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The increasing imperviousness of the watershed has been related to the decrease in the watershed re- sponse time by Carter (1961), Espey et al., (1965, 1968, 1969), Van Sickle (1969), Riley and Narayana (1968) Narayan. et.1., (1970), and Schulz (1971), The decrease in watershed response time results in an increase in the peak discharge of the unit hydrograph. In this investigation the percent of the watershed which is impervious is used to quantify watershed imperviousness. However in view of the results of Dempster (1974), Carter (1961) and Riley and Narayana (1968), the actual parameter used in the analysis must be modified to obtain values which are always greater than one: U = 1 + IA where U is the dimensionless Lopez coefficient of imperviousness, IA is the percent of impervious water- shed. The reason for advocating the use of either the Lopez or the Carter equation is that it is never zero and always greater than one. This characteristic is advantageous when using a log transformed multiple re- gression analysis. In many urban communities, the roof drainage is captured in flower beds or grassed terraces which never really result in any surface runoff. Carter's equation contains a coefficient which can be varied to account for the ultimate disposition of the roof drainage. Wat~hed Ro~ a~d ~e~. -- The proliferation of the ubiquitous roads and streets is the most obvi- ous feature in the evolving urban scene. Hydraulic- ally the highway and the street may perform quite different functions. The road, highway, expressway or freeway evolves from the country road. These arteries of commerce are developed by the placement of a spe- cially designed subgrade on top of a base which some- times is built on top of fill above the surrounding land. Jones (1971) has pointed out that the roadside ditches or borrow pits which result have a significant effect on the increase of what is normally called depression storage. In the very first stages of urban development, the street is an unpaved roadway, but the density of the soil is increased and the surface slope is developed such that there is little opportunity for rainwater to infiltrate into the soil. As rural development progresses into the suburban stage, the increasing use of the road causes paving the road with asphalt or concrete to solve the dust problem in dry weather and stabilize the surface in wet weather. Whether paved or unpaved, the projected area of the road no longer infiltrates rainwater, but the roadside ditches capture and may store storm runoff. In the case of the freeway or expressway in an urban environment, a porous gravel surface borders the paved surface. The shoulders of the roadway and the median strip are seeded with a suitable grass to con- trol erosion and to provide a pleasing appearance. At the immediate edge of the roadway, the water supply to the vegetated surface is enhanced by the additional runoff harvested from the impervious roadway surface. In many climates this additional water supply is a benefit to the grassed surface. The benefit may be partly offset by the adverse effect of some of the other constituents of the microclimate of the highway such as lead, nitrous oxides, rubber and asbestos dust, carbon monoxide resulting from the traffic. Hydro- logically an unpaved street or road or a paved roadway with a median strip or wide ditch probably has little effect on either runoff yield or the response time of the watershed. eMbed and Gu.tteJI.,d S,tMW. The curbed and guttered streets perform quite a different hydrologic function. Whereas the country road or urban freeway was built at an elevation above the immediate sur- roundings, the usual neighborhood street is set at an elevation below the immediate surroundings. This often results in the street functioning as a drainage way. Usually there is a crown at the center of the street so that water will drain toward the gutters at either side. The water will not drain from the surface be- cause of the curb. The gutter may also collect runoff from the adjacent sidewalk or adjacent property. The flow in the gutter is relatively deep in relatively straight-srnooth-channe1s. Super critical flow is often observed in gutter flow on moderate slopes. The gutters discharge into storm drains which also are relatively efficient carriers of storm runoff. Each mile of curbed and guttered street adds two miles of drainage channel to the watershed. The flood transit time in the curb-gutter-storm drain system is less than the transit time of the flood wave in the pris- tine natural channel system. watekhh~d Channel SY6tem.-- In the urban setting, the natural dra1nage ways existing in the pristine watershed are altered. The secondary drainage network is obliterated and may be replaced with a curb and gutter system. Larger channels may remain although the hydraulic efficiency may improve. ChannelS are straightened and often conform to subdivided property boundaries. Many times the banks are shaped to confine the flow. Sometimes steeper banks are stabilized. Higher velocities result from the straight channels and deeper flows. Drop structures are then constructed to stabilize the overall channel gradient. Gutter flow is discharged into these drainage ways when convenient. Sometimes storm sewers discharge into these drainage ways. The net result on the flood hydrology is to decrease the response time and to increase the peak discharge of the unit hydrograph. Espey et al. (1965) and Espey and Winslow (1968) used the channelization classification $ to quantify the change of the watershed response time for an urban watershed having both storm sewers and improved drain- age ways. (See suggested values of ~ in Tables 3 and 4.) Van Sickle (1969) proposed a estimating the watershed time to graph peak discharge: basin factor K for peak and unit hydro- K Ll IS where Lt is the total length of all drainage ways and storm sewers larger than 36 inches diameter in miles, is the mean basin length in miles, is the mean basin slope in feet/feet. L S Van Sickle used a ~rocedur~ described by Eagleson (1962) for finding L and S from a hyps~ric dia- gram for the watershed. The Van Sickle basin factor for an urban watershed is analogous to the watershed basin factor n as used by the Corps of Engineers for pristine watersheds. II