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An Integrated Approach to Sustainable Roadside Design and Restoration

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3. Design Strategies and Tools

• 3.3 Hydraulic Design
  • 3.3.1 Introduction
  • 3.3.2 Key Requirements
  • 3.3.3 Relationship with Other Disciplines
  • 3.3.4 Trade-offs and Considerations
  • 3.3.5 Recommended Approaches
  • 3.3.6 Hydraulic Design Checklist

3.3 Hydraulic Design

Hydraulic design considerations:

• Conveyance systems
• Channel behavior
• Erosion control

3.3.1 Introduction

The term "hydraulic" refers to the field of science and engineering dealing with liquids. For purposes of this guidebook, hydraulic design covers concepts such as conveyance systems, channel behavior, and erosion control.1

This section introduces a key issue that engineers face in working toward environmentally sustainable solutions: smaller, more frequent storms are commonly ignored when designing flood control facilities. And yet, these storms are most often responsible for shaping the long-term health of these facilities and the larger roadside. A number of trade-offs are introduced in this section, followed by a set of strategies to help mitigate smaller, more frequent storm events. Example strategies include microcatchments, micro- terraces, slope transitions, bioretention, and multi-cell box culvert design.

3.3.2 Key Requirements

The traditional approach to hydraulic design involves analysis of prescribed storm events and designing to mitigate such events. A Design Flood Event is typically either the 50- or 100-year storm. Smaller events, known as bank-full events, are responsible for shaping channels and ditches and defining where vegetation will grow. These bank-full events are typically characterized by the 2-year storm, which has a 50-percent chance of occurrence in any given year.

Traditional design practices require that hydraulic systems be designed for events ranging from the 10-year to the 500-year storm. Hydraulic design is based on roadway classification. Applicable design features for low standard roads are shown in Table 1.

Table 1: Low Standard Roadway Design Features

Design Feature	Standard	Flood Event
Culverts	High	50-year
	Low	25-year
Roadside Ditch	High	10-year
	Low	10-year
Longitudinal	High	50-year
Embankment	Low	25-year

Source: FHWA PDDM

Typically, standard design methodologies ignore the lower intensity, more frequent storms since safety and replacement concerns dominate the design requirements. This section details design features which will enhance the systems for smaller, more frequent events, reducing the need for maintenance and extending the life of the system.¹

Hydraulic design for roadways constructed by FHWA is dictated by the PDDM. Section 7.1.8 defines the Design and Check Flood for proposed drainage systems.

3.3.3 Relationship with Other Disciplines

Defining a sustainable roadside requires identifying the characteristics of a "traditional" roadway and comparing historic techniques to new methodologies. In order to establish new methodologies, relationships with other disciplines must be formed to create innovative techniques to enhance the roadside. This approach mimics a typical FHWA Cross Functional Team (CFT) meeting environment where the Hydraulics Team pulls ideas from each of the other disciplines.

Hydraulics must work together with each of the other disciplines to develop strategies for conveying runoff (rainfall that exceeds infiltration) safely and efficiently. Coordination with revegetation and geotechnical disciplines, among others, is critical.

VEGETATION: In order to create a stable conveyance area, the correct landscape material must be determined. Vegetation can protect slopes by reducing erosion and strengthening soil stability. Utilizing the correct land cover is critical to creating a sustainable roadside. The hydraulics group must determine flow rates and velocities and confer with landscape professionals about materials that can be established in the region and will be stable in the long term. In lieu of or in combination with vegetation, rock material can also be utilized to stabilize drainage ways. A soil-riprap mixture optimizes riprap protection by providing a growing medium that contributes to stability.

Landscape professionals may also be consulted where issues of water quality must be addressed. Bioretention areas provide environmental benefits such as creating ideal growth mediums for filtration processes. Impacts to adjacent vegetation and natural systems need to be considered to ensure that roadway run-off does not overload the landscape with grease, oils, metals, or chlorides. Sediment and increased run-off impacts should also be reviewed.

Planning is critical in mitigating environmental impacts to vegetation and designing a sustainable roadside. Identifying the protected or sensitive areas will help establish the drainage patterns in the project. General concern with roadway impacts, such as grease or oil, metals, and chlorides, can be addressed with proper planning. In addition, run-off from impervious surfaces can generate erosion that increases sediment conveyance. Slowing run-off and spreading flow will help replicate predevelopment patterns.

GEOTECH: Coordination with the geotechnical discipline is needed when addressing slopes and roadside structures. In general, flatter slopes are preferred but need to be balanced with potential impacts to the surrounding environment. Stable slopes that resist erosion and enhance vegetation growth are crucial to a sustainable environment. Detailed analysis and coordination is especially important when a wall is necessary, and optimal layout must be determined. Hydrostatic forces create complex systems. Weepholes are often required for drainage from backfill on the uphill side. Diversion of stormwater run- off using concrete curbs and gutters allows water to be safely diverted around the walls. In some instances, culverts are needed through the wall section to collect runoff. Overall, coordination between disciplines to ensure the proper wall design and stability is vital to enhancing the roadway.

The construction process needs to ensure that hydraulic features are feasible and sustainable. Design of controls to convey run-off during construction can provide a basis for permanent facilities that provide water quality. Utilizing features such as terracing of slopes can increase the length of travel that run-off follows, increasing the ability of run-off to infiltrate. BMPs such as rock check dams also slow velocities, which reduce erosion potential and increase infiltration. Many of the features installed during construction can remain as permanent features, as long as the materials will hold up over time.

3.3.4 Trade-offs and Considerations

Hydraulic design often requires trade-offs when working toward solutions. These trade-offs must consider project context and scale. For example, traditional design typically plans for peak flow; whereas a more sustainable design considers the full spectrum of storm events. Hydrologic criteria for sustainable design must include consideration of flows resulting from more than 10-, 50-, and 100-year storm events. Culvert and bridge hydraulic design should take into account an approximate flow or storm frequency for the channel forming discharge. This is often defined as a 1.5-year to 2-year event. These frequency events are typically published and in many cases are suitable substitutes for estimating the channel forming discharge. In order to minimize impacts to channels (hydraulic and ecological), design of culverts should also account for normal flow by minimizing expansion or contraction of the river or stream within the crossing.

Conventional designs at streams or drainage ways tend to focus on larger precipitation events. However, small events typically generate the highest concentration of pollutants and contribute the most towards erosion. Conventional methods tend to concentrate run-off flows in roadside swales and in culverts. In smaller drainages where no defined channel existed before, the increase in concentration can result in unexpected erosion. When rivers or streams are constricted at crossings, sediment is often accumulated upstream of the road and erosion is increased downstream of the road (Figure 3-15). More specific considerations include the following:

• Box culverts that are designed to convey a large storm event (such as a 50- or 100-year storm) without accounting for maintenance of the natural sediment transport condition can result in sediment accumulation in the culvert and increased erosion downstream. Allowing the channel forming discharge to widen and slow within the culvert results in sediment accumulation that will have to be removed frequently in order to maintain the design capacity of the culvert. In addition, increased erosion can result downstream of the box culvert as the flow velocity increases beyond it, picking up new sediment.
  
  Aquatic organism passage is an important consideration in the design of culverts that convey storm events. Traditionally, culverts have been installed to provide conveyance for stormwater. However, culverts often narrow the channel through the opening, which increases velocity and scour potential, and can create barriers to fish and other organisms. This narrowing reduces the distribution and habitat available and can lead to the inability of fish to access upstream spawning and rearing areas. This can result in decreased production and, in some cases, can eliminate fish populations altogether (Washington Department of Fish and Wildlife, 2003).
  
  Fortunately, recent guidance has been developed by numerous entities, including FHWA (2010), to provide technical approaches for enhancing the environment for aquatic organisms. In addition to standard culvert requirements, many variables need to be considered, including fish biology, fish passage hydrology, and stream geomorphology. By incorporating features that match the representative stream sections upstream and downstream of the proposed culverts, safe organism passage can be provided. Additional guidance from local jurisdictions can supplement the referenced documentation.

• Single, large-diameter culvert installations in drainage ways with wide, shallow flow properties can result in excessive ponding at the inlet and increased sediment accumulation upstream of the inlet. Increased erosion due to plunging flows from the culvert outfall and increased sediment carrying capacity of clear water flows may also result.

Figure 3-15: Large culvert following major storm event
Large culverts tend to collect sediment on the upstream end, limiting the ability to convey flows during major storm events.

View larger version of Figure 3-16.1

View larger version of Figure 3-16.2

Figure 3-16: Flow paths
Lengthening flow paths (bottom graphic) helps filter drainage before entering waterways.

