stormwater guide(1)

Unit CIV3264 Urban Water & Wastewater Systems: Topic 1: Urban Stormwater Management

Department of Civil Engineering, Monash University

CIV3264 Date: Feb 2012

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TOPIC 1: Urban Stormwater

TABLE OF CONTENTS

Chapter Topic

1.0 Introduction to Urban Stormwater

2.0 Design Rainfall Estimation

3.0 Flood Estimation Procedures

4.0 Minor Urban Drainage System Design

5.0 Major Urban Drainage System Design

6.0 Impacts of Urbanization on Stormwater and Receiving Waters

7.0 Water Sensitive Urban Design

8.0 Design and Modelling of Stormwater Quality Systems




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1.0 Introduction to Urban Stormwater Objectives 1. Understand the rainfall-runoff processes, and define components of

the process 2. Understand the effects of urbanisation on rainfall-runoff processes 3. Be aware of approaches to modelling rainfall-runoff processes. 4. Be aware of Australian guidelines relating to urban stormwater;

particularly Australian Rainfall and Runoff. 5. Understand the concept of Minor/Major drainage systems.

Reading

1. Australian Rainfall & Runoff Book 8: Urban Stormwater

1.1 Introduction As urban areas expand, there is a need to provide drainage, in order to protect infrastructure and people from flooding. The approach to providing urban drainage centres around two techniques: 1. the construction of impervious surfaces (roofs, paved areas and road surfaces)

leading to increased runoff volumes, 2. improvements in the hydraulic efficiency of the flow paths (pipes replacing natural

streams) leading to shorter flow times and reduced storage, making shorter higher duration storms critical.

had in many cases some negative outcomes, including environmental degradation (due to pollution and erosion of streams), as well as problems of shifting the flooding problem downstream. Later in the course, we will examine recent trends in urban stormwater management, which attempt more to treat it to improve quality, so that it may be either (a) used as an alternative water resource, or (b) discharged to receiving waterways.

In real practice: It is critical that you understand how to design urban drainage

real data from the drainage system in question (often design will occur prior to urbanisation with no possibility of data being available for the as constructed system).

Flood estimation methods must therefore be based on design rainfall events with a suitable rainfall-runoff model to produce design flows. Note that this must be a top down design process as elements of the design upstream of any component may have an influence on flow behavior downstream. In this topic, you will learn how runoff is generated (and thus how it can be estimated from rainfall), and understand how to model rainfall-runoff (using typical models). You will also become familiar with Australian guidelines for urban drainage, and consider the concepts of design floods and minor/major drainage.




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1.2 The Runoff Process

The runoff process describes that part of the hydrologic cycle between precipitation on land surfaces and subsequent discharge in waterways, or direct return to the atmosphere through evapotranspiration (Figure 1.1 and Figure 1.2). We talk of the

rocess as depicted in Figure 1.2-flow component. People often also use the term

(s) of that streamflow.

Figure 1.1. Simplified representation of rainfall-runoff in (a) natural and (b) urban

catchments.

Figure 1.2 - The runoff process




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Definitions of rainfall-runoff process components: Precipitation includes rain, snow, hail, sleet, mist, dew and frost. Channel Precipitation is that precipitation falling directly on the water surfaces of streams and lakes (usually a very small component of overall catchment precipitation). Interception is that part of precipitation that falls directly on trees, shrubs, grass, or other objects and does not reach the ground. Depression Storage (or Surface Retention) is the water retained in depressions on the surface of the catchment. It is later infiltrated or evaporated. For example, rainfall of




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Groundwater Flow is the flow in streams that comes from the groundwater reservoir. It accounts for the dry-weather flow in streams. Baseflow is a general term for the streamflow that seeps into the stream channels from below the ground surface. Mostly, it includes both groundwater flow and interflow, but sometimes is used to mean only groundwater flow. Streamflow is the total flow carried in a channel, creek or river. It is made up of channel precipitation, surface runoff, and baseflow. Evaporation is the process whereby liquid water is vaporised and diffused into the atmosphere. Transpiration is the process whereby vegetation extracts moisture from the soil, passes it through the plant and evaporates it into the atmosphere through the leaves. Evapo-transpiration is the total evaporative loss from soil and vegetation to the atmosphere through evaporation and transpiration. Modern usage sometimes uses the

-transpiration. Potential evapo-transpiration is that which would occur if there were no shortage of water to be evaporated, e.g. if the soil were always saturated. Actual evapo-transpiration may be less than potential if the soil is dry or dries out as evapo-transpiration proceeds.

1.3 When rainfall becomes runoff: an introduction to the hydrograph concept

falls, at a rate greater than the infiltration capacity of the soil, we will begin to get runoff, which will be conveyed through the catchment. We could then measure, at the outlet

example, that we have a storm, of uniform rainfall intensity (e.g. 10 mm/hr), that lasts 10 minutes. The resulting hydrograph (graph of discharge from the catchment over time) might look like this:

rainfall (10mm/hr)

Time

Dis

charg

e (

m3/s

ec)

simplified

hydrograph

rainfall (10mm/hr)

Time

Dis

charg

e (

m3/s

ec)

simplified

hydrograph

You will note here that the hydrograph shown is simplified, and assumes that the rate of rainfall over time is constant. In Chapter 3, we will examine how the shape of the hydrograph will vary, depending on the duration of rainfall.




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1.4 Effects of Urbanization on Rainfall-Runoff Urbanization, as previously described, involves:

1. Replacing pervious areas (soil) with impervious areas (roads, roofs, footpaths, etc)

2. Creating more hydraulically efficient flowpaths (from surface flow over vegetated soil, to gutters, pipes and constructed channels).

Urbanisation causes changes to catchment behaviour due to an increase in the impervious area and the reduction in catchment storages as waterways become channeled and piped (Laurenson et al, 1985, Schueler, 1987b). These effects are illustrated in Figure 1.3 and can be summarised in terms of the following changes to the characteristics of runoff hydrographs generated from a developing catchment;

increased peak discharges and runoff volume;

decreased time of concentration;

increased frequency and severity of flooding;

generally decreased baseflow (due to reduced infiltration), but not always (because of human inputs such as garden watering).

Each of the above listed factors account for the observation that urban catchments are more responsive to rainfall resulting in flash floods of high magnitudes and short duration. The characteristics of typical rainfall intensity-frequency-duration relationships are well understood. As catchment urbanisation results in a decrease in the time of concentration, the catchment response to rainfall becomes more sensitive to the higher rainfall intensities/short duration events.

Figure 1.3. Effect of urbanization (development) on streamflow.

The peak discharge generated from an urbanised catchment can be as much as 35 times that generated from a rural catchment (Figure 1.4), with the relative difference between rural and urban conditions being most pronounced for frequent storm events. These findings are consistent with the findings of Laurenson et al. (1977) based on observed data from a paired catchment study (ie. the Giralang and Gungahlin catchments) in the ACT. The bankfull discharge of a rural upland creek that would normally be exceeded at an average recurrence interval of approximately 5 years




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would occur on average twice a year following catchment urbanisation with just 20% of its area becoming impervious.

0.1

1

10

100

0.1 1 10 100

Average Recurrence Interval (years)

Pea

k D

isch

arg

e (

m3

/s)

60% Imperviousness

40% Imperviousness

20% Imperviousness

Rural Conditions

Figure 1.4. Flood frequency curves for varying degrees of urbanization.

These urbanizing processes tend to simplify the modeling process with runoff volumes increasing and catchment travel times reducing. This in turn creates downstream difficulties with the natural drainage channel having insufficient capacity to carry the increased flows and enhanced flooding problems. Historically this process has lead to the replacement of natural channel with concrete lined drains. The realisation of the negative effects of this process has seen the construction of retarding basins and infiltration systems to increase storage and decrease runoff. Increasing concern for the environment has seen increased attention given to water quality control of urban runoff. Initially there has been a concentration on retrofitting pollution control infrastructure into the drainage system (gross pollutant traps, wetlands etc.).

