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Western Dam Engineering Technical Note 14 Procedures for Estimating Unit Hydrographs for Large Floods at Ungaged Sites in Montana (Holnbeck and Parrett, 1996) Figure 5:Unit hydrograph methodology (Source: Cudworth, 1989). The development of a unit hydrograph, regardless of the methodology, is largely based on the watershed lag time, which is a measure of the watershed response time with regard to the translation of excess rainfall to a hydrograph. Estimation of the lag time varies according to methodology, but is generally a function of physical watershed characteristics. Different methodologies define and use parameters such as lag time and “time of concentration” differently and the reader should reference the specific use in each of the methodologies described below. When developing a synthetic hydrograph, regardless of method, the unit duration used to develop the hydrograph should be appropriately small so as not to miss or underestimate the peak by averaging it over too large of a calculation interval. Different methodologies provide guidance for unit duration. Reclamation Synthetic Unit Hydrograph The lag time, as defined by Reclamation, is based on physical watershed measurements (i.e. watercourse length, slope, etc.) and the watershed average Manning’s roughness value for the principal watercourses, “Kn”. While the physical measurements can be easily estimated using topographic data, the watershed average Kn value is more subjective and difficult to estimate. Reclamation provides guidance on adopting the watershed average Kn values using a series of plots based on watersheds with appropriate gaged and watershed calibration data, which are sub-divided into the following hydrologic groups: ·Rocky Mountain ·Great Plains ·Colorado Plateau ·Agricultural Fields ·Urban With an emphasis on dam safety, a conservative approach is recommended with regard to Kn value selection. As such, Kn values are often selected from the lower half of the proposed range of Kn values for a particular hydrologic group. Kn value selection from the upper half of a range is not recommended without appropriate justification. Upon selection of a Kn value, the lag time can be estimated and applied to a set of time and flow ordinates, for the appropriate hydrologic group, to estimate the synthetic unit hydrograph. See reference [1]and [6]for more detailed discussions regarding the use and application of the Reclamation synthetic unit hydrograph methodology, including guidance on Kn selection. Clark Synthetic Unit Hydrograph The Clark synthetic unit hydrograph approach is similar to that of the Reclamation methodology; however, the Clark approach uses a synthetic runoff time-area relation, which is similar to the time and flow ordinates of the Reclamation synthetic unit hydrograph, but is based on ratios of contributing area to total area. The Clark approach also considers the effect of runoff storage on the unit hydrograph shape using a storage coefficient. See reference [6] for more information. USGS Synthetic Unit Hydrograph (for Montana) The USGS also developed a synthetic unit hydrograph specifically for application within Montana. The USGS unit hydrograph for Montana was developed by compiling the predicted unit hydrograph results using the Reclamation and Clark methodologies for 26 watersheds throughout Montana. These results were further analyzed and averaged to produce a unit hydrograph that is representative of the watershed conditions specific to Montana. For more information regarding the use and application of the USGS synthetic unit hydrograph within Montana and also worked examples, see reference [3]and [4]. Baseflow Separation Although not usually critical for infrequent and very infrequent rainfall events, the natural stream baseflow D i s c h a r g e Unit Rainfall Excess Hydrograph Lag Time Unit Duration