3. Design Procedures. Design Procedures. Introduction

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1 Design Procedures 3. Design Procedures Introduction This chapter presents a procedure for the design of natural channels. The chapter primarily focuses on those physical properties of the channel required for hydraulic stability. The design procedure is presented only as a guide and in most circumstances final channel dimensions should be appropriately varied along the channel to form a natural rather than constructed appearance. Note: This chapter should only need to be reviewed if the proposed stream rehabilitation works require the relocation, reconstruction or alteration of the main channel. In any case, wherever possible, all appropriate channel features (Appendix A) should be incorporated into the existing or reconstructed channel. Depending on the type of creek works proposed, there can be a number of design steps that should and should not be performed. In some cases the channel location may be preset, in other cases the purpose of the creek works may only be to improve the ecological values of the channel bed or banks. In any case, the design philosophy should be to review only those design steps relevant to the given circumstances. The design procedure presented below is based on the following design philosophy: (i) (ii) (iii) (iv) The channel s geometry, bed and floodplain are influenced by different design flow rates, those being: bankfull flow, low flows and flood flows. The channel width, depth, meander radius and allowable channel slope are determined by the adopted bankfull flow rate and the allowable bankfull flow velocity at various stages of revegetation. The bed design is determined by the adopted channel slope and low flow conditions. The overbank geometry and vegetation density is governed, in part, by high level flood flows. Remember: There is more than one way to solve a hydraulic problem, and there is more than one way to solve a ecological problem. It is important to be creative, thoughtful and flexible. Also, aesthetics is often not the primary controlling factor. Good aesthetics does not necessarily result in good ecological values. 24

2 Natural Channel Design Guidelines Data collection Where possible, obtain a long section survey of the channel bed. This will assist in the selection of channel gradient and in locating pools and riffles. Soil tests should be conducted to determine if the soil is dispersive (Emerson Aggregate Test) and what type of soil it is, e.g. sandy, sandy loam, or clay. Design steps The following steps are used in this design procedure. Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 Step 9 Step 10 Step 10-1(a) Step 10-1(b) Step 10-2(b) Step 10-3(b) Step 10-4(b) Step 10-5(b) Step 10-6(b) Step 10-7(b) Step 10-8(b) Step 11 Select the reach of watercourse to be rehabilitated Determine channel fall Determine bankfull flow rate Select the type of channel Determine channel width and channel depth Channel bank slope Determine typical channel meander radius Select trial channel location Review riparian and floodplain vegetation Design of channel bed and low flow channel Bed design for channel with little or no bed vegetation - determine bed form Bed design for channels with bed vegetation determine bed form Determine the length of the low flow channel Determine the number of riffles Determine typical riffle dimensions Determine typical pool dimensions Determine pool length Determine riffle rock size Design low flow channels Incorporate channel features 25

3 Design Procedures Symbols D [m] Channel depth from bankfull water level to channel invert D N [m] Nominal rock diameter of which N% of the rock is smaller, i.e. 10% of the rock is smaller than the D 10 rock size. Rock size is measured as the diameter of a sphere with an equivalent volume to the individual rock D p [m] Maximum pool depth d/s [-] Downstream F r [m] Riffle fall measured from riffle crest to the low flow pool level within the downstream pool F t [m] Total channel fall across the proposed rehabilitation area equals upstream channel invert level minus downstream channel invert level L 1 [m] The valley length measured along a smooth straight or curved path (not including the channel meanders) from the upstream end of the channel reach to the downstream end of the channel reach L 2 [m] Length of a meandering watercourse channel measured along the channel centreline L 3 [m] Length of the low flow channel measured over a given reach of the watercourse L m [m] Straight line distance between successive channel meanders, or meander inflection points L p [m] Length of a low flow pool measured at the low flow water level L r [m] Riffle length measured from the riffle crest to the upstream end of the downstream low flow pool N [-] Number of riffles within a reach n [-] Manning s roughness value q [m 3 /s/m] Discharge per unit width for a nominated design event Q [m 3 /s] Discharge Q f [m 3 /s] Bankfull discharge Q fds [m 3 /s] Bankfull discharge immediately downstream of the works Q fus [m 3 /s] Bankfull discharge immediately upstream of the works Q [N] [m 3 /s] The peak design discharge at a given location that is expected to occur on average once every N years, i.e. the Q100 is the expected peak discharge during the 1 in 100 year design storm 26

