ewr stormy water solutions by HC12021016480


									                     1 – 9 Lygon Street, Brunswick
       Amendment C81 to the Moreland Planning Scheme

                               Expert Witness Report

  Flood Flow Implications of Site Development

                                         17 September 2009

Report by:        Valerie Mag, B.E. Civil (Hons), M. Eng. Sci.
                  Stormy Water Solutions
                  Ph 9511 5911, M 0412 436 021

 Proud to be the winner of the Stormwater Industry Association 2008 State Excellence Award for Education awarded for the
          design, development and presentation of over 40 Drainage Design and WSUD Courses (2004 – 2008)

1.     STATEMENT OF EVIDENCE...................................................................................... 1

2.     SITE DESCRIPTION AND FLOOD IMPACT ........................................................... 3

3.    HYDROLOGICAL ANALYSIS REVIEW ................................................................... 5

4.     100 YEAR ARI FLOOD IMPLICATIONS .................................................................. 6

     4.1    DEVELOPMENT EFFECT ON LOCAL FLOOD LEVELS ..................................................... 6
     4.2    PROBABLE REQUIREMENTS FOR GROUND FLOOR LEVEL DESIGN ............................... 6
     4.2    PROBABLE REQUIREMENTS FOR BASEMENT ENTRY DESIGN ....................................... 8
     4.3    SITE AND ACCESS FLOOD SAFETY ............................................................................... 9

5.    CONCLUDING REMARKS ........................................................................................ 10

6.    ABBREVIATIONS ........................................................................................................ 11

1.       Statement of Evidence

I am the author of this report. My name is Valerie-Joy Sally Mag. I am a Civil Engineer and Principal of
Stormy Water Solutions. I practice as a consulting hydraulic and hydrologic Engineer.

My educational qualifications are as follows:
        Bachelor of Civil Engineering, Monash University
        Master of Water Resources and Environmental Engineering, Monash University

I have twenty years experience and expertise in hydrologic and hydraulic engineering, particularly in the
areas of:
        Preparing complex urban and rural flood plain strategies,
        Major catchment analysis, including flood flow and flood level estimation,
        Planning and assessment of development within flood plain and overland flow path systems,
        Regularly preparing and conducting training in drainage and WSUD for the Municipal Association of
         Victoria, Vic Roads, Melbourne Water, the Department of Tourism Arts and the Environment
         (Tasmania) and others.

As part of the preparation for this report I inspected the site (1 – 9 Lygon Street, Brunswick) on 21 August
2009. This site inspection included making estimates of active flow path cross sections and hydraulic
roughness. I also visually assessed the overland flow path dynamics and probable obstructions.

I have also reviewed the following material:
        The original amendment request submitted to Council April 2007;
        The draft Urban Context Report, Peddle Thorp, October 2007;
        Concept Design Plans, Peddle Thorp, May 2008;
        A Melbourne Water Corporation letter to Moreland City Council, March 2009
        Council Officer Report, July 2009;
        Various MWC drainage design plans (Design Set 4415);
        Exhibited amendment documentation.

My report is also based on:
        My extensive experience in prescribing flood protection conditions on similar developments in
         Victoria during my ten year tenure as Senior Hydrologist within the Planning Section of Melbourne
         Water’s Waterways and Drainage Section;
        Melbourne Water Corporation’s (MWC’s) current flood mapping information;
        The existing MWC 0.5 metre contour information in the area of interest;
        Verbal advice from Taylors Surveyors that the existing building flood level is 38.07 m AHD;
        Hydrological and hydraulic investigations conducted by myself utilising the probabilistic Rational
         method to assess flood flows, and Mannings Formula to assess flood levels and underground asset
         capacities. These calculations were used to determine:
              o   The suitability of the current MWC recommended design flows and flood levels,
              o   The possible flood impact on site in relation to the development proposal, and
              o   The possible impact of site development on local flood levels.

This report specifically addresses the overland flow issues regarding the Lygon Street Main Drain overland
flow. The property is also serviced by local Council drains. However all of the council drains in the area are
assumed to drain directly to Lygon Street Main Drain. As such, the council system and Melbourne Water
system can be assumed to act a one lumped drainage system in the area of interest during major flood
events (greater in magnitude than the 5 Year ARI event). As such, this report specifically addresses the
consequence of site proposals due to the Lygon Street Main Drain overland flow effect.

I have had no previous involvement in the site. I have some knowledge of the hydraulics and the hydrology
of the Lygon Street Main Drain though my previous experience at MWC.

In summary, I conclude that the site is flood prone and affected by the Lygon Street Main Drain overland flow
path in extreme flood events.

