Suspended Sediment Transport and Exchange In. Port Reitz Creek with Special Focus on the Mangrove Fringed Mwache Wetland, Kenya

Abstract

This study deals with the dynamics of water exchange and sediment flux in the 17 Km2 mangrove-fringed Mwache mangrove wetland in Kenya. The study was implemented in the period between March 1998-March 1999. The creek experiences semi-diurnal tides with a spring tidal range of 3 m. It receives freshwater only in rainy seasons, mainly from Mwache and Borneriverswhosetotalcatchmentareais 1900Km2andriverdischargeislessthan 10m3/sin normal rainfall years. There is usually no river discharge in dry seasons. The aim of the study was to determine the influence of tidal circulation on sediment transport in mangrove fringed creek systems. The study involved measurements of suspended sediment concentrations (SSe), salinity, temperature, current velocities and sea levels both in the tidal channels and in the mangrove swamp forest. Long-term sse, sea level and current velocities were measured with an Orbital Backscatter Sensor, MicroTide pressure gauges and a SD-6000 current meter respectively. A pressure gauge connected to a backscatter sensor was also used to measure sse along the creek. In addition sse were determined by filtering water samples drawn from different levels of the water column with a Hydrobios water sampler. Water temperatures and salinities were measured in situ using an Aanderaa SalinityTemperature sensor. The results show that the sse is higher in the upper mangrove fringed creeks and lower in the frontwater zones bordering the Indian Ocean. The mean near surface sse in the upper mangrove region is 0.16 gfl while that in the lower region is 0.03 gil. Near bottom sse (in the upper zone) were usually higher and reached 1.40 gIl. In the lower frontwater zones, the near bottom sse was of the order 0.10 gil. At the creek entrance, the mean tidal volume fluxes in spring and neap are 2233 m3/s and 937 m3/s respectively. The (near bottom) mean tidal suspended sediment fluxes in spring and neap are 1220 Kgls and 400 Kgls respectively. The tidal sediment flux during flood tide ranged from 460 to 1740 Kgls as observed at a crosssection near the entrance of the creek. The ebb ones were much lower being in the range 330690 Kg/s. Major resuspension of bottom sediments which raises sse to values higher than 1.0 gil, occurs only during flood tide at spring when the current velocities reach 1 mls or more. The turbidity maximum zone (TMZ) with the highest sse occurs in the upper middle region of the creek (Stn 3-6) where sse is in the range 0.07-0.16 gil. This zone coincides with the salinity maximum zone (SMZ) with salinity in the range 36-38 in the dry season. Calculations using formulas for erosion and deposition within the main tidal channel for the period between 15 and 23 February, 1999 showed that erosion reach 6.0 x 10-4 Kglm2/s while deposition reach 5.0 x 10-5 Kg/m2/s. Erosion dominates during periods of high current velocities and deposition during periods of low current velocities which occur at low and high waters. A coarse comparison between total sedimentation and river sediment supply indicates that most of the riverborne sediments are trapped within the creek. In dry season with low sediment supply from the rivers, sediments are imported into the creek from Kipevu basin. Sediments re-suspended during spring flood tide enter the mangrove swamp where they are trapped due to the dense mangrove vegetation and sluggish current velocities of the order 0.05mls which cannot keep sediment particle in suspension. Mean sse is less than 0.05 gil in the mangrove swamp. Sedimentation rate in the mangrove swamp is of the order 250 glm2 per spring tide, corresponding to an accretion rate in the mangroves of the order 30 cml100 years. This is higher than the estimated local sea-level rise of 22 cml100 years, implying that Mwache mangrove swamp is keeping up in pace with sea level rise.