MODULE - 8 LECTURE NOTES 6 URBAN STORMWATER MANAGEMENT

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1 Water Resources Systems Plannng and Management: Water Resources Systems Modelng Urban MODULE - 8 LECTURE NOTES 6 URBAN STORMWATER MANAGEMENT INTRODUCTION Urban stormwater management systems are meant to gude, control and mody the quantty and qualty o surace runo. There are bascally ve subsystems whch characterzes the urban dranage system: () surace runo subsystem (2) storm sewer subsystem (3) detenton subsystem (4) open channel transport subsystem and (5) recevers such as rvers, lakes or oceans. In ths lecture we wll dscuss about the varous subsystems and the desgn o storm sewers usng varous methods. SUBSYSTEMS The surace runo subsystem transorms the ranall nput nto surace water runo. The outputs runo hydrograph rom surace runo subsystem s the nput to the storm sewer subsystem. Storm sewer subsystem transports runo to ether a detenton subsystem or an open channel transport subsystem or a recever subsystem. Output releases rom a detenton subsystem can be the nput to an open channel subsystem or a recever subsystem. Output releases rom open channel subsystem can be the nput to a detenton subsystem or a recever subsystem. In urban stromwater management, the determnaton o runo yeld and the optmal desgn o storm sewer networks are very mportant. Strom water runo allevaton s a major task. STORM SEWERS Strom sewers play an mportant role n urban stromwater management.a storm sewer system may consst o a number o sewers, junctons, manholes and nlets n addton to regulatng and operatng devces. The desgn o storm sewer ncludes determnng the dameter, slopes and crown elevatons o each ppe n the network. The desgn models can be dvded as hydraulc desgn models and optmzaton desgn models. The hydraulc desgn models determne the sewer dameters by consderng only the hydraulc parameters. In optmzaton desgn models, the mnmum sewer sze that s able to carry the desgn dscharge under ull ppe gravty condtons s determned. In these desgn models, the sewer system layout s predetermned and the sewer slope s assumed to be same as that o ground slope. The assumptons and constrants commonly used n storm sewer desgn are:

2 Water Resources Systems Plannng and Management: Water Resources Systems Modelng Urban 2 () Sewer s usually desgned or gravty low, No need o pumpng statons or pressurzed sewers. () The ppes used are commercally avalable crcular ones wth a mnmum dameter o 20 cm. () The desgn dameter should the smallest avalable ppe wth low capacty equal or greater than the desgn dscharge and satses all constrants. (v) The storm sewers must be placed well below the ground level to prevent rost, dran basements and also to allow sucent cushonng aganst breakage due to ground surace loadng. Thereore, mnmum cover depths should be speced. (v) At junctons, the crown elevaton o the upstream sewer should not be lower than that o the downstream sewer. (v) A mnmum permssble low velocty at desgn dscharge or at barely ull ppe gravty low should be speced to prevent excessve deposton o solds n the sewers. (v) A maxmum permssble low velocty should be speced to prevent scourng eects. (v) The downstream sewer should not be smaller than any o the upstream sewers at any juncton. (x) The sewer system s a dendrtc network convergng towards downstream wthout any closed loops. DESIGN OF STORM SEWERS () Ratonal method: Ths s the most popular method or the desgn o storm sewers. The surace runo peak s estmated usng the ratonal ormula: = C A where s the peak runo, C s the runo coecent, s the average ranall ntensty and A s the dranage area. I there are m subcatchments, then m j C j A j The ranall ntensty s the average ranall ntensty over a partcular basn or sub-basn. The tme o concentraton, t c s the tme at whch the peak runo reaches the pont o nterest. The t c to any pont n a storm sewer system s the sum o the nlet tme t 0 and the low tme n the upstream sewers t..e., t c =t 0 +t, where t 0 s the longest tme o overland low to reach

