ABE 325 Surface , Land Grading, and Open Ditch Design (Chapters 12 & 13)

Terms cut = ground elevation minus grade elevation cut sheet = sheet given to contractor showing stations along the route of a proposed drain, waterway or ditch along with the respective cuts drainage coefficient = inches of water to be removed from an entire drainage area in 24 hours drainage group = group of types with essentially the same drainage characteristics fill = grade elevation minus ground elevation grade elevation = bottom elevation of a proposed drain, waterway or ditch land forming = the process of changing the natural topography so as to control the movement of water onto () or from (drainage) the land surface limiting velocity = maximum allowable velocity above which channel scouring will occur slope stakes = stakes set at the intersection of the ground and side slopes of a drain or earth fill to show the contractor where to begin excavating or adding fill

INTRODUCTION Surface drainage is accomplished using broad, shallow surface channels which may either connect random low spots in a field or are constructed across long gentle slopes. Crops are planted across the drains and the drains are then unnoticeable. Sometimes the land is smoothed between drains using land planes to facilitate row drainage into the surface drains. Water is moved away from the field with open ditches. (Subsurface drains may also empty into open ditches.)

SURFACE DRAINAGE PATTERNS 1. Random Field Drains:

1.

2. 3.

4. If farming operations cross ditch: minimum side slope ratio - and depth less than or equal to 30 cm (1 ft) 8:1

- and depth greater than or equal to 60 cm (2 ft) 10:1 If farming operations are parallel to ditch:

- minimum side slope ratio 4:1 Depth - based on topography, outlet conditions and channel capacity Grade - sufficiently steep to prevent sedimentation but gentle enough to prevent erosion Maximum velocity - given in Table 13.2. Note that drainage water is silty, not clear. Minimum velocity - 0.3 - 0.6 m/s Area to drain > 2 ha - base capacity, q, on 10 year return period for 24 hour storm. Want to remove water in 12 - 24 hours in order to protect crop

2. Bedding

- combines land grading & parallel ditches - for flat slopes (< 1.5%) and slowly permeable soils where pipe drainage is not economical Plowing -

All other farming operations -

Bed length - 90 to 300 m Bed width - very slow internal drainage 7 - 11 m slow internal drainage 13 - 16 m fair internal drainage 18 - 28 m 3. Parallel Field Drain System

- for flat, poorly drained with numerous small depressions - distance separating ditches can vary. Maximum lateral flow length to ditch (row length) = 180 m if nonerodible soils and 90 m if highly erosive soils. - Plow -

- Other operations -

Ditch: - cross-section can be triangular, trapezoidal or parabolic, min. area = 0.5 m2 - minimum depth 0.2 m, minimum side slope 8:1 - if trapezoidal: minimum bottom width b = 2.4 m

4. Parallel Lateral Ditch System

- similar to parallel field drains but ditches are deeper and cannot be crossed with farm machinery - to control and provide surface drainage (provide same subsurface drainage as pipe drains at same depth) - add collection ditch running across "beds" to lateral drains for collecting row runoff

- All operations -

Ditch: - minimum depth = 0.3 m - side slopes 6:1 or steeper - Table 12.1 for watertable control dimensions

LAND GRADING: Profile Method 1. Plot ground profile from one end of field to the other. 2. By trial-and-error, select slope and grade lines for each profile that create a desired cut-to-fill ratio. Desired range of cut-to-fill volume is typically 1.3 to 1.6. Cut and fill volumes can be adjusted by either changing the design grade elevation or the slope of the plane. Want extra fill material to compensate for compaction of soil during cutting and filling operations.

Calculating Earthwork Volumes: 4 Point Method (from SCS) Volume of Cut: Volume of Fill:

Make sure to include "partial grid" squares as well as full grid squares when doing earthwork calculations. Example: Grid length = 50 m 10.0 9.8 9.5 9.4 9.2

9.9 9.7 9.6 9.4 9.3

9.7 9.5 9.7 9.6 9.4

9.8 9.7 9.6 9.4 9.2

OPEN CHANNELS Types Of Channels Canals = open channels that carry irrigation water to the farm. Open Ditches = open channels that carry drainage water from surface and subsurface drains away from the farm. [Drainage District = a local unit of government established under state law to construct and maintain open ditches. Most of northern Indiana is in some sort of drainage district.] Can be unlined (i.e. earthen) or lined. Typically have trapezoidal cross-section

Channel Discharge Capacity Based on Manning’s Equation: Check limiting velocity in Table 13.2. Velocity must be low enough to prevent scour yet fast enough to prevent sedimentation. Drainage water carries silt from the field and so is not clear. - for windy/curved channels, reduce limiting velocity from Table 13.2 by 25% - for flow depths > 0.9m, can increase limiting velocity from Table 13.2 by 0.15m/s - if water carries powerful abrasive, reduce limiting velocity from Table 13.2 by 0.15 m/s Values of n can be obtained from Appendix B. medium sized earth ditch, b = 1.5 - 3 m n = 0.035 smaller earthen ditches, b = 1.2 m n = 0.04 Suggested sideslopes are given in Table 13.1. Bottom width can be computed for a given discharge using the following equation. Note that hydraulic radius, R, will equal half the depth when this equation is used.

where: b = bottom width d = design depth theta = sideslope angle in degrees z = sideslope ratio (horizontal / vertical) Want freeboard = 20% of final depth, so:

Required rate of water removal in drainage ditch (m3/s):

where: C = constant from Figure 13.6 M = watershed area in km2 7 Causes of failure: