Low Flow, Mid-Level Stream and Ditch Crossings With Culverts

Agdex#:	751
Publication Date:	10/92
Order#:	92-143
Last Reviewed:
History:	Original Factsheet
Written by:	R. P. Stone - Engineer (Soil)/OMAFRA

Table of Contents

1. Introduction
2. Design Considerations
3. Construction Procedure
4. Approvals
5. Assistance
   

Introduction

Many Ontario farms have watercourses dispersed throughout them, making accessibility between the parcels of land difficult. Often livestock have access to land on both sides of the stream making the crossing of the watercourse necessary. When livestock have access to streams and ditches, bacteria levels in the water may increase and bank erosion can take place. As a result, water quality for down-stream users may be degraded and herd health for the livestock producer jeopardized. Livestock trampling the stream banks may increase the sediment load entering the water-course which can smother out aquatic habitat and sometimes result in expensive drain clean outs. A low flow, mid-level crossing is an environmentally acceptable structure for livestock to use as a ditch or stream crossing (Figure 1).

Figure 1. A low flow, mid-level crossing is an environmentally acceptable stream crossing for livestock.

Low flow, mid-level crossings allow normal water flows to pass through the culverts, keeping the crossing surface dry for most of the year. High flows, i.e. spring runoff, flow over the top of the crossing, as the culverts are not sized large enough to carry these flood-type runoff events. The crossing surface must be constructed of an erosion resistant material to combat the effect of water flowing over the crossing top and withstand cattle traffic. The low flow, mid-level crossing allows cattle to cross the watercourse on the dry crossing surface above normal water levels. The water course is completely fenced to restrict livestock entry. Gates at the crossing accesses allow for the complete closing of the crossing during high flow conditions.

The low flow, mid-level crossing also provides an excellent stream crossing system for farm machinery. Although more costly to build than a bed-level type crossing, the low flow, mid-level crossing allows for the movement of machinery across the stream above the water level.

Figure 2. The culverts carry normal flows while high flows move over the top of the crossing.

Design Considerations

Rigid design criteria are not clearly defined for the sizing of the components for a low flow, mid-level crossing. Flows that are greater than the culvert capacity will flow over the erosion-resistant crossing top, creating minimal problems upstream or in the immediate area. However, the following design guidelines should be considered when planning this type of crossing:

1. A topographic survey of the stream crossing area should be done to ensure proper installation of the crossing and its future operational success. From the standpoint of construction, the most desirable stream section for a crossing has a defined bed and banks and is of a moderate width, i.e. 3 - 6 m (10 - 20 feet), at that location. A shallow stream depth will back up less water behind the crossing, causing less stress on the banks and decreased erosion. 

2. The crossing location should be investigated in relation to potential backwater effects created by the crossing installation. Neighbouring properties, roads, etc., should not be affected as a result of water being backed up.

3. The stream should be investigated as to whether fish migration takes place or not. These crossings may not be very suitable on fish migration watercourses where high flow velocities in the culverts occur. If this type of crossing is installed on a fish migration stream, the design may require pipes at different elevations or larger diameter pipes with baffles to reduce flow velocities.

4. The crossing culverts are to be sized to carry normal flows. The use of stream flow data, if available, will be valuable for this purpose. The Ministry of Natural Resources will review the proposed design for potential effects on natural resources.

5. Several smaller diameter culverts are preferable to fewer, larger culverts. The small diameter culverts result in a lower elevation for the crossing surface, requiring less bedding materials and backing up less water in the stream. Also, a larger number of smaller diameter culverts located across the streambed will not converge the upstream flow as much at the culvert entry. On the other hand, smaller culverts are more prone to blockage by drifting debris, ice or through the work of beavers and, thus, could require more maintenance. Culverts should be selected according to the site, keeping in mind that fish migration may occur

6. The culverts must be long enough to permit construction of stable, erosion-resistant end wall slopes. Both ends of the culverts must be protected from erosion. Fill materials, i.e. crushed stone, are placed around the culverts in layers not thicker than 150 mm (6 in.), and each layer is well compacted. A filter cloth is placed under the culverts and fill material and extended up over the end wall slopes under the erosion-resistant surface, i.e. rock riprap, onto the top surface of the crossing immediately below the erosion-resistant top. The filter cloth allows the water to drain through while holding the fill material in place.

7. The crossing surface must be constructed of a material that is erosion-resistant and will not deter cattle from crossing. Surface materials must have an adequately roughened surface so as not to be slippery for cattle. Typical surface materials are linked modular concrete blocks and cast-in-place concrete.

8. The crossing should have a minimum top width of 3.65-m (12 feet) and be fenced on both sides to prevent cattle from entering the watercourse. A portable fence/gate system is desirable since it can be removed before incurring damage by heavy fall/spring flows (Figure 3).

