[Community Watershed Guidebook Table of Contents]
The Forest Practices Code regulations and Forest Road Engineering Guidebook contain many clauses that are intended to reduce the impact of road activities on water quality. The regulations and guidelines for activities in community watersheds emphasize increased standards for inventorying, planning and monitoring roads. But road construction and maintenance procedures are not expected to be different in community watersheds.
Requirements:
- The water licensee must be informed at least 48 hours before the start of road building or road modification or rehabilitation activities.
- Roads, proposed in a forest development plan or access management plan must not be located in the riparian management area of a community watershed, unless there is no practical option or a location elsewhere will result in a higher sediment hazard.
- Before road construction or modification, terrain stability must be field-assessed in any area of a community watershed with a high or very high potential for landslides.
- A detailed surface erosion hazard assessment must be carried out for all areas where road construction or modification is proposed. (After a 1996 amendment to the regulations: this assessment is restricted to areas identified on terrain maps as having a high or very high surface soil erosion hazard).
- A road must not be located within 100 m upslope of a community watershed water intake (i.e., point of diversion) unless the designated environment official agrees that water quality objectives can be better met by locating the road closer.
- Roads must not interfere with the known subsurface flow paths of a source area for a spring.
- Stream culverts must pass the 100-year return period instantaneous peak flow without the stream surface rising above the top of the culvert inlet, unless a professional engineer has designed inlet and outlet protection measures.
- Road construction, modification, maintenance or deactivation activities must not cause water quality to fall below the water quality objectives established by Ministry of Environment, Lands and Parks.
- Select timber harvesting systems in accordance with the long term forest development plan to minimize the requirements for roads.
- Do not locate roads and trails on areas of terrain stability class IV or V (moderate or high likelihood of landslides), or on class III with high landslide delivery potential, unless a detailed geotechnical assessment and road design by a qualified professional engineer or professional geoscientist demonstrate that slope stability and water quality impacts can be prevented.
- Minimize road width wherever possible in community watersheds.
- Minimize the number of stream crossings. Select stream-crossing locations with stable channels and banks, to minimize channel and bank disturbance.
- Roads must be located outside riparian management areas, except at stream crossings, unless no other feasible engineering option exists or unless a location elsewhere will create a higher risk of sediment delivery.
Prior to starting road construction or modification in a community watershed, the forest licensee must inform the water licensee, or a representative of the water users' community, and the designated environment official of the start date of road construction. This must take place at least 48 hours before road construction starts. Contact the local BC Environment office for a current list of contacts in each watershed.
Roads in community watersheds must not be located within a 100-m radius upslope of the water intake. Exceptions to this regulation are:
- roads built to service the water intake or its infrastructure
- when the designated environment official and forest district manager agree that a road farther away than 100 m would be on unstable or erodible soils that would create a greater hazard to water quality.
Refer to section 7 for information on terrain hazard mapping and field assessment requirements for roads.
In some cases, road construction across sensitive soil (i.e., the soil has a moderate or high likelihood for landslides or a high or very high surface soil erosion hazard) will be proposed because there is no alternate route. Such a road cannot be approved unless a person experienced with road construction across unstable soils prepares a field assessment and detailed design. After 1997 the design must be signed by professional engineer or a professional geoscientist.
The design must include plans and profiles of the entire section of road that crosses the sensitive soils, and must clearly show the measures that will be taken to minimize the hazard. The district manager, after reviewing the assessment report and proposed road design, still can refuse to approve the road plan if the risk is considered unacceptable.
If the design is approved, the engineer or geoscientist will indicate whether a construction or post-construction inspection is required.
Where water quality objectives or criteria are established, road construction, modification, maintenance and deactivation must not cause water quality, measured at the community water intake or other designated monitoring station, to fall below these standards.
