annexure--vb

8/6/2019 ANNEXURE--Vb

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DHI Water Environment Health December 2009

One and Two Dimensional Model Studies for Prediction of Changes in Flow Conditions in Panvel Creek due to Development

of Proposed International Airport at Navi Mumbai, Maharashtra

Review of CWPRS Draft Report



Review of CWPRS 1 and 2D Modelling December 2009

Client

City and Industrial Development Corporation of Maharashtra

Client’s representative

Er Satyendu Sinha

Project

One and Two Dimensional Model Studies for Prediction of Changes in Flow Conditions in Panvel Creek due toDevelopment of Proposed International Airport at Navi Mumbai,Maharashtra

Project No

11806447

Authors

Alasdair Macdonald

Date

December 2009

Approved by

Ajay Pradhan

Revision Description By Checked Approved Date

Key words

Data Collection

Hydrologic Modelling

One and Two Dimensional Hydrodynamic Modelling

Impact Assessment

Classification

Open

Internal

Proprietary

Review of CWPRS Draft Report B-220, lower ground

Chitaranjan Park

New Delhi 11 0019

India

Tel: +91 11 2652 0425

Fax: +91 11 2660 2276

e-mail: [email protected]: www.dhigroup.com

Distribution No of copies

CIDCO

CWPRS

DHI

Er Satyendu Sinha

Dr Z S Tarapore

Er D N Deshmukh

AJP, KWO

1

1

1

1



Review of CWPRS 1 and 2D Modelling i December 2009

LIST OF CONTENTS

1 INTRODUCTION ....................................................................................................... 1-1 1.1 Background ................................................................................................................ 1-1 1.2 Previous Studies ........................................................................................................ 1-1 1.3 Present Study ............................................................................................................ 1-1 1.4 Review Assignment ................................................................................................... 1-1 2 DATA ............................................................................................................................ 3 2.1 Topographic and Hydrographic Surveys ....................................................................... 3

2.1.1 Cross Sections and Bathymetry ...................................................................... 3 2.1.2 Sediment Samples .......................................................................................... 3

2.2 Hydrologic Data ............................................................................................................ 3 2.3 Daily Discharge Data .................................................................................................... 3 2.4 Tidal Water Levels and Velocity .................................................................................... 4 3 ONE AND TWO DIMENSIONAL MODEL REPORT ...................................................... 5 3.1 General ......................................................................................................................... 5 3.2 Rainstorm Analysis ....................................................................................................... 5 3.3 Rainfall-Runoff Analysis ................................................................................................ 6 3.4 Tidal Data Analysis ....................................................................................................... 6 3.5 One Dimensional Model ................................................................................................ 7

3.5.1 Introduction ..................................................................................................... 7 3.5.2 Calibration ....................................................................................................... 8 3.5.3 Impact Assessment ......................................................................................... 8

3.6 Two Dimensional Model ................................................................................................ 9 3.6.1 Introduction ..................................................................................................... 9 3.6.2 Model Domain ................................................................................................. 9 3.6.3 Calibration ..................................................................................................... 10 3.6.4 Impact Assessment ....................................................................................... 10 3.6.5 Safe Grade Elevation .................................................................................... 10

3.7 CWPRS Conclusions .................................................................................................. 11 4 OBSERVATIONS OF REVIEWER .............................................................................. 12 4.1 General ....................................................................................................................... 12 4.2 Diverted Channels ...................................................................................................... 12 4.3 Morphological Impact .................................................................................................. 12 4.4 Physical Model ............................................................................................................ 13



Review of CWPRS 1 and 2D Modelling 1 December 2009

1 INTRODUCTION

1.1 Background

The City and Industrial Development Corporation of Maharashtra Ltd (CIDCO) isdeveloping a new city called Navi Mumbai across Mumbai Harbour along Thaneand Panvel Creeks, and upstream rivers. In addition to the development of newtownships covering 343km2, the Corporation is developing a New InternationalAirport near Panvel by reclaiming 1,615ha of land, which is partly submerged athigh tide. The Gadhi and Ulwe Rivers will be diverted around the boundary of thereclamation: This will entail a total diversion of Ulwe river; for the Gadhi River a cut-off will be provided within the tidal zone between high water and low water and theriver will be trained.

