Simulation of flood hydrographs based on lumped and semi-distributed models for two tropical catchments in Kenya

Abstract

The design and operation of flood management systems require computation of flood hydrographs for both design floods and flood forecasting purposes, since observed data are usually inadequate for these tasks. This is particularly relevant for most developing countries, i.e. mainly for tropical catchments. One possible way of obtaining information about flood hydrographs is through the use of rainfall—runoff models. Two such models, namely the Bochum model and the Nash Cascade—Diskin Infiltration model, which are semi-distributed and lumped models, respectively, were used in the present study. These models were applied to two catchments in Kenya with drainage areas of 6.71 km² and 26.03 km². A set of 13 selected rainfall—runoff events was used to calibrate and validate the models. The physical parameters required by the models were derived from catchment characteristics using GIS and remote sensing data while the conceptual parameters were obtained by optimization. The flood hydrographs simulated using the parameters so derived indicated that it is possible to use the two models in this tropical environment.     

River sediment supply, sedimentation and transport of the highly turbid sediment plume in Malindi Bay, Kenya

The paper presents results of a study on the sediment supply and movement of highly turbid sediment plume within Malindi Bay in the Northern region of the Kenya coast. The current velocities, tidal elevation, salinity and suspended sediment concentrations (TSSC) were measured in stations located within the bay using Aanderaa Recording Current Meter (RCM-9), Turbidity Sensor mounted on RCM-9, Divers Gauges and Aanderaa Temperature-Salinity Meter. The study established that Malindi Bay receives a high terrigenous sediment load amounting to 5.7 × 10⁶ ton·yr^?1. The river freshwater supply into the bay is highly variable ranging from 7 to 680 m³·s^?1. The high flows that are > 150 m³·s^?1 occurred in May during the South East Monsoon (SEM). Relatively low peak flows occurred in November during the North East Monsoon (NEM) but these were usually <70 m³·s^?1. The discharge of highly turbidity river water into the bay in April and May occurs in a period of high intensity SEM winds that generate strong north flowing current that transports the river sediment plume northward. However, during the NEM, the river supply of turbid water is relatively low occurring in a period of relatively low intensity NEM winds that result in relatively weaker south flowing current that transports the sediment plume southward. The mechanism of advection of the sediment plume north or south of the estuary is mainly thought to be due to the Ekman transport generated by the onshore monsoon winds. Limited movement of the river sediment plume southward towards Ras Vasco Da Gama during NEM has ensured that the coral reef ecosystem in the northern parts of Malindi Marine National Park has not been completely destroyed by the influx of terrigenous sediments. However, to the north there is no coral reef ecosystem. The high sediment discharge into Malindi Bay can be attributed to land use change in the Athi-Sabaki River Basin in addition to rapid population increase which has led to clearance of forests to open land for agriculture, livestock grazing and settlement. The problems of heavy siltation in the bay can be addressed by implementing effective soil conservation programmes in the Athi-Sabaki Basin. However, the soil conservation programmes in the basin are yet to succeed due to widespread poverty among the inhabitants and the complications brought about by climate change.     

Geochemical evidence of hydrothermal recharge in Lake Baringo,central Kenya Rift Valley

Lake Baringo, a freshwater lake in the central Kenya Rift Valley, is fed by perennial and ephemeral rivers, direct rainfall, and hot springs on Ol Kokwe Island near the centre of the lake. The lake has no surface outlet, but despite high evaporation rates it maintains dilute waters by subsurface seepage through permeable sediments and faulted lavas. New geochemical analyses (major ions, trace elements) of the river, lake, and hot spring waters and the suspended sediments have been made to determine the main controls of lake water quality. The results show that evaporative concentration and the binary mixing between two end members (rivers and thermal waters) can explain the hydrochemistry of the lake waters. Two zones are recognized from water composition. The southern part of the lake near sites of perennial river inflow is weakly influenced by evaporation, has low total dissolved species (TDS), and has a seasonally variable load of mainly detrital suspended sediments. In contrast, waters of the northern part of the lake show evidence for strong evaporation (TDS of up to eight times inflow). Authigenic clay minerals and calcite may be precipitating from those more concentrated fluids. The subaerial hot‐spring waters have a distinctive chemistry and are enriched in some elements that are also present in the lake water. Comparison of the chemical composition of the inflowing surface waters and lake water shows (1) an enrichment of some species (HCO₃^?, Cl, SO₄^2?, F, Na, B, V, Cr, As, Mo, Ba and U) in the lake, (2) a depletion in SiO₂ in the lake, and (3) a possible hydrothermal origin for most F. The rare earth element distribution and the F/Cl and Na/Cl ratios give valuable information on the rate of mixing of the river and hydrothermal fluids in the lake water. Calculations imply that thermal fluids may be seeping upward locally into the lake through grid‐faulted lavas, particularly south of Ol Kokwe Island. Copyright © 2006 John Wiley & Sons, Ltd.     

