Cross-drain Placement to Reduce Sediment Delivery from Forest Roads to Streams
Chair of the Supervisory Committee:
A model has been developed which allows a designer to place ditch relief culverts at various locations and subsequently evaluate their impact on sediment delivery to streams. The main feature of the model is its immediate feedback to the forest engineer in visual as well as quantitative form. It allows the designer to dynamically assess the sediment impacts associated with each culvert as it is placed on the road network. Sediment delivery and routing algorithms are based on accepted methodologies. Current as well as planned roads can be evaluated and the potential for improvements documented in a quantifiable and repeatable way.
The model was tested on a portion of the Tahoma State Forest, situated south of Mt. Rainier. Two existing road systems with 28 and 39 stream crossings and 82 and 86 cross drain culverts respectively, were analyzed. Interactively relocating 20 and 35 of the cross drains resulted in a three quarter reduction in sediment delivered to the stream system. The last culvert was usually placed about 100 - 200 ft of a stream according to local conditions, challenging one of the regulatory recommendations to place a cross drain within 100 ft of a stream crossing. Forest engineers and regulators now have a design tool to assess effectiveness of a cross drain system rather then simply relying on culvert spacing and count.
Road Prism Structure and Drainage Patterns
Three essential prism components determine the water flow within
the roadway: the road crown, the road ditch and the cross-drains.
The crown represents the side sloping of the road surface. Its
main role is to disperse water laterally away from the tread and
prevent harmful flow routing along the travel surface. The side
ditch, when present, collects and routes water longitudinally along
the road alignment towards the nearest stream crossing or cross-drain
structure. The cross drains are routing elements that empty the
side ditch and redirect its accumulated water across and away from
the road prism, onto the side slope, where it will reenter the
natural flow regime. Frequently, cross drains are implemented by
drainage pipes built into the road bed but other, less common implementations
exist as well (Figure 1).
According to their drainage configuration forest roads can be classified in the following categories: insloped with a ditch, crowned with a ditch, outsloped with ditch and outsloped with no ditch (Figure 2). Each of these types generates different drainage patterns and impacts the original watershed drainage accordingly.
The insloped roads with a side ditch are a common occurrence among forest roads due in part to their ease of maintenance and increased traffic security. The drainage system of an insloped road involves a side ditch, cross drains and stream crossings. The surface of the road, being sloped inwards toward the cut slope, reroutes all water it captures to the side ditch. Consequently, the surface runoff intercepted by the cut bank is concentrated with the rerouted road capture. This can potentially lead to the accumulation of high volumes of water in the ditch. If no cross drains are present this water spills out at the end of the ditch, often directly into a stream at the nearest stream crossing. In cases where the road grade is relatively high, the energy of the moving water can reach potentially destructive levels. Cross drains are placed along the road alignment at various locations to empty the side ditch and reduce the possibility of water induced road and environmental problems.
At the opposite pole of road prism drainage types are the outsloped roads with no side ditch. The outlsoped roads with no ditch disperse all the water they capture on their fill slope. In the absence of a side ditch, concentration and rerouting of large volumes of water does not take place. As the energy of the flowing water is kept to lower levels, its damaging potential is also reduced. Moreover, the lack of direct ditch drainage into a stream network induces less hydrologic disturbance. Harmful processes of erosion and sedimentation generated by moving water are minimal. From an environmental standpoint, the outsloped road type represents a more suitable design. However, traffic security concerns raised by log trucks and heavy equipment, critically restrict its usability. Outsloping is normally applied to minor roads and spurs.
Other intermediary road types inheriting physical characteristics from both the insloped and outlsoped roads do exist (Figure 2). Their hydrologic impacts on the environment are closer to one or the other base types described above, with variations dictated by their elemental differences.
Sediment Production and Delivery Mechanisms
Sediment production refers to the physical generation of sediment, the process of detaching soil and parent material particles under the influence of various agents. Different premises govern the generation of sediment on each individual road component. The cut and fill slopes are subject to surface erosion occurring when detachable soils are exposed to erosion factors such as: overland flow, raindrop splash, freeze-thaw, dry ravel, and other biogenic processes (WA Forest Practices Board 1997). The litter cover, typically present in a forested environment, protects the soil against all these factors by dissipating erosive energy before it reaches the surface. Road construction operations however, expose the soils on the cut and fill banks, increasing the potential for particle detachment. New roads tend to generate significant quantities of sediment for the first few years of their existence but in time, as side slopes re-vegetate, the sediment production drops. The road’s running surface is another major sediment producer. Traffic is the destructive agent acting upon the surface material. The action of tires against the road surface will grind and dislocate particles into smaller units that can eventually be carried away by other agents like water and wind. Some researches consider traffic to be the single most essential factor influencing sediment generation, capable of increasing sediment amounts by one order of magnitude or more (Reid and Dunne 1984). The side ditch can also be considered as a source of sediment where the erosion is caused by moving water. In the cases where water gains sufficient energy to overcome the soil’s sheer strength, particle detachment is observed.
