Hurricane Hortense dropped up to 24 inches of cumulative rainfall in eastern interior Puerto Rico in September 1996. The rainfall amounts on the U.S. Virgin Islands ranged from 7 to 10 inches. This volume resulted in a destructive flood event in Puerto Rico. Hurricane Hortense made landfall on the southwestern coast of the island as a Category 1 Hurricane. The heaviest hit watershed was the Rio Grande de Loiza, which flows through the most populated portions of northeast Puerto Rico. This watershed measured 24.6 inches.

While the rainfall maxima were in the eastern interior of Puerto Rico, serious flooding was widespread across the island, with many locations experiencing high stages. The headwaters of the Rio Grande de Loiza measured record peaks at many streamgages along the river basin. During this flood event, the discharge of 83,000 cfs (32.32 feet) at Rio Gande de Loiza at Caguas was the largest recorded since 1945. Lago Loiza/Carraizo Dam also had the highest peak discharge ever recorded (223,000 cfs). This dam supplies potable water to more than 750,000 people in the San Juan metropolitan area. This overwhelming flow resulted in uncontrolled releases from Lago Loiza/Carraizo Dam, causing additional severe flooding in the towns of Trujillo Alto, Carolina and Loiza along the north Atlantic coast.

The most devastating damage seen with the passing of Hortense was flash flooding and mudslides. Hortense caused 21 deaths, damage to over 11,000 homes and agricultural losses of over $125 million (equivalent to $186 million 2013 dollars). FEMA declared Hortense damage a major disaster later that year.

 

Rainfall map Hurricane Hortense

Rainfall map from NWS San Juan

Four people in one family died at this site along the Guamani River near Guayama in southeast Puerto Rico, making it the location of the single worst loss of life during Hurricane Hortense. Photo: Israel Matos

Lago de Loíza Reservoir during Hurricane Hortense. Photo by Matthew Larsen, USGS

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Three Kings Day (el dia de Reyes), also known at the Three Wise Men, falls on January 6 and is the most celebrated and important official and religious holiday in Puerto Rico during the Christmas season. As a result, it is a highly traveled day across this mountainous and flash flood plagued island. The celebration begins on the eve of Three Kings, January 5, because, according to tradition, the Three Kings arrive sometime during the night.

A quasi-stationary cold front interacting with moist tropical air and Puerto Rico’s mountainous topography created the ingredients for slow moving convection with locally heavy downpours, resulting in flooding rainfall. Rainfall totals across the island averaged 8 to 12 inches with localized reports of 19.5 inches in Cayey and 20.3 inches in Toro Negro. The runoff from this rainfall resulted in severe flash flood and mainstem river flooding across most of Puerto Rico.

The USGS experienced gage house and equipment damages from this event. Many of the measurements were made indirectly but clearly they determined that the most severe floods occurred along the Rio Grande de Patillas and the Rio de La Plata from Comerio in the central interior headwaters to Toa Baja and Dorado along the north coast. At Rio de La Plata at Proyecto and Rio Grande de Patillas near Patillas, the peak flows exceeded the historical maximum stages and discharges. At Rio de La Plata at La Plata Dam, the water level rose 19 feet in 6 hours and downstream at Highway 2, the 110,000 cfs flow was a new record. Fajardo, Salinas, Ponce and Cayey were a few of the towns that experienced extensive flood damage during this event.

When the catastrophic event was over, there were 23 dead (20 in vehicles) and 167 injured. FEMA estimated total damages of $155 million (equivalent to $258 million 2013 dollars). Most of the deaths were due to drivers on winding mountainous roads during the flash floods, many of whom drove pass the civil defense emergency barricades.

“The catastrophic flash floods and river flooding which occurred in the Commonwealth of Puerto Rico on January 5 and 6 presented Federal, Commonwealth, and local officials with a worst-case scenario in which all aspects of the integrated warning program were severely tested.” (Excerpt from the Natural Disaster Survey Report- Puerto Rico Flash Floods January 5-6, 1992–U.S. Dept of Commerce, NOAA, and National Weather Service)

Three Kings 1992- NWS Rainfall amounts with Puerto Rico River Basins

Three Kings 1992- Towns with extensive Flood Damage (USGS)

The floods of October 1985 were a couple of tragedies rolled into one. This event is remembered as the Mameyes Landslide which occurred on October 7 when a slab of calcareous sandstone (sandstone which turns to mud when saturated) detached from the side of a hill and slid into a downhill community, resulting in the destruction of 90 homes and the death of 130 people. This landslide is a subset of a larger event and resulted from saturated soils and heavy rains that occurred over the period of October 5–8, 1985.

