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<br />
<br />BDF is the basin development factor, an index of
<br />lbc prevalence of the urban drainage improvements;
<br />IA is the percentage of the drainage basin occu-
<br />pied by impervious surfaces, such as houses, buildings,
<br />sueets, and parking lots; and
<br />RQT, are the peak discharges, in cubic feet per
<br />second, for an equivalent rural drainage basin in the
<br />same hydrologic area as the urban basin, for a recur-
<br />rence interval of T years; equivalent rural peak dis-
<br />charges are computed from the rural equations for the
<br />appropriate State, in the NFF program, and are auto-
<br />matically transferred to the urban computations.
<br />The basin development factor (BDF) is a highly
<br />significant variable in the equations, and provides a
<br />measure of the efficiency of the drainage basin. It can
<br />be easily determined from drainage maps and field
<br />inspections of the drainage basin, The basin is first
<br />divided into upper, middle, and lower thirds on a drain-
<br />age map, as shown (fig, la-c). Each third should con-
<br />tain about one-third of the contributing drainage area,
<br />and stream lengths of two or more streams should be
<br />approximately the same in each third. However, stream
<br />lengths of different thirds can be different. For
<br />instance, (fig. Ic), the stream distances of the lower
<br />third are all about equal, but are longer than those in the
<br />middle third. Precise definition of the basin thirds is not
<br />considered necessary because it will not have much
<br />effect on the final value of BDE Therefore, the bound-
<br />aries between basin thirds can be drawn by eye without
<br />precise measurements.
<br />Within each third of the basin, four characteris-
<br />tics of the drainage system must be evaluated and
<br />assigned a code of 0 or I, Summation of the 12 codes
<br />(four codes in each third of the basin) yields the BDE
<br />The following guidelines should not be considered as
<br />requiring precise measurements. A certain amount of
<br />subjectivity will necessarily be involved, and field
<br />checking should be performed to obtain the best esti-
<br />mates.
<br />
<br />1. Channel improvements.nlf channel improve-
<br />ments such as straightening, enlarging, deepen-
<br />ing, and clearing are prevalent for the main
<br />drainage channels and principal tributaries
<br />(those that drain directly into the main chan-
<br />nel), then a code of 1 is assigned. To be consid-
<br />ered prevalent, at least 50 percent of the main
<br />drainage channels and principal tributaries
<br />must be improved to some degree over natural
<br />conditions. If channel improvements are not
<br />prevalent, then a code of zero is assigned.
<br />
<br />2, Channellinings.--If more than 50 percent of the
<br />length of the main channels and principal trib-
<br />utaries has been lined with an impervious sur-
<br />face, such as concrete, then a code of I is
<br />assigned to this characteristic. Otherwise, a
<br />code of zero is assigned. The presence of chan-
<br />nel linings would obviously indicate the pres-
<br />ence of channel improvements as well.
<br />Therefore, this is an added factor and indicates
<br />a more highly developed drainage system.
<br />
<br />3. Storm drains or storm sewers.--Storm drains are
<br />defined as those enclosed drainage structures
<br />(usually pipes), frequently used on the second-
<br />ary tributaries where the drainage is received
<br />directly from streets or parlcing lots. Many of
<br />these drains empty into open channels; how-
<br />ever, in some basins they empty into channels
<br />enclosed as box and pipe culverts. When more
<br />than 50 percent of the secondary tributaries
<br />within a subarea (third) consists of storm
<br />drains, then a code of I is assigned to this
<br />aspect, otherwise a code of zero is assigned.
<br />
<br />4. Curb-and-gutter streets.--If more than 50 percent
<br />of the subarea (third) is urbanized (covered
<br />with residential, commercial, and/or industrial
<br />development), and if more than 50 percent of
<br />the streets and highways in the subarea are con-
<br />structed with curbs and gutters, then a code of
<br />1 would be assigned to this aspect. Otherwise,
<br />a code of zero is assigned. Drainage from curb-
<br />and-gutter streets frequently empties into storm
<br />drains.
<br />
<br />Estimates of urban flood frequency values
<br />should not be made with the seven-variable equations
<br />under certain conditions. For instance, the equations
<br />should not be used for basins where flow is controlled
<br />by reservoirs, or where detention storage is used to
<br />reduce flood peaks. The equations should not be used if
<br />any of the values of the seven variables are outside the
<br />range of values used in the original regression study
<br />(except for SL which is limited to 70 ftlmi). These
<br />ranges are provided in the NFF program, and the user
<br />is warned anytime a variable value exceeds the range.
<br />The program will compute urban estimates even
<br />though a parameter may be outside the range; however,
<br />the standard error of estimate may be greater than the
<br />value given for each equation.
<br />
<br />URBAN R.OOD-FREOUENCY ESTIMATING TECHNIQUES 9
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