Chapter 10 Open Channels
Chapter 10 Open Channels
Table of Contents
10-1 Introduction ...... 1 10-1-1 Chapter Overview ...... 1 10-1-2 Design Flows ...... 1 10-1-3 Channel Types ...... 2 10-1-4 Sediment Loads ...... 5 10-1-5 Permitting and Regulations ...... 6
10-2 Natural Stream Corridors ...... 7 10-2-1 Functions and Benefits of Natural Streams ...... 8 10-2-2 Effects of Urbanization ...... 9 10-2-3 Preserving Natural Stream Corridors ...... 11
10-3 Stream Restoration Principles ...... 12
10-4 Shared-Use Paths Adjacent to Streams ...... 14 10-4-1 Path Use ...... 14 10-4-2 Frequency of Inundation ...... 15 10-4-3 Path Geometry ...... 16 10-4-4 Public Safety Project Review ...... 20
10-5 Hydraulic Analysis ...... 21 10-5-1 Preliminary Channel Analysis ...... 22 10-5-2 HEC-RAS Modeling ...... 22 10-5-3 Evaluation of Erosion at Channel Bends ...... 28
10-6 Design Guidelines ...... 28 10-6-1 Major Drainageways ...... 29 10-6-2 Minor Drainageways ...... 37
10-7 Grade Control Structures ...... 39 10-7-1 Low-Flow Drop Structures ...... 39 10-7-2 Full-Channel-Width 100-Year Drop Structures ...... 40 10-7-3 Drop Structure Types ...... 41 10-7-4 Drop Structure Placement ...... 42
10-8 Riprap and Boulders ...... 42 10-8-1 Void-Filled Riprap ...... 42 10-8-2 Soil Riprap ...... 43 10-8-3 Riprap Sizing ...... 45 10-8-4 Boulders ...... 46 10-8-5 Riprap and Boulder Specifications ...... 46
October 2018 City of Durango 10-i Storm Drainage Design Criteria Manual
Open Channels Chapter 10
10-9 References ...... 47
Tables
Table 10-1. Characteristics of Common Types of Open Channels ...... 5 Table 10-2. Frequency of Inundation Criteria Summary ...... 16 Table 10-3. Recommended Roughness Values ...... 26 Table 10-4. Hydraulic Design Criteria for Natural Unlined Channels ...... 30 Table 10-5. Hydraulic Design Criteria for Grass-Lined Channels ...... 38 Table 10-6. Grade Control Drop Height Limits ...... 39 Table 10-7. Comparison of Void-Filled Riprap and Soil Riprap ...... 44
Figures
Figure 10-1. Functions and Benefits of Natural Stream Corridors ...... 9 Figure 10-2. Impacts of Stream Degradation ...... 11 Figure 10-3. Example of HEC-RAS Cross Section Placement and Alignment ...... 24 Figure 10-4. Design Elements Associated with Major Natural Drainageways ...... 29
10-ii City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
10-1 Introduction
10-1-1 Chapter Overview
This chapter addresses the preservation, enhancement, and restoration of stream corridors as well as the design of constructed channels and swales using natural channel concepts. Guidance is provided for the hydraulic evaluation of open channels and the design of measures to improve the stability and health of stream systems. The criteria described in this chapter are geared toward channels that are tributary to the Animas River, rather than the Animas River itself.
Much of the information in this chapter is presented in a brief, summary manner with references provided to direct the reader to additional sources of information. The Urban Drainage and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual (UDFCD Manual) is frequently cited.
This chapter is organized as follows:
Section 10-1 – Introduction. The Introduction section provides this overview of the chapter as well as background information relevant to open channel design, including design flows, sediment loads, types of open channels and permitting/regulatory requirements.
Section 10-2 – Natural Stream Corridors. Functions and benefits of natural stream corridors are summarized, along with a brief discussion of adverse effects that urbanization can cause to natural stream systems. This section also introduces the concept of preserving natural stream corridors and implementing techniques to restore stream functions.
Section 10-3 – Stream Restoration Principles. Eight principles of stream restoration are briefly introduced to provide design guidance for developers, engineers, ecologists, and others involved in the protection of stream resources. The restoration principles are valid for a variety of stream conditions, whether the corridor has been preserved or constrained and impacted through urbanization.
Section 10-4 –Shared-Use Paths Adjacent to Streams. Design considerations and requirements of the City of the Durango Development Code are summarized for shared-use paths adjacent to streams. In addition, water’s edge public safety is discussed.
Section 10-5 – Hydraulic Analysis. Summary guidance is provided for the hydraulic analysis of natural and constructed stream systems with an emphasis on the use of HEC-RAS for hydraulic modeling.
Section 10-6 – Design Guidelines. Design guidance is provided for major and minor drainageways.
Section 10-7 – Grade Control Structures. Guidance is provided for the design of grade control structures in channels.
Section 10-8 – Riprap and Boulders. The use of soil riprap, void-filled riprap, and boulders in stream restoration and constructed channels is addressed.
10-1-2 Design Flows
Open channel designs must account for a range of design flows, including baseflow, low flow and flood flow conditions. Full descriptions of these design flows and methods for estimating them are described in the Hydrology Chapter of this Design Manual. Brief descriptions of these flow conditions are provided below:
October 2018 City of Durango 10-1 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Baseflows - Baseflows are not directly related to storm events. In some cases, baseflow exists after urbanization occurs as a result of new sources such as irrigation return flows. Baseflows may not be present in undeveloped drainage basins. The presence or absence of baseflow can be a determining factor in the feasibility of implementing certain channel features, such as wetland bottoms.
Low flows – Low flows are normally contained within a well-defined channel that is overtopped only when a significant storm event occurs (e.g., a 2-year event or larger). Low flows typically establish the main channel section and the slope of the stream bed. The range of flows between baseflow and bank full capacity generally cause the geomorphic (channel shaping) activity and sediment transport.
Flood flows – Flood flows include any flows that exceed the low-flow or main channel capacity. As a result of exceeding the channel capacity, such flows have the potential to create unsafe or damaging conditions.
10-1-3 Channel Types
Two major channel types are addressed in this chapter: 1) major drainageways, and 2) minor drainageways. Brief descriptions of these channel types are provided below. Design guidance for major and minor drainageways is provided in Section 10-6.
10-1-3-1 Major Drainageways
In general, major drainageways are streams with contributing drainage basin areas greater than approximately 100 acres. This threshold generally corresponds to the threshold for regional detention facilities as described in the Storage Chapter.
As a watershed urbanizes, providing detention storage (and volume reduction practices) upstream of or in the headwaters of major drainageways is advisable to minimize changes to hydrology that have the potential to affect stream stability and capacity needed in the drainageway. The amount of sediment transport in major drainageways can vary greatly depending on the location of upstream detention storage and the level of development; therefore, sediment transport estimates and stable slope considerations can also be important factors for designing major drainageways.
Projects affecting major drainageways must preserve or restore natural drainageway features and benefits, and enhance them where feasible, unless otherwise designated in an approved master plan. A key consideration in the preservation of natural drainageways is obtaining an easement that is sufficiently large enough for the drainageway to provide the natural function of flood storage and also allow for the creation of open space that can provide habitat. This approach to channel design can also reduce the need to modify floodplain maps used in the administration of the National Flood Insurance Program (NFIP).
To the extent practical, major drainageway projects should protect and preserve the following features, if present:
Aquatic and riparian habitat, Jurisdictional wetlands, Riparian vegetation, Baseflows, Overbank flood storage, 10-2 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
Bedrock outcroppings or unique landforms, Historic, cultural, or archeological resources.
To complete the design of an open channel project, baseflows, low flows, and flood flows should be evaluated. At the discretion of the City Engineer, sediment transport evaluation may also be required, as discussed in Section 10-6.
The evaluation of flood flows will normally include delineation of the floodplain for land planning purposes and for maintaining adequate freeboard at structures on adjacent developments. The flood flow evaluation may also include scour calculations for utility crossings, bridge abutments and other structures. When the floodplain for the project reach is defined on a Flood Insurance Rate Map (FIRM), a revision to the regulatory floodplain may be necessary as described in the Floodplain Management Chapter of this Design Manual.
Types of major and minor drainageway projects are discussed below:
Major Drainageway - Modified Natural Channel
Most major drainageway projects can be described as modified natural channels. Such projects require limited modifications to drainageways in order to preserve or enhance most of the benefits of natural channels. Improvements are normally limited to stabilization of the low-flow channel (unless a meandering low-flow channel is planned), crossing structures, grade control structures and limited stabilization of the banks to manage unstable areas or protect infrastructure. Loss of flood storage resulting from encroachments should be mitigated by providing compensatory storage.
Considerations for modified natural channels are briefly summarized below:
1. Preserve streams not yet significantly impacted. Drainageways that have not been subject to degradation or other forms of erosion as a result of increased urban runoff should be preserved by implementing improvements such as grade control structures, vegetated overbank benches, stabilized low-flow channel, bank stabilization, and supplemental vegetation.
2. Restore Impacted Streams. Eroded, incised channels should not be stabilized in a configuration that retains the incised geometry with steep side banks. Instead, incised channels should be restored by raising the channel invert up to or near its historic elevation, allowing flood flows to spread out onto the natural floodplain, avoiding deep, concentrated flood flows within the main channel.
3. Channel Crossings. When changes to a natural channel are limited to a structural crossing such as a roadway and the upstream drainage basin is not expected to change significantly over time, the design process must fully consider historic basin conditions and the natural conditions of the drainageway. In such a case, the project should minimize impacts to the natural functions of the drainageway by avoiding encroachment into the adjacent floodplain and interfering with the natural tendencies of the drainageway such as meandering and sediment transport. This is typically best achieved by structures that span all or most of the floodplain (at least near crossings where there is typically contraction and expansion of the flow).
When floodplain encroachment cannot be avoided, mitigation should be provided to incorporate hydraulically efficient transitions upstream and downstream of the structure to minimize changes to the adjacent channel features and to the floodplain. The stabilization of eroded low-flow channels or banks to protect property or infrastructure may also be part of the project design. As
October 2018 City of Durango 10-3 Storm Drainage Design Criteria Manual Open Channels Chapter 10
part of these efforts, fill in the historic floodplain should be minimized to the extent practical so that the flood storage function of the channel is preserved.
By respecting natural historic drainage patterns and flood-prone areas in early planning and by implementing water quality and detention practices, drainageways and floodplains can be preserved that are stable, cost-effective, of high environmental value and that offer multiple use benefits to surrounding urban areas, including providing adequate capacity during storm events. In the absence of historic beneficial features, it may be desirable to design natural functions into projects.
10-1-3-2 Minor Drainageways
In general, minor drainageways or minor channels are streams with contributing drainage basin areas less than approximately 100 acres. Although minor drainageways may be reconstructed, relocated, or replaced with a storm sewer in combination with flood conveyance in the street network, the creation of vegetated surface channels is encouraged wherever practical in the minor drainageway network.
Minor drainageways are typically located upstream of detention storage facilities and design flows will be based on developed conditions that produce flows which are much greater than undeveloped conditions. Although natural channel features may not be present in minor drainageway channels, it is desirable to create natural features such as base-flow channels, low-flow channels and vegetated overbank areas to provide some of the beneficial functions of natural channels.
The amount of sediment transported in minor drainageways is expected to be limited when the upstream watershed has been developed and is stable. Sediment loads may be high while the drainage basin is under development, but the elevated loads are unlikely to continue as the drainage basin becomes more developed.
Two types of minor drainageway projects, constructed natural channels and constructed channels, are described below:
Minor Drainageways - Constructed Natural Channels
A modified natural channel is desirable to construct when adequate land is available to provide the benefits of a natural channel, such as flood storage, aesthetic benefits and habitat. Such “constructed natural channels” should be designed to emulate the functions of natural drainageways shown in Figure 10-1.
Constructed natural channels include grass-lined and composite channels, and may include bioengineered and wetland bottom channels as well. Where practical, existing natural features should be incorporated into the design. For these types of projects, the primary design considerations are to emulate natural channels, avoid flooding of adjacent structures, provide stable channel conditions during flood flows, and pass sediment to reduce maintenance.
Minor Drainageways - Constructed Channels
A channel that primarily provides flood flow conveyance may be necessary when upstream drainage basin conditions have already been significantly altered or are expected to be in the future, where the floodplain has already been significantly reduced, or where existing flooding is occurring. Constructed channels may also be necessary where right-of-way is limited.
Constructed channels will typically be fully lined with riprap, soil riprap, concrete, or manufactured linings. Some types of channel linings such as concrete, riprap and manufactured liners provide few benefits of a natural channel. The design of these channel types primarily depends on flood flows, but low flows and baseflows may be needed if sediment load passage is desired. The evaluation of flood flows provides the
10-4 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
delineation of the floodplain for land planning purposes and provides the basis to maintain adequate freeboard at structures and for adjacent developments but will not normally be shown on the FIRMs for minor drainageways.
Most channel projects will be either a modified natural channel or a constructed natural channel. The conditions necessary to maintain a channel in fully natural conditions rarely occur in an urbanizing drainage basin.
Table 10-1 summarizes typical characteristics of common types of open channels.
Table 10-1. Characteristics of Common Types of Open Channels Typical Floodplain Channel Design Sediment Drainage Preservation Vegetation Stabilization Type Flows Loads Area / ROW Major > 100 ac Qf ~ Qh Sf < Sh Preservation Limited Generally limited to Drainageway: (approx.) of most of the disturbance/ native areas of instability floodplain and or compatible plant and low-flow grade Modified natural species and control, soil riprap, Natural channel wetlands. boulders, sculpted Channel functions/ concrete, available bioengineering, other ROW. compatible materials.