1.2 Previous Studies

The Central Water and Power Research Station (CWPRS), Pune, conducted a onedimensional mathematical model study based on secondary data to predict theimpact of the development of the International Airport on the flow in Panvel Creek inJuly 2001. The study concluded that the proposed airport and townshipdevelopment will raise flood levels in Panvel Creek marginally: around the airport by0.1 to 0.15m, and in the upstream reaches around Taloja Creek by about 0.1m.

1.3 Present Study

Following “In Principle” approval of Navi Mumbai International Airport by theGovernment of India, CWPRS has conducted a one and two dimensionalmathematical modelling study, and is in the process of conducting a physical modelstudy, extending the existing Mumbai Model. CIDCO provided historical rainfalldata, a topographical survey of the upstream rivers, a new bathymetric survey of Panvel and part of Thane Creek, and tidal and velocity observations over onemonth.

1.4 Review Assignment

The objective of the assignment is to review the studies being carried out byCWPRS, and suggest any additional work required, if any, to ensure effective flood

and water management in and around the proposed airport. The scope of worktentatively identified is as follows:

(1) A site visit followed by examination of the data available to CWPRS for

carrying out the one and two dimensional model studies, and review of the

draft abstract report submitted by CWPRS indicating the major findings

of the studies.

(2) Interaction with concerned officials of CIDCO, CWPRS and the Advisor

(Hydraulic) at Navi Mumbai and Pune for full understanding of the studies

and findings.

(3) Assessment of the adequacy of the studies and advice on the need for

further studies, if any.




(4) Report on the best practice for flood and water management in and

around the airport.




2 DATA

2.1 Topographic and Hydrographic Surveys

2.1.1 Cross Sections and Bathymetry

A cross section survey of Panvel Creek and the five rivers upstream was carried outin 2008. The cross sections were taken at an interval of 200m upstream in therivers, and 100m in the tidal zone downstream. Cross sections from the mouth of Panvel Creek to the MLD Jetty were taken from a bathymetric survey of the entireThane Creek carried out in 1993/94. The areas covered by the early and recentsurveys are not clearly shown.

The cross sections were surveyed up to the maximum recorded water level (either

flood level or highest tide level). For the purpose of model analysis, the sectionshave been extended based on Survey of India topographic maps above themaximum predicted water level. (The model assumes that the cross section has aconstant width above the maximum cross section level.)

The report mentions cross section transverse distances looking upstream. Theconvention is for cross sections looking downstream, with left and right bankassigned accordingly. CWPRS state that the data have been used correctly in theone and two dimensional models.

It is mentioned that there are holding ponds balancing the stormwater runoff. Thereis no indication of the location and capacity of the holding ponds.

2.1.2 Sediment Samples

Sediment bed material samples and suspended sediment sampling are mentionedin the report, but the location (at discharge and velocity measurement sites?) is notgiven. Analysis of the samples for particle size distribution and concentration is notreported.

CWPRS provided a table with the analysis of three bulk sediment samples (20litres) collected over a period of 2 to 4 hours at three locations in Panvel Creek.The time and date of survey, tidal stage and precise locations are not known, onlythe names of the creeks (Taloja, Panvel and Reti Bunder) have been mentioned.The average concentration is from 310 to 370mg/l.

2.2 Hydrologic Data

A thorough exercise has been conducted to obtain available rainfall data from thearea around Panvel Creek and the region. Mention may be made of the differentraingauge types used, and analysis of the consistency of these data. Data fromdifferent sources using different techniques may give rise to different characteristicsof the rainfall data.

2.3 Daily Discharge Data

Daily discharge data are available at three locations on the upstream rivers. It is

not reported how the discharge data were obtained - presumably computed fromcurrent metering and stage:discharge rating curves. Water levels are recorded from




manual gauges. CWPRS states that the discharge data at the three upstreamgauging stations do not capture rainstorm runoff, and have therefore not beenuseful for the study.

