|
|
Department of Agriculture and Aquaculture
|
|
|
1.0 |
DEFINITION |
|
Terraces are earth embankments, channels or
combination of embankment and channels constructed across the slope to reduce
the slope length at suitable spacing and with acceptable grades for one
or more of the following purposes:
- To reduce soil erosion by water
- To provide for maximum retention
of moisture for crop use
- To remove surface water at a
non-erosive velocity
- To improve topographic conditions
and farmability
- To reduce sediment content in
runoff water
- To reduce peak runoff rates to
installations downstream
- To improve water quality
|
|
|
2.0 |
GENERAL |
|
2.1 |
All terrace system
design and construction shall be in accordance with the American
Society of Agricultural Engineers Engineering Practice - ASAE S268.3
- Design, Layout, Construction and Maintenance of Terrace Systems
and as specified in the following. |
2.2 |
The NB Department
of Environment Watercourse Alteration Regulations and Guidelines
will apply to all projects including Regulation 90-80 and 90-120
under the Clean Water Act as revised in 1993. |
2.3 |
To adequately
control erosion on sloping land terraces are used in conjunction
with other water control systems, such as: grassed waterways,
underground outlets, drop structures, and sediment control basins.
Complementary conservation practices are usually recommended in
conjunction with terrace systems such as: contour farming, crop
rotation, contour strip cropping, conservation tillage, residue
management and good soil management. |
2.4 |
The most common
type of terrace system used in New Brunswick is the Parallel
Variable Grade Diversion Terrace (PVGDT) system. This system concept
was adapted to New Brunswick and is very similar to the
narrow-base-terrace recommended in the ASAE standard: ASAE S268.3.
Design, layout, construction and maintenance of terrace systems are
also in accordance with Chapter 8, "Terraces" of the USDA-SCS,
Engineering Field Manual Handbook for Conservation
Practices. |
2.5 |
Parallel Bench
Terraces (PBT) and Parallel Rigeless Diversion Terraces (PRDT) are
constructed occasionally where conditions apply. PRDT's may be used
on uniform slopes (<6%) and for short lengths. These types of
terrace systems are also combined with grassed or rock-lined
waterway outlets, or specially designed pipe
outlets. |
|
|
|
3.0 |
SPACING |
|
3.1 |
Terraces shall be constructed at recommended
spacing using the Universal Soil Loss Equation (USLE) to meet acceptable
soil loss tolerance levels which were set for New Brunswick at a maximum
of 9t/ha/yr (4 t/ac/yr). |
3.2 |
Terrace spacing
design shall consider the soil - crop management and conservation
practices used by the landowner. Where no comprehensive crop
management exists, use the most intensive land use rotation for
spacing design. |
3.3 |
Terraces shall not
be constructed at a spacing less than 36 meters (40 rows - 36" per
row). |
3.4 |
Final spacing must be adjusted to multiples
of present and expected future row crop equipment widths (i.e. 4-row
planter, 20-row sprayer). Ideally, spacing should allow for an even
number of passes of each piece of equipment. |
|
|
|
4.0 |
ALIGNMENT |
|
4.1 |
Terrace system
alignment shall be planned for compatibility with modern farm
equipment and beneficial to soil and water conservation. The design
must consider the natural features, permanent boundaries, runoff
disposal outlets, and location of auxiliary factors, such as fences,
and field roads. |
4.2 |
The farmable row
lengths in a terrace system shall be a minimum of 200 meters to
maintain a high efficiency of machinery operation. |
4.3 |
Diversion terraces
shall be parallel to each other. Curves should be long and gentle to
accommodate farm machinery. Land forming, necessary cuts and fills
and variations in grade shall be used to maintain proper alignment
and a parallel system. |
|
|
|
5.0 |
CHANNEL GRADE |
|
5.1 |
Channel grades
shall range from 0.5% to 2% for the key-diversion terrace line in
the terrace system. The slope may be increased up to 3% for a length
less than 75 m (250 ft) when required to improve alignment and
farmability. |
|
|
|
6.0 |
CAPACITY & CROSS SECTION |
|
6.1 |
The diversion terrace shall have enough
capacity to control runoff from a 1 in 10 year frequency, 24-hour
storm without overl topping Terrace systems designed for sediment
or flood control shall have enough capacity to control a storm of
higher frequency consistent with the potential hazard involved. The
S.C.S. - E.F.M.H. method, or any equivalent method, is recommended
for designing capacity and cross-section. |
6.2 |
The design
discharge used to determine the terrace capacity shall be calculated
using the USDA-SCS EFM2 method or the TR55 method. |
6.3 |
PVGDT's shall have a minimum freeboard
of 0.3 meter and PRDT's a minimum freeboard of 0.1 meter to prevent
over topping |
6.4 |
Terrace
cross-sections shall be as shown in figures 1 and 2. Terrace width
shall range from 4 to 8 metres; width may be adjusted for slippage
and other topographic conditions to improve
farmability. |
|
|
|
7.0 |
PLANNING SYSTEM LAYOUT |
|
7.1 |
Terrace systems
should be planned with the aid of a topographic survey used to
generate a topographic contour map with maximum 1.0 meter contours.
If a survey cannot be completed topographic maps at scales 1:1000,
1:1200, or 1:2400 shall be used to determine the optimum system
layout. The recommended contour interval for design purposes will
vary from 0.5 to 1.0 meter. If topographic maps at these scales are
not available the minimum scale to be used shall be 1:4800 or better
for general layout pattern. |
|
|
|
8.0 |
CONSTRUCTION AND MAINTENANCE |
|
8.1 |
Terraces shall be
properly limed, fertilized and seeded with a conservation grass
mixture and maintained with a permanent, weed-free sod. |
8.2 |
Terrace outlets
shall be protected with an erosion control mat where required to
prevent damage to the terrace channel. |
8.3 |
Topsoil shall be
saved during construction and placed over the terrace in the final
construction phase to facilitate good grass growth/sod
cover. |
8.4 |
Terraces shall be
properly compacted at low, critical areas to prevent breaching and
erosion of berm. |
|
|
|
9.0 |
PROJECT COMPLETION |
|
9.1 |
Upon notification
of project completion, the project engineer or designate will carry
out final inspection and report of the conservation
site. |
9.2 |
Terrace projects
will only be consider complete if they meet these standards and work
is suitable to the project engineer or
designate. |
|
|
|
10.0 |
REFERENCES |
|
10.1 |
The Atlantic
Committee on Agricultural Engineering (A.C.A.E.) Publication No. 26,
1992 - Integrated Erosion Control on Potato Land in Atlantic Canada
contains additional information on rationale, methods and equipment
and should be referred to prior to commencing a terracing
project. |
10.1 |
Watercourse
Alteration Guidelines issued by the N.B. Department of
Environment. |
|
|
|
The New Brunswick Department of Agriculture, Fisheries and Aquaculture reserves the right to revise the above standard at any time.
Fig. 1 & 2. Typical Diversion Terrace Cross Sections
|