Minor < 640 ac Qf > Qh Sf < Sh Limited to full Limited to Low-flow stabilization Drainageway: (approx.) floodplain significant and grade controls preservation/ revegetation, some and possible full- Constructed provide preservation of channel-width grade Natural natural natural vegetation, controls. Channel channel revegetation using functions native or when compatible plant feasible/ROW species and availability wetlands. varies. Minor < 100 ac Qf >> Sf << Sh Almost no Limited Fully stabilized with Drainageway: (approx.) Qh floodplain revegetation/ linings (riprap, soil preservation/ normally hard- riprap, concrete, Constructed limited to no lined. grouted boulders, Channel natural etc.) and full-width channel drop structures. function. 1Typical drainage areas may vary depending on approved master plans. 2 Qh = historic flows, Qf = future flows 3Sh = historic sediment loads, Sf = future sediment loads
References for additional information:
UDFCD Manual, Volume 1, Planning and Open Channels Chapters for major and minor drainageway planning
10-1-4 Sediment Loads
The amount of sediment carried by channels is affected by several factors, including conditions and flows in the upstream drainage basin, impacts to the channel due to crossings or other modifications, and development activity. Temporary sediment loads may differ from longer-term sediment loads. It is normally
October 2018 City of Durango 10-5 Storm Drainage Design Criteria Manual Open Channels Chapter 10
desirable to pass sediment through a design reach by designing the low-flow channel with sufficient hydraulic capacity to ensure that excessive sediment is not deposited in the reach over time.
When evaluating a “design reach,” the engineer should consider upstream and downstream potential for channel erosion and deposition, as hydraulic characteristics of the upstream and downstream reaches will affect the “design reach.” Estimates of the sediment load entering the project reach can be made by an analysis of the capacity and type of material conveyed in the upstream reaches. However, applying these methods can require extensive data collection and expertise that is often not available. Any project that requires these types of analyses must include a thorough description of the data sources and methodology to be used and submitted for approval. Additional discussion on evaluating sediment transport is provided in Section 10-6.
10-1-5 Permitting and Regulations
Major drainage planning and design along existing natural channels can be a multi-jurisdictional process and must comply with regulations and requirements ranging from local criteria and regulations to federal laws. Discussions with the relevant permitting authorities should be held early in the design development process and throughout construction to ensure that permitting and regulatory requirements are being met. Some of the more significant permitting processes required for typical channel projects are listed below. The list is not all-inclusive and additional permits may be required.
10-1-5-1 Floodplain Development Permit
A Floodplain Development Permit is issued by the City’s Floodplain Administrator and is required for all projects proposed within the regulatory floodplain as mapped by the Federal Emergency Management Agency (FEMA). Refer to the City of Durango Land Use and Development Code (LUDC), Section 6-3-4- 2, and to the Floodplain Management Chapter of this Design Manual, for additional information on floodplain permitting and regulations.
10-1-5-2 Section 404 Wetlands Permit
Streams designated by the U.S. Army Corps of Engineers (USACE) as “jurisdictional” under Section 404 of the Clean Water Act are subject to specific protections established during the 404 permit process. The 404 permit may impose limits on the amount of disturbance of existing wetland and riparian vegetation, may require disturbed areas to be mitigated, and may influence the character of proposed stream improvements.
Additionally, sites located upstream of water quality facilities may require protection in the form of temporary (construction) and permanent on-site water quality measures, including reducing directly connected impervious area before discharging to the waterway. On-site measures for water quality are described in the Water Quality Chapter.
The USACE should be contacted early in the design process to determine if proposed channel modifications will require a 404 permit.
10-1-5-3 Endangered Species Act
Construction of improvements along drainageways may also be subject to regulation under the federal Endangered Species Act. The USACE, as part of the 404 permit process, will typically coordinate with the U.S. Fish and Wildlife Service (USFWS) to assess potential impacts to threatened and endangered (T&E) species. The USFW may require a Biological Assessment to determine impacts. Significant mitigation measures may be required if impacts are expected. In some areas, “Block Clearances” may be in place so
10-6 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
that some environmental assessments are not necessary. The designer should determine whether a “Block Clearance” is effective for the project.
Additionally, for projects located in a regulatory floodplain where a FEMA Conditional Letter of Map Revision (CLOMR) is required, T&E species must be addressed as part of the CLOMR process. If T&E species will not be affected by work associated with a CLOMR, the applicant typically submits a letter with a finding of “no likely impact” that has received concurrence from the USFWS. If T&E species will be affected by work associated with a CLOMR, FEMA requires documentation that the appropriate permits have been obtained before they will issue a Letter of Map Revision (LOMR).
10-1-5-4 Erosion Control/Stormwater Management Permitting
Projects that will disturb one or more acres of land require the development of a Stormwater Management Plan (SWMP) and submittal of a Notice of Intent (i.e., application) to obtain certification of coverage under the Colorado Department of Public Health and Environment (CDPHE) General Permit for Stormwater Discharges Associated with Construction Activity.
References for additional information:
UDFCD Manual, Volume 1, Planning Chapter for information on permitting for drainage projects 10-2 Natural Stream Corridors
Natural stream corridors often contain a primarily non-vegetated bankfull channel that may flow continuously or ephemerally. Outside of the bankfull channel are adjacent vegetated floodplain terraces (also called benches or overbanks) and higher outer banks.
Photograph 10-1. Natural channel cross section illustrating floodplain terraces.
An appropriately sized single-thread channel with floodplain terraces creates favorable conditions at baseflows, producing greater depth, lower temperatures, and better aquatic habitat. During higher flows, when water spills out of the bankfull channel onto wider floodplain terraces, it provides important interaction between water, soil, and vegetation. During such conditions, as floodwaters spread out onto the
October 2018 City of Durango 10-7 Storm Drainage Design Criteria Manual Open Channels Chapter 10
floodplain terraces, energy is dissipated, riparian vegetation receives water, and sediment can be conveyed through the system.
Natural channels take other forms besides the appearance of the cross section in the photograph above. Some channels may be influenced by relatively high sediment loads with areas of degradation (erosion) and aggradation (deposition), or they may be vegetated across the entire channel bottom, either with wetland species if the channel is normally wet or transitional or upland species if it is normally dry.
Photograph 10-2 shows a dry, vegetated stream common to upland areas. These channels tend to function best when high flows are allowed to spread out over a wider floodplain; however, this is not always feasible in an urban area. Natural steam channels are dynamic and change over time in response to hydrology, watershed conditions, and other factors. Large floods can result in rapid channel evolution/avulsion. When the floodplain is preserved, these natural changes have space to occur with lower potential for damage than channels that are constrained.
During high flow events, the water level rises and spreads to a width and depth associated with a specific return period. City of Durango, State, and Federal floodplain criteria are most commonly associated with the 100-year event. In some cases, the 500-year event is mapped and human development is limited with respect to this criterion. The overall width of the stream corridor should be planned and designed to convey these large flood events that can and will occur.
Photograph 10-2. Example upland channel.
10-2-1 Functions and Benefits of Natural Streams
Natural stream systems are dynamic, responding to changes in flow, vegetation, geometry, and sediment supply. In the absence of urbanization, stream systems are generally free to undergo dynamic change with little negative impact.
Healthy streams and floodplains provide a number of important functions and benefits which are summarized below and illustrated in Figure 10-1.
1. Provide stable conveyance of baseflows and storm runoff.
10-8 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
2. Support riparian vegetation.
3. Create habitat for wildlife and aquatic species.
4. Manage energy during a wide range of flow rates.
5. Promote infiltration, groundwater recharge, and exchange of surface and subsurface water in the hyporheic zone located under and adjacent to the low-flow channel.
6. Enhance water quality through reduced erosion and through vegetative filtering and soil-water interactions.
7. Provide corridors for trails and open space.
8. Enhance property values and quality of life.
Figure 10-1. Functions and Benefits of Natural Stream Corridors (Source: Arapahoe County)
10-2-2 Effects of Urbanization
The functions and benefits of a natural channel are frequently degraded in an urbanized watershed. Urban impacts often involve realigning or straightening streams, narrowing the width of natural floodplains, and even replacing surface streams with underground conduits. Urbanization also typically increases the frequency, duration, volume, and peak flow rate of runoff.
Increases in runoff as a result of urbanization can contribute to degradation, aggradation, loss of vegetation and habitat, and impaired water quality. Sediment movement, channel geometry, and vegetation can undergo significant and rapid changes in developing urban environments, exaggerating and accelerating the adjustments that streams make as conditions vary.
Common negative effects of urbanization on stream channels are summarized below and shown on Figure 10-2:
October 2018 City of Durango 10-9 Storm Drainage Design Criteria Manual Open Channels Chapter 10
1. Removal of riparian vegetation and wildlife habitat. Erosion can strip natural vegetation from the bed and banks of streams. This disrupts habitat for aquatic and terrestrial species and makes the stream vulnerable to further erosion damage.
2. Increase in flow velocities. An incised channel concentrates runoff in a narrow, deep channel which increases flow velocities and shear stresses.
3. Damage to infrastructure. Channel erosion can threaten utility lines, bridge abutments, and other infrastructure. Utility pipelines originally constructed several feet below the bed of a creek can become exposed as the bed lowers.
4. Lowering of water table and drying of terrace vegetation. Lowering of the channel thalweg and baseflow elevation can lead to a corresponding lowering of the local water table and less frequent flows on the floodplain terraces. Besides the loss of water storage, lowering the water table can “dry-out” the floodplain terraces and harm the ecology of terrace areas by causing a transition from wetland and riparian species to weedy and upland species. It should be noted that raising the degraded channel to approximate the natural elevation will raise groundwater levels closer to the surface and may impact properties adjoining the floodplain.
5. Impairment of water quality. The sediment associated with the erosion of an incised channel can lead to water quality impairment in downstream receiving waters. One mile of channel incision 5- feet deep and 15-feet wide produces almost 15,000-cubic yards of sediment that can be deposited in downstream lakes and stream reaches. Sediments typically contain naturally occurring phosphorus, a nutrient that can lead to accelerated eutrophication of lakes and reservoirs. Also, channel incision impairs the “cleansing” function that natural floodplain terraces can provide through settling, vegetative filtering, wetland treatment processes, and infiltration.
6. Increase in capital and maintenance costs. Projects to repair eroded streams require significant capital and maintenance investment. The more erosion occurring, generally the higher the cost for repair and maintenance.
7. Loss of flood storage. Incision of the low-flow or main channel portion of the drainageway prevents flood flows from spilling into the overbank area where natural storage exists to reduce downstream peak flows.
10-10 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
Figure 10-2. Impacts of Stream Degradation (Source: Arapahoe County)
10-2-3 Preserving Natural Stream Corridors
The opportunity to preserve natural stream corridors frequently occurs only at the outset of planning in a watershed. Preserving natural stream corridors not only preserves valuable habitat and vegetation; it can also reduce impacts and provide for future stream management at a lower cost and smaller footprint compared to constrained floodplains where elevated discharges must be conveyed in narrow corridors.
Ample space needs to be provided to enable high flows to spread out over the floodplain. As the wetted channel width increases, relative roughness increases and flow velocity and erosive force decreases for a given flood discharge. Therefore, wide floodplains are generally more stable than narrow floodplains for a given flow rate.
It is also critical to recognize that higher water surface elevations can and will occur as a result of increased channel vegetation and roughness, aggradation, raising degraded inverts, and flood debris. As a result, providing ample freeboard is essential. Freeboard is the vertical distance above a referenced floodplain water surface to a specific elevation associated with constructed infrastructure.
Employing runoff reduction techniques can mitigate the impacts of urbanization. Such techniques include minimizing directly connected impervious area and using features such as grass buffers, grass swales and permeable pavement (see the Water Quality Chapter for design criteria for these types of features).
References for additional information:
UDFCD Manual, Volume 1, Open Channels Chapter for information on natural stream corridors
October 2018 City of Durango 10-11 Storm Drainage Design Criteria Manual Open Channels Chapter 10
10-3 Stream Restoration Principles
“Stream restoration is the process of assisting the establishment of improved hydrologic, geomorphic, and ecological processes in a degraded watershed system and replacing lost, damaged, or compromised elements of the natural system.” (Bledsoe, 2013)
In general, the objective of stream restoration is to re-establish the natural and beneficial functions of a stream corridor. Although a degraded channel can be left in a narrow, deep configuration (see Photograph 10-3) and perhaps protected with heavy rock lining, stabilizing the invert in its existing, incised condition will potentially perpetuate a low water table, dried out terrace vegetation, high flood velocities, and reduced water quality filtering and infiltration in the terraces.
It is ideal, and often less expensive, to raise the channel invert to re-connect the channel with its floodplain, as shown in Photograph 10-4. It is better to promote healthy floodplain terrace conditions that can handle periodic flood flows and control increased runoff from development rather than “force” a degraded channel into a stabilized condition using extensive structural measures.
Photograph 10-3. Degraded channel Photograph 10-4. Same channel as Photograph 10-3 (before restoration). (after restoration).
The stream restoration principles provided in this section are not a “cookbook” or “one size fits all” set of design steps, but instead are principles to be applied to channel reaches with the experience, judgment and collaboration of a multi-disciplinary design team.
Eight principles for stream restoration are summarized below. Additional detail on stream restoration principles can be found in the Open Channels Chapter of the UDFCD Manual).
Stream Restoration Principle 1 - Understand Existing Stream and Watershed Conditions
Before any design work on a stream reach takes place, it is necessary to understand the existing conditions associated with the stream and its watershed, as well as understand development plans and planning/zoning documents that are relevant for the watershed. Comprehensive field reconnaissance should be performed as part of this analysis. Stream Restoration Principle 2 - Apply Fluvial Geomorphology Principles to Manage Sediment Balance
An alluvial channel is usually considered stable and in equilibrium if it has adjusted its width, depth, 10-12 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
slope, and other factors so that the channel neither aggrades nor degrades, resulting in no significant change in channel cross section over time. This is a dynamic equilibrium in which the sediment supply from upstream is generally equal to, or in balance with, the sediment transport capacity of the channel for the full range of flows. Evidence of aggradation and degradation over time can be documented by comparing current survey information of the stream invert to any prior survey or mapping information that may be indicated in past floodplain or master plan profiles, taking into considerations any datum differences from different surveys.
