STREET INLET IN SUMP

Transcription

1 Gu, J.C.Y., MacKenzie, K. and Mmmandi, A (2007) Sump Inlet Hydraulics, submitted t ASCE J. f Hydraulic Engineeirng. STREET INLET IN SUMP James C.Y. Gu 1, Ken A. MacKenzie 2, and Amanullah Mmmandi 3 1. Prfessr, Civil Engineering, University f Clrad at Denver/HSC, Clrad, James.Gu@cudenver.edu 2. Senir Prject Engineer, Master Planning Prgram, Urban Drainage and Fld Cntrl District, Denver, Clrad. kam@udfcd.rg 3. State Senir Hydraulic Engineer, Clrad Department f Transprtatin, Denver, Clrad. amanullah.mmmandi@dt.state.c.us Abstract Sump inlets are used t cllect strm water n the streets r t release stred water in detentin basins. The cmplicatin f sump inlet hydraulics is manifested by the unclear intermingling prcess between weir and rifice flws and the vrtices in the flw pattern. Over the years, the capacity f a sump inlet has been assumed t be either weir r rifice flw, whichever is smaller fr a given water depth. Althugh this practice might nt result in a failure t the strm drain, it has led t randmly versize r undersize inlets. This paper presents a labratry investigatin f the interceptin capacities f several different types f sump inlets, including bar and vane grates, and 3- and 5-ft curb pening inlets. The bserved data revealed significant differences frm the recmmended HEC 22 design prcedure. In this study, new frmulas and prcedures are develped with the cefficients calibrated by the labratry data. In cmparisn, this new methd well agrees with the bserved data and can cnsistently size the varius types f inlets accrding t the design cnditin. Key wrds: Street hydraulics, Sump inlet, Grate, Curb-pening, Cmbinatin, Type 13, and Type 16. INTRODUCTION Water n the street generates ptential safety hazards - such as hydrplaning - t mving vehicles (Agrawal et. al. 1977). The principal cncern in street drainage is t remve strm water frm the street as quickly as pssible (Huebner et. al. 1986). The gemetry f a street s crss sectin plays a key rle in strm water drainage design. In general, strm water n the street is mdeled as a wide, triangular channel that has variable depths acrss the tapered street gutter, with a deeper depth at the face f the curb and zer depth at the radway shulder r driving lane. The street hydraulic cnveyance capacity (SHCC) is prprtinal t the lngitudinal slpe (McEnre et. al. 1999) while the street hydraulic strage capacity (SHSC) is related t the gutter gemetry (Gu 2000A). Strmwater n the street flws away frm the street crwn tward the curb. The street gutter then cllects the strmwater and cnveys it dwnstream t the inlets. T prvide psitive drainage by gravity, a recmmended minimum lngitudinal grade fr a street is (+/-) 0.50% (AASHTO, 1990). Hwever, lw pints are created where a radway segment f psitive grade jins with a segment f negative grade. Likewise, a sump is usually created at a street crner that is cnfined by street curbs and crwns (Gu 2000B). Hydraulically, inlets are classified int tw categries: in-sump and n-grade. In current practice, mst street drainage criteria were develped fr the SHCC. Fr instance, the allwable water spread and inlet interceptin capacity are slely determined by the water flw n a slping street (USWDCM 2001). At a sump cnditin, where ttal interceptin is required, an inlet is assumed t perate like either a weir r an rifice (HEC 12). The demarcatin between weir and rifice flws is nt well understd. As a result, drainage manuals simply suggest that fr a given flw depth, the sump inlet capacity is selected as the smaller value between rifice flw and weir flw (USWDCM 2001). Over the years, the Clrad Department f Transprtatin and metr Denver gvernments have jintly spent millins f dllars t install inlets n streets and highways (CODT 2004). The field reprts in- 1