Sediment delivery refers here to the transportation of sediment generated by the road prism into neighboring stream networks. Fine sediment travels in suspension, carried by overland flowing water along its paths towards the river system. As an active component of watershed hydrology, the road drainage has a fundamental impact on the sediment delivery process. By determining the local water flow within and about the roadway, the road drainage implicitly controls the sediment routing (Section: Road Crowning and Drainage Patterns). The various road drainage configurations in conjunction with the actual road location drastically affect the amount of sediment reaching the stream. For example, in the cases of roads that have numerous water crossings, the presence of a side ditch establishes a direct connectivity between the road drainage and the stream network. The side ditch typically empties at stream crossings, unloading all accumulated sediment into the water. This configuration results in a high potential for stream sedimentation. Other high delivery scenarios include valley bottom roads that run parallel and within close distance to a stream. Large quantities of sediment can reach the stream laterally through surface runoff and water diverted from existing cross drain culverts. On contrast, roads with an outsloped surface geometry and ridge top roads located far away from any streams, have minimal impacts on sedimentation and can be considered as disconnected from the natural watershed drainage.
An important aspect when examining overland delivery on lateral slopes is the site’s filtering potential. Through filtering, parts of the sediment produced by the road are deposited before they have a chance to enter the stream network. Filtering is a complex process, fundamentally based on energy loss and infiltration. The carrying water’s transport capacity is proportional to its velocity. By reducing water velocity soil particles are dropped from suspension and settle down. The porosity of the soil surface influences the infiltration rates and further contributes to the deposition and absorption of fine sediment. In practice, a reduction of sediment delivery is obtained by diverting the water onto vegetated side slopes where the litter layer, plants, and gentle slopes can slow down the surface runoff enough to trigger these filtering effects. Various researchers have shown that the nature of the parent material and the local micro-topographical features have a major influence on the distance the sediment can travel downhill on a side slope. In some extreme cases these distances can be fairly significant (Ketcheson and Megahan 1996), but usually the filtering effects are seen within the first 200 ft from the road (WA Forest Practices Board 1997).
Current Management Techniques for Reducing Sediment Impacts from
A large part of these techniques address the sedimentation at the production end, acting directly onto the sediment producing factors:
Other methods are centered on the actual sediment transport, designed to decrease sediment impacts by obstructing the physical flow of sediment into streams:
Cross Drain Systems and Sediment Reduction
Designing cross drain systems to meet this new functionality asks for an analysis of the potential amount of sediment delivered by each culvert. In order to reduce sediment delivery, a cross drain must divert the sediment-laden water from the road ditch onto the side slope where it can be dispersed and filtered prior to reaching a stream (Figure 3).
The filtering capabilities of a side slopes vary with location
as micro-topography and vegetation conditions fluctuate (Section:
Sediment Production and Delivery Mechanisms). As sediment
accumulates continuously along the stretches of road between culverts,
amount available at each location is directly affected by the cross
drain spacing. An effective reduction of the sediment delivery
from a road network requires a certain number of culverts strategically
placed to take advantage of local filtering capabilities (Figure
4). It is important to note that these locations may not always
coincide with optimal locations for prism drainage but are not
mutually exclusive. When the amount of sediment available for delivery
at a particular culvert location exceeds the lateral slope’s
filtering potential sediment could still reach the stream. These
cases are often met
where a road is located too close to the valley bottom or a cross
drain is placed too close to a stream crossing.
Policy and Design Restrictions
Cross drain spacing is seen as an essential aspect of sedimentation reduction. Generally, cross drains are to be spaced at regular intervals along the road. The distance between culverts is provided as a function of road grade, side slope, average distance above streams, road surface condition and use, precipitation, and soil erosion potential. The rules also specify that the distance between a stream crossing and the first upslope cross drain is important to the volume of sediment delivered and recommend that a culvert should be installed 50 to 100 feet above all stream crossings (WA Forest Practices Board Manual 2000).