4 Natural Channel Design Guidelines Symbols R c [m] Channel bend radius measured to the centre of the channel, or the low flow channel meander radius measured to the centre of the low flow channel R o [m] Bend radius measured to the outer bank of the channel at the elevation of bankfull flow S [m/m] Slope S o [m/m] Average channel slope over a selected reach S r [m/m] Slope of downstream face of riffle S i [-] Sinuosity = L 2 /L 1 or L 3 /L 2 u/s [-] Upstream V [m/s] Average flow velocity W [m] Channel top width measured at the height of the lower bank Wp [m] Maximum pool width measured at low flow water level W r [m] Riffle bed width [m] Average meander displacement, or half the amplitude of the meandering channel centreline or low flow channel centreline [deg] The angle measured between two consecutive inflection points Step 1. Select the reach of watercourse to be rehabilitated Waterways are normally rehabilitated in stages. Often the length, or reach, of a watercourse to be rehabilitated is determined by external factors beyond the control of the designer; however, where practicable, the length of rehabilitation should be restricted to a hydraulically and financially manageable reach length. A hydraulically manageable reach length is one where the occurrence of a 1 in 2 year flood during the revegetation phase does not result in unacceptable bank erosion or damage to newly established plants. A financially manageable reach length relates to the initial establishment cost as well as the short-term maintenance costs (i.e. weed control). The 1 in 2 year (or Q2) flow is important because it is considered that if most new plants are able to survive the first 2 years without suffering damage from a flood event, then they are likely to reach maturity. In a channel where the bankfull flow rate was around, say, the Q10 event, it would be unlikely for a Q10 event to occur within the first two years. Discussion on selecting the maximum reach length is provided in Technical Note

5 Design Procedures TECHNICAL NOTE 3.1 Often the governing factor in natural channel design is the allowable bankfull velocity during the early revegetation phase when the soil is disturbed and susceptible to erosion, and the effective channel roughness is low. Typically, during this post construction phase the Manning s n roughness of the channel is around n = 0.04 depending on the irregularity of the channel, the degree of meandering, and the size of the planted seedlings. If bankfull flow is less than say Q2, then it is likely that a bankfull flow will occur during the early growth stage of the plants. Therefore, if the bankfull flow velocities are high, say greater than 1 m/s (depending on soil type), then it is likely that significant damage will occur to the plants before they reach maturity. To avoid such problems, it may be desirable to rely on existing high channel roughness downstream of the rehabilitation works to reduce bankfull flow velocities throughout the work area. However, this will only be successful if the area of the proposed works is within the backwater region of the downstream channel. This can be confirmed with the use of a hydraulic backwater model, otherwise restrictions may need to be placed on the maximum allowable channel fall across a proposed section of creek works. In the absence of any other data or model results, if bankfull flow velocities are expected to be high, then the length of the proposed rehabilitation works should not extend over a total channel fall equal to half the channel depth (D). Under these conditions, it is likely that the channel works will occur within the backwater (ie low velocity) region of the downstream, vegetated channel. Step 2. Determine channel fall (F t ) It is important to walk the full length of any proposed channel works prior to commencing the design or even planning the works. Take note of any hydraulic controls, such as bed rock, pipe crossings, culverts or drop structures (natural or constructed). Any fixed bed features, such as bedrock, will affect the allowable fall of the channel bed and thus the profile of the channel bed as shown in Figure 3.1. Measure the elevation of any fixed bed structures and the total channel fall (F t ) across the proposed rehabilitation length (L 2 ). F t L 2 Figure 3.1 Channel long section 28

