A Comparison of different methods for Bathymetric Survey and Sedimentation Evaluation of a Small Reservoir in Nigeria for Sustainable Management

Abstract

Sustainable management of small reservoirs requires adequate information on reservoir volume, sedimentation load and rates. Echo Sounder (ES) is a modern technique in bed measurement, especially in clear waters. This work compared the use of ES and manual survey using BATHYBOX, which was a concrete metal box with a unit weight of 24kN/m³ in bathymetry of a small reservoir for depth measurement, reservoir volume, sediments level and sedimentation rate estimations. Grid lines of 10m x 10m following X and Y axes respectively served as a datum. With GPS, the geographical detail of each survey point was recorded. Calibration of the ES transducer dual High Frequency (HF) and Low frequency (LF), as well as BATHYBOX, ensured the accuracy of measurement. The reservoir capacity with LF and HF were estimated to be 104,292.49m³ and 96,457.41m³ which is about 94.81% and 87.69% of the designed reservoir capacity (110,000m³) respectively. The observed reservoir capacity from BATHYBOX was 105,104.03m³. This is about 95.55% of the original dam capacity.  A difference of 814.54m³ (0.74%) was recorded between reservoir capacity obtained from BATHYBOX and Echo Sounder at LF. Sediment deposition of ES was 7835.08m³ (sediment rate of 652.9m³/yr) and BATHYBOX was 4895.97m³ (sediment rate of 407m³/yr). A difference of less than 10% existed between the volume estimation from the two. Although ES saves time, it is limited in highly vegetated reservoirs. BATHYBOX performed better under such a condition. BATHYBOX is an affordable alternative especially in small reservoirs even though it has higher labour need.