Beyond the stream and drainage way crossings, creating a sustainable roadside involves the application of selected low-impact development (LID) strategies. Not all LID techniques work for rural roads and, for those that do, they may not work in every location. When applying LID or other methods in order to achieve increased roadside sustainability, each method should be evaluated carefully. Sample considerations include:

• Can directly connected impervious areas be reduced or altered on the project? Rural roads and highways provide very little opportunity for influencing directly connected impervious areas, except in a situation such as a roadway-widening project that has a paved median. Directly connected impervious areas can be reduced when designing safety rest areas and traveler service locations.
• Can flow paths be lengthened on the project? In cases where roadways are running perpendicular to the prevailing drainage, flow paths are often lengthened as runoff is intercepted by the road and routed to culverts (Figure 3-16). Velocity of run-off can be influenced by using contour bench terraces, run-off strips, and microcatchments (FHWA Roadside Revegetation, 2007).
• Can infiltration be increased without nuisance flooding or creating standing water in inappropriate locations? The overall change in imperviousness caused by a single road crossing a rural watershed is small. Loss of infiltration is generally caused by an increase in the concentration of run-off and the resulting increase in flow velocity. Measures can be taken to minimize the premature concentration of flow, such as the use of native trees or brush where safe lines of sight and the climate allow. Trees and brush help increase the interception of rainfall and slow the accumulation of run-off.
• Does the road interrupt natural drainage patterns, possibly impacting adjacent land extending upstream or downstream? Can the natural drainage patterns be re-established or at least approximated? Toe erosion along embankments and in receiving channels can be reduced with the use of additional smaller culverts. The placement of these additional culverts can re-establish an approximation of the natural drainage patterns. In addition to reduced erosion, approximating the natural drainage patterns more closely could help avoid the loss of small habitat communities due to changes in available moisture (Figure 3-17).

Figure 3-17: Drainage directly into a creek without treatment/infiltration strategies

3.3.5 Recommended Approaches

Hydraulic facilities must be designed for multiple precipitation events starting with traditional major events but concentrating on the more frequent stream-forming events. This section outlines the design and approach for the hydraulic concepts previously discussed.

After the key requirements have been met for the hydraulic facilities, additional concepts can be designed to enhance the sustainability and the long-term function of the system. It is best to focus on smaller scale strategies, such as microcatchments, micro-terraces, terracing, vegetated buffers, filter strips, pervious paving, bioswales, and native vegetation.

• Microcatchments: Roadways typically create a blockage for natural drainage patterns and concentrate flows at discrete locations. One way to minimize the impacts of the roadway section is to place additional, smaller culverts at intervals along the road alignment to allow smaller frequency events to cross the roadway intermittently. This would also allow for smaller facilities at the main culvert crossings.
  
  Installing culverts in discrete locations can help replicate preconstruction flow patterns. As can be seen on Figure 3-18, returning flow to the downstream side of the road embankment can help restore vegetation and the ecology that often is severed. Combining this feature with micro-terraces can also assist in restoring flow patterns.

View larger version of Figure 3-18

Figure 3-18: Microcatchments
The installation of microcatchments allows smaller frequency events to cross the roadway, and can help reduce overall infrastructure costs.

• Micro-terraces: Runoff from the impervious portion of the roadway tends to concentrate and create erosion along embankments. A simple and relatively inexpensive technique utilizes terraced and roughened surfaces parallel to the road surface to slow storm run-off and lengthen the flow path. This also creates a more natural looking surface as opposed to the perfectly symmetrical traditional road embankments.
  
  Combining the micro-catchment technique with micro-terraces will help restore natural flow patterns. Surface roughening helps to create micro-terracing through replicating the natural undulation in the ground surface with small pools and riffles. Replicating this natural environment not only softens the look of the roadway slope, but promotes infiltration and vegetation growth and reduces erosion due to run-off. The two pictures in Figure 3-19 show the visual difference between a standard, flat, graded roadway embankment versus roughened slopes.

Figure 3-19: Micro-terraces (right) Micro-terraces replicate the natural environment to soften the roadway slope, promote infiltration and reduce run-off erosion.

