Hydrological data transposition by ratio approach for flood & rainfall frequency analyses for ungauged catchments

Abstract

This study involves the application of ratio method of hydrological data transposition for flood and rainfall frequency analysis for ungauged catchments. For the flood transposition, a gauging station, River Niger at Onitsha about 166.9km to an ungauged station, Taylor creek, both in Niger Delta were selected. Onitsha station has been established since 1914 with historic discharge records. For the Taylor creek there was evidence of bank erosion that prompted the early study on bank protection design options, either revetment or vertical face retaining wall. The Design element would require flood frequency analysis for determination of flood events such as 10, 50 and 100 year flood flows. Through bathymetric survey and velocity measurements rating curves were established for Taylor creek; at various depths discharges for Taylor creek and River Niger at Onitsha were computed with corresponding ratios. The overall average ratio is 9:100, that is, 9 percent of River Niger is equivalent to flow in Taylor creek. This ratio permitted 30years of records at Onitsha station to be transposed to Taylor creek. Analogous to ratio approach on stream flows, rainfall intensity duration frequency (IDF) models in Port Harcourt were transposed to Peremabiri by ratio 1:1.2. Both rainfall stations are in Niger Delta, Nigeria. The resulting transposed discharge data for Taylor creek were adopted for design of its bank protection. The ratio approach, though is new and is expected to be tested for more hydrological catchment areas to prove its worth and this is the challenge for researchers in hydrology