6 Natural Channel Design Guidelines Step 3. Determine bankfull flow rate (Q f ) Estimate bankfull flow rate upstream (Q fus ) and downstream (Q fds ) of the proposed works. In the unlikely situation where the bankfull flow rate is greater than the estimated Q100 discharge, then adopt the Q100 discharge. Calculate the Q1, Q1.5, Q2 and Q10 flow rates. As a first trial, nominate the bankfull flow rate (Q f ) based on the most appropriate of the following recommendations: (i) Average of Q fus and Q fds (if the length of the rehabilitated channel is small and there is a desire (aesthetically or otherwise) for the new channel to be of a similar size to the upstream and downstream channels. (ii) A calculated bankfull flow rate (Q f ) based on the monitoring of similar, stable streams and catchment land uses in the local region. (iii) Q1.5, if no specific regime equations have been developed for the local area, noting that in the Brisbane area, bankfull flow rates vary from less than Q1 to greater than Q10. (iv) A bankfull flow value based on local site constraints. In some circumstances the channel capacity may be determined by the requirements for flood control. In such cases a hydraulic model and study will be required for the complete floodplain. There is significant interaction between the channel and its floodplain and this should not be forgotten throughout the full design procedure. In long reaches it may be necessary to vary the bankfull discharge down the reach. Note: at Step 8 it may be necessary to reduce the nominated bankfull flow rate in order to obtain a stable channel (eg to reduce the bankfull velocity). Step 4. Select the type of channel The desirable style of drainage channel can vary from a grass-lined overland flow path for very small catchments to a fully established river channel for large catchments. Desirable bed conditions in a reconstructed watercourse usually depend on the following factors: catchment area catchment soil type (infiltration capacity) canopy cover Table 3.1 provides typical bed form conditions that are likely to occur in the Brisbane (Clayey Soil) region. The desirability of these bed form conditions of course depends on local hydraulic and ecological factors. 29

7 Design Procedures Table 3.1 Typical (desirable) bed conditions for the Brisbane region Catchment area Channel slope Desirable bed condition Less than 30 ha Open canopy Grassed bed urbanised catchment Closed canopy Rocky and/or vegetated bed Urbanised catchment between Steep channels Rock lined bed between 30 and 100 ha Grades > 1 in 20 Mild slopes Pool/riffle system, however, low flows Grades between may not be sufficient to maintain dry weather flows 1 in 20 and 1 in 100 and adequate pool water quality Flat slopes Grades < 1 in 500 Sandy channels Open earth or vegetated channel bed Sandy beds, some pools and riffles Greater than 100 ha mainly Steep channels Rock lined bed urbanised catchment Grades > 1 in 20 Mild slopes Grades between 1 in 20 and 1 in 100 Flat slopes Grades < 1 in 500 Sandy channels Pool/riffle system Open earth or vegetated channel bed Sandy, irregular bed with pools Less then 50 ha bushland catchment Open canopy Grassed bed Closed canopy Rocky and/or vegetated, bed Greater than 50 ha rural or Steep slopes > 1 in 150 Rock lined bushland catchments Mild slopes < 1 in 150 Pool/riffle system Flat slopes < 1 in 500 Sandy channels Open channel Sandy bed, some pools and riffles 30

