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Comment on Resource Consent application for Construction of a Swale Drain
Sponge Bay Development Wainui Rd
Background:
This consent application is for the construction of a swale overflow drain from the Sponge Bay development. It is not an application for the Sponge Bay Development itself, which was subject of a consent application in early 2008. A list of source material is attached at the end of this report.
Comment on the source material:
The reports supporting the consent application and the ensuing correspondence are self explanatory and I won’t waste time and space in explaining them in detail. Most of the information being requested and being provided was related to the impact of the Sponge Bay development on the Wainui stream which was necessary for background. However, as this consent application is not for the development itself, but only the proposed swale drain, only some of this is directly relevant to the present consent application.
Comment on the standards against which RC applications can be assessed:
Almost all infrastructure design uses standardised recurrence intervals of the events for which the design is being carried out. In structural engineering design the events would include wind and earthquake, but in stormwater design it is rainstorms which are the key design events.
The common stormwater design philosophy takes a two step approach. It acknowledges that it would be unrealistic to design all infrastructure to cope with a probable maximum event which may only occur in 10,000 years time, so it sets an intermediate level which could be anything from 1:5 years to 1:50 years (depending on the type of development in the area) and a primary stormwater system is designed to cope with that event. The primary system would include all the normal pipes and/or open drains which people expect of a stormwater disposal system.
However, in acknowledgement that it is possible for events to occur which could exceed this event it has become common for secondary flow paths to be designed to accommodate the overflow from these larger events without flooding houses or sewer gully traps. These secondary flow paths are normally roads, reserves and other land where a temporary period of submersion would not cause serious damage, just inconvenience and some cleanup afterwards. Housing platforms are therefore designed to be a bit higher than the roads and other open space so that in a very large flood they would be like wide shallow streams which would be taking the bulk of the flow for those few hours that the flow exceeds the capacity of the primary system.
Different codes of practice set different standardised recurrence intervals for the design of these systems, and it is common for any particular code of practice to set different recurrence intervals for different areas. The rationale is that in some areas the consequences of flooding are less severe than others.
In Gisborne we have an Engineering Code of Practice which includes general requirements for designing stormwater drainage.
Clause 3.2.1 (a) to (n) provides the design requirements for new developments, and sub clauses (a) & (b) requires that the primary system should provide protection from a 1:10 year storm, while the primary and secondary flow paths should together provide protection from a 1 in 100 year storm.
Clause 3.3.2 deals with the downstream stormwater system, and includes the sentence “the developer is required to ensure that the development does not exceed the capacity of the downstream stormwater system”.
What would be the rules governing the maxima in this existing “downstream system”?
An answer to this may be found in the table included in clause 3.2.1 of the Code titled “appendix/urban stormwater criteria”, which appears to apply to existing property (ie not property within the new subdivision development) and provides the minimum standard design storms to be used for different levels of service. For the highest level of service these include the requirements that gully traps and houses are to be 100mm above the 1:100 year storm water level, and it is acceptable for gardens, lawns and sheds to be flooded by a 1:5 year storm.
For more area specific requirements, chapter 5 of our District Plan covers hazards and sections 5.18 to 5.22 are the rules for specific areas and overlays within the district. In areas where they apply these rules would take precedence over the more general rules in the Code, but at present there are no overlays which specifically include the Wainui stream catchment.
Comment on the original consent design:
The first half of the second page of the 12 March 2009 MTEC report makes a statement about the standards to which the original design and the present design have been carried out. Presumably because the Sponge Bay development did not fall into any specific District Plan overlays the original 2008 resource consent conditions are more or less consistent with generally accepted practice, and also with our Code. Para 23 of the original resource consent reads as follows “The consent holder shall provide an overland flow path capable of handling a one in one hundred year flood in accordance with the details described in the letter from MTEC Consultants dated 30 January 2008”. Para 24 goes on to say “All dwellings shall be constructed with a minimum floor level of 8.35m”. As the calculated 1:100 year flood level was 8.2m, this floor level allowed 150mm of freeboard. However, neither the original consent application nor the 30/1/08 MTEC letter made any specific references to the Code requirement that the developer “is to ensure that the development does not exceed the capacity of the downstream stormwater system”. The analyses focussed on the flood levels within the new development itself.
