City Issues Assessment of Five Points Flooding

Flood of July 30, 2005 Assessment ReportIn and Around the Five Points Area - Columbia, South CarolinaAugust 8, 2005By: Al Cope, City of Columbia Department of Construction ManagementThe purpose of this document is to present known facts and circumstances on the storm and corresponding flood that took place on the afternoon of Saturday, July 30, 2005, proving that the Columbia and Five Points area flood was an unpreventable occurrence. Information was gathered from National Weather Service data, research from local meteorologists, various news media reports, local rain gauge collection data, and interviews with storm eyewitnesses, residents, and business owners in the Five Points area. The SettingFive Points is a focal point for Columbia commerce and nightlife. This area was built on top of what would be categorized today as a protected wetland, on the site of an ancient riverbed/lake. A tributary known as Rocky Branch ran from modern-day MLK Park to Maxcy Gregg Park, and also from Gervais Street area north of Food Lion to Maxcy Gregg Park. An underground tunnel system was built in the early 20th Century to carry the natural-flowing creek water along with storm water runoff from the surrounding neighborhoods. Five Points is one of the lowest points in the city of Columbia, surrounded by hills in all directions. The Physical AspectsAs mentioned, Five Points is in a very low-lying area that has historically been flood-prone. It is “fed” with storm water from east Devine Street, College Street (both directions), Greene Street (both directions), Saluda Avenue, and both directions of Harden Street. Storm water from as far away as Beltline Boulevard finds its way into the Five Points drainage system. The ongoing city streetscape/rehabilitation project was approaching the 50% completion mark on July 30th. The most flood-prone area on Harden Street, the 600 block, has a new, completely functional storm drainage system and the new inlets were clean and unblocked by debris (pre-storm). Saluda Avenue, the most flood prone area in Five Points, was about 25% complete storm system-wise on that day. However, this 25% was at least equal to the drainage system before construction. The new storm inlets were lined with fabric material as required by DHEC during construction to prevent sand, silt, and other foreign materials from entering into the storm system. The fabric used was designed to allow 1 inch of rain per hour penetration, normally very adequate for a summer thunderstorm.Previous storms this month had dropped nearly 3 inches on the area on July 3rd and 2+ inches on July 19th overnight with no flooding, as reported by business owners and articles in The State newspaper. The StormTimeline: A large summer thunderstorm storm hit Columbia on Saturday afternoon, July 30th. Work crews reported that when they left the site at 5:00pm, rain was imminent, but had not begun in the Five Points area. The rain began around 5:10pm according to an eyewitness in the area at the time. A Five Points building owner remarked that it was “as hard a rain as I have ever seen”. By 6:00pm, the rain had lessened to a slight drizzle, making the storm duration around 45-50 minutes.Rainfall Amount: Unfortunately, there are no official records kept in the Five Points area by the National Weather Service. Additionally, the local radar cannot accurately detect rainfall amounts in this area due to the close proximity of the radar, in “the cone of silence”, according to a local meteorologist. An unofficial rainfall amount taken from a rain gauge in Five Points measured 4 inches. Another taken in Cayce near the Blossom Street Bridge measured the same. It is safe to assume that a similar amount fell two miles away on Shop Road at The State newspaper, where flooding rains caused a crippling outage. When provided with these measurements, the NWS indicated that this assessment of amount of rainfall is believable and probably very close to accurate. Using all data gathered, it is reasonably estimated that 4 inches of rain fell in less than one hour within a one-mile radius of Whaley Street, another low point of the city where the worst flooding occurred, and the approximate midway point between Five Points, the Blossom Street Bridge and Shop Road). For reference’s sake, the average monthly rainfall in July is 5.54 inches. This amount was surpassed in two days on this weekend.Astounding Volume of Water: Doing some simple calculations, some amazing numbers are presented. A one-inch rain over a 1 square mile area (27.9 million square feet!) results in over 17 million gallons of water falling from the sky. Subsequently, a 4-inch rain over a 2 square mile area will produce 139 million gallons of water. In a city environment with 75% or more hard-covered acreage (streets, parking lots, and buildings), almost all of this water must find its way into the city storm system. When this much rain falls in less than a one-hour span, this is an overwhelming volume for the system to handle. Thus, a flooding situation was inevitable with the circumstances of July 30th. Rare