8 Natural Channel Design Guidelines Step 5. Determine channel width (W) and channel depth (D) The channel width (W) is defined as the channel s top width measured at the elevation of its lowest bank (Figure 3.2). The channel depth (D) is defined as the elevation difference between bankfull water level and the channel invert. When short sections of a watercourse are to be reconstructed or rehabilitated, it is often desirable to maintain a channel depth similar to the existing channel depth to reduce earthworks and to allow the channel bed to have a constant fall to the existing downstream channel invert. For the reconstruction of gravel bed channels, the desired channel width and depth may be determined using one of two methods: the Hey and Thorne equations, or the Brisbane equations. For the reconstruction of sandy bed channels, the Simons and Albertson equation should be used. W D Figure 3.2 Channel and floodplain cross section TECHNICAL NOTE 3.2 The Hey and Thorne equations presented below for gravel bed waterways suggest that the channel width is significantly greater for an open canopy channel (e.g. grassed channel) than for a closed canopy channel with a dense stand of bank vegetation. This may be the case for streams with little or no bed vegetation, such as rivers and streams fed by snow melts, but for creeks in the Brisbane region, observations have shown that closed canopy creeks are often wider than open canopy creek systems. This is because open canopy creeks generally contain more groundcover vegetation (usually grasses and weeds) and the presence of this groundcover greatly reduces erosion. If bed vegetation is not significant within the given type of watercourse, then the Hey and Thorne equations should be used to estimate bank width and depth. If bed vegetation is a common feature of the type of watercourse you are designing, then the Brisbane-based equations are recommended. However, the final judgement on how channel width and depth are determined, is left to the designer s experience and judgement. It is always useful to compare the results of the two sets of equations with existing channels in the region. 31

9 Design Procedures (i) Hey and Thorne equations for gravel bed streams W = 4.33 x (Q f ) 0.5 W = 3.33 x (Q f ) 0.5 W = 2.73 x (Q f ) 0.5 W = 2.34 x (Q f ) 0.5 for vegetation type 1 grassed riparian zone, no trees and shrubs, Manning s n typically 0.03 to for vegetation type 2 scattered trees and shrubs, dense grass and weeds, Manning s n typically 0.04 to 0.06 for vegetation type 3 light to medium stand of trees and shrubs, Manning s n typically 0.07 to 0.09 for vegetation type 4 medium to dense stand of trees and shrubs, Manning s n typically 0.1 to 0.15 (ii) Brisbane s clay-based creek systems with significant sand and gravel bed deposits Q f 100 m 3 /s - typical bank width, W = 2.41 (Q f ) 0.5 and typical depth, D = 0.75 (Q f ) 0.5 Q f < 100 m 3 /s - typical bank width, W = 4.37 (Q f ) and typical depth, D = 1.07 (Q f ) The typical range of bank widths: 4.33 (Q f ) 0.5 > W > 1.78 (Q f ) 0.5 for all flow rates. The typical range of bank depths: W 0.6 > D > W 0.6 (iii) Simons and Albertson equations for sand bed waterways The equations may be applied to all sand bed streams in the Brisbane area. It should be noted that in some cases, this may be outside the range of data from which the equations were derived. (a) For sand bank (sandy loam) Bed width (m) = 5.72.Q f Mean depth (m) = Q f (b) For cohesive banks (some clay content or internal strength) Bed width (m) = 4.29.Q f Mean depth (m) = 0.59.Q f Surface width (m) = 1.1 x Bed width + 2. This should yield bank batters in the range 2H:1V for cohesive banks and 3H:1V for sandy banks. (c) For sand and cohesive banks Meander arc length (m) = 6.31.W 32

10 Natural Channel Design Guidelines Step 6. Channel bank slope The desirable channel bank slope on reconstructed channels may be determined from one of the following methods: (i) Obtained from Table 3.2. (ii) Obtained from geotechnical investigations. (iii) A bank slope similar to existing stable upstream and/or downstream bank slopes. In short reaches it is often necessary to blend in with the bank slope immediately upstream and downstream of the proposed channel works. However, in most cases trying to re-establish a steep bank slope similar to existing conditions will result in poor revegetation of the banks. Introducing a wide variation of bank slopes can be an effective tool in making a constructed or modified watercourse appear as natural as possible. It is usually highly desirable to avoid a regular trapezoidal channel shape down the reach of a watercourse. Steep banks of the outside of bends may be natural, but they are also highly unstable. Avoid placing steep, unprotected banks in areas where watercourse stability is desired. Berms (Figure 3.3) may need to be introduce to steep banks that are higher than 3 metres, or where erosion is expected along the toe of the bank. A typical berm width is one metre. It is noted that bank erosion is frequently caused by deepening of the channel (bed erosion). Therefore, there is often no point in treating the banks unless the bed is stabilised first. Where possible, when designing a modified watercourse, restrict the gradients on the 'over-bank' or 'high-bank' areas to 6(H):1(V) with a maximum grade of 5(H):1(V). Generally, for safety reasons, grassed bank grades of 4(H):1(V) and steeper are not recommended adjacent to a watercourse where: public access exists; the watercourse is considered a safety hazard during dry weather conditions or during regular flood events (up to the 5 year ARI); and satisfactory safety barriers such as fences, shrubs, bushland or flat landings do not exist between the embankment and floodway. 33

