Roads and stream crossings are commonly associated with land use and development on public lands in the mountainous regions of the western United States. When inappropriately placed or sized, stream crossings can have long-lasting detrimental effects on water quality, channel stability, and aquatic habitats (Furniss et al., 1991). Consequently, it is important that aquatic specialists work closely with forest engineers to design and select a crossing structure that is consistent with expected channel dimensions.

On the Fortine District, we are finding that many culvert failures occur because they are either underdesigned or not consistent with prevailing stream geomorphology. In an effort to improve upon the existing situation, we are exploring the use of geomorphic concepts in the design of forest road crossings. Implicit in this approach is the recognition that stream channels adjust to prevailing flow and sediment conditions in the watershed and that stream crossings must allow for the orderly passage of flow and sediment to maintain the long-term integrity of the channel.

This article describes an evolving methodology that we are using on the District to design stream crossings on forest access roads. The process provides an avenue for aquatic specialists to work with engineers to design stream crossings that preserve constant channel dimensions. The technique has been used mostly on small, incised headwater streams (drainage area < 5 km2 with an average bankfull width < 3 meters) as well as on larger fourth order channels (drainage areas of 90 km2 with bankfull width of 7 meters). The process relies upon accurate field measurements of the equilibrium channel and on hydraulic calculations derived from the computer program XSPRO (Grant et al. 1992).

Planning a Stream Crossing
The District uses the following steps to design stream crossings for forest access roads:
(1) assessment of channel equilibrium,
(2) calculation of hydraulic variables,
(3) determination of design flows, and
(4) post-project monitoring.

Assessing Channel Equilibrium
An assessment of watershed condition is a step often omitted in the design process. Many watersheds have experienced some modification in their natural hydrology, riparian, or channel conditions. A watershed assessment determines how the stream may be adjusting to upstream conditions. The assessment provides information that can be used to design a stream crossing that is consistent with existing basin conditions.

Dave Rosgen (1996) describes a channel classification scheme and a hierarchy of inventory and assessment that can be used to determine the existing and potential state of a stream reach. The construction of a stream crossing on an reach that is out of equilibrium often puts the crossing at risk.

Calculating Hydraulic Variables
A hydraulic analysis based on good site surveys, proper reach selection, and careful field techniques is essential (Grant et al., 1992). At a minimum, the site survey should consist of several cross-sections located near the crossing site. Cross-sections should be located in representative reaches that are uninfluenced by the road or outside factors. Information collected at each site should include: cross-section dimensions that extend out onto the floodplain, bankfull dimensions of width and depth, slope, longitudinal profile, particle size distributions, and entrenchment. Harrelson et al. (1994) provide an excellent discussion on how to conduct a site survey.

The data collected during the site survey is entered into the computer program XSPRO. The program determines hydraulic variables such as: cross sectional area, wetted perimeter, hydraulic radius, slope, roughness, velocity, and shear stress. Collecting discharge at the site over a range of flow conditions can be useful to verify the accuracy of the hydraulic modeling.

Understanding bankfull dimensions is an important part of designing a stream crossing. The crossing should be designed to maintain sufficient shear stress to perpetuate sediment transport through the reach. In many cases, this requires that the inlet configuration and crossing structure preserve the bankfull dimensions of width, depth, and slope. Significant deviation from bankfull dimensions can lead to undesirable geomorphic adjustments.

Forest Service direction in the Columbia River Basin currently requires that stream crossings be capable of passing a design flow equivalent to the 100-year flood (Q100) in key watersheds (USDA Forest Service, 1995a). Empirical relationships, channel cross sections, and the computer program XSPRO can be used to estimate the flow volume and cross sectional area of flow events.

The bankfull depth can be related to design flood depth using a dimensionless rating curve (Dunne and Leopold, 1978). The curve shows the relationship between depth (d) at a specific recurrence interval flow to the mean depth at bankfull (d_bkf). The ratios of d/d_bkf have been constructed by various authors in different parts of the country and are shown in Table 1.


Recurreance Intervals

Table 1. Values of the ratio d/d_bkf, for various values of recurrence intervals, from four regions in the United States. Adapted from Donne and Leopold 1978:648. (Note: these units use mean d_bkf).

