Urban erosion control in the Triângulo pilot study area Nacala city, Mozambique, Equatiorial Africa: A multidisciplinary approach with specific emphasis on control of gully erosion by labour intensive construction work: Licentiate thesis

This study is a multidisciplinary approach to erosion control in urban areas with specific emphasis on control of gully erosion by labour intensive construction work. The study was executed in cooperation with another study, "Erosion control by vegetative measures". Both studies were executed in the Triângulo pilot study area in Nacala, Mozambique. The aim of the study is to evaluate the impact of intrinsic and extrinsic variables on the erosion processes affecting the coastal Pleistocene sand slopes of the city. Besides analysis of the erosion processes, also dealt with are conservative methods and protective measures aimed at preventing gully initiation and development and finally at restoration of the terrain. The study includes the following parts: field inventory, rainfall analysis, estimation of discharge and estimation of soil losses. Based on the estimations, a drainage and erosion control design is presented. Execution of the work during 1990-92 is briefly described and experiences gained so far are introduced. The research methods used are time-area, rational and Cook's method for the estimation of discharge, and the methods of Elwell and Poliakov and a volumetric method for estimation of soil losses. The state of erosion in the Triângulo pilot study area is fairly severe. The main type of erosion is rill and gully erosion, the total length of erosion channels being over 5,000 m. Intrinsic variables make the system highly susceptible to disturbances in the pilot study area. The impact of extrinsic variables in accelerating the erosion process has been considerable. Calculated soil losses are about 13,000 m3/a. Measured soil loss from the largest gullies is about 12,000 m3. Cook's method gave the lowest values for discharge, the results of the time-area method gave approximately the same values, and the rational method slightly higher values. The drainage and erosion control plan includes cut-off, discharge and collection ditches, as well as check dams, rock protections, weirs and other gabion structures. The general plan and design values are based on the hydrological data and erosion sensitivity of the soil. The possibility to use labour intensive work methods was also a main target. Construction work was executed simultaneously with vegetative measures. Other main measures were land-use planning, resettlement of people, extension work and participation of residents. Based on experience from the Triângulo pilot study area, the erosion control method to be used in Nacala should be a combination of vegetative measures, low cost construction and some heavy construction work. The role of vegetation is crucial and a resettlement component is also needed. The term "Land husbandry" can be applied to this type of combination of urban erosion control measures.