Abstract
The area north of the Dar es Salaam City is endowed with beautiful sandy beaches and high concentration or tourist beach hotels that are threatened by coastal erosion. In order to protect the buildings and land, coastal erosion must be controlled by mitigation options that are based on scientific data, wave and current climate along the East African region are generally controlled by the seasonal monsoon winds. During the north-east monsoon period when Winds blow from the Northeast, the wind-generated waves approach the coast from the northerly direction and produce long-shore Currents With a southerly component. During the southerly monsoon period the Wind direction is reversed and so is the wave and current clvmate. As such thc erosion of the coast is expected also to be periodic in pattern and intensity. study area is Kunduchi Beach, which is located IO km north of Dar eS Salaam city. The beach lies on the west of the Zanzibar Channel, which separates Zanzibar from the Mainland. Study of the near-shore waves was carried out by deploying wave gauges on the tidal flat at about 100 m from the beach and in 5 — IO m deep water, about half a kilometre from the beach. pressure gauges can measure up to 2.5 bar of absolute pressure. Each pressure sensor was connected to a 12•bit XR440-M data logger with a capacity Of 86400 readings. Waves were recorded at a sampling frequency Of I Hz on different dates. data set used was recorded in April (during north-east monsoon winds) and September (during the Southeast monsoon winds). According to the measurements, the highest tide level is +2.5 m in thc period Of September. During Iow tide, the tidal nat is dry. Results show that the deviation of the surface elevation from the Gaussian distribution does not vary very much during flooding and ebbing (std (flood) 0.0867, std (ebbing) 0.0954). However, it increases almost twice during high water (std (high water) = 0.14), such that the probability density function of the surface elevation becomes broad and extended. The power spectrum at high tide is 3-peaked and is divided into three frequency regions; long waves (f < 0.04), swells (0.04< f < 0.1) and wind waves (f > O. I). During flooding and ebbing, the power spectra are broad and Iow-peaked, which means waves interact with the flooding and ebbing tides to induce breaking. Significant wave height is linearly related to mean water level. It is during high water that wave activity is most intense and therefore sediment suspension is expected to be highest. It is during thc south-east winds that waves are predominant for the beach processes. The highest waves arrive at the shore and break directly on the beach during high water. Since the shorelme is oriented nearly in the north-south direction and the waves reach thc shoreline from the north-east during the north-east winds and reach the shore from the south-east during the southerly winds, long-shore wave induced currents will be generated. These Currents will carry the suspended sediments along the shore.
Recommended Citation
Dubi, A. (2001). SOME ASPECTS OF EROSIVE WAVES AT KUNDUCHI BEACH, DAR ES SALAAM, TANZANIA. Tanzania Journal of Engineering and Technology (TJET), 24(1), 59-90. https://doi.org/https://doi.org/10.52339/tjet.v24i1.307
Publisher Name
University of Dar es Salaam