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Stormwater Management

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Sustainable ArchitectureProf. Avlokita AgrawalDepartment of Architecture and PlanningIndian Institute of Technology, RoorkeeLecture – 24Sustainable Sites - IVWelcome back to this new lecture for the online ongoing course on SustainableArchitecture, and we have been discussing about the Sustainable Sites and how to designand develop them. So, this is the 4th lecture in the series where we have been discussingabout the sustainable sites.In the previous one, we have talked about the selection of sites, the development, thenwe also talked about the design of these sites to mitigate the urban heat island impact.And in today’s lecture we will be talking about the management of stormwater andreducing the runoff from the developed sites.(Refer Slide Time: 01:09)So, to start with, what is stormwater? Stormwater is the water that actually originatesduring precipitation. So, whenever there is rain or there is snowfall or the ice melts sothat is when these storm water comes in. And we have been seeing around us that moreand more cities are actually getting flooded during the monsoons. So, unfortunatelywhen rains used to be the time which everybody would enjoy and welcome, our citiesand the people in cities are actually not enjoying rains. Simply because the rains and the stormwater which is a resultant of these rains that is notwell managed. It is not a problem, but it is a problem when it is not managed well. Andthat is more pronounced in our cities because we have more and more of hard surfaces.Let us see what are the components when we talking about these stormwatermanagement and what all are the parameters which go into consideration.(Refer Slide Time: 02:17)But before that if we properly manage our stormwater, it brings about a lot of benefits tous. So, we have proper drainage of surface runoff. Whatever runoff actually happens wecan see a lot of draft, but we can see that there is not much of accumulation ofstormwater anywhere else because these it has been well managed. So, there is amplevolume, sufficient volume of the drain to take care of the stormwater.So, there is proper drainage, it does not create problems for the day-to-day life in cities,urban areas largely. Then this collected water can actually be used for groundwaterrecharge or urban landscape or farming for other purposes. So, it can be further reused.While in other cases it will actually go unused.Then we can treat the water in very early stages. Now, here if it is not properly managed,it may actually the stormwater which is good water can actually merge with the greywater or black water in the drains and that cannot be treated or if it has to be treated thenit will come at a very high cost, at a price which is much more than if we intercept if wecollect the stormwater early and then treat it. The next benefit is it avoids damage of infrastructure. For example, the streets, thebuildings, private properties or public properties. So, if there is storm water which iswhat we see that the low lying areas of the cities actually get flooded because the stormwater was not managed and the public property actually gets damaged which is what wecan avoid. And that is a substantial cost, it adds a lot to the economic cost.And this stormwater if properly managed can be integrated into urban landscape and itcan help provide for the green and recreational areas. So, some of the sustainable cities, Iam not talking about just a neighborhood level or a small building site level, but at a citylevel people have created green parks in such a manner that those parks serve as pondsduring the monsoons during the rains and by the time it is summers all that water haspercolated down and it allows green areas for people to use during the summers andsprings. So, stormwater management actually provides us a lot of benefits.(Refer Slide Time: 04:45)Now, when we are talking about stormwater runoff, we are talking about essentially thewater which is generated which is there because of precipitation, rains and that actuallyflows into the drains. Now, these drains have to be separate drains for sewer systems andfor stormwater, but unfortunately in cities both these drains are often combined. So, thesame drains would carry the storm waters as well as the grey water or the black water.And then all these drains are opening into the water bodies, hopefully after infiltrations,after treatments. So, we are talking about the stormwater runoff as the precipitation that leaves the project site boundaries and it is getting collected through the drains and in aproperly organized manner that is what the stormwater runoff is.(Refer Slide Time: 05:51)Now, when we are talking about the stormwater runoff, we are talking about theplanning and design of the site, we are talking about the kind of materials which aregoing to be used. So, largely two components where we talking about the design andplanning aspect of it and we are talking about the right selection of materials.