28 ENGINEERED WATERWAYS

28.1        INTRODUCTION..........................................................................................................28-1

28.2        WATERWAY TYPES......................................................................................................28-1

28.3        DESIGN STORM..........................................................................................................28-1

28.4        LOCATION..................................................................................................................28-1

28.5        RESERVES..................................................................................................................28-2

28.6        FREEBOARD................................................................................................................28-2

28.7        GRADES.....................................................................................................................28-2

28.7.1     Minimum Grades............................................................................................28-2

28.7.2     Maximum Grades...........................................................................................28-2

28.7.3     Drop Structures..............................................................................................28-2

28.8        ROUGHNESS COEFFICIENTS........................................................................................28-3

28.8.1     General.........................................................................................................28-3

28.8.2     Sensitivity Analyses........................................................................................28-3

28.8.3     Composite Waterways....................................................................................28-3

28.9        NATURAL WATERWAYS...............................................................................................28-4

28.9.1     General Guidelines.........................................................................................28-4

28.9.2     Waterway Improvements................................................................................28-5

28.9.3     Maintenance..................................................................................................28-6

28.10      GRASSED FLOODWAYS................................................................................................28-6

28.10.1   General.........................................................................................................28-6

28.10.2   Geometry......................................................................................................28-6

28.10.3   Grass Cover...................................................................................................28-7

28.10.4   Low Flow Provision.........................................................................................28-7

28.10.5   At-grade Crossings.........................................................................................28-8

28.10.6   Landscaping..................................................................................................28-9

28.10.7  Advisory Signs...............................................................................................28-9

28.11      LINED CHANNELS.......................................................................................................28-9

28.11.1   General.........................................................................................................28-9

28.11.2   Cross-Section.................................................................................................28-9

28.11.3   Concrete Linings............................................................................................28-9

28.11.4   Superelevation...............................................................................................28-10

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28.11.5   Earthwork......................................................................................................28-10

28.11.6   Bedding.........................................................................................................28-10

28.11.7   Subsoil Drainage.............................................................................................28-10

28.11.8   Pressure Relief Weep Holes.............................................................................28-10

28.11.9   Lateral Protection and Cut-off Walls..................................................................28-10

28.11.10Downstream Protection...................................................................................28-10

28.11.USafety Requirements.......................................................................................28-10

28.11.12Access Requirements......................................................................................28-11

28.12      ROADS AS FLOODWAYS...............................................................................................28-11

28.12.1   General..........................................................................................................28-11

28.12.2   Surface Flow Criteria.......................................................................................28-11

28.12.3   Low Side Verges.............................................................................................28-11

28.13      EROSION AND SCOUR PROTECTION.............................................................................28-11

28.13.1   General..........................................................................................................28-11

28.13.2   Protection Measures........................................................................................28-12

28.14      MAINTENANCE............................................................................................................28-12

28.15      DESIGN PROCEDURE...................................................................................................28-12

APPENDIX 28.A DESIGN CHARTS............................................................................................28-15

APPENDIX 28.B WORKED EXAMPLE.........................................................................................28-23

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28.1      INTRODUCTION

Drainage should be designed in an environmentally responsible way to minimise disruption of the natural environment of the nation's streams and waterways.

"Engineered waterways" are a preferred means of meeting the above objective by providing a drainage system that more closely resembles natural streams. The word "engineered" in the title indicates that the waterways are carefully designed to achieve this condition. Design considerations for grassed floodways and natural waterways should include the principles of watercourse management, which are discussed in Chapter 43.

These waterways are all components of the major drainage system designed to collect and convey flows from the minor drainage system and to provide for the safe passage of larger flows up to the major design storm.

This chapter provides guidelines for the design of grassed floodways, natural waterways, lined channels and roads as floodways.

28.2      WATERWAY TYPES

The types of engineered waterways available for urban drainage systems are almost infinite, depending only upon good hydraulic practices, environmental design, sociological impact, and basic project requirements. However, from a practical standpoint, the basic choice to be made initially is whether or not the waterway is to be a hard-lined one for higher velocities, a grassed floodway, or a natural channel already existing. The following types are applicable in urban areas:

Natural Waterways: These are waterways carved or shaped by nature before urbanisation occurs and are the most desirable of the various types of constructed or modified waterways. They often, but not always, have mild slopes and are reasonably stable. As the tributary catchment urbanises, natural waterways often experience erosion and may need grade control checks and localised bank protection to stabilise.

Grassed Floodways: These are soft-lined waterways designed to lower flow velocities, provide channel storage, and offer various multiple use benefits. Low flow areas generally need to be concrete or rock lined to minimise erosion and maintenance problems.

Concrete Lined Channels: Concrete lined channels are high velocity artificial waterways that are not encouraged in urban areas. However, in retrofit situations where existing flooding problems need to be solved and where width available for a drainage reserve is limited, concrete channels may offer advantages over other types of engineered waterways.

Rock Lined Channels: Riprap lined channels offer a compromise between a grassed floodway and a concrete lined channel. They can significantly reduce drainage reserve needs, but are more difficult to keep clean than other types and are recommended for consideration only in retrofit situations where existing urban flooding problems are being addressed.

Other Channel Liners: A variety of artificial channel liners may be used to protect the channel walls and bottom from erosion at higher velocities. These include gabions, interlocked concrete blocks, concrete revetment mats formed by injecting concrete into double layer fabric forms, and various types of synthetic fibre liners. As with rock and concrete liners, all of these types are best considered for helping to solve existing urban flooding problems, but are not recommended for new developments. Each type of liners has to be scrutinised for its merits, applicability, how it meets other community needs, its long-term integrity, and maintenance needs and costs.

Roadways : Roadways will act as overland flow paths, whether or not they are specifically designed that way, whenever flows exceed the capacity of the minor system ARI conveyance or whenever stormwater inlets become blocked with debris. Roadways, if designed to satisfy specific surface flow criteria related to public safety, can offer a safe and effective means of conveying overland flows to one of the preceding types of engineered waterways.

28.3      DESIGN STORM

Engineered waterways shall be designed to cater for flows up to and including the major system design ARI (refer Table 4.1).

Adjoining low-lying land may need to be acquired and/or reclaimed to ensure effective surface drainage and containment of the design ARI flow within an engineered waterway.

