Hydrologic Issues for Dams - Association of State Dam Safety Officials

Temporal patterns used to distribute the rainfall depths in time are based on methods thatwere derived explicitly with the objective of AEP-neutrality (Pilgrim et al., 1969; Nathan,1992). Importantly, “pre-burst” patterns are provided that allow intense bursts of designrainfalls to be converted to storm rainfalls (see next section). Procedures are alsoprovided on how to spatially distribute the rainfalls across the catchment.Estimation of Rainfall ExcessA loss model is needed to partition the design rainfall input into rainfall excess (runoff)and loss. The most common loss models used are the simple Initial Loss – ContinuingLoss (IL-CL) model, and the Initial Loss – Proportional Loss (IL-PL) models. The initialloss is that which occurs prior to the commencement of surface runoff. The continuingloss is the average rate of loss, and the proportional loss the average fraction of loss,throughout the remainder of the storm. The continuing loss rate is often conceptualised asan average rate of potential infiltration over the catchment, while the proportional lossrate (equivalent to 1-C, where C is the runoff coefficient) can be understood to be theunsaturated proportion of the catchment which produces very little runoff.One important distinction made in the application of loss rates is whether or not designbursts of rainfall are used or complete storms; a schematic diagram illustrating thedifference between these two concepts is shown in Figure 3. Design bursts are generallypreceded by some lower intensity rainfall (pre-burst rainfall), they do not representcomplete storms. Accordingly the larger the rainfall the larger the expected cumulativedepth of pre-burst rainfall. This expected variation means that if losses are used withdesign rainfall bursts then they are decreased in some fashion with increasing rainfalldepths. On the other hand, if losses are used with design rainfall storms then the availableempirical evidence (Hill et al, 1997) suggests that they may be held constant over the fullrange of rainfall depths considered.Design StormRainfall / streamflowPre-stormrainfallPre-burst rainfallBurst InitialLoss (IL B )Storm Initial Loss (IL s )Design burstStart ofhydrographriseTimeFigure 3: Schematic diagram illustrating concepts used in design loss formulation(adapted from Hill et al, 1997).Paper 13 - Nathan 78