4.6.2 Finish Grading 



Since land surface alterations may increase the volume and rate of 

 runoff, finished grades should be designed to direct water through natural 

 terrain and drainage courses, where the vegetation and soils can cleanse 

 the runoff waters and reduce erosion. Barren soils should be stabilized 

 by replanting vegetative cover as quickly as possible after grading. 



4.6.3 Paving 



Paved surfaces bypass the natural purification system of soil and 

 vegetation (Figure 26). Seepage into subsurface aquifers is blocked, and 



Figure 26. Typical hydrographic changes due to increasing the area 



of impermeable paved surfaces in a developing area (Source: Reference 71) 



Runoff 

 <i 



Shallow 

 infiltration 



A 



30% 



Evapo— 

 transpiration 



Deep 

 infiltration 



A 



20% 



TX 



3?7„ 



10% 



25% 



35-50% 

 Paved 

 Surfaces 55% 



A 



40% 



Natural 

 Ground 

 Cover 



A 



20% 



25% 21%/? 



Y A 30% 



36% 



10-20% 



Paved 



Surfaces 



21% 



V 



15% 



10% 



75-100% 



Paved 



Surfaces 



M 



5% 



rainfall moves directly into surface waters. Among the continuing 

 effects of paved surfaces are (1) sedimentation, resulting from the 

 erosion of soil by runoff from impermeable surfaces, (2) reduction in 

 groundwater recharge due to increased impermeable surface area and 

 accelerated runoff, and (3) addition of pollutants such as lead, zinc, 

 and oil to coastal waters. In a developed, or developing, area the amount 

 of runoff increases proportionately with the extent of impervious surfaces 

 and rooftops. 



Alternatives to paving include gravel, crushed rock, or crushed shell. 

 Gravel can be used for some parking lots and secondary roads. Crushed 

 stone, because its shape and larger size limits compaction, may be suitable 

 in soft muddy areas or on sites prone to erosion. 



There are other suitable paving materials made of concrete, brick, and 

 metal. Examples range from lattice concrete blocks to standard paving 



118 



