History of Road-building Techniques

Road techniques being promoted today are nothing new. Pre W.W.II, rolling dips installed on outsloped roads that followed the topography of the land were not uncommon because the heavy equipment and pipes were expensive or not available. Post W.W.II large earth moving equipment facilitated the construction of flat wide insloped and crowned roads. Crossings were often constructed with large fills and corrugated metal pipes. Roads connected to streams via inside ditches, which often emptied into culvert inlets contributing sediment and more water than the channel had evolved to handle causing accelerated erosion.

Why do we reshape roads?

Insloped roads are often drained by cross drainage culverts, which are notorious for being infrequently spaced and concentrating water on slopes causing accelerated erosion, contributing sediment to the watershed. Flat wide roads can make travel quicker and sometimes safer. These goals continue to drive projects for main transportation networks that support high volumes of traffic. However, more recently, additional goals drive the design of transportation networks, which are: 1) reducing the chance of sediment delivery as a result of episodic events; and 2) reducing the chronic delivery of sediment and to reduce maintenance.

Workshops and Demonstration Projects

In an effort to incorporate these goals into road design, the State Coastal Conservancy funded for the second year a road workshop for heavy equipment operators. The RCD and NWWG co sponsored the workshop facilitated by Danny Hagans of Pacific Watershed Associates. 27 workshop attendees participated in this year's workshop. A section of improved Hungry Hollow Road, Nash Mill Road and Holmes Ranch Road were visited in the Mill Creek watershed. The work on Hungry Hollow Road and Nash Mill Road was completed by Dean Titus. The work on Holmes Ranch Road was done by Wayne Hiatt.

Storm-proofing roads

Well-designed crossings, capable of passing the discharge and LWD associated with a one hundred-year storm, will reduce the chance of sediment delivery because of episodic events. Disconnecting roads from streams will reduce the chronic delivery of sediment. Danny suggests that 80 to 90% of roads can be disconnected from the streams. Disconnecting roads from streams is the opposite of collecting and concentrating road drainage. During the workshop, two road sections, on which different improvement techniques were employed, were visited.

Case Study: Hungry Hollow Road Project

Gully created by flow from first cross drain culvert on Hungry Hollow Road

The Hungry Hollow section of road was improved from an insloped road, with infrequent cross drains , and poorly designed undersized culverts at stream crossings to an outsloped road with frequent rolling dips and culverts at stream crossings designed for a 100 year flood flow. To address the issue of safety and to improve drivability, little to no outslope was incorporated into the road on outside curves, a heavy outslope was installed on inside curves and straight stretches were outsloped normally. Rolling dips augmented the drainage provided by the outslope and were installed on straight stretches to avoid the unsavory driving condition created when a rolling dip is located on a curve. Culverts at stream crossings were designed for a 100 year flood flow. On the smaller diameter culverts a fence post was placed upstream to align woody debris before it entered the culvert. The post was placed upstream from the mouth of the culvert a distance equal to that of the diameter of the culvert sunken deep into the ground. Sturdy fence posts serve as an effective trash rack on culverts less than 36", above which a sturdier setup should be investigated.

48 inch culvert, the fencepost is on the inadequate side for this large of a culvert

Case Study: Nash Mill Road Project

The improvements made on the Nash Mill section of road also succeeded in reducing the amount of water collected and concentrated. Before improvement, infrequent cross drain culverts drained the wide crowned (meaning water flows to the outside of the road and the inside of the road) road. Culverts at crossings were poorly designed, undersized and some were reaching the end of their life expectancy. Instead of changing the shape of the road to be outsloped and installing rolling dips the shape of the road was maintained, frequent cross drain culverts were installed and the large outside berm was breached. The Nash Mill project stakeholders ultimately decided on this scenario, even though it requires more maintenance then the scenario on Hungry Hollow Road because the inlets of the culverts must be cleaned prior to each rainy season. Similar to Hungry Hollow, structures at stream crossings were designed for a 100-year flood flow. Culverts were installed at all of the tributary stream crossings. At the main Mill Creek crossing, a bridge was installed (show picture of bridge) because the stream is fish bearing and the size of the replacement culvert would have been greater than 60".