11 Design Procedures Berm or terrace Figure 3.3 Berm located on steep, high bank Table 3.2 Suggested bank slopes [1] (V):(H) Bank description 1:2 Good, erosion resistant clay or clay-loam soils with a healthy, deep-rooted bank vegetation formed by suitable riparian groundcover species, shrubs and trees 1:3 Sandy-loam soil with groundcover vegetation or a closed canopy channel with shaded banks and sparse bank vegetation 1:4 Sandy soils 1:6 Mowable, grassed banks (undesirable in Natural Channel Design) Note [1] The bank slopes presented in this table should be considered as a general guide only. Final bank slope should be based on bank stability and revegetation requirements. 34

12 Natural Channel Design Guidelines TECHNICAL NOTE 3.3 Establishing a berm on a watercourse bank can provide the following benefits: (i) If bank erosion occurs at the toe of the bank and this erosion results in soil slumping on the bank, then the berm can reduce the area of the bank over which this slumping occurs. (ii) Bank berms can be used to increase the stability of steep banks. (iii) Berms can be used to delay the effects of soil erosion around the root system of establishing trees. (iv) Wide berms can be used to provide pedestrian access to the watercourse banks for recreational and educational purposes. Step 7. Determine typical channel meander radius If a channel is to be reconstructed or relocated (ie when replacing an existing concrete drain with a natural channel) it is often desirable, if not necessary, to introduce some channel meanders. Meandering the channel can improve its aesthetics, increase habitat and channel diversity (through large-scale hydraulic turbulence) and can increase the effective channel length. Increasing the channel length is one of the most effective ways of reducing the bankfull flow velocity. The meander radius to the centre of the channel (Figure 3.4) is generally greater than three times the normal channel width. Select a desirable meander radius (to centre of channel) such that: R c 3W Alternatively, the meander radius to outer bank is given by: R o 3.5W. If a sharper meander radius needs to be constructed, then it may be necessary to rock-line the outer bank of the meander. W R c R o Waterway channel Channel meander Figure 3.4 Channel meander 35

13 Design Procedures Step 8. Select an initial channel location Often stream rehabilitation projects commence with a desired channel layout already known. However, as the design procedure progresses and hydraulic checks are done, it may become necessary to adjust the channel location until a stable channel geometry and flow condition is obtained. Some of the issues that should be considered when determining the desirable plan form of a watercourse are listed below: (i) historical or natural location of the channel; (ii) existence of steep, high channel banks or adjacent land forms; (iii) flood flow; (iv) bankfull velocity; (v) bend radius; (vi) location of essential or valued floodplain features (eg existing football fields, existing bushland or mature trees); (vii) current channel location. Discussion on each of these issues is provided in Chapter 2, Planning Aspects. The channel normally follows the invert of the valley floor so that the floodplain can drain freely into the channel. Ponding on the floodplain can cause maintenance problems (eg mowing) and mosquitoes; however, in some locations, ponding may be desirable to promote frog breeding. Expert advice should be sought. The channel location is important because it establishes the effective channel slope, and it is this slope that determines bankfull velocity. In many cases the ideal channel slope and dimensions cannot be achieved due to site constraints such as the proximity of existing structures, the desirable meander radius or the required minimum bed width. The location of the channel is often controlled by existing site constraints and urban structures such as bridges, culverts, stormwater outlets, rare or otherwise valued trees, including canopy trees, and other public assets. These items should be clearly identified on field or survey plans. A channel system should never be designed without performing a site visit and walking the length of the valley. In cases where the location of the channel is defined by site constraints, consideration should be given to the formation of minor channel meanders or variations in channel bank slope to provide some degree of irregularity to the channel. Two procedures are recommended for the establishment of a trial channel location: (a) (b) the allowable velocity method; and the allowable channel slope method. Each of these methods is detailed over. 36