(Refer Slide Time: 06:17) When we talking about the materials, we are concerned about the property of thesematerials which is called as runoff coefficient. Now, this table gives the typical runoffcoefficients for these different types of surfaces, different types of surface type options.Now, the higher runoff coefficient implies that most of the water which will be availablewhich will be there during the precipitation through the precipitation will be running offand there will be very little which will be either infiltrated to the ground or evaporated orretained or intercepted.Now, here when we are looking at only the surface property, we are largely concernedabout how infiltration and evaporation happens retention and interception is by virtue ofthe design. So, how the material is used on the site, how, what are the different levels,how the flow is going to be intercepted, whether there is a pond, whether there is a waterbody for attention or not. So, these two are largely taken care of by virtue of design andhere we are talking about the different surface types.Now, here if you see we are also talking about the slopes. So, we are talking about theretention capacity. The steep the slope is higher is the runoff coefficient. So, if we have aturf which is steep, so we can see that it increases from a flat turf to a steep turf and thererunoff increases which is commonly seen and understood phenomena as well. But weknow that around 45 of the water the precipitation will we will actually runoff.If you look at the water bodies which are unlined for example, water ponds which arenaturally present because of the topography and being there we see that the runoffcoefficient is 0. So, there is no water which will actually be running off from this waterbody. So, a lot of infiltration will take place infiltration to the ground, there will be someevaporation and it is retaining most of it and it is also intercepting it before it flows out.So, there is a runoff coefficient of 0 which is the ideal case ideal scenario. (Refer Slide Time: 08:55)So, when we are designing a site, we are looking at the pre and post developmentscenarios. What we have to try doing is we have to have the same amount of runoff evenafter the development as it was before the development and construction had taken place.So, if this was the undeveloped site and there was a lot of vegetation, so there were therewas water which was going towards evapotranspiration and then there was surfacerunoff, there was interflow which was right beneath the surface and then there was baseflow where the water was actually percolating to the ground. However, if we look at theafter construction scenario we see that the volume of surface runoff has substantiallyincreased because the vegetation is not there as much.So, the surface type allows more water to be runoff and there is less which is there in thebase flow and largely inter flow. So, this is not a good scenario. More surface runoffimplies more water which can actually be percolated to the ground and can be used isbeing run off to the drains in urban areas we are saying. So, it will largely be running offto the drains and become water which is non-usable or which requires treatments beforeit can actually become usable. (Refer Slide Time: 10:31)So, we are talking about imperviousness. Now, imperviousness as it is literary meaningalso indicates is the capability of a surface to allow water to permeate through it to thelower surfaces. That is what imperviousness of a surface would be.Now, when we are looking at a site development, we should promote that the run off andthe perverseness of the surfaces is less than 50 percent, so that more and more water isallowed to stay on the side and percolate down to the ground and not runoff to the drains.We are talking about the capability of a material to allow percolation of the stormwaterto the lower strata of the ground, the reverse of which is imperviousness. So, we wantmore and more surfaces to be pervious. We want less than 50 percent of the site to havehigher perviousness or for perviousness of more than 50 percent and this will promotemore and more water to be retained on the site and not run off from the site going intothe drains.So, if we look at the intent which we have just talked about, we want to retain the predevelopment hydrology and allow less and less of surface runoff and the water to reachthe surface aquifers through the drains or otherwise. (Refer Slide Time: 12:05)So, in case of a predevelopment scenario where a lot of built surfaces, lot of hardsurfaces, impervious surfaces were not added we would see that a lot of groundwaterrecharge was happening naturally.So, in this case we see that there is a lot of ground water recharge there is a lot ofinterflow which is right beneath the topsoil, that is because of the presence of these theroots of the trees which make this top soil and of the top layer of the soil morepermeable, so that there is more recharge and there is greater interflow and there is agreater evapotranspiration happening. So, the total amount of runoff to the surfaceaquifer was only 0.3 percent as compared to a post development scenario where we seethat almost 30 percent of the surface runoff is received in the surface aquifers.Now, if this was the case where the urban area whatever city is being developed had thepossibility of or the proximity of these surface aquifers and that the surface aquifers hadenough capacity to hold all that surface runoff, that would still be a manageable scenariowhere we have most examples of cities where we do not run off to larger water bodieswe have even eaten up, taken up these water bodies. So, where does this water go? Itactually enters our homes. And since there is less and less of groundwater rechargepercolation to the ground, there is less and less of groundwater available. The groundwater table is further going down. So, earlier where we would have groundwater at around 35 feet, 40 feet, now we havegroundwater table which has gone down to 150, even 200 feet we cannot find water thatis what the effect of this increase runoff is. And for those same reasons we have to limitthe imperviousness and also the runoff from our developed sites.So, the kind of strategies that we are looking at when we are talking about increasing theperviousness and reducing the runoff, so these are some of the strategies which iscommonly used, which are commonly implemented when we are talking aboutdevelopment of site for reducing the runoff.