2017 Queensland Urban Drainage Manual - Hydrology Chapter 4 PDF

4. Catchment hydrology 4.1 Introduction When undertaking a hydrologic analysis of a drainage catchment for the purpose of designing a drainage system, the ‘intent’ should be to: select and utilise appropriate hydrologic methods take all necessary steps to understand the appropriate application of the chosen hydrologic methods with respect to the catchment conditions;; including the selection and usage of key variables—in this respect, this Manual must not be treated as a ‘prescriptive standard’ (refer to the Glossary, or the Professional Engineers Act, 2002) apply all hydrologic methods in an manner that best allows the determination of appropriate design discharges for the drainage catchment being considered determine a design discharge that best protects public and private assets throughout the expected working life of the drainage system being designed;; that is an appropriate ‘worst case’ condition, whether such a case exists at the beginning, middle or end of its expected working life, or at any stage of a staged development. Achieving the intent of the last dot point can be complicated by the difficulties of assessing the likely future drainage conditions of an upstream catchment when such ‘future’ conditions are outside the control, and possible knowledge of, the drainage designer. Unless there is other more reliable information, designers may reasonably assume that the future drainage conditions of an upstream catchment (including land use category and the utilisation of flow mitigation systems) are those defined by the local government, typically within: the current Planning Scheme an approved Local Government Infrastructure Plan (LGIP) current stormwater/drainage codes used in the assessment of development applications. If the purpose of the catchment hydrology is to be used to set minimum fill or floor levels, then the nominated peak discharge must consider the worst case scenario. It is usually assumed that this is represented by a fully developed catchment;; however, circumstances can exist where current peak discharges and flood levels could exceed expected future conditions. For example, when a major regional stormwater detention/retention system exists within a planning scheme, but as yet has not been built. Best practice drainage design usually requires the designs to cater for flows discharged from a ‘fully developed catchment’. If such conditions are not adequately defined within a planning scheme or code, then the designer should assume the following catchment conditions, but only so far as they do not contradict the requirements outlined within an approved planning scheme or code: Ultimate land use in accordance with the current Planning Scheme or Strategic Plan for the catchment. The incorporation of flow attenuation systems within the upstream catchment (e.g. flow detention and WSUD principles) only if such flow attenuation mechanisms are considered mandatory within the current Planning Scheme or Strategic Plan. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-1 The incorporation of existing surface storages (e.g. detention systems and hydraulic chokes) only if the sustainability (longevity) of these surface storages is protected by appropriate measures;; for example, the flood storage system is contained within a drainage easement or reserve. Within the limits of reasonable investigation (relative to the design risk), the drainage designer must not take into consideration any surface storage or flow attenuation system that has the following characteristics: − surface storage that is contained within land not under the control of an easement, reserve or local government Planning Scheme − surface storage that primarily results from ‘hydraulic choking’ caused by an undersized drainage system that does not comply with current design standards (i.e. any surface storage that could reasonably be expected to be removed once the catchment’s drainage system is upgraded to current design standards) − surface storage that consists of flooded land (e.g. existing urban areas) where it is reasonable to expect that future flood mitigation works will remove, or at least substantially reduce, such flooding (e.g. flood mitigation works identified within Master Drainage Planning). The drainage designer must not assume, without appropriate investigation, that upstream inflows will not be altered from pre-development conditions once the catchment is fully developed. The last dot point refers to the realisation that it is unreasonable to expect that the full development of a large creek catchment will not result in at least some change (usually an increase) in peak discharges and runoff volumes within the lower catchment. Even if best practice stormwater management principles are applied throughout the catchment, an increase in peak discharges can still occur because: there will be increases in runoff volume (which are generally impossible to fully mitigate) stormwater detention systems established on privately developed land typically do not compensate for increased flows resulting from associated state and council infrastructure stormwater detention systems are usually designed not to increase flows resulting from design storms, not real storms (which typically have a longer ‘rising limb’ and runoff volume) the upgrading of existing ‘below standard’ infrastructure (e.g. pipes and culverts) can reduce surface storage within the upper catchment flood mitigation works may have been carried out along the waterway that reduce the effective in-channel flood storage. Further discussion on the limitations of modern flow mitigation systems is provided in Chapter 5 – Detention/retention systems. 