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N t o . . ~ ~ ~- Z e iaf Ii t;/fle "''''--'' c........l Wotershed BDundary Subrone8oundory Figure 2.2. Dividing the flood plain area into subzone~ I. Natural topography and streel configurations. 2. Location of rainfall and stream- flow gages. 3. Objectives of the study, for ex- ample, different boundaries might be chosen for investigations involving (a) storm characleris- tics, (b) land use, and (c) the design of flood control structures. 4. Locations and densitities of di- versions. B. A subwalershed model requires thai all outflow from a subzone be de- fmed and preferably be at a single point. A single oulflow point is not essenlial, but it simplifies model de- velopment. II. Determine the impervious cover associated with roads, buildings, parking iots, and sidewalks. The use of large scale aerial pholographs (in this study, a scale of I" = 400' was used) greatly reduced the work involved. It is possible to work directly on the aerial photographs, delineating boundaries, subzones, and units within subzones by means of wax pencils of vari- ous colors. The imporlant parameter is the lotal impervious area, but the addi- tional work necessary to differentiate among roads, buildings, parking lois, and sidewalks often is worthwhile. With this information, the research can examine the effects of a parlicular kind of impervious cover on the runoff characleristics of the watershed. In addition, the infornation is useful for economic analysis. The fol- lowing procedure is suggested for deler- mining average values of various kinds of impervious cover within a study area. A. Choose a set of residential blocks that includes a represen tative of each type of block within Ihe water- shed. I. For each block chosen, carefully measure each type of impervious cover. The total area of the block is considered to be the area en- closed within lines joining the midpoints of Ihe inlersections of adjacent roadways (see Figure 2.3). Linear measurements can be made wilh a scale and a roto- meter. A planimeter is also use- ful. ( ) l.. .. I I I I I I I I I I I THoretl"1 ~OUf.ldory of on urban bJock I,,*ottd .., --- CUM aGUTTE SIDEWALK I I I I 1 I I I I I I +- - --ROAD-------+ ~( DRIVE WAY ') ( Figure 2.3. Typical urban residential block showing the pervious and the impervious areas. 16