In real practice: Awareness of these difficulties is leading to the development of urban water sensitive design concepts being applied to new developments to provide a better catchment wide approach to improving urban runoff quality.

1.5 Modelling Rainfall-Runoff Being able to predict runoff in response to a given pattern of rainfall is an important requirement of managing urban drainage systems, and many models have been developed. Most attempt to provide a (simplified) representation of the rainfall-runoff process as described in Figure 1.1. Their accuracy depends on available data (to




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calibrate the model), and models range from very simple to highly complex. Different models are suitable for different situations.

Useful link and further information: www.toolkit.net.au/rrl provides access to a range of rainfall- -runoff

Common rainfall-runoff models used in Australia include: AWBM (Australian Water Balance Model), Sacramento, Tank

A commonly-used rainfall-runoff model in Australia is SimHyd (Chiew & McMahon, 1999) -named because it represents water movement into, out-of, and within the surface, soil, and groundwater, as a series of stores (Figure 1.5), with given properties for each store (e.g. capacity, maximum inflow rate, maximum outflow rate, etc). For example, the version presented (in a simplified form) in Figure 1.5 has been adopted in MUSIC (the Model for Urban Stormwater Improvement Conceptualisation: see www.toolkit.net.au/music). In that model, for example, the impervious store will have properties related to its area, and its initial loss (the depression storage or amount of rainfall necessary before runoff results). The pervious store will have properties related to its area, its depth, maximum infiltration rate, field capacity, and rate of loss of water to groundwater).

DEEP SEEPAGEDEEP SEEPAGEDEEP SEEPAGE

Figure 1.5. Conceptual rainfall-runoff model used in MUSIC (Source: Tony Weber, WBM Oceanics).
http://www.toolkit.net.au/rrlhttp://www.toolkit.net.au/music




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Accurate modelling of rainfall-runoff requires short time-steps (ie. temporal frequency with which the model calculates the water balance). The smaller the catchment being modelled, the smaller the required timestep (typical application ranges from 6 minutes up to 24 hours).

In industry practice: Application of rainfall-runoff models is can be very straightforward with modern software and computing technology. However beware!! Model results are only as good as the input data on which they are based. Ideally, models should be calibrated to observed rainfall and streamflow data. In practice, this is often not done, and in such cases, the model results should be viewed with great caution (or scepticism)!!

1.6 The role of Australian Rainfall-Runoff (ARR) Guidelines Management of urban drainage, and of flooding, is a critical issue to urban communities, and comes at great cost (both for the drainage/flood management infrastructure, and also for the potential damage caused by flooding). It is role as the professional body, Engineers Australia has thus, through its National

(AR&R), as the national guidelines for drainage and flood management. Originally released in 1958, and now in its 4th Edition (1998), the 5th Edition is due out in 2008. AR&R is made up of eight books:

Book I Introduction (on MUSO Book II Design rainfall considerations (on MUSO Book III Choice of flood estimation methods and design standards Book IV Estimation of design peak discharges (on MUSO under

Book V Estimation of design flood hydrographs Book VI Estimation of large and extreme floods Book VII Aspects of hydraulic calculations Book VIII Urban stormwater management (on MUSO

SUGGESTED

WEBSITE

http://www.arr.org.au/ncweARR/arrSummary.htm

As a practising engineer, you will be expected to

be familiar with the contents of AR&R.

You can get access to all volumes of ARR through

the Monalisa service at Monash library.

In industry practice: Rainfall and runoff, called Australian Runoff Quality, which includes guidance on design of water-sensitive stormwater management techniques (e.g. swales, wetlands, ponds, infiltration systems, biofiltration systems, etc): see www.arq.org.au.
http://www.arr.org.au/ncweARR/arrSummary.htmhttp://www.arq.org.au/




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1.7 Design Floods and the Minor/Major Drainage Concept

The selection of appropriate design standards for hydraulic structures varies considerably dependent on satisfying engineering standards, the cost of infrastructure and the costs associated with uncontrolled surcharging. Determination of an appropriate standard is often based on cost-benefit analysis (ie. what will be the reduction in flood damage cost, compared to the cost to achieve it?). Typically, a doubling in the design Average Recurrence Interval (ARI) will increase the drainage system cost by around 10% (Pilgrim, 1998a). However, the appropriate standard will depend on a range of issues such as (after: Pilgrim, 1998a):

- the level of hydraulic performance required - construction and operating costs (including maintenance requirements) - safety - aesthetics - regional planning goals - legal and statutory requirements - politics!

Consequently, standards have become established over time as acceptable (to both engineers and the public). Typical Australian standards (Design Average Recurrence Intervals) are:

Intense Business, Commercial, Industrial 20-50 year ARI Other Business, Commercial, Industrial, Intense Residential 10 year ARI Other Residential or Open Space 5 year ARI

These vary by region and organisation (for example, different Municipalities may use a different standard). However it is important to note that many existing areas (e.g. around Melbourne) contain drainage infrastructure which either (a) did not meet these standards (having been designed when different standards were in place, or (b) no longer meets these standards, as a result of further development (increased impervious area). Designs developed using these standards are generally concerned with inconvenience and disruptions rather than major damage, which can eventuate when infrequent, or extreme events occur. It is therefore necessary to have two separate standardsflooding, and one for major flooding. The concept of the major/minor drainage system involves two distinct but interacting drainage networks (Figure 1.6). The Minor system is the network of house drains, road gutters, pits and pipes with capacity limited to the flows generated by the selected minor flood recurrence interval (e.g. 5 years). The Major system provides overland flow paths (roads, reserves open spaces) generally following the natural catchment flow paths, which will allow the passage of a




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major flood. The design will essentially be implemented in the selection of the layout of the urban system (i.e. positioning of roads and free space to provide clear pathways for large flows). Retrofitting existing urban areas to meet requirements of Major drainage, often involve lowering road surfaces, to provide an overland flow path.

Figure 1.6 Concept of minor (top) and major (bottom) drainage system. Note that major system makes use of overland flow paths (roads, floodways), whilst minor system uses pipes, gutters, etc. (Source: Australian Rainfall & Runoff: Book 8)

1.8 Recent Advances in Urban Stormwater Management Traditional approaches to stormwater management are based on a single management objective that considers stormwater as a source of potential hazard to public safety. Stormwater management was essentially that of stormwater drainage using two general methods, ie. (i) conveyance of stormwater to receiving waters in an hydraulically efficient manner; and (ii) detention and retardation of stormwater. The wide ranging impacts of urban stormwater have led to the implementation of urban stormwater management strategies that address multiple objectives. These objectives include stormwater drainage, managing stormwater as a resource, protection of receiving water quality, protection of downstream ecological health, etc. There have been a number of initiatives to change the conventional means by which urban stormwater is managed. One such initiative was the development of the Water Sensitive Urban Design (WSUD) guidelines under the auspices of a number of government departments in Western Australia (Whelans et al.., 1994). Other states followed, with Victoria, NSW and Queensland all developing environmental guidelines for stormwater management. Most recently, Engineers Australia released Australian Runoff Quailty (Wong, 2005), the companion to Australian Rainfall and Runoff. The change in design focus over the last 30 years is illustrated in Figure 1.7; the overall trend is one of increasing the range of objectives for which stormwater is managed. The management of urban stormwater to meet these objectives can fundamentally be categorised into stormwater quantity and stormwater quality management and incorporating stormwater management infrastructure into urban landscape features.




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Figure 1.7 Evolution of (Source: Whelans & Halpern Glick Maunsell, 1994).

The evolution of WSUD has sparked a revolution in the way land is developed, and in

systems are now being replaced by drainage systems which: - retain water on site, to allow infiltration, or to provide for stormwater

harvesting - filter and clean water before discharge to receiving waters - integrate drainage infrastructure into the landscape, to provide

increased amenity - treat stormwater as a resource, rather than a waste to be disposed of.

This evolution has created great opportunities for innovation in design and the urban water industry currently has a drastic shortage of skills in this area! There is great demand for creative engineers who can develop and implement WSUD solutions!