28.4      LOCATION

Continuous designated overland flow paths shall be provided from the top of the catchment through the entire urban area.

Engineered waterways may be located within designated drainage reserves, roadways, parkland and open space areas, and pedestrian ways. All engineered waterways shall be located wholly outside of privately owned lots. If circumstances arise where this arrangement cannot be provided, prior agreement to locate engineered waterways within privately owned areas must be obtained from the Local Authority and the private landowners affected.

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Piping of major system design flows may be considered as an alternative to an engineered waterway, but acceptable provision against the pipe being blocked or its capacity being exceeded will be required.

Engineered waterways shall be provided along the alignment of existing watercourses and drainage depressions. Diversion of engineered waterways away from their natural paths will only be permitted in exceptional circumstances and only with the approval of the Local Authority.

Wherever possible, landuse within engineered waterway corridors should be designated as public open space. Other types of landuse may be considered, but they must be fully compatible with the primary role of the waterway to convey flood flows up to and including the design storm.

28.5 RESERVES

Reserves are required for all engineered waterways. These must be clearly defined on all development plans to ensure that future development does not encroach upon land inundated by flows up to and including the design storm.

The prime function of reserves is to give ready access to personnel, plant and materials, which may, from time to time, be required for waterway and berm maintenance. No encroachment, especially earth fill that may inhibit such access or make such maintenance unduly difficult, shall be allowed on reserves.

The minimum drainage reserve width shall be the top waterway width for the major storm ARI flow plus a 300 mm freeboard requirement. Maintenance width requirements may be incorporated within this reserve width by benching. If this cannot be achieved, the reserve width must be increased to include maintenance width requirements. Minimum widths to be provided for maintenance access shall be in accordance with Table 28.1.

Table 28.1 Minimum Requirements for Maintenance Access

Top Width of Waterway

Minimum Requirements for Maintenance Access

W< 6m

One side 3.7 m, other side 1.0 m

W> 6m

Both sides 3.7 m

When planning development along a waterway for which a master plan is not yet available, a drainage reserve width shall be estimated based on the premise that the design storm flow will be catered for by a grassed floodway. This premise ensures that sufficient land will be available for the design of the waterway when carried out in conjunction with detailed landuse planning at a later date.

28.6      FREEBOARD

The freeboard above the design storm water level for all engineered waterways shall be a minimum of 300 mm. A higher freeboard should be considered at locations where superelevation or hydraulic jumps are anticipated.

28.7      GRADES

28.7.1   Minimum Grades

Engineered waterways shall be constructed with sufficient longitudinal grade to ensure that ponding and/or the accumulation of silt does not occur, particularly in locations where silt removal would be difficult.

The minimum longitudinal grade for engineered waterways shall be as follows:

      0.5% Grassed floodways and natural channels

      0.2% Lined channels

Longitudinal grades shall not produce velocities less than 0.8 m/s if low flow inverts flowing full.

28.7.2   Maximum Grades

Engineered waterways shall be designed with longitudinal grades that minimise the incidence of hydraulic jumps, dangerous conditions for the public, and erosion of surface linings and/or topsoil.

Longitudinal grades shall be chosen such that the design storm average flow velocity will not exceed:

      4 m/s in lined channels and low flow inverts

      2 m/s in grassed floodways and natural waterways

28.7.3   Drop Structures

Drop structures should be provided to reduce waterway longitudinal grades such that the design storm average flow velocities do not exceed the limits specified in the previous section.

Drop structures shall be designed to ensure that the structures do not get 'drown out' due to high tailwater levels under the major system design flow plus freeboard. Design requirements for drop structures are provided in Chapter 29, Section 29.3.

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28.8 ROUGHNESS COEFFICIENTS

28.8.1   General

Care must be taken in the estimation of the waterway roughness coefficient sn' for use in the Manning equation. The choice of an appropriate value for the roughness coefficient of an open waterway is often critical in the overall design procedure and requires a considerable degree of judgement.

The Manning's roughness coefficient is dependent on a number of variables including surface roughness, vegetation, waterway irregularity, the presence of obstructions, waterway alignment, and the likelihood of sedimentation or scour. It is important to note also that it is dependent on the relative depth of flow at a cross-section. Generally, as flow depths increase, the relative effect of bed roughness decreases, while the effect of bank roughness may increase. The dependent variables can be considered as individual components of an overall effective Manning's roughness coefficient for a particular waterway.

Manning's roughness coefficients for the various types of waterways presented in this chapter are provided in Design Chart 28.1. The designer should use judgement in selecting an appropriate design value from the range of values given. However, the maximum and minimum values should be used to check the sensitivity of waterways to varying roughness value conditions as discussed in the following section.

28.8.2   Sensitivity Analyses

The designer should ensure that the parameters adopted in the design of an engineered waterway system accurately represent the range of anticipated conditions that could reasonably be expected to occur throughout the design life of the waterway. In the case of hard-lined channels, this anticipated future condition can usually be estimated with a reasonable degree of certainty. However, in the case of soft-lined waterways such as grassed waterways and natural channels, the anticipated future conditions are generally less quantifiable.

Soft-lined waterways would be expected to experience much greater variation in waterway surface roughness, compared to hard-lined waterways. The surface roughness of soft-lined waterways is dependent on a number of variables including vegetal type, time or season of year, and degree of maintenance. The Manning's roughness coefficient could obviously be expected to be higher if no waterway maintenance was undertaken at all.

It is important in soft-lined waterways to assess their capacity for both the lowest and highest likely values of Manning's roughness coefficient during the design life of the waterway. The lowest coefficient will give the highest velocity and therefore the most critical situation in relation

to waterway scour. The highest coefficient will give the lowest velocity and therefore the highest flood levels and the greatest likelihood of silt deposition.

Sensitivity analyses enable the designer to model a range of possible future conditions, and to subsequently assess the relative impacts of each condition. Sensitivity analyses for engineered waterway design generally include modelling the waterway system with a range of assumed Manning's roughness coefficients, or modelling with a modified cross-section shape to account for the effects of sedimentation and scour. In most cases, a sensitivity analysis involving an examination of the relative effects of a variation in assumed Manning's roughness coefficient will provide the designer with a reasonable indication of the consequences of possible future conditions and an incorrect assessment of the Manning's roughness coefficient. The designer should consult the Local Authority to ascertain the level of analysis required.