Stream Restoration Principle 3 - Establish Effective Cross-Sectional Shape
It is desirable that floodplain terraces adjacent to the bankfull channel be relatively wide, flat, well- vegetated, and not excessively steep with respect to longitudinal slope. Terraces with these characteristics assist with reducing flow velocities and provide adequate capacity for larger storm events.
Stream Restoration Principle 4 - Maintain Natural Planform Geometry
Natural streams offer variety and complexity in form; they are seldom straight and uniform. Outer banks move in and out and bank heights, slopes, and widths vary. Bankfull channels exhibit a degree of meandering and sinuosity, moving right and left across a section in an alternating manner. The shape of the bankfull channel varies as well, tending to widen slightly in bends; side slopes tend to steepen at the outside of bends and flatten as point bars form on the inside of bends. Increasing sinuosity decreases longitudinal slope.
Stream Restoration Principle 5 - Develop Grade Control Strategy to Manage Longitudinal Slope
A typical stream response to increased urban runoff is to trend toward flatter longitudinal slopes, which, if left unmanaged, leads to degradation and channel incision. A primary management approach to limit degradation is the installation of grade control structures along the length of a stream. The structures hold grade so if the stream wants to flatten its equilibrium slope, incision is limited.
Stream Restoration Principle 6 - Address Bank Stability
Existing steep, unstable banks at the edges of the bankfull channel or at outer channel banks should be addressed.
Stream Restoration Principle 7 - Enhance Streambank and Floodplain Vegetation
It is desirable to re-establish or supplement vegetation in stream corridors, especially along the banks of the low-flow channel and on the adjacent floodplain terraces to build up a sturdy, durable cover to help retard flood flows, resist erosion, and enhance habitat.
Stream Restoration Principle 8 - Evaluate Hydraulics of Streams over a Range of Flows
Detailed hydraulic modeling of stream corridors with proposed restoration improvements is required to assess flow depths, velocities, Froude number, imposed shear stress and other relevant parameters. The hydraulic analysis should consider a range of flows including the bankfull discharge, 2-year, 10-year, 100-year, and perhaps other intermediate and larger flows.
October 2018 City of Durango 10-13 Storm Drainage Design Criteria Manual Open Channels Chapter 10
References for additional information:
UDFCD Manual, Volume 1, Open Channels Chapter for information on stream restoration 10-4 Shared-Use Paths Adjacent to Streams
Paths are an integral part of recreational channels, providing access for the public and channel maintenance and also are typically part of the active conveyance area for the channel during a flood. Guidance for the design of recreation trails and other shared-use paths adjacent to streams is provided in this section.
Design of trails within the City of Durango must comply with the City’s LUDC, Division 4-2-3, Sidewalks and Trails and, to the extent they are consistent with the LUDC, the Parks, Open Space, Trails, and Recreation Master Plan, the most recently published volume of the American Association of State Highway and Transportation Officials (AASHTO) Guide for the Development of Bicycle Facilities, and the Americans with Disabilities Act (ADA).
In addition to addressing design criteria for public trails, this Section also provides guidance for water’s edge public safety.
10-4-1 Path Use
Paths are often constructed along streams to provide access for maintenance vehicles. However, if public access is provided to the path, it should be assumed that the path will be used by the public. For this reason, it is important to design paths with the health, safety, and welfare of the public as a primary design objective.
It is also important to evaluate when it is appropriate for a path to conform to accessibility criteria. Accessibility is a requirement for all paths described in this section with rare exceptions such as, for example, a gated section of path not intended for any public use. Depending on the design, users may include bicyclists, pedestrians, runners, equestrians, dog walkers, people with baby carriages, people in wheelchairs, skate boarders, and others. Not all paths will be designed for all of these users, but the following can be considered when determining type of use of the path:
. Does this segment of path fit into an existing master plan where use has been determined?
. What connections are made with the path? Who are the likely users?
. How can the path best provide continuity between its connection points? Alternating segments (in regard to intended use, material, or geometry) should be minimized.
Determining the expected types of path users expected will help in establishing geometry, selecting construction materials and techniques, and understanding safety considerations.
10-14 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
10-4-2 Frequency of Inundation
The frequency of inundation is one of the most important considerations for the design of a path adjacent to a stream. This criterion directly affects safety and maintenance and frequently impacts cost, conveyance capacity, and the users’ path experience. Less frequent inundation is better from a safety and maintenance perspective. The public safety threat is especially high in channels susceptible to flash flooding and where egress from the channel section is limited (e.g., walled channels). Frequently inundated paths also require more frequent maintenance due to sediment deposits on the path surface and erosion at the path edges. Photograph 10-5. Frequently inundated channels pose a Paths constructed with new channel or roadway high threat to public safety, especially in a walled channel. improvements should be constructed above the 5- year water surface elevation or higher. For highly used paths, an elevation above the 10-year water surface elevation is preferred.
For a retrofit project, the same standards should be met when practical; however, existing conditions may not allow this for the entire length of the path. In this case, the City strongly recommends that the design elevation remain above the 2-year water surface elevation at all locations.
Changes in channel section can occur over time resulting in the increased frequency of overtopping in the future. For this reason, it is also good practice to set the surface of the path a minimum of two feet Photograph 10-6. A well designed recreation path raised above the level where it is frequently inundated. above the estimated base flow elevation. When existing conditions do not allow for a path elevation meeting either of these two criteria, consider alternative alignments.
Exceptions to the above criteria may be appropriate in the area of a low-flow stream crossing where the crossing could be designed to pass up to a 2-year event before overtopping. This should be evaluated on a case-by-case basis taking into consideration frequency of use and the importance of the crossing as a path connection component. Benefits of constructing a low-flow crossing include conserving flood capacity for higher flows, improving user experience by bringing the user in closer contact with the stream, and potentially eliminating railing that could otherwise catch debris, become a maintenance issue, and further impact the floodplain. However, low-flow crossings have attendant safety risks of their own.
Underpasses, where users frequently seek shelter in a storm event, present a more critical case for public safety as it relates to frequency of inundation. If the geometry of the surrounding area and configuration of the underpass combine to allow the user to see the water and seek higher ground, more frequent inundation may be acceptable.
Frequency of inundation criteria for paths is summarized in Table 10-2. October 2018 City of Durango 10-15 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Table 10-2. Frequency of Inundation Criteria Summary
Recommended Minimum Water Surface Water Path Type Elevation Other Considerations Surface (when Elevation practicable)
Stream Crossings 2 to 5-year 2-year
Bridge Underpass 5-year 2-year
Culvert Underpasses less The user should be able to see when 5-year 2-year than 100 feet in length water is rising and climb to safety. Culvert Underpasses The culvert should be straight. The greater than 100 feet in 10-year 5-year user should be able to see when length water is rising and climb to safety. Elevating the path to the 10-year All Other Locations (New) 10-year 5-year WSE is preferred.
All Other Locations Where practicable also elevate the 5-year 2-year (Retrofit) path two feet above the baseflow.
Removal of sediment after runoff events typically involves collection and disposal of sediments. Washing the sediment back into the channel would violate typical MS4 permit requirements. Additionally, sediment deposition between the channel and the path can impede drainage away from the path and result in water or ice on the path.
10-4-3 Path Geometry
10-4-3-1 Typical Sections
Path geometry requirements are described in the LUDC, Sec. 4-2-3-3, Paved Multi-Use Trails. The multi- use trail shall be a minimum of 10 feet wide, six-inch thick concrete paving with a two foot, soft-surface shoulder on each side of the trail. A reduced width typically results in edge damage from maintenance vehicles. This is also consistent with AASHTO’s width recommendations for two-directional shared-use paths.
In many cases it may be desirable to increase the width to 12 or even 14 feet to accommodate conflict points or when high volumes of users are anticipated. In very high-use areas, multiple trails allow separation of uses that might conflict, such as cyclists and pedestrians. Rub rails on bridges are horizontal members that help mitigate injury to cyclists crashing into them.
On each side of the path, the adjacent grade (shoulder) should be no steeper than 6(H):1(V) for a minimum width of two feet. This is regardless of the edge treatment and provides a place for the user to safely move off the path and also protects the path from potential damage due to adjacent sloughing grade. Sloughing grade adjacent to the path can eventually undermine the path or cause a rumble strip to become separated from the path. It is best to provide a section in the construction drawings that shows the shoulder and 10-16 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
specifically calls out for backfilling the sides of the path. When the site does not allow for a shoulder, a thickened edge can protect the path from being undermined and allow maintenance personnel time to identify and repair the problem.
In some cases, a safety rail parallel to the path is recommended. Rails are appropriate where a dangerous condition would otherwise exist. Common locations include steep side slopes, vertical walls, steep longitudinal slopes, bends, areas where cross drainages create isolated hazards, and where combinations of the above circumstances exist.
Photograph 10-7. Signing and striping help segregate bicyclists and pedestrians.
10-4-3-2 Path Overtopping Protection
Provide adequate protection to avoid damage caused when flows overtop the path. As a path turns perpendicular to the stream, or anywhere significant overland flows are likely to cross the path (e.g., downstream of side-channel spillways or at undersized culvert crossings), scour can occur along the downstream edge. This causes the path to act like a drop structure. Flows across the path accelerate, potentially damaging the upstream edge of the path, while scour downstream can eventually undermine the path (see Photograph 10-8). For these reasons, a thickened edge on both the upstream and downstream sides of a path approaching a low crossing is recommended. Soil riprap placed adjacent to the path can be used to provide additional Photograph 10-8. A soil cement path undermined on the downstream side due to overtopping and inadequate scour protection. protection.
The length of the overtopping protection is site
October 2018 City of Durango 10-17 Storm Drainage Design Criteria Manual Open Channels Chapter 10
specific. If the bank of the channel is well defined, protection should extend from the crossing into the bank. If the bank is not well defined, extend the protection to a point where the path is more parallel with the stream than it is perpendicular. In either case, the length of overtopping protection typically does not need to extend higher than the 10-year surface elevation.
10-4-3-3 Horizontal and Vertical Trail Alignment
In order to avoid a crash, a cyclist must have time to identify potential conflicts and react accordingly. For all hard paths, or where bicyclists are otherwise anticipated, in accordance with the LUDC, horizontal and vertical trail alignment shall be per the most recently published volume of the AASHTO Guide for the Development of Bicycle Facilities. Trail grades shall meet all AASHTO recommendations and requirements of the Americans with Disabilities Act (ADA).
10-4-3-4 Use of Rails, Curb Rails, and Rumble Strips
Rails, curb rails, rumble strips, increased path width, changes in texture and/or color, signage and striping are all tools that can be used to improve path safety and heighten user awareness of a new or changing condition. For the purpose of thus manual, the term “edge treatment” refers to rails, curb rails, and rumble strips. All above-grade stream crossings should include an edge treatment.
10-18 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
Use of full rails (typically 42 inches when bicyclists are anticipated and 54 inches when the path provides equestrian passage) can cause adverse flooding conditions and should only be used when a curb rail or rumble strip does not provide an acceptably safe condition for the user. When rails are used, the hydraulic model should consider the full area of the rail to be clogged with debris. Based on the experience of UDFCD, “break-away” rails which are designed to collapse during high flow, are often ineffective over time and should not be relied on for floodplain analysis (i.e., they too should be modeled as fully blocked).
Considerations for Designing Safety
Rails
. Minimize the likelihood of the rail catching debris. This is a
maintenance issue and, if not maintained, can reduce capacity in the stream and cause flooding or damage to the safety rail
. Place horizontal members on the users’ side of the posts. This provides a safer surface, less likely to catch clothing, a bike pedal, or a stirrup. Photograph 10-9. Curb rails are typically no higher than 12 inches. . Provide a rail height of at least 42 inches when cyclists are anticipated and 54 inches when the trail provides equestrian passage.
. Consider snow removal either by designing the rail to allow movement of snow through the bottom of the rail (without creating a safety hazard for small children) or
by planning for snow storage in an alternate location.
Photograph 10-10. Rumble strips warn the user of the path edge without reducing capacity for flood flows. (Photo Courtesy Architerra Group)
October 2018 City of Durango 10-19 Storm Drainage Design Criteria Manual Open Channels Chapter 10
References for additional information:
City of Durango LUDC, Division 4-2-3, Sidewalks and Trails
UDFCD Manual, Volume 2, Stream Access and Recreational Channels Chapter for information on shared use paths
AASHTO Guide for the Development of Bicycle Facilities
Americans with Disability Act (ADA) Standards for Accessible Design
FHWA Evaluation of Safety, Design, and Operation of Shared-Use Paths
Architectural Barriers Act (ABA) Accessibility Standards
National Trails Training Partnership website
NACTO Urban Bicycle Design Guide
www.bicyclinginfo.org
10-4-4 Public Safety Project Review
As an increasing number of design professionals and developers promote the natural and beneficial functions of the floodplain, encouraging passive recreation in the floodplain and drawing people toward the water’s edge, public safety becomes even more critical.
Although the engineer should consider public safety throughout the design process, the following siting and design components should trigger a comprehensive project review for public safety:
Projects in densely populated areas and with populations that may require specific site requirements (e.g., high populations of children or elderly); Projects adjacent to schools, playgrounds, or within a public park; Projects designed with the intent to draw the public toward water; Drop structures taller than 3 feet from crest to stilling basin floor; Vertical drop structures of any height; Walls (including boulder walls and channel edging) exceeding 3 feet; Channel side slopes steeper than 4:1 (H:V); Detention basins and outlet structure; Retention ponds and outlet structures; Inlets to storm drains and long culverts; Below grade paths, and Low-flow crossings.