(Refer Slide Time: 14:53)So, first is permeable pavement. So, we are talking about a pavement which allowspercolation of water through it and into the ground. We are, unfortunately most of thematerials which we use these days are so dense that they allow 0 permeability, there is nopercolation of rainwater and all of it goes into the drains thereby we require more drainsize, greater wall to hold greater volume of the stormwater, unlike a permeable pavementwhere this will allow not 100 percent of it, it will not still not be comparable to avegetated green surface, but it would still be better. So, we are looking at pavementswhich are permeable.Now, this is a very good strategy for our streets for our roads which have to be linedwhich have to be paved. But we can have permeable pavements there where there will bemore percolation to the ground. (Refer Slide Time: 16:01)A lot of research is actually happening when we are talking about the permeablepavement, so that our streets our pavements on roads actually allow for groundwaterpercolation.(Refer Slide Time: 16:17)So, there are permeable pavers, there are permeable concretes, we also have grass paverswhich is a very common strategy where we actually see that the paver is havingpunctures and the vegetated area it. So, it allows for growth of grass and that also allowsfor percolation of water down to the ground. (Refer Slide Time: 16:37)Another strategy which is a design strategy. So, this is a mix of strategies we are talkingabout materials and we are talking about design which are the two aspects here when weare talking about stormwater and its management.So, the second major strategy which is a design strategy is to maintain the naturaltopography. Now, this is at the building level, a micro scale of development to a verylarge level which is a city development we have to respect and retain the naturaltopography which is unfortunately not happening. Earlier all the low lying areas in thecities were actually used as surface aquifers, water bodies, that is why we see in majorityof the villages even when we go today the ponds are actually the low lying areas.So, naturally all the water is going to go and get retained there. So, with the help ofcreation of a small bund or a small boundary around the pond or the low lying area it willbe able to hold all the water which will come running to it and since it is not lined, thebottom of it will not be lined, so it will allow percolation.Now, the design feature here was besides the engineering of not lining it or adding abund to it or adding a boundary to it, the main point was topography and selection ofsuch sites which will actually receive a lot of water. Now, if you look at newspapersdays, in during the monsoons you would find the newspapers flooded with the news ofevery city I am saying that this area got flooded, so there is water in the homes people had to be evacuated. So, there is a new term which has been coined which is called urbanflooding.Now, there was nothing like urban flooding earlier, there was just flooding. Now, todaywe have urban flooding because this has become so typical to urban areas because wehave constructed houses, we have constructed buildings even in the low lying areas, thatis because we have not respected the topography. So, we respect the topography and weretain the natural slopes as much and not build in the low lying areas is what the intent ofthis one would be.Another important strategy is green roof. So, we have seen in the previous lecture thatthe green roofs helped in reducing the amount of radiation which is absorbed by the hardsurfaces, because these plants this greenery which is there on the roofs takes up all thatsolar radiation and the heat to make their food to carry on with their processes likephotosynthesis and others.(Refer Slide Time: 19:17)Green roofs also help from storm water point of view because they absorb one is theyabsorb this water. Another is they reduce the flow of the water on a surface. So, theyhelp these green roofs they help in both the ways. (Refer Slide Time: 19:45)The next design strategy is constructed wetland. So, there are naturally occurringwetlands which we can preserve and retain if we are following and retaining the naturaltopography. But there is also this constructed wetland. So, even when there was not andexisting water body within the site if we have a large site.So, this is usually doable on a large site, so if we have a large site and on the lowest levelof the site, the lower part of the site we create water bodies and we allow the runoff fromthe site to collect there in that part of the site which will hold which will retain the water,it can be lined depending upon the quality of the soil and then it will be allowed tochannelize if it is full. So, that is what constructed wetland would be. (Refer Slide Time: 20:55)Then we also have infiltration