28.8.3 Composite Waterways

Estimation of an equivalent or composite Manning's roughness value in a waterway of varying roughness is often required where there is a marked variation in the boundary roughness across an individual cross-section. In the case of an engineered waterway designed within a typical urban environment, examples of this situation include a grassed floodway containing a concrete low flow invert, or a waterway containing a low level access road along one side of the waterway.

Equation 28.1 may be used to estimate the overall roughness coefficient in engineered waterways of composite roughness. It involves the determination of flow area, wetted perimeter, and Manning's roughness coefficient for each segment representing the varying zones of roughness across the waterway section.

Zj p2/3 ;=1 ri

where,

n = equivalent Manning's roughness coefficient for the whole cross-section

/7,   =    Manning's roughness coefficient for segment /

A,   =    flow area of segment / (m2)

P,   =    wetted perimeter of segment / (m)

m   =    total number of segments

It is important that the designer check that the composite Manning's roughness coefficient value obtained using Equation 28.1 is reasonable. A distorted or inaccurate value will result in inaccurate predictions of waterway flow conditions.

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28.9 NATURAL WATERWAYS

28.9.1 General Guidelines

Natural waterways are either in the form of steeply banked streams, which have erodible banks and bottoms, or mild channels, which are reasonably stabilised. For either type of waterway, if it is to be used for carrying storm runoff from an urbanised area, it can be assumed initially that the changed runoff regime will result in highly active erosional tendencies. In nearly all cases, some form of modification of the waterway will be required to create a somewhat stabilised condition for the waterway.

The design guidelines for grassed floodways do not necessarily apply to natural waterways, but such criteria can be utilised in gauging the adequacy of a natural waterway for future changes in runoff regime.

Design criteria and techniques, which should be used in the design of natural waterways, include the following:

      channel and overbank capacity shall be adequate for the design storm

      channel velocity shall not exceed the lesser of 2 m/s or the critical velocity for any particular section. Manning's roughness factors, n, which are representative of maintained channel conditions shall be used for determining critical channel velocities

      water surface limits shall be defined so that the floodplain can be zoned and protected. Manning's roughness factors, n, which are representative of unmaintained channel conditions shall be used for the analysis of water surface profiles

      drop structures or check dams should be constructed to limit flow velocities and control water surface profiles, particularly for the initial storm runoff

Figure 28.1 Typical Natural Waterways (ASCE, 1992)

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Figure 28.2 Composite Waterways (ASCE, 1992)

28.9.2 Waterway Improvements

Figures 28.1 and 28.2 provide examples of slightly improved natural waterways. Stabilisation measures in Figure 28.1(a) include check structures, riprap, minor grading, and short sections of retaining walls. In general, little or no waterway capacity improvements are included. In Figure 28.1(b), waterway capacity has been increased to lower or confine the design storm flow by excavating outside of the environmentally sensitive area and constructing retaining walls.

Figure 28.2 shows possible drainage improvements for composite waterways.          Stabilisation measures in

Figure 28.2(a) include check structures, riprap, grading, and retaining walls. Improvements to the main channel increase capacity for minor flood flows and may confine or reduce the depth of the design flood.

In Figure 28.2(b), the main channel area has been left undisturbed (i.e. that area containing the base flow plus the immediate vegetation area) and the overbank conveyance capabilities improved by excavating the

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floodplain area. This 'improved' natural waterway conveys the base flow and increases the capacity of the total waterway to convey the major system design flow. Provision should be provided for maintenance access to the waterway. In stabilising the main channel and overbank, as much vegetation as feasible should be retained consistent with the objectives of enhancing stability and capacity. Multiple uses of the overbank flooding area should be encouraged, especially if the main channel capacity is substantial, i.e. if overbank flooding is infrequent.

28.9.3 Maintenance

Periodical maintenance will be required to maintain the hydraulic capacity of a natural waterway. Overgrowth of shrubs and unwanted growths of exotic species should be removed from the main channel and overbanks. Debris washed from downstream will generally become snagged against dense thickets and culvert entrances and should also be removed.

Grassed overbank areas should be mown if they are part of recreational facilities. Sediment, litter, and debris deposits should be removed, particularly at flow restrictions such as bridges and culverts.

28.10 GRASSED FLOODWAYS

28.10.1 General

Grassed floodways are typical types of waterways included in the soft-lined category. Waterways of this type are generally regarded as more aesthetically pleasing than the hard-lined channels and they can be designed to blend into the surrounding natural environment. Grassed floodways can be used in situations where:

      flow velocities are insufficient to produce scour,

      the drainage reserve width is not a problem, and

      aesthetics is an important design consideration

Whilst grassed floodways generally have significant construction cost advantages over corresponding hard-lined waterways, this benefit is in some cases offset by their additional land-take requirements. However, the effect of this additional land-take can be minimised by designing the floodway for multi-purpose use.

Grassed floodways shall conform to the following general requirements:

      floodways shall be grassed with provision for low flows

      access road and footpath paving within a floodway shall be designed to withstand the design discharge in areas of high velocity such as adjacent to bridges and underpasses

The following factors may need consideration:

      drop structures may be included in the design to reduce velocities to acceptable levels

      in some instances, tree and shrub planting within the floodway may be desirable to reduce velocities and therefore should be considered as an integral part of the stormwater system. The waterway area of such floodways shall be increased to allow for tree and shrub planting (See Chapter 43)

28.10.2 Geometry

(a) Cross-Section

The preferred cross-section for grassed floodways is shown in Figure 28.3.

Side slopes for grassed floodway areas must be adequate to ensure drainage without localised ponding occurring. Batter side slopes should not be steeper than 6(H): 1(V) for reasons of public safety. However, steeper side slopes up to a maximum of 4(H): 1(V) may be provided in special circumstances, such as to preserve existing trees and other natural features, or in existing areas where the land available for a grassed floodway is limited.

The base width of a grassed floodway should be designed to accommodate the hydraulic capacity of the floodway with due consideration given to limitations on flow velocity. The width must be sufficient to allow access to the base for maintenance purposes. The floodway base side slopes shall not be less than 50(H): 1(V).

