Australia; Source Control and Distributed Storage – A Cost Effective Approach to Urban Drainage for the New Millennium

Australia; Source Control and Distributed Storage – A Cost Effective Approach to Urban Drainage for the New Millennium

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    8 th International Conference on Urban Storm Drainage, Sydney, Australia, 30 August – 3 September 1999, pp1997-2005   SOURCE CONTROL AND DISTRIBUTED STORAGE – A COSTEFFECTIVE APPROACH TO URBAN DRAINAGE FOR THE NEWMILLENNIUM?  R. Y. G. Andoh* and C. Declerck**  *  Hydro International plc, Shearwater House, Victoria Road, Clevedon, N. Somerset, BS21 7RD, UK  **  La Missonnais, 35560 Noyal-sous-Bazouges, France ABSTRACT The paper describes the results of a study investigating the cost and operational benefits of source control anddistributed storage. Hypothetical catchments based on the aggregation of (20Ha) sub-modules derived fromanalysis of combined sewer networks in the UK were used as the basis for assessment. Source Control,distributed storage and conventional techniques of sewerage rehabilitation were assessed in a staged approachregarding levels of service provision. The study also reviewed impacts resulting from a general increase incatchment impermeability from 30 to 40% for flat and steeply sloping catchments.The results show a general trend of decreasing costs for alleviating flooding, the further upstream therehabilitation measure is effected within the drainage system. Cost savings in the region of 25 to 80% comparedto conventional solutions, are realised depending on the type of rehabilitation scheme adopted. The paper advocates a shift in focus from end-of-pipe solutions to upstream controls as an effective alternative urbandrainage strategy, especially to resolve the challenges of urban drainage infrastructure provision for the newmillennium. KEYWORDSSource control; distributed storage; stormwater management; developing countriesINTRODUCTIONA review of urban drainage practice shows that the traditional (conventional) approach has evolved from the philosophy of conveying municipal wastewater and storm run-off away from the urban areas as quickly as possible. As a result, sewers are designed on the basis of providing hydraulic capacity to convey the peak flowsfor a given return period storm associated with the desired level of service provision (e.g. no surcharge for the 1in 2yr ‘critical storm’). The ‘critical storm’ here refers to the duration producing the maximum runoff rate – typically the time of concentration for the catchment.Conventional piped systems have developed over many centuries in response to a changing problem. Originallyopen channels were used to transport rainfall runoff (water) away as quickly as possible and prevent localflooding. However, over time, these open channels became contaminated and caused objectionable odour  problems. This led to the use of closed sewers (combined systems) and as more and more paved areas were built, these sewers in turn became overloaded. Overflows (acting as pressure relief valves) were then providedto prevent flooding from the overloaded sewers.In time, these overflows have caused pollution of local receiving watercourses resulting in the current focus onimprovements to combined sewer overflow (CSO) structures. The urban drainage infrastructure resulting from  the necessary improvements associated with increasing urbanisation and industrialisation, coupled withdevelopments in flood plains, has resulted in downstream flooding and heavy pollution of receiving waters.The current focus and emphasis on resolving problems associated with CSOs is a natural progression in the“traditional” water control policy which has been to cure problems when they arise (i.e. a reactive-curative)rather than the more holistic approach (proactive-preventative) in tune with nature’s way. The traditionalcurative approach has tended towards “end-of-pipe” solutions for resolving problems associated with our urbandrainage systems. These include the provision of costly large interceptor / relief sewers, massive storage tanks(or basins) in downstream locations to alleviate flooding, and centralised wastewater treatment facilities toreduce the pollution of receiving waters.Flooding from combined sewers in most urban centres is caused by increase in runoff rates and volumesresulting from expansion and growth beyond the core area. The search for conventional solutions of larger relief sewers or detention basins in the areas where the problems are manifest (i.e. the urban centre), are fraughtwith problems of lack of space and congestion of services and inevitably leads to very costly schemes.It has been estimated that by the year 2,000 half of the world population (of over six billion) will be living inurban areas and of the estimated 90 million people currently added to the global population, each year, 94% arein developing countries. A large proportion of the urban population in these countries has no access to sewagedisposal systems and urban drainage infrastructure. Most of the existing collecting systems discharge directly tothe receiving waters without any treatment. The problem of urbanisation is growing worse on a global scalewith the rural poor in the developing countries moving into urban areas. As a result, these urban centres (thefastest growing areas of these countries) will suffer unprecedented demands on the already inadequate urbandrainage infrastructure and will be subjected to immense growing strains, both in its physical and socio-economic dimensions.Sadly, the resolution of problems associated with urban drainage infrastructure provision in the developingcountries currently most invariably follows along the traditions of the developed countries even though theseschemes are known to be inappropriate and often unaffordable (Sonuga, 1993). The need for more cost-effective, affordable and sustainable urban drainage in the new millennium is clear. This poses severechallenges for all.SOURCE CONTROL AND DISTRIBUTED STORAGESource control and distributed storage present an alternative preventative approach to urban drainage, more intune with “Nature’s Way” and sustainable development principles. The concepts of source