trenches. Now, infiltration trenches are actually sort ofdrains, but they do not actually carry the water. These are just shallow areas as you cansee in this picture. So, this is slightly shallow which will allow the water from both thesides to enter and this is this is vegetated. So, this is the filter strip which will allow forthe suspended particle solids to be retained here with the help of this grass green a littlebit of vegetation that you can see here.And then, there will be the top will be lined with the small size gravel and the bottom ofthis trench is actually filled up with the clean gravel and bigger size gravel, and inbetween it is a geomembrane. So, the finer particles finer solids will actually notpermeate through it and choke the lower strata.So, when the water actually gets collected because it also it is shallow and it is acontinuous trench usually we would see it along the highways. So, when the water fromthe highway actually reaches this trench it will be held here, retained and it will beallowed to percolate down to the ground because this media is actually loosely packedgravel and it will allow for a lot of permeability, a lot of percolation. This is whatinfiltration trench does. (Refer Slide Time: 22:23)A very similar of this thing is this is again you would see near the highways and roadswhich is a grass filter strip. So, what we have is that we have a grass strip and this isgently sloping and there is a berm which is created, so it will hold the water. So, it isallowing for percolation. It is also allowing for retention.So, it will be retained, this berm allows for all the solids to be retained here, and when itflows after retaining it which is again further taken into a forest filter where the roots ofthis these trees allow more and more of percolation down and then we can have furtherhave an infiltration trench or a drain which may carry the excess of the stormwater. So,this is what grass filter strip does. (Refer Slide Time: 23:15)Similar is the concept of grass swales. So, just as we had we have seen infiltrationtrenches and these strips we also have these whales which are very common alongsidethe highways. So, we just allow for these areas to be covered with grass not huge treesand we also add these bunds, small barriers which will help the water to be retained here,so it will not flow and all the solids will actually be retained here.Another function that all these swales and infiltration trenches have is to reduce theamount of pollutants going into the water bodies. Now, all the pollutants which areemitted by these running vehicles on these highways, so some will be unburned fuel, sounburned particles, then there will be other you know combusted particles, there will befuel, petrol, diesel and all other things. So, these when they are washed with the waterwhich is coming as storm water and if it is taken directly to the surface aquifer, so thesurface aquifer will actually be flooded with a lot of these pollutants.While, if it is retained here all these pollutants will actually be written retained here andonly the water will allowed to percolate down to the ground. And even when it isflowing; even when it is flowing through the swale to the drain when the excess watercomes most of the pollutants will actually be held here with the help of these barriers,these berms. (Refer Slide Time: 25:11)A bigger version of infiltration trench is an infiltration basin where instead of a channelwe have a larger basin, but more or less similar concept where we have very looselypacked substrata and it allows water to percolate down. In addition to just gravel whichwe saw in infiltration trench we would have a lot of plants and vegetation the soils ofwhich help in maintaining the looseness of this substrata and it will help us to percolatethe water down.(Refer Slide Time: 25:47) Then natural drainage. Natural drainage is retaining the drainage pattern which wasnaturally occurring which we have also talked about when we were talking about thenatural topography.So, if you look at the beautiful case of Auroville. So, this is only Auroville Earth Institutewhich is shown in the picture here, but if we look at the concept and how the entireAuroville came up developed. So, initially people followed the natural pattern of howwater was flowing and then they looked at where the water needs to be intercepted, sothat it does not take away the soil along and where those retention ponds, the infiltrationbases and the water bodies have to be created. So, all of that will be happening, thedesign will be happening when we actually understand and study the natural drainagepatterns which is part of natural topography study.(Refer Slide Time: 26:51)So, to summarize these measures and strategies, the first one is post development runoffcharacteristics where we are aiming at developing the site in such a manner that the postdevelopment runoff and the conditions are same as that of pre-development ones. So,there should not be any change in the runoff as far as possible in the volume, rate, timingand pollutant load for a given site. The second one is that the final design shouldmaximize on site storage, infiltration and evaporation of storm water, and not letting itgo to the drain. The third is to encourage reduction of pollutant load and groundwater recharge, all ofthese are leading to the first point which comprises of these. Then avoid dischargingstormwater directly to a surface water body such as stream. Now, this will be through thedrains. So, the drains will carry it to the stream or some other water body. And the mostimpactful of these strategies is to reduce the imperviousness and maximize thevegetation.