STORMWATER DRAINAGE AND SEWERAGE OR THE GREATER FREETOWN AREA MASTER PLAN REVIEW AND RELIMINARY ENGINEERING DESIGN    A high proportion of the rainfall in the Freetown region is discharged rapidly to the sea as surface runoff as a result of the steep topography of the hills surrounding the town, low permcability cLay soils and the high proportion of paved surfaces within the urban area. The dty has an extensive stormwater drainage system dating from the early 19th Century. comprising road gutters, collector drains and deeper main drains discharging into natural stram channels and thence into the Sierra Leone EsnTy. Most of the drainage network is reported to have been built by the mid 1950s and there have been no significanc improvemenms since then. Road gutters arc constructcd of precast sections with curved inverts whilst the main screet drains are of standard precast concrete open channel sections or concrete or brick-lined channels of various ,; 2L91Ho3110 "1 5 - 9 sections construaed in sitU. larger drains may incorporate a low flow high-velocity centre section e.g. Sanders Brook. Although the system has developed in a piecemeal manner. with no overall planning, in general it performs satisfactorily and major flooding is resricted to only a few flatter : localities in the dty. Flooding tends to be associated with short duradon storms and rarely persists for more than a few hours. Overtopping of the drainage network is in many case due to inadequate channel capacity to accommodate less frequent flood flows or to constaints at road crossings or channel intersections. However, in other cas flooding arises from a lack of regular maintenance operations, particularly the clearance of silt and debris in the chanels and the rpair of colapsed or damaged sections. Soil erosion from the steep undeveloped parts of the ca!chments and the urban residential areas produces high loads of suspended material which, according to hydraulic conditions, contributes either to the erosion of the concrete or brick-lined drainage channels or accumulates in the channels. Since the DbUM studies in 1977 and 1980 there has been considerable urban development to the west and east of the traditional boundary of the city. In paticular, the completion of the Freetown-WelUington trunk road in the mid 1970's has led to extensive ribbon development through Iissy and further to the casL New residential development on the steep slopes of the suburb of Tengbeh Town, west of the city centre, has also taken place in recent years. In general no fofrmal drainage system has been constructed alongsidc these developments and both flooding and erosion of road pavements are increasingly serious problems. There are unfortunately no quantitative data on flood depths or duradons during major flood events and no flow measurenents are undertaken on either the natural or artificial channels in the Freetown area. Infonnation on flood damage Is also only anecdocal but undoubtedly flooding in die main channels has resulted ln the loss of several lives over recent years. The risk of death arises predominandy fiom persons alling into and drowning in the high-velodty flows in the deeper channels. Propert di-nage figures are not available from GOSL, but insurance companies have been contacted and surveys underaken to confirm the exrent and value of damagc aising From the most recent significant flood evenr, in August 1992, in which a number of people are reported to have drowned. lnadequacies in the existing stormwater drainage system also have inporunt advese impacts on public health. Stagnant ponded watm in blocked or damaged drains and channels are favourable breeding grounds for Anopbeks gamlbiae, the mosquito vector for malaria, whilst slow moving water contaminted by sullage and other wastes creates conditions favourable to the breeding of Culexfatigans, the mosquito vcwor for filarsis and the common house fly Musca domestkca, vector for vatious aetiological agents of infecdous diseases parcularly those of a diarrhoeal nature. bhe incidence of fy-bomc disease is strongly associated with blockages In drains causing stagnant or slow moving water, uncovered drains and the presence of solid waste, sewagc and sulage in the drainage system. Many open drains are used for bathing or washing dothes and there is consequendy a high level of diret contact with water coutaminated by suDage and sewage from pit latrines and scptic tanks. The problem is most severe in the densely populated low income areas of the city and particularly in the slum area   5.3.4 Stormwater Drainage A number of immediate remedial works to the storm drainage system have been idendfied. In the longer term, measurcs to Improve the whole network to reduce flooding and improve the sanitry conditon of the drains may be bnpplemented. Thc immediate and long term works comprise the following:. * repairs to main storm drains of adequate capacity works to increase the hydraulic capacity of main drains and channcls where this is inadequate * new drainage measures in areas lacking adequate facilities      Stornmwter Drainage The impacts of the construction of storinwater drainage works are similar in most respects to those arising from the extension of the sewerage system. Traffic disrupdon, noise and dust impacts would be significant in the short tern but would be mitigable by ensuring that the coanracaor employs best practice procedures. Many of the open drains in the road ncrwork present a hazard to pedestians and providc a convenient means for illegally disposing of solid and lquid wastes. The improvement works allow for removable covers to be provided for fooc traffic in areas where no alternative pedestrian access is available- However, whilst it would be desirable to provide covers to all road drmai within the network. such works are not considered to be cost-effecrive. Better protection of the drains from dumping may bc achieved by public education programmes stressing the function and importance of the dr;iinage network. In the vicinity of the improved maii river channels, visual improvements may be effected through the involvement of self-help groups and community orpnisations. The sense of responsibility and guardiamship engendered through loca partidpation schemes would encourage protection of the structures and reduced use of the channels for dumping. At its best, local parriApation in simple landscape design along the linear routes of the channels may provide significant environmiiental enhancements whilst maintaining the waterways in good hydraulic condition. At high discharges the main channels are hazardous to public safety. Loss of life through drowning has occurTed in recent flood events. In critical high velociy sections access to the channels may be restricted by increasing the height of retaining wals or installing protective railing but such measures will have ondy a limited effect on the total risk. The problem may be best addressed by public awareness programmes and the involvement of community organisations.