Pavement warping uses natural terrain in roadway design as a permanent best management practice to enhance drainage and reduce erosion from concentrated stormwater runoff.

By Christopher M. Crowley

Facility designers preparing drainage plans throughout the United States have several goals in mind when accomplishing their tasks: reducing erosion potential from concentrated stormwater runoff, enhancing drainage in low areas, protecting improvements that would otherwise be damaged by standing water, and providing the best affordable drainage solution. The techniques selected are generally a combination of channels, piping, and natural conveyances. Pavement warping (PVW) designs address sub-basin and microbasin outfalls utilizing the natural topography to significantly reduce erosion potential and the cost of infrastructure in implementing a drainage plan by elevation of concentrated stormwater flows. This case study illustrates how PVW techniques can be applied to perimeter roads, light-duty accesses, campgrounds, and trails with paved or unimproved surfaces.

At Mueller State Park near Divide, CO, PVW significantly reduced the amount of stormwater infrastructure required to complete 1.25 mi. of new camping road, 32 campsites, and parking lot areas used on a seasonal basis. Forty PVW sections were incorporated into the design to help reduce erosive forces from a two-year, 5-in./hr. storm while maintaining overall driver safety and comfort. Detailed site topography and CADD systems aided in the decision-making process of laying out road alignments, finished grade profiles, and site facilities. The project was successful at maintaining the site's aesthetic beauty while permitting access to backcountry and conserving the natural resources present.

Application

The application of the PVW technique reduces the number of culverts required for an access road. The reduction in required piping reduces construction costs, lowers maintenance costs, and partially eliminates concentrated flows, which have high erosion potential. Although not a direct replacement for storm sewer systems, the PVW technique can be used in seasonal, semipermanent, light-travel applications and as part of large drainage projects.

At Mueller State Park, culvertless drainage was used successfully in 1996 on a 1.25-mi. camping loop, which now serves 32 recreational-vehicle camping sites and 50 day-use and long-term parking slots (Figure 1). The overall site-design goal was to have the lowest number of construction zone impacts while providing the highest available aesthetic value to future campers. The major constructed feature is the roadway loop situated on the sideslope of a prominent ridge. The most intensive use of this loop is by campers and vehicles during the summer camping season (Memorial Day through Labor Day), followed by foot travel during the remainder of the fall and early winter when vehicular access is limited to maintenance activities. No public access is allowed during the early spring while elk calving occurs. This particular area has an unusual dual role as both patron area and wildlife habitat.

Situated at 9,000 ft. above sea level and receiving approximately 24 in. of precipitation per year on the western slope of Pikes Peak, the fragile ecosystem can be easily disrupted by human activity. Soils of a thin organic layer atop easily erodible decomposed granite subsoil were of grave concern. Although the annual precipitation might be considered low by some standards, substantial peaks occur in early spring during snowmelt and in summer during flashy storms. These events often have drastic effects on the easily eroded soil and required thorough consideration. The effects are compounded by the removal of vegetation through patron usage and construction activities and by a relatively short growing season, which limits replacement potential for living ground cover. Because of the short growing season, the design process included an effort to save as many of the large trees and existing ground cover as possible. The project was successful in minimizing the clearing limits and preserving the forest cover.

The approximately 15-ac. site was divided into six sub-basins, separated by a prominent ridge longitudinally and smaller hogback cross ridges. Microbasins (0.25—1.0 ac.) generating less than 1 cfs for the two-year storm occurrence were outlined on a detailed topographic map before the road and facilities were laid out. The Rational Method was employed to develop the peak flows at discrete design points.

The overall goal for the road alignment and layout was to follow the natural contour without exceeding 10% longitudinal slope and while holding a 2% cross-slope. These criteria were in response to large recreational-vehicle usage and site limitations. All of the campsites were located downstream of the proposed road alignment at the request of the seasoned onsite park staff. This was done so campers would feel they were in a more natural setting. This requirement somewhat complicated the drainage design, because the quickly draining impervious areas were upgradient of the campsites. Stormwater routing, in many cases, dictated the position of camping facilities in the landscape and helped identify those areas that would require additional erosion and runoff protection and more detailed planning.

