Spreadsheet modelling of unit and storm hydrographs for ungauged watersheds

Abstract

This paper describes a spreadsheet model developed for deriving both unit and storm hydrographs for ungauged watersheds based on the Soil Conservation Service (SCS) method. Essentially, the model implements computational procedures that involve developing a 1-hour unit hydrograph for a given ungauged watershed, then applying it to estimate 1hour incremental rainfall excess to derive a design storm hydrograph through the convolution technique. The model, which is intended to remove computational drudgery in several calculations that are involved in the synthetic unit hydrograph derivation, excess rainfall estimation and hydrograph convolution, was applied in deriving a 1-hour unit, 25-yr, 24 hr and 100-yr, 24 hr storm hydrographs for an ungauged watershed of the Department of Agricultural Engineering, University of Ilorin, Ilorin, Nigeria. The weighted curve number and the potential maximum storage of the watershed estimated by the model were 73 mm and 94 mm respectively. The model computed the watershed’s lag time and time of concentration as 0.25 hr and 0.42 hr. The peak flow from the unit hydrograph was 0.05 m3/s at time to peak of 0.8 hr while the 25-yr, 24 hr and the 100-yr, 24 hr storms had peak flows of 1.73 m³/s and 2.38 m³/s, at times to peak of 1.20 hr and 1.12 hr respectively. This work will enhance the computational efficiency in unit and storm hydrograph development.

Keywords: Spreadsheet modelling, Unit hydrograph, Storm hydrograph, Ungauged watersheds

Journal of Applied Science, Engineering and Technology, Volume 8, 2008