Even though the site filtering potential is not explicitly referenced by the current regulation, the cases where culverts are too close to a stream crossing and diverted sediment could still reach the stream network are recognized. The manual recommends either avoidance of such situations or implementation of additional measures such as sediment traps or ponds, rock armored ditches, and vegetated ditches.
A sufficient number of cross drains should be installed in order to prevent ditch scour, over flowing cross drain capacity or erosion at cross drain outlets. The manual requires designers to make use of natural swales that the road crosses in order to avoid rerouting water along the ditch, where it can pick up and transport sediment.
Using these guidelines makes it possible for an experienced professional to design a functional cross drain system. However, one potential draw back of spacing cross drains at regular intervals is that the site filtering potential might not be fully exploited. In areas highly susceptible to sedimentation, a non-uniform spacing of culverts to take advantage of local terrain and lateral sediment retention capabilities might be more suitable. For a more rigorous analysis of the sediment production and delivery from forest roads some specific software tools exist.
Sediment Modeling and Existing Tools
The Water Erosion Prediction Project (WEPP) is a simulation program, originally developed for agricultural purposes as a replacement for the Universal Soil Loss Equation (Elliot et al. 1999b). It is a complex program that models the processes that lead to erosion including infiltration and runoff, soil detachment, transport and deposition, plant growth and residue decomposition. WEPP works with a given slope profile and runs simulations over a specified period of time under a multitude of customizable parameters. The Forest Service Moscow Lab has developed a set of specialized interfaces in order to simplify WEPP use for erosion and sediment delivery from forest roads.
X-DRAIN is a basic interface to accessing predicted sediment yields from over 130,000 WEPP simulations ran by soil erosion specialists (Elliot et al. 1999a). It was developed to simplify and speed up the application of WEPP for simple forest road settings. The end user has limited control over climate, soil, side slope and distance to streams parameters. Road geometry and distance to streams are assumed uniform along the analyzed road segment. The total sediment yield in lb/year is presented on a tabular form for a fixed number of combinations of road gradient and cross drain spacing values.
WEPP:Road is meant to be a more refined sediment modeler capable of modeling one road segment at the time. As inputs, it accepts road surface information and a customizable climate description. The modeling can be done over a user defined period of time (Figure 5). The sediment yield to the stream network in lb is reported as a single number together with the additional average precipitation, runoff, and the amount of sediment leaving the eroding portion of the road prism. There is also the option of an abbreviated hillslope output presenting a distribution of erosion and deposition, the presence of a sediment plume in the forest, and the particle size distribution of sediment delivered to the channel (Elliot et al. 1999c).
One major limitation of the WEPP based programs is that they are spatially non-explicit and thus incapable of distinctly placing the sedimentation processes within a road network. Other organizations have approached this problem within a more spatially aware context using Geographic Information Systems (GIS) as a base for their analysis. GIS have become a standard in environmental modeling. Their capacity of modeling overland flow is what makes them indispensable to spatially distributed phenomena involving streams and water routing.
SEDMODL is a GIS based, road erosion and delivery model developed by Boise Cascade Corporation in cooperation with the National Council on Air and Stream Improvement. The model identifies road segments with a high potential for delivering sediment to streams in a given watershed. It uses spatial information to determine the proximity of the roads to the stream network. Sediment delivery is then calculated for the roads that drain to streams using methods derived from the Washington Department of Natural Resources Standard Method for Conducting Watershed Analysis and WEPP. The program is designed as a flexible, multipurpose tool that can be used both for screening purposes or a more detailed sediment analysis. For more reliable results a set of specific road attributes is required. They can include: road use, surface type, road width, construction year, cutslope height, road geometry type, and road gradient. If culvert locations are known they can be inputted as GIS layer and will affect sediment computations accordingly (National Center for Air and Stream Improvement 2002).
The Washington Road Surface Erosion Model (WARSEM) is new software from Washington Department of Natural Resources intended as a long term road management planning tool. It can model sedimentation and drainage at four different levels from the broad basin scope to the individual road segment. An increasingly complex amount of road information is required with each superior level. The model stresses out the importance of accurate, field verified input data towards a successful sediment budget. The model is implemented as an Access database without a spatial component. SEDMODEL2 results can be imported to generate performance metrics in a long term best management practices analysis (Watershed GeoDynamic et al. 2003).
