| 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: |
Terrace
- 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 |
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. |
9.0 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