14 Natural Channel Design Guidelines (a) Allowable velocity method (i) Estimate channel roughness for both short-term (post-construction, say n 0.04) and long-term conditions (fully vegetated). (ii) Determine the allowable bankfull velocity for both post-construction condition (low vegetation roughness Table 3.7) and long-term channel conditions (high vegetation roughness Table 3.3). (iii) Calculate the desired maximum channel slope (S) using the Manning s Equation or a hydraulic backwater model for the following conditions: (a) (b) initial channel roughness during Q2 or bankfull flow whichever is lower; and long-term channel roughness during Q100 or bankfull flow whichever is lower (in this case the 1 in 100 year flood event is used because it is the standard design event in Brisbane for major channels). Table 3.3 Recommended maximum velocity for various Manning s roughness to avoid significant vegetation damage [1] Average Manning s n roughness n = 0.03 n = 0.06 n = 0.09 n = 0.15 Recommended maximum velocity during bankfull flow and 1 in 50 year flood event [2] 2.0 m/s 1.7 m/s 1.5 m/s 1.0 m/s Notes: [1] As the vegetation density increases, and thus the channel roughness increases, it becomes harder for flood flows to pass around or through the vegetation without causing significant damage. A Manning s roughness of 0.03 represents a typical, deep water, grass channel where high flow velocities of around 2 m/s are expected to cause only minor damage. However, a Manning s roughness of 0.15 represents typical natural bushland for the Brisbane region. If a flow velocity of 2 m/s was allowed to flow through such bushland, significant damage to the vegetation would be expected. 37 [2] The 1 in 50 year flood event is chosen because it represents an extreme flood event during which some vegetation damage would be expected, but it is desirable to minimise this damage where possible. It is considered that a 1 in 20 year flood event would in most cases be too small to act as the design event for vegetation damage. The 1 in 100 year flood can be chosen, but it may be considered unrealistic to have minimal vegetation damage during such an extreme event.

15 Design Procedures (iv) Determine the channel length (L 2 ): where L 2 = (F t /S) Refer to Figure 3.5. (v) Measure the valley length (L 1 ) (vi) Determine the required channel sinuosity: S i = (L 2 /L 1 ) (vii) Determine the average meander displacement ( ) using Table 3.4 and Figure 3.6. (viii) (ix) Sketch the trial channel centreline on the valley plan. If the required channel length is excessive, then revise the channel location using Method (b) below. TECHNICAL NOTE 3.4 In the above design method it is assumed that bankfull velocity is controlled by the average channel slope (i.e. the pool/riffle system is totally drowned out). Such a flow condition can be assumed to exist if the channel depth (D) is at least twice the average riffle fall (F r ). In some cases it may be desirable to sketch an undistorted long section of the channel bed to visualise what possible impact that the pools and riffles may have on bankfull flow velocity. If the riffle height is significant, then the pools and riffles should be included in the hydraulic backwater model and the cross section spacing should be appropriately reduced. L 2 Figure 3.5 Channel reach length 38