An initial estimate of the minimum base width for a floodway cross-section may be obtained from Design Charts 28.2 or 28.3, depending on the Manning's roughness. These base widths have been calculated to limit the average flow velocity in the floodway to the maximum allowable velocity of 2 m/s as specified in Section 28.7.2. If a low flow invert is provided in the floodway, the base width may have to be increased in order to keep the average flow velocity within the specified limit.

Alternative arrangements may be provided to widen a grassed floodway adjacent to public open space or recreational areas. In such cases, the minimum side slope for the floodway shall be 50(H): 1(V).

Terracing, as indicated in Figure 28.4, may be introduced across the floodway to contain more frequent flood flows, or to utilise adjacent public open space and recreational areas for flood storage or to reduce flood levels and velocities for the major system design flow. Flooding of the adjacent open space and recreational areas should only be incorporated in such areas where flood protection less than the design storm is satisfactory. The minimum side slope for the terrace base shall be 50(H): 1(V).

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Batter

d----------------------------------------------3*

Figure 28.3 Typical Grassed Floodway Cross-Section

Terrace Base Batter

<------------------------------X-----

Figure 28.4 Typical Grassed Floodway Terracing

28.10.3 Grass Cover

28.10.4 Low Flow Provision

The grass species chosen for lining of floodways must be sturdy, drought resistant, easy to establish, and able to spread and develop a strong turf layer after establishment. A thick root structure is necessary to control weed growth and erosion.

A protective cover consisting of mulch and grass seeding is necessary to protect newly constructed floodways immediately after construction. If possible, disturbed areas should be seeded with a permanent grass seed mix. To provide quick ground cover, the seed mix may include a perennial ryegrass. The perennial ryegrass germinates quickly and will not compete with the sod-forming grasses later on. When immediate seeding of the permanent grass is not practical, an annual crop may be planted with the perennial grass seeded later in the stubble or residue. Rye, oats, or ryegrass give fair temporary protection for waterways, but the crop should be clipped before it matures to seed.

Permanent grassing for floodways shall be in accordance with Chapter 26, Section 26.2.6.

A grassed floodway must be provided with a low flow system to facilitate drainage, pest control, and maintenance. Urban waterways normally have a continuous dry-weather baseflow mainly due to runoff from domestic water usage. Continuous flow over the grass lining in the floodway invert will destroy the grass stand and promote weed growth. The invert may also become eroded and form pools of water, which may cause soggy ground conditions, a breeding ground for mosquitoes.

The low flow provision may take the form of a surface invert or an underground pipe. The choice of which type to use may depend on a number of factors that are specific to local conditions. However, some general factors that should be considered are:

      aesthetics: it may be desirable to use an invert to create an open water landscape in open space and recreational areas

      floodway longitudinal grade: very flat waterway grades could result in problems with siltation of pipes

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      expected debris and litter loadings: small diameter pipes can block easily and are not recommended

      utility services: low flow inverts provide more clearance from other services, such as sewers

      construction costs: inverts may be more economical than pipes where pipe excavation would be in rock

(a)     Design Capacity

Low flow inverts and pipes shall be sized for a minimum capacity of 50% of the 1 month ARI flow.

(b)     Invert

Low flow inverts may be designed in accordance with Design Charts 28.4 or 28.5 and Standard Drawing SD F-41. Proprietary precast products such as 'pudu-cut' and U-shaped units may also be used. The size of a low flow invert has been limited to prevent its size becoming excessive in large floodway systems. If the required design capacity of the invert exceeds the maximum allowable size, a low flow pipeline will need to be used instead.

Careful consideration shall be given to minimising the possibility of scour at the interface between the invert edge and the grassed surface of the floodway. It may be necessary to provide a transition zone using a stabilisation system such as reinforced grass.

Attention shall also be given to making the transition of branch pipelines as smooth as practicable to minimise turbulence and potential scour. Stabilisation may also be required on the downstream side of branch pipeline transitions.

Designated crossing points for maintenance machinery shall be provided at regular intervals of not more than 200 m.

A subsoil system shall be provided for the invert. This may take the form of a standard subsoil drain, geofabric wrapped aggregate, or a strip drain. Refer to Standard Drawing SD F-41 for typical details and Chapter 44.

Inverts shall be constructed of reinforced concrete, mortared stone, or other materials as approved by the Local Authority.

(c)      Pipeline

A low flow pipe system may be either a single or multiple pipeline, depending on the required design capacity.

Low flow pipes shall comply with the following size limits:

      the minimum pipe diameter shall be 450 mm

      the maximum pipe diameter shall be 1800 mm

To minimise the likelihood of the pipe becoming silted up over time, the pipe slope shall be chosen to ensure that the half-full pipe flow velocity is not less than 1.0 m/s.

Floodways utilising a low flow pipeline shall be sized for the entire major system ARI design flow based on the assumption that the low flow pipeline is fully blocked.

A surcharge structure shall be provided at a branch pipeline connection wherever the combined capacity of the branch pipeline and upstream low flow pipeline is greater than the capacity of the downstream low flow pipeline. Surcharge structures should be designed in accordance with Standard Drawing SD F-3.

Plantation inlets shall be provided to facilitate positive drainage of the floodway. Care must be taken with grading immediately upstream of the plantation inlets to ensure that low flows enter the low flow pipeline, but do not bypass the inlets.

Careful consideration must be given to minimise the likelihood of blockage of the low flow pipeline from litter and debris.

To minimise the likelihood of the floodway invert remaining soft, a pervious backfill and subsoil drainage system shall be provided. Refer to Standard Drawing SD F-41.

28.10.5 At-grade Crossings

At-grade footpath and cycleway crossings of floodways will be permitted and should be designed in accordance with Standard Drawing SD F-42.

Crossings should follow the shape and grade of the floodway, as far as practical, to avoid any flow restrictions and minimise changes to the flow regime.

(a)      Grassed Invert

A hardstand area shall be provided at the floodway invert on the downstream side of the crossing to prevent the likelihood of water ponding on the crossing due to the build-up of grass over time.

(b)     Low Flow Invert

Crossings spanning low flow inverts shall have a minimum 200 mm clearance above the base of the invert to minimise the likelihood of blockage by litter and debris.

Adequate provision for public safety shall be included in the design.