Comment on the July 2008 storm and this proposal:
As explained in the recent consent application and in Dave Peacocks peer review report, the July 2008 storm prompted a review of the flood levels on site, and also downstream as well. I think it is important from a process point of view to identify what triggered this review. Did the storm cause the developer and his consultant to have second thoughts about the flood flows and levels which would result from the 1:100 storm specified in the resource consent? If this was the case, and they had found that the flows/levels which would result from the 1:100year storm had been underestimated, then it would mean the review was initiated by the developer trying to comply with the original consent. I asked Stephen Bos if this was the case, but he said no, the review was not initiated by them or by the developer, but apparently by council. It would appear that even if the swale overflow were not installed the development would still meet the original resource consent conditions, that is “the consent holder shall provide an overland flow path capable of handling a one in one hundred year flood …” They did find that there was more head loss through one of the batteries of culverts under SH35 than they had estimated, but the minimum floor levels of the houses within the development had been raised to compensate. The fact that the proposed swale overflow is not designed to start operating until a bigger than 1:100 year storm confirms that its existence is not necessary to comply with the original consent.
The question therefore has to be asked, what triggered the swale overflow?
Reading the file and reports, it would appear that the trigger for the swale proposal was a request made after the July 2008 event that Regional Rule 5.22.1 from the district plan (which applies to flood hazard overlay 7, not Sponge Bay) should apply to this area. Para (b) of this rule requires that house floor levels are to be 200mm above the 1977 and/or 1985 flood levels. The application of this rule had the effect of increasing the compliance level from the 100 year flood to something between a 200 and 1000 year flood and it was the resulting increase in levels which made it necessary to have the swale overflow.
Comment on the impact of the proposed swale overflow on downstream levels.
There is little to comment on that is not already covered in the reports. There is no difference in flood flows downstream for all storms up to 1:100 year, because the swale will not have begun to operate at these storms. MTEC estimate that the max downstream flow during a 1:100 year storm would be about 13.78 cumecs. At some storm just above the 1:100 year the swale would commence flowing and with bigger and bigger storms the maximum flow through the swale would increase until at the equivalent 1977 storm it would be passing about 7.6 cumec (much of which would not have originated within the development) and the total flow in the stream just downstream would be 24.36 cumecs. MTEC have provided estimated flood levels from the swale discharge to Murphy Road at this extreme flow.
Comment on the impact of the Development on downstream levels.
This comment is not strictly speaking relevant to this consent application, but it would have been relevant to the original consent application. We have however asked for and received quite a lot of information as background to this application so some comment is now possible.
MTEC have calculated the effect of the Development on downstream flow, before and after development, for the 50 year and 100 year storms. They have also provided a review of the before and after flows resulting from an “extreme event”, being the 1977 storm.
Approx peak flow
100 year 1977 storm
Pre development 13.78 cumec 22 cumec
Post Development 13.948 cumec 24.36 cumec
% increase 1.2% 10.7%
However, it is not so much the rate of flow which manifests itself on downstream inhabitants, but the water level and, to a certain extent, the velocity of flow.
The increases in water level “before” and “after” the development resulting from the 100year storm specified in the original resource consent would be too small to plot on a map. In fact the calculations and assumptions required to estimate the flood levels have a margin of error bigger than 1.2%. So it would be fair to say that had we requested more information on the effects of the development on the downstream stormwater system at the time of the first consent application, it would have been concluded that it was not a significant issue.
The effects of the development (including the swale) at the extreme (1977) flood are measureable and MTEC have provided before and after levels at 40m intervals along the 1km section of the stream, from the state highway culverts to Murphy Road. They have also provided a longitudinal section. To summarise, the “before” and “after” levels are given below at three locations.
Peak levels during extreme (1977) storm
Chainage/location pre development post development
22 cumec 24.36 cumec variance
0 at state highway 6.95m 7.14m 190mm
480m halfway between 5.79m 6.07m 280mm
980m Murphy road 3.27m 3.50m 230mm
It can be seen that the above increases average about 250mm. MTEC have provided cross sections through the stream at 40m intervals and it appears that a typical section is triangular in shape, with side slopes about 1:2 to 1:10. This means that on average the additional area inundated at the peak flood would be a strip on both sides between 0.5m and 2.5m wide. In the lower reaches this will mostly be in GDC esplanade reserve land, but where there is no esplanade reserve, then provided that the additional private property inundated as a result of the development only covers “gardens, lawns and sheds” then (bearing in mind that the recurrence interval of this additional inundation will be well in excess of 1:100 years) this additional
inundation should be acceptable if the table in 3.2.1 of our Code is used as the standard. It will be recalled that this table allows inundation of this sort of property at anything down to 1:5 years recurrence intervals.
