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PEACOCK D H LIMITED
5 Benjamin Street; Makaraka, Gisborne.
GST Reg No 97918708 email: dave2mar@gmail.com Phone 06 868 3259
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PEER REVIEW OF THE POTENTIAL EFFECTS OF THE PROPOSED
SPONGE BAY SUBDIVISION ON FLOOD LEVELS WITHIN THE SUBDIVISION, AND IN THE WAINUI STREAM FROM THE SUBDIVISION TO THE SEA.
1.0 Introduction and Background:
The Sponge Bay subdivision, being a subdivision of Lot 2 DP 370338, is a new 12.1 ha. housing subdivision on the eastern fringe of Gisborne city. This peer review is to assess whether the design complies with GDC resource consent requirements Nos. 22, 23 and 24 in GDC consent No SG 207092A.; see Appendix 1.
The original resource consent conditions in Appendix 1 have subsequently been modified due to further flood information becoming available during the course of this peer review.
This peer review commenced in June 2008 with the gathering of information from the designer (MTEC Consultants Ltd; of Tauranga), and substantial progress had been made when a significant rainstorm fell over much of the North Island on the 30th July 2008, including Gisborne city. It became apparent to the designer that tailwater levels in the Wainui Stream at the twin 900 mm culvert outlets to the subdivision were substantially higher than anticipated given the amount of rainfall.
Both the designer (on behalf of the applicant) and the client (GDC), agreed to widen the scope of the peer review to investigate why flood levels on the 30th July within and downstream of the subdivision were so high, and during the course of the investigation it was discovered that a major flood had occurred in this area on the 21st June 1977 which was substantially higher again than the 30th July flood.
The scope of this review was again widened to include a review of the June 1977 flood and the potential impact of a flood of the same magnitude on the area of the subdivision and downstream.
2.0 Scope and Purpose of this Peer Review; and Disclaimer:
The purpose of this peer review is to give the client, the GDC, assurance that resource consent conditions 22 to 24 (incl.) have been fulfilled by the designer (on behalf of the applicant); and that any increase in discharge to the Wainui Stream as a result of the proposed subdivision will not significantly adversely affect properties downstream and adjacent to the stream.
The original resource consent conditions 23 and 24 (see Appendix 1); have effectively been modified by subsequent agreements between the GDC and the applicant, and my understanding of the three conditions being reviewed are as follows:
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Condition No 22 (unchanged); Note that only the first sentence of this condition is being reviewed herein:
A flood retention pond to store excess run-off from impervious areas on this site shall be provided to the satisfaction of the Consent Authority. The run-off retention shall be designed to ensure that sufficient open space is retained for recreational purposes.
Condition No. 23 (modified):
The consent holder shall provide a secondary flow path capable of handling a flood equal in magnitude to the flood of 21st June 1977 without exceeding a flood level of RL 8.45m within the subdivision.
Condition No. 24 (modified):
All dwellings shall be constructed with a minimum floor level of RL 8.65m.
Note that the June 1977 flood is considered to be a more severe event in terms of run-off than the standard (chart value) “one in 100 year” flood event.
The design methods and design assumptions have been reviewed to see that such methods and assumptions conform to generally accepted engineering practice and, if not, any alternative methods are reasonable and suitable for the purpose.
This peer review is not however a step by step check of the designer’s calculations, nor is it a review of the capacity of the internal subdivision stormwater reticulation system.
Data provided by the designer for the calculations has been taken at face value; in other words I have relied on the designer to provide reasonably accurate design data.
3.0 Potential downstream effects of increased stormwater run-off due to the subdivision:
The subdivision will result in a significant increase in the proportion of impermeable or nearly impermeable surfaces in the 12.1 ha. area due to roofs, roads, footpaths, driveways and other
paved areas. This can be expected to increase the peak runoff during storm events. Standard runoff coefficient charts have been used by the designer and reasonable assumptions as to soil type and permeability have been made.
The increase in runoff coefficient for the 12.1 ha. subdivision has been calculated by standard engineering methods and in accordance with the GDC Engineering Code of Practice (ECOP).