The proposed site development will not adversely affect flood levels upstream or downstream of the site. In
addition, provided the detailed design accounts for Melbourne Water ground floor level requirements, it can
be developed to ensure it’s own flood protection during the 100 Year ARI flood event.

I have made all the inquiries that I believe are desirable and appropriate and that no matter of significance,
which I regard as relevant, have to my knowledge been withheld from the Panel.

Valerie – Joy S Mag, 16 September 2009
B.E. Civil (Hons), M. Eng. Sci.
10 Paxton Drive
Glen Waverley 3150
2.      Site Description and Flood Impact
The subject site is located on the south western corner of the intersection of Brunswick Street and Lygon
Street. Melbourne Water Corporation (MWC) have flood mapped the area in question. Figure 1 shows the
estimated 100 Year Average Recurrence Interval (ARI) flood extents in the area.

Figure 1         100 Year ARI Flood Extent
                 Melbourne Water Corporation Flood Mapping Information

In events in the order of the 5 Year ARI flood event, flood flows should be conveyed within the underground
drainage system (both MWC and Council’s drainage systems). In events of greater magnitude, surcharge
flows engage the road system. Overland flow will occur along the roads conveying flow, in general, in a
south easterly direction.

The flood extent will affect properties adjacent to the active (road) flow paths. However, in general the MWC
calculated flood levels probably assume that the surcharge flow affecting the properties is inactive. In
essence, it is assumed that the buildings, fence lines etc associated with the properties severely restrict the
flow though the properties.

This is the case for the subject site. The existing building occupies the entire site. As such, no flow can be
conveyed within the subject site itself. All flow will be conveyed within Brunswick Street and Lygon Street in
the extreme event. However, backwater effects may result in ponding over the site.

Directly downstream of the site is the Park Street linear park. Lygon street rises about 0.5 metres (compared
to the level adjacent to the subject site) at this point. This rise in road level probably directs overland flows
around the southern boundary of the site. Overland flow crosses the linear park directly south of the subject
site before engaging Lygon and Park Street again south of the area of interest.

Melbourne Water have advised the following in respect to the subject site:
       The 100 Year ARI flood level for the subject site is 38.2 m AHD. This is assumed to be the flood
        level along the northern portion of the property,
       The applicable flood depths are in the order of 100 – 200 mm above the natural surface level within
        the site, and
       All finished floor levels, basement car park windows, windows, vents, openings, entry/exit driveways
        and ramp apexes must be 300 mm above the applicable flood level (i.e. 38.5 m AHD).

This report outlines an independent hydrologic analysis which assesses the overland flow path issues which
MWC have highlighted.

The following tasks were completed as part of the analytical process:
       A review of the adopted MWC 100 Year ARI (Average Recurrence Interval) design flows in the area
        of interest,
       A review of the adopted MWC 100 Year ARI design overland flow path flood levels,
       Determination of the expected off site impact on local flood levels given the site proposals, and
       Determination of the probable ground floor and basement carpark requirements given the MWC
        requirements detailed above.

3.       Hydrological Analysis Review
MWC have indicated that the overland flow used to assess the site flood impactions is 10.0 m /s. They also
advise that the asset capacity (i.e. the underground box culvert capacity) is 4.9 m /s resulting in a 100 Year
ARI overland flow of 5.1 m /s.

Appendix A details a hydrological review performed by Stormy Water Solutions to assess if the above flows
and flood levels are reasonable. The results of the analysis are detailed below.
        A Mannings formula check on the 0.9 by 1.8 m box culvert (Lygon Main Drain adjacent to the site)
         has confirmed that the assumed capacity is 4.9 m /s,
         A rational method review of the design flows indicate that a 100 Year ARI design flow of 10 m /s is
         realistic, and
        Given an approximate natural surface level of 37.5 m in the Park Street reserve directly downstream
         of the site, flood levels over the site of 38.0 m AHD (at the southern boundary) and 38.2 m AHD at
         the northern boundary are realistic.

As such, it is considered that the MWC flood mapping in the area is a reasonable representation of the flood
effect within and around the subject site during the 100 Year ARI flood event.

4.      100 Year ARI Flood Implications

4.1     Development Effect on Local Flood Levels

The existing building occupies the entire site. Under current conditions no flow can be conveyed within the
subject site itself. However, the site may be affected by backwater ponding. All active flow (flow exhibiting
measurable velocities) will be conveyed within Brunswick Street and Lygon Street in the extreme event.

It is assumed that the MWC flood level has been set assuming this constriction. Replacing the existing
building with a new building occupying the entire site footprint will not change this situation. As such, the new
building will not affect flood levels upstream or downstream of the site.