The following considerations may be helpful when conducting this review:
At what locations and with what frequency might a person become trapped by flood water? At what locations could signage be beneficial to public safety? What dry weather and wet weather risks exist in the project area?
10-20 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
What locations present potential fall hazards during dry weather, wet weather, or when snow or ice is present? Do maintenance personnel have safe access to all required areas? How will channel degradation impact safety associated with various elements of the project?
With respect to open channels, considerations related to safety include, but are not limited to, the following:
Channel side slopes steeper than 2.5H:1V are considered unacceptable under any circumstances because of stability, safety, and maintenance considerations.
Vertical drop structures can cause dangerous hydraulic conditions, including keeper waves, during wet weather and are generally discouraged. In addition, vertical drop structures are to be avoided due to impingement energy, related maintenance and turbulent hydraulic potential (ASCE and WEF 1992).
o Vertical drops are not appropriate where fish passage is needed, design flow (over the length of the drop) exceeds 500 cfs or a unit discharge of 35 cfs/ft, net drop height is greater than 2 feet, or the stream is used for boating or there are other concerns related to in-channel safety.
o Drop faces should have a longitudinal slope no steeper than 4(H):1(V) to avoid the formation of overly retentive hydraulics. Longitudinal slope, roughness and drop structure shape all impact the potential for dangerous conditions.
When designing underground conveyance systems with flared-end sections that are larger than 36 inches in diameter, pedestrian railing may be warranted if public access will occur. If this is the case, railing can be more easily mounted to a combination headwall/wingwall.
Safety grating should be required for a culvert or storm drain when any of the following conditions are or will be true:
o It is not possible to “see daylight” from one end of the culvert to the other.
o The culvert is less than 42 inches.
o Conditions within the culvert (bends, obstructions, vertical drops) or at the outlet are likely to trap or injure a person.
It should also be noted that retention ponds pose a greater risk to the public compared to detention basins and should be evaluated for unintentional entry by the public. Additional safety information for storage facilities is included in Chapter 8, Storage.
References for additional information: UDFCD Manual, Volume 2, Stream Access and Recreational Channels Chapter Public Safety Guidance for Urban Stormwater Facilities (ASCE 2014). 10-5 Hydraulic Analysis
Evaluating channel and floodplain hydraulics is a key component of any stream project. Hydraulic modeling
October 2018 City of Durango 10-21 Storm Drainage Design Criteria Manual Open Channels Chapter 10
provides insight into flow properties including water surface elevation, depth, velocity, shear stress, and Froude Number. Understanding these flow properties is necessary to assess risks associated with structure flooding and channel erosion and can help guide the design of stream capacity and stabilization improvements.
10-5-1 Preliminary Channel Analysis
There may be times when a preliminary or “quick” analysis is needed, rather than using hydraulic modeling software, to evaluate channel properties in uniform, steady open flow conditions. For these cases, Manning’s Equation should be used. Manning’s Equation describes the relationship between channel geometry, slope, roughness and discharge for uniform flow conditions and is expressed as:
1.49 Q AR 2/ 3S1/ 2 Equation 10-1 n
Where:
Q = discharge (cfs) n = roughness coefficient (see Section 8.2.3) A = area of channel cross section (ft2) R = hydraulic radius = A/P (ft) P = wetted perimeter (ft) S = friction slope (ft/ft) (approximated by channel invert slope for normal depth calculations)
Manning's Equation can also be expressed in terms of velocity by employing the continuity equation, Q = VA, as a substitution in Equation 10-1, where V is velocity (ft/sec).
For wide channels of uniform depth, where the width, b, is at least 25 times the depth, the hydraulic radius can be assumed to be equal to the depth, y, expressed in feet, and, therefore:
1.49 Q by5/ 3S1/ 2 Equation 10-2 n
The solution of Equation 10-2 for depth is iterative, therefore using a software program to assist with this calculation can be beneficial. A number of additional software packages are available to solve Manning’s Equation by inputting known channel properties.
The designer should realize that uniform flow is more often a theoretical abstraction than an actuality (Calhoun, Compton, and Strohm 1971), namely, true uniform flow is difficult to find. Channels are sometimes designed on the assumption that they will carry uniform flow at normal depth, but because of ignored conditions, the flow actually has depths that can be considerably different. Uniform flow computation provides only an approximation of the hydraulic conditions that will actually occur.
10-5-2 HEC-RAS Modeling
The most commonly used tool for open channel hydraulic modeling is the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) from the US Army Corps of Engineers. HEC-RAS is non-proprietary software that can be downloaded and used free of charge from the USACE via the following link: http://www.hec.usace.army.mil/software/hec-ras/downloads.aspx.
10-22 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
The summary information provided in this chapter focuses on HEC-RAS’s ability to perform one- dimensional steady flow analysis using a series of input parameters. References that provide more detailed guidance on HEC-RAS modeling are provided at the end of this section.
The application of the HEC-RAS computer software shall use model parameters described in this Manual or in the program documentation. Justification shall be provided for values used that are not consistent with these documents. Typical input parameters include flowrate, channel cross section geometry, roughness coefficients, main channel bank stations, etc. HEC-RAS has the capability to model bridges, culverts, weirs and spillways as well as address unsteady flow computations. In most cases, a subcritical HEC-RAS run is appropriate for natural channels. This section provides only general guidance on determining appropriate input parameters and reviewing output information when using HEC-RAS.
10-5-2-1 Cross Section Location
Cross section placement should be governed by changes in discharge, channel width, slope, shape, roughness, and the location of hydraulic structures (bridge, culvert, grade control structure, etc.). Typical cross section spacing may be in the range of 200 to 400 feet or closer if conditions warrant.
In addition to spacing of cross sections along a stream reach, the designer must consider the alignment of individual cross sections. Cross sections should generally be oriented to be perpendicular to the channel centerline and the water flow path. At times it may be necessary to include deflections in the cross section in order to be perpendicular to flow in the channel terraces. Figure 10-3 illustrates an example of cross section placement and alignment to capture channel flow paths perpendicular to the cross section.
October 2018 City of Durango 10-23 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Two-Dimensional Flow Modeling
Two-dimensional hydraulic modeling is not addressed in this Design Manual, although its use is becoming more widespread for evaluating complex hydraulic conditions. Discussion in this Design Manual is limited to one-dimensional modeling using HEC-RAS. This is the primary tool for modeling stream restoration improvements.
Figure 10-3. Example of HEC-RAS Cross Section Placement and Alignment
10-5-2-2 Cross Section Geometry
Required cross section input data includes station (x) and elevation (y) coordinates of the cross section, main channel bank stations, roughness coefficients, and contraction/expansion coefficients.
Cross Section Coordinates and Main Channel Bank Stations
Entering cross section coordinates can be accomplished in several different ways. Several software packages can generate cross section data from a digital terrain model and import it directly into HEC-RAS. Cross sections can also be entered manually. Regardless of the method used, it is critical that the input coordinates accurately represent the horizontal and vertical geometry of cross sections, so back-checking for quality assurance is recommended.
Once the station and elevation coordinates for the channel cross section have been input into HEC-RAS,
10-24 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
the main channel bank stations must be determined. It is generally recommended that the main channel be interpreted as a relatively narrow portion of the cross section. Photograph 10-11 illustrates the main channel and terraces in a typical cross section. The bankfull channel comprises the deepest part of the cross section, often has a lower roughness value than the vegetated terraces, and typically experiences the highest flow velocities. Sometimes small headwater channels and especially swales may have a vegetated bankfull channel as a result of minimal or no baseflow.
Required input for HEC-RAS includes reach lengths to the next downstream cross section. The downstream reach length for the main channel is measured along the established channel centerline. The downstream reach lengths for the left and right overbanks are measured following the flowpath of water in the overbanks from the centroid of flow in the overbank of one cross section to the centroid of flow in the next section downstream. This means that the overbank distance on the inside of the bend will be less than the overbank distance on the outside of the bend.
Photograph 10-11. HEC-RAS cross section definitions
10-5-2-3 Roughness Coefficients
Required HEC-RAS input also includes defining hydraulic roughness coefficients, or Manning’s n values, for the main channel, left overbank and right overbank. For channel cross sections that are best described with varying values for Manning’s n in the overbanks, the “Horizontal Variation in n Values” feature can be used. This feature allows the designer to specify a Manning’s n value at each cross section coordinate.
Selecting roughness values for the main channel and overbanks of each cross section in the model is an important task. Because this tends to be somewhat subjective rather than deterministic, it is recommended that hydraulic modeling be conducted in two ways. First, conservatively low roughness values should be used for assessing velocities, Froude numbers, and shear stresses. Second, conservatively high roughness values should be used for assessing water surface elevations and depths. The lack of vegetation in post- construction conditions will result in higher channel velocities and greater potential for erosion. Channels with fully established vegetation will have reduced velocities but higher flow depths.
October 2018 City of Durango 10-25 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Table 10-3 provides low and high roughness values that are suitable for initial approximations of hydraulic conditions; however, it is the designer’s responsibility to conduct a field reconnaissance of the stream reach being analyzed, characterize roughness conditions along the main channel and overbanks, and select appropriate roughness values. Additional information on estimating roughness values for grass overbanks and cobble channels is discussed below.
Table 10-3. Recommended Roughness Values When Assessing When Assessing Water Surface Location and Cover Velocity, Froude No., Elevation and Water Shear Stress Depth Main Channel (bankfull channel)
Sand or clay bed 0.03 0.04
Gravel or cobble bed 0.035 0.07
Vegetated Overbanks
Turfgrass sod 0.03 0.04
Native grasses 0.032 0.05
Herbaceous wetlands (few or no willows) 0.06 0.12
Willow stands, woody shrubs 0.07 0.16 (Source: Chow 1959, USDA 1954, Barnes 1967, Arcement and Schneider 1989, Jarrett 1985)
Roughness of Grass Overbanks
A common procedure for determining Manning’s n for vegetated channels is documented in the Handbook of Channel Design for Soil and Water Conservation (hereinafter referred to as the NRCS Method). The NRCS Method uses the vegetation properties to establish a degree of retardance. The retardance is based upon the type of plants, the length and condition of the vegetation. Finding a solution for Manning’s n becomes an iterative process using the following channel properties: slope, velocity and hydraulic radius. The documentation for the NRCS method contains a series of curves that provide solutions for Manning’s n values based upon the vegetation retardance. Refer to the NRCS Method documentation for additional detail and guidance.
Roughness of Cobble (Rock) Channels and Riprap Areas
There are multiple methods available for determining Manning’s n values for cobble/rock lined channels and significant areas of riprap. Two relationships are shown below; it is the responsibility of the designer to evaluate the methods available and determine the approach most appropriate for the specific project conditions.
10-26 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
Determination of Roughness Coefficients for Streams in Colorado (Jarrett 1985)
n 0.39S 0.38R0.16 Equation 10-3
Where:
S = channel slope (ft/ft)
R = hydraulic radius (ft)
The Manning's roughness coefficient, n, for a void-filled or soil riprap-lined channel may be estimated using:
1 6 n 0.0395D50 Equation 10-4
Where:
D50 = mean stone size (feet)
This equation is appropriate for computing channel capacity and associated flow depth, but when soil riprap is vegetated, velocity and shear computations should be based on the roughness provided by the vegetation and not the riprap.
This equation does not apply to grouted boulders or to very shallow flow (where hydraulic radius is less than, or equal to 2.0 times the maximum rock size). In those cases the roughness coefficient will be greater than indicated by this equation.
10-5-2-4 Design Storms
HEC-RAS refers to design storms as “profiles” and allows a designer to add multiple profiles. Boundary conditions are defined for each profile and options consist of known water surface elevation, critical depth, normal depth or rating curve.
It is recommended that the designer evaluate multiple return periods (profiles) when evaluating a stream reach. These may include the “bankfull” event, 2-year, 5-year, 10-year, 100-year, and perhaps larger events. Evaluation of multiple design storms allows the designer to see variations in flow patterns for different storm events and the resulting velocities, flow depths, etc. In some cases it may be appropriate to modify Manning’s n values based on the flow depth at a specific design storm to more accurately depict the flow conditions.
The 2-year through 10-year profiles are important when a shared-use path is planned adjacent to the stream to ensure proper elevation of the past. See Section 10-4 for criteria regarding trails including low-flow crossings.
10-5-2-5 Output Variables
Results from a HEC-RAS steady flow analysis can be viewed in both tabular and graphical format. Tabular output can be generated at an individual cross section or a summary table can be produced that includes multiple cross sections and multiple storm events (profiles).
The following is a list of key output variables that the designer should review during analysis (abbreviations used by HEC-RAS are indicated in parentheses). October 2018 City of Durango 10-27 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Water surface elevation (W.S. Elev); Critical water surface elevation (Crit W.S.); Froude Number (Froude #); Total flowrate within the cross section (Q Total), left overbank (Q Left), channel (Q Channel), and right overbank (Q Right); Average velocity in the main channel (Vel Chnl), left overbank (Vel Left), and right overbank (Vel Right); Hydraulic depth in the main channel (Hydr Depth C), left overbank (Hydr Depth L), and right overbank (Hydr Depth R); Specific Force for the cross section (Specif Force); Shear stress in the main channel (Shear Chan), left overbank (Shear LOB), and right overbank (Shear ROB).
The list above is a small sampling of the variables that HEC-RAS can provide. The designer is responsible for selecting output variables, evaluating all aspects of the channel hydraulics, and determining the acceptable values for the channel parameters based upon the specific project. Refer to the User’s Manual within the HEC-RAS Help menu for definitions of the model’s output variables.