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2.0 Design Rainfall Estimation Objectives 1. To understand how rainfall intensity-frequency-duration (IFD)

relationships can be derived. 2. To be able to derive IFD relationships for any location in Australia,

using (i) Australian Rainfall and Runoff Book 2 and (ii) AusIFD software.

3. To be familiar with typical storm temporal patterns and understand how the pattern will affect flood peaks.

Reading

1. Australian Rainfall & Runoff Book 2: Design rainfall considerations 2. AusIFD website:

http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htm

2.1 Introduction The first step in being able to predict the magnitude of storm flows or floods is being able to predict the amount and pattern of rainfall, for a given recurrence interval (ARI). For example, as a drainage engineer, we may be required to construct a drainage system which can safely convey the 5 year ARI flow. To do this, we need to be able to

Rainfall data have been collected by the Bureau of Meteorology (www.bom.gov.au), as well as water and drainage management agencies (e.g. Melbourne Water: www.melbournewater.com.au) for many years. Using that data, statistical analysis has been undertaken to derive design rainfall estimates for storms of any duration from 6 minutes to 72 hours, with average recurrence intervals from 1 to 100 years. This analysis is called Intensity-Frequency-Duration (IFD) analysis, because it considers:

- The rainfall intensity (mm/hr) - The frequency (average recurrence interval, years) - The duration of the storm (minutes)

Procedures to undertake this analysis are provided in Book Two (Design rainfall considerations) of Australian Rainfall and Runoff (Pilgrim, 1998b). These have also been compiled into easy-to-use computer software packages such as AusIFD (see http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htm). By the end of this topic, you should be able to derive IFD relationships using either ARR Book 2, or using the IFD software.

2 Most methods of flood estimation for the design of hydraulic structures consist of models that convert rainfall into runoff and estimate either the peak discharge or the complete hydrograph of a design flood. To use such methods requires a "design storm", a hypothetical storm that represents some critical "loading" on the catchment and the hydraulic structure (e.g. pit, pipe, retarding basin).
http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htmhttp://www.bom.gov.au/http://www.melbournewater.com.au/http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htm




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2 -

is not always possible (for

represent the post-development scenario. Therefore, rainfall data are used to provide flow estimates, in cases such as:

1. On ungauged catchments or those with only a short streamflow record, rainfall records combined with procedures for converting rainfall into streamflow may give reliable flood estimates.

2. Areas of meteorological homogeneity are about two orders of magnitude greater than areas of hydrological homogeneity. Transposition of rainfall over long distances can give much more reliable estimates of extreme rainfall than could be obtained by regionalizing flood data (some loss of this advantage is

; ie. the model is a simplification of reality, so dome detail is lost).

3. In flood forecasting, the warning time can be increased by basing the flood forecast on rainfall rather than streamflow (ie. there is a lag between the rainfall and the subsequent flow).

4. Rainfall-based methods provide a convenient way of deriving the complete flood hydrograph as distinct from just the peak discharge of the design flood. This is necessary when modification of the hydrograph by storage occurs (ie. storage attenuates the peak). It is not conveniently done using streamflow data and, as noted in 1 above, the streamflow data may not be available.

2.4 Specification of design storms To define a design storm, the following factors must be specified:

i) duration of storm rainfall; ii) mean intensity of rainfall; iii) temporal pattern of rainfall; iv) areal pattern of rainfall Strictly, all four of these factors are random variables, which are partially correlated with each other. Theoretically, their conditional probability should be determined (ie. we should consider the influence of each variable on the others), but this is computationally very difficult. In practice, the general approach is to:

- ignore correlations between the factors and treating them as independent; - ignore

val Sometimes, the whole concept of design storms is rejected and replaced by a long term simulation of rainfall at a short enough time step to simulate floods. This particularly the case for water quality modelling, where the performance of a wetland (for example) during a wide range of storm events needs to be assessed. The methods of such simulation of rainfall are beyond the scope of these notes and are not well established.




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Usually, a design storm is specified with factors (i) to (iv) above determined as follows:

1. A range of storm durations is assumed, giving a range of design storms, each of which is converted into a design flood. The storm duration that gives the

critical duration

2. The mean rainfall intensity for each of the trial storm durations and the design Average Recurrence Interval (ARI) [or Annual Exceedence Probability (AEP)] is determined from Book 2 of ARR (or by using the AusIFD software.

Note that the ARI [or AEP] value used for the rainfall is the same as that defined for the design flood. Accordingly, all factors other than mean rainfall intensity that affect the size of the flood are assumed to be at their mean values.

3. For Australia, these are given in Book 3 of ARR.

4. pattern based on rainfall records on the catchment of interest is used; this is better than the assumed uniform pattern but is not always possible, and requires special study.

2.5 Rainfall Intensity-Frequency-Duration (IFD) analysis The aim of IFD analysis is to provide the rainfall intensity data for design storms for use in flood estimation. Such analyses are primarily frequency analyses of recorded rainfall data, similar to flood frequency analyses. However, whereas flood discharge at a given station is a function only of frequency, rainfall intensity at a station depends on frequency and storm duration. The intensity is lower for longer storms at constant frequency of exceedance. Flood peaks are instantaneous maxima, whereas the rainfalls analysed are mean intensities over given periods of time.

Typical Analysis Procedure

1. Select a range of durations, e.g. 6 minutes, and 1, 12, and 72 hours. 2. For each duration, determine, from the rainfall record, the annual series of

rainfall intensities. 3. Fit a lognormal distribution to each annual series. 4. For all durations, adopt a consistent set of parameters (means and standard

deviations of the logarithms of the rainfall intensities) to give a consistent set of frequency curves.

The result of this analysis is a set of frequency curves as in Figure 2.1:




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Figure 2.1 - Frequency curves of rainfall for various durations.

General form of IFD relationships

The intensity-frequency curves for each duration are usually transformed into intensity-duration curves for various frequencies expressed as average recurrence intervals (ARIs) as illustrated in Figure 2.2. The curves have a hyperbolic shape when plotted to natural scales (Figure 2.2a), but are usually plotted on log-log scale, which transforms them into approximate straight lines at the higher durations, with a flattening slope at the shorter durations (Figure 2.2b).

Duration, D, hours

(a)

LINEAR

Mean Inte

nsity

,I, m

mh

-1

Average

Recurrence

Interval,

ARI, yrs

100101

Duration, D, hours

(b)

SCALES

Mean Inte

nsity

,I, m

mh

-1

AverageRecurrence

Interval, ARI, yrs

100101

LOGLOGSCALES

Figure 2.2 - Rainfall intensity-duration curves for various frequencies.

Three methods are used to derive rainfall intensities (see page 1 of ARR Book 2):

1. Graphically, using curves (as per Figure 2.2) 2. Algebraically, using equations (fitted to the curves) 3. Computer software (using the equations) such as AusIFD

In industry practice, you will most commonly use a computerised technique such as AusIFD to determine IFD relationships. However, application of a tool such as AusIFD without understanding its basis, is likely to lead to embarrassing (and very expensive!!) errors!

ARI (yrs)

I

mm/h

6 min.

1 hr.

12 hr. 72 hr.




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You are therefore strongly advised to study the graphical and algebraic methods for IFD analysis given in Sections 1.3 and 1.4 of ARR Book 2. In your project, you will use AusIFD to calculate rainfall intensities and durations.

Determining rainfall intensities using AusIFD

AusIFD is a computer program, written by Graham Jenkins (formerly of Griffith University, now at Queensland University of Technology), available at: http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htm.

Figure 2.3 Help-screen (left) and opening screen of AusIFD (right).

An introduction to AusIFD, from the website, is given below:

AUS-IFD is a program which calculates the design average rainfall intensities and temporal patterns for any location in Australia.

The procedure used to calculate the average rainfall intensity at a site is described in Chapter 2 of Australian Rainfall and Runoff, 1987 (AR&R, 1987). The Log Normal rainfall intensities for the 2 year and 50 year ARI, the geographical factors F2 and F50, plus the average regional skewness G at the site must be entered by the user.