Design

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The first step of design was to lay down the alignment, both vertically and horizontally, for the main camping loop. This loop had a 25-mph design speed with a posted speed limit of 10 mph. Several iterations were required as the typical roadway template was applied to the centerline profile to accommodate a near-balanced cut/fill situation. It became clear early on that fill material from an outside source would be necessary to avoid severe cuts in potentially erodible banks and to avoid damaging existing plant root systems. The finished grade profile made use of existing low points, draws, and drainages in the topography along the roadway. In essence, the project team "chased" grade along the existing topography in order to best fit the new road into the existing conditions.

The layout process, although time-consuming, was significantly enhanced by the use of CADD systems. After several iterations were completed, park staff and engineers agreed upon the best solution. The end result was that approximately 90% of the roadway and parking areas were laid down at or near existing grade. This necessitated removing existing topsoil and 3-7 in. of subsoil to accommodate the aggregate base and paved section. The topsoil was stockpiled for later revegetation efforts, and the subsoil materials were used in the construction of tent and RV pads for the campsites, restroom facilities, and parking areas. Select fill from a predesignated area outside the park boundaries was used for the roadway.

The typical roadway section constitutes a 2% cross-slope from edge to edge, pitching with the existing-topography sideslope. This configuration gently drains the roadway into the forest's existing topography. This design was acceptable for single-lane, single-vehicle use, although this practice can be applied to traditionally crowned sections with low design speeds. No roadside ditch was incorporated into the typical section; however, 3:1 cut/fill sideslopes were used where necessary to groom and transition the roadway section into the natural topography. By carefully analyzing detailed cross-sections of the proposed facilities, the project team achieved a significant reduction in the amount of fine grading outside the paved section.

All campsites were located in accordance with the wishes of park staff, resulting in small ramps or loops of 50-200 ft. in length that exit the main camping loop, serve a camp pad—picnic area combination, and reenter the traffic flow. Although this particular project was for campsites, these areas could very easily have been electrical, pumping, communications, water-supply, or oil-exploration/production facilities.

Pavement Warping

The actual PVW tool is a modified application of the paved swale urban stormwater best management practice (BMP). As the roadway approaches a drainage point, the vertical profile is lowered to closely match the existing drainage invert. The typical section's 2% cross-slope is gradually pitched over to a near-flat portion (0—0.5%) of 10-25 ft. in the center, allowing for any flow concentration along the trailing edge of the roadway to be slowed while it spreads across the pavement area, thus reducing its erosive capacities. Once through this flat portion, the pavement transitions back to the typical section. Overall, for drive comfort and design speed specifications, we found that 40- to 75-ft. transition-to-transition sections were required on the main roadway. The minimum vertical curve length was dictated by design speed, driver comfort, and safety considerations. The length was increased to pass a maximum two-year, 5-in./hr. storm flow of 1 in. in depth across the section, as solved by Manning's Equation. The two-year return-period event was chosen as the design storm because it was determined that the intensity and duration characteristics of this storm would be the most common during the heaviest use of the park and needed to be handled easily by the PVW design.

Design of PVWs for greater storm intensities and durations can be computed by solving for bottom width across the PVW using Manning's Equation for the necessary peak flow. Lengthening the PVW section and modifying the drainage-basin delivery area based on an anticipated storm-event yield can result in shallower storm-flow depth across the roadway. In the Mueller State Park PVW design, a maximum depth of 1 in. was arbitrarily chosen. At this site the design criteria have been successfully implemented; however, site-specific design inputs would need to be evaluated for future applications. Implementation of any BMP must be based on site reconnaissance, engineering judgment, and field experience.

In this project, the downstream side of the PVW received no specific protection, such as riprap or ditching, but instead a considerable effort was made to protect existing vegetation and match existing grade. The spreading of storm flow across the length of the PVW reduces the concentration of stormwater and relies heavily upon the natural hydrology of the forest and range soils at the outlet end to absorb the energy and moisture inputs. For this project, the trailing edge of the finished grade was designed at the existing ground elevation, creating a depression in the roadway and using the immediate area in the forest downstream as a sediment trap.

