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The TR-55 Tabular Method approximates TR-20, which is a more detailed hydrograph procedure. This method can develop partial composite runoff hydrographs at any point in a watershed by dividing the watershed into homogeneous subareas.
Computation of travel time and time of concentration. Water moves through a watershed as sheet flow, shallow concentrated flow, open channel flow, or some combination of these. The type that occurs is a function of the conveyance system and is best deter-mined by field inspection. Travel time ( Tt ) is the ratio of flow length to flow velocity:
Technical Release 55 (TR-55) presents simplified procedures for estimating runoff and peak discharges in small watersheds. In selecting the appropriate procedure, consider the scope and complexity of the problem, the available data, and the acceptable level of error. While this TR gives special emphasis to urban
Hydrograph Method. This chapter presents the Tabular Hydrograph method of computing peak discharges from rural and urban areas, using time of concentration (Tc) and travel time (Tt) from a subarea as inputs. This method approxi-mates TR-20, a more detailed hydrograph procedure (SCS 1983).
Soil name and hydrologic group (Appendix A) Table 2-2. CN 1/. Fig. 2-3 Fig. 2-4 Product of CN x area. Area acres mi2 %. Cover description ( Cover type, treatment, and hydrologic condition; percent impervious; unconnected/connected impervious area ratio) 1/ Use only one CN source per line. Totals =. CN (weighted) = total product = total area.
In most cases, the designer will NRCS methods to develop the base use the hydrology (CN, Tc, graphical peak discharge q. p], etc.[), and use that data in one of the numerous hydrologic/hydraulic computer models (including TR-55, TR-20, HEC 1, etc.).
Table 2.1 TR‐55 Manual. What Happens When Adjusted HSGs Are Compared to Non‐Adjusted. These are only hydrologic models, but they show substantial differences from the what is considered “acceptable” engineering practices today. Overall runoff volume increases. Peak flow rates increase. How Much More?