10-5-3 Evaluation of Erosion at Channel Bends
Special erosion control measures are often needed at bends. Riprap sizing should be based on locally higher velocities at the outside of a bend. An estimate of velocity along the outside of the bend can to be made using the following equation.
r V (0.147 c 2.176)V Equation 10-5 a T
Where:
Va = adjusted channel velocity for riprap sizing along the outside of channel bends (ft/sec)
V = mean channel velocity for the peak flow of the major design flow (ft/sec)
rc = channel centerline radius (ft)
T = Top width of water during the major design flow (ft).
References for additional information: UDFCD Manual, Open Channels Chapter, for information on hydraulic analysis and HEC-RAS modeling USACE, HEC-RAS User’s Manual 10-6 Design Guidelines
Each reach or each segment of the project reach must be evaluated to determine the basin conditions that will influence its function within the drainage basin or watershed and the applicable design standards. Channel design requirements are determined based on their being categorized as either major or minor channels and by the particular characteristics of the project reach. References for specific design guidelines for major and minor drainageways are provided at the end of their respective sub-sections sections.
10-28 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
10-6-1 Major Drainageways
The natural channel design criteria described herein shall be used for all major drainageways unless otherwise approved by the City Engineer. Typical design elements included in a major natural channel design project are summarized as follows and shown in Figure 10-4:
1. Create low-flow channel.
2. Establish a low-flow design longitudinal slope.
3. Utilize vegetated benches to convey overbank flow.
4. Stabilize eroding banks.
5. Analyze floodplain hydraulics.
6. Consider aquatic ecology.
7. Undertake major drainageway plan improvements if required.
Figure 10-4. Design Elements Associated with Major Natural Drainageways
These seven steps are summarized in the following sections and comprise the recommended design approach for preserving, restoring, or modifying natural healthy drainageways. References that provide detail on the design approach for major drainageways are listed at the end of this section. Designers shall address these seven elements and submit their proposed approach for drainageway stabilization for review and approval by the City.
October 2018 City of Durango 10-29 Storm Drainage Design Criteria Manual Open Channels Chapter 10
10-6-1-1 Create Low-Flow Channel
One of the primary design tasks for a major drainageway is to preserve or establish a low-flow channel that is appropriately sized in relation to the adjacent overbank geometry and the design low-flow rate. In general, shallow low-flow channels with adjacent well-vegetated overbank benches are best suited to spread-out and attenuate flood flows. The top of low-flow channel banks shall normally be established along the edge of the historic overbank. This may require filling degraded incised channels, excavating overbank benches adjacent to the low-flow channel, or some combination of the two.
Usually, filling a degraded channel is the option that results in the least disturbance to existing floodplain vegetation and restores the relationship between the low-flow channel and the floodplain, although filling generally will impact biota that are able to inhabit the degraded channel. Sometimes, it may be difficult to raise the invert of a degraded channel. Existing storm sewer outfalls may have been installed near the bottom of the incised channel and constrain how much the channel bed can be raised. It may be necessary to remove the downstream end of low storm sewer outfalls and reconstruct them at a higher elevation.
Raising the invert may also cause a rise in a critical floodplain elevation if the regulatory floodplain was based on the degraded channel condition (it is recommended that floodplains be determined for restored, not degraded channel conditions). There may be a need for compensatory excavation in other portions of the floodplain to offset rises in the floodplain caused by filling in the eroded low-flow channel.
The width of the low-flow channel shall approximate the width of the historic low-flow channel within the design reach or in stable reference reaches upstream or downstream. Normally, a low-flow channel exhibits some meandering and sinuosity in natural channels. Modified channels should feature a meander pattern typical of natural channels. Side slopes for low-flow channel banks shall be no steeper than 4H:1V for unlined banks. Lesser slopes are encouraged and may provide improved vegetative cover, bank stability and access. Allowable velocities for unlined low-flow channels are shown in Table 10-4.
Table 10-4. Hydraulic Design Criteria for Natural Unlined Channels Design Parameter Erosive Soils Erosion Resistant Soils or Poor and Vegetation Vegetation Maximum Low-flow Velocity (ft/sec) 3.5 ft/sec 5.0 ft/sec Maximum 100-year Velocity (ft/sec) 5.0 ft/sec 7.0 ft/sec Froude No., Low-flow 0.5 0.7 Froude No., 100-year 0.6 0.8 Maximum Tractive Force, 100-year 0.60 lb/sf 1.0 lb/sf 1 Velocities, Froude numbers and tractive force values listed are average values for the cross section. 2 “Erosion resistant” soils are those with 30% or greater clay content. Soils with less than 30% clay content shall be considered “erosive soils.”
Baseflow Channel
If baseflows are present within the low-flow channel or are anticipated to be present in the future, it must be determined how the baseflows will be accommodated. Two common approaches include:
1) The invert of the low-flow channel can be shaped to accommodate a defined baseflow channel and a lower secondary overbank area, or
2) The baseflow can be allowed to meander in the bottom of the low-flow channel without modifying 10-30 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
the low-flow channel section.
The baseflow rate may be based on available records from gage data, when available, but can be estimated based on field observations, seasonal hydrology and channel characteristics. The invert of the baseflow channel is typically unvegetated if a constant baseflow or frequent ephemeral flow is present. If baseflows are less frequent, the invert of the baseflow channel can be vegetated with riparian or wetland species.
Wetland Bottom Channels
There are circumstances where the use of a wetland bottom may be appropriate within the low-flow channel of a natural channel reach. Riprap bank protection will generally not be required in wetland bottom channels. Freeboard requirements for wetland bottom channels shall be the same as those given for grass- lined channels.
Bioengineered Channels
Elements of bioengineered channels may be used in the design or stabilization of natural channels. Freeboard requirements for bioengineered channels shall be the same as those given for grass-lined channels.
10-6-1-2 Establish a Low-Flow Design Longitudinal Slope
Watershed development tends to cause channel degradation and a reduction in channel slopes. Therefore, the long-term stable slope of the low-flow channel is expected to be less than for undeveloped conditions and less than the longitudinal slope of the adjacent overbanks. To accommodate this anticipated change, grade control structures are required in the low-flow channel to create a “stairstep” profile to stabilize the low-flow channel and maintain the natural relationship between the low-flow channel and the floodplain.
The estimated design slope determines how many grade control structures are required. A flatter design requires more grade control structures and increases costs. The spacing of drop structures depends on the original natural channel slope and the design slope necessary to stabilize the channel. The design and placement of grade control structures is described in Section 10-7, Grade Control Structures.
Ultimate Design Slope
Several methods have been developed to estimate channel slopes for ultimate (full) build-out upstream drainage basin conditions. When sediment loads are expected to decrease significantly and flows are expected to increase significantly, estimates of the ultimate stable slope tend to be flat. Even when flows are properly regulated through detention storage ponds upstream, the reduction in sediment load will still result in very flat estimates of the ultimate channel slope.
Interim Design Slope
When a long-term sediment supply may be present or when the time required for channels to reach their ultimate design slope can be long, methods that estimate an ultimate design slope based on a very limited sediment supply may be too conservative and increase the cost of channel stabilization. Therefore, intermediate design slopes may be used to construct fewer grade control structures initially if the need to ultimately construct additional structures is recognized and funded. Estimating design slopes in developing watersheds is complicated by difficulties in estimating interim sediment supplies, flows, channel dimensions and floodplain encroachment. Understanding these development impacts on channel slope can be important for financing long-term stabilization needs and designing effective structures.
October 2018 City of Durango 10-31 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Various methods for estimating interim channel slope changes as a result of development can be applied; however, due to the lack of availability of historic data, the uncertainty of changes in key input parameters (such as sediment load and flow), the experience required to apply them correctly and the uncertainty of the results, a geomorphologist or civil/water engineer with expertise in sediment transport should be consulted. Details of the methods used for these analyses are not provided in this Manual. References on this topic are provided at the end of Section 10-6-1.
If an applicant is considering sediment transport analysis to determine an interim channel slope, a meeting with the City Engineer should be held to discuss the proposed methods of sediment transport/channel stability analysis before any formal submittal.
Estimating Historical Slopes
If field investigations or analyses of historical data indicate that channel conditions are currently in equilibrium, then measurements of the existing bed slope in the field or from topographic mapping can be used to provide a starting point for evaluating changes that may occur due to increased volume, flow rates and changes to sediment supply in the future. Channel slopes can vary along a stream reach, so care must be exercised to utilize a slope value representative of the entire reach under design. Potential indicators of historical or on-going degradation include exposed infrastructure (pipe crossings or bridge foundations), extensive bank erosion and steep channel banks where the channel invert is below the roots of adjacent bank vegetation or has begun to expose them. Historical topographic mapping, FEMA studies, bridge or other structure design drawings can also provide insight on changing conditions. If field investigations or historical data indicate that channel conditions may not currently be in equilibrium, then data (aerial photos, topographic data or maps, photos, historical design drawings, etc.) or studies from a historical time when equilibrium conditions existed should be used to estimate the historical slope.
Stable reference streams or reference reaches can be used to estimate a stable slope. Geomorphic analysis of channel bank and valley slopes can be used to estimate channel slope for pre-development (undisturbed) conditions. The selection of reference reaches and geomorphic analysis of bank and valley slopes may be highly subjective and should be carried out only by qualified professionals with experience in geomorphology. The key is to select reference reaches that have approximately the same sediment supply, valley setting and boundary conditions. In many instances these criteria are only met just upstream if they are met at all. Downstream reference reaches are sometimes adjusted to the increased sediment delivery provided by unstable design reaches that are upstream. A reference on the subject of geomorphic analysis of stream channel reference sites is provided at the end of Section 10-6-1.
Detailed Sediment Transport Analysis
A detailed sediment transport analysis may be appropriate when potential cost savings and available data are sufficient to justify the level of expertise and technical analyses required to produce reasonable results. These approaches to sediment transport analysis generally require using computer-based modeling. The most commonly used one-dimensional sediment transport model is HEC-6; however, most of its functions have now been incorporated into HEC-RAS. Other models that represent two-dimensional or even three- dimensional conditions are available, but are computationally intensive and are not generally applicable for most routine channel design projects. A computer-based approach for modeling sediment transport analysis offers the following benefits:
10-32 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
. Geometry: Variations in channel geometry along reach can be modeled (mobile bed options only).
. Sediment Data: Sediment gradation data is utilized in the model and can be varied along the length of the channel reach depending on the number of samples taken. Most sediment transport models route sediment by size fraction and can simulate armoring (mobile bed option only).
. Sediment Inflow Data: A critical input into sediment transport models is the amount of sediment that is expected to enter the upstream end of the study reach or that might enter through tributaries along the study reach. This can be very difficult data to obtain or estimate.
. Hydrology: Measured or synthetic long-term hydrology (years) or hydrographs for single events can be discretized and modeled.
In spite of the greater level of detail, sediment transport modeling results can still have a wide margin of error and must usually be evaluated for reasonableness by comparisons with more conventional methods. Even with the greater level of detail, both data and modeling will have significant limitations and results should generally be interpreted only as indicating trends or ranges of potential change rather than exact future stream grades.
The HEC-RAS Version 4.1 Hydraulic Reference Manual introduces the discussion on sediment transport modeling by noting: “Sediment transport modeling is notoriously difficult. The data utilized to predict bed change is fundamentally uncertain and the theory employed is empirical and highly sensitive to a wide array of physical variables.” In keeping with this cautionary statement, uncertainty associated with modeling results should be considered when interpreting results. One of the most significant limitations of HEC- 6/HEC-RAS modeling is that lateral bank erosion processes are not effectively modeled.
Detailed sediment transport modeling has some significant practical challenges, including:
. Considerable cost is typically required to develop model input data (hydrology, sediment, geometry) and to carry out the modeling itself.
. The method does not lend itself to standardized or “cookbook” approaches that can be concisely presented in a criteria manual. Considerable expertise and experience related to sediment transport modeling, hydrology and geomorphology are required.
In general, the importance of conducting a sediment transport analysis increases with higher sediment supply, the extent to which sand dominates the channel bed material, and the overall liability of the channel, (i.e. flow energy relative to the erodibility of the channel boundary and floodplain materials). Estimates of sediment transport capacity-based empirical relationships are often highly uncertain when the results are used as absolute magnitudes of sediment transport. However, application of a consistent and appropriate sediment transport relation can be very useful and quite accurate in estimating relative transport capacities among stream segments.
10-6-1-3 Utilize Vegetated Benches to Convey Overbank Flow
For existing natural channels, vegetated benches often exist just above the tops of the eroded baseflow channel. When the historic natural floodplain is preserved and flows from upstream of the project reach are not expected to increase, it is likely that the undisturbed overbank areas of natural channels will be stable and require little or no stabilization. Raising the invert of degraded channels usually establishes a favorable overbank geometry. If necessary, benches can be excavated adjacent to the low-flow channel, especially if impacts to existing vegetation are minimal. It may be necessary to re-establish or supplement vegetation on the overbanks to build up a sturdy, durable cover to help retard flood flows and resist erosion.
October 2018 City of Durango 10-33 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Except for the delineation of the floodplain limits, the hydraulic characteristics of this portion of the natural channel should not be a design consideration when the natural floodplain is stable and preserved.
10-6-1-4 Stabilize Eroding Banks
Steep unstable banks existing within the 100-year floodplain should be sloped back and stabilized. On a plan-view topographic map, designers shall indicate the location, height and existing slope of any unvegetated, steep, or otherwise unstable banks within the 100-year floodplain, along with the proposed approach for stabilizing the banks. This may occur where the low-flow portion of the channel has meanders that impinge on the outer channel banks.