Need for a Specialized Cross Drain Design Tool
There is presently no methodology that allows a road engineer
to quantify the effects of varying culvert spacing or selecting
specific culvert locations, on sediment delivery to streams. The
absence of such a design tool makes it more difficult to take culvert
spacing into account as an effective solution for reducing sediment
impacts from forest roads.
The Cut-Off Culvert
The volumes of overland delivery (OD) from culvert C after filtering and direct delivery from ditch (DD) are represented by the two colored areas. The total sediment delivered is given by the summation of these two areas and fluctuates with culvert placement. Assume that the amount of sediment produced by the road prism is constant in all three cases, the side slope has uniform filtering capabilities and the lateral filtering is proportional with the distance the sediment must travel downhill this side slope. It can be noticed that by placing culvert C closer to the stream intersection increases its lateral delivery potential while it decreases the direct delivery from ditch (Figure 6b). Conversely moving culvert C away from the intersection gives more filtering power but also leaves more contributing area for direct delivery (Figure 6c). The question of ideal placement of a cut-off culvert becomes a question of maximizing the filtered sediment (ND) at the expense of direct delivery (DD) and overland delivery (OD).
of Sediment Delivery for a Simple Case
The water dispersed by the culvert in question usually travels towards the stream on a sinuous path following the local topography (flow distance). For simplification purposes assume that the side slope’s topography is uniformly flat and this flow path is identical to a straight perpendicular to the stream (Euclidian distance). The generic term: overland distance (o) refers here to the Euclidian distance between culvert and stream (Figure 7).
The sediment filtering potential expressed here as the proportion of total available sediment delivered to stream (F), varies with the overland distance (o) as well as other site-specific factors like: soil types, local gradient and vegetation cover. Because the overland distance (o) is in this case proportional with the direct distance (d) it can be stated that the filtering potential (F) is also proportional with the direct distance (d). By expressing F as a function of d the total sediment delivered to the stream can be budgeted with Equation 1:
K = sediment production rate (kg/unit length of road prism)
L = total road length
F = fraction of sediment delivered (non-dimensional)
The minimum delivery T is obtained when:
The optimal location for cut-off culvert C can be obtained by solving differential Equation 2 for d. It can be noticed that the optimal location of C is not dependent on the sediment production rate K but only on the road-stream geometry and the local filtering characteristics.
In actuality, absolute deterministic models of side slope filtering
potential to take into account every combination of factors encountered
on forested terrain are not currently available. As the fraction
of sediment delivered F cannot easily be determined a completely
analytical approach to culvert optimization is not feasible.
By plugging equation 4 into the sediment budget equation the total sediment delivered for our case scenario can be plotted as a function of the culvert location (Figure 8). A family of curves is shown for various sediment production rates. A minimum delivery is obtained with culvert C at 50 meters away from the stream intersection all across these different sediment production regimes. It is important to note that empirically derived equation 4 cannot be universally applied. However, assuming that in most cases overland sediment delivery follows analogous invert exponential distributions (Burroughs and King 1989) it is possible to effectively approximate near optimal locations for culverts.
Similar approaches have already been incorporated into some more advanced sediment modeling programs. Therefore in order to further investigate the optimal placing of a cut-off culvert, the F.S. WEPP:Road interface was called upon to provide the modeling for the sediment production and delivery for the proposed case scenario. An experiment was conducted where culvert C from the set-up in Figure 7 was incrementally moved along the road segment. The sediment impacts associated with each of these locations were quantified by WEPP simulations run at the Forest Service web site. The sediment production parameters were: 4m wide, insloped bare ditch road, 5m long fill slope at 50% gradient and silt loam soils. Olympia Station described the local climate. The forested buffer had a uniform 25% slope. All simulations were performed for a 1-year period. Multiple runs were completed for various stream crossing angles α . Figure 9 presents a graph of the results of this experiment. The total sediment delivered exhibits a similar behavior to the Ketcheson and Megahan approach presented in Figure 8. It follows a right skewed distribution with a minimum in the first 1/3 of the road segment. The optimal culvert location for this particular case is located 50-60 m away from the stream crossing. The geometry of the road-stream intersection has a major impact on the filtering distance and implicitly affects the optimal cross drain location.
Exploring Optimization of Cross Drain Systems
Elaborating on the simple geometry single culvert case scenario presented above, the question of further minimization of sediment delivery with the introduction of additional culverts was explored. Another experiment was conducted where two adjacent culverts were moved away and toward each other in an attempt to obtain a reduction in sediment delivery. The setup for this experiment is seen in Figure 10. Note that the essential simplification of flow path distance being identical to the Euclidian distance was maintained from the previous case (Section: Minimization of Sediment Delivery for a Simple Case).