3 Water Resources Systems Plannng and Management: Water Resources Systems Modelng Urban 3 the storm sewer nlet and t n j L V j j s the low tme requred wthn a ppe o length L j and average low velocty V j. (2) Hydrograph method: In ths method, the nput desgn hydrograph at the upstream end o the sewer s propagated to the downstream end o the sewer through some routng methods. Ths sum o the routed hydrographs rom all the upstream sewers s added to the surace runo hydrograph at the downstream juncton. Ths represents the desgn nlow hydrograph to the downstream sewer ppe. Now, based on the commercally avalable ppes, those ppes whch can handle the peak dscharge o the nlow hydrograph and mantan a gravty low are selected. A smple hydrograph desgn method s the hydrograph tme lag method. whch s a hydrologc routng method. In ths method, the nlow hydrographs o each sewer are shted by a sewer low tme wthout dstorton t to generate the outlow hydrographs. These outlow hydrographs are added at a manhole wth the drect nlow hydrograph o the manhole to generate the nlow hydrograph o the downstream sewer. Ths can be expressed as j j 0 ds dt where j s the nlow rom the th upstream sewer nto juncton j, j s the drect nlow nto the juncton, 0 s the outlow rom the juncton to the downstream sewer and S s the water stored at the juncton. (3) Mnmum cost desgn: In ths method, the mnmum cost o sewer system s acheved by a trade-o between the ppe cost and the excavaton cost. A specc amount o dscharge can be carred usng a steeper ppe, one wants to reduce the ppe cost. Ths may n turn ncrease the excavaton cost (or ncreased slope). The desgn varables n ths problem are the ppe dameters, u/s and d/s crown elevatons o sewers and the depth o manholes. Oten, dynamc programmng (DP) s used to solve the least cost desgn o storm sewer systems. The stage, state, decson, return and transormaton o DP are explaned below: Stage: the entre sewer network s dvded nto several stages by drawng magnary sonodal lnes. These lnes pass through manholes whch are separated rom the system outlet. A stage ncludes the ppes connectng u/s manholes on lne to d/s manholes on lne +. Hence,

4 Water Resources Systems Plannng and Management: Water Resources Systems Modelng Urban 4 there are I sonodal lnes, there wll be I - stages. The manholes on any lne are connected to the outlet by I-I ppe sectons. The sonodal lnes are constructed startng rom the outlet and proceedng u/s. The numberng s done reversely startng rom u/s and proceedng to d/s. A smple network wth sonodal lnes s shown n gure. 2 Isonodal lne Outlet Manhole Fg. Isonodal lnes o a smple network States: At each stage, the states are the crown elevatons o the ppes. Let S m, m+ be the nput states.e., crown elevaton at the u/s end o the ppe connectng manholes m and m + and Ŝ m, m+ be the output states.e, crown elevaton at the d/s manholes m +. Decson: The decson D m, m+ s the drops n the crown elevatons o the ppes across the stage. The drop n elevaton represents the slope. Now, by usng Mannngs eqn. the ppe dameter can be determned or desgn low rate, assumng ull ppe low. Return: The cost o each ppe and the respectve u/s manhole. Each manhole on u/s sonodal lne s draned to the d/s sonodal lne. Thereore there are M ppes connectng across stage, then the total return at each stage can be expressed as M r rm m S m m D, m, m m,,. Transormaton: Ths denes the transormaton o nput states.e., crown elevatons S m, m+ nto output crown elevatons Ŝ m, m+, through the decson varables D m, m+. Ths can be expressed as Ŝ m, m+ = S m, m+ - D m, m+ The recursve equaton or each ppe at each stage s m, m mn Dm, m r m, m m, m S m, m, D m, m,2,... I where (Ŝ m, m+ ) s the mnmum cost o the system that s connected to manhole m + through manhole m. Hence, the recursve equaton or all ppes n the state can be expressed as

5 Water Resources Systems Plannng and Management: Water Resources Systems Modelng Urban 5 T m, m T m, m mn D m, m r S, D,2,... I where T m, m+ s the combnatons o ppes m rom sonodal lnes to +. DETENTION SUBSYSTEM The runo volume ncreases due to urbanzaton. Detenton s meant to reduce the peak runo rates by holdng the runo or a short perod o tme. The water s then released to natural water course as shown n gure 2. The detenton structures can be a road culvert or even a reservor wth control devces. Retenton s the storage o water or a long perod o tme. The water may never be released to a natural water course. The water n retenton may be consumed by plants, evaporaton or even nltraton. Inlow Detenton pond Outlow Fg. 2 Detenton Structure Detenton storage can be near the precptaton ste or n the storm sewers or n d/s mpoundments. Detenton may not sgncantly reduce the total runo; but t reduces the peak runo by redstrbutng the runo over tme as shown n gure 3. Inlow ater urbanzaton Outlow ater detenton Inlow beore urbanzaton Fg. 3 Inlow and Outlow hydrographs

6 Water Resources Systems Plannng and Management: Water Resources Systems Modelng Urban 6 BIBLIOGRAPHY / FURTHER READING. Jan, S.K. and V.P. Sngh, Water Resources Systems Plannng and Management, Vol. 5, Elsever Scence, Loucks D.P. and van Beek E., Water Resources Systems Plannng and Management, UNESCO Publshng, The Netherlands, Loucks, D.P., J.R. Stednger, and D.A. Hath, Water Resources Systems Plannng and Analyss, Prentce-Hall, N.J., Mays, L.W. and K. Tung, Hydrosystems Engneerng and Management, Water Resources Publcaton, Vedula S., and P.P. Mujumdar, Water Resources Systems: Modellng Technques and Analyss, Tata McGraw Hll, New Delh, 2005.