4.2 Choice of hydrologic method When choosing a hydrologic method the ‘intent’ must be to choose an analytical procedure that: is appropriate for the catchment conditions and the required degree of accuracy is capable of assessing a critical ‘change’ in a specific catchment condition when the modelling is required to assess the likely impact of a proposed development (e.g. assessment of pre- and post-development runoff, or the augmentation of a drainage channel) is capable of being reviewed by any regulator, or a nominated third party, that is required to review the hydrology/development proposal. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-2 4.2.1 Rational Method Utilisation of the Rational Method, as described within the Manual, may be considered appropriate for the following uses and catchment conditions: determination of peak design discharge for urban catchments of less than 500 hectares that do not incorporate detention storage facilities estimation of peak discharge for rural catchments of less than 25 km 2 (refer to Section 4.6.11 for reference to other documented procedures for the assessment of rural catchments) determination of peak design discharge in conditions where the bulk of the stormwater runoff is contained within a drainage system (i.e. conduit, channel or overland flow path) that does not provide significant flow-attenuating flood storage (i.e. the nominated ‘time of concentration’ approximately equals the actual travel time of the flow, which approximately equals the travel time of the ‘flood wave peak’) as a ‘checking tool’ (not calibration) of numerical models developed for small ungauged catchments (i.e. checking for potential gross errors) the design of cross drainage structures (e.g. culverts), where the consequences of errors in the determination of a design discharge are not significant (e.g. overtopping flows do not threaten the flood immunity of adjacent buildings) the design of local government drainage systems where it is not considered feasible to develop a comprehensive runoff-routing model conducting local government drainage investigations (e.g. investigating drainage complaints) the design of temporary drainage, erosion and sediment control measures in association with the preparation of construction site Erosion and Sediment Control Plans the design of spillways (bywash) for minor farm dams. Utilisation of the Rational Method for the determination of ‘peak’ stormwater discharges should not be considered appropriate for use in the following circumstances: urban catchments with an area greater than 500 hectares determination of design discharges for the determination of minimum flood level for new buildings (minimum flood levels should be based on appropriate runoff-routing modelling) determination of design hydrographs or runoff volumes for flood mapping or the analysis of flood storage systems the analysis or design of those components of the drainage system which are volume-based, such as detention and retention basins (it is noted that some local authorities may have design procedures for small on-site detention systems which are not volume-based, but instead are based solely on pre and post-development peak discharges) determination of peak discharges associated with historical (real) storms assessment of unusually shaped drainage catchments (refer to Section 4.7) assessment of catchments containing significant, isolated, areas of vastly different hydrologic characteristics, such as a catchment with an upper forested sub-catchment and a lower urbanised sub-catchment assessment of catchments with significant floodplain storage, detention basins, or catchments with wide-spread usage of on-site detention systems. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-3 4.2.2 Utilisation of the Rational Method within complex catchments The Rational Method is a simple hydrologic method with a modest degree of accuracy compared to numerical runoff-routing models;; however, it is through this ‘simplicity’ that the Rational Method is able to avoid the gross errors that can occur from time to time in non-calibrated numerical models. It is for this reason that some practitioners choose to use the Rational Method as a means of checking (not calibrating) numerical models. There are numerous catchment conditions where use of the Rational Method would be considered inappropriate. There are other conditions where the application of the Rational Method requires special consideration and adaptation. The following list provides examples of catchment conditions where care must be applied in the application of the Rational Method. Further discussion and detailed recommendations are provided within the Background Notes. An overland flow path passing through low gradient land, oval or park that provides significant surface storage during major storms (i.e. acting as an unofficial detention basin). Catchments where the travel time for the minor drainage system is significantly different from that of the major drainage system. The upstream catchment is zoned for urban use, but is currently undeveloped. Catchments containing significant on-site stormwater detention (OSD). Sub-catchments containing flow-attenuating surface storage systems (e.g. lakes, wetlands or detention/retention basins). Catchments containing water supply dams or weirs. Catchments containing private dams (e.g. farm dams). Urban catchments with an area greater than 500 ha. Catchments containing significant isolated areas of land exhibiting highly diverse stormwater runoff characteristics (e.g. land with significantly different discharge coefficients and runoff velocity). Partially urbanised, ungauged catchments. Irregular shaped catchments (also refer to Section 4.7). Catchments with a significant change in catchment slope or stream slope. 