In 2004 and 2005, several Caribbean islands including Haiti, Dominica, Grenada, Jamaica, St. Lucia, and Trinidad and Tobago were affected by sediment-water flows which were simply recorded and treated as common water floods (Ahmad and Baban, 2004; Ahmad, 2005 a). This subject has not received due attention of either the local hazard professional community or disaster management officials in the Caribbean. Sediment floods are amenable to avoidance and correction and economic losses may be significantly reduced. It is important to distinguish different flow types to adopt appropriate mitigation. Costa and Jarrett (1981) have shown that protective measures for water floods may not be effective for debris flows and indirect-discharge estimates in sediment flow channels may not be accurate. Also, hydrological processes and channel dynamics in relatively small and steep mountain watersheds of the Caribbean are not comparable to those operating in the large river basins. Since flooding processes are markedly different in the two environments, the response and management for two scenarios is also different. Costa and Jarrett (1981), Costa (1984), and Hungr et al. (2001) have described physical geomorphology, material properties, rheology, differentiation of water floods and debris flows, and classification of sediment-water flow types (Table 1.2). Table I.2. Sediment – water flow types in channels, modified from Costa, 1984. FLOW TYPE:
WATER FLOOD Stream flow
Sediment load by weight Bulk Density g/cm3 Strength dynes/m2 Fluid Type
1-40% 1.0-1.3 <100
Deposits and Landform
Sorted, stratified sheets and bars
Newtonian
MUD/DEBRIS FLOOD Hypoconcentrated flow 40-70% 1.3-1.8 100-200
DEBRIS FLOW
Approximately Newtonian Poorly sorted; weakly stratified, no sharply defined margins
Visco-plastic
70-90% 10-20 >200
Levees and lobes of very poorly sorted, largely unstratified debris, large clasts on top and at face of lobe
Debris flows may be regarded as gravity-induced mass movement comprising poorly sorted rock debris, 70-90% by weight, and a process intermediate between landslides and water floods. Its flow properties depend on the sediment size varying between clay to boulders,
R.Ahmad
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