Water and Sediment Control Basins

Agdex#:	751
Publication Date:	1989
Order#:	89-167
Last Reviewed:	1997
History:	Original Factsheet
Written by:	R.P. Stone - Engineer, Soil Management/OMAFRA

Table of Contents

1. Introduction
2. Advantages of a Water and Sediment Control Basin System
3. Disadvantages of a Water and Sediment Control Basin System
4. Design Considerations
5. Water and Sediment Control Basin Location
6. Construction
7. Maintenance
8. Assistance

Introduction

Water and sediment control basins are erosion control structures commonly installed to prevent bank and gully erosion on farmland. These structures control erosion due to concentrated water flows and are not effective in combating sheet erosion. This erosion control structure may also be referred to as a "channel terrace" or a "floodwater storage for a smaller watershed".

Figure 1. A water and sediment control basin reduces gully erosion.

A water and sediment control basin in a conservation system can achieve the same objective as a grassed waterway. This structure is comprised of an earthen berm, constructed across a low draw in a field, and built to a height to pond the runoff water from the upstream watershed for the maximum expected storm which may occur once every 10 years.

Figure 2. A water and sediment control basin holds back water and soil. Photo courtesy of U.T.R.C.A.

The runoff water is temporarily stored behind the berm eliminating its erosive capabilities further down slope. This ponded water is slowly released through an inlet riser pipe to an underground tile exiting at an adequate outlet.

Figure 3. Water and sediment control basin system.

Advantages of a Water and Sediment Control Basin System

1. The peak runoff flows from a watershed are buffered reducing their erosive capability downstream.

2. Sediment settles out of the runoff water in the storage pond decreasing pollution and silting problems in watercourses below.

3. Outlet flows from the storage pond are small in comparison to a system with no storage available, resulting in smaller diameter, lower cost outlet pipes.

4. Potential plugging of the pond tile inlet system is minimized, as compared to a tile/catch basin system with no storage, since debris has time to settle out before reaching the inlet pipe.

Disadvantages of a Water and Sediment Control Basin System

1. Flooding of the land in the ponding area will occur for a period up to 24 hours after the runoff event.

2. A higher risk is associated with this structure because of the possibility of berm washout or water overtopping the berm causing problem further down in the watershed.

3. There is an inconvenience factor associated with working around the berms with farm equipment.

4. Berms (especially narrow-based) are vulnerable to rodent damage which could result in failure.

5. A water and sediment control basin system requires a thorough design by a qualified person before being constructed.

Design Considerations

1. Before a water and sediment control system can be designed, a field survey should be undertaken to determine the topographical features which will affect the berm locations and ponding volumes available. Soil types, watershed design features and outlet availability should also be investigated.

2. Having established the berm locations in the field the following factors should be considered in the design of this structure:

3. The peak flow and total volume of runoff water for the watershed is to be determined. For further information see OMAFRA Fact sheet Determination of Watershed Characteristics and Their Effect of Peak Flow.

4. A single water and sediment control basin system can be feasible designed for a maximum watershed acreage of 50 acres (20 hectares) with an upper storage volume of 100,000 cubic feet (2800 cubic meters) and a maximum ponding depth of 7 feet (2.0 meters). Exceeding one of these criteria results in a relocation of the berm to a more desirable location in the gully or the consideration of a completely different type of erosion control structure. More than one water and sediment control basin may be located on the same tile outlet and gully control system.

5. The ponding volume available is to be determined from a field survey of the area. The necessary berm height can be calculated knowing the volume of water and sediment that must be stored. Normally, the storage is designed to hold 15 years accumulation of sediment that erodes from the watershed as well as the runoff water expected.

6. The tile outlet system for the ponding area is sized on the maximum duration of time, ie. Up to 24 hours. For assistance in the design of the outlet tile see OMAFRA Publication 29, Drainage Guide For Ontario. The maximum flooded time for the ponded area depends upon the crops grown in this area and their susceptibility to water lying on them.

7. An emergency spillway is to be included in the berm design to safely handle unusually large flows of water. A level-top vegetated berm crest, where the water flows over the entire berm length in a thin layer, is the most common emergency overflow for a low risk structure. A. A grass or rock-lined notch spillway in the berm may also be used as the emergency overflow.

8. Where multiple water and sediment control basins are used with a common outlet tile, consideration must be given to excessive flows out of the upper storage and into the lower storage are causing a possible overtopping of the lower berm. Where the outlet tile does not restrict the flow out of the upper basin to accommodate the problem, orifice plates can be installed in the perforated stand pipe to restrict intake flow.

Water and Sediment Control Basin Location

The topography of the erosion prone area must be suitable for the installation of a water and sediment control basin system. There is a requirement for a low draw through the gully area with proper grades on the side slopes so as to provide for a berm of a practical length and height to achieve the necessary storage volume. Similar requirements apply to the upstream ponding slope if a reasonably sized storage pond is to result. The exact berm location will be decided by the side slope and upstream slope of the ponding area in the field. As well, there berm location and number of berms necessary to install on a single gully system will be determined by the slope of the gully or draw and the area of watershed contributing to the fully in question. With steep gradient gullies, the berms will have to be located closer together to eliminate rills forming between the berms and to provide for adequate storage volumes behind the berms.