4.2.3 Runoff-routing models The following recommendations are provided in relation to the use of numerical models for the design of urban drainage systems. For guidance on the use of numerical models for the purpose of flood estimation and flood mapping, refer to the latest version of Australian Rainfall and Runoff. Preference should be given to the use of computer-based, runoff-routing, numerical models when analysing the following drainage conditions: urban catchments with an area greater than 500 hectares determination of minimum flood level for new buildings the analysis or design of drainage systems that are volume-dependent, such as detention and retention basins determination of peak discharges associated with historical (real) storms assessment of complex drainage catchments (refer to Section 4.7). When utilising computer-based runoff-routing models to analyse urban drainage systems, the following practices should not be adopted: Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-4 the extraction of peak discharges at model nodes that have fewer than 5 contributing sub- catchments, unless the model’s user manual identifies that fewer sub-catchments are acceptable (e.g. XP-RAFTS) the adoption of a ‘critical storm duration’ based on the assessed Rational Method ‘time of concentration’;; these are two different hydrologic terms that should not be interchanged the adoption, within ‘design storm’ runs, of those initial loss and continuing loss values determined from historical storm calibration runs without appropriate consideration of the likely effects of pre-storm rainfall. A critical aspect of runoff-routing modelling is the choice of loss rates (e.g. initial and continuing losses). Designers should refer to the latest version of Australian Rainfall and Runoff for guidance on the choice of loss models and initial and continuing loss rates. A short discussion on the selection of loss rates is also provided within the Background Notes for this chapter. 4.2.4 Regional flood frequency analysis The latest edition of Australian Rainfall and Runoff provides guidance on the use of regional flood methods for ungauged rural streams. These equations are expected to be suitable for small to medium sized (8 to 1000 km2) rural catchments (< 10% urban) for both coastal and semi-arid regions of Queensland. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-5 4.3 The Rational Method In its general form (using the non-standard units of Q (m3/s), I (m/s) and A (m2)) the Rational Method is based on the following formula: Q = C . I . A (4.1) For design purposes the more common units are used;; Q (m3/s), I (mm/hr) and A (ha): Qy = (Cy . tIy . A)/360 (4.2) where: Qy = peak flow rate (m3/s) for annual exceedence probability (AEP) of 1 in ‘y’ years Cy = coefficient of discharge (dimensionless) for AEP of 1 in ‘y’ years A = area of catchment (ha) t Iy = average rainfall intensity (mm/h) for a design duration of ‘t’ hours and an AEP of 1 in ‘y’ years t = the nominal design storm duration as defined by the time of concentration (tc) The value ‘360’ is a conversion factor to suit the units used. Calculation of the flow at the various inlets and junctions along the drainage line is carried out from the top of the system progressively downstream. The design discharge at any given location is not the sum of the individual sub-area peak discharges, but is a value calculated for that location based on the assessed time of concentration at that location. 4.4 Catchment area When determining the catchment area, drainage designers must: utilise a catchment plan that best represents the historical, existing or future conditions as the case may be give appropriate consideration to the likelihood that the catchment area for the minor drainage system may be different from that of the major drainage system give appropriate consideration to the possibility that land contouring and piped drainage systems may extend the catchment boundary beyond the natural catchment boundary give appropriate consideration to the possibility that roof water from a given property may discharge to a different location from that of the ground level runoff give appropriate consideration to the potential effects of constructed flow diversion systems, such as property fences and roads, that may extend the catchment boundary beyond the natural catchment boundary give appropriate consideration to proposed future road layouts that may extend or reduce the catchment boundary beyond the natural catchment boundary give appropriate consideration to the likelihood that road upgrades or resurfacing could alter the direction of overland flow paths (e.g. a high-crown road is replaced by a low-crown road, or one with two-way fall replaced with one-way fall or vice versa). Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-6 4.5 Coefficient of discharge The coefficient of discharge, ‘C’ as used within the Rational Method should not be confused with the volumetric runoff coefficient ‘CV’, which is a direct ratio of total runoff to total rainfall. The nominated coefficient of discharge must account for the future development of the catchment as depicted in the current Planning Scheme, with appropriate consideration of the authority’s current detention/flow-control policies. The recommended steps for determining the coefficient are listed below: determine the fraction impervious (fi) for the catchment under study from Table 4.5.1 determine the 1 hour rainfall intensity (1I10) for the 10 year ARI (10% AEP) at the locality determine the frequency factor (Fy) for the required design storm from Table 4.5.2. determine the 10 year discharge coefficient (C10) value from tables 4.5.3 and 4.5.4. multiply the C10 value by the frequency factor (Fy) to determine the coefficient of runoff for the design storm (Cy). Cy = Fy . C10 (4.3) It is recommended that the value of Cy should be limited to unity (1.0) within urban areas. Table 4.5.1 – Fraction impervious vs. development category Development category Fraction impervious (fi) Central business district 1.00 Commercial, local business, neighbouring facilities, service industry, 0.90 general industry, home industry Significant paved areas e.g. roads and car parks 0.90 Urban residential – high density 0.70 to 0.90 Urban residential – low density (including roads) 0.45 to 0.85 Urban residential – low density (excluding roads) 0.40 to 0.75 Rural residential 0.10 to 0.20 Open space and parks etc. 0.00 Notes: 1. The fraction impervious should be determined for each development. Local governments may specify default values. 