2 dicate that sme f the sump inlets were fund t be unnecessarily lng while thers were inadequate. It has becme an increasing cncern as t hw t prperly design a sump inlet accrding t the flw nature (Gu 2006). This study was cnducted as an attempt t refine the current design methds fr sump inlets. A 1/3-scale labratry mdel was built in a 3.5-meter flume that was used t simulate the street flw arund a sump inlet. Several sets f data were cllected fr grate, curb pening, and cmbinatin inlets. In this study, it was fund that the current design prcedure applies incnsistent parameters t size an inlet under varius sump cnditins. Based n the labratry data frm this study, the prcedure fr sump inlet design has been significantly revised and all design parameters have been cnsistently calibrated and then recmmended fr sizing varius types f street inlets. FLUME TESTS FOR SUMP INLET As recmmended, an inlet must be installed wherever the water spread exceeds the allwable limit n a slping street (CDOT 2004). The majr parameters fr a street sectin include street width, water spread, gutter width, gutter depressin, and slpes. In an urban area, a straight street sectin has a unifrm transverse slpe while a cmpsite crss sectin includes an additinal gutter depressin. The street transverse slpe is a cmprmise between drainage efficiency, driver cmfrt, and vehicle stability. Andersn (1993) utlined varius radway crss sectins and the necessary parameters invlved in the determinatin f a transverse slpe. As recmmended, the transverse slpe is recmmended t be 1 r 2.0% n radway pavements (Mays 2001). At a sump, the accumulated water vlume is dictated by the incming street flw and the inlet interceptin. The water depth and the inlet size are the key factrs t determinate the flw interceptin. In the labratry, a 3.5-m wide flume was built t mdel the street curb and gutter n a lngitudinal slpe f % and a transverse slpe f 1%. All mdel parameters were determined using Frude number similarity with a length scale f 1/3. A sump was created 3.5-m upstream f the clsed end f the flume. The water depth at the sump reflects the equilibrium cnditin between the inflw and the utflw. Inlets mdeled in the labratry tests include: (1) Bar grate (Type 13 inlet), (2) Vane grate (Type 16 inlet), (3) 3-ft Curb-pening, (4) 5-ft Curb-pening (Type R inlet), (5) Cmbinatin 13 using a bar grate and 3-ft curb pening, and (6) Cmbinatin 16 using a vane grate and 3-ft curb-pening. Their prttype dimensins are listed in Table 1 and cnstructin details can be fund elsewhere (Standard Plan 2001, Standard Details 1994). Pht 1 is the prttype Cmbinatin 13 inlet using a bar grate and 3-ft curb-pening inlets. Grate Dimensin Bar Vane 5-ft Curb-pening 3-ft Curb-pening Cmbinatin 13 Cmbinatin 16 Grate Grate Length in ft Width in ft N/A N/A Opening Height in ft Steel Bar in ft Vane angle in degrees Nte: ne ft = meter Table 1 Prttype Dimensins f Inlets Investigated in Labratry Tests The water depth at the curb face in the mdel gutter was measured by a pint gauge that was lcated 2.2 m upstream f the mdel inlet. Usually, a sump inlet n a residential street is allwed t have a pnding water depth up t 0.72 cm (18 inches) n a 2.4 cm (6-inch) curb. Therefre, the range f water depth in the mdel gutter was measured up t 2.4 cm in the mdel, r 0.72 cm in the prttype. Fr each run, water flw was circulated frm a pump thrugh the flume, and then intercepted by the sump inlet. Befre a measurement was taken, the system was first perated t establish a steady state. Fr each run, the water depth and discharge were recrded fr analyses. When water was shallw in the mdel gutter, the flw smthly vertpped the edges f the mdel grate (Pht 2). When the water depth was clse t the curb height, the grate surface area was abut t be cmpletely submerged. The flw began t frm vrtices alng the edges f the grate (Pht 3). As sn as the curb was submerged, a large single vrtex 2

3 was frmed, dminating the flw pattern. This is bviusly an indicatin f rifice flw because a strng suctin was develped n the grate surface (Pht 4). Pht1 Illustratin f Cmbinatin 13 Inlet Pht 2 Weir Flw thrugh Mdel Vane Grate 3