2.4 Tidal Water Levels and Velocity

Tidal water levels and velocities were measured at five locations in the tidal zone of Panvel Creek from mid May to mid June 2009. Three gauges were installed byMumbai Maritime Board (MMB) using an acoustic device from above the water level, and two by CWPRS using pressure sensors located below low water level.Both gauge types record the average water level and velocity since the previousrecord. The expected accuracy of the water level gauges as informed by CWPRSis ±1cm. CWPRS advises that the velocity meters were deployed near the deepestpart of the channels, where the maximum velocity may be expected

The CWPRS gauges were installed to supplement the MMB gauges, and cover thelast two weeks of May, one spring-neap cycle. It appears there was somemalfunctioning of the velocity meter at Ulwe Bridge as constant zero velocity isindicated following low tide (mentioned in Technical Report No 4627 of March2009). The Waghiwali gauge indicates lower HWL and LWL than the other gauges,especially at neap tides. There was no hard foundation for the tidal gauge, whichmay have subsided.




3 ONE AND TWO DIMENSIONAL MODEL REPORT

3.1 General

The draft report abstract on the One and Two Dimensional Model Studies waspresented by CWPRS at CIDCO office in Navi Mumbai by CWPRS on 15th October 2009. This report comprises:

• A review of the available rainfall data in the area

• Estimation of a range of hourly rainstorm profiles

• Estimation of the runoff to the rivers discharging into Panvel Creek

• One dimensional mathematical model study of the flow conditionsin the rivers and creek for a range of rainstorms, for existing

conditions and conditions with the reclaimed land for the proposed

airport

• Two dimensional mathematical model study of the tidal regime in

the rivers and creeks, for a range of rainstorms, for existing

conditions and conditions with the reclaimed land for the proposed

airport

• Estimation of the flood safe grade elevations for the airport area.

CWPRS has informed that the full report will include a List of Contents, List of Figures, List of Tables and be structured with more sub-headings to improvereadability. Units will be shown on all Figures and Tables, and a better base mapwill be incorporated.

GIS will be required for the EIA. All the data should be georeferenced andtransferred to GIS. Nearly all advanced modelling tools utilise data in GIS format.CWPRS advises that all data shown in Google Earth and AutoCad aregeoreferenced and can be transferred to GIS format.

3.2 Rainstorm AnalysisThe rainfall data analysed comprise:

• Yearly maximum one day rainfall for 13 gauges in and around the

study area

• Hourly rainfall data for seven gauges for the exceptional storm of

26th July 2005

• Yearly maximum hourly rainfall for Colaba and Santa Cruz from

1969 to 2004

Figure 1 of the draft report shows raingauges around Panvel Creek, while figure 2shows selected regional gauges (both from the CIDCO Hydrological Data Report).




The two figures may be combined to show in one figure the spatial distribution of allthe gauges used in the analysis.

In the rainfall analysis, emphasis is placed on Santa Cruz and Colaba data which

are IMD stations. Gauges of other organisations, eg Irrigation Department, wereused to support the analysis.

Data up to 2008 could have been collected, though in the opinion of CWPRS, asthere were no major storm events, adding a few more years’ data would not make asignificant difference to the rainstorm analysis. Recent events would nonethelessextend the trend of the influence of any environmental changes in the region.

Frequency analysis has been conducted on the daily data. The study has adoptedthe Log-Pearson III distribution, excluding the exceptional event of 26th July 2005.This yields daily rainfall of 456mm and 531mm for the 50 and 100 year return periodevent respectively.

The LP III distribution excluding the July 2005 event fit this event better than theGumbel distribution (adopted as standard by IMD). LP III gives more conservativevalues, and was therefore selected. In the full report, the table on page 2 will becompleted in respect of LP III, though these periods of records are relatively short.

The derivation of the hourly rainfall distribution in table VI is made according tostandard IMD procedures, reference for which should be given. Actual and derivedhourly rainstorm profiles will be plotted in the full report.

The rainstorm analysis has been thorough and competent, and the derivation of the50 and 100 year return period rainstorm profiles can be taken as applicable to therainfall-runoff analysis. Bringing the data up to date with rainfall from 2005 to 2008may not affect the results, but would round off the study.

3.3 Rainfall-Runoff Analysis

The rainfall-runoff analysis of the upstream catchments has been carried out usingthe time:area method, which is appropriate for the analysis of single events whereno runoff discharge data are available for calibration. A water balance check hasbeen carried out for Gadhi catchment. The water volume computed on the basis of the derived flood hydrograph (hourly flood discharges) is in good agreement withthe computed water volume of on the basis of rainfall, catchment area and runoff coefficient. This check may be repeated for the other catchments of the rainfall-runoff model.