Mill Creek Bridge Crossing

Determining Culvert Size

The Forest and Ranch Roads Handbook offers methods to determine culvert size. A rough field culvert size determination method is to estimate the cross sectional area of the channel during a 100 year event, outside of the zone of influence of the road, divide by 3.14 and take the square root of the result. The final number will be the radius of the culvert. Increase the culvert size by a minimum of one step to account for error. Passage of Large Woody Debris(LWD) should also be incorporated into the final culvert size. In grassland, LWD is a non-issue. However, in forestland, it may play a significant role. If the size of the pipe required is greater than 60", especially if the stream is fish bearing, the cost of installing a bridge becomes competitive.

Project Timing

Project timing is critical to protect fish and downstream water quality. On the Northern California Coast, the preferred work window in fish-bearing streams is between June 15 and October 15. For non fish-bearing streams the preferred work window is when the creek is dry. The best time to shape the road is when the road is not rutted by heavy equipment but, not so dry that the road turns to powder when worked. Sometimes this scenario is unavoidable and the road will need to be watered.

Rock-Armored Crossings

Rock Armored Fill Crossing

A rock armored fill crossing on Little Mill Creek Road, which connects to Nash Mill Road, was the last stop made before wrapping up the workshop on the Holmes Ranch Road. It is installed at the crossing of a steep ephemeral draw. The apron, built to armor the road fill over which water passes, was built to be 6 times the width of the natural channel. This design specification is a good rule of thumb. The apron extends into the roadbed 1/3 to 1/2 the width of the roadbed and to the base of the fill connecting to the channel to allow for the seamless flow of water over a stable surface. The apron consists of rock large enough such that it will not be mobilized by large storm events and smaller rock which fill in the interstitial spaces, such that water will not work in between the larger rock and begin eating away at the underlying fill. A keyway can be built at the base of the fill to provide a more stable toe for the apron. Some maintenance is required, depositional fans will form when the bedload carried during high flows drops out when it passes over the less steep roadbed. Therefore, it is important to make the dip associated with the rock armored fill crossing deep enough so that if maintenance is forgone for several years the depositional fan does not build up to the point that water is able to divert down the road. Culverts are prone to plugging on steep ephemeral draws because when bedload including sticks and rocks arrives at the change in channel gradient, the gradient at which the pipe is set, the load will drop out at the culvert inlet plugging it. Therefore, steep ephemeral draws are prime candidates for rock armored fill crossings.

Summary—The "Take-Home" Message

It is important to note that after road improvement, unless the road is rocked or paved, the number of yards of sediment generated (chronic erosion) is the same. However, the improvements are intended to redirect the sediment generated to stable locations and filter strips, such that the sediment has a chance to drop out before reaching the waterway. The take home message from this workshop is that in understanding that roads interrupt natural drainage patterns enabling decision makers including, landowners, farmers, heavy equipment operators and natural resource professionals with practical solutions episodic and chronic erosion and maintenance can be reduced to ultimately benefit fish. There are many ways to accomplish the same goal. Addressing the following points in your road management decisions will insure the ultimate goal of making roads as invisible on the landscape as possible is met. Drain roads well and frequently onto stable surfaces, diversion proof crossings, design crossings to pass fish and design crossings for the large episodic event.