The runoff coefficient for the 105 ha. catchment upstream of, and including the subdivision; has been calculated by the designer to increase from a value of 0.3 to a value of 0.326; the method used also conforming to standard practice and the ECOP.
The effect of this increase in runoff coefficient is expected to have some downstream effect during floods and freshes, and a number of computer runs have been provided by the designer to gauge this potential increase in runoff to the Wainui Stream. This has been investigated for small to moderate floods, and also for an extreme event equal in magnitude to the June 1977 event.
4.1 Downstream effect for small to moderate floods:
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The designer has modelled the effects of the increased runoff coefficient for the 105 ha catchment (upstream of and including the subdivision) on the discharge into the Wainui stream downstream of the twin 900 mm culvert outlet to the subdivision under SH35. A computer model (a standard single lake food routing model) has been used by the designer and the results compared for three storm events; a “one in 5 year”, a “one in 10 year” and a “one in 50 year” event . These are rainfall events using rainfall statistics for Gisborne from the NIWA HIRDS 1b model, an acceptable source of rainfall data used widely throughout NZ. These rainfall statistics have also been increased for potential effects of climate change using the standard MfE guidelines for the year 2040 HT (high temperature) scenario.
The Rational method has been used to calculate runoff using chart values for runoff coefficients as in the NZ Building Code document E1- Surface Water Verification Method 1.
The inflow to the subdivision will effectively be increased in direct proportion to the increase in runoff coefficient, ie; from 0.30 to 0.326, which is equal to an increase of 8.7%. However the discharge from the subdivision is influenced by two factors, firstly attenuation (ponding) in the subdivision lagoon, and secondly by the tailwater level at the outlet to the twin 900mm diameter culverts under SH35.
The pre-development discharge at the outlet to the twin culverts for three storm events has been compared to the discharge for the same three storms for the post-development case. The results are as follows:
i) One in 5 Year Storm Event: The peak runoff at the twin culvert outlet into Wainui Stream for the post-development case has been shown by the model to be slightly less than that for the pre-development case. (1.93 cumecs vs 2.0 cumecs respectively). This is not an unexpected result since up to RL 7.0 there is substantially more storage available in the post-development lagoon than in the pre-development lagoon.
ii) One in 10 Year Storm Event: The peak runoff is 6% higher as a result of development (2.124 vs 2.005 cumecs). The ponding level is higher than for the 5 year event and the storage volume available post-development is slightly less than pre-development.
iii) One in 50 Year Storm Event: The peak runoff is just under 10% higher (2.615 vs 2.381 cumecs) as a result of the proposed development.
In the worst case above, an increase of about 10% in the peak 50 year storm discharge could be expected. This increase of 0.234 cumecs (or 234 litres/sec), becomes an even much smaller fraction of the total Wainui stream flow downstream of the confluence with the tributary from Tamarau, and particularly downstream of Wheatstone Road, where a tributary from a larger catchment joins the Wainui Stream.
4.2 Downstream Effects of Extreme Events:
Following the discovery of the 1977 flood levels in the Sponge Bay area the applicant was requested by GDC to consider how a storm of the same magnitude as the 1977 event could affect the subdivision and what changes would be necessary to protect the subdivision houses from flooding. Following discussions between the designer, GDC staff and the author, a proposal was submitted by MTEC to increase the size of the proposed overflow culvert from 825 to 1800 mms diameter and to test the design by using the single lake flood routing model
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but extended to both sides of SH35 to cover the extent of the 1977 flood . The June 1977 storm rainfall data and the calculated runoff from the whole contributing catchment (including the Wheatstone Road catchment) was used as input to the model. The model was set to pre-development conditions and calibrated to the 1977 flood by increasing the runoff coefficients for the contributing catchment until the ponding level reached the same peak level as during the 1977 event. The runoff coefficient had to be increased to a maximum of 0.9 to enable this match of levels to be achieved.
Subsequently the designer proposed an 1800mm diameter overflow culvert to discharge directly to the Wainui stream from the NE corner of the subdivision.; and raising the minimum ground level for the house sites to RL 8.50 and the minimum house floor levels to RL 8.65. This entailed the importation of more fill material to the site.