If water quality does fall below these standards, BC Environment will survey the contributing streams in the watershed or do detailed water quality monitoring to determine the cause. If road activities are the cause, all contributing road-building activity in the community watershed must cease until remedial measures are completed. See section 5 "Water quality monitoring" for how to establish water quality objectives.
Stream culverts in community watersheds must be designed to pass the 100-year return period instantaneous peak flow (Q100) without the water surface rising above the top of the culvert inlet (with zero head), unless a professional engineer has designed inlet and outlet protection measures.
This requirement is intended to avoid several problems.
- It ensures that the culverts are big enough to prevent overtopping and scouring road surfaces.
- It reduces the risk of road destabilization, by seepage pressure in the road prism around the stream crossing.
- It reduces culvert outlet velocities.
- It provides a margin of safety for sedimentation and passage of debris at the culvert entrance.
Because of the lack of streamflow information for most of the province, especially for small watersheds, it is very difficult to estimate the 100-year return period instantaneous peak flow. Confidence limits are generally very wide and the accuracy of estimates low. However, this criterion provides a consistent benchmark of relative risk against which audits can be conducted.
Soon, new methods based on British Columbia conditions will more directly provide design data on peak flows and culvert sizes. Once these have been completed and field tested, this guidebook will be updated to document the results. More information can be obtained from BC Environment's Hydrology Branch, Water Management Program, and Ministry of Forests Research Branch.
The following procedures must not preclude other reasonable and generally accepted methods for determining design flows and culvert size. Bridges or major culverts (2000 mm or greater, or design peak flow greater than 6 m3/s) must be designed by professional engineers who are ultimately responsible for designing adequate stream-crossing structures.
Culvert size based on site information
Basing culvert size on stream dimensions (described in the Forest Road Engineering Guidebook) assumes that the flow at bankfull stage is the mean annual peak flow, or Q2. This assumption may not apply for all situations. Also, the area of culvert opening required for a 100-year return period flow is assumed to be three times the area required for Q2.
The bankfull level often is not readily apparent. Figure 7 shows how to identify this feature; it is where there is a distinct change from unvegetated channel to streamside vegetation.
Figure 7. Identifying bankfull width (after Church, 1992).
A key element in obtaining a flow estimate is to select a reach of stream with a length about five times the bankfull width, in which flow is fairly uniform and shows no backwater effect.
The following steps should be followed (see Figure 8).
- Measure the width of the channel at the top of the bank (W1) and the stream bottom (W2) in metres. Be careful that the measurements are of the streamflow channel, not of the channel after scouring by debris flow.
- Measure the depth of the channel at several spots to obtain the average depth (D) in metres.
- Calculate the cross-sectional area of the channel: A = (W1 + W2)/ 2 x D
- Calculate the area of the required culvert opening: Ac = A x 3
- Size the pipe, using Table 8. Use the smallest pipe area that exceeds the required area. For a log culvert, select an opening size greater than Ac.
Figure 8. Stream cross-section.
Table 8. Round pipe diameter and cross-sectional area required for various discharges
This method for determining culvert size cannot be used on channels that have been affected by debris flow. Instead, culvert size should be based on the estimate of Q100 described in the next section.
Culvert size based on hydrologic information
On some streams it may be difficult to measure channel dimensions or to estimate the bankfull flow. In those cases, the culvert size should be based on an estimate of Q100 deduced from analyzing historical flows.
Estimates from gauged watersheds
For each BC Environment region, there is a Guide to Peak Flow Estimation for Ungauged Watersheds. The guide includes regional peak flow data analysis for gauged watersheds. Copies are available at forest region offices, or can be ordered from the Surface Water Section, Hydrology Branch, BC Environment. These guides can be used to estimate flows for culverts to be installed in watersheds similar to the gauged watersheds. Because gauging is typically conducted on larger streams, this information may be less reliable for small streams.
A regional approach (Church, 1992) similar to that in the BC Environment Guide to Peak Flow Estimation is described below. Q2 is estimated using an equation which includes a factor to account for regional variations. For culvert sizing, it is assumed that Q100 is three times Q2.