AUS-IFD can also access a data base of locations within Australia at which the required design parameters have been estimated from Volume 2 of AR&R, 1987. The data base file is an ASCII based text file, which can be updated by the user either from within AUS-IFD, or using a text editor.

The average rainfall intensity can be calculated for specific duration events with a specific ARI. Rainfall intensities for individual ohours can also be calculated. These values are presented in either a graphical or tabular format and can be printed as a table or graph of ARI versus duration and intensity.

The design temporal patterns at the site are calculated in accordance with the procedure described in Chapter 3 of AR&R 1987. The temporal pattern zone number is determined from the latitude and longitude of the site. In the program, no allowance is made for transition zones at the boundaries of the temporal pattern zones. If the selected site is within the transition region between temporal pattern zones, the user should check page 49 of AR&R 1987 and make the required corrections to the calculated temporal pattern. When calculating the design temporal pattern, the program calculates the average design rainfall intensity using the procedure described in Chapter 2 of AR&R 1987.
http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htmmk:@MSITStore:C:/Program%20Files/AUSIFD/ausifd.chm::/html/aush1cvd.htmmk:@MSITStore:C:/Program%20Files/AUSIFD/ausifd.chm::/html/aush6e3x.htmmk:@MSITStore:C:/Program%20Files/AUSIFD/ausifd.chm::/html/aush2eci.htmmk:@MSITStore:C:/Program%20Files/AUSIFD/ausifd.chm::/html/aush2i40.htm




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A fully advanced storm pattern can also be generated for the selected site. The fully advanced storm pattern has been derived such that it preserves the rainfall intensity versus duration characteristics that are included in the IFD data for the site. The procedure used to derive the fully advanced storm pattern is based on the method described by Jenkins, Goonetilleke and Black, "Estimating Peak Runoff for Risk-Based Assessment in Small Catchments", Australian Journal of Water Resources, (In Press).

Both the calculated temporal patterns and the rainfall intensity-frequency-duration values can be saved to an ASCII based text file, for use in other application programs.

Source: http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htm

Below (Figure 2.4) is an example of output from AusIFD for Melbourne, for standard ARIs (1,2,5,10,20,50, 100 years). Output can be specified as either graphical or tabular.

Figure 2.4 Example of output from AusIFD: IFD curves for Melbourne.

SUGGESTED

WEBSITE

http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htm

Go to the website and download AusIFD for

Melbourne and Darwin. Where would you prefer

to work as a drainage engineer??

2.6 Temporal pattern of storm rainfall

Rain rarely falls at a constant intensity throughout a storm. Thunderstorms usually reach a high peak intensity very early in the storm and then tail off (see Figure 2.5a).
http://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htmhttp://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htmhttp://www.ens.gu.edu.au/eve/research/AusIfd/AusIfdVer2.htm




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General or frontal storms are more variable in their temporal patterns but most often reach a moderate peak intensity just before the mid-point of the storm (see Figure 2.5b).

Time, hours

Mean Inte

nsity

Inte

nsity

5

4

3

2

1

0 21

Time, hours

Mean Inte

nsity

Inte

nsity

5

4

3

2

1

0 246 1812

Figure 2.5 - Typical storm temporal patterns, (a) Thunderstorm, (b) General Storm.

Design temporal patterns

Average temporal patterns for various rainfall durations and various regions of Australia for use in design storm synthesis are given in ARR Book 2 (section 2, page 31 onwards). These patterns were derived of a study of the temporal patterns of storms throughout Australia. Each rain period was divided into a number of time increments. Using all rain periods of a given length at a given location, the average magnitude and average position within the rain period of the most intense time increment were then determined. The same was done for the 2nd, 3rd, . most intense time increments. This gave an average temporal pattern for the given duration and location (the regions are provided in Figure 3.2 of Book II of ARR (1998).

Effect of temporal pattern on flood hydrographs

A hydrograph is a graph of discharge (rate of streamflow) (m3s-1) over time. A heavy storm on a catchment produces a flood hydrograph in the stream. The shape of the hydrograph is affected by the temporal pattern of the storm rainfall. The more non-uniform the rainfall, the higher the peak discharge of the hydrograph, as illustrated in Figure 2.4. Since the peak discharge is usually the critical design parameter in a flood investigation, this effect is important.




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I,Q Storm B - non-uniform

Storm A - uniform

Hydrograph B

Hydrograph A

t

Figure 2.6 - Effect of storm temporal pattern on hydrograph.

AusIFD can be used to determine rainfall patterns for locations throughout Australia (Figure 2.7).

Figure 2.7 - Effect of storm temporal pattern on hydrograph.

2.7 Areal pattern and application of Areal Reduction Factors Rainfall IFD curves derived from standard ARR methods (and AusIFD) are applicable strictly only to a point, although can be reliably used for small areas of up to 4km2 (ARR




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Book 2, section 1.7). Over large areas, it is unrealistic to assume that intensity will be uniform, there will be spatial heterogeneity. So, we define an Areal Reduction Factor (Figure 2.8).

Figure 2.8 Area reduction factors (Source: Erwin Weinmann)

Areal reduction curves, which turn point estimates into estimates appropriate for broader areas, are provided in ARR Book 2 (page 27). The curves work by providing a reduction ratio for a given storm duration and catchment area. For example, from Figure 2.9, it can be seen that a 100km2 catchment, for a 1 hour storm duration, will have a reduction ratio of 0.86 for the derived rainfall intensities. However typically in urban drainage we are dealing with much smaller catchments, and this reduces the need for an areal reduction factor to be applied.

1

Figure 2.9 Areal reduction factors for Australia.

1 Extracted from Australian Rainfall and Runoff - Volume 1, Book II Section 1.




22

Key Concepts and Definitions Design Storm: A particular design storm with a given site, rainfall duration and

recurrence interval. IFD: Statistically derived design rainfall intensities for a given frequency

and duration. Areal Pattern: The distribution of rainfall over the land surface. (Storms have

centres with peak rainfalls). Rainfall based Flood estimation: For ungauged catchments the derivation of flood

estimates using a runoff model and design rainfall information. Temporal Pattern: The variation of rainfall intensity during a storm.

Review Questions 1. The variability in design rainfalls can be demonstrated from the 6 key rainfall

estimates of the capital cities in Australia. Use AusIFD to tabulate these values and comment on the differences and suggest possible reasons for these variations.

2. What are the general implications for drainage system design?

Answers to Review Questions 1. e.g. Canberra (mm/hr) 2i1 = 22.0

2i12 = 4.30 2i72 = 1.14

50i1 = 43.0 50i12 = 8.00

50i72 = 2.25 You do the other cities! Reasons for differences are predominately meteorological e.g. storm type, aspect, air temperature & humidity which determines moisture volume which can be released as rainfall. 2. Drainage systems will either need to have a higher capacity in cities with higher rainfall intensities, or local standards will need to be lower (e.g. minor system designed to convey only the 2 year ARI storm, rather than the 5 year ARI).




23

3.0 Flood Estimation Procedures Objectives 1. To understand the concept of the rainfall-runoff process upon which

the rational method is based. 2. To be able to estimate the time of concentration, rainfall intensity, and

runoff coefficient and apply them in the rational formula, for urban and rural catchments.

3. To know the limitations of the rational method. 4. To understand the principle of hydrograph generation using the time-

area method (translation) 5. To understand the concepts of hydrograph attenuation and translation

and the physical conditions which generate each. 6. To appreciate the effects of storage in attenuating a hydrograph and in

particular the action of retarding basins in urban drainage systems

Reading

Australian Rainfall & Runoff (1998) Book 4 and Book 8 (both available on MUSO under Resources)

3.1 Introduction This topic will allow you to be able to predict flood peaks and magnitudes. This is one of the most commonly required skills in urban drainage. In order to obtain design flood estimates, a representative model of the Rainfall-Runoff process (see Chapter 1) is required. The model can then be employed to convert design rainfall (as derived in Chapter 2) into design flood estimates. Selection of an appropriate model is dependent on data availability, flow processes, time scale, catchment size and other parameters. In this lecture a number of simple models are represented. More complex models are available, although the general procedures to employ them are similar.




