CWN-EN-Stormwater-Report-FINAL

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INNOVATIVE STORMWATER MANAGEMENT:TRANSLATING SCIENCE INTO ACTIONHANS SCHREIER, FACULTY OF LAND & FOOD SYSTEMS, UNIVERSITY OF BRITISH COLUMBIABACKGROUND:More than 3 billion people will move into cities over thenext 30 years and to accommodate them all will requireincreased densification that dramatically changes theurban landscape. This built up landscape is dominated byimpervious surfaces which no longer allow the rainfall toinfiltrate into the soil and will instead result in increasedoverland flow that contributes to more widespread flooding.At the same time climatic variability is increasing and thecombination of urban densification and climate changewill result in increasing runoff events that will result inincreased flood risk. When we change land use we changethe way the water is distributed in the hydrological cycle.As shown in Figure 1, a portion of the precipitation caneither be evapotranspired, converted into surface runoff,stored in the soils or percolated into groundwater. When weconvert a forested watershed into a paved urban landscapea much larger portion of the rainfall becomes surfacerunoff, which in traditional management is then conveyeddirectly into urban streams though an elaborate system ofstormwater pipes. It is becoming increasingly apparent thatconventional stormwater drainage systems are ill preparedto deal with increasing rain events and a drastically changedland surface.% of Total PrecipitationLand Use Effects on How Rainfall Moves through FourComponents of the Hydrological Cycle1009080706050403020100ForestPastureLawnsRural Res.LegendEvaporationSurface RunoffSub-UrbanMulti-FamilyCommercialSoil InterflowGroundwaterImperviousFigure 1. Land use impacts on the distribution of precipitationwithin the hydrological cycle.Most municipalities base their stormwater design on the100 or 200 year storm return period that uses the maximumhistoric rainfall event as a basis and then develops thereturn period graph that is known as the rainfall intensityduration-frequencycurve (IDF). If long term streamflow datais available this can be converted into storm runoff returngraph, which shows to probability of a storm flow of a certainmagnitude to reoccur over a different time period (Figure 2).However, such graphs are no longer valid because the landuse and the climate have changed over recent times and nolonger represent the historic conditions. This means that aStorm Peak Flow in m 3 /Sec25020015010050Current and Future Storm Return Period Chart012Based onEmergingConditionsStorm Return Periods in YearsFigure 2. Historic and emerging stormflow return period dueto increased climatic variability and increased impervioussurfaces. The historic line formed the basis for designingstormwater and flood protection infrastructure (In this examplea 100 year historic storm would now likely occur every 7years – based on blue line).previously expected one in one hundred rainfall or streamdischarge event is now occurring at a much more frequentrate. With these new environmental conditions, we anticipatethat much larger rainfall events will occur in the near futureand the frequency of flooding events over the past few yearsalready show and increasing trend. The questions that needsto be asked is how can we deal with this new reality andhow do we change the traditional stormwater managementsystem to cope with more frequent and higher flood events?TRADITIONAL STORMWATER MANAGEMENTThe traditional approach in stormwater managementhas been to convey the surface runoff directly into urbanstreams, without any consideration of contaminants thataccumulate on urban surfaces from land use activities,transportation and spills. This management scheme hasbeen very successful in removing water from roads andparking lots but has resulted in a stream flow regimethat became flashier, increased stream bank erosion andgenerated increased sediment transport. To cope with thesenew problems many urban streams were channelized usingprotective walls and protective structures, which pushed thewater through the stream more efficiently. As a result morewater ended up in the lowland portion of the watershed,which then required floodplain protective structures andland use regulations in the flood zone. As land becamescarce in many cities the pressure on development withinthe flood zone increased, which then required improvedflood proofing of the new infrastructure.If we hope to create greener and more environmentallyfriendly cities we now need to address not only the increasedflood risk but also the neglected urban pollution problemwhich resulted in deteriorating conditions in most urbanstreams.510Based onHistoric Data2050100200Canadian Water Network 1