The designer shall consider the existing bank conditions and angle of attack, the estimated potential for future erosion, and the proximity of infrastructure that could be impacted by the bank erosion as a basis for determining the appropriate method for bank stabilization. Other channel characteristics such as channel geometry, longitudinal slope, existing vegetation, underlying soils, available right-of-way and expected flow conditions shall be considered and analyzed with respect to the various potential improvements.
Unstable banks shall be protected using one of the following approaches.
1. Sloping Back Banks: Steep, unstable banks shall be cut back to a flatter slope and revegetated. The maximum permissible slope shall generally be 4H:1V (horizontal:vertical). Reducing bank slopes to 6H:1V or flatter will assist in the establishment and viability of vegetation, the stability of channel banks and accessibility of the waterway for recreation. Designers are encouraged to utilize flatter slopes whenever possible. In some locations, right-of-way constraints may dictate steeper slopes. In such areas, slopes up to 3H:1V may be permitted with appropriate slope protection and approval.
2. Riprap Bank Protection: Riprap bank protection is widely used to stabilize channel banks along the outside of existing channel bends and along steep banks that cannot be graded back sufficiently due to right-of-way constraints, where flow velocities are too high, or where overbank grades are too steep. Riprap bank protection shall be designed in accordance with the Open Channels Chapter of the USDCM. All riprap bank protection shall consist of soil riprap that is buried with topsoil and revegetated.
The riprap need only extend up the slope to where shear stresses do not exceed those for natural unlined channels as defined in Table 10-3. By applying those allowable shear stress limits to the equation for shear stress, the vertical distance from the 100-year water surface to the upper limit of the riprap layer can be calculated as follows:
If τ = γdS, then
d = τ/γS Equation 10-7
For Erosive Soils, τ = 0.6 lb⁄sf and if γ = 62.4 lb/cf, then
d = 0.0096⁄S Equation 10-8
For Erosion Resistant Soils, τ = 1.0 lb⁄sf and if γ = 62.4 lb/cf, then
d = 0.0160⁄S Equation 10-9
Where:
10-34 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
d = vertical distance below 100-year water surface
S = channel overbank slope in ft/ft
3. Bioengineered Bank Protection: Experience with the application of bioengineering techniques to protect channel banks is growing along the Colorado Front Range. Bioengineering techniques are discussed in the Open Channels Chapter of the UDFCD Manual.
10-6-1-5 Analyze Floodplain Hydraulics
The floodplain associated with existing or modified natural channels shall be analyzed using HEC-RAS to delineate the 100-year floodplain and evaluate flow velocities to assess drainageway stability based on flow rates for the range of design flows. It is important to analyze floodplain hydraulics based on conditions that are likely to cause the greatest resistance to flow and the highest water surface elevations in the short term and over time. Some of these conditions may include the following:
. Increased baseflows and runoff from development that promote increased growth of wetland and riparian vegetation, making drainageways hydraulically rougher.
. Stream restoration work that raises the bed of incised channels to levels that existed prior to degradation or flattens channel slopes.
. Upstream bank erosion or watershed erosion, flatter slopes, and increased channel vegetation that lead to sediment deposition and channel aggradation, raising streambed and floodplain elevations.
An accurate delineation of the floodplain is also necessary for laying out development projects and setting lot and building elevations adjacent to the floodplain according to the freeboard requirements defined in the Floodplain Management Chapter. For facilities that are not structures (typically not requiring a building permit) such as roadways, utility cabinets, parks and trails improvements, etc., a minimum of 1 foot of freeboard is desirable. Assessments of freeboard at bends shall take into account super elevation. The required freeboard should be contained within a floodplain tract and/or easement.
Incised or eroded channels shall not be analyzed based on their existing geometry, but on the geometry representative of a restored natural channel. Otherwise, the floodplain may be inappropriately low, constraining future restoration efforts such as installing grade control structures that raise the channel bed back to earlier conditions.
Floodplain Encroachments
Floodplain encroachments that reduce natural channel storage and increase downstream flows or velocities are discouraged. However, when encroachments are approved and proper documentation is submitted and approved as described in the Floodplain Management Chapter, channel hydraulics must be fully analyzed to ensure that the remaining natural channel features or designed low-flow channel are stable during flood flows.
10-6-1-6 Consider Aquatic Ecology
When streams or major drainageways have conditions that are favorable for supporting fish, additional consideration should be given to the baseflow and low-flow channel designs to provide conditions that are consistent with good aquatic ecological conditions, fish habitat and fish passage.
Aquatic habitat is degraded in a variety of ways by watershed urbanization and stream modification. Potential impacts include water quality, water quantity, loss of bank vegetation, bank erosion and channel October 2018 City of Durango 10-35 Storm Drainage Design Criteria Manual Open Channels Chapter 10
invert degradation. Implementation of the natural stream design principles presented in this Manual can significantly help preserve or improve aquatic habitat. Important aquatic habitat design considerations include:
1. Water Temperature. Water temperature is one of the most important factors in determining the distribution of fish in freshwater streams (FISRWG 2001). Feeding and spawning activities are often keyed to water temperatures, and high water temperatures can be lethal to some species. Often in degrading stream systems, bank erosion results in a loss of perimeter vegetation and a widened channel bottom that produces shallow-flow depths. Limiting baseflow channel widths to increase typical flow depths and providing bank vegetation for shading can reduce solar heating of the water.
2. Cover and Refuge. Providing cover in the form of overhead vegetation, boulders, large woody debris, pools and other irregular features provides fish with spawning areas, protection from predation, and habitat for species that are critical to the food chain. Channel design elements that can contribute to enhanced cover include pool and riffle sequences, a variety of vegetation types along the channel edge, variations in baseflow channel geometry, scour holes, groupings of boulders, and woody debris such as root wads and logs in various configurations.
3. Habitat Diversity. Diversity of habitat and hydraulic conditions allows for a greater diversity of species and a richer ecosystem. Channel designs can incorporate riffles, pools, small drops, boulders, large woody material, changes in channel geometry and a variety of riparian plant types to create diversity.
4. Water Quality. High organic matter and chemical content is common in urban stream systems. Channel designers typically have limited ability to change or rectify these conditions; however, identifying and understanding the characteristics of these sources should be incorporated into the project design. Sources typically include wastewater treatment plant discharges and urban runoff carrying various chemicals, fertilizers, yard cuttings and other organic matter. High organic content can lead to low dissolved oxygen levels and the death of aquatic organisms. Shading of channels with vegetation to reduce water temperatures and creating riffle and drop structures to induce aeration can help with this problem.
5. Substrate. Sand and silt substrates are generally the least favorable alluvial materials for supporting aquatic organisms and support the fewest species and individuals (FISRWG 2001). Smooth bedrock surfaces devoid of alluvium, are even less favorable. Raising degraded channel inverts with grade controls can naturally restore alluvial channel bottoms. Riffles and other rock structures can also add diversity to the substrate.
6. Hydrology. Both increases and decreases in natural channel flows can have adverse impacts on aquatic habitat. Withdrawals of water for agricultural, industrial and municipal uses can reduce stream flows to essentially dry conditions at some times of the year. Increases in flows from lawn watering return flows, runoff associated with increased imperviousness, and wastewater treatment plant discharges increase velocity and shear and can erode channel banks and bottoms which results in deterioration of habitat features and cover.
7. Stream Crossing Structures. Stream crossing structures, such as grade controls, culverts or bridges, which create high velocity flows or discontinuities in the water surface, can be an impediment to migration. Disconnecting stream segments with impassible hydraulic structures results in genetic isolation, which also degrades species viability.
Maintaining natural stream systems and corridors is the best way to provide adequate and sustainable habitat for
10-36 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
fish. Where restoration is taking place or where natural stream functions are limited by urbanization impacts, structures specifically constructed to enhance fish habitat may make sense. The references listed at the end of Section 10-6-1 provide a summary of details for the basic techniques most commonly employed to preserve and protect aquatic habitat, when they may be appropriate and cautions in their use.
10-6-1-7 Undertake Major Drainageway Plan Improvements if Required
In addition to the six mandatory design elements discussed above, additional major drainageway plan improvements may be required on a case-by-case basis.
References for additional information:
UDFCD Manual, Volume 1, Open Channels Chapter, for major drainageways design criteria
Stream Channel Reference Sites: An Illustrated Guide to Field Technique (USDA 1994) provides guidance on field measurement techniques.
10-6-2 Minor Drainageways
Constructed natural channels, including grass-lined channels or composite channels, shall generally be used for minor drainageways. However, constructed channels that are riprap-lined, concrete-lined or manufactured lining types may be necessary due to project constraints. The use of conduits is discouraged and must be approved on a case-by-case basis.
10-6-2-1 Constructed Natural Channels
Because the upstream drainage basin conditions are expected to change dramatically for minor drainageways, resulting in higher flows and low sediment loads, it is likely that creating a naturalistic channel design will require significant regrading of unimproved channels. This will generally require the removal and reestablishment of natural vegetation, rather than its preservation.
For constructed drainageways designed to emulate unlined natural channels, the parameters in Table 10-4 shall be achieved for both the low-flow and the 100-year event. Existing natural features should be protected to the extent practical. Hydraulic modeling shall be based on the channel and overbank definition shown in Figure 10-4 and on the roughness information identified in Table 10-3. Constructed natural channels must be analyzed for both higher velocity conditions, when projects are newly completed and vegetation may not have matured, and for higher flood potential and capacity conditions, when vegetation has fully matured and creates the greatest resistance to flow.
10-6-2-2 Grass-Lined Channels
Grass-lined channels are an option for minor drainageways, especially where the tributary area is relatively small and minimal baseflows are expected. Sod-forming native grasses suited to wetter conditions are recommended for grass-lined channels. If irrigated bluegrass sod is proposed, a small baseflow channel shall be provided and vegetated with the wetter, sod-forming native grasses. Hard-lined baseflow channels are not desired in grass-lined channels. Grade control structures or rock stabilization in the bottom of the channel may be necessary if velocities or longitudinal slopes exceed the values in Table 10-5.
October 2018 City of Durango 10-37 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Table 10-5. Hydraulic Design Criteria for Grass-Lined Channels Grass: Grass: Erosion Design Item Erosive Soils Resistant Soils Maximum 100-year velocity 5.0 ft/s 7.0 ft/sec Minimum Manning’s “n” for capacity check 0.035 0.035 Maximum Manning’s “n” for velocity check 0.030 0.030 Maximum Froude number 0.5 0.8 Maximum 100-year depth outside low-flow zone 5.0 ft 5.0 ft Maximum channel longitudinal slope 0.6% 0.6% Maximum side slope 4H:1V 4H:1V 2 x top width 2 x top width Maximum centerline radius for a bend (200 ft min.) (200 ft min.) 1 Velocities, Froude numbers and tractive force are average values for the cross section. 2 “Erosion resistant” soils are those with 30% or greater clay content. Soils with less than 30% clay content shall be considered “erosive soils.”
10-6-2-3 Composite Channels
Composite channels include a low-flow channel and a constructed floodplain that will normally convey flows much greater than undeveloped flows.
10-6-2-4 Wetland-Bottom Channels
There are circumstances where the use of a wetland-bottom channel may be appropriate. These channels are a special case of composite channels where it is intended that the lower portion of the low-flow channel be designed to support wetland plants.
10-6-2-5 Bioengineered Channels
When bioengineered channel treatments are included in composite channels, they shall be designed using the guidance provided in the Open Channels Chapter of the UDFCD Manual.
10-6-2-6 Constructed Channels
Constructed channels may be necessary when the upstream drainage basin is highly developed and design flows are significantly greater than undeveloped flows, when sediment loads are low, and where available right-of-way is restrictive. These channels retain few of the benefits of natural channels and primarily function as flood conveyance structures. Because these channels are generally steep and the flow is confined, design velocities tend to be higher, requiring a hardened channel lining to maintain stability. However, there are maximum velocity limitations on these channels; therefore, drop structures must be used to reduce design slope and lower velocities to acceptable limits. These structures will typically be designed for 100-year flows and will most often be lined with riprap, soil riprap, or concrete, but may also be lined with manufactured systems.
Because these types of channels eliminate any overbanks or floodplains, base-flow channels or low-flow channels do not normally provide a benefit. The use of base-flow or low-flow channels in these types of channels can help to pass sediment through the system and reduce maintenance requirements if sediment loads are present; however, in many cases, the available sediment load will be limited.
10-38 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
10-6-2-7 Riprap-Lined and Concrete-Lined Channels
The use of plain (not buried) riprap-lined or concrete-lined channels is generally discouraged, but they will be considered for minor drainageways on a case-by case basis. Design criteria for concrete-lined and riprap- lined channels are provided in the Open Channels Chapter of the UDFCD Manual. Freeboard requirements for riprap and concrete-lined channels shall be the same as those given above for grass-lined channels. Additionally, if supercritical flow is present in concrete-lined channels, freeboard shall be computed in accordance with the Open Channels Chapter of the UDFCD Manual.
References for additional information:
UDFCD Manual, Volume 1, Open Channels Chapter
UDFCD Manual, Volume 3, Treatment BMPs Chapter, for guidance for wetland channel bottom 10-7 Grade Control Structures
Grade control structures provide energy dissipation and are used to flatten longitudinal channel slopes and moderate flow velocities. This chapter provides general guidance for grade control structure.
Table 10-6 provides typical maximum drop heights for grade control structures. Grade control structures are normally constructed as hardened drop structures, but may be implemented in other forms, such as rock riffles, with approval. Common approaches shall be considered when implementing grade control structures, as discussed below.
Table 10-6. Grade Control Drop Height Limits Capacity of Grade Control Maximum Drop Height Structure (feet) Low-flow Discharge 1.5
Between Low-flow and 100-year 2.5
100-year and Greater 4.0
10-7-1 Low-Flow Drop Structures
Low-flow drop structures are grade control structures that extend only across the low-flow channel to provide control points to limit degradation at specific locations and to establish flatter thalweg slopes. During a flood event, portions of the flow will circumvent the structure and travel in the overbank portion of the channel. These structures are only appropriate for natural channel types or for constructed natural channels when overbank conditions do not exceed the allowable limits so that full-width drop structures are not necessary.