24

3.2 The Rational Method for flood estimation The rational formula expresses a relationship between rainfall intensity and catchment area as independent variables and the peak flood discharge resulting from the rainfall as the dependent variable. It has been in use for over 150 years! It is widely used in the design of stormwater drainage systems, farm dam spillways, and small culverts in road and railway embankments.

CAUTION: The rational method (or rational formula) is the most commonly used equation in urban hydrology! It is viewed by many practising

simplification of a complex process. Consequently, you should apply the rational formula where appropriate, but be aware of its limitations and uncertainties.

In industry practice: The rational method is used for peak discharge estimation in many consulting and government organisations. You will be reimperfections!!).

Basis of the Rational Method

The rational method is based on a simplification of the rainfall-runoff process, as follows: Consider a catchment of area, A, bounded by drainage divides (ridges) all around, and draining through a network of channels to a single outlet point on a channel.

Stream

Outlet

Drainagedivide

Catchment

Stream

Outlet

Drainagedivide

Catchment Assume a rainstorm occurs, commencing suddenly at one instant of time and finishing suddenly at another (ie. simplified to make calculations easier!!), the rainfall intensity (I) being uniform over the entire catchment area (ie. spatially uniform) and temporally constant (steady) throughout the duration of the storm (D):




25

Time t

I

0 D

RainfallIntensitymm/h

Duration

At the ground surface of the catchment, spatially uniform and temporally constant infiltration loss is assumed to occur throughout the duration of the storm; the remainder of the rainfall, the part not lost to infiltration, is called rainfall-excess, effective rainfall, or runoff. The ratio of the rate of rainfall-excess to the rainfall intensity is called the runoff coefficient (C). Expressed simply, the runoff coefficient is the ratio of runoff to rainfall (ie C=runoff/rainfall).

At the commencement of the storm, time zero, the runoff all over the catchment commences to "run-off", towards the outlet, flowing always in the direction of steepest slope. Initially, the flow (discharge) at the outlet is derived only from the area closest to the outlet. As time goes on, the (instantaneous) discharge at the outlet is derived from an increasing area as runoff arrives from elements of area further away from the outlet. The runoff is said to be "concentrating" to the outlet.

After a certain time, the first element of runoff to have occurred on the element of area most remote from the outlet has had time to travel to the outlet. This time is called the time of concentration (tc) and represents the time it takes for all of the catchment to be contributing runoff to the outlet. If the rainfall has lasted this long, the instantaneous discharge at the outlet will comprise elements of runoff from all over the catchment, the first drop of rain from the most remote point having just arrived at the same time as the last drop of rain from the point adjacent to the outlet. At this time, since the area contributing to discharge at the outlet can no longer increase, a steady state has been reached with discharge Q (volumetric rate of flow) at the outlet equal to the volumetric rate of generation of runoff on the catchment.

storm. What will be the effect of storm duration (assuming uniform intensity) on the maximum discharge measured at the catchment outlet?

Time t

I

0 D

Rainfall - excess

LossCI




26

Time

Dis

cha

rge (

m3/s

ec)

Resulting

Hydrographs

Storm Duration (relative to Tc)

=Tc >TcTc




27

It is usually assumed that the reduction in contributing area outweighs the increase in rainfall intensity relative to that of a storm of duration equal to tc and the peak discharge from the shorter storm is, therefore, the lesser. Based on that assumption, we can say that the time of concentration is normally the critical storm duration, i.e. the duration producing the highest discharge for a given average recurrence interval (ARI), rainfall intensity (I) and runoff coefficient. This is shown below.

The Rational Formula

To summarize, the rational formula for estimation of the peak discharge to be used in the design of a hydraulic structure is:

Q = 0.00278CIA (or CIA/360) (3.2) where Q = peak discharge, m3/s; C = a dimensionless runoff coefficient; I = mean rainfall intensity mm/h of a storm of the design ARI and duration equal to the time of concentration tc; A = catchment area, ha. The coefficient 0.00278 [= 104/(103.3600)] converts ha.mm/h to m3/s.

3.3 Application of the Rational Method To estimate a flood using the rational formula, the following factors must be determined: A (area), I (rainfall Intensity) and C (runoff Coefficient). Therefore, to determine the peak discharge using the rational method we must:

1. Determine the catchment area 2. Determine the design ARI we wish to use 3. Determine the rainfall intensity, by: a. Calculating the time of concentration of the catchment b. Using that to determine the storm duration c. Calculating intensity for the given duration and required ARI. 4. Determine runoff coefficient (C)

t

I

0 Duration D

ARI

Intensity

c




28

We should already know the catchment area, A. To determine I, we need first to know both: tc, and the design ARI.

Rainfall Intensity (I)

Determining the Time of Concentration (tc)

A number of empirical formulae (ie. based on observed measurement data) have been derived for determining time of concentration. However, it is important to realise that these methods are, by necessity, a simplification of reality. Time of concentration will vary from storm to storm, depending on factors such as season, antecedent soil moisture (ie. time since last storm), and the pattern of rainfall. Rural Catchments An empirical formula relating time of hydrograph (ie. the time taken for flow, under constant rainfall conditions, to reach its maximum) rise to measurable catchment characteristics is used. A commonly used example is the Bransby Williams formula (Eqn. 1.3, p. IV-3, ARR Book Four, 1998).

c 0.1 0.2t

= 58L

A S (3.3) where tc = time of concentration, minutes; L = length of main stream, km; A = catchment area, km2; S = average slope of main stream, m/km. Note L is the dominant independent variable (and so needs to be accurately determined, including any meandering within the stream. Another example, recommended for use in rural Victorian areas, provides an even greater simplification, and avoids the need for determining stream length (See ARR Book IV, section 1.4.3, page IV-9.), is: tc = 0.76A

0.38 (3.4) where tc = time of concentration in hrs; and A = catchment area in km2.

Note that, if L A0.5 as is a reasonable assumption, the L/A0.1 of the Bransby Williams formula (Eq. 3.3) is similar to the A0.38 of the Victorian one (Eq. 3.4).




29

Urban Catchments A common approach for urban catchments is to adopt: Time of concentration (tc) = overland flow time + gutter flow time + pipe flow time for the point most remote, in terms of travel time (not necessarily distance), from the catchment outlet. The three component times are estimated from the measured flow distances and estimated flow velocities. In each case, the velocity is an average value obtained by applying Manning formula (Eq. 3.5) V = (R2/3S1/2)/n (3.5) where R = hydraulic radius (= wetted area / wetted perimeter), S= slope ratio, and n is

.

to verify/modify the initial estimates), a very simplified method may be used. One such method is to assume the catchment is a square shape, and therefore that the longest

flow-path is the hypotenuse of an equilateral triangle (= A, where A = catchment area):

Selecting the ARI

Selection of the appropriate ARI will be based on relevant guidelines. For example, each municipality will have its own guidelines (although many are likely to be similar). This reflects the degree of security required in a design (ie. it relates to the consequence of flooding in that case). e.g. culverts on remote roads 1 years culverts on resource railways - 50 years drains in residential streets and open spaces - 5 years drains in business districts - 20 or 50 years. You should read Section 1.10 of Book Three (Choice of Flood Estimation Methods and Design Standards) of ARR, and section 1.3 of Book Eight (Urban Stormwater) for guidance on selecting standards.




30

In industry practice: If you are working for a council, you will probably be required to design to existing standards. If you are a consultant, you will be required either to (a) design according to the standards specified in the brief, or (b) advise the client on an appropriate standard. To do this, you will need to understand the relevant guidance in

your decision).

Determining the Rainfall Intensity

Once you know the critical storm duration, given by the time of concentration, tc and recurrence interval ARI, you can determine the appropriate rainfall intensity, I, as per the methods described in Chapter 2. In common practice, this would be done using a computer software package such as AusIFD, or HydroTools (HydroExpert Sofware).