Magnetotelluric images of the crustal structure of Chyulu Hills volcanic field, Kenya

Electromagnetic experiments were conducted in 1995 as part of a multidisciplinary research project to investigate the deep structure of the Chyulu Hills volcanic chain on the eastern flank of the Kenya Rift in East Africa. Transient electromagnetic (TEM) and broadband (120–0.0001 Hz) magnetotelluric (MT) soundings were made at eight stations along a seismic survey line and the data were processed using standard techniques. The TEM data provided effective correction for static shifts in MT data. The MT data were inverted for the structure in the upper 20 km of the crust using a 2-D inversion scheme and a variety of starting models. The resulting 2-D models show interesting features but the wide spacing between the MT stations limited model resolution to a large extent. These models suggest that there are significant differences in the physical state of the crust between the northern and southern parts of the Chyulu Hills volcanic field. North of the Chyulu Hills, the resistivity structure consists of a 10–12-km-thick resistive (up to 4000 Ω m) upper crustal layer, ca. 10-km-thick mid-crustal layer of moderate resistivity (50 Ω m), and a conductive substratum. The resistive upper crustal unit is considerably thinner over the main ridge (where it is ca. 2 km thick) and further south (where it may be up to 5 km thick). Below this cover unit, steep zones of low resistivity (0.01–10 Ω m) occur underneath the main ridge and at its NW and SE margins (near survey positions 100 and 150–210 km on seismic line F of Novak et al. [Novak, O., Prodehl, C., Jacob, A.W.B., Okoth, W., 1997. Crustal structure of the southern flank of the Kenya Rift deduced from wide-angle P-wave data. In: Fuchs, K., Altherr, R., Muller, B., Prodehl, C. (Eds.), Structure and Dynamic Processes in the Lithosphere of the Afro-Arabian Rift System. Tectonophysics, vol. 278, 171–186]). These conductors appear to be best developed in upper crustal (1–8 km) and middle crustal (9–18 km) zones in the areas affected by volcanism. The low-resistivity anomalies are interpreted as possible magmatic features and may be related to the low-velocity zones recently detected at greater depth in the same geographic locations. The MT results, thus, provide a necessary upper crustal constraint on the anomalous zone in Chyulu Hills, and we suggest that MT is a logical compliment to seismics for the exploration of the deep crust in this volcanic-covered basement terrain. A detailed 3-D field study is recommended to gain a better understanding of the deep structure of the volcanic field.     

Sedimentation and recent history of a freshwater wetland in a semi-arid environment: Loboi Swamp, Kenya, East Africa

Loboi Swamp is a 1·5 km² freshwater wetland situated near the equator in the Kenya Rift Valley. The climate is semi‐arid: precipitation is ≈ 700 mm year^?1, and evapotranspiration is ≈ 2500 mm year^?1. Some of the wetland water is currently used for irrigation. An interdisciplinary study was conducted on the geology, hydrology, pedology and biology of the wetland to determine its origin and history and to assess its longevity under present hydrological conditions. Sedimentary records from two piston cores (1·8 and 4 m long) indicate that the present wetland developed during the late Holocene on a low‐relief alluvial plain. Floodplain deposits (sandy silts) are capped with wetland sediments (organic‐rich clay and peat), which began to form at ≈ 700 BP. The swamp is dominated by Typha domingensis Pers. (≈ 80%) and floating Cyperus papyrus L. (20%). It is fed by warm springs (T ≈ 35 °C; pH ≈ 6·4–6·9) emanating from grid faults of the rift floor. Water compositions suggest that sources are dominated by shallow meteoric water, with little contribution from deeper geothermal fluids. Siderite concretions in the floodplain silts reflect the Fe‐reducing conditions that developed as the surface became submerged beneath the water table. The pollen record captured both local and more regional vegetation, showing the prevailing dry rift valley climate despite development of the wetter conditions on the valley floor. The diatom record also suggests a dramatic change in local hydrology. The combined biological records of this semi‐arid wetland indicate an abrupt change to wetter conditions, most probably as a result of a regional change in climate. Rift tectonics provided accommodation space, maintained the wetland at or below the water table and enabled spring recharge. The size of the modern wetland has been reduced by about 60% since 1969, which suggests that the system may now be under hydrological stress due to anthropogenic impacts from land‐use change.     

Analysis of survival curves in seasonally mated pastoral goat herds in northern Kenya using logistic regression techniques

In order to assess the effect of controlled seasonal breeding on survival performance of pastoral goat flocks, a systematic breeding programme was initiated in a herd of small East African goats over a period of 4 years in Isiolo District, northern Kenya. This papers presents an analysis of survival rates observed in six different mating seasons. The statistical analysis is based on modelling hazard functions parametrically over time using logistic regression and polynomial spline functions. With respect to kid survival, litter size and parity of dam failed to reach statistical significance after adjustment was made for the effects of birth weight and milk yield until weaning. Highly significant interactions were found between time factors and the effect of mating season, indicating non-proportionality of death risks across mating seasons. Confining mating to the period between June and November is likely to confer a distinctive advantage in terms of young livestock survival. In contrast, joining does during the short dry season leads to kid mortality rates of about 50%, and should therefore be avoided. Doe survival was evaluated in terms of mating season, production cycle, parity, and reproductive status. The ranking of mating season groups with respect to doe survival was similar to that observed in kids, although the differences were much smaller and non-significant. The lowest annual mortality rate was observed when breeding females were mated during the long dry season. It is concluded that confining breeding to the long dry season can be an effective management intervention to reduce mortality rates in pastoral goat flocks.     

