North Carolina’s dams in the US experienced the full force of Hurricane Floyd. Suzanne Moxon reports
Flying out of Washington Dulles airport I felt relieved to be heading towards the Atlantic Ocean, homeward-bound for the UK. On 16 September 1999, the day Hurricane Floyd cast a foreboding shadow over the US east coast, my flight was one of few that even attempted to take off into the darkening skies. With the biggest task before me the choice of my inflight movie, I was glad to be leaving the raging wind and blinding rain behind.
Such an opportunity, however, was not open to Jim Simons, a dam safety engineer in North Carolina. He and his colleagues witnessed the full brunt of Floyd’s fury for themselves — and were left to pick up the pieces.
Described as the worst natural disaster in the history of North Carolina, Hurricane Floyd visited the state only a few months after Hurricane Denis had left its mark. Streams and rivers did not have a chance to return to normal levels before Floyd unceremoniously dumped 20in of rain in 24 hours. This was then followed by tropical storms which led to another 8in of rainfall.
As Simons explained, the severe weather affected 44 of North Carolina’s 100 counties. With 1200 dams under their remit, and with a significant proportion of these being classified as high hazard structures, the state’s dam safety engineers knew that their job was only just beginning.
Catalogue of events
On 16 September North Carolina experienced 40 dam failures. At least 91 dams were overtopped. Sixty-one dams which have been damaged are still impounding water and are a cause for concern. Most dams failed through overtopping, although several of the high hazard dams failed through head cutting in the emergency spillway. ‘Some dams did not fail during Floyd,’ Simons said, commenting on the catalogue of weather events that surrounded the hurricane. ‘Some owners were just not able to repair their dams before the tropical storms followed. And the structures couldn’t cope.’ There were no fatalities caused by the dam failures. Several secondary roads were washed out by the failing dams and some houses and shops were under water but, fortunately, they had already been evacuated. Six counties in the state said that this had been their 500-year flood event. To sum up the enormity of the flooding, Simons marvelled at the fact that the main Interstate 40 was 3.4m under water.
Before the winds had died down Simons and his colleagues were out in the field to assess the damage. They inspected 160 dams in a two-week period — a job which was made increasingly difficult as some areas were turned into islands.
Upon arrival at the dams the engineers carried out reconnaissance inspections. If a dam had failed their main concern was to ascertain how high the overtopping had been, how long it had lasted and what size the breach section was. Eye witnesses were asked for information which could then be analysed at a later date. Some homeowners were even able to provide video footage of failing dams.
It was also necessary to determine if dams had failed completely. Some which had experienced partial failure were still impounding water and needed to be assessed. If additional flooding occurred the hazard potential had to be calculated, taking into account any downstream development and risk to human life.
After damage assessment Simons’ biggest task was to locate all dam owners and break the bad news to them. Owners of dams which have only partially failed or are damaged and still holding water, are being encouraged to continue inspections along with the dam safety engineers.
‘The hurricane season is now over,’ says Simons, ‘but we still have the potential for heavy rain and flooding. Some dams do not have bottom drains and so it has been difficult to lower the water level. We are still inspecting the dams and are now getting engineers to start repair plans.’ Repairing the dams may have just as devastating an effect on North Carolina as the hurricane. The cost of repairs of some structures has been estimated at hundreds of thousands of dollars. All of the dams are privately owned 3-8m high earthen impoundments. Many were millponds built in the 1800s which are mainly used for recreation now. ‘The problem is that many of the private owners are not wealthy,’ Simons explains. ‘Many are also still in a distressed state as their homes have been flooded or damaged. So at the moment they’re not jumping at the bit to spend money on their dams when they have a tree through their house.’ Simons adds that at the present time there is no immediate danger from the hurricane-stricken dams. State engineers will continue to work with the owners to repair the structures but everyone is aware this will not happen over night. ‘It’s worth noting,’ he says, ‘that we have only just finished repairing dams which were damaged by Hurricane Fran in 1996.’
Watershed repair funds
From a financial perspective, one positive note may be the small watershed repair fund bills which are currently working their way through the US legislative system. These are requesting government funding to refurbish dams built under various public law (PL) programmes. (See article on pp26-27 of this issue). About six PL566 dams have been severely damaged by the hurricane and so may get funding if this bill goes ahead.
The importance of the National Dam Safety Programme was also highlighted by Floyd. ‘We were much better prepared for this than we were for Fran,’ says Simons. The programme, which came into force in 1996, enabled North Carolina’s dam engineers to utilise programme-funded equipment in their pre- and post-hurricane efforts.
So what were the lessons from Hurricane Floyd? On dam safety, Simons says that they were as well prepared as they could be. But one lesson has been learnt. ‘Before Floyd,’ he explains, ‘we were worrying about coastal destruction from hurricanes, which happened with Fran. But Floyd taught us that hurricanes can create terrible flooding. The state has now learnt about the implications of development close to the flood plain.’ He also spoke about the increasing awareness of deaths from driving on flooded roads and the potential for tragedy on roads downstream from dams. ‘To be honest,’ Simons adds wryly, ‘I think we’re just more afraid of hurricanes now.’