Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 205

and slope. A frequently used guide to maximum furrow flow rates is Q = 10/ s ,

where s is furrow slope in percent and Q in gallons per min per furrow.

6.4.4.4 Other Considerations

• Land grading is a major source of sedimentation and must be carefully planned

and carried out.

• The use of phasing, natural buffers, mulching, and temporary and permanent

seeding should be the primary methods of addressing erosion control for land
grading projects.

• Fall and winter erosion control measures must be upgraded and refined to protect

the site from spring runoff and snowmelt.

In estimating the cost of grading, the field boundaries and the type of surface
irrigation (small basins, large basins, furrow, and border strip lengths, etc.) must
be determined. There is an interrelationship between the irrigation field size and
the amount of land grading required. Where grading will cause damage by expos-
ing subsurface horizons and hardpans, alternative development options should be
considered.

6.4.5 Methods of Land Grading and Estimating Earthwork Volume

6.4.5.1 Planning and Early Surveying

Grading plan and installation should be based upon adequate surveys and soil inves-
tigations. In the plan, show disturbed areas, cuts, fills, and finished elevations of the
surface to be graded. All practices necessary for controlling erosion on the graded
site and minimizing sedimentation downstream should be included in the plan. Such
practices may include the following:

• Vegetation, vegetated- and lined-waterways

• Grade-stabilization structures

•

Surface and subsurface drains

There are four aspects of topography that have special bearing on land leveling
and grading for surface irrigation: (1) slope, (2) microrelief, (3) macrorelief, and
(4) cover. The land classifier and/or surveyor must achieve competence in distin-
guishing and evaluating those topographic features that are significant. Considerable
experience is required to achieve acceptable accuracy in estimating the costs of
leveling from field observations. Topographic maps do not always give sufficient
information for accurate assessments. Guidance and training may be provided by an
experienced agricultural engineer engaged in detailed layout studies. Detailed farm
layouts of representative areas showing the costs of land grading can provide the
best guidelines. If done properly, evaluation of the topography based on experience
and field layout studies is adequate for most planning studies.