Analysing river bank seepage with a synthetic design hydrograph

The design conditions of a river embankment are usually computed in a steady state, in a similar way to those due to the maximum river level of an assigned return period. The seepage analysis of the water pressure that stresses the embankment, with an increase in the risk of failure owing to both static instability and piping effects, results in an overly prudent evaluation that can be recognised as a non-economic design. The seepage steady state condition is, however, rarely verified in most river embankments, owing to the limited duration of floods and the poor conductivity of the soils that are usually adopted to construct river banks. Attempts to design in an unsteady regime have been made in recent years by means of mathematical models, but the selection of a design hydrograph in a river remains an ongoing problem. Over recent years, a new procedure for the identification of an efficient synthetic design hydrograph (SDH), with Q4 an assigned return period, has been developed by Maione and co-workers. This procedure has been successfully applied in the design of flood control reservoirs and in the evaluation of flood routing in rivers with large flood plain Q5 storage. In the current paper this SDH is applied in the analysis of seepage through a river embankment, as an attempt to design the river embankment in an unsteady regime; the SDHs of a given return period are applied to a levee with a simple geometry using a semi-analytical model and the main hydraulic quantities (maximum water level, time of maximum, etc.) of the seepage through the soil can be determined. All the real floods are then used as input for the model and the main hydraulic quantities of each flood are determined. The reliability of the design flood methodology has been tested by way of a comparison between the outcomes from the SDHs and the application of real floods.