Tonkin & Taylor Ltd - Environmental and Engineering Consultants, 43 Halifax St, Nelson, New Zealand PO Box 1009, Nelson, Ph: 64-3-546 6339, Fax: 64-3-546 7619, Email: [email protected], Website: www.tonkin.co.nz
T&T Ref: 871003 29 October 2012 Network Tasman Ltd
C/- McFadden McMeeken Phillips PO Box 656
Nelson 7040
Attention: Nigel McFadden
Dear Nigel
Flood assessment report for Network Tasman, Main Road Hope, Richmond
This letter report presents the results of our flood assessment for the Network Tasman Limited (NTL) site at 52 Main Road Hope, Richmond. Authority to proceed with the assessment was provided in writing by you on 16 October 2012. Tonkin & Taylor Ltd’s (T&T) proposal dated 12 October 2012 sets out the scope of work and conditions of engagement.
1 Background
An assessment of flood hazard at the NTL site has been requested by Tasman District Council (TDC), in support of NTL’s application for a private plan change. NTL is seeking the change to allow for further development of the site to meet their future needs.
Refer T&T Figure 1 in Appendix A for a plan of the site including legal boundaries and proposed plan change. Currently one out of the five NTL-owned lots at the site is covered by Schedule 17.5A of the Tasman Regional Management Plan (TRMP). It is proposed to expand Schedule 17.5A to incorporate adjoining land owned by NTL.
For the purposes of supporting this application, this flood assessment considers:
Present-day and future flooding at the site, due to overtopping of the Borck Creek and/or Reed/Andrews Drain channels;
The effect of development of the site on total runoff potential from the site during extreme rainfall events.
1.1 Location
Figure 1 in Appendix A shows the site and the secondary drainage network in the immediate
surrounds. The site is located in an area known as Richmond South, and sits at the confluence of the Reed/Andrews Drain and Borck Creek. The area is comprised of alluvial (floodplain) gravels and is generally flat, sloping to the north at an overall floodplain slope of approximately 1 m in 200 m. The
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site is bounded by SH6 (Main Road Hope) to the south east, and a slightly raised cycleway to the north-west.
1.2 Flooding history and drainage improvements
The Richmond South area has historic flooding issues. Most notably, the area was subjected to widespread flooding during a storm event in January 1986 (refer Figure 2 in Appendix A). The event produced approximately 150 mm of rainfall within 24 hours, which gives it an average recurrence interval (ARI) of 30-40 years. Since that event, Council has undertaken various studies and drainage network improvements. The most comprehensive of these studies are:
i. South Richmond Development Area Study – Stormwater Concept Design, MWH Jan 2006;
ii. Richmond Stormwater Land Designations – Stormwater Summary Report, MWH Sept 2009.
In order to provide robust flood protection for existing and future development in the Richmond South and Richmond West areas, Council has undertaken some network improvement works already, and designated land for further improvements. Network improvements already undertaken which most directly affect the Network Tasman property are as follows:
i. Construction of a culvert under SH6 and a new drain (extension of Reed/Andrews Drain) alongside the new Network Tasman building. This work was undertaken in 1998-99;
ii. Capacity improvements to Borck Creek, including a major channel realignment north of the White Road / Main Road Hope intersection.
Proposed future upgrade works are recommended in the MWH 2009 report, and allow for maximum probable development in the Richmond South/Richmond West areas, and the effects of climate change to the Year 2080. These works involve substantial widening of existing waterways. The upgrade of Borck Creek is currently scheduled for 2014-2022 in Council’s 10 year Long Term Council Community Plan (LTCCP), with works in other drains provided for in the 20 year plan.
2 Hydrology
2.1 General
The main watercourses with the potential to produce flooding at the Network Tasman site are Borck Creek and the Reed/Andrews Drain (refer Figure 1 in Appendix A).