The holding ponds which could attenuate the runoff hydrograph and reduce thepeak have not been taken into consideration. The design of the holding ponds wasbased on total rainfall of 240mm in 4 hours. For the severe storms considered inthe NMIA analysis, their effect would be limited. As a conservative assumption,they are not included in the analysis.

Though it is not expected to have a significant impact on the tidal hydraulics of thecreek, for the sake of completeness, the developed airport will be included in therainfall-runoff analysis, and presented in the full report.

3.4 Tidal Data Analysis

It would be useful for comparison among the tidal water gauges to present themoving averages of the water levels over 25 hours (two tidal cycles) and 175 hours




(spring-neap cycle) to filter astronomical effects and visualise the longitudinal river gradient, starting from Mumbai Port. It will also serve as a check on the datums of the gauges (see for example the figure below).

0.0

1.0

2.0

3.0

4.0

5.0

6.0

15 /0 5 0 0: 00 2 0/ 05 00 :00 25 /05 0 0: 00 3 0/ 05 00 :0 0 04 /06 00 :0 0 09 /06 00: 00 14 /06 0 0: 00 1 9/ 06 00 :0 0 24 /06 0 0: 00 29/ 06 00: 00 0 4/ 07 0 0: 00

W a t e r L e v e l ( C h

a r t D a a t u m )

Tidal Predictions from MIKE C-Map at Revas Bandar

showing Filtered Diurnal Tidal Variation

CWPRS advises that the upstream discharges in both Panvel and Thane Creek aresmall relative to the tidal flux, and figures will be presented in the report todemonstrate this for Panvel Creek (Thane Creek is outside the present scope, andoutside the model domain). Panvel and Thane Creek may be considered to be wellmixed vertically, and though no salinity measurements have been taken, salinestratification is not expected to have an effect on the tidal hydraulics.

3.5 One Dimensional Model

3.5.1 Introduction

The one dimensional modelling software applied is Charima, developed by Prof FMHolly at Iowa Institute of Hydraulic Research, University of Iowa. The modelincorporates the full hydrodynamic flow equations for the conservation of mass andmomentum, as is appropriate for both the tidal reaches and the upstream rivers.The software is not supported, and does not have options for structures such asbridges.

The model was established for an earlier study of the 1992 flood event, and wasvalidated for the Gadhi River reach between NH4B bridge and Expressway bridgefor the July 1991 flood event, when the 24 hour rainfall in the upstream catchmentwas over 400mm.




3.5.2 Calibration

For calibration, the downstream boundary is the record at the MDL Jetty in ThaneCreek. For predictive runs, the downstream boundary is synthesised from Apollo

Bundar with an addition for the upstream gradient (according to CIDCO).

For calibration, the upstream runoff was taken as zero. CWPRS has confirmed thatfor the calibration of the Panvel Creek reach during the pre-monsoon period, usingthe tidal data collected from 15th May to 15th June 2007, due to a delayedmonsoon there was no rainfall, which normally begins in the second week of June.The flow in the five rivers discharging into Panvel Creek was practically zero.Upstream dry weather runoff could have been estimated from the population,present and future. CWPRS consider this would not be significant for the hydraulicanalysis. It may be more relevant if water quality analysis is required for the EIA.

The calibration of the model shows lower simulated water levels (approximately0.3m) and earlier peaks and troughs downstream. The difference at the criticalmaximum tidal water level is more in the order of 0.1m lower than observed. Thetime axis on the plots comparing simulated and observed should show the calendar time, rather than the relative time. The calibration shows four days around springtide. In the full report, CWPRS will show the calibration for the four days aroundspring tide, and four days around neap tide. Consideration may be given toshowing the full month of observations, and summary statistics prepared.

CWPRS advise that owing to low velocities, the water levels were found not to besensitive to changes in the bed roughness, and no adjustments were made to thebed and bank roughness (transverse and longitudinal) to improve the calibrationobtained in 1991. The differences at low water level between the simulated andobserved may be partly owing to lack of detailed resolution of the rocky bathymetry.

The tidal gauge installed by CWPRS at Waghiwali appears to have subsided owingto lack of a solid foundation, and has not been used in the calibration. This isunfortunate, as it represents a key location in the looped section of the creek.