9. The crossing approaches should be designed with the proper grade to serve their intended purposes. For a cattle crossing, the grade should not exceed 4 (horizontal):l (vertical). An 8 (horizontal):l (vertical) or flatter grade should be used for machinery crossing installations. The entry to the approaches must be graded so that surface water does not flow onto the approaches from the surrounding lands. This water, if allowed to flow over the approaches, can cause a potential washout. A good surface material, i.e. crushed stone, underlain by a geotextile or filter cloth, is required on the approaches. All disturbed soil areas around the stream and approaches should be graded at a 2 (horizontal):1 (vertical) slope or flatter and be seeded immediately after construction. 
   

Figure 3. A plan view of a stream crossing for livestock.

Construction Procedure

Before construction begins, approval from the Ministry of Natural Resources and the local Conservation Authority may be required. The timing for the construction work and other specifications related to the project may be stipulated by these agencies.

Large backhoes should be used to excavate the streambed area, and install the culverts and other crossing components because they can be set up at a distance from the stream to do the work.

To protect downstream fish and wildlife, the watercourse may have to be diverted or dammed up while construction takes place. Control measures should be implemented to retain sediment at the construction site.

The streambed is excavated, if necessary, and shaped to obtain a firm bed to place the culverts on. It is important that the base (invert) of the culvert be placed at or slightly below the stream's bottom. The structure should be keyed into the streambanks at least 450 - 600 mm (18 in. - 24 in.) to prevent water from eroding the crossing ends. A filter cloth is placed on the streambed beneath the fill material.

Figure 4. A typical stream crossing with three culverts to carry the normal flows.

The culverts are installed at a slope equivalent to the stream gradient. When more than one culvert is used, a minimum spacing between culverts of 300 mm (12 in.) should be maintained (Figure 4).

Fill material is placed around the culverts in layers not exceeding 150 mm (6 in.) and compacted as it is placed. Enough fill material is added above the culverts to provide an adequate base for the crossing top. The total height of the crossing should be kept to a minimum--typically the diameter of the culverts plus approximately 300 mm (12 in.)

The filter cloth extends up over the end-wall slopes of the crossing and is placed immediately under the erosion-resistant crossing surface (Figure 5).

An erosion-resistant crossing surface which extends down over the crossing end walls and around the culverts is then installed on the top.

If a cast-in-place concrete surface is selected, it should be steel reinforced and roughened to make it less slippery for livestock. Other erosion-resistant materials may be selected provided that the animals are not afraid to walk across them. The erosion protection should extend adequately up into the streambanks so that overtopping flows move over this protected surface. Erosion protection, in the form of rock riprap, should extend along the streambed at both ends of the culverts to resist the scouring action of water converging into the culverts or exiting at the end of the culverts.

Figure 5. A view of the crossing showing installation details of filter, culverts, fill material and erosion-resistant crossing surface.

The approaches must have a surface material that livestock and machinery can travel over at any time of the year. A 150 - 200 mm (6 - 8 in.) layer of crushed stone will be adequate. If wet conditions are experienced at the approaches, a filter cloth should be placed below the gravel to stabilize the base and prevent fine soils from moving up through the gravel bed.

The watercourse must be fenced to restrict cattle entry along the stream and direct them to the crossing. A fencing system must be installed on the crossing to prevent cattle from entering the watercourse from the crossing surface and/or to completely eliminate access to the crossing if water flows are high, etc. A portable type of fencing system is encouraged because it can be removed during high water flows when cattle are not using the crossing, thus providing a method to avoid potential damage to the fence.

Approvals

Lakes and Rivers Improvement Act

Landowners planning to construct a stream crossing must first obtain a "work permit" from the Ministry of Natural Resources to ensure that the requirements of the Public Lands Act and the Lakes and Rivers Improvement Act have been complied with.

Fisheries Act

No one may carry out work that harmfully alters, destructs or destroys fish habitat unless there is clear authorization. Also, no one is permitted to deposit a harmful substance in water frequented by fish. Silt is considered a harmful substance under this Act. Violating the provisions of this Act can mean substantial fines and the risk of imprisonment; you may also be required to cover the costs of returning the site to its natural state.

The Fisheries Act is administered by the Ontario Ministry of Natural Resources. Discuss your project while it is in the planning stage with local Ministry of Natural Resources staff. They will examine your proposed work and suggest the necessary mitigative measures so that your project will meet the requirements of this Act.

Drainage Act

If the watercourse is a municipal drain under the Drainage Act, approval must be granted from the Municipality before construction begins.

Conservation Authorities Act

The landowner should also consult the local Conservation Authority, as they may have other Regulations under their jurisdiction that must be adhered to before a stream crossing is installed.

Assistance

Qualified erosion control contractors are available for the design and construction of stream crossings. Consulting engineers can also provide design expertise. In some areas of the province, Conservation Authorities may offer technical assistance and/or construction supervision. Other erosion control Factsheets and information are available from the Ministry of Agriculture, Food and Rural Affairs. The Ministry of Natural Resources has information pertaining to stream crossings and the legal requirements of landowners working around water.