INNOVATIVE STORMWATER MANAGEMENT: TRANSLATING SCIENCE INTO ACTIONSHans Schreier, Faculty of Land & Food Systems, University of British ColumbiaTHE NEED FOR INNOVATIONSAs part of the Canadian Water Network (CWN) researchinitiative a number of workshops were held in Vancouver,Calgary and Toronto to discuss the current problems inurban stormwater management, and to learn from casestudies how the increasing flood risks can be reduced, andhow best to address the non-point sources of pollution.Based on this cross-city experience it became evident that anumber of new initiatives are under way to radically changethe way we manage urban stormwater. Three key issuesemerged from these workshops:1. How effective are many of the new innovations andis there enough scientific evidence to show that theinnovative approaches are more effective than thetraditional measures;2. How can these innovations be incorporated intomainstream urban development to reduce the risk offlooding and contamination; and3. What are the most appropriate combinations ofinnovative approaches that work at all scales of watershedmanagement and different environmental settings?A wide range of innovative measures were identified andexamined and it became clear that not all of the proposedinnovative solutions are appropriate in all cities becauseeach urban watershed differs in geology, surface conditions,climate and land use. To document the effectiveness ofsome of the proposed innovative measures a numberof research projects were initiated with CWN support toaddress the above mentioned issues. We are now confidentthat many of the proposed changes are effective not onlyfrom the environmental point of view but also in terms ofcosts, particularly if they are introduced into new expandingurban areas. A major effort is now needed to promote theseinnovations so that they can become the main adaptivemeasures taken to cope with the increased risk of floodingand stream contamination.THE NEW STORMWATER MANAGEMENTPARADIGMWhen people move into cities they assume that the cityengineers will provide them with all the key services relatedto water management including safe drinking water, sewagetreatment, stormwater removal and flood protection. Giventhese new issues of densification and climate change it isnow apparent that the sole reliance on the engineeringstructural approach is no longer sufficient and the propertyowner will now have to be part of the solution in solving theemerging stormwater problems.The fundamental changes that are required in orderto cope with the new rainfall/runoff regime are to shiftfrom draining and removing water from sites, to storing,detaining and infiltrating rainwater and by enhancing thenatural processes that provide these functions. At the sametime these innovative approaches need to address both thehydrological processes as well as the impacts from nonpointsources of pollution. Enhancing soils, wetlands andplant cover in the watershed and within riparian bufferzones is essential in order to provide sufficient capacity toabsorb, convert and immobilize many of the contaminantsfrom different land use activities.As the water moves from the property to the neighborhoodand into the watershed, the processes change and so do thecontaminants. At the property scale it has been the normto drain all water away from the housing structure and intostormwater pipes. Pollutants from gardening, spills andhousehold activities are all removed through storm drains. Atthe neighborhood scale we have to deal with roads, parkinglots and commercial facilities and many of the transportrelated activities create a wide range of new contaminantssuch as hydrocarbons, oils, grease metals and industrialchemicals. Again these contaminants are directly releasedinto the storm sewer system. At the end of the pipe all ofthe runoff and contaminants end up in the urban streamand we hope that nature will provide sufficient remediationand dilution so as not to harm the ecosystem. To reducethe hydrological and pollution impact on urban streamsrequires a new approach, shown in Table 1, and this differssignificantly from what has traditionally been done.Table 1. Differences between the traditional approach versusthe new innovative approach at the three spatial scales.TRADITIONALAPPROACHINNOVATIVEAPPROACHPROPERTY SCALE Drain and remove rain & runoff Retain rain on site, slow releaseof water though infiltrationsystemsNEIGHBORHOODSCALEWATERSHED SCALEDrain and remove rain & runoffStore water in ponds, developprotective structures (dams &dykes), channelize streamsLand use restriction in floodplainStore and delay runoff usingdetention and filter systemsDelay runoff in wide bufferzone and naturalized streamchannels, improve flood storagewithin the watershed.INNOVATIVE ACTIONS AT DIFFERENT SPATIALSCALESAs shown in Table 2, there are a wide range of innovativeapproaches that need to be adapted in order to reduce theflooding and pollution risks. No single action will be ableto address all issues but a combination of actions at thedifferent spatial scales will be the best recipe for success.Not all of the listed actions in Table 2 will succeed in allareas due to the differences in climatic and site conditions.However, there is now sufficient evidence that using acombination of options can go a long way in reducing therisk of flooding and the amount of contaminants reachingthe urban streams.Canadian Water Network 2
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CWN-EN-Stormwater-Report-FINAL

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