The floods of October 1985 proved disastrous because of the duration of the rainfall, the terrain and soil type. A nearly stationary tropical depression dropped over 31 inches of rain from October 4–8. The heaviest rainfall occurred October 6–7, when over 24 inches was recorded. The most severe floods were along the south central and eastern sections of the island. The rivers along the south coast of Puerto Rico are short with steep slopes and the intense rapid runoff during this event, when combined with the saturated sandstone, proved to be catastrophic in October 1985.

In addition to the deaths from the Mameyes Landslide, there were also approximately 40 additional fatalities directly and indirectly related to flash flooding. A number of people died when cars fell into Rio Coamo in the middle of the night after a portion of a bridge collapsed from raging waters. Property damage was estimated at $125 million dollars (equivalent to $271 million 2013 dollars).

Widespread flooding was reported from Ponce to Santa Isabel with record breaking flows recorded on Rios Coamo, Inabon, Descalabrado , Cerrillos and Portugues. The northern towns of Barceloneta, Toa Baja and Arecibo, with headwaters in the Cordillera Central Mountains, also experienced significant flooding during this event. The worst flooding was in the Rio Coamo basin, just west of Ponce.

This tropical wave moved northwestward and became Tropical Storm Isabel on October 8, 1985.

Images from Dept. of the Interior, USGS, Open-File Report 87-123 (see link below)

NWS rainfall analysis


Rainfall map Oct 4-8, 1985 for Mameyes Event, NWS data

Aerial View of Mameyes Landslide, Puerto Rico, R.W. Jibson, U.S. Geological Survey

Another case of a slow moving tropical depression, resulting in rainfall over multiple days across the island, set the stage for the devastating floods of October 1970. The focus of the rainfall core shifted from day to day, but some areas experienced copious amounts of rainfall on consecutive days, causing rainfall amounts that could be measured in feet. The highest total over those 6 days was 38.42 inches at Jayuya and 41.68 inches at a station near Jayuya. Jayuya had a 24 hour total of 17 inches.

Flooding was widespread during the period. One of the hardest hit areas was the densely populated San Juan area. There were reports of bridge washouts, stranded vehicles, flooded homes, electrical outages and road closings due to both flood inundations and landslides.

The basins with the highest recorded flows were the Rio Grande de Manati at Ciales along the north coast and the Rio Espiritu Santo near El Verde in the northeast. The volume of runoff was a record for Puerto Rico since there were numerous peaks at various times during the flood duration. With the small basin sizes and the typical rains that occur across the region, rivers usually flood within 6 to 12 hours of the heaviest rain. These floods lasted days, creating unique challenges for the affected communities.

There were 18 confirmed fatalities, 34 people missing, $65 million (equivalent to $375 million 2013 dollars) worth of damage, 10,000 homeless citizens, 600 houses destroyed and another 1,000 damaged. The damage to agriculture alone was approximately $8 million with $12 million tallied for road and bridge damage. President Nixon declared the territory a disaster area; the worst in Puerto Rico’s history.



NWS Rainfall Data, October 2–10, 1970

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Historical records indicate that the floods across eastern Puerto Rico in September 1960 were some of the highest on record despite the fact that the rainfall amounts were not that extreme for the El Yunque rainforest, where the average yearly rainfall is around 120 inches. The key to these devastating floods was the intensity of that rainfall. A few stations measured 3 inches an hour for 2 to 3 hours during the night of September 5 into the morning of September 6. The highest rainfall amount of 18.76 was reported near Sabana, in the Luquillo Mountains (El Yunque rainforest). This high intensity rainfall covered a 15 to 40 square mile area from the central interior to the east coast of the island and was responsible for the overnight flash floods that drowned residents and demolished houses.

The most severe flash flooding occurred along Rios Grande de Manati, Grande de Loiza (and its tributaries), Humacao, Turabo, Gurabo, Valenciano, Blanco and Guayanes. The runoff from the Rio Grande de Loiza at San Lorenzo destroyed an entire community along its banks and contributed to the 42 foot rise just below the Loiza Dam in Trujillo Alto.

The flood wave continued downstream through Carolina, where a peak discharge of 197,000 cfs was measured at the Highway 3 Bridge, with continued high flow and flooding to the Atlantic Ocean along the north coast. Significant flooding was also noted on the Rio del La Plata from Comerio in the central interior to Dorado on the north coast, with a rise of 15 feet in one hour at the Toa Alta station. Many damaging landslides also occurred along the mountainsides during this event.

In Humacao, 90 people drowned from flash flooding on Rio Humacao during the early morning hours of September 6. There were a total of 117 deaths, 30 missing and 136 injured. Damage estimates exceeded $7 million (equivalent to $55 million 2013 dollars) with 484 houses destroyed, 3,600 damaged and over 8000 acres of agriculture ruined.