Figure 4. Multiple water and sediment control basins on the same gully.

If a gully has two or more branches (Figure 3), it may be necessary to construct water and sediment control basins on each of the branches. This decision will be determined by the size of the contributing watershed to each branch, the storage requirement, the ability to achieve this storage volume at each location and the erosion potential between berms.

Construction

The land beneath the storage ponding area should be tile drained to provide for a more rapid dry out period after ponding has occurred. All subsurface drainage is to be installed prior to the water and sediment control basin installation and in a manner so as not to interfere with the outlet tile for the structure.

The exact field location for the berm should be identified and staked. The point at which the berm top meets the original ground line on both sides is to be marked as this will show the berm length and height.

The outlet tile for the storage basin(s) is installed from the outlet up through the water and sediment control basin locations. This tile line is located in the low area but 20' - 30' (6-9 m ) off to one side of the main fully to reduce further washout problems over the line. The outlet tile is sized on the basis of the quantity of water inflow from the storage basins. In most installations, this outlet tile flows under pressure. Thus, extreme caution should be taken when installing the tile so that a stable tile outlet results.

The construction of the berm(s) is the next installation step. Three types of berms may be used, namely:

1. Narrow-based berm

The narrow-based berm is the most common berm having a top of 4 feet (1.2 m) and side slopes of two horizontal : 1 vertical (Figure 5). This berm is the most economical to build as it requires the least material.

Figure 5. Narrow-based berm.

The steep side slopes of this type of berm eliminate the possibility of working on it with farm machinery. 

2. Broad-based berm

The broad-based has side slopes of 8 horizontal :1 vertical and can be worked on with farm machinery (Figure 6).

Figure 6. Broad-based berm

This berm is more costly to construct because of the extra material required. 

3. Steep-back slope berm

This berm has a broad-based slope of 8 horizontal : 1 vertical on the front side and a steep slope of 2 horizontal:1 vertical on the back side (Figure 7).

Figure 7. Steep backslope berm.

The front side can be worked over with farm machinery while the steep-back is left permanently vegetated. This berm type could be located at the edge of a lane, gully, etc. where the front side is to be worked. 

The berm construction comprises of the scraping back of the topsoil from the area where the berm and storage basin is to be built. A subsoil material, free of stones, rubble, stumps etc. and with at least 10% clay should be used to construct the berm. The material for the berm may be obtained from the area behind or in front of the berm location or moved from a borrow pit located elsewhere on the farm. An earth mover will be required to move material from a pit located a distance away while a bulldozer would be used if material is located in the berm area.

The berm material is to be placed in layers and compacted as each layer is added.

Figure 8. A berm under construction.

 After the berm has a final shape the sides of the berm should also be compacted.

To provide for larger-than designed peak flows, two types of overflow system are used, namely:

1. "The level-top vegetated berm crest" which provides for very flat berm top so that the water will overflow along the berm length in a thin layer so as not to create erosion problems (Figure 9).

Figure 9. Water overflowing a berm in a thin layer. Photo courtesy of U.T.R.C.A.

 2. "A notched spillway" which is located at the one end of the berm, ie. Where the berm meets the original ground, and is lined with rock rip rap or vegetation (Figure 10).

Figure 10. A rock rip rap emergency spillway at the end of the berm. Photo courtesy of U.T.R.C.A.

 The emergency spillway is to be designed so that it will safely carry the maximum expected flow from a storm that will occur once every 25 years.

The perforated riser pipe is installed in the lowest area of the storage basin and offset from the main outlet tile line. To reduce erosion of the front face of the berm the riser pipe is set back 8 feet - 10 feet (2.4 - 3.0 m) from the berm base. The perforated riser pipe and underground outlet tile line is selected on the basis on the basis of removing the ponded water over a maximum duration of 24 hours. If high value, water-susceptible crops are grown in the ponding area a reduced flooding time of 12 hours or less should be designed for. If a reduced flooding time is used, a larger perforated riser pipe and outlet tile will result. If more than one water and sediment control basin is located on a single tile line, consideration may have to be given to restricting water inflow on the upper basins so as not to flood out the lower basins. Either orifice plates in the riser pipe or a restrictor outlet tile line will perform this function.

The final installation step is the moving of the topsoil back to the berm and ponding area and establishment of vegetation on the berm. Various seed mixtures are available to choose from. The seed should be planted immediately after the berm construction and fertilizer applied to ensure a quick, thick stand. For more information on vegetation establishment see OMAFRA Fact sheet, Seeding of Erosion Control Projects. A mulch may be applied to the seeded berm or the spillway to provide erosion protection and conserve moisture to aid in quick seed germination.

Maintenance

1. The narrow-based berms should not be tilled but left in permanent sod cover and mowed at least once or twice annually.

2. Regular checks should be made for cracks in the berm, settlement of the berm or obstructions in the storage basin outlet system.

3. Repair any berm washouts immediately. Reseed and fertilize as necessary.

4. Watch for rodent damage in the berms and repair as necessary.

5. The notch-type emergency spillway is to be maintained and kept in good condition.

6. A good conservation tillage and cropping program must be practiced by the farmer in order to reduce accumulation of sediment in the storage basin over the life of the structure.