16 Natural Channel Design Guidelines Table 3.4 Meander dimensions Meander Sinuosity /R c L m /R c Meander Sinuosity /R c L m /R c angle ( ) (L 2 /L 1 ) angle ( ) (L 2 /L 1 ) Lm R c Waterway channel W Figure 3.6 Channel meander (b) Allowable channel slope method (i) (ii) Select a trial channel location based on typical channel meander patterns upstream and downstream of the proposed works (assuming the u/s and d/s channel reaches are in the form of a natural channel design). Estimate the channel roughness for both short-term (post-construction) and long-term conditions. (iii) Given the measured channel fall (F t ) and channel length (L 2 ), determine the channel slope (S). It is noted that the channel length is measured along the centreline of the channel, not along the low flow channel. 39

17 Design Procedures (iv) Calculate the short-term (post-construction) bankfull or Q2 flow velocity (whichever is the lower discharge) and long-term bankfull or Q100 flow velocity (whichever is the lower discharge). Note: In the short term we are interested only in flood events less than the Q2 because it is desirable to minimise vegetation damage during the early growing phase. In the long term we need to consider the Q100 event because there may be factors other than vegetation damage that we need to consider, such as public safety. If there are no other stated velocity control issues other than vegetation damage, then the Q50 event should be checked rather than the Q100. (v) Compare the above channel velocities with the desirable flow velocities for the given channel conditions. If channel velocities are excessive, then modify the trial channel location or bed slope using one or more of the following options: (a) (b) (c) Increase the channel length. Increase the channel roughness - usually by increasing the post-construction channel roughness by planting more plants during the post-construction period or by planting more mature plants. Decrease the channel gradient by installing a rock chute or a series of rock chutes (vertical or near-vertical drop structures should be avoided in locations where fish migration is important). It is noted that installing a pool-riffle system will not decrease the effective channel gradient; however, it may slightly increase the channel roughness. (d) (e) Decrease the bankfull discharge, i.e decrease the channel width and depth. Maintain the selected bankfull discharge, but decrease the channel depth and consequently increase its width. When locating a trial channel location, consideration should be given to the following points: (i) (ii) The typical spacing of inflection points (i.e average spacing of meanders, L m ) is in the range 5W<L m <7W. When the valley length (L 1 ) and the desired channel length (L 2 ) are known, the average meander spacing (L m ) and amplitude can be determined from Table 3.4, where amplitude = 2. (iii) The valley topography and the encroachment of existing and/or future buildings, roads or other public or private assets can affect the location of the waterway channel. Where practicable, public assets such as roads and buildings should not be located within 15 m of the channel bank, or within the zone defined by a 1(V):3(H) gradient from the toe of the bank, unless the bank is formed from stable rock (Figure 3.7). 40

18 Natural Channel Design Guidelines Note that if a channel meander approaches a steep bank, then any future movement of the channel towards the bank may result in significant erosion, bank instabilities and sediment loss to the watercourse. 15m 1 3 Figure 3.7 Minimum placement of structures from channel bank (iv) Significant benefit can be achieved by starting rehabilitation in the upstream reaches of a watercourse thereby using the existing downstream fully vegetated or weed-infested creek to reduce bankfull velocities within the rehabilitated reach. Unfortunately, this usually does not apply to projects involving the reconstruction of concrete lined drainage channels where it may be more desirable to start at the downstream end of the watercourse. Step 9. Review riparian and floodplain vegetation When an acceptable trial channel alignment and cross section has been obtained, check that riparian vegetation can be introduced along the edges of the watercourse without causing unacceptable flood levels during the peak design flood event, usually the 1 in 100 year flood. Chapter 4 provides information on the design of riparian and floodplain vegetation. If the proposed channel meanders across the floodplain, then the establishment of riparian vegetation along the banks of the channel may significantly affect the hydraulic capacity of the floodplain. If this is the case, then the options are: (i) Decrease the density of riparian vegetation in selected areas to allow overbank flood flows to pass down the valley with few restrictions (refer to Chapter 4). (ii) Relocate the channel along one side of the valley (where channel velocities allow) to reduce the potential interference on overbank flood flows. 41