Steps:
- Determine the watershed drainage area A (km2) from the best available topographic map (usually 1:20 000 scale TRIM).
- Determine the regional factor k from the map (Figure 9).
- Estimate the two-year return period daily peak flow Q2 for the watershed by using the following equation:
- Multiply Q2 by 3 to estimate Q100.
- Size the pipe, using Table 8. Use the smallest pipe diameter that exceeds the corresponding Q100. For a log culvert, select an opening size Ac corresponding to Q100.
Figure 9. British Columbia k contours: k values for streamflow estimation.
Example
A watershed near Nelson has a drainage area of 2 km2.
From Figure 9, the watershed has a k factor of 1.0.
Q2 = 1.0 (2.0)0.68 = 1.6 m3/second.
Q100 = 3 x Q2 = 4.8 m3/second.
From Table 8, the required pipe diameter is 1800 mm.
Estimates from British Columbia Rational Formula
The rational formula is a method of estimating flows in small watersheds from regional rainfall intensity data and gross watershed characteristics. For each forest district, curves of Q100 against drainage area have been plotted, based on the B.C. Rational Formula. Copies of the graphs can be obtained from Surface Water Section, Hydrology Branch, BC Environment.
Estimates from culvert surveys
Another approach (which will add to the currently small peak flow data base) is to survey existing culverts in the forest district to record evidence of past peak flow events at each culvert.
Steps:
- Select culverts for which installation dates are available, on streams of various sizes (based on drainage area and channel size). They should show clear evidence of high water marks (debris, sand or scour lines) and not have been overtopped.
- Using culvert nomographs, determine the peak flow passed by the culvert as indicated by the high water marks.
- Based on hydrometeorological data for the region for the period the culvert has been in place, estimate a return period for the culvert high water mark flow.
- Determine the watershed area A (km2) from the best available topographic map (usually 1:20 000 scale TRIM).
- Peak flow estimates can be plotted against drainage areas for zones within a forest district which are assumed to be homogeneous with respect to peak flow variation (Figure 10). Depending on the reliability of the data, it may be possible to define a curve which relates peak flow of some return period to drainage area. Regional peak flow data can be used to adjust these flows to Q100.
Figure 10. Sample plot of culvert unit discharge versus drainage area.
Culvert size based on debris flow potential
If the stream has a moderate or higher potential for debris flow, the design of the drainage system should be based on the maximum debris flow rather than the water flow. There is not a strong correlation between expected peak flows in a watershed and the possible debris flow torrent volumes. Debris flow volumes can exceed ten times the peak flow discharge.
The Gully Assessment Procedure Guidebook provides a hazard rating system for coastal streams that have debris flood or flow potential. Where there is a significant debris flood or debris flow hazard, refer to the Ministry of Forests Working Paper 22 – Debris Flow Control Structures for Forest Engineering (Van Dine, 1996). The drainage system for the crossing should incorporate the following:
- regular maintenance after storms
- debris racks
- possibly, deflection or catchment berms
- sediment control fences or settling basins
- fords instead of culverts on streams with frequent debris flows (ford construction procedures are in the Forest Road Engineering Guidebook)
- contoured road approaches. These ensure that passing debris torrents will be contained in the gully and prevented from running down road grades.
- Before the start of road construction, brief all construction workers in the watershed on plans, special design and construction requirements, sediment control requirements and special water quality concerns.
- Provide portable chemical toilets at all major construction sites (e.g., bridges). Toilets must be at least 50 m from any water source and emptied outside of the watershed.
- Suspend all road construction activities when they imminently threaten water quality, especially during periods of intensive rainfall or heavy runoff. Use road construction windows to avoid high runoff seasons and winter snow conditions.
- Construct temporary drainage structures to control sediment and runoff during temporary work stoppages.
