Rainfall data for the design of sewer detention basins

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1 1 INTRODUCTION Detention basins in sewer systems are used to reduce the runoff peak flows downstream of the basins or to control the amount of overflow of untreated sewage water to the recipient, or to reduce the amount of pollution in the sewage water Rainfall data for design of the size of a basin must be selected to meet the aim of the basin. As a genera 1 rule basins are designed to store a specified volume of water while sewer pipes are designed to carry peak flows. The same rainfall data may not be appropriate to use for the design of basins as for the design of pipes. Other factors influencing the selection of rainfall data are if the basin is located off-line or on- ine. Off-line basins are located separately from the sewer system and the water enters the basin via, for example, a weir. On-line basins are a part of the hydraulic flow system where the water is flowing through the basin and is detained by controling the outflow from the basin by, for example, a nozzle, a weir, or a pump. This report describes the selection of rainfall data for design of on-1 ine basins with an outlet of a nozzle-type located at the bottom. The permitted maximum outflow and the permitted maximum water depth in the basin are limited and flooding is not permitted more quently than stated by the design return period used. Two different kinds of rainfall data can be used for the design of detention basins: * * Design storms estimated from Intensity-Duration Frequency (I-D-F) relationships or from historical rainfall data. Historical storms or time series generated by statistical methods. A design storm is a rainfall which is developed for a certain design return period, and the flow value which is calculated by means of the storm is said to obtain the same return period as the storm.
2 When historical storms are u a number of storms are run through a runoff model and stati ical analysis is applied to the simul flow values to find the flow value coupled with the ign return period. The conclusion is that when ign storms are u , the design is based on rai 11 statistics and when historical storms are used, the ign is on flow istics The latter must be more since flow values are the ting ign varia bles The ign storms have the advantages of ing easy and inexpen sive to use They are, however, evaluated for normal runoff areas only, and can give di rent results for different types of areas The historical storms give more correct results for all types of areas, and non-linear effects in the runoff process that influence the flow statistics can be taken into account. Historical storms can be more expensive and complicated to use, but that problem can minimized through well-developed manuals and computer routines A number of earlier investigations about detention basins are reported in the literature but only a few deal with the selection of rainfall data for design of the basins. Johansen (1979) found that the use of historical rainfalls, combined with a unit-hydrograph to the basin, gave approximately the same sizes of the basins as by using the constant rainfall intensity obtained from the I relationships as direct inflow to the basin. For outflows less than 10 1/s hared the use of the I-D-F curves gave a small underestimation and for outflows larger than 10 1/s hared the use of the I-D-F curves gave overestimations. The outflow was assumed to be constant as a function of time. The use of the his torical rainfalls as di inflow to the basins gave overesti mated volumes compared to volumes obtained by using the I F curves. Thus, it was found important to use some sort of runoff model to calculate the inflow to the basin. Stahre (1981) found that the use of a model like ILLUDAS (see Terstriep and Stall, 1974) gave smaller basins than the use of a linear time-area diagram when using the same rainfall data in the
Page 1 and 2: CHALMERS TEKNISKA HOGSKOLA GEOHYDRO
Page 4 and 5: PREFACE This report includes the re
Page 6 and 7: i i i SUMMARY This report describes
Page 10 and 11: 3 form of a constant rainfall inten
Page 12 and 13: 2 REALIZATION OF IGN OF I BASINS 5
Page 14 and 15: 7 The method used for screening of
Page 16 and 17: 9 is just the constant rainfall int
Page 18 and 19: where t = time T = total duration c
Page 20 and 21: 13 in which the definition of the s
Page 22 and 23: 15 Infiltration f ---, I ~ ....J Di
Page 24 and 25: 17 the pipe Figure 2.8 The outlet f
Page 26 and 27: 2 5 As catchments for tests of the
Page 28 and 29: 21 One detention basin is located a
Page 30 and 31: 23 Table 2.6 Detention basin data.
Page 32 and 33: 25 ign Using ign Storms The differe
Page 34 and 35: where 27 M = volume of the basin (m
Page 36 and 37: 29 Table 2.9 Values of constants fo
Page 38 and 39: 31 000 GJ E :J 0 > 2 000 ~ w
Page 40 and 41: 3.2 The complete results of design
Page 42 and 43: 35 mary of the result as mean value
Page 44 and 45: 37 Table 3 4 Deviations in percent
Page 46 and 47: The overestimations for Sifal storm
Page 48 and 49: 41 LEGEND 30 XX Results reported by
Page 50 and 51: 43 * * * The Chicago design storm o
Page 52 and 53: 45 After calculation of the basin v
Page 54 and 55: BERGSJO xtmum basin depth 2.0 m f[o
Page 56 and 57: ERGSJO axtmum stn 2.0 m flow 30 l/s
Page 58 and 59:
LIN 1mum mum G s1n 2.0 m flow 10 l
Page 60 and 61:
s LINKOP G 1mum s1n 2.0 m outflow 3
Page 62 and 63:
LINKOP G 2 x1mum basin depth 2. 0 m
Page 64 and 65:
LIN G 2 X m s1n 2.0 m s1n volumes m
Page 66 and 67:
mes RG x1mum mum Each point (x) cor
Page 68 and 69:
volumes m 1mum s1n 3.5 m 2 flow 30
Page 70 and 71:
es LINK x1mum 1 sn 3.5 m flow 10l/s
Page 72 and 73:
LINK G 1mum s1n 3.5 m vo mes mum po
Page 74 and 75:
LINK G 2 ax1mum s1n 3. 5 m outflow
Page 76 and 77:
2 vo Ll K 1mum (x) corresponds to t
Page 78 and 79:
71 Marsalek J., 1978: rch on the De
Page 80 and 81:
73 nr 22 nr 23 nr 24 nr 25 nr 26 nr
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Rainfall data for the design of sewer detention basins

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