A Comparison of Hydrologic Methods Compiled by Peter Smart for HydroCAD Training Seminars Also see the HydroCAD Owner's Manual HydroCAD Software Solutions LLC Chocorua, NH 03817 U.S.A. These notes are intended as an outline for the understanding and comparison of various hydrologic methods. They are designed primarily to put the various hydrologic concepts and techniques into proper perspective so that their differences (and similarities) may be better understood. These notes should not be used as the sole basis for applying theseThere are many texts available which provide accurate, detailed information on the specifics of each technique. These sources should be consulted before attempting to employ any of these methods. One source that is highly recommended is A Guide to Hydrologic Analysis Using SCS Methods by Richard H. McCuen. Other sources include SCS Technical Release Number 55 (TR-55), and Basic Hydrology by Sharp & Sawden.

rainfall observations, IDF curves can be compiled for any location: This curve indicates the relationship between the intensity (i) and duration (d) of a rainfall event with a given return period (T). Instead of return period, it is more accurate to think in terms of the exceedance probability (p), where p=1/T. Thus, a "25 year storm" actually designates a rainfall event which has a 4% chance of occurring in any given year. The Rational Method For Predicting The Rational method may be used to predict the peak runoff according to The method derives its name from the fact that the units have beenThat is, 1 CFS = 1.01 in-ac/hr. Although the rational method appears straightforward, it is totally dependent on the "correct" selection of C and i: C is based on the soil, ground cover, and other i is obtained from the local IDF curve for a given return period and duration. One of the major challenges of the rational method is choosing the

The duration must be just long enough for maximum runoff to occur. longer duration will yield a lower intensity from the IDF curve, and thus a lower runoff. For a single subcatchment the "correct" duration is usually equal to the time-of-concentration. However, when several subcatchments are combined in a complete drainage system the correct duration can have any value between the shortestac window unit spitting out water The rational method is intended only to determine peak runoff. outside ac unit hissing not yield cumulative runoff (volume) and therefore cannot be used when subsequentac unit hurricane straps volume-sensitive routing is required. By studying the Weather Bureau's Rainfall Frequency Atlases, the Soil

Conservation Service determined that the entire country could be represented by just four dimensionless rainfall distributions of 24-hour duration. Each distribution is expressed as a mass curve indicating what fraction of the total 24-hour precipitation has fallen at These curves were developed from the same depth-duration-frequency data used for IDFUsing 30 minute increments, the incremental rainfall was calculated for durations of 30 minutes to 24 hours. For example, the 30 minute depth was subtracted from the one hour depth, and the one hour depth from the 1� hour depth. Then the largest 30 minute increment was placed at the middle of the hypothetical storm, which is 12 hours. The second largest 30 minute incremental depth is placed in the next 30 minutes, and the third largest in the previous 30 minutes. process is repeated until the entire 24-hour curve is developed. Peak Air Conditioning is an established mechanical installation business specialising in the design, supply and installation of air conditioning and ventilation systems across a diverse range of commercial sectors.

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