• Slope Transitions: Placing a roadway in a natural setting will require both cut and fill slopes throughout the project. Typically, slopes are tied into the natural grade as quickly as possible. In theory, this minimizes the impact to the surrounding area, but in reality it separates the improvements from the natural area. Where run-off is concerned, when sheet flow travels across the natural area, it is slowed by native grasses and natural terracing. When run-off encounters the engineered slope, it accelerates, creating erosion. Providing slope transitions that smooth the interface between the existing and proposed slope lessens the acceleration typical to the interface. This helps promote and accelerate vegetation re-establishment, providing lasting benefit and a more naturally blended system (Figure 3-20).
• Bioretention/Bioslopes: In locations where run-off from rural roadways may impact ecologically sensitive areas, bioretention or bioslopes may be a practical addition to the standard systems. Both systems provide a filtration medium that can remove unwanted sediment and pollutants before releasing the flows back to the open system. Utilizing ditch sections as a part of the roadway template are key requirements standard to every rural roadway. Enhancements to the typical ditch section can provide tremendous benefits over the standard section. By implementing a filter section of natural materials, such as sand or organic materials, filtering of run-off from the road section can take place before the run-off enters ecologically sensitive areas. Utilizing an underdrain system can increase the efficiency of the system by removing filtered water quickly.

View larger version of Figure 3-20

Figure 3-20: Slope transitions Providing slope transitions that link the interface between the existing and proposed slope by mimicking natural undulations can help lessen acceleration of stormwater and increase natural infiltration.

• Culverts: There are multiple techniques that can be used at culvert crossings to create systems that will meet key requirements and help create a sustainable system. Culverts can be designed to flow in a manner that mimics natural flow conditions for the full spectrum of flows (normal flow to design flood) as much as possible. In locations where existing box culverts are continually filling with sediment, inlet improvements can improve the sustainability of the existing crossing by preventing sediment deposition during minor events. One way this can be accomplished is by designing multicell box culverts with one cell floor lowered to keep normal channel flows and the channel forming discharge (usually equivalent to a 1- to 2-year event) confined to one cell (Figure 3-21). This reduces the impact on the natural sediment and bedload transport balance. This also reduces the need for maintenance, such as removal of sediment accumulated in the culvert, and reduces the upstream and downstream impact on the water way. Conveying run-off that more closely matches the natural channel will also enhance the area surrounding the roadway. By keeping the bench associated with a natural channel, native vegetation can establish itself, increasing biological benefits.

View larger version of Figure 3-21

Figure 3-21: Multi-cell box culvert This image shows the concept of a multi-cell box culvert, which places one cell floor lower than the other to keep normal channel flows and discharge confined to one cell.

There are multiple opportunities to apply techniques above and beyond the key requirements to create sustainable hydraulic solutions. Soil improvement is one example - when soil is improved, infiltration increases, which absorbs and stores water that would normally run off into water courses. It is important to keep water on the roadside to increase the amount of water available for vegetation, which in turn means better growth and less flooding.

3.3.6. Hydraulic Design Checklist

Detailed Strategies/Low Impact Development Techniques

• Microcatchments
• Micro-terraces
• Slope transitions
• Bioretention
• Bioslopes
• Innovative culvert design
• Vegetated buffers

Vegetation

• Identify protected or sensitive areas along corridor to help maintain natural drainage patterns.
• Use native trees, shrubs, and grasses to maintain natural biodiversity and increase success in establishment in specific environmental conditions.
• Design revegetation to both follow and mimic natural drainage patterns and to encourage stable slopes that intercept rainfall, slow run-off accumulation, and allow for on-site infiltration.
• Plan for water movement and capture to sustainably and naturally irrigate vegetation, minimizing the use of additional irrigation past the establishment period for revegetation plantings.

Geotech

• Evaluate the balance in slopes to preserve natural environment.
• If right-of-way is available, design drainage ditches to be traversable so that a vehicle leaving the roadway can cross over them without overturning or abruptly stopping.
• Coordinate between disciplines concerning need for walls and design of walls. Evaluate options to handle or divert run-off with wall design.

Aesthetics

• Minimize visual impact of water quality features and structures.

Construction Practices

• Restore streambanks.
• Provide for soil erosion control.
• Meet National Pollutant Discharge Elimination System (NPDES) requirements.
• Stage construction to minimize soil exposure.
• Provide stormwater detention.
• Minimize water ponding on the edge of pavement which can contribute to deterioration of the pavement edge and rutting of the soil.

 

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