Floodwater destruction

Floodwater destruction

(The Groundsman, August 2007)

Floodwater moves silently and quickly – it can be menacing and destructive.

Time has no meaning and all and everything succumbs to its power and disregard. There are two main aspects to flooding.

  • the deluge of water flow following an indiscriminate path on an unplanned water course seeking escape to a definite outfall destination. This water is the product of catchment water that accumulates over an area into a concentrated flow that eventually forces an exit route.
  • the accumulation of surface water with no immediate and adequate route of escape. It is confined to low lying areas and can be far more destructive. The water level rises relative to the capacity of the unplanned collection area that has inadequate outfall.

At the time of flooding, nothing much can be done besides moving people and property to higher ground – if there is time.

Erosion and deposition

Flooding brings destruction to the landscape and lessons can be learnt to prevent or reduce the damage in future occurrences. Water flow has the power to erode, displace and destruct. Denudation begins when existing grass cover cannot tolerate the high velocity water flow. Quickly, grass cover, loose stones, sand, silt and clay particles become dislodged and are carried away in surplus water flow where they are suspended owing to the speed of water flow. Heavy compacted clay soils are not as easily disturbed.

While water is moving quickly it deposits little. When there is rapid flow all forms of suspended matter including debris of virtually any size can be conveyed along the most direct line of water course to lower lying land. It is logical therefore that, where flooding is expected, such water should be directed where possible through a reticulation of ditches, swales and remodelled land form in such a way as to conserve structures along the exit route to the water flow.

In the case of flooding resulting from high intensity short duration storms, provision to direct water flow can be planned ahead at the time of construction. The ultimate destination outfall of the surplus water flow should be clearly defined. If it does not exist it should be provided for in earthworks design in the form of an excavated pond or wet land area. Attenuation (temporary storage of water) can also be installed in the form of buried reinforced plastic cells or rock-filled excavations. This measure prevents a peak flow and controls the rate of water discharge. Where water is trapped in depressions, settlement and deposition is inevitable. Large expanses of flood water with no route of escape will deposit as soon as the rate of water flow is unable to retain the suspended particles.

Health and Safety

With the rapid flow of storm water run-off following the path of exit over localised areas, there is usually little risk of contamination. The water is generally entirely surplus rain water finding an exit route. However, when flooding is extensive and entire areas of industrial and domestic structures are inundated, there is the expected incorporation of foul water and industrial water material. This changes the scene dramatically and demands special care and attention.

None of this water or contaminated soil should be handled or dealt with without protective clothing. In fact little can be done before the flood water subsides. The only effort that can be made is the immediate protection of property and structures by whatever method is practical. This would include sand bagging and where feasible, the use of earthworks machinery to channel or divert water flow.

In depressed areas where water collects and flow rate is reduced, there is the accumulation of silt and sand, but of more concern, foreign and objectionable matter that pose a danger to human health. Once water levels subside there are three immediate concerns:

  • soil deposits contaminated with infectious bacteria
  • soil contaminated with heavy metal and toxic substances
  • surplus deposits of clay, silt, sand, vegetative and foreign matter

Associated with these deposits are areas of eroded and denuded land from where the deposits originate. Depending on the speed of flow, the distance between the two may be considerable. Drying out is relatively quick and fortunately the health hazard of bacteria infected material soon diminishes with exposure to the sun and atmosphere. These levels of contamination are rapidly reduced. The presence of heavy metal minerals and toxic substances however, can remain where the source of water and its passage of flow have followed unplanned water courses. Soil analysis following standard guidelines should be carried out on samples of soil taken from a representative grid within the area of deposition.

The major concern is not one of health and safety but is the sheer physical effort needed to remove surplus silt and sand deposits and undertake the restoration to the denuded and eroded areas that have suffered from the rapid surface water flow. Where silt and sand have been deposited this material must be scraped off with earthmoving equipment and shovels in restricted areas. In addition, the eroded areas need imported soil and have to be reseeded. Grass cover not buried for more than a week is likely to soon recover.

Further Consideration

After the flood serous thought should be given to altering the land form to create routes of water escape and collections areas for storm water run-off. Where low elevations make the potential for the rise of flood water levels inevitable due to inadequate outfall, there should be some remodelling of land from to provide wherever possible the eventual exit of the surplus water.

The utilisation of flood plains for sport and recreation is cost effective where damage inflicted is not a major concern and human life is not threatened. However, within these areas it is expedient to plan water courses with a ditch reticulation and in the case of recreational areas such as golf courses, sports and leisure sites to elevate features which include greens and tees. In addition, there should significant surface gradient to sports pitches and golf course fairways. Wherever rising water levels can be diverted away from features and water run-off can be channelled out of harms way, there is merit in undertaking the earthworks necessary to achieve this.

In summary, whether preparation is made for excessive storm water run-off or localised flooding, the planning of water flow outfalls is a vital consideration. Creating surface gradients, adequate water channelling and elevated features all contribute to reducing the extent of damage that follows temporary or more indeterminate flooding. Though contamination may be minimal on recreation sites, land locally exploited industrially and commercially warrants special attention with regard to coping with storm water run-off or flooding. With the planned remodelling of land form wherever there is the fear of surplus water flow, there can be significant reduction in destruction and restoration.

Gordon Jaaback

August 2007