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Synthetic UH

Synthetic UH. Definition: Synthetic Hydrograph is a plot of flow versus time and generated based on a minimal use of streamflow data. Example: A pending land use change and the resulting runoff hydrograph is thus unknown, but nevertheless must be estimated. Synthetic UH.

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Synthetic UH

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  1. Synthetic UH • Definition: Synthetic Hydrograph is a plot of flow versus time and generated based on a minimal use of streamflow data. • Example: A pending land use change and the resulting runoff hydrograph is thus unknown, but nevertheless must be estimated.

  2. Synthetic UH • Developed for basin that were un- gauged • Based on data from similar gauged basins • Most methods are very similar in nature • Revolutionized ability to predict hydro response

  3. Gauged and ungauged watersheds • Gauged watersheds • Watersheds where data on precipitation, streamflow, and other variables are available • Ungauged watersheds • Watersheds with no data on precipitation, streamflow and other variables.

  4. Need for synthetic UH • UH is applicable only for gauged watershed and for the point on the stream where data are measured • For other locations on the stream in the same watershed or for nearby (ungauged) watersheds, synthetic procedures are used.

  5. Synthetic UH • Synthetic hydrographs are derived by • Relating hydrograph characteristics such as peak flow, base time etc. with watershed characteristics such as area and time of concentration. • Using dimensionless unit hydrograph • Based on watershed storage

  6. Synthetic UH Methods • Snyder’s Method (1938) • Clark Method (1945) • Nash (1958) • SCS (1964, 1975) • Espey-Winslow (1968) • Kinematic Wave (1970s)

  7. Snyder’s Method • Synder (1938)- based on study of large catchment in eastern US developed a set of empirical equation for synthetic UH. • The equation are use in USA and with some modification in many other countries.

  8. Snyder’s Method Methods to characterize ungaged basins - 1938 Use data and relationships developed from gages Variety of approaches but most based on tp and Qp, Where tp=lag time (hr) and Qp = peak flow in cms/cfs

  9. Snyder’s Equation tp = Ct (LLca)0.3 • tp=basin lag in hour • L= basin length in km • Lca= distance along the main water course from gauging stn. To a point opposite to the watershed centroid in km • Ct = regional constant represent watershed slope and storage effects.

  10. Lag time • The most important characteristic of basin due to storm- basin lag (lag time) • Lag time- time difference between the centroid of the input (rainfall excess) and the output (DRH) • Represent time of travel of water from all parts of watershed to the outlet during a given storm.

  11. Snyder’s UH Method

  12. SCS Method • Dimensionless UH • Based on a study of large number of UH • Developed by US Soil Conservation Services (SCS)

  13. The SCS Unit Hydrograph • For large watersheds, time of concentration tc duration (D) of constant rainfall intensity • Rainfall cannot last long enough that the peak flow, Qp, will occur at time tc • Instead, the peak flow, Qp, will occur at time tp, which is a function of rainfall duration D and the watershed characteristics represented by tc

  14. Triangular UH SCS Methods Dimensionless UH

  15. SCS dimensionless hydrograph • Synthetic UH in which the discharge is expressed by the ratio of q to qp and time by the ratio of t to Tp • If peak discharge and lag time are known, UH can be estimated. Tc: time of concentration C = 2.08 (483.4 in English system) A: drainage area in km2 (mi2)

  16. Ex. 7.7.3 • Construct a 10-min SCS UH. A = 3.0 km2 and Tc = 1.25 h 0.833 h q 7.49 m3/s.cm Multiply y-axis of SCS hydrograph by qp and x-axis by Tp to get the required UH, or construct a triangular UH t 2.22 h

  17. Clark Unit Hydrograph (UH) Computation • Based on the use of time-area method. • Concept of instantaneous unit hydrograph (IUH)

  18. Clark Unit Hydrograph (UH) Computation • Figure 6.23 shows the shape of different hydrographs for different D values. • As D is reduced, the intensity of rainfall excess being equal to 1/D increases and the UH becomes more skewed. • A finite UH indicated as D 0. • The limiting case of UH of zero duration is known as instantaneous unit hydrograph (IUH).

  19. Method of IUH • Develop a time area (TA) curve • Route the time area curve through a linear reservoir with a Clark routing parameter

  20. Example 6.16

  21. Nash Linear model

  22. Kinematic Wave Overland Collector Channels Main Stream Manning’s Eqn

  23. Kinematic Wave

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