4 Pht 3 Mixing flw thrugh Mdel Vane Grate Pht 4 Orifice flw thrugh Mdel Vane Grate SUMP GRATE and CURB-OPENING HYDRAULICS The analyses f data suggest that the flw thrugh a sump inlet is varied with respect t the pnding depth and cntinuusly changes frm weir flw, thrugh mixing flw, t rifice flw when the water becmes deep enugh. A grate is frmed by steel bars r vanes. Therefre, the riginal frmulas fr rifice and weir flws are mdified with weir length r area pening ratis as: Q Q 3/ 2 = N C 2g (2W L ) d fr weir flw thrugh grate (1) w w w g g = N C 2gW L d fr rifice flw thrugh grate (2) g g Where Q w = weir flw, Q = rifice flw, W g =grate width, L g =grate length, d=water depth, N w = weir length pening rati after subtracting steel bars, N = rifice area pening rati, C w = weir discharge cefficient, and C = rifice discharge cefficient. Similarly, the mdified weir and rifice frmulas fr a curb-pening are: 3/ 2 Qw = Cw 2g Lcd fr weir flw thrugh curb pening (3) Q 1 2 = C 2g L H d fr rifice flw thrugh curb pening (4) c c Where L c = length f curb-pening and H c = height f curb-pening. The transient prcess between weir and rifice flws is termed mixing flw that is mdeled as: Q = C Q Q fr mixing flw (5) m m w Where Q m = mixing flw and C m = mixing flw cefficient. In practice, fr the given water depth, it is suggested that the interceptin capacity be the smallest amng the weir, rifice, and mixing flws as: Q = min( Q, Q, Q ) (6) p w m 4

5 Where Q p = predicted interceptin capacity. A grate inlet has three parameters that need t be calibrated, Including C w, C m, and C ; s des a curb-pening. In this study, the calibratin prcess is implemented by the least square methd as: i = n i= 1 E = min [ Q ( i) Q ( i)] (7) e p 2 Where E = sum f squared errrs and Q e = measured interceptin capacity. The slutin fr Eq. 7 is frmulated under the fllwing cnditins: E C w = 0 subject t C w < (8) E C = 0 subject t C < (9) IE = 0 subject t C m < (10) C m In this study, the numerical prcess f the least square methd was executed using the slver in Micrsft Excel Sftware. Fr instance, Figure 1 presents the cmparisn between the bserved and the predicted interceptin capacities fr the bar grate. As shwn in Figure 1, the HEC22 prcedure is fund t cnsistently ver-estimate the grate capacity. In general, Eq 6 gives gd agreement with the bserved data, except fr shallw water depths. In fact, the shallw gutter flw is mre like a wide pen channel flw that vertps the grate edges under a M2 prfile (Chw 1959). The parameters fr the mdel bar grate were calibrated using Eq s 1, 2, 5, and 6. As listed in Table 1, C = 7 and C w 2 g =2.38 and the mdel bar grate has an pen-area rati f 44%, r 56% f the grate surface area ccupied by steel bars. Figure 2 is the cmparisn between the bserved and the predicted capacities fr the mdel vane grate. Under the cnditin f n clgging, the HEC22 prcedure verestimates the capacity f a vane grate until the water depth becmes adequately deep. A vane grate is shaped with the highest hydraulic efficiency t intercept the gutter flw. The mdel vane grate is frmed with curved blades that have an inclined angle f 45 degrees. The pen-area rati fr a vane grate varies with respect t the directin f flw. In this study, tw variables, N w in Eq 1 and N in Eq 2, were intrduced t the least square methd. As shwn in Table 2, nly 62% f the wet perimeters arund the three edges f the mdel vane grate remained pen t intercept weir flws while nly 31% f the surface area f the vane grate remained pen fr rifice flws. 5

6 Type 13 Grate in Sump Flw Interceptin in cfs Figure 1. Cmparisn between Observed and Predicted data fr Bar Grate. Type 16 Grate in Sump Flw Interceptin in cfs Figure 2. Cmparisn between Observed and Predicted data fr Vane Grate. Figure 3 and 4 present the perfrmance curves fr 3-ft and 5-ft curb pening inlets. A curb pening acts like a side weir. As shwn in Table 2, the data reveal that a curb pening is a mre efficient weir than the grate because bth 3-ft and 5-ft curb pening have a higher value fr C w. In cmparisn, the HEC-22 prcedure verestimates the capacity f a curb-pening inlet when water depth is shallw, and then becmes underestimating when water depth exceeds 18 cm. On the cntrary, the prpsed new equatin agrees with the bserved well. 6

7 3-ft Curb Opening in Sump Flw Interceptin in cfs Figure 3. Cmparisn between Observed and Predicted data fr 3-ft Curb Opening. 5-ft Curb Opening in Sump Flw Interceptin in cfs Figure 4 Cmparisn between Observed and Predicted data fr 5-ft Curb Opening. Type f Inlet C Cw 2 g C m N w N Bar Grate Vane Grate ft Curb Opening ft Curb Opening Table 2 System Variables fr Grate and Curb Opening Inlets 7