The consistency of the relationship between bankfull depth and depth of a recurrence interval flood is very useful to the design engineer or hydrologist. Once the bankfull elevation has been carefully identified in the field (USDA Forest Service, 1995b), the depth of the design flow can be estimated using the d/dbkf relationship. The d/dbkf relationship, cross sectional data, and XSPRO can then be used to determine the elevation (and cross sectional area) inundated by a given recurrence interval flood. The computer program XSPRO is used to estimate the discharge, cross sectional area, depth, and hydraulic characteristics of the design flow event. The designer must be aware however, that there is variation in the dimensionless ratio values for depth as a function of watershed conditions, stream type, and floodplain shape.

Comparing the geomorphic estimates of design flows determined with this approach with regional empirical flow estimation equations such as Parrett et al. (1983), Omang (1992), or with other techniques is strongly recommended. The comparison will assure that the geomorphic estimates of flood flows are reasonable.

Just as important as determining the magnitude of the design flow is the selection of the appropriate type of crossing structure to be employed. Ideally, the crossing structure should allow overbank flows (floods) to pass unimpeded at the appropriate floodplain elevation and slope. Structures that constrict the flow of water along the floodplain can precipitate upstream deposition and downstream scour. Therefore, streams with a defined floodplain should receive a crossing structure that avoids constricting overbank flows. The identification of channels with a defined floodplain can be made using the entrenchment ratio (Rosgen, 1996).

The entrenchment ratio compares the width of the flood-prone area to the surface width of the bankfull channel. The entrenchment ratio identifies streams with defined floodplains. Streams with a well developed floodplain (entrenchment ratio > 2.2) should have flows greater than bankfull conveyed on the floodplain using either a bridge or a multiple culvert system. Bridges can easily be designed to pass flows along the floodplain and are the preferred choice. However, economic constraints on forest access roads often preclude their use. Therefore, multiple culverts may be an option to consider on small streams with a well defined floodplain. Multiple culvert designs should be used with caution because large flows extending onto the floodplain often carry a large load of organic debris that may plug culvert entrances.

Streams without a well defined floodplain or incised streams (entrenchment ratio 1.0 - 1.4) can be expected to accommodate design flows in a single culvert since entrenched channels tend to increase in depth more rapidly than in width as discharge increases. Figure 1 shows a culvert installation in an entrenched stream.

Entrenched Culvert

Figure 1. Example of a culvert installation in an incised channel typical of the types of streams where geomorphic design principles are being applied on the Fortine Ranger District.

Post-project Monitoring
To prevent the additional engineering and resource costs associated with failed stream crossings, monitoring should be incorporated into stream crossing designs. The establishment of a permanent benchmark and monumented cross-sections during the site survey facilitates future monitoring. Monitoring information collected through time will aid designers in future projects and may suggest problems before structures fails.

References
Dunne, T. and L.B. Leopold, 1978. Water in Environmental Planning. W.H. Freeman & Co., San Francisco, CA.

Furniss, M.J., T.D. Roelofs, C.S. Yee, 1991. Road construction and maintenance. American Fisheries Society Special Publication 19:297-323.

Grant, G.E., J.E. Duval, G.J. Koerper, J.L. Fogg, 1992. XSPRO: A channel cross-section analyzer. U.S. Dept. of Interior, Bureau of Land Management, Technical Note 387. Denver, CO.

Harrelson, C.C., C.L. Rawlins, J.P. Potyondy, 1994. Stream channel reference sites: An illustrated guide to field technique. Gen. Tech. Rep. RM-245, USDA, Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.

Omang, R.J. 1992. Revised techniques for estimating magnitude and frequency of floods in Montana. U.S. Geological Survey, Water-Resources Investigations Report 92-4048.

Parrett, C., R.J. Omang, J.A. Hull, 1983. Mean annual runoff and peak flow estimates based on channel geometry of streams in northeastern and western Montana. U.S. Geological Survey, Water-Resources Investigations Report 83-4046.

Rosgen, D.L. 1996. Applied River Morphology. Wildland Hydrology, Pagosa Springs, Colorado.
USDA Forest Service, 1995a. Inland native fish strategy environmental assessment and decision notice (INFISH). USDA Forest Service, Coeur d'Alene, ID.

USDA Forest Service, 1995b. A guide to field identification of bankfull stage in the western United States. Rocky Mountain Forest and Range Experiment Station, Stream Systems Technology Center, Fort Collins, CO (Video).

An improved Windows-based version of XSPRO, called WinXSPRO, is under development by the Stream Systems Technology Center. The software will be available for distribution later this year. Look for an announcement in a future issue of STREAM NOTES.