Crossings that are likely to be used by maintenance vehicles shall be structurally designed for a 7 tonne wheel loading.

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28.10.6 Landscaping

Allowance shall be made for the effects of landscaping in the hydraulic calculations of engineered waterways. To minimise ongoing maintenance:

      no trees other than those with clean boles, strong crown structure, and no propensity for root suckering may be planted in the floodplain

      minimum spacing of trees shall be 3 m

      maintenance free 'thicket' zones used for hydraulic reasons shall have a minimum of 3 m clearance from lot boundaries to provide access for mowing

      no vegetation other than grass shall be planted within 3 m of a concrete invert in a floodway

28.10.7 Advisory Signs

(a)      Type

For grassed floodways and natural waterways, proper signs should be provided to advise people to take care at certain times rather than inhibit them from using the environs of the stream or floodway. The recommended sign is shown on Standard Drawing SD F-43.

(b)     Location

Signs should be located at points of congregation and generally at about 500 m intervals along floodways within the 2 year ARI floodplain. Points of congregation would be, for example, pathways leading to low-level pedestrian crossings.

Where possible, signs should be illuminated.

(c)      Installation

The signs shall be erected back to back on a 75 mm galvanised steel pole and be located 1.5 m above the ground.

28.11 LINED CHANNELS

28.11.1 General

Waterways lined with concrete, stone pitching, and rock mattresses are typical of the type of channel included in this category. Lined waterways of this type should only be used for upgrading works in existing areas where:

      the flow velocity would cause scour in a grassed floodway

      the drainage reserve width is restricted

      continual maintenance of the waterway is a problem

      other factors dictate that a grassed floodway is not practicable

28.11.2 Cross-Section

The slope of the channel sides shall be no steeper than 1.5(H): 1(V) unless designed to act as a structurally reinforced wall to withstand soil and groundwater forces.

The inverts of lined channels should have a nominal invert Vee' of at least 10(H): 1(V) or a precast 'pudu-cut' section, such that low flows remain concentrated along a single location within the channel invert. The low flow invert may be either centrally located or offset to one side of the channel. Channels lined with rock mattresses should be provided with a cast-in-situ or precast concrete low flow drain located within the channel invert. The requirements for this low flow drain are the same as those for grassed floodways (refer Section 28.10.4(b)).

28.11.3 Concrete Linings

All concrete linings shall be designed to withstand the anticipated hydrodynamic and hydrostatic forces. The channel base shall be designed to withstand maintenance machinery loadings.

(a)     Materials

The minimum concrete compressive strength for all channel sections shall be concrete grade 30.

Proposed admixtures shall be approved by the Local Authority prior to use.

(b)     Lining Thickness

The minimum lining thickness shall be 175 mm for cast-in-situ sections and in accordance with the Manufacturer's specifications for precast sections.

(c)     Jointing

Contraction and expansion joints should be provided to cater for differential movements and minor temperature and environmental movements, thereby minimising the risk of cracking and subsequent undermining and failure.

Longitudinal joints, where required, shall be constructed on the side walls at least 300 mm vertically above the channel invert.

Construction joints are required for all cold joints and where the lining thickness changes. Reinforcement shall be continuous through the joint.

(d)     Finish

The surface of the concrete lining may be finished in any of the finishes listed in Table 28.1. The designer should check with the Local Authority to determine which finishes are acceptable.

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(e) Reinforcement Steel

The minimum size for steel reinforcement shall be T32 bars with a tensile strength fy= 460 N/mm2. Wire mesh shall not be used.

Reinforcing steel shall be placed at the centre of the section with a minimum clear cover of 75 mm adjacent to the earth. Additional steel shall be provided as needed to meet retaining wall structural needs.

28.11.4 Superelevation

Superelevation of the water surface must be determined at horizontal curves and the design of the channel cross-section adjusted accordingly.

An approximation of the superelevation can be obtained from the following equation:

V1 Wt

grc

where,

h =    required superelevation (m)

V =    velocity (m/s)

rc =    centreline radius of curvature (m)

WT =    top width of channel (m)

g =    acceleration due to gravity (9.81 m/s2)

28.11.5 Earthwork

The following areas shall be compacted to at least 95% of the maximum density as determined by ASTM D698 (Standard Proctor):

      the top 300 mm of subgrade immediately beneath the channel bottom and side slopes

      the top 300 mm of earth surface within 3 m of the top edges of the channel

      all fill material

The subgrade under the channel must be of acceptable strength for the expected loadings, i.e. weight of concrete and water at maximum flow depth. The following may be used to strengthen or compensate for deficient subgrades:

      piling

      concrete blinding layer

      geotextiles

28.11.6 Bedding

Granular bedding of 150 mm thickness, equivalent in gradation to 20 mm concrete aggregate, shall be provided under the channel bottom and side slopes.

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28.11.7 Subsoil Drainage

Longitudinal subsoil drains shall be provided at 3 m centres, shall be free draining, and shall daylight at drop structures (when applicable). A check valve or flap valve shall be provided at the outlet to prevent backflow into the subsoil drain.

28.11.8 Pressure Relief Weep Holes

Pressure relief weep holes shall be provided in channels within the channel invert sides. The extent and density of pressure relief weep holes should be sufficient to prevent hydraulic uplift of the channel.

28.11.9 Lateral Protection and Cut-off Walls

The channel lining should extend horizontally beyond the top edge of the channel, at least 0.45 m wide on both sides. This horizontal strip will assist in providing scour protection against lateral inflows to the channel, and in preventing undermining.

The lining should also be continued vertically downward as a cut-off wall at the upstream and downstream extents of the lined channel. These cut-off walls should be provided along the channel invert and side slopes and will provide some scour protection against high velocity flows entering and leaving the channel. The required depth of the cut-off walls is dependent on a number of factors including channel flow rate, flow velocity and the type of material upstream and downstream of the lined section. However, it is recommended that cut-off walls be constructed to a minimum depth of 0.9 m below the upstream and downstream surface levels.

28.11.10             Downstream Protection

Designers should ensure that scour beyond the downstream end of lined channels is minimised by the provision of erosion protection structures or measures (refer Chapter 29).