8 SUMP COMBINATION INLET HYDRAULICS A cmbinatin inlet cnsists f a hrizntal grate placed in the gutter and a vertical curb pening inlet n the curb face. The advantage t adpt a cmbinatin inlet is t reduce the risk f being cmpletely clgged by debris. Fr instance, if the grate becmes clgged, the curb pening remains functinal r vice versa. When water flws thrugh a cmbinatin inlet, the grate intercepts the shallw flw. As a result, the curb pening will nt functin until the grate is submerged. Different appraches were develped t size a cmbinatin inlet. Fr instance, it has been recmmended that the capacity f a cmbinatin inlet be the larger interceptin between the grate and the curb pening r a reductin n the algebraic sum f the ttal interceptin (Gu 1999). Hwever, n clear recmmendatin has ever been made r verified fr such a capacity reductin. In practice, the street flw is first intercepted by a grate as if the curb pening did nt exist, and then the remaining flw is applied t the curb pening inlet as if the grate did nt exist (USWDCM 2001). Nevertheless, the hydraulics f a cmbinatin inlet remains unclear even thugh hundreds f cmbinatin inlets have been installed in metr areas every year. In this study, a new apprach was frmulated t mdel the interceptin capacity f a cmbinatin inlet. It is suggested that a reductin factr be applied t the algebraic sum f the ttal interceptin as: Q t = Q + Q K Q Q (11) g c g c Where Q t = interceptin capacity fr cmbinatin inlet, Q g = interceptin fr grate, Q c = interceptin fr curb pening, and K= reductin factr. The value f Q g is determined using Eq 6 aided with Eq s 1, 2, and 5. The value f Q c is als determined using Eq 6 aided with Eq s 3, 4, and 5. Tw cmbinatin inlets were investigated in the labratry. Cmbinatin 13 inlet is cmpsed f a hrizntal bar grate and a 3-ft lng curb pening. Similarly, Cmbinatin 16 inlet was frmed with a vane grate and 3-ft lng curb pening. Having cllected several sets f data, the least square methd was set up t minimize the squared errrs using the reductin factr, K. It was fund that K=5 fr Cmbinatin 13 inlet as shwn in Fig 5, and K=0.31 fr Cmbinatin 16 inlet as shwn in Fig 6. A higher reductin factr implies that the higher interference between the grate and the curb pening. Fr instance, the vane grate is mre susceptible t inundatin because f its lw area-pen rati. As a result, the vane grate is mre likely t perate under high water depths r bth the vane grate and its curb pening can cnstructively functin tgether. On the cntrary, a bar grate in the Cmbinatin 13 inlet can intercept the majrity f the gutter flw. Its curb pening is therefre nt fully utilized till the bar grate is submerged under an verwhelming inflw. The HEC22 prcedure assumes that the grate and curb pening can independently wrk. As a result, it cnsistently verestimates the capacity f a cmbinatin inlet. In this study, a capacity reductin is intrduced t Eq 11. Of curse, the value f K is a lumped, average parameter representing the range f bserved water depths in the labratry. During the mdel tests, it was bserved that when the grate surface area is subject t a shallw water flw, the curb pening intercepted the flw at its tw lw crners, r it did nt behave as a side weir t cllect the flw alng its full length. Under a deep water flw, the vrtex circulatin dminates the flw pattern. As a result, the central prtin f the curb pening seems t mre actively draw water int the inlet bx. Althugh Eq 11 appears simple fr use, it best represents the range f the bserved data. 8

9 Type 13 Cmb Inlet in Sump Flw Interceptin in cfs Figure 5. Cmparisn between Observed and Predicted data fr Cmbinatin 13 Inlet. Type 16 Cmb Inlet in Sump Flw Interceptin in cfs CONCLUSION Figure 6 Cmparisns between Observed and Predicted data fr Cmbinatin 16. (1) The mdel tests were cnducted in a 3.5-m wide flume t investigate the nature f the sump flw and the interceptin capacities f grate and curb pening inlets. It is cnfirmed that the capacity f a sump inlet increases with respect t water depth, starting with weir flw and then switching t rifice flw. In between, a transient mixing flw exists. In this study, the mixing flw cefficient is fund t be 0.93 fr bth the grate and the curb pening inlets investigated. 9