Typically, low flows are contained within the hardened portion of these structures and fill the full cross section of the structure without freeboard. Low-flow drop structures are not appropriate within completely incised floodplains or very steep channels where the velocities shown in Table 10-3 cannot be achieved.
To provide a stable structure, secondary design flows must also be evaluated. The secondary design flow is the flow that causes the worst condition for flow around the sides of the structure, stability within the structure, or as flows return back into the low-flow channel downstream (i.e., during a 5-year, 10-year, or October 2018 City of Durango 10-39 Storm Drainage Design Criteria Manual Open Channels Chapter 10
100-year event). Designers must evaluate site-specific hydraulics to determine the extent of surface protection and where in the cross section it may be appropriate to transition to softer types of protection such as vegetated soil riprap. One approach to analyze the hydraulics of low-flow drops is to estimate unit discharges, velocities and depths along overflow paths. The unit discharges can be estimated at the crest or critical section for the given total flow. Estimating the overflow path around the check can be difficult and requires judgment. The flow distribution option in HEC-RAS may be used to assist in evaluating minimal or reasonable damage to the floodplain below.
The minimum crest depth (from the invert of the crest to the top of the structure at the beginning of the overbank area) for low-flow drop structures is 1.5 feet. The maximum drop height of low-flow channel grade control structures shall generally be limited to 1.5 feet.
Seepage control is also an important consideration because piping and erosion under and around these structures can contribute to their failure. It is essential to provide a cutoff wall that extends laterally at least 5 to 10 feet into undisturbed bank and that has a depth appropriate to the profile dimension of the drop structure.
Check structures described in the USDCM are implemented within the City of Durango as temporary devices with the expectation that drop structures will replace the check structures as the channel degrades. This approach is not appropriate when long-term improvements must be completed with limited capital funds or for cost estimates for long-range basin plans. Rather than constructing temporary check structures, it is more appropriate to construct fewer permanent drop structures within a project reach with the goal of adding additional structures later. However, this approach is only appropriate if a funding source is available for completing the later improvements. In any case, channels must be designed for ultimate conditions so that adequate funding can be identified for permanent channel improvements as needed.
10-7-2 Full-Channel-Width 100-Year Drop Structures
Full-channel-width drop structures are structures that are designed to convey the major flood flow within the structure and to provide a stepped invert profile so that channel velocities (both in the low-flow channel and in the overbank area) do not exceed allowable limits. These structures are necessary in constructed natural channels and constructed channels when 100-year flood flow velocities exceed allowable limits.
Each drop structure location is unique and designers should evaluate the required extent of hardened drop structure materials across the floodplain for each individual structure. Often grouted boulders do not need to extend to the limit of the 100-year floodplain, even where channels are incised to some degree and the floodplain has been encroached upon. Shear and velocity values typically decrease with increasing distance from the main channel; therefore, transitions to soil riprap and then to vegetation may be feasible. These floodplain hydraulic characteristics should be evaluated and hardened surfaces and soil riprap used only where necessary to minimize costs and enhance aesthetic and environmental qualities.
10-7-2-1 Constructed Natural Channel Drop Structures
When deep channel incision and/or development in the floodplain or increased flood flows have already occurred, the potential for channel restoration may be limited. In such cases, drop and grade control structures may be necessary to convey the major flood without causing significant damage. The maximum allowed height for such structures is 4 feet. This criterion has been established to limit channel incision, minimize the amount of bank stabilization required, avoid developing excessive kinetic energy, avoid lowering of the groundwater table, and minimize the obtrusive appearance of massive structures.
In addition to these standard criteria, designers should consider the necessary extent of grouted rock or other hardened surface material. It may not be necessary for the hardened surface to extend across the entire 100-
10-40 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
year waterway to provide 100-year protection. Instead it may be possible to transition to softer treatments such as vegetated soil riprap at the point in the floodplain where velocities and shear stresses are sufficiently reduced according to the criteria defined in Table 10-3.
10-7-2-2 Constructed Channel Drop Structures
Constructed channel drop structures are placed in channels that are fully hardened and under significant hydraulic stresses. These conditions require full-width, 100-year drop structures and shall be designed in accordance with the guidance provided in the UDFCD Manual.
10-7-3 Drop Structure Types
The use of drop structure types and configurations that are functional, natural-looking, provide for fish passage, and blend-in with the drainageway and surrounding environment are encouraged. Grouted boulders can be used to develop unique, natural looking configurations such as a horseshoe-arch shape or stepped configurations. Other drop types commonly used include sheet pile drops, sculpted concrete drops, and soil cement drops. The sculpted concrete drops have become more popular for aesthetic reasons, particularly in upland prairie settings. The concrete is shaped, sculpted, and colored with earth tones to emulate natural rock outcroppings. Use of the following drop structure types is preferred:
. Grouted sloping boulder
. Grouted boulder in natural configurations
. Sculpted concrete.
Design guidance, detailed design criteria, and construction details have not been developed by the UDFCD for sculpted concrete drop structures. It is the responsibility of the design engineer to develop and provide detailed construction drawings, based on previous experience in the design of sculpted concrete drop structures or review of past designs that have been constructed.
The use of soil cement and roller-compacted concrete drop structures may be allowed, but only on a case- by-case basis. Steady baseflows can quickly erode soil cement, especially when there is significant sediment being transported. Soil cement structures may be provided with a hardened low-flow channel to prevent erosion or should be reserved for ephemeral or intermittent channels. Specifications and construction quality control needed for soil cement and roller-compacted concrete are extensive and generally must be in accordance with standard specifications developed by organizations such as the Portland Cement Association.
Vertical drops greater than 2 feet in height are not permitted for safety reasons. In dry conditions, the vertical face presents a fall hazard. Under flowing conditions, reverse flows on the downstream face can form dangerous “keeper” hydraulic conditions. Vertical drops greater than 2 feet in height may be permitted, but drop heights should consider fish passage if the stream supports a fishery. Additionally, they should be constructed using natural or natural-appearing materials such as grouted boulders. The use of sheet pile or cast-in-place concrete walls for these structures is generally discouraged for aesthetic reasons.
Other methods of constructing low-flow drop structures, including rock riffles, ungrouted boulder drops and boulder cross vanes, may also be acceptable when floodplain and hydraulic conditions are appropriate for their use and when properly designed. These types of structures will generally not be appropriate in situations where there has been significant encroachment into the floodplain, where an incised channel condition will exist, or where urbanization has significantly increased peak flood flows. Approval of the use of such structures will be on a case-by-case basis.
October 2018 City of Durango 10-41 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Where fish passage is a concern at grade control structures, additional information can be found in the references provided at the end of this chapter. Designing to accommodate fish passage must first identify target species and then establish adequate flow depths, meet maximum allowable flow velocities and distances between refuges and meet maximum vertical drop heights (if any). A variety of configurations are possible, but given the limited swimming and jumping capabilities of some fish species, use of separate fishways or ramps that allow steeper slopes across the main channel portion of a drop structure will often be the most economical approach. In addition to the swimming and jumping performance criteria previously mentioned, the design of separate fishways requires careful attention to flows and a crest design that ensures the entrance to the fishway has adequate depth and does not become obstructed by sediment or debris over time. A high level of care, attention to detail, and supervision will generally be required during construction of any fish passable structure to ensure the constructed passage meets stringent criteria.
10-7-4 Drop Structure Placement
The distance between drop structures varies with the difference between the bank slope and the design slope and the height of the upstream structure. The distance between drop structure crests is determined by dividing the height of the upstream structure by the difference between the top of bank slope and the invert design slope. By intersecting the design slope with the toe of the face of the upstream drop structure, the proper relationship between the drop structures will be maintained. Drop structures must extend down below the design slope to provide protection from local scour and long-term degradation that might extend below the estimated design slope.
Drop structures may also need to be placed where necessary to protect upstream infrastructure or to control water surface elevations to divert flood flows into detention facilities or diversion channels.
References for additional information:
UDFCD Manual, Volume 2, Hydraulic Structures Chapter 10-8 Riprap and Boulders
In conditions where rock protection is required, it is recommended that soil riprap, void-filled riprap, or boulders be used. For small installations, and where vegetation is not anticipated, riprap over bedding material may also be used. This chapter provides a general discussion of the usage of riprap and boulders for channel armoring.
10-8-1 Void-Filled Riprap
Void-filled riprap is designed to emulate natural rock riffle material found in steep gradient streams. It contains a well-graded mix of cobbles, gravels, sands, and soil that fills all voids and acts as an internal filter. As a result, a separate bedding layer between subgrade and rock is not required.
10-42 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
In applications where it is difficult to establish vegetation, void-filled riprap is better able to resist the direct, prolonged impingement of water on the riprap installation compared to soil riprap. However, void-filled riprap is more difficult to properly mix and install compared to soil riprap.
In addition to specifying the D50 rock size, individual material components that will make up the void-filled riprap mix needed to be specified. The gradation of each material component should be specified by identifying a variety of particle sizes (from large to small) and the range of allowable “passing” percentages for each particle size. The designer should specify any mix adjustments based on Photograph 10-12. Void-filled riprap is designed to emulate natural rock riffle material. the requirements of a particular project.
10-8-2 Soil Riprap
Soil riprap refers to riprap that has all void spaces filled with topsoil with the intention of supporting vegetative growth. Soil riprap is intended for use in applications where vegetative cover can be established and where the shear stress, imposed by frequently occurring flows, is less than the resistive strength of the vegetation and soil. The riprap layer is designed to remain stable and provide protection during the extreme events.
When installed outside of the low-flow channel, UDFCD frequently specifies 4 to 6 inches of topsoil on top of soil riprap to help establish vegetation. Soil used in the voids and placed on top of the soil riprap should meet the description for viable topsoil composition for Colorado native plant establishment and upland areas as defined in the Revegetation Chapter of this Design Manual. (See the Revegetation Chapter in the UDFCD Manual for a fabric staking detail that can be used where fabric is specified over soil riprap). The combination of straw and coir mat is frequently used to help retain soil and seed. This is especially useful when topsoil is placed on top of soil riprap and then seeded. Specifications for mixing and installing soil riprap are further addressed in the UDFCD Construction Specifications.
A comparison between void-filled riprap and soil riprap and is provided in Table 10-7.
October 2018 City of Durango 10-43 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Table 10-7. Comparison of Void-Filled Riprap and Soil Riprap Advantages/ Void-Filled Riprap Soil Riprap Disadvantages . Better emulates natural streambed . Requires mixing of only two material. different materials and, therefore requires less effort to order, . Provides better stability and stockpile, and mix materials. armoring in riverine environments. . Requires less expertise and . Creates a dense, interlocking mass oversight during mixing and Advantages and functions as an effective internal placing. filter, keeping water flows on the surface and reducing the likelihood . Organic material within the growing that flows will displace the material medium supports riparian and expose a weak spot in the vegetation. subgrade.
. Provides a growing medium that supports riparian vegetation. . Requires mixing up to five or six . Does not provide the same level of different aggregates in the proper stability and armoring in areas of proportions. This requires additional direct, continuous flow effort in ordering, stockpiling, mixing, impingement. and placing materials compared to soil riprap.
. Difficult to inspect after installation because small void-material can cover larger riprap. Need to inspect during placement.
. Requires construction observation for submittal and sample material Disadvantages review, adjustments to the mix proportions to compensate for varying material gradation, approval of test fields mixing operations, and observation of placement and compaction.
. Costs more than soil riprap.
. If not well mixed, pockets of small void material, especially near surface can wash out and unravel void-filled riprap installation. Need to continually monitor and make sure larger riprap is not displaced and located at surface to provide sufficient D50.
10-44 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
10-8-3 Riprap Sizing
Procedures for sizing rock to be used in soil riprap, void-filled riprap, and riprap over bedding are the same.
10-8-3-1 Mild Slope Conditions
When subcritical flow conditions occur and/or slopes are mild (less than 2 percent), the following equation is recommended (Hughes, et al, 1983):
2 VS 0.17 d 50 0.66 Equation 10-10 4.5(Gs 1)
Where:
V = mean channel velocity (ft/sec)
S = longitudinal channel slope (ft/ft)
d50 = mean rock size (ft)
Gs = specific gravity of stone (minimum = 2.50, typically 2.5 to 2.7), Note: In this equation (Gs -1) considers the buoyancy of the water, in that the specific gravity of water is subtracted from the specific gravity of the rock.
Note that Equation 10-10 is applicable for sizing riprap for channel lining with a longitudinal slope of no more than 2%. This equation is not intended for use in sizing riprap for steep slopes (typically in excess of 2 percent), rundowns, or protection downstream of culverts.
Rock size does not need to be increased for steeper channel side slopes, provided the side slopes are no steeper than 2.5H:1V (UDFCD 1982). Channel side slopes steeper than 2.5H:1V are not recommended because of stability, safety, and maintenance considerations. At the upstream and downstream termination of a riprap lining, the thickness should be increased 50% for at least 3 feet to prevent undercutting.
10-8-3-2 Steep Slope Conditions
Steep slope rock sizing equations are used for applications where the slope is greater than 2 percent and/or flows are in the supercritical flow regime. The following rock sizing equations may be referred to for riprap design analysis on steep slopes:
. CSU Equation, Development of Riprap Design Criteria by Riprap Testing in Flumes: Phase II (prepared by S.R. Abt, et al, Colorado State University, 1988). This method was developed for steep slopes from 2 to 20 percent.