28.11.11             Safety Requirements

(a) General

Where the design storm flow depth within a lined channel exceeds 0.9 m, a 1.2 m high handrail fence shall be provided on both sides of the channel to discourage public access. Handrail fencing should be parallel to the channel as far as practicable and should be located within 1 to 3 m from the channel edge. Lockable gates shall be placed at appropriate locations to permit access for maintenance.

Ladder-type steps or step irons should be provided where the channel side slope is steeper than 2(H): 1(V), and where the channel depth exceeds 1.2 m. These should be

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Engineered Waterways

installed no more than 120 m apart on alternating sides of the channel. The bottom rung of the ladder or bottom step iron shall be placed approximately 300 mm vertically above the channel invert.

Warning signs should also be provided at locations such that they will be clearly visible to the public (refer to Section 28.10.7 and Standard Drawing SD F-43).

(b) Adjacent to roadway

Where a lined channel is adjacent to a public road, guardrailing shall be provided instead of a handrail fence. Guardrails shall be provided in accordance with the requirements of the relevant Authority and/or those for the JKR Roads.

28.11.12             Access Requirements

Access ramps shall be provided at a maximum spacing of 500 m for maintenance machinery to gain access to the channel bottom. Ramps shall be a minimum of 3.5 m wide with a longitudinal slope not steeper than 10(H): 1(V) and have a non-slip surface.

28.12 ROADS AS FLOOD WAYS

28.12.1 General

Major or minor floodways should be provided in preference to using roads as floodways wherever possible. However, where road floodways are deemed necessary, the following aspects shall be adopted:

      the roadway flow capacity shall be sized for the 'gap' flow which is the difference between the major and minor system design flows

      the catchment area feeding a road floodway shall be kept as small as possible

      conditions creating high water velocities and excessive water depths in a road floodway shall be avoided. The capacity of the minor drainage system shall be increased if necessary to limit the volume and depth of surface flow

      for longitudinal and cross flow stability of vehicles and pedestrians, the following criteria shall apply,

° the maximum depth of flow shall not be greater than 50 mm above the top of kerb

° the product of maximum depth and average velocity shall not exceed 0.4 where depth is in metres and velocity is in m/s (see Chapter 4)

      ready discharge from road floodways at low points or other relief points shall be provided to remove water quickly, avoid ponding, and prevent deposition of gravel and silt on the roadway

      a drainage reserve or open space area incorporating a designated overland flow path shall be provided on

the downstream side of road low points. The overland flow path shall be sized for the 'gap' flow and located such that flows do not encroach onto adjacent lots. A depressed kerb and verge may be required to limit the depth of ponding on the roadway

      the floodway cross-section and continuity shall be maintained; i.e. the level of drive entrances and low-side banks shall be above the 'gap' flow flood level

      materials used in the construction of medians, paths, nature strips, etc, shall be able to withstand inundation at anticipated design flow velocities. Pine bark, gravel, and other loose materials shall not be used as they are likely to scour and cause blockage of inlet sumps

The design of roadways to carry surface flow is described in Chapter 24.

28.12.2 Surface Flow Criteria

Road gutters and pavements shall be designed to comply with the surface flow criteria specified in Table 4.4 in Chapter 4.

28.12.3 Low Side Verges

Lots on the low side of roadways must be protected from inundation by flows overtopping verges. The high point of all low side road verges including driveways shall be at least 50 mm above the level of the major storm design flow in the road gutter.

28.13 EROSION AND SCOUR PROTECTION

28.13.1 General

The design average flow velocity limits specified in Section 28.7.2 have been selected to prevent erosion and scour of waterway surfaces under normal conditions. However, waterways may be subject to intense local erosion or scour at obstructions (e.g. bridge piers, pipe headwalls), sudden changes in waterway cross-sections, drops, regions of changes in waterway bed materials, and other similar conditions. The following factors should be considered wherever significant changes in flow regime occur and appropriate measures provided to protect the waterway surface from local scour.

The main factors that provide favourable conditions for erosion and scour in a waterway are high flow velocities, particularly at shallow depths, and soft and/or fine bed materials. Velocities are higher in steep waterways, at changes in waterway configuration, in smooth waterways, and at higher discharges. Soil type largely determines the erosion potential of bed materials.

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Local scour occurs in non-uniform flow regions where pressure forces, lift forces, and shear forces fluctuate. For example, local scour around bridge piers is caused by the vortex resulting from water piling up on the upstream edge and subsequent acceleration of flow around the nose of the pier.

Local scour is a function of a combination of several of the following factors:

      slope of the waterway

      characteristics of the bed materials

      characteristics of the flood hydrograph

      direction of the flow in relation to its depth

      direction of the flow in relation to its structures

      characteristics of the transported materials

The following locations are the most common areas where localised erosion can occur and will require careful consideration of the need for erosion protection measures:

      Transitions: Any changes in cross-section or changes in waterway lining material. Particular attention should be paid to the region immediately alongside low flow inverts.

      Bends: The outside bank of bends will be subject to higher flow velocities.

      Drain tributaries: Waterways usually have many small capacity tributary drain and pipe connections. Flows from these tributary connections will normally be of relatively high velocity and the angle of entry will cause turbulence in the waterway.

      Waterway tributaries: Other waterways entering the main waterway system may cause turbulence and erosion of the waterway botom and opposing bank.

      Energy dissipator structures: Changes in the flow regime will usually occur immediately upstream and downstream of drop structures and energy dissipation basins.

      Culverts: Exit velocities from culvert crossings will normally be supercritical.

      Bridges: Flow velocities around bridge piers and abutments may be higher than the waterway limit.

Localised scour or general degradation can quickly lower the bottom of a channel. Erosion protection facilities must have deep toe protection or they can fail by being undermined.

28.13.2 Protection Measures

Waterway protection must be provided to suit the local physical and scour characteristics. Erosion protection is required for waterway linings in reaches where the maximum permissible flow velocities or critical tractive forces are exceeded under the design storm flow conditions.

Waterway vegetation is perhaps the simplest erosion and scour control measure. However, where the flow velocities will exceed the velocities at which the vegetation is effective, other erosion protection measures will need to be considered (see Chapter 29).

28.14   MAINTENANCE

The owner of an engineered waterway should establish a routine maintenance inspection program once the facility has been completed and placed in service. The inspections should be conducted on an annual or semi-annual basis, as well as following major storms.