The total sediment delivered was recorded again using WEPP:Road. All WEPP sediment-modeling parameters set in the previous experiment (one culvert scenario) were kept unchanged. The stream intersection angle ? was set at 45?. Starting with the cut-off Culvert1 in the optimal position previously determined at 60 m from the stream, the total sediment delivered was computed with Culvert 1 and Culvert 2 at several different locations. Due to the high number of possible combinations of 10m increments over a 150 m road segment only a few were fully explored. Table 1 present the results of this experiment. The sediment delivery to stream S was reduced by as much as 58 % with the cross drains at 30 and 90 m away from the stream crossing as compared to the case of a single cut-off culvert in optimal location. Other possible combinations may further improve this result. It can be inferred that the addition of a third culvert at a key location could decrease sedimentation even more.
Full optimization of cross drain systems becomes more complex with the addition of each new culvert but the importance of the cut-off culverts closest to the stream crossing cannot be overstated. In reality, departures from the simple triangular geometry assumed here, further complicates this analysis. As road alignments and stream paths curve and weave randomly, the road’s proximity to the stream and implicitly the side slope’s filtering potential become more difficult to model analytically (Figure 11). The flow path of the water dispersed by the cross drain follows the local micro-topography and can be significantly different from the straight line distance between culvert and valley bottom. Moreover sediment generating factors, vegetation and filtering characteristics vary along the road alignment and side slope respectively adding further complexity to sediment delivery modeling.
In addition to their stream crossings areas, roads are also susceptible to sediment delivery in all cases where they run parallel, in close proximity to streams (e.g. valley bottom roads). Whenever the volume of sediment diverted onto the side slope exceeds the local filtering potential, sediment delivery will occur. The process of finding best culvert locations in these situations follows similar analysis principles with the evident omission of the direct delivery from ditch component.
Culvert Location Analysis for Design Purposes
Interactive Culvert Placement
The major difference between the two cases is given by the feedback the user receives during design, as a validation of a placement decision. This feedback is essential to producing a good solution as users can quickly improve upon each proposed step and get closer to an optimum cross drain setup.
CULSED- A Decision Support Tool for Cross Drain System Design
CULSED is implemented as an ArcGIS extension that seamlessly integrates with the standard ArcGIS package being able to access all the existing functionality and providing a familiar interface and ease of use. Running CULSED requires at the minimum a GIS road layer, a stream layer, and a digital elevation model. Additional information such as: road surface, road age, road grade, soil and parent material and side slope vegetation cover may also be used during the sediment modeling stages. Appendix A contains a more detailed explanation of this software’s capabilities.
Modeling Cross Drain Systems with CULSED
The controller module’s role is to provide user input to various operations related to the culvert analysis. It essentially drives the activities of other modules that require user interaction such as: generation of network topology, cross drain movement and description of sediment producing factors. The controller module is a part of the ArcMap user interface on which CULSED is developed.
The view module is also part of this interface and is represented by the standard ArcMap graphical output. It is at this level where the results are presented to the user for evaluation and decision support. This module receives information from the ditch model, dynamically rendering all changes made by the end user.
At the core of CULSED is a suite of three modeler modules: the Road Ditch Model, Sediment Production Model and Side Slope Filtering Model. The Road Ditch Model is a simplified representation of the road’s drain ditch, modeling the flow of water along it. This module makes the following assumptions:
The road ditch is described by the interplay of a geometric network and its associated logical network. The geometric network stores the physical location and geometry of the roads alignments, stream crossings and cross drain culverts together with their geometric connectivity. These network components are the elements that the end user sees and interacts with on the computer screen.
The logical network portrays the sediment flow within the system,
storing the directionality of water movement. It is conceptually
similar to a generic graph, being composed of interconnected edges
and nodes. There is a direct correspondence between the elements
of the logical network and the geometric network. An edge is associated
here to a one dimensional stretch of road of uniform sediment producing
characteristics. Edges are sources of the sediment that flows along
them. A node is the abstract representation of a connection point
between edges. Culverts are always node points and play the role
of sinks within this architecture. Sinks capture all physical flow
routed to them. From a topological perspective each edge can be
characterized by parents, children, sinks and a flow direction
(Figure 15). A parent is a connected ditch segment located directly
upstream on the flow path, routing its sediment-laden water into
the current segment. For implementation reasons the maximum number
of parents is restricted to 4. Similarly a child is a connected
ditch segment downstream along the flow path, receiving sediment-laden
water from the current segment. Because in reality ditch water
is never split onto multiple roads at intersections, the model
only allows one child per segment. The presence of a sink at the
end of an edge implies no children connectivity as in reality a
cross drain intercepts and re-routs all the water it captures.
