Chville. Flood Levels in the Reach from Birchville to Te Marua Were Not Recorded During the May 1981 Event but Were Pegged Following the Flood of 13 March 1990
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18 5. CALIBRATION OF MODEL. Two floods were chosen for calibrating the model:- the2l May 1981 and the 13 March 1990. The 2I May 1981 flood is the largest to occur since the recorders at Birchville and Taita Gorge were installed. A comprehensive set of flood levels was recorded in the reach from the mouth to Birchville and an almost complete river survey was carried out prior to the flood. This flood was therefore used to calibrate the river below Bi¡chville. Flood levels in the reach from Birchville to Te Marua were not recorded during the May 1981 event but were pegged following the flood of 13 March 1990. Berrill Critchlow had recently surveyed the cross sections so this flood was used to calibrate the upper reach. The May 1981 flood had a return period of 11 years at Taita and 13 years at Birchville. The March 1990 was a smaller flood with a 3 to 4 year retum period. 5.1 Flood Discharges. Mav 1981. There has been some doubt as to the actual size of the May 1981 flood discharge recorded at the Taita Gorge site, due to uncertainties over the flow rating. This is because the site was only installed in 1979 and very few larger floods have occurred (and been gauged) over this period. Originally the rating indicated a peak flow of 1405 cumecs, however this was subsequently revised, as part of the current review, to 1228 cumecs, This revision would appear to be too low after comparison with both the peak flows at Birchville of 1227 cumecs (audited data DSIR 1989) and the V/hakatiki at Dude Ranch of 50 cumecs and the tairly uniform and high intensity rainfalls that were recorded at Wallaceville. The flow hydrographs recorded at Birchville and in the Whakatiki (making an adjustment I for that part of the lower catchment not measured at Dude Ranch) were routed, using the MIKE 11 computer model, down the Hutt River to the Taita Gorge site. The model predicted a flow at Taita Gorge of 1288 cumecs. Inclusion of the 60 square kilometres I of various other catchments in the Upper Hutt area would raise the size of this flood to at least 1300 cumecs. Over the total reach downstream of Taita Gorge it can conservatively be assumed that any attenuation of the flood peak would have been compensated for by the various minor tributary inflows. Thus a flow of 1300 cumecs was used to calibrate the model over the re¿ch from the Mouth to Birchville. This value for peak flow was used to f,rx the rating curve for Taita Gorge in the review of rating curves carried out as patr of the Climatology and Hydrology package. March 1990. The revised rating curves for both Taita and Birchville were available to size this flood. Taita Gorge reached a peak flow of 1017 cumecs and Birchville of 876 cumecs. To determine the peak flow in the reach upstream of the Akatarawa confluence the flood hydrograph for the Akatarawa river was subtracted from the Birchville hydrograph. The resulting hydrograph was used with the Mangaroa River hydrograph to determine the flow in the Hutt upstream of the Mangaroa confluence. The Mangaroa hydrograph was lagged by 15 minutes but this had minimal effect on the peak flow in the Hutt reach. I t9 I For the reach upstream of the Akatarawa confluence the peak flow was 579 cumecs and upstream of the Mangaroa it was 440 cumecs. I Starting Level Mav 1981 + I The flood pe¿ked at the mouth at 0915 hours NZST at approximately mid-tide. Associated with the rainfall were very high southerly winds that could conceivably cause storm surge and wave setup as large as 0.5 metres. The control level at the mouth for I the model simulations was therefore set to 0.5 metres. March 1990 I The flood peak at the mouth occurred at 1800 hours NZST coinciding with a high tide of 0.37 metres. The rain was associated with a northerly front which does not produce large wave set up at the Hutt mouth. To allow for some storm surge the control level I at the mouth for the model simulation was set to 0.5 metres. 5.J Cross Sections. Mav 1981 I The 1980/1981 survey of the Wellington Regional Water Board cross sections were used supplemented with 1977 survey data where sections we¡e missing. The WRWB sections did not always cover the entire flood channel between the stopbanks. The berm shape I was taken off the 1987-1989 Berrill Critchlow survey with the exception of the State Highway 2 Upper Hutt Bypass where the berm has changed considerably between 1981 and 1987. Fo¡ the Upper Hutt Bypass contour maps and Ministry of Works surveys I were used to provide berm data. (Refer Appendix 3) Bridge cross sections were taken from the 1987-1990 survey and a few 1987-1990 I sections were used in the model to provide an approximate 400 metre spacing. March 1990 I The Berrill Critchlow 1987-1989 sections were used. 5.4 Flood Marks. I Following the May 1981 flood 24 flood marks were recorded, 16 downstream of I Silverstream and 8 upstream. 