A hydrological analysis of the Borck Creek and Reed/Andrews Drain is provided in the MWH 2009 report. The peak 1 in 100 year flow estimates have been computed using the Rational and Modified Rational Methods. We have reviewed the assumptions made in this report, and make the following observations and comments:
We note that the hydrological assessment and drain design has been made without the benefit of any gauged drain flows. Assumed catchment parameters can therefore not be validated. If drain upgrades are proposed over a number of decades, we suggest it would be sensible for Council to start monitoring Borck Creek catchment flows;
The 2009 report presents peak flows and proposed drain widths based on maximum probable development (65%) within the catchment. The current level of development is less than this;
The peak flows in the 2009 report are based on climate change forecasts to the Year 2080.
This is appropriate, and in accordance with current Ministry for the Environment (MfE) guidelines and the Council’s Engineering Standards 2008;
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2.2 Rainfall
The 1 in 100 year peak flow estimates in the MWH 2009 report were based on rainfall depth values given to TDC by Opus in a 2007 report. The Opus 2007 report considered present day (2007) and 2080 climate conditions and presented design rainfall depths for both scenarios. We have adopted these rainfall depths for our assessment, and used them to compute 1 in 20 year, 1 in 50 year and 1 in 100 year rainfall depth estimates. These estimates are presented in Table 1 below.
Table 1 – Design rainfall depths for Richmond (from Opus 2007 report) Average
recurrence interval (ARI)
(yrs)
Rainfall depth (mm)
Richmond 2007 Richmond 2080
1 hr 2 hr 6 hr 1 hr 2 hr 6 hr
5 32 46 71 37 52 80
10 38 53 85 44 61 97
20 43 62 98 49 71 111
50 49 73 114 46 83 131
100 54 80 127 62 92 145
2.3 Peak flow estimates
Table 2 below presents the peak flow estimates for the Reed/Andrews Drain alongside the existing Network Tasman site and Borck Creek immediately to the north of the Network Tasman site. Note that the Borck Creek flows include flow contribution from the Reed/Andrews Drain. Catchment areas, times of concentration, runoff and development parameters are as per the MWH 2009 report.
Refer Appendix B for full runoff calculations.
Table 2 – Design peak flows (m³/s) Average
recurrence interval (ARI) (yrs)
Reed/Andrews Drain adjacent Network
Tasman Limited site Borck Creek Drain on the northern boundary of the Network Tasman Limited site
2007 2080 2007 2080
20 16.4 20.7 37.9 45.4
50 18.7 23.0 44.5 53.5
100 20.6 25.7 49.9 58.9
3 Drain capacities
3.1 Existing drains
The capacity of existing drains has been assessed using TDC LiDAR data, supplemented with measurements taken during our site visit. The Reed/Andrews Drain and Borck Creek channels are uniform along the site, with approximate dimensions as presented in Table 3 below. The computer modelling software HEC-RAS v4.1.0 was used to model flows in the existing drains. The model was extended approximately 400 m upstream and 400 m downstream of the site to minimise the influence of assumed boundary conditions. Drain capacities are presented in Table 3.
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Table 3 – Assessment of existing drains Base width
(m)
Side slopes (1V:xH)
Typical depth (m)
Flow capacity
m³/s Approx. 1 in x year (present day)
Reed/Andrews Drain 2 1 1.4 8.5 3 - 5 year
Borck Creek 3.5 0.5 1.9 16 3 - 4 year
In addition, we note that a 3 m by 1.5 m concrete box culvert conveys flows in the Reed/Andrews Drain to the Network Tasman site under Main Road Hope (SH6). Assuming a heading depth of 0.2 m above the culvert soffit (i.e. up to road level) at the upstream end, the capacity of this culvert is approximately 10.3 m³/s, or about a 1 in 4 year present-day peak flow.
3.2 Proposed drainage network improvements
Our assessment above confirms that the drains currently do not provide the target level of service as set out in the TDC’s Engineering Standards. For major watercourses and secondary flowpaths (including Borck Creek and Reed/Andrews Drain), TDC have adopted a target level of service of passage of the 1 in 100 year flow with a minimum of 250 mm of freeboard (refer Sections 2.5 and 2.6, MWH 2009).
