Scribd
Upload
78 views3 pages

Drainage

Drainage ditches are designed to provide overall drainage for a watershed area by rapidly removing water to prevent crop damage. The design capacity of ditches is determined using empirical formulas that relate discharge to drainage area and other factors. Channel slope is determined by topography and should be as steep as possible without violating depth or velocity limits. Changes in channel direction should be gradual depending on flow velocity and erodibility. There are different types of ditch layouts including random field ditches, bedding systems, parallel ditch systems, and cross-slope ditch systems suited to different land types and slopes. Design details can be found in the SCS Engineering Field Manual.

Uploaded by

Hamza Aldaeef
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
78 views3 pages

Drainage

Drainage ditches are designed to provide overall drainage for a watershed area by rapidly removing water to prevent crop damage. The design capacity of ditches is determined using empirical formulas that relate discharge to drainage area and other factors. Channel slope is determined by topography and should be as steep as possible without violating depth or velocity limits. Changes in channel direction should be gradual depending on flow velocity and erodibility. There are different types of ditch layouts including random field ditches, bedding systems, parallel ditch systems, and cross-slope ditch systems suited to different land types and slopes. Design details can be found in the SCS Engineering Field Manual.

Uploaded by

Hamza Aldaeef
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
You are on page 1/ 3

Drainage Ditches

A drainage ditch is a particular type of open channel waterway designed to provide


overall drainage for a watershed area. The emphasis in these ditches are not
primarily peak flow rates, but on removing water rapidly enough to prevent serious
crop damage. Empirical formulas for the design capacity of drainage ditches have
been developed by the ASAE and presented in ASAE Engineering Practice: ASAE
EP302.3. The basic equations are of the form

q=K.C.Mx

where Q is the design discharge, K is a conversion factor depending on the units


used, C is a coefficient relating to the desired level of drainage, M is the drainage
area, and x is an index, normally 5/6.

Channel Slope
The channel slope is determined by topography and the depth of the waterway. In
flat areas, the channel slope should be as steep as possible, as long as the limiting
depth or the limiting velocity criteria are not violated. The channel should be deep
enough to collect all the drainage water from the area. In some cases, the depth is
constrained by the depth of subsurface drains that empty into the waterway.

Alignment
Where it is necessary to change the direction of an open channel, the change should
be gradual, with the degree of curvature dependent on the velocity of flow and the
erodibility of the side slopes. The degree of curvature is defined as the angle
subtended at the center of a circle by a 100 unit chord.

100
D/2
R D R
Curvature recommendations for open channel waterways (SCS, 1973)
Approximate Maximum
Channel Top Width Slope Curvature (deg)
(m) (%) English Units SI Units
4-6 < 0.06 19 62
4-6 0.06-0.12 14 46
5-11 < 0.06 11 36
5-11 0.06-0.12 10 33
11 < 0.06 10 33
11 0.06-0.12 7 23

Drain Layout
Random Field Ditches
These are best suited to the drainage of scattered depressions or pot holes. These
are usually parabolic or triangular in shape, and may be grassed to maintain channel
stability, or may be cultivated in some instances depending on the nature of the
crop. When farming operations cross the channel, the side slopes should be in
excess of 8:1. (The side slope is here defined as the ratio of the horizontal distance
to the vertical distance). The side slopes may be reduced to a minimum of 4:1 in
cases where it is possible to farm parallel to the ditch. The outlet for a system of
random drains may be a natural water course, a constructed drainage ditch or a
protected slope.

Bedding Systems
This system consists of narrow width plow lands with equally spaced dead furrows
running parallel to the prevailing land slope. The area between two adjacent dead
furrows is known as a bed. The beds may be of different shapes. Each bed is
cultivated separately either parallel or normal to the dead furrows. A collection ditch
must be provided to collect the water from the dead furrows and channel it to some
outlet ditch.
Parallel Ditch Systems
These are similar to bedding systems but with the channels spaced further apart and
with a greater capacity. The ditches are usually trapezoidal in shape. The systems
may be further subdivided based on their side slopes and their depths. The minimum
side slopes are of the order of 1:1 in sandy soils and 1.5:1 in other mineral soils. The
walls may be vertical in histosols.

Cross-Slope Ditch Systems


These are mainly used for the drainage of sloping lands. The drains run across the
slope and intercept water flowing from upslope.

Details of the design of open channel waterways and ditch systems can be found in
Chapters 7, 9, and 14 of the SCS Engineering Field Manual.

You might also like