Flood Risk Management Report March 2016

53 Highgate Street, Stratfield

J. No. 3024

1

Kozarovski and Partners

14/52 Kingsway

Cronulla NSW 2230

0412 997767

pavelk@optusnet.com.au

Date: 9 March 2016

Flood Risk Management Report for

No. 53 Highgate Street, Stratfield

Overview A relatively small catchment with an area of 3.54 ha gravitates to Highgate Street. The catchment can

generate some 3.95m3/s during the 100 year flood event. The draiange systems have been designed to

convey the runoff during the 1 in 5 year storm. It was assumed that draiange system would be

operating at 50% capacity. The 1 in 100 year overland flow is therefore: 3.95 – 1.73/2=3.08 m3/s.

The corresponding to the 100 year flow depth of 240 mm was calculated using Izaard’s equation.

Most of the 100 year flow would be contained within the street with only a portion of the front yard

being inundated by the 100 year flood. The extent of flood and the estimated flood levels are shown on

Drawing C-3024-01.

The minimum floor level of the development would have to be set 500 mm above the 100 year flood

level. The relevant flood level is at 31.29 m AHD and the minimum Floor level is 31.79m AHD. The

adopted floor level is at RL 32.0 m AHD which is higher than the PMF level.

The minimum hump level protecting the basement from flooding must be set 300 mm above the 100

year flood level. The relevant flood level is at RL 31.14 and the minimum hump level should be set at

RL 31.44 m AHD. The adopted hump level is at RL 31.50 m AHD which is higher than the PMF

level.

Categorization of the Flood Risk The subject site is partially affected by the 100 year flood, with low hydraulic hazard and it is classified to be in a medium flood risk precinct.

Floor level The proposed floor level is at RL 32.00 m AHD which is more than 500 mm above the 100 year flood

level.

Flood impact The proposed dwelling will not have any impact on flood behavior. The existing dwarf wall between

No. 51 and No. 53 will be removed, increasing the conveyance. No flood storage will be lost due to

the development

Evacuation Evacuation from this site would not be required because the proposed floor level would be above the

PMF level.

Building material The proposed dwelling would have to be constructed form flood compatible materials up to RL 32.00m

AHD.

Flood warning/signage The flood depths and velocities through the site are negligible and flood warning signage is not

required.

______________

Flood Risk Management Report March 2016

53 Highgate Street, Stratfield

J. No. 3024

2

Pavel Kozarovski, MIEAust, CPEng, NPER-3

Appendix A

Catchment Map

Rational Formula Calculations

Kerb Flow Calculations

Flood Risk Management Report March 2016

53 Highgate Street, Stratfield

J. No. 3024

3

Flood Risk Management Report March 2016

53 Highgate Street, Stratfield

J. No. 3024

4

Kinematic Wave Time of Concentration - Rational Method

Rainfall Grid Reference : Strathfield

Average Recurrence Interval (ARI) years : 100

Catchment Area (Ha.) : 7.54

Impervious Area (%) : 75.0

Gutter Flow (mins) : 0.00

Length (m) Slope (%) Roughness

300.0 2.40 0.030

1 in 5 y ARI

Co-efficient "C" 0.76

Intensity (mm/hr) 108.8

Time (mins) 12.2

Runoff (l/s) 1730

1 in 100 y ARI

Co-efficient "C" 0.96

Intensity (mm/hr) 196.5

Time (mins) 9.6

Runoff (l/s) 3946

Q=0.375F(Z1/n1(d1^8/3-d2^8/3)+z2/n2d2^8/3))*S^0.5 (Izzard's equation)

F=0.9 for triangular channels and = 0.8 for composite channels as the one drawn

n= Maning's n

S=Slope m/m

Catchment Area 3.54 ha

Q100y= 3.95 m3/s

Q5y= 1.73 m3/s

Q100y_overland 3.085 m3/s Long. Slope 0.023

H (m) Qsingle_kerb

Q_2 kerbs PMF 18.2075 m3/s

Z1 10 0.1 0.163604 0.327207 100 y Depth 240 mm

n1 0.025 0.12 0.260952 0.521905 PMF depth 480 mm

Z2 33.3333 0.14 0.386896 0.773791

n2 0.025 0.16 0.544124 1.088248

F1 0.9 0.18 0.735228 1.470456

F2= 0.8 0.2 0.96271 1.92542

b 0.5 0.22 1.228992 2.457985

B 4.5 0.24 1.536427 3.072854 100y

d1-d2 0.05 0.48 9.219722 18.43944 PMF