2. Typically for urban residential high density developments: townhouse type development fi = 0.7 multi-unit dwellings > 20 dwellings per hectare fi = 0.85 high-rise residential development fi = 0.9 3. In urban residential low density areas fi will vary depending upon road width, allotment size, house size and extent of paths, driveways etc. 4. Refer to Table 7.3.3 for the definition of development categories as used in this Manual. Notes (tables 4.5.3 & 4.5.4, over page): 1 I10 = One hour rainfall intensity for a 1 in 10 year ARI (10% AEP) C10 = Coefficient of discharge for a 1 in 10 year ARI (10% AEP) fi = Fraction impervious Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-7 Table 4.5.2 – Table of frequency factors AEP (%) ARI (years) Frequency factor (Fy) 63% 1 0.80 39% 2 0.85 18% 5 0.95 10% 10 1.00 5% 20 1.05 2% 50 1.15 1% 100 1.20 Table 4.5.3 – Table of C10 values Intensity Fraction impervious fi (mm/hr) 1 I10 0.00 0.20 0.40 0.60 0.80 0.90 1.00 39-44 0.44 0.55 0.67 0.78 0.84 0.90 Refer to Table 4.5.4 45-49 0.49 0.60 0.70 0.80 0.85 0.90 50-54 0.55 0.64 0.72 0.81 0.86 0.90 55-59 0.60 0.68 0.75 0.83 0.86 0.90 60-64 0.65 0.72 0.78 0.84 0.87 0.90 65-69 0.71 0.76 0.80 0.85 0.88 0.90 70-90 0.74 0.78 0.82 0.86 0.88 0.90 Refer to notes on previous page. Table 4.5.4 – C10 values for zero fraction impervious Medium density bush, or Light cover bushland, or Land Good grass cover, or Poor grass cover, or Dense bushland description High density pasture, or Low density pasture, or Zero tillage cropping Low cover bare fallows Intensity Soil permeability Soil permeability Soil permeability (mm/hr) 1I10 High Med Low High Med Low High Med Low 39–44 0.08 0.24 0.32 0.16 0.32 0.40 0.24 0.40 0.48 45–49 0.10 0.29 0.39 0.20 0.39 0.49 0.29 0.49 0.59 50–54 0.12 0.35 0.46 0.23 0.46 0.58 0.35 0.58 0.69 55–59 0.13 0.40 0.53 0.27 0.53 0.66 0.40 0.66 0.70 60–64 0.15 0.44 0.59 0.30 0.59 0.70 0.44 0.70 0.70 65–69 0.17 0.50 0.66 0.33 0.66 0.70 0.50 0.70 0.70 70–90 0.18 0.53 0.70 0.35 0.70 0.70 0.53 0.70 0.70 Developed from Department of Natural Resources and Mines (2004);; see Background Notes for further discussion. These coefficients are not suitable for soils compacted by construction activities. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-8 4.6 Time of concentration (Rational Method) 4.6.1 General Time of concentration (tc) is a catchment parameter that is used in the selection of an appropriate average rainfall intensity (tIy). The time of concentration of a catchment is defined as the time, measured from the start of a design storm, for surface runoff to collect and flow from the most remote part of the catchment to the point at which a design discharge is being calculated. It is noted that the ‘time of concentration’ as used in the Rational Method is not the same as the ‘critical storm duration’ or ‘time to peak’ as determined from runoff-routing models, such as RAFTS, RORB and WBNM. It is therefore inappropriate to interchange or compare these catchment parameters. In some states, time of concentration is determined as a function of catchment area. Within this Manual, time of concentration is determined directly from flow travel times. This approach has been adopted because it is considered to provide a better representation of potential changes in catchment hydrology caused by urbanisation. Time of concentration must be based upon: a period of time rounded to the nearest minute the sum of the individual travel times of stormwater runoff, at the peak of the storm, passing along the individual drainage segments that make up the longest (with respect to time) flow path a total travel time that extends only so far as the stormwater runoff remains confined within a conduit, swale or open channel where the individual drainage segment does not provide significant surface storage that could attenuate peak discharges (i.e. the segment does not contain a pond, lake, or floodplain). The exceptions being at the top-of-catchment where the runoff is likely to consist of wide-spread sheet flow. at the top of the catchment, the initial travel time should be defined by which ever of the following best represents the upper catchment (the exception being the procedures presented in Section 4.6.11): − the term ‘standard inlet time’ (refer to Section 4.6.4) − the roof runoff travel time (refer to Section 4.6.5) − the duration of sheet flow prior to flow concentration (refer to Section 4.6.6). However, consideration of ‘partial area effects’ may require the adoption of an alternative (shorter) time of concentration (refer to Section 4.7). 4.6.2 Minimum time of concentration The minimum recommended time of concentration for the design of urban drainage systems (not roof water drainage—refer to building codes) is 5 minutes. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-9 4.6.3 Methodology of various urban catchments The means of determining the time of concentration must be appropriate for the type of drainage catchment. There are typically five different types of drainage catchments, those being: (a) Predominantly piped or channelised urban catchments less than 500 ha with the top of the catchment being urbanised. (b) Predominantly piped or channelised urban catchments less than 500 ha with the top of the catchment being bushland or a grassed park. (c) Bushland catchments too small to allow the formation of a creek with defined bed and banks. (d) Urban creeks with a catchment area less than 500 ha. (e) Rural catchments less than 500 ha. Table 4.6.1 provides a summary of the typical components that make up the ‘time of concentration’ for each type of drainage catchment. This table outlines the typical components that make up the total travel time. Unusual