The discharge data at the three upstream gauging stations did not capturerainstorm runoff, and have therefore not been useful for the study. CWPRS willshow the simulated discharge and water level hydrographs at key locations on therivers upstream in the full report.

Strictly, bridge losses should be simulated as structural controls replacing themomentum equation with an appropriate bridge loss formula, rather than using aconstricted cross section. This will not have a major impact on the water levelsupstream, and probably no impact downstream. This is a software limitation, but

the calibration of the water levels upstream of bridges appears reasonable.

CWPRS has informed that separate studies were carried out earlier to assess theafflux in Gadhi River upstream of NH4B bridge, without and with the bridgesimulating the clear waterway of the bridge. Validation of the model for the July1991 flood showed that the estimated water levels upstream of the bridge near Panvel port, SH54 bridge, NH4 bridge and New Panvel were in good agreementwith the observed flood levels, and that the water levels downstream of the bridgeremain unaffected.

3.5.3 Impact Assessment

The longitudinal plots should show the maximum and minimum water levels, or upper and lower envelopes of water level, and bank levels. While not affecting the




overall picture, it is suggested that “smoothing” the locus of the plots in thelongitudinal profiles is not entirely appropriate.

The plots show a 4m head loss over 650m horizontal distance (figure 18). CWPRS

explain that in this reach the bed levels increase by about 2m, there are constrictingrail and road bridges, and the bed has uneven rock outcrops which offer highroughness. The details will be examined for the full report, as will the largedrawdown of around 0.5m followed by a sudden drop of 1.7m at around 4km (figure20).

The report states that there is negligible change in the water levels in Panvel Creekwith the airport reclamation. While this may not appear on comparativehydrographs and longitudinal sections of maximum water levels with and withoutthe airport, this will be illustrated with a table comparing the maximum water levelsat key locations with the left and right bank levels in the full report.

The maximum ebb velocities appear to be higher than the maximum flood

velocities, indicating that sedimentation may not be a problem. Also the velocitiesare relatively low.

3.6 Two Dimensional Model

3.6.1 Introduction

The two dimensional model in plan has been set up with MIKE 21, which has aconsistent development history over 40 years by DHI Water Environment Health.The tidal water levels predicted from the Charima model and the MIKE 21 modelare in close agreement (figures 12 to 14).

3.6.2 Model Domain

For orientation, the model domain should be shown on a geographical background.Rather than a boundary along the centre line of Thane Creek, a tidal boundaryacross Thane Creek from the MLD Jetty and including Thane Creek upstreamwould have given a larger area in which to demonstrate any far field effects of thereclamation, particularly with respect to morphological impact. Upstream, ThaneCreek narrows, and CWPRS has explained that the upstream catchment area, abranch of the Ulhas River, is small and there is little inflow. This boundary may betaken as zero flow.

It may be reasonable to assume that, considering that Panvel creek is relatively

small compared to Thane Creek, hydraulic conditions in Thane Creek areindependent of flows in Panvel creek. Nonetheless, while hydraulic impacts aretransient, morphological impacts may accumulate over time. Initial model test runscould be carried out using the early bathymetric survey of Thane Creek, which isbeing used for the physical model.

The grid size of the model is 25m (N-S) x 50m (E-W). This gives a higher resolutionin the N-S direction than the E-W direction. Since the primary flow directionchanges with the alignment of the creeks, it would have been better to interpolatethe bathymetry on a square 25 x 25m grid.

The initial condition applied to all runs was a uniform high tide level. The roughnessvalues applied were the same as used in the one dimensional model. Since the

calibration was found satisfactory, and owing to low velocities the levels were not




sensitive to roughness values, no adjustments were made to improve thecalibration.

3.6.3 CalibrationAs for the one dimensional model, for calibration from mid May to mid June theupstream river inflow was taken as zero. CWPRS advise that the 2007 monsoonarrived late, and there was no significant rainfall during the calibration period.Upstream dry weather runoff could be estimated from the population distribution,present and future. This may be more relevant if water quality analysis is requiredfor the EIA.

The Gadhi River reach between NH4B bridge and Expressway bridge wascalibrated with data of 1992 with high flows from upstream, and subsequentlyvalidated for the high flood of July 1991 (CWPRS Technical Report No 3815 of August 2001), as well as the 26th July 2005 extreme rainfall event.