The following guidelines are general requirements for the maintenance of all engineered waterways:

      Mowing: Grassed waterways should be mowed often enough to maintain appearance and to control weeds.

      Debris control: Debris blockage at drainage structures often contributes to flooding problems. Structures such as inlet pits, headwalls, trash racks, and debris traps should be regularly cleaned. Debris should also be regularly removed along the length of the waterway. This should include trimming and thinning of trees if they encroach on the waterway main channel or if they have become overgrown.

      Sediment and silt removal: Some silt accumulation in energy dissipation structures and around waterway obstructions is inevitable and is harmless in limited amounts. Silt should be removed if it is severe enough to alter the water surface or affect the function of structures such as drops and culvert inlets. Silt accumulations can also cause trouble by supporting undesirable or obstructive vegetation.

Sediment traps will need regular removal of trapped material to protect downstream facilities.

      Access road and footpath repair: damaged access road and footpath sections should be repaired to ensure continued maintenance access and better pedestrian use.

       Vandalism: Drainage facilities can be attractive nuisances and can be damaged by those who use the area. Preventive measures may be necessary, to keep graffiti off walls, to keep rock riprap from being relocated, or to keep gabion baskets from being cut open.

28.15    DESIGN PROCEDURE

After a waterway type has been selected, the general procedure outlined in Figure 28.5 may be used for locating and sizing the waterway. Design charts for use in the procedure are given in Appendix 28.A and a worked example is provided in Appendix 28.B.

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Engineered Waterways

Select alignment

I

Determine design ARI and discharge

J

[ Select bed slope (50) J

Select waterway cross-section

Reduce base width (B)

and/or increase bed slope (5 )

Compute maximum depth (/) and top water width (Wr)

I

j

No

Compute average flow velocity

No

Compute required

waterway reserve width

(7~+ freeboard)

Check flow velocity and depth

using minimum and maximum

expected rvalues

Increase base width (B)

and/or reduce bed slope (5 )

I

Lower waterway invert and adjust reserve width

Figure 28.5 General Design Procedure for Engineered Waterways

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APPENDIX 28.A DESIGN CHARTS

Title                                                                                                                 Chart No.                      Page

Suggested Values of Manning's Roughness Coefficient, n                                           28.1                           28-16

Grassed Flood ways

Floodway Base Width - Preliminary Estimate (Manning's n = 0.035)

Floodway Base Width - Preliminary Estimate (Manning's n = 0.050)

Low Flow Invert Size (Type 1 - Variable Depth)

Low Flow Invert Size (Type 2 - Variable Width) Lined Channels

Solution to Manning's Equation for Lined Channels of Various Side Slopes                 28.6                           28-21

28.2

28-17

28.3

28-18

28.4

28-19

28.5

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Surface Cover

Suggested rvalues

Minimum

Maximum

Grassed Floodways

Grass cover only

Short grass

0.030

0.035

Tall grass

0.035

0.050

Shrub cover

Scattered

0.050

0.070

Medium to dense

0.100

0.160

Tree cover

Scattered

0.040

0.050

Medium to dense

0.100

0.120

Natural Channels

Small streams

Straight, uniform and clean

0.025

0.033

Clean, winding with some pools and shoals

0.035

0.045

Sluggish weedy reaches with deep pools

0.050

0.080

Steep mountain streams with gravel, cobbles, and boulders

0.030

0.070

Large streams

Regular cross-section with no boulders or brush

0.025

0.060

Irregular and rough cross-section

0.035

0.100

Overbank flow areas

Short pasture grass, no brush

0.025

0.035

Long pasture grass, no brush

0.030

0.050

Light brush and trees

0.040

0.080

Medium to dense brush

0.070

0.160

Dense growth of trees

0.110

0.200

Lined Channels and Low Flow Inverts

Concrete

Trowelled finish

0.011

0.015

Off form finish

0.013

0.018

Shotcrete

Trowelled, not wavy

0.016

0.023

Trowelled, wavy

0.018

0.025

Unfinished

0.020

0.025

Stone Pitching

Dressed stone in mortar

0.015

0.017

Random stones in mortar or rubble masonry

0.020

0.035

Rock Riprap

0.025

0.030

Roadways

Kerb & Gutter

0.011

0.015

Hotmix Pavement

Smooth

0.012

0.014

Rough

0.015

0.017

Flush Seal Pavement

7 mm stone

0.017

0.019

14 mm stone

0.020

0.024

Design Chart 28.1 Suggested Values of Manning's Roughness Coefficient, n

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Design Chart 28.2 Floodway Base Width - Preliminary Estimate

(Manning's n= 0.035, Average Velocity = 2 m/s)

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Floodway reserve width, R (m) (including required freeboard )

50           50

Base width, B (m)

Flow depth, y (m)

Base width, B (m)

0.5             0.6          0.7         0.8 0.9 1.0              1.2           1.4         1.6 1.8 2.0

2.5             3.0           3.5 4.0 4.5 5.0

Longitudinal Grade, S (%)

Design Chart 28.3 Floodway Base Width - Preliminary Estimate

(Manning's n= 0.050, Average Velocity = 2 m/s)

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Design Chart 28.4 Low Flow Invert Size (Type 1 - Variable Depth, Manning's n = 0.013)

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Design Chart 28.5 Low Flow Invert Size (Type 2 - Variable Width, Manning's n = 0.013)

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0.05

01 >

0.01

Z= 3 ^^>^<^\^^^

z=

2.5

------

^

z

'= 2

Z= 1.

