The effects of hurricane hugo on suspended-sediment loads,Lago loíza Basin,Puerto Rico

Runoff generated from large storm events, such as hurricanes, is expected to transport large quantities of suspended sediment. Observations of suspended-sediment loads transported during Hurricane Hugo in the Lago Loìza (Lake Loiza) basin, Puerto Rico, indicate that loads were lower than expected. In the two main tributaries that enter Lago Loìza, Rio Grande de Loìza and Rio Gurabo, 99 600 tonnes of suspended sediment was transported by 58·2 × 10⁶ m³ of runoff in a 48 h period. The storm-average suspended-sediment concentration in the Río Grande de Loìza for Hurricane Hugo was 2290 mgl⁻¹, the second lowest for the 12 storms that have been monitored at this site. In Río Gurabo the storm-average suspended-sediment concentration was 1420 mgl⁻¹, the sixth lowest recorded out of 15 monitored storms. In Quebrada Salvatierra, a small tributary to Río Grande de Loìza, suspended-sediment concentrations were as low as 33 mgl⁻¹ during the peak runoff of 20 m³ s⁻¹. Normally the suspended-sediment concentrations at this discharge are 3000 mgl⁻¹. Hurricane force winds seem to be the most important factor contributing to the lower than expected suspended-sediment loads in the Lago Loìza basin. The high winds caused vegetation and debris to be dislodged and displaced. Debris accumulated on hillslopes and in small channels, blocked bridges and formed debris dams. These dams caused local backwater effects that reduced stream velocities and decreased suspended-sediment loads.