Runoff Coefficient (C)

As defined earlier, the runoff coefficient is the ratio of runoff to the rainfall. Or, to put it more completely, it is the rate at which rainfall-excess, or runoff, is generated, compared to the rate of rainfall. However, the runoff coefficient varies between storms, depending on factors such as the storm size, and the antecedent conditions (e.g. soil moisture). For example, the value of the losses in a particular rainfall-runoff event depends very much on the "antecedent wetness" of the catchment (ie. how long since the last rainfall, and thus how moist the soil is). Consequently, the runoff coefficient varies from event to event, depending upon the initial or antecedent wetness. To calculate the Y year ARI flood peak from the Y year ARI rainfall intensity, which is the usual application of the rational formula, it is necessary to use some kind of average value of CQ as the runoff coefficient. This average value is called a statistical or probabilistic runoff coefficient (CY).

Conceptually, the statistical runoff coefficient is derived from the rational formula as follows: QY = 0.00278CYIYA (for catchment area in ha) or ... CY = QY/(0.28IYA) where the subscript Y refers to the average recurrence interval (ARI) in years. There are three approaches to determining the coefficient of runoff:

1. For a gauged catchment, it is determined by preparing a flood frequency curve to give QY values, and using the relevant rainfall intensity-frequency-duration values for IY.

2. For design on an ungauged rural catchment in Victoria, C10 is read from Map (a) or (b) of Fig. 5.3 in Vol. 2 of ARR, and adjusted to the required CY using a factor from Table 5.4 of Vol. 1, ARR 1987. In Melbourne, the C10 varies between 5 and 30, with an average value of 10. Note that CY increases with Y.

3. For urban catchments, CY is estimated as a function of rainfall intensity, and the amount of impervious area in the catchment. This is covered in greater detail in Chapter 4.




31

Limitations and application of the Rational Method

- -runoff processes. Specifically:

1. The assumption of uniform, steady rainfall may underestimate peak discharge, but

2. The ignoring of storage effects within the catchment (ie. ignoring routing behaviuour) over-estimates the peak discharge.

The rational method can be applied with confidence in (a) urban drainage systems or (b) small rural catchments, because these will have relatively uniform rainfall, and little channel storage. It is often assumed that in urban areas, the runoff coefficient of a catchment impervious proportion of the catchment, because the impervious area is assumed to have a runoff coefficient of 1.0. This is typically an over-estimate (and will thus lead to over-design). For example, ARR Book Four recommends that the runoff coefficient from urban should be set at 0.9 (unless there are local data to show otherwise), to

It is important to note also, that the rational formula gives only the peak discharge, not the complete hydrograph (ie. to estimate the total stormflow volume). The Time-Area Method (Section 3.4) is used to produce the complete hydrograph.

3.4 Estimating Flood Hydrographs: the Time-Area Method

SUGGESTED

Viessman & Lewis Introduction to Hydrology, pp. 247-249

Shaw (2002) Hydrology in Practice: Sections 13.2, 18.5.

The rational method allows us to calculate peak discharge. However, we may wish to know not only the peak discharge, but the entire storm hydrograph (we will need this to size retarding basins or stormwater treatment wetlands, for example). The Time-Area Method allows the hydrograph to be determined. The Time-Area Method may be considered as an extension to the Rational Method.

Note: The Time-Area Method may produce a hydrograph with a different peak discharge to that calculated from the rational formula.

The Time-Area Method considers a catchment as a number of discrete sub-areas, and uses this to calculate the contribution of each area to catchment discharge, at each timestep through a storm. Like the rational formula, the time-area method is based upon a kinematic view of the runoff process (i.e. it is assumed that no horizontal dispersion of flood waves nor temporal attenuation of flood peak occurs). However, whereas the rational formula predicts only the peak discharge resulting from a temporally uniform rainfall-excess,




32

the time-area method calculates the complete hydrograph from a temporally varying rainfall-excess. Given the same steady rainfalls, the rational and time-area methods would give identical peak discharges. However, with a temporally varying rainfall of the same average intensity interacting with catchment shape, the time-area method is likely to give a higher peak discharge than the rational method. The kinematic assumption can also be explained in terms of the effects of storage. Catchment storage (overland flow storage, channel storage, and lake storage) has two effects on an inflow hydrograph - (i) Translation (Related ideas: lag, time lag, convection, kinematic wave) (ii) Attenuation (Related ideas: diffusion, storage effect, concentrated storage effect)

The time area-method models the translation effect but neglects the attenuation effect. Alone, therefore, it may be used only for situations with little attenuation effect, e.g. stormwater drainage systems. For more complex situations, software such as RORB is used. The hydrograph calculated by the time-area method may be routed through a significant storage such as a retarding basin (see Chapter 6).

The Time Area Diagram (TAD)

Under the assumed runoff process of translation with no attenuation, the time-area diagram displays the variation over time of the area contributing to flow at the catchment outlet (assuming that rainfall over time is constant and uniform over the entire catchment area). The form and method of construction of the TAD are different for rural and urban catchments.

Inflow

t

Q

Q

t

Outflow

Inflow

Outflow




33

Rural Catchments: Isochrones (lines of equal travel time of water to the catchment outlet) are constructed. To construct the isochrones, various assumptions about travel time are possible, the most common being:

(i) Travel time travel distance (since travel distance is the dominant factor affecting travel time)

(ii) Travel time Distance

slope

Travel time

To attach travel times to the isochrones, we first need to know the Time of Concentration for the total catchment (as discussed earlier in this Chapter). The TAD is a histogram of area between isochrones against Travel Time.

0

Areabetween

Isochrones

1 2 3 4 5

A1

3

2

4

A

AA

5A

Travel Time Urban Catchments: Sub-division of the catchment is on the basis of areas contributing to individual inlets (to the drainage network) rather than areas between isochrones. This is because the

-(ie. the pipe network may flow in a different direction to the natural surface). In urban areas, the effective area (= actual area x runoff coefficient) is usually used instead of actual area. For each sub-area (inlet area), we require:

(i) effective area (= CA) (ii) time of concentration of sub-area

Travel

0

132

5A1

3

2

4

A

A

A5A

4Time to Outlet




34

(iii) travel time from the drain inlet (i.e. the outlet of the sub-area) to the outlet of the entire catchment.

For a set of sub-areas A, B, C, D, & E as shown in the catchment plan below, the contribution over time can be plotted, and this can be used to produce the time area diagram. Note that sub-areas may overlap in their contributions over time (note E and D particularly, but also C, D, E, etc).

A

B

C

D

ET1

T2

A

B

C

D

ET1

T2

A

B

C

D

E

T1

T2

A

B

C

D

E

T1

T2

Urban catchment with five sub-areas (left) and plot of flow contributions over

time Note that the shape of individual sub-areas is not modelled; each sub-area is treated as if it were rectangular.

In industry practice: Software packages such as RORB are used to determine hydrographs from catchments with sub-areas. Whilst such software packages are easy to use, this depends on understanding the underlying principles.