Fossil diatoms and the mid to late holocene paleolimnology of Lake Turkana,Kenya: a reconnaissance study

A 12 m sediment core recovered from the south basin of Lake Turkana, northwestern Kenya, reveals four major diatom assemblages that span approximately 5450 to 1070 years BP based on AMS radiocarbon analyses. The oldest assemblage, Zone D (5450 to 4850 yr BP), is dominated by Melosira nyassensis and Stephanodiscus spp. and is interpreted to reflect higher lake levels, fresher water and more variable seasonal mixing of the water column than the modern lake. Melosira dominates the assemblage in Zone C (4850 to 3900 yr BP) with some Surirella engleri and Stephanodiscus. This assemblage indicates a continuation of relatively high lake levels and seasonal mixing of a stratified lake. The brief peak of Surirella, interpreted as benthic, suggests an episode of slightly lower lake level. Thalassiosira rudolfi and Surirella predominate since the beginning of Zone B (3900 to 1900 yr BP), reflecting a decrease in lake level and increase in water column salinity. Increasing dominance of Surirella in Zone A (1900 to 1070 yr BP) may suggest that the lake continued to decrease in depth. Salinity probably rose to levels comparable with the modern lake. These results are consistent with paleoclimatic interpretations based on carbonate abundance, lamination thickness, oxygen isotope and bulk geochemistry profiles from this core and cores recovered from the north basin. It extends the known paleolimnology beyond 4000 yr BP of the earlier research to 5450 yr BP and into the early to mid Holocene pluvial phase in northern intertropical east Africa.     

Geochemistry and Tectonomagmatic Affinities of the Mozambique Belt Intrusive Rocks in Matuu-Masinga Area, Central Kenya

The Matuu-Masinga study area, located about 70 km north-east of Nairobi, is predominantly underlain by rocks of the Neoproterozoic Mozambique Belt (MB) of Kenya. The rocks vary from medium to high grade gneisses and granulites, that are intruded by granites, anorthosites, diorites and gabbros. Important high-grade tectonothermal events in the belt took place between about 845 and 715 Ma B.P. The subsequent cooling and uplift of the basement has been traced by K-Ar dates on biotites, which range between 438 and 528 Ma. The belt has been inferred to mark the sites for several superimposed Proterozoic subduction zones and collisional sutures (Muhongo, 1998, and references therein).The general structural trend in the survey area varies from NNW-SSE to NW-SE direction, with westerly dips and localized concentric trends around granitoid intrusions. Competent mafic lensoidal layers that have undergone ductile deformation and associated with rotated boudins and displaced micro-faults define a sinistral sense of shear. The several shear zones, cleavage patterns, boudins and thinning suggest severe strains.The mafic and granitic rocks are interpreted to be mainly metaluminous to slightly peraluminous. Geochemical data and field studies suggest that the granites are of calc-alkaline origin. The discriminant diagrams suggest a dominantly island arc-tectonic setting with subordinate within plate environment.Rb-Sr whole-rock age of 558 ± 16 Ma is given on the granite from Mavoloni hills, Matuu area. From the low initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70398, the age is interpreted to indicate the time of emplacement for granite magma from a deep crustal or upper mantle material.The gabbroic and granitic rocks in Matuu-Masinga area, with high Ba (av. 1331 ppm) and Zr (av. 370 ppm) contents, are interpreted to have contained hornblende and biotite mineral assemblage phases that had a relatively high distribution coefficients for Ba and Z. The relatively high concentration of Cu (188–5810 ppm, av. 1960 ppm) and Zn (88–264 ppm, av. 155 ppm) in the mafic rocks of the study area invokes further exploration of their ore minerals.     

Ground subsidence and its socio-economic implications on the population: a case study of the Nakuru area in Central Rift Valley, Kenya

 Several areas of Nakuru Town and its environs often undergo subsidence along the parallel fault zones during and after heavy rainfall. During the rainy season, when most of the subsidence occurs, the overlying unconsolidated volcanoclastic sediments become oversaturated with water. The water reduces the shear strength of the sediments and also introduces extra loading through saturation leading to subterranean erosion along faults. The unconsolidated sediments then collapse into the subsurface water channels which closely follow the fault zones, leading to formation of “sinkholes”. The frequent incidences of ground subsidence in the study area, have caused several fatalities, destroyed settlements and physical infrastructure. Furthermore persistent subsidence has increased the cost of construction and the repair of the destroyed properties. Apart from being hazardous, ground subsidence degrades environment when sewage water, refuse and garbage enter into the groundwater systems through the sinkholes. The fissures formed after subsidence also stand prominently as ugly features from the rest of the terrain. Mitigation measures including control, channelizing of drainage, proper engineering practices and appropriate land use are suggested in this paper. Received: 1 December 1998 · Accepted: 8 March 1999