TDC proposes to carry out improvements to the Richmond South drainage network over coming decades, as outlined in the MWH 2009 report. The report presents design channels for the various watercourses in the Richmond South area, including Borck Creek and Reed/Andrews Drain.
We have checked the proposed watercourses in our HEC-RAS model, and confirm that the proposed watercourses provide a minimum of 250 mm freeboard above the Year 2080 1 in 100 year flood level. We note that the upgrade of the Reed/Andrews Drain will need to include upgrade of the culvert under Main Road Hope to increase its capacity by approximately 150% from 10.3 m³/s to 25.7 m³/s.
4 Flood extents and levels
As noted in the previous section, once the proposed channel improvements are implemented, the Network Tasman site will be protected from 1 in 100 year flows in Borck Creek and the
Reed/Andrews drain, and so long as suitable on-site drainage is provided, there is no flood hazard at the site in up to the 1 in 100 year event.
Council plans to implement the Borck Creek improvements over the next ten years, and the
Reed/Andrews Drain improvements over the subsequent ten years. Therefore, by 2032 the Network Tasman site is expected to be outside the 1 in 100 year floodplain, and remain that way until at least 2080, depending on the rate of climate change and effects on rainfall.
Between present day and 2032, the site is expected to be prone to some degree of flooding in events that exceed the capacity of the Reed/Andrews Drain and Borck Creek watercourses. These events are expected to occur once every 3 to 5 years on average.
In order to accurately predict flooding extents and levels, a full 2-dimensional computational model of the wider drainage network would be required. This level of modelling is outside the scope of this current flood assessment. Instead, the approach taken in this assessment has been to adopt
conservative (upper bound) peak flow estimates as presented in Table 2 above, and use LiDAR data to assess any likely outbreak points and flowpaths.
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4.1 Expected 1 in 100 year flooding
Flooding is expected to occur as follows:
Existing drains will fill to capacity. Flows down the Reed/Andrews Drain through the Network Tasman site will become increasingly controlled by the capacity of the SH6 culvert at the upstream end, and the rising water level in the Borck Creek channel at the downstream end.
Flows in excess of the capacity of the SH6 culvert (approximately 10.3 m³/s) are expected to flood property to the south of SH6, and flow in a north-easterly direction towards Three Brothers corner. In a present-day 1 in 100 year event, this would split the Reed/Andrews Drain catchment flows in half. That is, up to 10.3 m³/s would flow through the culvert at the peak of the storm, and 10.3 m³/s would spill across property south of SH6 towards Three Brothers corner;
High water levels in the Reed/Andrews drain are expected to overtop the right bank during the peak of the storm, at an existing low point in the left and right banks located adjacent to the existing bridged access road shown in Figure 1 in Appendix A. The peak outbreak flow at this point is expected to be in the order of 1.8 m³/s. LiDAR data indicates that the low point in the right bank at this point is at approximately RL 16.45 m, some 0.4 m lower than ground levels along the left bank at this point. The outbreak location is approximately 10-15 m wide. Some ponding is expected over the left bank within the existing cul-de-sac and carpark at Lot 1 DP 20183 (52 Main Road Hope). Flows over the right bank are expected to discharge along the existing carriageway through Lot 1 DP 435942 and the northern end of Lot 3 DP 435942, before flowing into Norman Andrews Place and beyond, as per the January 1986 storm event (refer Figure 2 in Appendix A). All other NTL lots are expected to be outside the 1 in 100 year flooding extents;
Borck Creek outbreak flows are expected to spill northward into the adjacent paddock and drain under the SH60 bridge. A cycleway is located along the north-western site boundary and provides a slightly elevated strip of land between Borck Creek and the Network Tasman site.
This elevated strip is expected to protect the site from any outbreak overflows, as it did during the January 1986 event.