. USDA- Agricultural Research Service Equations, Design of Rock Chutes (by K.M. Robinson, et al, USDA- ARS, 1998 Transactions of ASAE) and An Excel Program to Design Rock Chutes for Grade Stabilization, (K.M. Robinson, et al, USDA- ARS, 2000 ASAE Meeting Presentation). This method is based on laboratory data for slopes from 2 to 40 percent.
. USACE Steep Slope Riprap Equation, Hydraulic Design of Flood Control Channels, EM1110-2-1601,
October 2018 City of Durango 10-45 Storm Drainage Design Criteria Manual Open Channels Chapter 10
(June 1994). This method is applicable for slopes from 2 to 20 percent.
All three of the steep slope methods are based on two key parameters: unit discharge and slope. Flow concentration is one of the main problems that can develop along steep riprap slopes; both the CSU and USACE methods recommend that the design unit discharge be increased by a flow concentration factor. When using the CSU equation or the USDA method, increase the largest rock size by approximately 30% when specifying standard UDFCD riprap gradations. This increase accounts for the fact that the steep slope equations were developed using poorly graded rock (uniform in size) unlike the well-graded gradations in UDFCD specifications.
Additionally, it is typical to also apply a design safety factor of 1.5 or more times the calculated D50 riprap size when using any of these steep slope riprap sizing methods, because of multiple uncertainties in the field, including the imprecision of installing rock in the field, variability in rock size delivered from quarries, flow conditions in streams, and others. When using the CSU equation or the USDA method, apply the safety factor after increasing the largest rock size by 30%.
10-8-4 Boulders
Boulders may be placed and grouted or placed without grout. When not grouted, boulders require careful design to provide a firm foundation and stable configuration as well as properly graded backfill material sized to prevent migration of fine subgrade material through voids in the boulders. All stacked boulders require consideration of stability and any stacked boulder configuration over six feet in height requires a structural analysis to confirm proper design. Additionally, some municipalities require structural analysis and a building permit for walls greater than four feet.
10-8-5 Riprap and Boulder Specifications
Design details for riprap and boulders, including material and installation specifications, can be found in UDFCD’s Construction Specifications, available at www.udfcd.org.
References for additional information:
UDFCD Manual, Volume 1, Open Channels Chapter, for details and specifications for riprap and boulders
UDFCD Manual, Volume 2, Hydraulic Structures Chapter, for rundowns, grouted boulder grade control structures.
UDFCD Manual, Volume 2, Culverts and Bridges Chapter, for protection downstream of culverts
Technical paper titled Demonstration Project Illustrating Void-Filled Riprap Applications in Stream Restoration (Wulliman and Johns 2011). This paper provides background on the derivation of void-filled riprap and its applications in stream restoration.
10-46 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
10-9 References
American Association of State Highway and transportation Officials, 1999. Guide for the Development of Bicycle Facilities
American Society of Civil Engineers (ASCE). 2014. Public Safety Guidance for Urban Stormwater Facilities. New York: ASCE.
American Society of Civil Engineers (ASCE). 1975. Sedimentation Engineering. American Society of Civil Engineers Manuals and Reports on Engineering Practice No. 54. New York: ASCE.
American Society of Civil Engineers and Water Environment Federation (ASCE and WEF). 1992. Design and Construction of Urban Stormwater Management Systems. American Society of Civil Engineers Manuals and Reports of Engineering Practice No. 77 and Water Environment Federation Manual of Practice FD-20. New York: American Society of Civil Engineers.
Arcement, G.J. Jr., and Schneider, V.R., 1989, Guide for Selecting Manning’s Roughness Coefficients for Natural Channel and Flood Plains, US Geological Survey Water-Supply Paper 2339.
Barnes, H.H. Jr. 1967. Roughness Characteristics of Natural Channels. Geological Survey Water-Supply Paper 1849. Washington, D.C.: U.S. Government Printing Office.
Biedenharn, D.S., C.M. Elliot, and C.C. Watson. 1997. The WES Stream Investigation and Streambank Stabilization Handbook. Vicksburg, MS: U.S. Army Corps of Engineers, Waterways Experiment Station.
Bledsoe, B. 2013. Overview of Stream Restoration Paradigms, Approaches, and Tools. Powerpoint Presentation from Stream Restoration Workshop.
Bohan, J.P. 1970 Erosion and Riprap Requirements at Culverts and Storm Drain Outlets. WES Research Report H-70-2. Vicksburg, MS: U.S. Army Corps of Engineers, Waterways Experiment Station.
Calhoun, C.C., J.R. Compton, and W.E. Strohm. 1971. Performance of Plastic Filter Cloth as Replacement for Granular Filter Materials, Highway Research Record No. 373, pp. 74–85. Washington, D.C.: Highway Research Board.
City of Durango. 2016. Code of Ordinances. Durango, CO: La Plata County, CO.
City of Durango. 2001. Development Standards for Public Improvements and Construction Specifications. Durango, CO: La Plata County, CO.
City of Durango. 2014. Land Use and Development Code. Durango, CO: La Plata County, CO.
Chow, V.T. 1959. Open-Channel Hydraulics. New York: McGraw-Hill Book Company.
Daugherty, R.L. and J.B. Franzini. 1977. Fluid Mechanics with Engineering Applications. New York: McGraw Hill.
Dunne and Leopold. 1943. Water in Environmental Planning. New York: WH Freeman and Company.
Fischenich, C. 2001. Stability Thresholds for Stream Restoration Materials. Vicksburg, MS: U.S. Army Corps of Engineers, Ecosystem Management & Restoration Research Program (EMRRP).
Fletcher, B.P. and Grace, J. L.. 1972. Practical Guidance for Estimating and Controlling Erosion at October 2018 City of Durango 10-47 Storm Drainage Design Criteria Manual Open Channels Chapter 10
Culvert Outlets, WES Miscellaneous Paper H-72-5. Vicksburg, MS: U.S. Army Corps of Engineers, Waterways Experiment Station.
Fortier, S. and F.C. Scobey. 1926. Permissible Canal Velocities. Transactions of the American Society of Civil Engineers, Volume 89, pp. 940-956.
Guo, J. 1999. Roll Waves in High Gradient Channels. Journal of Water International 24(1)65-69.
Hipschman, R.A. 1970. Erosion Protection for the Outlet of Small and Medium Culverts. Master Thesis, Civil Engineering Department, South Dakota State University.
Jarrett, R., 1985, Determination of Roughness Coefficients for Stream in Colorado, US Geological Survey, Water-Resources Investigations Report 85-4004.
Lane, E.W. 1952. Progress Report on Results of Studies on Design of Stable Channels. Hyd-352. Denver, CO: Department of Interior, Bureau of Reclamation.
Lane, E.W. 1953. Progress Report on Studies on the Design of Stable Channels by the Bureau of Reclamation. Proceedings of the American Society of Civil Engineers. Volume 79. Separate No. 280, pp. 1-31. 1953.
Lane, E.W. 1955a. Design of Stable Channels. Transactions of the American Society of Civil Engineers, Volume 120, pp. 1234–1260.
Lane, E.W. 1955b. The Importance of Fluvial Morphology in Hydraulic Engineering. Proceedings of the American Society of Civil Engineers.
Leopold, L.B. and T. Maddock Jr. 1953. The Hydraulic Geometry of Stream Channels and Some Physiographic Implications. Washington, DC: U.S. Geological Survey.
Leopold, L.B. 1994. A View of the River. Cambridge, MA: Harvard University Press.
Murphy, T.E. 1971. Control of Scour at Hydraulic Structures, WES Miscellaneous Paper H–71–5. Vicksburg, MS: U.S. Army Corps of Engineers, Waterways Experiment Station.
Newbury and Gaboury. 1993. Stream Analysis and Fish Habitat Design: A Field Manual. Newberry Hydraulics Ltd. and the Manitoba Habitat Heritage Corporation.
Posey, C.J. 1960. Flood Erosion Protection for Highway Fills. Bulletin No. 13. Ames, IA: Iowa Highway Research Board.
Rhoads, B.L. 1995. Stream Power: A Unifying Theme for Urban Fluvial Geomorphology. In Stormwater runoff and Receiving Systems, E.E. Herricks, ed. Boca Raton, FL: CRC Press.
Riley, A.L. 1998. Restoring Streams in Cities: A Guide for Planners, Policymakers, and Citizens. Washington, D.C.: Island Press. Washington.
Rosgen, D. 1996. Applied River Morphology. Pagosa Springs, CO: Wildland Hydrology.
Schiechtl, H. 1980. Bioengineering for Land Reclamation and Conservation. Edmonton, Alberta: University of Alberta Press.
Simons, D.B. 1957. Theory and Design of Stable Channels in Alluvial Materials. Ph.D. dissertation,
10-48 City of Durango October 2018 Storm Drainage Design Criteria Manual Chapter 10 Open Channels
Colorado State University. Fort Collins, CO.
Simons, D.B. and F. Senturk. 1992. Sediment Transport Technology. Littleton, CO: Water Publications.
Smith, C. D. 1975. Cobble Lined Structures. Canadian Journal of Civil Engineering, Volume 2.
Smith, C.D. 1974. Hydraulic Structures. Saskatoon, SK: University of Saskatchewan Printing Services.
Society of Ecological Restoration, Science and Policy Working Group. 2004. SER International Primer and Ecological Restoration.
Stevens, M.A., D.B. Simons, and F.J. Watts. 1971. Riprapped Basins for Culvert Outfalls. Highway Research Record No. 373. Washington, D.C.: Highway Research Service.
Stevens, M.A., D.B. Simons, and G.L. Lewis. 1976. Safety Factors for Riprap Protection. Journal of the Hydraulics Division 102 (5) 637–655.
U.S. Army Corps of Engineers (USACE). 1994. Hydraulic Design of Flood Control Channels. USACE Design Manual EM 1110–2–1601.
U.S. Army Corps of Engineers. 1991. HEC-2, Water Surface Profiles, User's Manual. Davis, CA: Army Corps of Engineers Hydrologic Engineering Center.
U.S. Army Corps of Engineers. 1995. HEC-RAS, River Analysis System, User's Manual. Davis, CA: Army Corps of Engineers Hydrologic Engineering Center.
U.S. Army Corps of Engineers. Copeland et.al. 2001. Hydraulic Design of Stream Restoration Projects. Coastal and Hydraulic Laboratory. ERDC/CHL TR-01-28.
U.S. Army Corps of Engineers. P.J. Soar and C.R. Thorne. 2001. Channel Restoration Design for Meandering Rivers. Coastal and Hydraulic Laboratory. ERDC/CHL CR-01-1
U.S. Bureau of Reclamation (USBR). 1984. Computing Degradation and Local Scour. Washington, DC: Bureau of Reclamation.
U.S. Department Of Agriculture (USDA). 2007. National Handbook Part 654: Stream Restoration Design.
U.S. Department Of Agriculture. 2007. Technical Supplement 14I: Streambank Soil Bioengineering.
U.S. Department Of Agriculture.1994. Stream Channel Reference Site: an Illustrated Guide to Field Technique.
U.S. Department Of Agriculture. 1987. Agriculture Handbook Number 667: Stability Design of Grass- Lined Open Channels.
U.S. Department of the Interior, Bureau of Reclamation. 2006, Fish Protection at Water Diversions; A Guide for Planning and Designing Fish Exclusion Facilities, Water Resources Technical Publication.
USDA Natural Resources Conservation Service, September 2014, Guidance for Stream Restoration, Technical Notes, Engineering – No. 27.3.
U.S. Department of Agriculture, Soil Conservation Service, revised June 1954, Handbook of Channel Design for Soil and Water Conservation, Stillwater Outdoor Hydraulic Laboratory.
October 2018 City of Durango 10-49 Storm Drainage Design Criteria Manual Open Channels Chapter 10
U.S. Department of Transportation Federal Highway Administration, July 1999. Designing Sidewalks and Trails for Access.
U.S. Department of Transportation Federal Highway Administration, last modified April 23, 2011. Equestrian Design Guidebook for Trails, Trailheads and Campgrounds.
U.S. Environmental Protection Agency (USEPA). 1983. Results of the Nationwide Urban Runoff Program: Final Report. Washington D.C.: U.S. Environmental Protection Agency.
U.S. Federal Interagency Stream Restoration Working Group (USFISRWG). 1998. Stream Corridor Restoration: Principles, Processes, and Practices. Washington, DC: U.S. Federal Interagency Stream Restoration Working Group.
Urban Drainage and Flood Control District. 1982. Communications between Dr. Michael A. Stevens and UDFCD staff.
Urban Drainage and Flood Control District. 1984. Guidelines for Development and Maintenance of Natural Vegetation. Denver, CO: Urban Drainage and Flood Control District.
Urban Drainage and Flood Control District. 1986. Comparison of Measured Sedimentation With Predicted Sediment Loads. Technical Memorandum in response to District Agreement No. 85–02.078 from WRC Engineering, Inc., to the Urban Drainage and Flood Control District. Denver, CO: Urban Drainage and Flood Control District.
Vallentine, H. R. and B.A. Cornish. 1962. Culverts with Outlet Scour Control. University of New South Report No. 62. Sydney, Australia: Wales, Water Research Laboratory.
Watson, C.C., D.S. Biedenharn, and S.H. Scott, 1999. Channel Rehabilitation: Process, Design, and Implementation. Washington, D.C.: Environmental Protection Agency.
Watson, C.C., D.S. Biedenharn, B.P. Bledsoe. 2002. Use of Incised Channel Evolution Models in Understanding Rehabilitation Alternatives. Journal of the American Water Resources Association, American Water Resources Association.
Wulliman, J. and D. Johns. 2011. Demonstration Projects Illustrating Void-Filled Riprap Applications in Stream Restoration.
Yang, C.T, 1996. Sediment Transport: Theory and Practice. New York: The McGraw-Hill.
10-50 City of Durango October 2018 Storm Drainage Design Criteria Manual