This proposal was subsequently agreed by GDC, however following further detailed work by the designer it was found that the excessive energy losses from the 90 degree bend at the entrance to the proposed 1800mm culvert pipes could not practically be reduced because of the limited space available, so this design was abandoned and replaced with an open channel design, eventually deciding on a trapezoidal shaped grass lined open channel.
The method used to model the 1977 flood for both the pre and post- development cases I believe is an acceptable method which could be expected to give reasonably accurate differences between the pre and post-development results; and the design of the grass lined open channel (as proposed in MTEC drawing No 603552-M-E-D001; Sheet No. 23; Issue E ) follows generally accepted engineering practice and should be expected to meet the design objective of a capacity of 7.64 cumecs at RL 8.45 at the upstream end. However the grass channel will need careful maintenance, be kept free of any impediments to the free flow of water, and the grass be kept trimmed short.
The overflow channel will only operate when flood levels are above RL 8.0. This will only happen during major flood events or in the unlikely event of one of the culverts under SH35 becoming blocked. It should be noted that floodwaters at the peak of the 30th July 2008 flood only reached about RL 7.5.
Floodwater depths and velocities indicate that at peak flood levels within the overflow channel “high hazard” conditions (according to the NSW Flood Manual, 2005) would exist. In my opinion it will be necessary for the overflow area from the road kerb within the subdivision to the outlet to Wainui Stream to be designated as “F9 Urban Floodway” in the GDC Combined Regional and District Plan; and danger signs be erected to warn the public.
To the casual observer the flood flows in the Wainui Stream downstream of the proposed overflow channel outlet would appear to (potentially) increase by an amount up to the capacity of the overflow channel (ie; up to 7.64 cumecs). However, this is not the case because flood ponding levels in the subdivision will be substantially lower, thereby reducing the discharge through the twin 1800mm diameter culverts under SH35 near Wheatstone Road.
However, the potential increase in flow downstream of the overflow channel outlet to Wainui stream has been modelled for a flood equal in magnitude to the 1977 event for both the pre and post-development cases. In the pre-development case, the maximum discharge from the twin 1800 mm diameter culverts is 22 cumecs; whereas for the post-development case the maximum discharge for the same flood is 24.36 cumecs (16.72 cumecs for the twin culverts and 7.64
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cumecs for the overflow channel).. Hence the potential increase in flow for a flood equal in magnitude to the extreme 1977 event is 2.36 cumecs, or an increase of 10.7%.
This increase however is only downstream of the overflow channel outlet. Upstream of the twin 1800 mm culverts under SH35 (as far as the tributary on the downstream side of Wheatstone Road), in an event equal to the 1977 storm, the discharge in the post-development case is 24% less than the pre-development case (16.72 vs 22 cumecs respectively). Thus flood levels between the outlet to the subdivision and the twin 1800mm diameter culverts under SH35 will be significantly less for any flood which ponds to over RL 8.0 and bypasses the Wainui Stream via the overflow channel.
The Wainui Stream downstream of the overflow channel outlet is deeply entrenched and cross sections derived by MTEC from an aerial laser survey, and hydraulic parameters calculated by MTEC, show that there is ample capacity for an increase in flow of 2.36 cumecs (as for an extreme event equal to the 1977 flood). The stream however, over most of its length is heavily overgrown and obstructed, and even relatively minor channel and berm clearing works in my opinion would more than offset any potential increase in flood levels as outlined above. Even without the subdivision, this work should be undertaken as part of the maintenance of one of the Council’s Land Drainage assets. It is an important watercourse and is recognised as such by its designation as an “F9 Urban Floodway overlay” in the GDC Combined Regional and District Plan.
Ideally the stream channel should have all obstructions in the channel and any sharp bends eased, and the berm on both sides cleared for a few metres from the stream bank. Preferably one berm should be cleared for at least four metres to allow occasional access for channel clearing machinery.
The Wainui stream downstream of the subdivision to the sea, will require regular maintenance by GDC to clear obstructions, and to remove debris which could block culverts during flood events.