If the detailed design results in a connection between Brunswick Street and Park Street being constructed
along the western boundary of the site, flood levels may actually reduce slightly. This is because active
surcharge flows will now have a greater cross section to occupy.

4.2     Probable Requirements for Ground Floor Level Design

Verbal advice from Taylors Surveyors (17/9/09) indicates that the existing floor level is constant over the site
and is 38.07 m AHD. Therefore, the existing floor level is about 130 mm below the 100 Year ARI flood level
as defined by MWC.

The MWC flood mapping information is based on survey obtained for the flood mapping project in the late
1990’s. This survey can be a combination of detailed survey information (primarily in the road reserves) and
aerial survey to fill in information within properties. As such, the information within the roads is usually
relatively accurate. This survey information (as documented by MWC flood mapping plan) documents a flood
depth in both Lygon Street and Brunswick Street adjacent to the site of 400 mm.

The new building will require a ground floor level higher than the existing floor level. The new floor level must
be constructed to 38.5 m AHD. This will provide 300 mm freeboard to the 100 Year ARI flood level. This level
is 430 mm above the existing floor level.

As the ground floor level may have implications in regard to final building height (depending on the number
of floors) the design team should fully assess the implications of this requirement early in the detailed design
stage of the project.

Figure 2        Existing Floor Level Relative to Lygon Street (Left) and Brunswick Street (Right)

MWC have expressed concern about incorporating an active street frontage in streetscapes subject to
flooding in extreme events. However, as shown in Figure 3, active street scapes can accommodate shop
access and appropriate floor levels without impacting on the building use. It is anticipated that the design
team will adopt simular treatments at the detailed design stage of the project to protect any future shops (or
simular) from danger to occupants or stock damage.

Figure 3         Possible Active Street Front Floor Level Treatments for Flood Protection

4.2      Probable Requirements for Basement Entry Design

Preliminary design plans indicate that access to an underground carpark is proposed along the Brunswick
Street frontage. Again, this can be achieved if entry driveways incorporate ramps which exhibit an apex (i.e.
highest point) 300 mm above the applicable flood level (i.e. Apex of ramp = 38.5 m AHD).

This type of design occurs quite regularly in design of buildings within flood prone areas. Of course, height
clearances and ramp slope requirements must be clearly understood be the building design team so that this
aspect of the design can be incorporated well. In general though, it is considered that a ramp design is not
required until the detailed design stage of the project.

Figure 4 shows how access ramps can be accommodated to provide adequate flood protection to basement

Figure 4         Possible Basement Ramp Entry Treatment for Flood Protection
                 In this example the apex of ramp is outside the building and is about 700 mm above the
                 footpath level

4.3     Site and Access Flood Safety

As defined by the Melbourne Water Guidelines for Development in Flood Prone areas, the existing site is
defined as a having a low flood hazard risk. For example, if it is assumed that the existing 100 Year ARI
flood depths within the property are 130 mm (maximum depth as currently advised by MWC (d av)) exhibiting
low velocities (0.3 m/s assumed (Vav), probably much less than this), the corresponding velocity/depth
criteria (Vav × Dav) is 0.04 m /s. As such, new development on the subject site will not create a flood risk
hazard defined by a velocity depth criteria of 0.35 m /s.

In the roads, flood velocities will be greater. Assuming a road width of 20 metres (which is, conservatively,
less than the road reserve width) and a flood depth of 0.4 m (as defined by MWC), the active flow area will
                      2                                                                                 3          2
be in the order of 8 m . This results in a flow velocity in the 100 Year ARI event in the order of 5.1 m /s/ 8 m
= 0.65 m/s. The corresponding velocity/depth criteria (Vav × Dav) is 0.26 m /s. As such, even if pedestrians
are within the road reserve during the 100 Year event, although they will get wet, they should be able to walk
and transport themselves to safety.

Given the above, it is considered that the proposed use of the site will not place people using the site or
passing the site in adverse danger during a 100 Year ARI flood event.
5.       Concluding Remarks
The analysis contained within this report indicates that:

        The existing flood levels and flood flows predicted by MWC in the area of interest are reasonable,

        The proposed building on the subject site will not increase local 100 Year ARI flood levels, and

        The detailed design will be required to ensure that the ground floor level and the apex of the
         basement access ramp are constructed to at least 35.5 m AHD to ensure adequate flood protection
         in the 100 Year ARI flood event.

6.          Abbreviations
Table 6 lists some common abbreviations and drainage system descriptions and their definitions which are
referred to in this report.