For calibration, the downstream boundary is the record at the MDL Jetty in ThaneCreek, assumed to be valid along the centre line of the creek upstream. For predictive runs, the downstream boundary is synthesised from Apollo Bundar records, with an addition for the upstream gradient (according to CIDCO).

3.6.4 Impact Assessment

The two dimensional plots in figures 27 to 33 show instantaneous water levels.They plots presented in the full report will show the maximum water level, which willnot occur at the same time at all locations.

Figures 27 and 28 indicate a significant increase in the water level upstream on the

Kasadi River for the 100 year return period with the airport development. CWPRSadvise that both one and two dimensional models show the same rise in the water levels in this reach, primarily owing to the rise in the bed levels, road and railwaybridges, and urban and industrial developments along the upstream narrow reachconfining the flow of Kasadi river. This may be further investigated, as the channeldimensions are not changed by the airport reclamation.

In figures 37 to 39, the ebb water levels are lower with the airport, presumablybecause the airport reclamation removes a significant high tide storage area, andthere is less water to drain on the ebb. This will be discussed in the full report.

To assist in determining the safe grade elevations for the airport, a plot should showthe maximum water levels around the airport boundary. It is suggested that this

together with the recommended freeboard should be the defining output of theCWPRS study, leaving the airport designers to determine the levels within theairport reclaimed area commensurate with balancing cut and fill, and stormwater drainage. CWPRS will give general guidelines for storm water drainage design.

3.6.5 Safe Grade Elevation

Local flood embankments along the Gadhi River and a wider diversion for the UlweRiver could lower the overall safe grade elevation for the airport, if this is desirablefrom a construction cost viewpoint.

CWPRS considers that lowering the safe-grade elevation and provision of a floodembankment will involve protection and maintenance of the embankment, and

present a constant threat of flooding from a breach in the embankment, though thisneed only be a low embankment. Secondly, the embankment along the periphery




would obstruct the storm water flow (surface runoff) to the creek, though the runoff could be directed in a westerly direction to Panvel Creek. Lower storm water drainage bed levels would imply a shallower gradient and a wider drain. Provision

of holding ponds would complicate the operation of the storm water drainagesystem.

In view of the importance of safety of the international airport and allied facilities,while reclamation to a safe-grade elevation above the predicted surrounding highflood levels may involve a high initial cost, long term maintenance costs will belower. In the opinion of CWPRS, maintaining safe grade elevations above thesurrounding maximum predicted water level is appropriate.

3.7 CWPRS Conclusions

CWPRS suggest that there is an overall trend of rising rainfall in the area. Study of the yearly maximum 1-day rainfall and yearly maximum intensity at Santa Cruz(1950 to 2005) and Colaba (1901 to 2004) (Tables I, II and VI) shows that there areincreasing instances of high one day rainfall in the past 30 to 40 years. Also theSanta Cruz record of intensities clearly indicates a rising trend of intensities. Thiswill be set out in the full report.

The July 2005 event was applied uniformly over the entire catchment. Therainstorm recorded at Santa Cruz while not recorded at Colaba was recorded assevere within the Panvel Creek area. Therefore it is justified to consider the casewhere the Santa Cruz event is distributed over the catchment as the most severecase.

The frequency analysis excluded the July 2005 rainstorm event at Santa Cruz. This

is reasonable when viewed alongside the fact that the impact of the airportreclamation was tested applying both the July 2005 event and the ProbableMaximum Precipitation determined by IMD in the model simulations.

The new cross section data do not differ significantly from the earlier survey. Hencethe model was simply validated against limited high flood levels from the July 2005event. The previously ascribed roughness values were found satisfactory, and nochanges were made.

With regard to the freeboard for the airport safe grade elevations, there is no historyof tsunamis in the Arabian Sea, nor until November 2009 of cyclone induced surgesin the water level striking the coast. Cyclone induced surges most frequently strikeeastern continental margins.

The impact of the airport on maximum velocities should be investigated. A twodimensional plot will be prepared showing the impact, ie the difference in maximumvelocity between the with and without airport cases.

The general procedure proposed by BIS for the design of the bank protection workshas been adopted: a more elaborate description and references will be given in thefull report.

The airport reclamation will not increase flood levels upstream. Floodembankments are required under existing conditions. The analysis has beenconducted assuming flood embankments upstream, as a conservative assumptionfor the with airport condition.