Since natural drainages were used in the roadway design to the greatest possible extent, in general the vegetation was naturally more dense and brushy than in the drier upland areas. A more developed soil structure is present in many cases, absorbing the storm flow and further increasing vigor in vegetation. These statements are true only as long as increased sedimentation is not a factor. Root systems have a hierarchy of design; the depth within the soil profile dictates the function and form of the roots. Sedimentation causes the depth ratio to change, inhibiting gas exchange and uptake strategies for moisture and nutrients. The overall approach of the project was to eliminate erosion potential, thereby reducing sedimentation as much as possible. Five years after completion of the project, we have observed no significant sediment buildup or loss of vegetation. Overall, vegetation is locally greener and thicker in the receiving areas.

On the camping turnouts, a similar typical section, averaging 25 ft. in length, was used to drain the infield areas, or "islands," cut off between the main road and the camping pullout. The pavement on the camping pullout, reversed at a 2% slope inward toward the existing topography, creates a swale against the slope that channels surface drainage from these small areas (often less than 500 ft.²) to a localized PVW drain, protecting the campsite facility downstream. Reversing the pavement inward toward the slope has the added effect of draining rainwater away from RV doorways and camping pads and helps create a more stable and level area for campers.

The vertical curves designed for these PVWs were developed iteratively through the use of the Vertical Curve Calculator within SoftDesk/AutoCAD software. Campsite PVWs were solved for length, and roadway vertical-curve solutions were design speed. The designer individually tailored each PVW to the specific site.

Other Considerations

It is easy to realize that in icy and heavy-precipitation conditions, the PVW approach might lead to a reduction in speed along the roadway. There are potential safety hazards from ice buildup or hydroplaning. For light-duty and seasonal-use accesses, the cost reduction in initial installation, piping maintenance, and repairs to eroded land are thought to offset these potential problems. Initial construction savings will need to be weighed against limited access and future maintenance during the design phase for future projects to determine applicability.

As with all tools, application is important. Should this project have crossed a wet or poor soil area, geotechnical fabrics might have been used to reinforce the pavement section. As water flow crosses pavement swale areas in poor soil conditions, moisture tends to collect in the soil under the pavement, reducing pavement service life. Reinforcing fabric can be used in the pavement design, in accordance with recommendations of a geotechnical consultant, to maintain a higher flexibility in the section and provide a binding quality that standard macadam does not possess.

On roadways that do not receive asphaltic or concrete treatments, the flat swale areas should be lengthened. Lengthening accommodates surface softening because the water runs over the gravel or soil base. Increased swale length reduces wetting depth, decreases drying time, and increases the service life of the maintained surface. Polymer adhesives that bind the surface together can be spray-applied to the groomed surface to enhance the service life of the roadway, decrease maintenance, and reduce the need for grading, which can lead to side-cast erosion.

Well-planned stormwater management is tantamount to success. Excess material from excavation and clearing operations can be used in landscape berms or as fill in shallow depressions in areas that are steep and will naturally concentrate stormwater. Erosion control BMPs–such as sediment basins, silt fencing, hay-bale barriers, and mulching–are necessary to maintain the environment, as is minimizing the total amount of disturbed area. A detailed erosion control plan was developed for the Mueller State Park project and implemented by the contractor to protect the forest ecosystem.

Prompt site restoration is critical to success with a minimal-impact approach. Permanent revegetation activities on this particular project were completed in this semiarid area within two months of the initial disturbance. Revegetation can be expensive and needs to be fully addressed in the costs of the project. Mulching with wood-fiber materials generated from clearing operations and chipped on-site were cost-effectively used to foster revegetation and enhance erosion control efforts for this project.

Developing a written set of well-defined design criteria before beginning detail design drawings benefited the overall project outcome. A detailed topographic survey is required to properly analyze the site characteristics for application of this tool. A knowledgeable grading and paving contractor, who is partner to the project, is a must in order to accurately implement the design.

Christopher Crowley is a forest hydrologist with Rocky Mountain Consultants Inc. in Longmont, CO.

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