This rainfall was associated with Hurricane Donna which passed to the northeast of the island of Puerto Rico.

NWS rainfall plot


NWS Rainfall Data, Sep. 3–6, 1960
 

Photos likely form the local newspaper, El Mundo

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The terrain of St. Thomas and St. John of the northern U.S. Virgin Islands is mountainous. Runoff from high intensity rainfall does not flow into rivers but into dry guts: the valleys between the mountains which are normally dry. The flow during extremely heavy or high intensity tropical rainfall events is rapid and destructive due to the steep slopes. One of these historical, high intensity tropical rainfall events occurred in April 1983. During the 19-hour period from 7 pm on April 17 to 2 pm on April 18, storm total rainfall amounts ranged from 14.5 inches in St. Thomas to 18.7 inches in St. John.

While the whole region recorded high rainfall amounts from the interaction between an upper level trough and a nearly stationary low pressure system, St. Thomas and St. John had an event for the history books, with rainfall intensities of up to 2.5 inches an hour. The dry guts became raging rivers causing widespread flooding with mud, rock and debris flow. In St. Thomas, the Harry S. Truman airport was flooded for 2 days, then buried under 2 to 3 feet of water and mud while landslides blocked roads and destroyed utility poles in St. John, leaving that island without power and telephone service for several days.

One death was recorded in St. Thomas, a person being swept away by flood waters. Flood related damages were $12 to $15 million (equivalent to $35 million 2013 dollars). The highest peak discharges were recorded on the Bonne Resolution Gut at Bonne Resolution and the Turpentine Run at Mariendal in St. Thomas and the Guinea Gut at Bethany in St. John.




Rainfall April 18, 1983 – St. Thomas – USGS Caribbean


Rainfall April 18, 1983, St. John, USGS Caribbean


Photos provided by Robin Koeppel Hepburn

 

Photos provided by Robin Koeppel Hepburn

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St. Croix is the largest of the U.S. Virgin Islands with mountains in the northwest with peaks from 1,000-1,100 feet above mean sea level. Similar to St. Thomas and St. John, St. Croix’s flooding is also associated with dry guts that become rivers during heavy rainfall. On November 10, 2010, heavy tropical rainfall engulfed the island of St. Croix.

The monthly rainfall total for St. Croix was 10.71 almost double the November normal of 5.55 inches. This departure from normal was exceeded in one day in Frederiksted, St. Croix on November 10, where a spotter reported 7.04 inches of rain in the span of a few hours. The runoff from this high intensity rainfall resulted in serious to catastrophic flash flooding across the island.

Government officials confirmed the death of a woman that was swept away in the floods. Historical flooding occurred within a couple of hours of the heavy rainfall and caused the closure of roadways, bridges, schools, stranded residents and destroyed infrastructure. The most extensive damage was in the historic west end of St. Croix.

A “On Nov. 24, 2010, the U.S. Department of Homeland Security’s Federal Emergency Management Agency, or FEMA, announced that federal disaster aid was available to supplement the U.S. Virgin Islands’ recovery efforts in areas struck by severe storms, flooding, rock slides, and mudslides associated with Tropical Storm Tomas from Nov. 8–12. The territory has previously received two disaster declarations this year, for Hurricane Earl and Tropical Storm Otto.” (FEMA REPORT)

 

Credit Dave Davis- From St. Croix Source Newspaper, Frederiksted Flooding

Credit Bill Kossler- From St. Croix Source Newspaper, Frederiksted Flooding

Credit Bill Kossler- From St. Croix Source Newspaper, Concordia Flooding

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Flash flooding is a rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (i.e., intense rainfall, dam failure, ice jam). More information...

River flooding occurs when river levels rise and overflow their banks or the edges of their main channel and inundate areas that are normally dry. More information...

At any time of year, a storm from over the ocean can bring heavy precipitation to the U.S. coasts. Whether such a storm is tropical or not, prolonged periods of heavy precipitation can cause flooding in coastal areas, as well as further inland as the storm moves on shore. More information...

A back-up of water into surrounding areas can occur when a river or stream is blocked by a build-up of ice or other debris. Debris Jam: A back-up of water into surrounding areas can occur when a river or stream is blocked by a build-up of debris. More information...

When heavy rain falls over extremely dry land, the water rushes towards low-lying areas, which may include dried up canyon or river beds. This can quickly turn a dry channel into a raging river.More information..

A break or failure can occur with little to no warning. Most often they are caused by water overtopping the structure, excessive seepage through the surrounding ground, or a structural failure. More information...