Design cross sections of the diverted channels will be provided in the full report.




4 OBSERVATIONS OF REVIEWER

4.1 General

The study of the impact of the proposed airport reclamation on the hydraulics of thePanvel Creek and upstream rivers by CWPRS has overall been carried out in athorough and competent matter. Improvements can be made to the draft report byway of presentation and discussion of the results and impacts.

As the impacts are small and do not show on plots, tables clearly showing theimpact of the airport reclamation on the hydraulics of the river channels upstreamand the creeks downstream will be presented in the full report. Longitudinal profilesas well as hydrographs showing the impact will be presented from the onedimensional model. Longitudinal profiles will be presented as maxima rather than

instantaneous in the full report.

The bathymetry of the two dimensional model will be plotted in the full report, for theexisting and with airport reclamation conditions. For the two dimensional model, theresults will also be presented as maxima, and as impacts. For the latter, grid outputmay show the difference in maximum water levels and velocities in the modeldomain with and without the airport. Tools for these calculations are available inMIKE Zero.

4.2 Diverted Channels

The diverted Gadhi and Ulwe Channels will run through rocky areas where potential

for replanting mangroves is limited. Both CWPRS and the Reviewer are of theopinion that the existing alignment of the Gadhi Channel should be retained throughthe fine sediment so that the mangroves can be saved and restored in this area.Consideration may also be given to preserving the alignment of the Ulwe Channelthrough the reclaimed area, with its mangroves, though CWPRS does not subscribeto this view.

4.3 Morphological Impact

Assessment of the impact of the airport reclamation on the morphology of Panveland Thane Creeks was not part of the scope of the CWPRS study, and scant dataare apparently available of the morphology of the creeks. Nonetheless, themorphological impact should not be neglected.

While the immediate impact of the airport reclamation on the tidal hydraulics may besmall, the accumulated impact on the morphology of Panvel and Thane Creeksover several years may be significant. There is some evidence that sedimentationin Thane and Panvel Creeks is not a major issue, eg the requirement for maintenance dredging for navigation access channels to the ports along ThaneCreek is low, once every few years, and that there is little visual evidence of sediment deposited in Panvel Creek.

Hard evidence should be obtained to support this. Information should be obtainedfrom ports regarding their dredging operations, in respect of quantity and location.

Information should be sought regarding the concentration of sediment over at leastone spring tidal cycle. If not available from published sources, a campaign should




be mounted to survey concurrently the water levels, velocities and sedimentconcentrations in the creeks.

Assumptions have been implied in the terms of reference for the CWPRS study

which need to be tested with a model covering Thane Creek. The domain of thetwo dimensional model should be extended to the mouth of Thane Creek to enablea comprehensive analysis of the sedimentation of both Thane and Panvel Creekswith and without the airport. Special techniques may be applied to extrapolatemodel results over decades. This will remove any uncertainties regarding thefurther field and long term effects of the airport reclamation on the hydrodynamicsand morphology of the area.

CWPRS is of the opinion that the Panvel Creek flow conditions are governed by theconditions in the Thane creek outside the mouth of the Panvel creek, and there willbe no effect of change in the flow conditions in Panvel creek on the flow conditionsin Thane creek. The studies indicate that the change in the flow conditions in

Panvel Creek subsequent to development of the international airport is marginaland not likely to affect the far field conditions in Thane Creek.

4.4 Physical Model

The proposed physical model of Thane Creek extended to include Panvel Creek willnot yield useful results for hydraulic and environmental analyses of the airport landreclamation. The present analysis has rightly focussed on mathematical modelling,and further more detailed investigations should extend the present study, and notincorporate physical modelling.

The physical model covers the entire Thane Creek up to the Ulhas confluence north

of Panvel Creek. The downstream boundary of this model is far away in the mainsea to the south west. CWPRS is of the opinion that this could be useful to studythe impact of the airport reclamation on the hydraulic flow conditions in ThaneCreek, if any. The reviewer considers it will be impossible to include morphology inthe physical model.

For demonstration of the results of the mathematical modelling to lay persons, threedimensional computer graphics based on the model results (using readily availablesoftware) should be utilised. This may be enhanced to include surroundingtopography, buildings, trees, etc for visual effect.

annexure--vb

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