests that, all else being equal, development-driven land-cover changes since 1985 are responsible for a large portion of the stormwater experienced that day.
Despite this 2.54-inch rainstorm being relatively moderate, it caused disruptive, damaging, and dangerous flooding in both St. Thomas and St. John (Figure 3). For reference, a 100-year rainfall event is 13.4 inches in 24 hours or over 3 inches in 1-hour.
Figure 2. Modeling results table. Click to enlarge.
Figure 3. Flooding caused by 2.54” of rainfall in November 8, 2018.
Fortunately, there is more to the riverine flooding story. Increased runoff doesn’t have to cause an increase in flooding. As we have developed, we have built stormwater infrastructure such as swells, drainage channels, and culverts to change the route of surface runoff so it does not cause damage and endanger people. When it works as designed (Figure 4), infrastructure allows more water to drain much more quickly and prevents riverine flooding from all but the most extreme storms. Unfortunately, stormwater infrastructure doesn’t always work as designed. Infrastructure can fail, especially when obstructed by debris (Figures 4 and 5). Problems also arise when development continues over time and eventually causes runoff to occur more quickly than the stormwater infrastructure was designed to handle.
Figure 4. Left: debris accumulation and plant growth form an obstruction to water flow. Right: A properly functioning drainage channel.
Figure 5. Debris obstructing the entrance to storm drains and culverts.
Just as more runoff doesn’t have to cause more flooding, more flooding doesn’t need to disrupt our lives. Riverine flooding presents a serious hazard in the USVI. Development now causes more runoff to occur more rapidly than ever before (Figure 2). Our stormwater infrastructure struggles, especially with the accumulation of storm debris (Figures 4 and 5). Reports of flooding occur regularly, even from relatively routine storms (Table 1). Perhaps most concerning, a great amount of infrastructure and people exist within the 100-year floodplain (Figure 6).
Figure 6. USVI flood zone map.
Riverine flooding presents a serious hazard in the USVI. Development now causes more runoff to occur more rapidly than ever before. Our stormwater infrastructure struggles, especially with the accumulation of storm debris. Reports of flooding occur regularly, even from relatively routine storms (Table 1). Perhaps most concerning, a great amount of infrastructure and people exist within the 100-year floodplain.
Table 1. Average probability of flooding, 1998-2019. Based on the Storm Events Database maintained by NOAA and historical hourly rainfall data provided the Local Climatological Data Tool maintained by NOAA. Quality-controlled, historical rainfall data are not readily available for St. John.
The good news is there is much that can be done to make the USVI resilient to riverine flooding. The first way is to reduce or delay runoff from occurring in the first place. Making informed and reasonable land-use and development decisions will help us avoid making the runoff problem worse. Mitigation projects can help reduce runoff problems caused by existing development. Construction of soakaway trenches, swales, check dams, and infiltration basins can greatly increase the amount of rainfall that soaks into the ground rather than becoming runoff and rapidly flowing downhill. Additional benefits of this approach are to reduce erosion and sedimentation of our reefs and increase groundwater resources that can then be used to support irrigation in times of drought.
Another way to be resilient to riverine flooding is to improve our stormwater infrastructure. This can involve projects to design and construct new stormwater drainage infrastructure, but it is often easier to start by maintaining our existing infrastructure. Simply clearing debris from swells, drainage channels, culverts, and guts is often all that is needed to prevent flooding problems (Figures 3 and 4).
The good news is there is much that can be done to make the USVI resilient to riverine flooding.
Finally, we can become resilient to riverine flooding by avoiding development of floodplain areas. This begins with making informed and responsible development decisions. In the case of the USVI, it also involves making difficult decisions between moving development out of floodplain areas and constructing infrastructure to prevent their flooding. The most immediate need is to consider moving critical infrastructure such as pumping stations for the public water system away from the floodplain.