control anddistributed storage are described in detail elsewhere (Smisson, 1980). Case studies describing the application of these alternative approaches to real catchments demonstrating the scope for cost savings of the order of 50%over traditional interceptor schemes are described by Andoh and Lamb (1996) and Barber et. al. (1994).In this paper, the term Source Control is used to describe mitigating measures implemented when the rain firstimpacts upon the catchment surface, typically upstream of piped drainage and sewerage networks. Theseinclude storage on flat roofs, in water butts or below ground, in temporary reservoirs formed by impermeablemembranes beneath permeable surfaces. Storm flows may also be routed back to the natural environment byenlisting the aid of natural processes such as infiltration and percolation. These techniques have been calledBest Management Practices (BMP’s) or compensating techniques (Urbonas and Stahre, 1993).Distributed storage on the other hand refers to storage facilities implemented in the upstream parts of stormwater or wastewater collection and conveyance systems where each storage unit is relatively small and islocated closer to inflow sources. The target of this approach is to provide a preventive solution to flooding andassociated pollution through distributed attenuation and treatment facilities in the upstream parts of the  catchment. Thus by controlling the flows at their source so that they match the downstream system capacity, best use is made of the existing drainage infrastructure.These solutions aim at counterbalancing and compensating the adverse effects of the increase in impermeability(resulting from the urbanisation process). This is effected through better control of flows in the upper parts of the catchments, close to their inflow sources. The application of upstream controls to mitigate downstream problems in urban drainage systems is illustrated simply by Figures 1 and 2. TOTREATMENTCSOTO RIVER  Pipe capacityof 3 unitsCombined flows of 6 unitsis greater than downstream pipe capacityof 4 units leading to flooding or CSO Spill Hydrograph Schematicof a CSO Spill or Downstream Flooding Pipe capacityof 4 unitsSpill or FloodVolume with a peak of 2 units 33 462   Figure 1   TOTREATMENTCSOTO RIVER  Hydrograph Schematicshowing effects of Flow Control and Attenuation.No CSO Spill or Flooding Pipe capacityof 3 unitsupstreamPipe capacityof 3 unitsupstreamDownstream flowscontrolled to 2 units by flow control A T  T  E  N  U  A T  I  O  N    A  T  T  E   N  U A  T  I  O   N Combined flow doesnot exceed 4 unitsFlow within capacity of pipetherefore no surcharge henceno CSO Spill or flooding 2332440   Figure 2    Figure 1 shows the combined flows from two sub-catchments on a network exceeding the capacity of adownstream sewer resulting in flooding or a CSO spill. The conventional solution to this problem wouldtypically involve providing storage at the downstream location where the flood or spill occurred or transferringthe problem further downstream by upgrading the capacity of the downstream sewer to cope with the combinedflows from upstream.In Figure 2, the combined flows from upstream are prevented from overflowing or flooding a downstreamlocation because the rate of release of water from upstream parts of the catchment is limited, by the use of flowcontrol devices, to the capacity of the downstream sewer. This results in a distributed storage system withinherent system storage in upstream sewers mobilised. Where the mobilised system storage volumes areinadequate, supplementary storage in the form of on or off-line storage tanks or over-sized sewers may be provided as appropriate . Source control and distributed storage systems generally result in a multiplicity of control elements andstructures within the catchment, the further upstream the control measure is implemented. Fears and worriesabout maintenance and issues relating to responsibilities has been one of the mitigating factors preventing thewide-spread adoption of these approaches in the UK (McKissock, et. al., 1999). In general however, adistributed system with passive robust control elements is inherently more reliable and less susceptible to failure than a centralised system . Failure of one component of a distributed system may not necessarily becritical whereas failure of a centralised system can lead to catastrophic effects.The key to the effective implementation of distributed storage systems within the context of urban drainagesystems lies in the availability of passive robust flow control devices such as vortex flow controls (Andoh,1994). These devices require no external power source to operate but rather harness the inherent energy withinthe flow to effect the required control. In addition, they have virtually no maintenance requirements and are lesssusceptible to blockage compared with other flow control devices such as orifice plates. METHODS   Catchment Characteristics and Modelling Tools The catchment models used for the analysis were based on the aggregation of sub-module catchments withcharacteristics derived from an assessment of information from a number of Drainage Area Studies conductedfor Water Service Companies in the UK. This resulted in the derivation of a 20Ha sub-module with thefollowing features: ã Total length of sewers: 11 000m ã Total number of manholes: 210 (one manhole every 50m on average) ã Pipes roughness: 1.5mm ã Slope: 1% for flat catchments and 4% for steep catchments. ã Average depths: from 2 to 4 metres. ã Pipe diameter: from 225mm to 900mmThe catchments were assumed to be evenly developed with an impermeability of 30% (medium densityresidential area – 19% for roofs and 11% for the paved surfaces). Further catchment development via theurbanisation process was represented by an increase in impermeability to 40% derived from (19% roofs, 21% paved).The Hydroworks simulation package was used as the network-modelling tool in this study. For each of theidentified rehabilitation options, a range of storm durations were simulated for each of the desired levels of service (return periods). This was to ensure that the design storms investigated included that for maximum rate
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