Table 6           Common Drainage Abbreviations
Abbreviation                  Definition
AHD - Australian Height       Common base for all survey levels in Australia. Height in metres above mean sea
Datum                         level.

ARI - Average Recurrence      The average length of time in years between two floods of a given size or larger
Hectare (ha)                  10,000 square metres
Hydraulic Roughness           The relative resistance to flood conveyance along a drainage conveyance
                              system. For instance, a flow path with obstructions is rougher then a flow path
                              without obstructions. Increasing hydraulic roughness is modelled by increasing
                              the “Mannings n” in Mannings Formula.
Kilometre (km)                1000 metres
m /s -cubic metre/second      Unit of discharge usually referring to a design flood flow along a stormwater
                              conveyance system

MWC                           Melbourne Water Corporation
Mannings Formula              Hydraulic formula used to calculate flood depths (usually to AHD) and extents
                              given a flood flow
Rational Method               Hydrological formula used to calculate flood flows (usually to m3/s) given rainfall,
                              catchment and runoff coefficient data

                                              APPENDIX A

Confirmation of Asset Flow Capacity
MWC advise Asset Flow is                        4.9 m3/s

Is this reasonable?

MWC design plan 4415/2/4
             Original design plan
             No date, however very old and dimensions in feet and chains

Box culvert, approximately 3 foot by 6 foot
                .=                  0.9 m by               1.8 m

Confirmed by MWC drawing 4415/3/6 - Lygon Street rehabilitation works
These works are downstream of the site but appear to connect to original drain
in Lygon Street
Rough section of original drain appears to match the above dimensions.

Fall 0.25 feet per chain

              1 chain =            100 links
           4.97 links =              1m

              1 chain =           20.1 m
           0.25 feet =            0.08 m

HGL rise before flow into road
                             .=                0.08 m (say)

HGL slope approx =            0.007763         .= 1/       129


Width =                            1.8   m
Depth =                            0.9   m
Wetted perimeter =                 5.4   m
Area =                            1.62   m2
Hyd radius =                       0.3   m
slope =                       0.007763
Mannings n =                     0.013

Capacity =                        4.92 m3/s

Therefore, MWC asset capacity appears reasonable
given a slight HGL influence
Design Flow Confirmation

MWC advise Q100 =                                  10 m3/s
and 100 Year Overland Flow =                      5.1 m3/s

Rational Method                                        Q = CIA/360

Catchment Area =                    109.6 ha
Catchment is partially industrial and partially residential (or equivalent)

C5 =             0.6          (based on MWC C5 = 0.45 residential and 0.7 industrial)
C100 =           0.75         (based on MWC C100 = 0.6 residential and 0.9 industrial)

Time of Concentration

For small pipes in upper and mid catchment, typical pipe velocity =              1.2 m/s (typ)
                                                               for 1/250 slope)
Main Reach Length =                                                             2500 m
Time in pipes =                     35 minutes
Time to initiate runoff =             7 mins (MWC land development manual)

Therefore 5 year time of concentration =                    42 minutes (40 mins say)
100 Year time of concentration is 50% longer than this due to part flow in street and part flow in pipes

Therefore 100 year time of concentration =                    63 minutes
                                                       60 Minutes (say)

Design Intensities for Melbourne
I5 =                    31.4 mm/hr
I100 =                    48 mm/hr

Q5 =                      5.7 m /s
Q100 =                   11.0 m /s

Check - DSE flood regression curves
Rough 100 Year ARI flow in urban catchments = 10.29×A0.71 =
                                                                                    11 m /s

5 year flow is a close match to asset capacity
100 Year ARI flow adopted by MWC of 10 m 3/s is close to rational method check and
                 rough check with DSE flood regression curves

Therefore, MWC design flood flows appear reasonable
100 Year ARI Flood Level Confirmation
Rough check only - based on flood mapping contours provided by MWC (August 2009)

Flood level of subject site is a direct result of ponding of water the linear park reserve
Minimum Level of 37.5 m directly south of site

Conservatively assume control section in linear reserve over 35 m
100 Year HGL = 1/400 as in MWC flood mapping

Flood Depth at control                        0.5     m
Assumed NSL at control =                     37.5     m AHD MWC flood mapping contours
Width of flow at control section =            35      m (MWC flood mapping contours - conservative)
  HGL Slope                                 0.0025
    Area                                     17.5     m
Wetted Perim.                                35.00    m
      R                                      0.50     m
  mannings n                                 0.050    (grass and parkland)
   Capacity                                  11.02    m3/s

Therefore flood level at Control =                38.0 m AHD

This matches the flood level south of the site.

MWC applying a flood level of 38.2m AHD at the northern boundary of the site appears reasonable

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