21 marks were recorded for the March 1990 flood in the reach from Birchville to the Hutt Gorge. I For both the calibration floods the flood level pegging was ca¡ried out the day after the flood using debris marks. When calibrating the model to the flood marks consideration was given to the fact that debris marks usually tend to underpredict flood levels as the debris can fall with the falling limb of the hydrograph. I I I 20 5.5 Calibration Method positions , of channel dividers and energy loss better matching with recorded levels. ún;; representation of the physical situation. he calibrated parameters from the 19gl s sections with channel dividers, offsets the 1991 positions. The l9g1 central as representative of the reach either side of the were treated in the same mann". un,":;",LÏrilål?L'ËJÏl';.,Ti'Ï.rT#åîî.å occurred. The 1987 model has section spacings o greater spacing the 1997 model *", *n metre section spacings. All levels were we¡e within 0.02 metres. The levels for metres above the model with the 100 m 5.6 Mouth to Silverstream Mouth m¿Ixlmum extent of def,rned by a line co angle of 10 degrees 2T Immediately upstream at XS 0040 the main channel narrows choking the flow. Critical flow may develop for floods as frequent as the I0% probability flow. The predicted levels through this reach must be smoothed for design purposes. Ava Railwav Bridee The piers of the Ava Railway Bridge are skewed at an angle of 26 degrees to the upstream flow alignment. Furthermore, the nine principal piers are some 13.7 metres long and present an effective obstruction width to the flow of some 6.0 metres. In practice the flow is to some degree "trained" through the piers, however, there will be an associated energy loss. The bridge was consequently modelled applying an appropriate pier ratio of 0.3 and an associated bend loss. Pomare Railway Bridge The piers of the Pomare Rail Bridge are skewed at an angle of 20 degrees to the upstream flow. The six principal piers are tapered at both ends thus increasing the available flow area. The overall degree of obstruction to flow (as expressed in the pier ratio factor) was determined after delineating a "no-flow" zone based on the pier shape, size and alignment. As for Ava Railway Bridge the flow is to some degree "trained" and an associated bend loss was applied. Silverstream Railway Bridge The Silverstream Rail Bridge is supported by 13 piers of 1.86 metre diameter cylindrical piers and one smaller pier. The pier ratio was based on the piers being aligned at 5 degrees to the upstream flow direction. Silverstream Road Bridge The piers of the Silverstream Road Bridge are skewed at an angle of 32 degrees to the upstream flow. This large misalignment has resulted from changes to the river channel alignment implemented during the construction of the River Road section of State Highway Two. Previous to this the bridge was aligned at 20 degrees to the upstream flow. The best calibration results were obtained using a pier ratio based on the flow misalignment of 20 degrees. Whilst this was the case for the 1981 flood , it may be that during flood events the effect of both the upstream meander and the "training" of the piers induces an alignment less than the average of 32 degræs. (Furthermore Henderson @eference 4) refers to flow angles of up to 20 degrees when using the projected area approach). General Areas of "dead water" where no flow occurs were excluded from the model. The larger of these areas were the right banks below Esfuary, Ava and Pomare bridges. Areas which received floodwater only in the larger events were included in the model for only these events (e.g the Shandon Golf course land immediately upstream of Estuary Bridge). Summarv For the Mouth to Silverstream reach the calibrated model predicted levels to within * 0.35 metres with 93% of the model levels within + 0.23 metres of the recorded levels and 65% to within + 0.07 metres. The levels were underpredicted by an average of 0.03 metres. Centrcl Chonnel MannTngs n vCIlues Figure 2 Historicol ond Presenl Monnings n volues Monnings n 1990 model 1977 model 1960 model 0. 05s 0 .05 0 .045 0.04 0.