(Refer Slide Time: 28:15)Coming back again to how green building rating programs assess your efforts throughdesign and selection of materials. So, it has to be a tangible way in which it can beassessed it can be evaluated. Here we do that, but through the calculation of capturedrunoff.Now, this captured runoff is actually a resultant of the rainfall, so it would vary fromplace to place. Places, dry places which have less amount of rainfall will not have asmuch problem as compared to the warm and humid areas which receive a lot of rainfall.And then, this is actually the runoff coefficient which is directly dependent upon theimperviousness of the surface.Higher is the imperviousness higher is the runoff coefficient which we have seen in theprevious table as well. And then the area of the collection surface. So, we are looking atreducing the total amount of runoff and if there was the runoff then all that shall be captured, captured in through creation of a tank or a retention water body, retention pondor any other thing.(Refer Slide Time: 29:37)Here if you look at the compliance options, we are looking at controlling the runoff andwe are looking at the stream channel protection. So, we are looking at the quantitative aswell as the qualitative aspects of it and we are looking at the reduction in the flow ratesof this stormwater.So, if you are looking at the runoff control we are looking at two cases. Now, these twocases are where the sites which we are going to develop it is for a specific project site,where the existing imperviousness is 50 percent or less. So, it is a site which is lessdeveloped and there could also be a case where the existing imperviousness is greaterthan 50 percent which is the largely developed site which is already developed.Now, the possibilities in this case, in the case 1, is that we would be able to retain theexisting pre development conditions post development also. There is a greater possibilityof doing so, while in case of a site which is already developed it is very difficult. That iswhy these two cases come up.So, in case 1, the post development rate and quantity shall be equal to or less than thepredevelopment values, while in case the existing imperviousness is already greater than50 percent we are looking at a post development scenario where the rate and quantity should be at least 25 percent less than the predevelopment values. So, that is what we arelooking at when we are looking at the runoff controls. So, this is one of the exampleswhere we are calculating the volume of captured runoff.(Refer Slide Time: 31:29)Now, this is for a roof area which is a non-vegetated roof surface. Now, this nonvegetated roof surface is 100 percent impervious. So, it allows 100 percent of the waterwhich it receives to runoff. So, using the same equation, equation 1 of the previous slidewe see that this is the amount of rainfall which is received, this is the total area of theroof and this is the runoff coefficient where through the given formula and thepercentage imperviousness which here in this case is 100 percent imperviousness. So, itdoes not allow any water to percolate through it, permeate through it. We calculate weget a runoff coefficient of 0.95, so it means that 95 percent of the water which is receivedwill actually be running off and we calculate the total captured runoff.Now, for this volume of, for this volume of runoff provisions have to be made to store it,to capture it that is what it indents. Out of this if we can show that around say 75 percentof it is being retained on site or captured through the construction of a tank or any otherstrategy, any other measure that is what will be used for the compliance. (Refer Slide Time: 33:11)We are also talking about the quality control here because stormwater is a biggerproblem when it carries a lot of suspended solids with it. So, all these strategies whichwe have just seen in previous slides for example, the infiltration basin, the infiltrationtrench, the vegetated strip, the grass swale, the permeable pavements and all of thesethey help in reducing the amount of suspended solids and total solids.So, when the water is retained through any of these strategies it allows for solids to settledown as we were seeing in case of the infiltration trench or the grass strip. So, the solidsthe soil which is carried, the pollutants which are carried along and a lot of other solidswhich will be carried with the storm water when it goes with the flow, they will be heldthere will be retain and they will be removed from the water which further flows. So, it isnot just about the quantity control, so not just capturing the storm water, but it is alsoabout the quality control.So, through these measures we will actually be able to manage the storm water whichwill be received on site, and the attempt has to be towards maintaining the predevelopment scenario even after the development has happened or post development aswell.So, here we stop today. And we will look at the remaining strategies and criteria whichwe should keep in mind when we are developing the site for sustainable development. Thank you so much for being with us today. And we shall meet for the next lecture onSustainable Site Development.Thank you.

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