The Sediment Production Module used by CULSED follows the methodology outlined in the Washington Department of Natural Resources Standard Method for Conducting Watershed Analysis to compute the amount of sediment generated by the road prism (Appendix A). However, in order to provide the means for other user-implemented sediment modelers to be used with the program, this module was developed on top of a generic sediment production framework. The framework enforces a certain common functionality needed by various methodologies to work together with the other program components (Appendix B). Its main function is to calculate the sediment produced by the road prism on a segment by segment basis according to their specific sediment producing parameters. These parameters have to be associated to each road segment prior to running the analysis by the end-users unless default parameters are used.
The third module in the modeler suite, the side slope filtering module, determines the sediment filtering potential associated with each potential culvert location in road the network. It is based on the work of Ketcheson and Megahan of USDA Forest Service describing the sediment deposition on a vegetated side slope as a function of proximity to streams (Ketcheson and Megahan 1996). The module computes the proportion of sediment that can reach the stream at any given distance along a vegetated side slope (Equation 3). The proximity to the stream is based on the physical flow path distance from the culvert to the nearest stream generated from a digital elevation model.
The sediment delivered by each culvert is calculated in response to various user triggered events as the sum of the sediment produced by all contributing road segments multiplied by the filtering potential at that particular location (Equation 4).
In order to proof the concept of the cut-off culvert and demonstrate its applicability in a real world case scenario, several road settings were examined within the context of a forest harvest plan. The goal of these experiments was to reduce the total sedimentation from an existing road network by redesigning its cross drain system to make better use of sediment dispersion and filtering potential on vegetated side slopes. As the cost of the resulting drainage system had not to exceed the original cost, the total number of cross drains could not be increased. CULSED was used to compute the amount of sediment delivered by the entire road network at each design alternative during the design process.
North Tahoma Planning Area
Original Sediment Delivery
All existing culvert locations were used as a starting point in
our sedimentation analysis.
Design Process Example
Sediment Reduction by Cross Drain System Redesign
The original culvert configuration for the East half of North Tahoma consisted of 39 stream crossings, 22 unmovable drainage culverts and 64 cut-off culverts. This configuration yielded a total of 42.10 tons/year of sediment to the stream network. Acting gradually upon the greatest sediment contributors, relocating the cut-off culverts involved at each of these road settings, we were able to achieve a substantial reduction in sediment delivery. After repositioning approximately 35 cut-off culverts, the total sedimentation of the East area was reduced to 10.04 tons/year, a 76 % percent drop from the original delivery. Figure 20 displays a graphic representation of the final sedimentation. Comparing the relative size of the proportional symbols with the original configuration (Figure 17) the sediment reduction becomes obvious.
An analogous design process was conducted for the West part of
North Tahoma. The original cross drain system containing 28 stream
crossings, 27 drainage culverts and 55 cut-off cross drains, was
potentially delivering 25.01 tons of sediment / year. Redesigning
this system involved relocation of approximately 20 cross drains.
The final sediment delivery was dropped to 6.33 tons/ year, achieving
a 74 % reduction from the initial amount (Figure 21). The graphic
quantification of this improvement can be easily noticed when contrasted
with the original setup (Figure 18). The total improvement for
the entire North Tahoma planning area can thus be quantified at
approximately 75 % decrease in sedimentation. A number of 55 cross
drains have been moved to new locations. No new culverts have been
introduced in the system.
The average distance from the first cut-off culvert to the stream crossing at the end of the design process was 140 ft. A minimum of 55, maximum of 289 and a standard deviation of 59 ft were recorded. These results contrast with the Forest Practices Board recommendation of placing the first culvert within 50 – 100 ft of a stream crossing. The wider range of values produced by CULSED stems from the variability of local conditions: sediment producing factors and delivery potential, characteristic to each road location. The interplay of the direct delivery from ditch and overland delivery from cross drain is what determines the amount of sediment reaching the streams in the near vicinity of a stream crossing. Although general guidelines can be successfully applied in certain average situations, cross-drain location design is best approached on individual bases.