- Do not place slash, organic debris or stripped topsoil in riparian management areas or intake management zones, if there is potential for the material to be eroded and transported to a watercourse.
- Locate pits or quarries outside riparian management areas, and preferably greater than 100 m from a watercourse. Do not discharge drainage from pits and quarries directly into a watercourse.
- Construct road subgrades well in advance of operational use.
- The time required to let the road bed stabilize will vary from six months to three years, depending on soil conditions.
- Construct subgrades immediately after stripping.
- An hydraulic excavator is recommended for road construction, particularly on any slope over 40 per cent and on easily erodible soils, such as sand or clay. Bulldozers are better for short horizontal movements but not for side-casting, where mass wasting is a concern.
- Construct the road subgrade deep enough to raise the road surface above the water table during the operating period.
- Ensure that drainage works (ditches, culverts, fords, cross-ditches and water bars) always work well during subgrade construction and temporary or seasonal construction shutdowns.
- Do not place unprotected erodible fill below the high water level of watercourses, whether seasonal or continuous. When placing fill next to a lake, river or stream (whether continuous or seasonal) is unavoidable, and high water may wash the toe of the fill, make sure the toe of the fill is clean riprap.
- When fueling or servicing construction equipment in a community watershed, do it outside of riparian management areas and the intake management area.
Road ballast and surfacing
- Road surface erosion is one of the main sources of fine suspended sediment to watercourses.
- All haul roads with a subgrade surface of sand, silt or clay should be covered with a compacted layer of clean gravel or rock crush, with 10 per cent fine to allow compaction.
- All roads should be sufficiently ballasted to carry logging trucks, loaded as designed, and other logging equipment without rutting or deforming the road surface, in all weather except spring breakup when the road is inactive. The ballast should be deep enough to raise the road surface above the water table during the operating season.
- Use road-surfacing materials of crushed rock ballast or gravel that will not break down into silt-sized material that will wash off the road surface.
- Do not use chemical dust suppressants during road surfacing.
Revegetation
- Seed or hydroseed (or other approved revegetation means) must be applied to all exposed mineral soils that will support vegetation. These include completed borrow pits, waste areas, road cuts, fill slopes and other areas disturbed in the cleared width within one year of disturbance, plus the area revegetated within two years of disturbance.
- The following guidelines are the best management practices for maintaining water quality in community watersheds. Many of these guidelines are described in the Forest Road Engineering Guidebook, but are consolidated here as a reference for water quality protection.
Drainage structures (general)
- Construct bridge, culvert and cross-drain systems to maintain natural surface drainage.
- Use rolling grades with outslopes, swales or French drains to break up lengths of road with long continuous grades, reducing water volume and velocity buildup.
- During construction of stream crossings, follow these guidelines:
- Keep machinery operated near a stream free of excess oil or grease.
- Operate machinery from the stream bank, not in the channel.
- Cross the stream with equipment at only one location.
- Remove immediately any fill material inadvertently placed within the high-water wetted perimeter of the channel. Be careful not to disturb the natural streambed and banks.
- Complete construction activities within the wetted perimeter quickly.
- Do not use erodible or toxic materials below the high water mark of any stream.
- Prevent sediment from entering streams at stream crossings.
- Where possible, slope bridge approach grades away from the bridge, except immediately adjacent to it.
- If adverse grades cannot be used, cap approach roads with clean gravel at least 20 cm deep (with approximately 10 per cent fines for compaction) for a distance of 30 m either side of the bridge.
- Cattle guard cross-drains can be used to control road surface runoff.
- Construct all embankments that encroach on watercourses with shot rock or with clean granular material surfaced with riprap.
- Where riprap is required for surface material, it must be clean, durable, well-graded, angular rock suitably sized to resist movement by the stream and to prevent migration of fines from the granular fill underneath.
Ditches and cross-drain culverts
- To reduce the need for ditches and cross-drains and their maintenance, construct swales or outsloped roads where possible.