3.5 Calculation of Hydrographs In order to calculate the hydrograph (ie. the flow rate at each time step), we need to multiply the effective rainfall (actual rainfall x runoff coefficient) during each timestep, by the area which will be contributing flow to the outlet at that timestep: Take for example the following catchment:

Travel

0

103020

50A1

3

2

4

A

A

A5A

40Time to Outlet (minutes)

Travel

0

103020

50A1

3

2

4

A

A

A5A

40Time to Outlet (minutes)




35

30 minutes on this catchment. We will use a 10-minute timestep. Now, calculate the flow at each timestep by multiplying the (effective) rainfall (I)l x area (A), for each sub-area (A) contributing flow at the outlet at that time: At time 0 minutes, Q0min = 0 (obviously!) At time 10 minutes, Q10min = I10.A1 ( At time 20 minutes, Q20min = I10.A2 + I20.A1 At time 30 minutes, Q30min = I10.A3 + I20.A2 + I30.A1 At time 40 minutes, Q40min = I10.A4 + I20.A3 + I30.A2 At time 50 minutes, Q50min = I10.A5 + I20.A4 + I30.A3 At time 60 minutes, Q50min = I20.A5 + I30.A4 At time 70 minutes, Q50min = I30.A5 This may seem like a long and convoluted process! Not surprisingly perhaps, this process is called convolution! It is important to try to conceptualise the process try drawing your own diagram of where the runoff is coming from, at each timestep. However, the underlying process is simple: the flow at any timestep

is the sum of the products of effective rainfall and area, contributing to the catchment discharge at that time. The general expression for calculating the hydrograph ordinate at the kth timestep, for a catchment with j isochrones, is:

kj 1

k

j k- j+1j 1

k

j k- j+1Q = A I = I A (3.6)

where: Qk = the flow ordinate at time k, Aj = area of isochrone j, I = effective rainfall. The use of convolution to calculate hydrographs assumes that the catchment acts as a linear system. This implies that the so-called kinematic wave speed is a constant, and is independent of discharge. Isochrones and the time base of the time-area diagram are, therefore, fixed (do not vary with discharge). If more complex situations are involved (where these assumptions do not apply), then a computer program such as RORB would be used.

SUMMARY This topic has given a brief introduction to the use of hydrologic models to generate flood estimates from rainfall data. The use of two simple models producing:

1. a peak flow estimate, 2. a complete hydrograph

have been reviewed. More complex (computer based) models are available (e.g. RORB), and are commonly used in industry practice.




36

KEY CONCEPTS AND DEFINITIONS

Isochrone a hypothetical line representing points with equal travel time to the

catchment outlet. Rational Formula a widely used simple equation relating peak design discharge to

rainfall Runoff coefficient a parameter relating the proportion of runoff from an area to the

runoff generated from an ideal impervious area. Time-Area method a method of deriving hydrographs from rainfall hyetographs. Time of Concentration: travel time from the most remote point of the catchment.




37

REVIEW EXAMPLES

Example 1

A culvert on a rural road is to be designed to pass a flood of ARI = 10 years; the

catchment area is 5 km2. The statistical runoff coefficient C10 is found to be 0.3. A

fragment of the rainfall intensity-duration relation for ARI = 10 years at the site is given

below. Calculate the design discharge by the rational method.

100

80

60

40

20

1 2

Duration, hours

Inte

nsity, m

m/h

1012631.4

-1

Intensity-duration relation for Average Recurrence Interval = 10 years

Example 2

A paved parking area is defined in the above diagram. Runoff occurs as sheet flow and is collected in a drain as shown. Time of concentration to the drain is 12 minutes, time of flow in the drain can be assumed negligible. It is required that the peak outflow discharge for a given design ARI be no greater than would have been the case if the area were in its natural condition. To achieve this, a small detention basin is being constructed at the outlet. For the design of this basin, because the attenuation of the peak discharge by the detention storage must be calculated, hydrographs under natural and proposed conditions must be calculated for assumed design storms.




38

50

50

100

50

Drain

Outflow

Dimensions in m.

This example shows the calculation of the inflow hydrograph by the time-area method for an assumed design storm of 12 minutes duration with rainfall-excess of 4, 8, 3 and 1 mm in the four three minute periods respectively. Use the time-area method to calculate the hydrograph of outflow from the drain. Compare the peak discharge with what would be obtained using the rational method.

ANSWERS TO EXAMPLE QUESTIONS

Solution to Example 1

Use Eqn. 3.2. Q = 0.28 CIA C = 0.3 A = 5 km2 To determine I, first calculate tc using Eqn. 3.4. tc = 0.76 A

0.38 hours = 0.76 x 50.38 = 1.40 h For a storm duration of 1.4 h, from the above intensity-duration diagram, I = 65 mmh-1

Q = 0.28 x 0.3 x 65 x 5 = 27.3 m3s-1 The design discharge is 27 m3s-1




39

Solution to Example 2

Since the time of flow in the drain is negligible, the hydrograph of outflow from the drain is the same as the hydrograph of overland flow into the drain. This is obtained by convolving the hyetograph of rainfall-excess with the time-area diagram.

Hyetograph: Time, min. PE, mm

0

4

3

8

6

3

9

1

12

Time-area Diagram:

50

50

100

25 25 2525

0 36 9

12Travel time,min.

Dimensions in m.

Isochrones

Travel Time, min. Area, m

2

0

2500

3

2500

6

1250

9

1250

12




40

Convolution:

Note that 1 ha x 1 mm/(180s)= 10

4 x 10

-3/180 m

3/s

= 104/180 L/s

= 55.56 L/s

Time,

min.

Area

ha

Discharges, L/s for

PE, mm/(180s)

Discharge

L/s

4 8 3 1

0

3

6

9

12

15

18

21

24

0.25

0.25

.125

.125

0

56

56

28

28

0

111

111

56

56

0

42

42

21

21

0

14

14

7

7

0

56

167

181

139

91

28

7

0

Rational Formula

Q = 0.28 CIA

CI = mean rate of rainfall-excess in mm/h

= (4 + 8 + 3 + 1) x 60/12

= 80 mm/h

A = catchment area in km2

= (100 x 100 - 50 x 50) x 10-6

= 0.0075 km2

Q = 0.28 x 80 x 0.0075 m3s

-1

= 0.28 x 80 x 7.5 Ls-1

= 168 Ls-1

This peak discharge, from a storm of constant intensity, is less than the 181 Ls-1 calculated by the time-area method for a variable intensity storm of the same mean intensity. This occurs because, in the convolution process (see above), the large product of a high hyetograph ordinate and a high time-area diagram ordinate more than makes up for the small product of two low ordinates. If either the time-area diagram or




41

the hyetograph has a constant ordinate, the time-area method and the rational formula will give identical peak discharges, as is illustrated in Exercises 1 and 2 below.

REVIEW QUESTIONS Question 1

Repeat the above Worked Example for a parallelogram shaped area 100 m x 100 m, i.e.

like the one used in the Worked Example but without the 50 m x 50 m corner cut out.

Question 2 Repeat the Worked Example for the original area, but with a constant intensity rainfall

excess equal to the mean intensity of the original storm.

Question 3

A plane triangular paved area drains to its

vertex as shown in the diagram. Its time of

concentration is 20 minutes.

Use the time-area method to calculate the

hydrographs that would be produced by two

storms, each of 20 minutes duration, but one

increasing in intensity, the other decreasing,

producing rainfall-excess as follows

Storm Rainfall-excess, mm, in 5 minute

increments

____________________________________

A 5 10 15 20

B 20 15 10 5

This can be done by hand, or by creating an Excel spreadsheet that represents the time-area

diagram and the convolution of the hydrograph (Study the previous worked example

before completing this exercise.)

80m

200m

Outlet




42

ANSWERS TO REVIEW QUESTIONS

Question 1

The time-area method and the rational method both give a peak discharge of 224 Ls-1

.

(This is because the time-area diagram for this case has a constant ordinate, i.e. the area

contributing to flow at the outlet increases linearly with time).

Question 2

The mean intensity of rainfall-excess is 4 mm per 180s, and the time-area method and the

rational method both give a peak discharge of 168Ls-1

.

Question 3

Time, Hydrograph, L/s

min. Storm A Storm B

0

5

10

15

20

25

30

35

40

0

8

42

117

250

342

342

233

0

0

33

125

258

417

283

158

58

0




43

4.0 Minor Urban Drainage System Design Objectives 1. To be aware of the general approaches to urban drainage and flood

mitigation 2.

a. To be able to compute design discharges for inlets and pipes (or alternative techniques such as swales) in an urban drainage system, using the rational method.

b. To be able to determine required pipe diameters, allowing for pit losses.

3. impact on design flows.