4.2 Estimated 1 in 100 year flood levels
Assuming the expected flooding mechanisms described above, and allowing for TDC freeboard requirements for new buildings, the estimated 1 in 100 year flood levels and minimum floor levels are presented in Table 4. TDC Engineering Standards and Policies 2008 requires that all buildings (regardless of use) are built with floor levels a minimum of 0.5 m above the 1 in 50 year flood level.
Since flooding within the site is via overflows of the Reed/Andrews drain, and since the
Reed/Andrews drain flows are limited to less than the 1 in 50 year through the SH6 culvert, 1 in 50 year and 1 in 100 year peak flows and levels are expected to be the same. Hence, the minimum floor levels shown in Table 4 are 0.5 m above the estimated 1 in 100 year flood levels.
Note that flood and floor levels are presented only at selected locations across the site, and are indicative only. Flood and floor levels for any particular proposed new building location would need to be assessed specifically during design. This would require further hydraulic modelling.
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Table 4 – Indicative 1 in 100 year flood levels at key locations
Location Estimated 1 in 100 year
flood level
Minimum floor level for new buildings
Lot1 DP20183 (52 Main Road Hope) RL 17.3 m RL 17.8 m South-western end of Lot 1 DP435942,
adjacent to the Reed/Andrews Drain
RL 17.3 m RL 17.8 m
Cul-de-sac at the end of the right of way between Lot1 DP435942 and Lot3 DP435942
RL 16.5 m RL 17.0 m
Cul-de-sac at the end of Norman
Andrews Place RL 15.8 m RL 16.3 m
5 Future re-development of the site
The proposed expansion of the Schedule 17.5A boundary would affect a number of lots currently owned by NTL (refer T&T Figure 1 in Appendix A). With the exception of Lot 1 DP 20183 (52 Main Road Hope) to the south-west, the lots are completely sealed (100% impervious cover). Thus, the runoff potential from these lots have already been fully-realised. Any redevelopment within this area (including construction of additional buildings) is therefore not expected to result in any increase in runoff from the site over present-day levels. Redevelopment could be allowed to occur without any adverse impact on the stormwater drainage network over and above the existing situation.
52 Main Road Hope to the south-west has only been partially developed (26% impervious), with the remainder of the lot currently grassed. Further development of this lot could therefore potentially lead to increased peak runoff (m³/s) and total runoff volumes (m³) from the lot. The additional water would need to be picked up by existing drains already identified as having inadequate capacity.
This, however, does not preclude any further development of the lot. In the long term (20+ years), the capacities of Borck Creek and Reed/Andrews Drain will be increased to provide suitable receiving watercourses for any site runoff in up to the Year 2080 1 in 100 year event. In the short term, the
6 Conclusions and recommendations
The conclusions and recommendations of our flood assessment are summarised as follows:
a The site is subject to overland flooding during a present-day 1 in 100 year flood event due to the inadequate capacity of the Borck Creek and the Reed/Andrews Drain;
b Excepting 52 Main Road Hope, the NTL-owned lots that are proposed to be included in Schedule 17.5A provisions of the Tasman Regional Management Plan are currently 100%