drainage catchments may require an alternative approach to the determination of an appropriate time of concentration. Table 4.6.1 – Summary of typical components of time of concentration Concentrated overland flow Channel flow Creek flow travel time sheet flow Kerb flow inlet time Pipe flow Standard Overland time time time Catchment conditions 4.6.7 & Subsection number: 4.6.4 4.6.6 4.6.8 4.6.9 4.6.10 4.6.11 4.6.10 (a) Urban piped catchments with urban development at Yes Yes Yes Yes the top of catchment (b) Urban piped catchments with park/bush at top of Yes Yes Yes Yes Yes catchment (c) Small, non-piped catchments, with no formal Yes Yes Yes creek (d) Urban creeks (< 500 ha) As above for the appropriate catchment Yes with no floodplain storage conditions (d) Urban creeks (< 500 ha) Standard inlet time, sheet flow and pipe flow time not with significant floodplain Yes included storage (e) Rural creek catchments Standard inlet time, sheet flow and pipe flow time not Yes included Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-10 4.6.4 Standard inlet time Use of standard inlet times for developed catchments is recommended because of the uncertainty relating to the calculation of the travel time of overland flow. Standard inlet times, as presented in Table 4.6.2, are considered to represent the travel time from the top of the catchment to a location where the first gully or field inlet would normally be expected to exist, as depicted in Figure 4.1. P ipe flow G utter flow Ov er la n d flo w P ipe flow Ov S wale flo e rla w nd Figure 4.1 – Application of standard inlet time Table 4.6.2 – Recommended standard inlet times Inlet time Location (minutes) Road surfaces and paved areas 5 Urban residential areas where average slope of land at top of catchment is greater 5 than 15% Urban residential areas where average slope of land at top of catchment is greater 8 than 10% and up to 15% Urban residential areas where average slope of land at top of catchment is greater 10 than 6% and up to 10% Urban residential areas where average slope of land at top of catchment is greater 13 than 3% and up to 6% Urban residential areas where average slope at top of catchment is up to 3% 15 Note: The average slopes referred to in this table are the slopes along the predominant flow path for the catchment in its developed state. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-11 4.6.5 Travel times from roof to main system connection In cases where the use of a standard inlet time is not considered appropriate, the following roof to drainage system flow travel times are recommended: Table 4.6.3 – Recommended roof drainage system travel times Time to point A Development category (minutes) Rural residential, residential low-density For the roof, downpipes and pipe connection system from the building to the 5 kerb and channel or a rear-of-allotment drainage system (Figure 4.2). Residential medium and high-density, commercial, industrial and central business district For the roof and downpipe collection pipe to the connection point to the 5 internal allotment drainage system abutting the building (Figure 4.3). Note: The flow time from point A (figures 4.2 & 4.3) through the internal allotment pipe system to the kerb and channel, street underground system or rear of allotment system for the more intense developments noted should be calculated separately. A K erb and channel S treet A A H ous e F actory drainag e s ys tem R ear of allotment Figure 4.2 – Typical roof drainage systems Figure 4.3 – Typical roof drainage systems (residential) (industrial) Note (figures 4.2 & 4.3): Point A is referred to in Table 4.6.3 Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-12 4.6.6 Overland flow travel times Overland flow travel times are used when: it is considered inappropriate to use standard inlet times, or surface conditions at the very top of the catchment are likely to produce sheet flow conditions. Procedure for the determination of overland flow times: Overland flow times typically consist of a combination of sheet flow and concentrated flow travel times (each component determined separately). The first step is to determine the distance over which ‘sheet flow’ will occur at the top of the catchment (based on field observations, location of walking trails, natural swales, etc.). Appropriate consideration must be given to the maximum sheet flow travel distances presented in Table 4.6.4. In urban areas, the length of overland sheet flow will typically be 20 to 50 metres. In rural residential areas the length of overland sheet flow should be limited to 200 m (Argue, 1986), however the actual length is typically between 50 and 200 m. Travel times for the ‘sheet flow’ segment can be determined either from the Friend’s equation (Equation 4.5), which is the preferred method, or the Kinematic Wave equation (Equation 4.6). Table 4.6.4 – Recommended maximum length of overland sheet flow Surface condition Assumed maximum flow length (m) Steep (say >10%) grassland (Horton’s n = 0.045) 20 Steep (say >10%) bushland (Horton’s n = 0.035) 50 Medium gradient (approx. 