Reading

Australian Rainfall & Runoff (1998) Book 8

4.1 Introduction As discussed in Chapter 1, urbanization has a major impact on hydrology of a catchment. Increases in impervious areas, and the clearing of vegetation create a

onding rapidly to rainfall. In order to cope with this change, constructed drainage systems are designed to safely convey flows up to the required standard (see details in Chapter 2). The constructed drainage system is commonly made up of gutters, pipes and gully pits (often just called

approaches such as vegetated swales (more detail will be given in Chapter 8). In industry practice (particularly in consulting), a common requirement is to design the pipe and pit network (ie. the minor drainage network) for a new urban development. This lecture aims to give you the skills to undertake this design. In a subsequent lecture (see Chapter 5), design of the major drainage system will be discussed in detail.

4.2 Effects of urbanization on hydrology: the need for drainage and flood mitigation

A study in Canberra (Codner et al., 1988) demonstrated the magnitude of changes to hydrology by urbanization. The Giralang catchment, a typical medium density urban catchment, had peak flows of around six times that of a similar, but undeveloped catchment (Figure 4.1).




44

Giralang, 93 ha, Urban

Gungahlin, 112 ha, Rural

10

8

6

4

2

1400

0

1500 1600 1700

5th February, 1981

Dis

charg

e, m

/s

3

Figure 4.1 Hydrographs from rural and urban catchments, from the Giralang study.

The effects of urbanization on hydrology can be summarised as:

- impervious areas)

- Peak discharges increase - The time of concentration decreases, as flow travels more quickly through

gutters, pipes, pits, etc - Pollution increases

As urban drainage engineers, our task is to design a drainage system capable of conveying flows under this changed hydrologic regime. In Chapter 8, we will also examine how to do this whilst minimising environmental impacts.

4.3 Approaches to urban drainage and flood mitigation There are a number of approaches to managing urban drainage and mitigating floods:

1. Planning and land use controls. 2. Conveyance systems (pipe and channel or Water-Sensitive Urban Design) 3. Retarding basins (for dealing with major storms)

A combination of these is typically required.

Planning and land use controls

prone areas, and to minimise impervious areas such that disturbance to hydrology is minimised! Planning policies and controls are usually development by planners (either in State or Local Government), but Engineers have a critical role in advising the planners of appropriate policy. Good planning policies will avoid very expensive capital works later!




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Pipe and channel systems or Water Sensitive Urban Design systems

through the natural drainage paths, by use of gutters, pipes and constructed channels. It removes water from the ground surface quickly. Whilst this approach is often necessary, there are a number of problems with it:

1. Concentration of flows downstream, such that an even bigger flooding problem is caused (and major works such as retarding basins are needed to overcome the problem)

2. Degradation of receiving waters (e.g. creeks, etc) due to erosion and water pollution.

water sensitive urban design (WSUD) is increasingly being used,

using systems such as swales, biofiltration systems, wetlands, and ponds, with the aim -

or at least cleaned before being discharged to receiving waters. These can actually help to reduce peak flows, and thus reduce flooding problems.

alternatives in the assignment, and in Chapter 8.

Retarding basins

Retarding basins are small dams constructed on drainage lines within the urban catchment. They work by temporarily detaining water, and releasing it at a controlled

e peak flow. As well as their construction cost, retarding basins have a very high land cost (particularly in urban areas where land is expensive), but often a retarding basin has multiple benefits: it may also contain a water quality wetland, along with recreational facilities such as walking paths, etc (Figure 4.2).

Figure 4.2. Example of multiple-use retarding basin with flood management, water

quality and recreational benefits.




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4.4 Design of urban drainage systems: minor In Chapter 2, we learnt how to estimate design rainfall, and its use in drainage design. Then, in Chapter 3, we used the Rational Method and the Time-Area Method to determine the peak discharge (and hydrograph) of a design. We will now use this knowledge to design the urban drainage system made up of

vegetated swale.

SUGGESTED

Australian Rainfall and Runoff (1998 Book 8 Sect. 1.5.)

Overview of the minor system design process

In the following sections of this chapter, we will step through the tasks necessary to design the minor drainage system. The steps can be summarised as:

1.0 Layout of the system 2.0 Pit design

- Determine design discharge for pit inlets - Selecting pit type and size

3.0 Pipe design: determining the pipe size o Calculation of head losses within the (a) pit and (b) pipe

We will now consider the detail of each of these steps.

Layout of the drainage network

When designing the drainage network, we need to consider the minor/major drainage concept (Chapter 1):

1. Minor drains generally follow the street pattern 2. Major drains follow natural drainage lines

Typically, the minor drainage system will be designed to convey up to a 2 or 5 year ARI storm, with floods above this, up to the 100 year ARI, to be conveyed within streets and/or drainage reserves. We start the drainage design by examining a contour map of the proposed layout (Figure 4.3)




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Figure 4.3. Example of proposed development layout.

We can then make a tentative layout of the pipes, drains, pits and swales. Assuming that the road network has already been planned (ie. we know where the roads are located, their lengths, etc), our first job is to work out where the pits should be located. Each pit has an entry, which may be an inlet in the kerb (often called a Side Entry Pit Trap, or SEPT), or a grating (more common in Brisbane, for example). The pit will also have an outlet pipe, and may have pipes discharging into it (e.g. direct from a house roof, or from a nearby carpark). An example of a pit is shown in Figure 4.4).

Take a Look! locations. How widely spaced are the pit? Is the spacing uniform? Do the pits use a grated entry, or a side-entry?




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Figure 4.4 Typical standard design of Side Entry Pit (Source: Dandenong City Council).

When choosing where to locate pits, our aim is to maintain the design gutter flow such that:

1. all or most of the discharge can be trapped by the pit inlet 2. the gutter flow at the design discharge is not excessively wide (so that roads

and/or footpaths are obstructed).

too many pits, because that will increase the cost! In order to locate the pits, we thus need to calculate the design discharge for each pit.

Calculating design discharges for drains and inlets, etc

Each inlet is fed by a gutter and therefore has its own catchment area, called a sub-area. We must determine the discharge capacity for:

1. Each pit inlet 2. The pipe drain immediately downstream of each inlet.

We do this by use of the Rational Formula (see Chapter 3). It is important to note that drain design discharges are NOT determined by summing the design discharges for all upstream inlets! This is because these discharges have been calculated for shorter times of concentration (and consequently, more intense rainfalls). In a typical drainage design, there may be many inlets, and so there are many calculations to be performed. Setting up a logically structured spreadsheet is critical to undertaking this task!




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Design calculations are performed pit by pit, starting at the upstream-most point (ie. a pit with no upstream pipe drain), and proceeding downstream (Figure 4.5). When a junction is reached, before the calculations are done for that pit, all upstream drainage lines must be designed. This applies at all junctions. When this requirement has been satisfied, the junction pit can be designed and the downstream progress of the design calculations resumed. At each pit, the inlet is designed first and then the outlet pipe. Where backwater effects at the final outlet may control water levels, it may be necessary to carry out a redesign working backwards from that downstream level. This is to ensure that upstream pits are not surcharging.

1

Direc

tion o

f flow

2

3

4

5

6

7 1

Direc

tion o

f flow

2

3

4

5

6

7

Figure 4.5. Sequence of pit inlet capacity calculations: Start upstream, working downstream,

but ensuring at each junction that all upstream contributions have been calculated.

Selecting the location of pits Determining a suitable location for pits depends on matching the flow for a given area to the inlet capacity of the pit. This process is outlined in the following section. However, you may also wish to refer to Technical Note 2 on p11 of ARR Book 8 (available on MUSO under Resources); this technical note provides an easy method for locating pits.

Pit Inlet design

Determine design discharge for pit inlet Step 1 Determine runoff coefficient (C) of the sub-area, based on design ARI and location. It is determined as an area-weighted average of the runoff coefficients for the pervious and impervious:




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C10=0.9f + Cperv10.(1-f) (4.1) Where C10 is the 10 year ARI runoff coefficient, f= fraction imperviousness of

sub-area, and Cperv10 is the 10 year ARI pervious area runoff coefficient. The adjustment of the impervious area C by 0.9 is to account for the fact that

-like a pervious area. The Cperv10 (usually designated as C

110) can be derived from map

stormwater guide(1)

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