10 (2) The calibratin results in a value f 7 fr rifice discharge cefficient, 2.38 fr the variable f C w 2 g when using a grate inlet. In cmparisn, the curb pening is mre efficient as a weir flw because it has a value f 3.51 t 3.58 fr C w 2 g. (3) The HEC 22 prcedure generally fllws the increasing relatinship between inlet capacity and water depth. Hwever, it tends t verestimate the capacities fr bth grate and cmbinatin inlets. Curb pening is fund t have a higher weir cefficient than that fr a grate. The HEC22 tends t verestimate the capacity f a curb pening inlet under a shallw water depth and then underestimates it fr deep depths. (4) Vane grate was invented t be safe fr bicycle and t be efficient fr flw interceptin. The labratry data indicate that the interceptin capacity f a vane grate is nly 75 t 80% f bar grate. The width f inclined vanes significantly reduces the area and width pening ratis. As a result, the efficiency f a vane grate is substantially cmprmised by its safety. In cmparisn, a cmbinatin inlet with a bar grate has a higher reductin factr than that using a vane grate. (5) All cases investigated in this study were under n clgging cnditin. It has been recmmended that a clgging factr be applied t the grate area when the grate perates as an rifice r t the wet perimeter when the grate perates as a weir (Gu 2000C, 2006). APPENDIX I. REFERENCES Agrawal, S.K., Henry, J.J., and Mayer, W.E., (1977). "Measurement f Hydrplaning Ptential, " Final Reprt t Pennsylvania Department f Transprtatin, Reprt N. FHWA-PA AASHTO (2007). Highway Drainage Guidelines, 4th Editin. CDOT (2004) Drainage Design Manual, Clrad f Department f Transprtatin, Denver, Clrad. Chw, V. T. (1959) Open Channel Hydraulics, McGraw Hills Bk Cmpany, New Yrk. Gu, James C.Y. (2000A). Street Strm Water Cnveyance Capacity, ASCE J. f Irrigatin and Drainage Engineering, Vl 126, N 2, Mar/Apr, Gu, James C.Y. (2000B). Street Strm Water Strage Capacity, J. f Water Envirnment Research, Vl 27, N 6., Sept./Oct. Gu, James C.Y. (2006). Decay-based Clgging Factr fr Curb Inlet Design, Vl 132, N. 11, ASCE J. f Hydraulic Engineering, Nvember. Gu, James C.Y. (2000C). Design f Grate Inlets with a Clgging Factr, Advances in Envirnmental Research, Vl 4, Elsvier Science, Ireland. Gu, James C.Y. (1999). Street Hydraulics and Inlet Sizing, published by WRP Cmpany, Littletn, Clrad. HEC 12, (1984). "Drainage f Highway Pavements", US Department f Transprtatin, Federal Highway Administratin Washingtn D.C., Virginia. HEC 22 (2002) "Urban Drainage Design Manual, US Department f Transprtatin, Federal Highway Administratin, Washingtn D.C., Virginia. Huebner, R.S., Reed, J.R., and Henry, J.J., (1986) "Criteria fr Predicting Hydrplaning Ptential," ASCE Jurnal f Transprtatin Engineering, Vl 12, N 5, September. Mays, L (2001) Strmwater Cllectin Systems Design Handbk, published by McGraw Hill Publicatin Cmpany in August. 10

11 McEnre, M.B., Wade, P. R. and Smith, A.K. (1999). Hydraulic Perfrmance f Curb and Gutter Inlets, Reprt N. K-TRAN: KU-99-1, Kansas Department f Transprtatin, September. Standard Plan (2001). Type 13 Cncrete Grate, N. M , Department f Transprtatin, Denver, Clrad. Standard Details (1994). N. 16 Inlet Grate and Frame, Adjustable Curb Bx, City f Denver, Clrad. USWDCM (2001) "Strm Water Design Criteria Manual," Vl 1 and 2, Urban Drainage and Fld Cntrl District, Denver, Clrad APPENDIX II. REFERENCES C = rifice discharge cefficient C m = mixing flw cefficient C w = weir discharge cefficient, d=water depth, E = sum f squared errrs H c = height f curb-pening K= reductin factr L c = length f curb-pening L g =grate length N = rifice area pening rati N w = weir length pening rati after subtracting steel bars Q e = measured interceptin capacity Q c = interceptin fr curb pening Q g = interceptin fr grate Q m = mixing flw Q = rifice flw, Q p = predicted interceptin capacity Q t = interceptin capacity fr a cmbinatin inlet Q w = weir flw, W g =grate width, 11