- Drain ditches and cross-drain culverts onto stable areas of low erosion or slope stability hazard. Any sediment transported from the road or ditches must filter out on the forest floor before the water reaches a watercourse. The slope below the culvert outlet must not erode or become unstable.
- Do not discharge ditch runoff off the end of a switchback, unless this is the natural drainage pathway. Put paired cross-drain culverts below and above the switchback. Prevent ditch water from discharging directly into a watercourse. Where it can not be avoided, build a settling pond.
- Do not use graders to clean ditches. A grassed ditch produces much less sediment than a graded ditch.
- Where ditch erosion is anticipated or observed, install devices to dissipate energy such as drop structures. The ditch can also be armoured with erosion-resistant material. Ditches can be stabilized using rock, coarse clean gravel, filter fabrics and vegetation, if water velocities permit.
- Construct a sump hole or settling basin within a size of 1 m3 by the inlet of every cross-drain culvert to control soil transport from ditches.
- Construct ditch blocks immediately downstream of all cross-drain culvert inlets, with the crest of the ditch blocks lower than the road shoulder.
- Protect unstable or erodible material at culvert outlets with flumes, riprap or other erosion-resistant materials.
- Install additional culverts at road junctions to help preserve the natural drainage patterns.
- Install a clearly visible culvert marker on the ditch side of any plastic or metal cross-drain culvert if the inlet or outlet could be hit by a grader or snowplow.
Stream-crossing culverts
- Install culverts in every natural gully, seepage-way, perennial or intermittent stream, and road dips that will not drain naturally, to maintain natural drainage.
- Install protective measures, such as trash racks, where debris cannot safely pass through the culvert.
- Limit fill heights to less than 2 m, measured on the intake side.
Bridges and major culverts
- In community watersheds all bridges should be fully decked with wood. Gravel decking is a second choice but should only be used with a concrete deck or over log stringers that incorporate geotextile. Place geogrid between stringers and the geotextile, to ease future removal of the gravel deck.
- Protect abutments and approach fills with riprap if they may erode.
- Make sure abutments do not impinge on a channel.
Fords
- Use ford crossings for stream crossings only where it is impractical to construct bridges or culverts because of debris loading or debris torrents in the stream.
- Fords should be pre-approved by the designated environment official.
- Limit industrial traffic to times when no water is passing over the surface of the ford. Refer to the section on "Ford design and construction" in the Forest Road Engineering Guidebook.
Metallic ores or coal strata commonly contain sulphide minerals that, when exposed to air and water (as liquid or vapor), become unstable. Potentially, this can generate acidic (low pH, high sulphate) runoff, commonly termed acid rock drainage. Where materials that generate acid rock drainage are exposed by road construction or used as aggregate in road construction, pH reductions and elevated concentrations of metals in water can result. These can harm aquatic life and exceed safe drinking water guidelines for humans.
Where an area of potential acid rock drainage is known in a community watershed, take these actions:
- alter a road location to avoid areas of acid rock drainage
- ensure that bedrock that could generate acid rock drainage is not exposed to the air
- do not use rock that generates acid for road construction or modification.
Where acid-generating rock formations cannot be avoided, manage acid rock drainage according to the Acid Rock Drainage Technical Guides (1992) published by the Ministry of Energy, Mines and Petroleum Resources.
- Regular maintenance inspections of forest roads must be done to assess road infrastructure conditions and the adequacy of ditches and culverts, particularly before and during high runoff periods. Remedial work identified by the inspections must be carried out as soon as warranted by the risk to water quality and road users.
- Inspect and maintain all roads in community watersheds until they are permanently or semi-permanently deactivated according to a deactivation plan.
- Carry out road maintenance inspections:
- after major storms
- during spring breakup
- prior to fall rains.
- Guidelines to follow during inspections and maintenance are listed below.