5%) bushland or grassland 100 Flat (0–1%) bushland or grassland 200 Equation 4.5 (Friend, 1954) may be used for determination of overland sheet flow times. This equation was derived from previous work in the form of a nomograph for shallow sheet flow over a plane surface (Figure 4.4). It is noted that values for Horton’s ‘n’ are similar to those for Manning’s ‘n’ for similar surfaces. Figure 4.4 – Overland sheet flow times (shallow sheet flow only) (source: ARR, 1977) Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-13 Friend’s equation: t = (107n L 0.333)/S 0.2 (4.5) where: t = overland sheet flow travel time (min) L = overland sheet flow path length (m) n = Horton’s surface roughness factor S = slope of surface (%) The kinematic wave equation for overland travel time, as developed by Ragan & Duru (1972), may also be used;; however, it should only be applied to planes of sheet flow that are homogenous in slope and roughness. Thus, travel times need to be determined separately for areas of different slope or roughness. As shown by McCuen (1984) it is best applied to large paved areas such as car parks and airports. t = 6.94 (L.n*) 0.6 /(I 0.4.S 0.3) (4.6) where: t = overland travel time (min) L = overland sheet flow path length (m) n* = surface roughness/retardance coefficient I = rainfall intensity (mm/hr) S = slope of surface (m/m) Typical values for n* are presented below: (i) As quoted by Argue (1986) p. 28. o Paved surfaces = 0.015 o Lawns = 0.25 o Thickly grassed surfaces = 0.50 (ii) As derived from ARR (1998), Book 8, Table 1.4. Table 4.6.5 – Surface roughness or retardance factors Surface type Horton’s roughness coefficient n* Concrete or Asphalt 0.010 – 0.013 Bare Sand 0.010 – 0.016 Gravelled Surface 0.012 – 0.030 Bare Clay-Loam Soil (eroded) 0.012 – 0.033 Sparse Vegetation 0.053 – 0.130 Short Grass Paddock 0.100 – 0.200 Lawns 0.170 – 0.480 Notes: 1. The surface roughness/retardance coefficient n* is similar but not identical to Manning’s n value for surface roughness. 2. For further details of this procedure reference should be made to Technical Note 3, Book 8, ARR (1998). 3. Experience both locally and as quoted by McCuen (1984) indicates that the kinematic wave equation tends to result in excessively long overland sheet flow travel times. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-14 4.6.7 Initial estimate of kerb, pipe and channel flow time An initial (trial) estimate of flow travel times along kerbs, pipes and channels can be determined from Figure 4.5 (Argue, 1986). The chart may be used directly to determine approximate travel times along a range of rigid channel types and, with the application of multiplier Δ for a range of loose-boundary channel forms. Technical notes for Figure 4.5 Flow travel time (approximate) may be obtained directly from this chart for: kerb-and-gutter channels stormwater pipes allotment channels of all types (surface and underground) drainage easement channels (surface and underground) Multiplier Δ, should be applied to values obtained from the chart as per: grassed swales, well maintained and without driveway crossings, Δ = 4 blade-cut earth table drains, well maintained and no driveway crossings, Δ = 2 natural channels, Δ = 3 Once a trial flow rate has been determined, the travel time determined from Figure 4.5 will need to be checked using either figures 4.6 or 4.7. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-15 Figure 4.5 – Flow travel time in pipes and channels (Source: Argue, 1986) Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-16 4.6.8 Kerb flow travel times Time of flow in kerb and channel should be determined by dividing the length of kerb and channel flow by the average velocity of the flow. The average velocity of the flow may be determined in either of two ways: Izzard’s equation—refer to Technical Note 4, Book 8, ARR (1998). Reference is also made to Section 7.4.6 (d) of this Manual for a more detailed explanation of Izzard’s equation. Figure 4.7 provides a quick solution to Izzard’s equation—accurate enough for travel time calculations. Using Figure 4.6. Figure 4.6 – Kerb and channel flow time using Manning’s equation Technical notes for Figure 4.6 Formula: t = 0.025 L / S 0.5 (minutes) where: t = time of gutter flow in minutes L = length of gutter flow in metres S = slope of gutter (%) Example Length of gutter flow = 100m Average slope of gutter = 3% Thus, time of travel = 1.5 minutes. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-17 Figure 4.7 – Kerb and channel flow velocity using Izzard’s equation Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-18 4.6.9 Pipe flow travel times Wherever practical, pipe travel times should be based on calculated pipe velocities either using a Pipe Flow Chart (e.g. n = 0.013 for concrete pipes), uniform flow calculations using Manning’s equation (Equation 4.7), or a calibrated numerical drainage model. An initial (trial) assessment of the pipe flow travel time can be determined using Figure 4.5. Alternatively, if the travel time within the pipe is small compared to the overall time of concentration, then an average pipe velocity of 2 m/s and 3 m/s may be adopted for low gradient and medium to steep gradient pipelines respectively. 4.6.10 Open channel flow travel times The time stormwater takes to flow along an open channel may be determined by dividing the length of the channel by the average velocity of the flow. The average velocity of the flow is calculated using the hydraulic characteristics of the open channel. Manning’s equation is suitable for this purpose: V = (1/n).R 2/3. S 1/2 (4.7) From which t = L/(60.V) = n. L / (60. R 2/3. S 1/2) (4.8) where: V = average velocity (m/s) n = Manning’s roughness coefficient R = hydraulic radius (m) S = friction slope (m/m) L = length of reach (m) t = travel time (min) Where an open channel has varying roughness or depth across its width it may be necessary to segment the flow and determine the average cross-sectional flow velocity in order to determine the flow travel time. Travel times along grassed swales can vary significantly depending on flow depth and surface roughness. The effective surface roughness should be determined from vegetation retardance charts (Department of Main Roads, 2002). For a grass length of 50 to 150 mm, typical Manning’s roughness values may be interpolated from Table 9.3.4. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-19 4.6.11 Time of concentration for rural and creek catchments Wherever practical, a regional flood frequency analysis (refer to ARR) should be used to determine design discharges from rural catchments in preference to the use of the Rational Method. The available methods for the determination of time of concentration for creeks include: The Bransby-Williams’ equation—however, various authorities have found this equation to be unreliable (refer to further discussion within the Background Notes). The modified Friend’s equation—this method was not found to be representative of the time of concentration for urban creeks within Brisbane (refer to further discussion within the Background Notes) The stream velocity method—this method has been modified from that presented within the 2013 Provisional edition of this Manual following recent (2015) calibration studies (refer to further discussion within the Background Notes). (a) Bransby-Williams’ equation tc = 58 L /(A 0.1. Se 0.2) (4.9) where: tc = the time of concentration (min) L = length (km) of flow path from catchment divide to outlet A = catchment area (ha) Se = equal-area slope of stream flow path (%) (b) Modified Friend’s equation (maximum catchment area of 25 km2) tc = 800 L /(Ch. A 0.1. Se0.4) (4.10) where: tc = time of concentration (min) L = Length (km) of flow path from catchment divide to outlet Ch = Chezy’s coefficient at the site = (1/n)R1/6 R = hydraulic radius = 0.75RS where stream slope is fairly uniform = 0.65RS where stream slope varies appreciably along the stream RS = hydraulic radius at the initially assumed flood level at the site n = average Manning roughness coefficient for the entire stream length A = catchment area (ha) Se = equal-area slope of stream flow path (%) Calculation of hydraulic radius (R) is based upon the peak level of the design flood at the site in question. If later hydraulic calculations show this level to be in error by more than 0.3–0.6 m, the value should be recalculated. Also, an ‘inlet time’ should not be applied to any of the procedures presented in this section. Equations 4.9 and 4.10 use different units from the original equations presented within the 1992 edition of this Manual, as well as other publications such as ARR (1998). The equation units were changed such that both equations were able to utilise the same units for time, area and equal-area slope. The adopted equation coefficients (‘58’ & ‘800’) have been appropriately adjusted (though rounded down from the exact unit conversion) for use of these revised units. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-20 Technical note for Figure 4.8: Warning : equa l area s lope (S e ) needs to be converted from Figure 4.8 demonstrates equal-area slope in units of m/km, which (m/km) to (% ) for us e in equations needs to be converted to percentage units (%) by dividing by 10. Figure 4.8 – Derivation of the equal-area slope (Se) of main stream (c) Stream Velocity Method The Stream Velocity Method is based on the use of an ‘assumed’ (i.e. not real) average stream velocity to determine a nominal ‘time of concentration’, that, when used with the Rational Method, compensates for the flow attenuation effects of floodplain storage. Table 4.6.6 – Modified Stream Velocity method for catchment areas of 5 to 100 km2 Catchment description Travel velocity (m/s) Flat country (0 to 1.5% average catchment surface slope) 0.3 Rolling country (1.5 to 4% average catchment surface slope) 0.7 Hilly country Significant floodplain storage exists along most of the 0.9 (4 to 8% waterway average Natural floodplain storage is limited by adjacent hill 1.5 catchment slopes, or the natural floodplain storage has been reduced surface by urbanisation and/or land filling slope) The waterway has experienced significant channelisation 2.0 that has removed most of the floodplain storage relevant to the flood event being studied Steep country (8 to 15% average catchment surface slope) with soil- 1.5 based waterway (i.e. not a rocky gorge) with floodplain storage limited by steep topography Steep rocky mountain country (>10% average catchment surface slope) 3.0 with rock-based waterway (i.e. rocky gorge) with minimal, if any, floodplain storage Fully channelised waterway with no floodplain storage Actual stream velocity Notes: ‘Travel velocity’ represents the ‘average stream velocity’ that should be adopted to determine the ‘time of concentration’ for use within the Rational Method ‘Catchment surface slope’ is a measure of the average slope of the full drainage catchment, not the stream bed;; thus it is not the same as stream slope or ‘equal area slope’. The extent of floodplain storage refers to the extent of floodplain storage during the flood event being studied;; and thus may not be relevant in the study of minor in-bank flows. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-21 4.7 The partial area effect Because the Rational Method has been developed for ‘typical’ drainage catchments, there is the potential for this method not to define the actual ‘peak’ design discharge for drainage catchment that exhibit ‘unusual’ drainage conditions. However, in some situations high flows can be generated from a shorter storm duration acting upon a reduced catchment area. This outcome is known as the ‘partial area effect’. When considering the potential impacts of partial area effects on the selection of the effective catchment area and time of concentration, the ‘intent’ must be to: give appropriate consideration of possible catchment characteristics that could cause partial area effects give appropriate consideration to benefits of the development of a runoff-routing model that would better model the runoff characteristics of an ‘unusual’ catchment instead of utilising the Rational Method check for partial area effects in drainage catchments that exhibit any of the following characteristics: − a long, narrow, upper catchment that contributes an ‘out-of-proportion’ contribution to the whole-of-catchment time of concentration − a significant change in creek slope − a significant increase in the runoff velocity or runoff volumes (expressed in terms of the coefficient of discharge) from the upper catchment to the lower catchment. However, if the catchment does exhibit unusual runoff conditions, then this should mean that a suitable runoff-routing model should be employed in preference to a questionable Rational Method analysis. Guidance on the treatment of partial area effects is provided within the Background Notes. 