Rock Armored Fill Crossing

Information about Building Armored Crossings

A rock armored fill crossing is a dip in the road at a stream crossing with a rock apron connecting the outside edge of the road to the natural channel. The dip should be built large enough to contain the stream at high flows. The rock apron is built to armor the road fill over which water passes. The apron should be 5 to 6 times the width of the natural channel. This design specification is a good rule of thumb. The apron should extend into the roadbed 1/3 to 1/2 the width of the roadbed and to the base of the fill connecting to the natural channel to allow for the seamless flow of water over a stable surface. The apron should consist of rock large enough such that it will not be mobilized by large storm events and smaller rock which fill in the interstitial spaces, such that water will not work in between the larger rock and begin eating away at the underlying fill. A keyway is built at the base of the fill to provide a more stable toe for the apron. Some maintenance is required, depositional fans will form when the bedload carried during high flows drops out when it passes over the less steep roadbed. Therefore, it is important to make the dip associated with the rock armored fill crossing deep enough so that if maintenance is forgone for several years the depositional fan does not build up to the point that water is able to divert down the road. Steep ephemeral draws are prime candidates for rock armored fill crossings. Culverts are prone to plugging on steep ephemeral draws because when bedload including sticks and rocks arrives at the change in channel gradient, the gradient at which the pipe is set, the load will drop out at the culvert inlet plugging it. Following is a diagram with specifications, intended to stand alone, for use in the field.

Culvert Installation Method

Outslope Installation Method

Rolling Dip Installation Method

Information about Building Rolling Dips

Rolling dips are simply breaks in the grade of a road. The rolling dip represents a change-in-grade along the road alignment and acts to discharge water that has collected on or is flowing down the road surface. They are sloped either into the ditch or to the outside of the road edge to drain and disperse road surface runoff. Rolling dips are installed in the road bed as needed to drain the road surface and prevent rilling and surface erosion, and are most frequently used on outsloped roads as in the case of Hungry Hollow Road in the hills above Anderson Valley California. As a road becomes steeper, rolling dips should be made deeper and placed at a steeper angle to adequately capture and divert road runoff. On a less steep road the cut is kept at a minimum. To disperse surface runoff quickly the roadbed is smooth and there is no outside berm.

It is easier to properly locate and construct rolling dips when they are designed into the original road plan. However, they may also be installed on existing roads to improve surface drainage where a single dip can be built in about one hour, or less, using a medium size bulldozer (D-7 size) and a hydraulic excavator. Unsurfaced roads are more easily reconstructed with rolling dips, but rocked road surfaces can also be reconfigured. Excavation for a rolling dip typically begins 50 to 100 feet up-road from where the axis of the dip is planned. Material is progressively excavated from the road bed, slightly steepening the grade, until the axis is reached. This is the deepest part of the excavation, with the overall depth being determined by the slope of the road. The steeper the road, the deeper the dip will have to be in order to reverse grade. In order to safely and effectively direct runoff to the side of the road, the axis of a rolling dip should be angled about 30 degrees to the road alignment. On the down-road side of the rolling dip axis, the road bed slope should actually rise slightly to ensure that runoff cannot continue down the road surface. This is called a "grade change". The rise in grade is carried for about 10 to 20 feet before the road surface begins to fall again at its original slope. This transition from axis bottom, through rising grade, to original falling grade is achieved in a road-distance of 15 to 30 feet. Unlike a waterbar, the reverse grade portion of a rolling dip is not composed of fill. The entire drainage structure is excavated into the roadbed. Rolling dips require very little maintenance if they are constructed properly and at an adequate spacing. They should not collect enough runoff to develop significant erosion. The length and depth of the rolling dip should be adequate to divert road runoff but not so great as to interrupt or endanger traffic at normal speeds. Care should be taken to ensure that grader operators do not fill the depressions with soil or cut deeply into the lower part of the rising section, thereby eliminating the change-in-grade. More information about rolling-dips and other methods of road building can be found in the excellent handbook prepared by Pacific Watershed Associates for the Mendocino County Resource Concervation District. Contact the Mendocino County Resource Concervation District at 707.468.9223. They are located at 405 Orchard Avenue, Ukiah, CA 95482. All work done on Hungry Hollow Road was done by Dean Titus of Boonville, CA. Dean can be reached at 707.895.2515.