One important aspect to mention is the scope of the analysis carried above. Given that an absolute optimal location is impractical to obtain through experimentation (especially when dealing with a high number of culverts over a large area) a three quarter reduction of sedimentation was considered satisfactory. Further culvert manipulation could lower sedimentation even more but major improvements should not be expected.
The benefits of the cut-off culvert are more apparent in regions with steep, fragmented topography, where higher amounts of sediment are produced and transported. Flat areas with low grade roads and few stream crossings do not gain from a complex culvert analysis. Furthermore, modeling flow patterns in these flat areas presents a challenge for the current GIS algorithms, yielding unreliable results.
A particular GIS problem affecting CULSED analysis is the modeling of the flow-path and generation of streams from a digital elevation model. The raster resolution strongly impacts the outcome of this analysis. The cell spacing determines the minimum increment of the flow path distance measurement and implicitly influences the number of valid culvert locations that can be evaluated along a road segment. Analyzing short road segments with wide cell spacing can render this technique impractical. The standard 10m and 30m DEM can only be used for smaller scale analysis with a larger tolerance for error.
Specific issues related to computer modeling of the cut-off culvert concept currently restrict its wide scale usability. CULSED was designed for insloped roads with a side ditch. The ditch model assumes ditch continuity along all roads segments. A road network containing road types where the side ditch may not be present would be misinterpreted as its sediment flows would erroneously be simulated.
Sediment production and delivery potential associated with each culvert are computed with empirical models derived for particular conditions in the Northwest of the United States (WA Forest Practices Board 1997, Ketchesson and Megahan 1996). These models were meant to operate on relatively large scales and adjust poorly to the micromanaging imposed by a culvert by culvert analysis. Their absolute results may not always reflect the reality of all case scenarios met in road design. Overestimation of sediment production seemed characteristic for the North Tahoma planning area. Nevertheless, as culvert locations are evaluated on a relative scale, these figures can serve as a basis for comparison and decision support.
Importance of Better Models
Another aspect of the current method that could benefit from improvements is the modeling of the sediment delivery process from a cross drain to the nearest stream. A deterministic model of water infiltration through the forest soil has the potential to increase the accuracy of our sediment filtering prediction. Coupled with the average magnitude of water flows outwards from a specific culvert, such a model would quantify sediment deposition over distance to stream. Local terrain conditions influencing this process could serve as inputs. This model would be easily incorporated into the present GIS based analysis routines which best represent spatially distributed phenomena on a fixed point in time.
A common occurrence in GIS environmental analysis also employed
by CULSED is the use of raster elevation models (DEM) to describe
terrain features. They are in essence a discrete pixel representation
of the ground topography. In terms of pixel resolution and generating
method, various DEM standards exist but their ability to capture
topographic details varies. When performing a detailed analysis
as required by cross-drain modeling, the topographic expression
is critical to the accuracy of the results. Natural terrain features
such as small stream valleys, draws, swells and other low spots
influence local hydrology, road layout and implicitly sedimentation.
More particularly, in a computer environment they affect the modeling
of the flow path, at the base of the cross drain design process.
Recently, high resolution DEM, a new standard in terrain modeling
have been introduced. Their ability to reveal a lot of the micro-topography
makes them suitable for cross drain analysis. The North Tahoma
Redesign Project has been carried out on a 6 ft high resolution
DEM. A shaded relief of this model clearly illustrates the micro-topographical
detail included in that analysis (Figure 22). Although new, this
kind of data is rapidly becoming available at lower costs. Organizations
such as Puget Sound LIDAR Consortium are developing datasets of
large extents. As this technology matures and turns more accessible,
the usability of tools like CULSED will increase.
As with most other computer-modeling of natural phenomena, cross drain modeling needs to be validated by field verifications. Since the local conditions at various culvert locations could be complex, input parameters to the model could be erroneous or only partially descriptive. Therefore field inspections are required in order to asses both the validity of the input factors and final outcomes at project completion. The North Tahoma Project has benefited from input validation performed by University of Washington forest engineering students, class of 2003. However, as this project was investigational and will not be implemented in its current form, no further attempts of validation have been made. A future expansion of this project might take into consideration field visits and/or a monitoring program to culvert sites in the event of a possible commission.
The CULSED toolbar is composed of 9 tools and 6 menu items that operate on the existing data and control the flow of the culvert analysis session. A small window on the toolbar presents the total amount of sediment delivered by the analyzed road network at each step during analysis. The sediment volume is expressed in tons / year.