4.8 Intensity-frequency-duration data Intensity-frequency-duration (IFD) data is required as input to the hydrologic model used for design. IFD charts can be obtained either through the local authority, the Bureau of Meteorology, or Australian Rainfall and Runoff. As of 2016, Book 1, Chapter 6 of AR&R is the best point of reference. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-22 4.9 Estimation of runoff volume 4.9.1 General In stormwater design, the estimation of runoff volume is often as important as the estimation of peak discharge. Runoff volume is used for a variety of purposes, including: sizing temporary and permanent sedimentation basins sizing stormwater detention/retention basins designing various urban stormwater treatment systems. Stormwater runoff volumes can be represented by either the ‘average annual runoff volume’, or the runoff volume expected from just a ‘single storm’. The procedures used to determine the average annual volumetric runoff coefficient are different from those used to determine the volumetric runoff coefficient for an isolated storm. It is also noted that the volumetric runoff coefficient (CV) is not the same as the Rational Method coefficient of discharge (C). 4.9.2 Estimation of annual average runoff volume The average annual runoff volume may be determined from continuous catchment modelling (preferred method), or through the use of a calibrated regional volumetric runoff coefficient. The average annual volumetric runoff coefficient for a given catchment is depend on: soil permeability local hydrology percentage of directly connected impervious area percentage of indirectly connected impervious surface area degree of stormwater harvesting, including the use of rainwater tanks. An estimation of the average annual volumetric runoff coefficient may be obtained using one of the following methods: analysis of long-term stream gauging and rainfall records (preferred option) continuous water balance modelling using a calibrated catchment yield model (second option) use of an annual average volumetric runoff coefficient from an adjacent catchment with similar soil, topographic and climatic conditions (third option). 4.9.3 Estimation of runoff volume from a single storm An estimation of runoff volume from a single (isolated) storm event may be obtained using one of the following methods: calibrated runoff–routing model (preferred method) use of the single storm event volumetric runoff coefficient (Table 4.9.1) direct extraction of estimated rainfall losses from a given rainfall hyetograph. The actual runoff volume will be dependent on a number of variables including soil type, depth of soil, land slope, the degree of surface storage, type and density of vegetation cover, and the degree of soil moisture at the start of the storm event (i.e. the lasting effects of previous rainfall). Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-23 (a) Single event volumetric runoff coefficient The volumetric runoff coefficient for a single storm event may be estimated using the U.S. Soil Conservation Service (1986) procedures. Volumetric runoff coefficients developed from these procedures are presented in Table 4.9.1. When using the coefficients presented in Table 4.9.1 the following issues should be noted: The coefficients apply to the pervious surfaces only;; therefore, an adjustment must be applied to determine a coefficient for urbanised catchments, as presented in Equation 4.12. The coefficients where originally developed for relatively flat agricultural land;; therefore, these coefficients are likely to under-estimate the runoff volume from steep catchments. CV ( pervious) . ( A − A(imp.) ) + A(imp.) (4.12) CV (composite ) = A where: CV (composite) = Composite volumetric runoff coefficient CV (pervious) = Volumetric runoff coefficient for pervious surface (Table 4.9.1) A = Total catchment area A (imp.) = Area of directly connected impervious surface, plus a percentage of the indirectly connected impervious surface area (assume 50% unless otherwise directed) Table 4.9.1 – Typical single storm event volumetric runoff coefficients for various Soil Hydrologic Groups Soil Hydrologic Group Rainfall Group A Group B Group C Group D (mm) Sand Sandy loam Loamy clay Clay 10 0.02 0.10 0.09 0.20 20 0.02 0.14 0.27 0.43 30 0.08 0.24 0.42 0.56 40 0.16 0.34 0.52 0.63 50 0.22 0.42 0.58 0.69 60 0.28 0.48 0.63 0.74 70 0.33 0.53 0.67 0.77 80 0.36 0.57 0.70 0.79 90 0.41 0.60 0.73 0.81 100 0.45 0.63 0.75 0.83 Source: US Soil Conservation Service (1986) Group A soils: soil with very high infiltration capacity. Usually consist of deep (> 1 m), well-drained sandy loams, sands or gravels. Group B soils: soil with moderate to high infiltration capacity. Usually consist of moderately deep (>0.5 m), well-drained medium loamy texture sandy loams, loams or clay loam soils. Queensland Urban Drainage Manual 2016 Edition Catchment Hydrology 4-24 Group C soils: soil with a low to moderate infiltration capacity. Usually consist of moderately fine clay loams, or loamy clays, or more porous soils that are impeded by poor surface conditions, shallow depth or a low porosity subsoil horizon. Group D soils: soil with a low porosity. Usually consists of fine-texture clays, soils with poor structure, surface-sealing (dispersive/sodic) soils, or expansive clays. Included in this group would be soils with a permanent high watertable. Landcom (2004) provides typical infiltration rates for the various Soil Hydrological Groups (A, B, C, and D) as presented in Table 4.9.2. Table 4.9.2 – Typical infiltrations rates for various Soil Hydrological Groups Soil Hydrological Typical infiltration rate (mm/hr) Ksat (mm/hr) Group Saturated Dry soil A 25 >250 >120 B 13 200 10–120 C 6 125 1–10 D 3 75

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