event: According to data from the SC State Climatology Office (http://www.dnr.state.sc.us/climate/sco/pcp_ints.html), a rain of this magnitude (4 inches in 60 minutes) in Richland County, SC is considered a 100-year rain event. Storm systems in SC are typically designed to handle a 10-year event (in Richland County, 2.5 inches in 60 minutes). The new system in Five Points, even in an unfinished state, has already been able to handle this without any problem, and will improve greatly in the near future. Richland         Return         Period5 min.10 min15 min30 min60 min6 hr12 hr24 hr           2 yr0.485 1.051.391.92.853.23.7 5 yr   1.752.243.54.154.75 10 yr   2.052.554.14.85.5 25 yr   2.32.944.855.66.3 50 yr   2.543.255.256.27.15 100 yr0.831 1.82.863.945.86.97.8 Sunday’s storm: Interestingly, another storm on the following day, Sunday, July 31st, brought 3.5 inches to downtown Columbia. This rain fell between 1:30pm and 3:00pm (an average of 2.33 inches per hour). Though less intense, the rainfall amount was similar. No flooding was reported. Again, the existing system is well equipped to handle a reasonably intense storm such as this. Other Contributing FactorsDownstream: Due to the previously discussed intensity of the rain, the city drainage system was compromised. Downstream from the Five Points area, specifically the South Main Street/Whaley Street area, it was overwhelmed, where the storm may actually have hit hardest. This area is where the Five Points storm water drains enroute to the Congaree River. With nowhere for the water to go, it pooled in the streets.Limbs, Pine Straw, and Magnolia Leaves: After the storm, several storm drains and grates were completely blocked with plant material not found in the immediate area. Since there are few magnolias and pine trees around, an investigation was done. From interviews with residents up the hills on Saluda Avenue and Harden Street south of Five Points, it was determined that garden work done earlier in the day resulted in piles of yard clippings, limbs, etc. being set curbside by residents for city pickup on Monday. These residents said that those piles were completely “gone” after the rainstorm. Not surprisingly, these materials were washed down into the streets and corrupted the drainage system capability.Mattresses, children’s toys, and other trash: Two storm inlets (one on Devine and one on Harden Street) were found after the storm to be blocked by a baby-sized mattress and set of box springs. There were also a number of plastic children’s toys (one example: a “Big Wheel” bike) and large cardboard boxes found washed into the drains, obstructing water flow. This can only be explained the same way as the plant material – these objects were washed down from neighborhood curbside garbage pickup areas. When removed, the water freely rushed into the drains. Needless to say, these items did not help the drainage situation.Revelers in vehicles causing wake: There were a high number of vehicles attempting to move through the streets at the height of the flooding. Even slow-moving vehicles displaced enough water to enter buildings. This caused untold damage to the interiors of the stores along these streets, especially on the west side of Saluda Avenue, where the curb and sidewalk had been removed for replacement. But perhaps the most disturbing event of all came from eyewitness accounts of a “contest” held shortly after the storm, where revelers took turns speeding around a “lap” consisting of Harden, Blossom, and Devine Streets through the 12+ inches of standing water in high-axled SUVs, jeeps, and trucks, attempting to cause the biggest “wake” possible onto the sides of buildings. This was all done with great glee, but easily caused the most damaging flooding to businesses, as substantial waves of water splashed over the curbs and hit porous doorstops and windows. At least one business owner has attested to this event being the primary cause of store flooding.SummaryAs much as the Five Points storm system has already been improved, and as much as it will continue to improve during the next year until the end of the construction project, the reality of the situation is that the City of Columbia cannot guarantee business owners and patrons of Five Points that flooding problems will ever be completely solved. Five Points was built in a low area of the city, and will remain in a low area. A storm like the one that hit on July 30, 2005 is a very rare occurrence, categorized as a 100+ year rain event because of the tremendous amount of rain in such a short span of time (4 inches in 45 minutes). Only this combination of rain volume and intensity proved to be catastrophic. Judging from early results of this summer’s rainstorms, a rain of less than 3 inches per hour is manageable, but the additional stress of another inch per hour (remember, millions of gallons of stormwater can result from just one inch of rain) seems to be too much for the drainage system to adequately handle. However, if the National Weather Service data is accurate, something like this should only occur once per century or thereabouts.