All CULSED operations must be performed within an analysis session. The session steps must be performed in order. If the session is not completed in one sitting the user can leave it open in order to be stored with the ArcMap project. When the session is stopped all progress is cleared and all internal variables reinitialized.
Note: the Spatial Analyst extension must be installed for CULSED to function properly.
CULSED requires the following layers: a digital elevation model, a roads layer, a culvert point layer and a stream layer.
Notes: for best results the digital elevation model must be free of sinks and the stream layer must align with it. One convenient method for generating streams from a digital elevation model is the minimum contributing area method.
CULSED assumes that each road segment presents a consistent combination
of sediment production factors (e.g. road grade, road width, surface
material etc). At this stage in the analysis the user must examine
the road segments and ensure that this condition is met.
Note: to properly estimate the road grade the vertical units of
the DEM must be the same with the horizontal units. If they differ
the user must correct the grade values in the attribute table appropriately.
This tool changes the grade of a road segment. Click on a segment to display grade. Type a new value and right click to commit the change.
This tool simultaneously changes the grade of all road segments at a certain intersection. Left click to increase grade. Right click to decrease grade.
This tool changes the flow direction along a road segment. The flow direction along the side ditch is represented with arrows. Use this tool when the DEM estimated grade is incorrect.
This tool splits a road segment in two parts. All attributes are copied onto the newly created segments.
This tool merges two road segments into one. The road grade is taken from one of the segments. When more than two segments meet at an intersection click the intersection with the left button and while keeping the button depressed press the right button to select which segments will get merged.
This command enforces geometric consistency, eliminating duplicate segments and forcing node connectivity (simplifies geometries) to ensure proper topology. Make sure to use this command when you are done with all edits and are ready to proceed to next step.
In certain cases, the network topology algorithms cannot automatically identify the parent-child relationships needed for modeling the water travel. A list of these cases is presented to the user during flow set-up. The user must click on each item on this list and using the ArcMap selection tool direct the water on its way to the next ditch segment (Figure 27).
The default sediment model provided with this version of CULSED
follows the procedures in the WA DNR Manual for Conducting Watershed
Analysis. The following sediment production parameters can be specified
as road attributes and are associated to each segment: age, grade,
width, surface material traffic and side slope cover. Other parameters
such as precipitation and parent material are considered uniform
over the entire study area (Figure 28). If no attributes fields
are specified, a set of default road characteristics are applied.
The can be viewed and modified in the option menu.
A minimum number of contributing cells is required for computing the flow paths to the nearest streams. A raster layer of streams is generated during this process. For accurate results this number should produce a stream layer identical to the one used in the input section.
The flow modeling section is useful when working with high resolution elevation models. A typical problem when modeling water flow in these cases is the stream capture by the road ditch. To reduce the stream capturing effects the elevation models can be smoothed with a circular neighborhood of given radius.
The maximum sediment travel distance represents is a generic number that influences the sediment deposition factor used for calculating a probability of sediment delivery. This number is particular to local conditions and should be based on empirical observations at the site. The user’s expertise is important for obtaining valid results.
Culvert Operation Tools
This tool inserts a new cross drain in the road drainage system.
This tool moves a cross drain culvert to a different location.
This tool removes a cross drain from the road drainage system.
A new ArcMap extension must be developed in order to use a different sediment model. This extension must be written in Visual Basic and must implement the ISedimentModel interface provided with the CULSED code (Appendix B). This interface specifies the methods necessary for CULSED to be able to integrate with a sediment model.
After inputting the required layers at the start menu the user must proceed to set up the road geometry (if needed) by estimating and adjusting grades and inspecting road attributes that drive sediment production (Figure 30). At this stage it is helpful to display the road grade associated to each segment.
The next step is to generate the road network topology. The user
may be asked for sediment routing information in cases where two
or more possible flow paths exist. Upon successful completion the
road network is ready to be analyzed for sedimentation (Figure
To take advantage of the intended graphic comparison the used must draw culverts with proportional symbols based on the field called “SED” in the culvert layer’s attribute table. If these values are spread over a wide range it is helpful to “stratify” the analysis and start by representing only the big contributors. As sedimentation is reduced by moving culverts to different locations culverts in the lower sediment ranges can also be included (Figure 32). Most of the sedimentation will be located near the stream crossings and it is these locations where users can make the greatest improvements to their drainage systems.