- Correct immediately any deficiencies that risk water quality.
- Do not disturb cut slopes when grading the road surface or cleaning ditches. Do not use graders to clean ditches. Grassed ditches are effective sediment filters but graded ditches add to the sediment load.
- When grading roads, do not push road surface material off the road into a watercourse or into a position where it may eventually slide into the watercourse.
- Remove all gravel berms or windrows during grading unless the berms or windrows are for a specific purpose.
- When plowing, put gaps in snow berms at cross-ditches and at frequent intervals, to avoid water buildup on the road.
- Provide swales or cross-ditches to drain road surface water that comes from snow plow banks.
- Ensure that all drainage structures are clean and that backup structures are constructed where necessary, before the fall rain and the winter snow seasons.
- Inspect, clean and, where necessary, repair all culverts, ditches, cross-ditches, waterbars and bridge openings to make sure that they will work during the fall rains and spring breakup.
- Roads should be deactivated when they are no longer in regular use or regularly maintained. Stabilize the roadway prism and cleared width and restore and maintain the natural drainage.
- Temporary deactivation, especially of cross-ditches, is required whenever regular maintenance is suspended for more than one season. Orient cross-ditches so water is redirected back into its original channel.
- Inspect after major storms, during spring breakup and before fall rains, to assess the adequacy of ditches and culverts, and the need for improved drainage works, road surfacing or revegetation.
- Complete remedial works identified by inspections before the onset of fall rains.
- Back up stream culverts with a cross-ditch on the downhill side.
- Repair bridges as necessary.
- Remove or put gaps in any windrows on the outer edge of the road surface.
- Install additional cross-ditches or water bars where there is a risk of adversely impacting the road and other water resources through erosion or by channeling runoff, when water is likely to run down the road surface:
- on sensitive or steep terrain
- in areas of heavy rainfall
- where there is much groundwater seepage
- at switchbacks and road junctions
- where ditches are prone to plugging.
Deactivate very high hazard roads semi-permanently when future access is required but regular use is suspended for more than three years.
Restore natural flow patterns by removing all stream culverts and installing fords where possible, if access is required during a period of deactivation. Inspections and maintenance are required on semi-permanently deactivated roads.
Remove all road fill material below the high water line of a channel.
On moderate to high hazard roads, the natural drainage pattern must be restored. Remove all existing stream culverts and restore channel stability. Install fords, where possible, if access is required during the period of deactivation. Alternatively, stream culverts must be backed up with similarly sized cross-ditches on the downhill side.
For all moderate to high hazard sites, remove cross-drain culverts and replace with cross-ditches. Ditch blocks must be intact. Make sure the cross-ditch angle redirects water back into its original channel. Alternatively, cross-drain culverts must be backed up with similarly sized cross-ditches on the downhill side. Add cross-drains at approximately twice the frequency of cross-drain culverts, in all but very dry locations.
Dig additional cross-ditches:
- on sensitive or steep terrain
- on areas of heavy rainfall
- where there is much groundwater seepage
- at switchbacks and road junctions
- where ditches are prone to plugging.
Remove or put gaps in any windrows on the road surface.
If there is a high risk of slope failure or stability of the fill is questionable, pull sidecast back onto the outsloped road grade to ensure drainage.
Forest Practices Code regulations require that road construction, modification, maintenance and deactivation must not cause water quality to fail to meet the objectives set by the Ministry of Environment, Lands and Parks. Particular water quality objectives apply to a certain site or reach of a watercourse and are designed to protect the most sensitive water use (see section 5.1 "Water quality objectives").
This requirement is only relevant where a water quality monitoring station has been established at the intake of the community watershed for the purposes of monitoring raw water quality. Water quality objectives must be established by Ministry of Environment, Lands and Parks for that watershed or for the hydrological zone within which the watershed is located.
For information on monitoring the baseline water quality during road engineering, see section 5 "Water quality monitoring."