impervious. Therefore, re-development within these lots can occur without causing any additional runoff during rainfall events. Any development within these lots should be subject to the flood levels and minimum floor levels as determined specifically for individual
development proposals;
c 52 Main Road Hope is currently only 26% impervious. Further development of this lot could result in increased runoff, which could not be accommodated by existing watercourses during rainfall events in excess of an approximately 1 in 5 year return period. Any development of
Appendix A: Tonkin & Taylor Figures
Figure 1 – Site location planFigure 2 – January 1986 Flood Mapping
Appendix B: Runoff Calculations
Network Tasman Flood Assessment
Rational method flow calculations - based on MWH 2009 report Computed: NRM Checked: DNV
Section Catchment
Area (ha) C Time of
Concentration (minutes)
Rainfall
Depth (mm) Intensity
(mm/hr) Q (m3/s) Qmodified (m3/s) Hart Drain
Diversion*
(m3/s) Qdesign
(m3/s) Rainfall Depth (mm) Intensity
(mm/hr) Q (m3/s) Qmodified (m3/s) Hart Drain
Diversion (m3/s)
Qdesign (m3/s) Rainfall
Depth (mm) Intensity
(mm/hr) Q (m3/s) Qmodified
(m3/s) Hart Drain Diversion (m3/s)
Qdesign (m3/s)
A 1440 0.51 150 92 36.8 75.5 61.9 61.9 82 32.8 67.3 55.2 55.2 70 28.0 57.4 47.1 47.1
B 1250 0.51 150 92 36.8 65.5 53.7 53.7 82 32.8 58.4 47.9 47.9 70 28.0 49.9 40.9 40.9
C 1160 0.51 150 92 36.8 60.8 49.9 0.0 49.9 82 32.8 54.2 44.5 0.0 44.5 70 28.0 46.3 37.9 0.0 37.9
D 950 0.51 150 92 36.8 49.8 40.8 40.8 82 32.8 44.4 36.4 36.4 70 28.0 37.9 31.1 31.1
E 264 0.52 60 54 54 20.6 N/A 0.0 20.6 49 49.0 18.7 N/A 0.0 18.7 43 43.0 16.4 N/A 0.0 16.4
F 257 0.52 55 49.5 54 20.1 N/A 0.0 20.1 44.9 49.0 18.2 N/A 0.0 18.2 39.4 43.0 16.0 N/A 0.0 16.0
G N/A
H N/A
I** 160 0.45 64 57.6 54 10.8 N/A 0.0 10.8 52.3 49.0 9.8 N/A 0.0 9.8 45.9 43.0 8.6 N/A 0.0 8.6
J N/A
Section Catchment Area (ha)
C Time of
Concentration (minutes)
Rainfall Depth (mm)
Intensity (mm/hr)
Q (m3/s) Qmodified (m3/s)
Hart Drain Diversion*
(m3/s) Qdesign
(m3/s) Rainfall Depth (mm)
Intensity (mm/hr)
Q (m3/s) Qmodified (m3/s)
Hart Drain Diversion (m3/s)
Qdesign (m3/s)
Rainfall Depth (mm)
Intensity (mm/hr)
Q (m3/s) Qmodified (m3/s)
Hart Drain Diversion (m3/s)
Qdesign (m3/s)
A 1440 0.51 150 105 42.0 86.2 70.7 70.7 95 38.0 78.0 63.9 63.9 80 32.0 65.7 53.8 53.8
B 1250 0.51 150 105 42.0 74.8 61.3 61.3 95 38.0 67.7 55.5 55.5 80 32.0 57.0 46.7 46.7
C 1160 0.51 150 105 42.0 69.4 56.9 2.0 58.9 95 38.0 62.8 51.5 2.0 53.5 80 32.0 52.9 43.4 2.0 45.4
D 950 0.51 150 105 42.0 56.8 46.6 46.6 95 38.0 51.4 42.2 42.2 80 32.0 43.3 35.5 35.5
E 264 0.52 60 62 62 23.7 N/A 2.0 25.7 55 55.0 21.0 N/A 2.0 23.0 49 49.0 18.7 N/A 2.0 20.7
F 257 0.52 55 56.8 62 23.0 N/A 2.0 25.0 50.4 55.0 20.4 N/A 2.0 22.4 44.9 49.0 18.2 N/A 2.0 20.2
G N/A
H N/A
I** 160 0.45 64 66.1 62 12.4 N/A -2.0 10.4 58.7 55.0 11.0 N/A -2.0 9.0 52.3 49.0 9.8 N/A -2.0 7.8
J N/A
Notes :- KEY
* Hart Drain Diversion is assumed to have occurred by 2080 but not currently. Catchment to Borck Creek adjacent Network Tasman site
** Flows from section I are already included in Section C. Catchment to Reed/Andrews Drain adjacent Network Tasman site
Refer MWH 2009 report for catchment locations
Year 2007 : Q100 (MWH) Year 2007 : Q50 Year 2007 : Q20
Year 2080 : Q100 (MWH) Year 2080 : Q50 Year 2080 : Q20