Conclusions:
With the (fortunate) discovery of the exceptionally high flood levels reached by the June 1977 flood in the Sponge Bay area, this subdivision has had to be re-designed while under construction to provide adequate protection to houses, while at the same time not significantly raising downstream flood levels.
As stated in an email from S Bos, MTEC Consultants, dated 13th January 2009; “In terms of consent condition 22 it is noted that the increase(s) in runoff are small and in terms of the overall catchment, flows are unlikely to provide any notable effect. The balance of still providing usable land for recreation, having the tailwater set by the downstream outlet pipes (which for the greater storms the ponding within the catchment negates most of the lower level storage) plus also raising the lots to mitigate the greater flood effect has resulted in limited opportunity to provide storage above RL 7.5 which is required to lessen the post development site discharge.” I agree with this statement and recommend to Council that the first sentence of resource consent condition 22 be accepted as being fulfilled by the current MTEC plans; these being: “Design Finish Levels”, Dwg. No. 603552-M-E-100; sheet 01; issue A; and (more specifically) the original design and revised post earthworks Storage Volumes, attached to an email to the author from S Bos, MTEC consultants, dated 05/01/09.
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I consider also that resource consent condition No. 23 (as modified in clause 2.0), will be fulfilled by the overflow channel as proposed in MTEC drawing No. 603552-M-E-D001; Sheet No. 23; Issue E .
Finally, it will be up to GDC building inspectors to confirm that all houses within the subdivision meet the requirements of consent condition No. 24 (as modified in clause 2.0).
D H Peacock; BE(Civil); MIPENZ
Director,
Peacock D H Ltd.
11th February 2009.
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Potential Effect of the Sponge Bay Subdivision Overflow Channel
on the Mouth of the Wainui Stream
At the downstream end of the proposed Sponge Bay Subdivision overflow channel, the discharge during an extreme rainfall event (equal to the June 1977 event) has been estimated to be 2.36 cumecs more than it would have been prior to development of the subdivision. This short report assesses the likely impact of this potential increase in peak discharge at the mouth of the Wainui stream.
First of all it should be noted that the conditions in which an increase in discharge of this magnitude could occur are expected to be extremely rare. In the storm of 30th July 2008, (a “1 in 2 year” rainfall event), this overflow would not have operated, the peak flood level within the subdivision being about 0.5 metres lower than the level necessary for the overflow channel to operate.
It is likely that this overflow will only operate in floods in excess of a “1 in 5 year” or “1 in 10 year” rainfall event; and the discharges during events of this size would be very much less than during a flood equal in magnitude to the 1977 flood which has been assessed as a “1 in 100 year” to a “1 in 150 year” event, for rainfall durations of 2 to 6 hours.
The assessed potential peak increase in flood discharge at the outlet to the proposed overflow channel of 2.36 cumecs would however be substantially less at the downstream side of the Murphy road culverts due to attenuation (a delay and moderation of the flood peak due to part of the increased flow going into storage; ie; a small increase in flood level in the channel and over the berms). I would suggest that the increase in flow at the outlet to the Murphy Road culverts would be substantially less then two cumecs and possibly even less then one cumec.
Due to this potential increase in discharge downstream of Murphy Road of say one to two cumecs, there could be expected be a small increase in scour at the stream mouth (between Murphy Road and the sea); however this would be short lived as the peak discharge would be transitory and wave action from the sea would start to infill any channel as the flood subsides.
The above can be put into perspective by comparing with the normal seasonal movement of sand at any beach; onshore in the summer and offshore in the winter. At Wainui Beach on average some 250,000 cubic metres of sand moves onshore and offshore each year. Over the 4 kms of beach this represents some 60 cubic metres of sand per metre length of beach, and at the mouth (between revetments) over 1000 cubic metres of sand is transported onshore and offshore quite naturally by wave action each year.
In conclusion, in my opinion the effect of the proposed overflow channel on coastal processes at the Wainui stream mouth would be insignificant compared to natural processes, and would not be measurable.
D H Peacock; BE(Civil); MIPENZ
Director,
Peacock D H Ltd.