5----

^h^P i 1

cr^I ^4 as

- z

= 0

0.06

0.1

0.15

0.2

0.25           0.3

Value of

B

Design Chart 28.6 Solution to Manning's Equation for Lined Channels of Various Side Slopes

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28-22                                                                                                                              Urban Stormwater Management Manual

Engineered Waterways

APPENDIX 28.B WORKED EXAMPLE

Problem:

Determine the size of a grassed floodway at the downstream end of a proposed 150 hectare medium density residential area in Ipoh, Perak based on the following assumptions:

      the floodway is to be designed to carry the 50 year ARI flow with no freeboard

      the post-development time of concentration tc\s estimated to be 30 minutes

      the floodway will be well maintained with an estimated design Manning's roughness coefficient of 0.035

      low flows will be conveyed by a concrete invert with a design capacity of 50% of the 1 month ARI flow

Solution:

Step (1) Determine design flows for the floodway

The 50 year ARI rainfall intensity is estimated using Equation 13.2 with fitting constants from Table 13.Al for Ipoh, Perak.

ln(50/30) = 5.231 +0.660 ln(30) + (-0.255)(ln(30))2 +0.014 (ln(30))3 = 5.077 50/30 = 160 mm/hr

The 1 month ARI rainfall intensity is estimated from the 2 year ARI rainfall intensity using Equation 13.5a.

ln(2/30) = 4.942 + 0.607 ln(30) + (-0.251)(ln(30))2 + 0.014 (ln(30))3 = 4.654 2I30 = 105 mm/hr

0083/30 = 0.4x 2I30 = 0.4x105 = 42 mm/h

Using the Rational Method, with a runoff coefficient C of 0.83 for 50 year ARI and 0.80 for 1 month ARI (Design Chart 14.3, category 3), the design flows are:

n 0.83 a-160 a-150 cc_ 3,

Q7 =--------------------= 55.3 m/s

V2              360

.,.,,          ... 0.80x42x150 _. 3.

0.5<?o.o83 = 0.5 x----------------= 7.0 m3/s

360

Step (2) Select floodway longitudinal bed slope

An initial longitudinal bed slope, S0, of 0.8% is selected to conform to the natural topography along the floodway alignment.

Step (3) Select floodway cross-section

1.            Low flow invert

From Design Charts 28.4 and 28.5, for a design flow of 7.0 m3/s and a longitudinal slope of 0.8%, the low flow invert selected is a type 2 invert with a base width of 1.2 m.

2.            Grassed floodway

From Design Chart 28.2, for a design flow of 55.3 m3/s and a longitudinal slope of 0.8%, the preliminary base width is 35 m. This may have to be increased to compensate for the effect of the concrete low flow invert on the average flow velocity.

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Step (4) Compute maximum flow depth and top water width

To calculate the flow depth and width of the total floodway section, a composite Manning's n value for the low flow invert and grassed sections needs to be estimated using Equation 28.1. The flow depth and width will then need to be determined by trial and error after assuming an initial value for the flow depth.

Assuming an initial total flow depth of 1.0 m, trial and error calculations are summarised in the following table. Figure 28.Bl shows the flow segments used to calculate the composite Manning's roughness n*.

(m)

Ya

(m)

Yb (m)

(m2)

Pl&3

(m)

A2 (m2)

Pi (m)

n*

Q

(m3/s)

T (m)

1.00

0.29

0.00

2.10

14.50

1.80

3.91

0.029

6.0

32.4

1.25

0.54

0.22

6.19

17.17

2.56

3.91

0.033

21.8

37.7

1.50

0.79

0.47

10.66

18.69

3.41

3.91

0.033

47.0

40.7

1.57

0.86

0.54

11.95

19.10

3.65

3.91

0.034

55.3

41.5

Figure 28.Bl Flow Segments for Calculating Composite Roughness

Step (5) Compute average flow velocity

The flow velocity may be determined from the above table using: .. _ QD _           55.3

2x11.95 + 3.65

2.01 m/s

This may be satisfactory as the average velocity is only just over the maximum limit of 2 m/s. If not, the base width should be increased. The flow details for a base width of 36 m are:

(m)

K

(m)

Yb (m)

(m2)

PlSJ

(m)

(m2)

Pi (m)

n*

Q

(m3/s)

T (m)

1.56

0.85

0.53

12.10

19.51

3.60

3.91

0.034

55.3

42.3

The average flow velocity is: 55.3

2x12.10 + 3.60

1.99 m/s

< 2 m/s (OK)

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Step (6) Compute required waterway reserve width

Adding a freeboard allowance of 300 mm to the flow depth gives a minimum reserve width of: T+ 2 x freeboard x batter side slope = 42.3 + 2x0.3x6 = 45.9 m

Step (7) Check flow velocity and depth using minimum and maximum expected n values

From Design Chart 28.1, the range of Manning's roughness values that could be experienced during the design life of the floodway range from a minimum of 0.030 to a maximum of 0.050. The change to the average flow velocity or the flow depth is determined from trial and error as before using the designed cross-sectional dimensions.

For n = 0.030, trial and error calculations are summarised in the following table.

(m)

Ya

(m)

Yb (m)

(m2)

Pl&3

(m)

A2 (m2)

Pi (m)

n*

Q

(m3/s)

V (m/s)

1.56

0.85

0.52

12.02

19.49

3.58

3.91

0.029

63.7

2.31

1.40

0.69

0.36

8.99

18.52

3.04

3.91

0.029

42.0

2.00

1.50

0.79

0.47

10.90

19.14

3.38

3.91

0.029

55.3

2.20

From the above results, the average velocity will be increased to 2.20 m/s and the flow depth will be reduced to 1.50 m.

For n = 0.050, trial and error calculations are summarised in the following table.

Yt

(m)

Ya

(m)

Yb (m)

(m2)

P\&3

(m)

A2 (m2)

Pi (m)

n*

Q

(m3/s)

V (m/s)

1.56

0.85

0.52

12.02

19.49

3.58

3.91

0.048

38.6

1.40

1.70

0.99

0.66

14.80

20.34

4.06

3.91

0.048

52.0

1.55

1.73

1.02

0.70

15.45

20.53

4.17

3.91

0.048

55.3

1.58

From the above results, the flow depth will be increased to 1.73 m and the average velocity will be reduced to 1.58 m/s.

If either of the above results are not acceptable, the floodway base width and/or longitudinal bed slope will need to be modified and the design procedure repeated from Step (2) until a satisfactory floodway configuration is found.

The composite Manning's roughness value for the floodway could also be increased by selective planting of shrubs and trees with clean boles in the waterway area and/or constructing the low flow invert with a rougher surface material such as stone pitching or rock riprap.

The design configuration for the grassed floodway and low flow invert is shown in Figure 28.B2.

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Minimum floodway reserve width = 45.9 m Flow width = 42.3 m

Base width = 36 m

(a) Grassed Floodway

3.6 m

(b) Low Flow Invert

Figure 28.B2 Grassed Floodway and Low Flow Invert Dimensions

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