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Slopes without vegetation must be protected from runoff and restabilized as rapidly as possible. Where possible, steep slopes should be left undisturbed. By limiting the length and gradient of slopes created or modified during development, runoff volumes and vel. ocities can be reduced and erosion hazards minimized, Climate The frequency, intensity, and duration of rainfall are fundamental factors in determining the amounts of runoff produced. As both the volume and the velocity of runoff increase, the capacity of runoff to detach and transport soil particles also increases. Where storms are frequent, intense, or oflong duration, erosion risks are high. Seasonal changes in tempera- ture, as well as variations in rainfall, help to define the high erosion risk period ofthe year. When precipitation falls as snow, no erosion will take place. In the spring, however, the melting snow adds to the runoff and ero- sion hazards will be high. Because the ground is still partially frozen, its absorptive capacity is reduced. Development should be scheduled to take place during the periods of low precipitation and low runoff. Ex- posed areas should be well stabilized before the period of high erosion risk in the spring. FACTORS INFLUENCING WIND EROSION Wind erosion occurs when a wind of adequate velocity blows across an unprotected soil surface that is smooth, bare, loose, dry, and finely granulated. Soil particles start to move when the forces of wind overcome grav- ity. The minimum windspeed required to start move- ment depends on the size and weight of soil particles and the friction provided by neighboring particles. Where very small particles are present, soil movement begins under field conditions when windspeed is about 12 to 15 miles per hour at a height of 1 foot above the soil surface. As the soil moves downwind, increasing numbers of soil particles are set in motion. Soil flow is zero at the windward edge of an eroding area, but the rate in- creases leeward until it reaches the maximum that a given wind can carry, The more erodible the soil, the shorter is the distance to maximum flow. Blowing soil can cause traffic problems, fill drainage ditches, block roads, bury fences and equipment, and create dust and health problems in residential areas. The factors which influence wind erosion from a given area are: soil erodibility, soil surface roughness, soil moisture, windspeed, wind barriers, and vegetative cover. 4