I have marked up a contour plan with the inundated areas and the location of all houses shown on the aerial photos on our GIS, and in general there are no houses within either 10m in plan, or 2m in elevation of the water except 29 Murphy Road. The carport of this house appears to be about 4m from the “before development” extreme event inundation contour, which reduces by about half for the “after development” inundation contour. In elevation the MTEC report indicates that according to the lidar survey the “before development” inundation contour would have been 3.27m, the “after development” inundation contour would be 3.50m while the carport floor is at about 4.75m (The updated floor level is a result of a survey carried out on 26/3/09 by Denis Crone, Paul Murphy and myself.)
While the MTEC report (and our subsequent survey) indicates that this house should be in no additional danger as a result of the development, some comment should be made of unpredictable events. There are anecdotes that during the 1977 flood a caravan was picked up from the side of a road, and eventually washed into the Mangapapa stream where it blocked a culvert and caused flooding upstream. These things can happen and because the carport floor of no 29 is about a metre below the level of Murphy Road at this point, the road embankment could potentially act as a dam. I have carried out my own calculations and while it should still not endanger the carport if one of the three culverts were blocked, there could be some risk if two were blocked. This is not an issue which has relevance to the sponge bay development because the risk would have been there even if the development had not taken place. It may be worth while for one of the operational departments of council to check from time to time that none of the residents upstream is storing/parking motor vehicles, caravans, boats, or any other sizable object which is capable of floating, in the 15m strip on either side of the stream.
(Note that in the case of No 29 Murphy road the house is above the carport, the carport ceiling forming part of the floor of the house so the risk referred to above does not apply to the house itself, only to the carport.)
Comment on the existing bush in the Wainui stream bed and banks.
It was pointed out in the present consent application that the existing stream bed is overgrown with vegetation and bush and suggested that the stream could take a lot more flow if it were cut back. The initial calculations supporting the application were accordingly consistent with this suggestion and used Mannings roughness coefficient n = 0.023, equivalent to a “natural stream channel with grass and weeds and little or no brush”.
I felt that the water velocities should also be taken into account, not only the inundation level, and while clearing the brush would certainly make a significant impact on reducing flood levels, the increased water velocities could cause erosion damage. I felt it would be better (if possible) not to make the clearing of the vegetation a condition of the consent. Better to at least try calculating the flood levels while leaving the vegetation “as is” as it will always be possible to clear the vegetation, but it would take years for it to grow again.
For this reason all the information and flood levels requested subsequent to the submission of the present application (including the flood levels given above) have been based on the downstream reaches having a Mannings n = 0.08, equivalent to a “natural stream channel with some weeds and dense willows on bank”, in other words more or less in its present condition. The typical velocities calculated by MTEC in this state are about 0.9 m/second, which is not erosive. In fact, there is probably latitude for some careful bush clearing.
Comment on the reach of the Wainui stream between Murphy Road and the sea.
Neither the present application nor the original application made any attempt to predict how the changes upstream would impact on this area. It is my opinion that the water levels in this short reach (not only flood levels but also normal levels) are more affected by the sea and the state of the beach, than the river flow. In addition the land on both sides of the stream in this reach is GDC esplanade reserve up to between 4m and 5m MSL, and these are probably why analysis was not carried out. If an analysis is to be carried out it
should be done by a specialist in marine systems, not river systems. Dave Peacock and the consultants he has employed over the years would be the best source of comment for this area.
Summary
I have not seen anything which would lead me to think the proposed swale overflow will have any effect on property below the development which would be outside of our Engineering Code of Practice. While its existence is not necessary to comply with the original consent, it represents a good safety valve against flooding in the Sponge Bay development in an extreme event, and also in the whole area upstream of the state highway culvert crossing in lesser floods, if one or more of those state highway culverts were blocked by, for example, a vehicle or a fallen tree.
I hope these comments are helpful.
Graham Tayler 2/4/09
MIPENZ ,BSc(Eng)
Source Material:
1. Report by “Insight” dated 11 February 2009 supporting the Consent application.
2. MTEC Hydraulic analysis: Stephen Bos.
3. Peer Review report: Dave Peacock
4. Email query from Graham Tayler to Ross Muir dated 23 February 2009.
5. Responses from MTEC dated 24/3/09 sent via Insight 25 February 2009.
6. Email query from Graham Tayler to MTEC via Ross Muir 3 March 2009
7. Response from Stephen Bos, MTEC, dated 13 March 2009.
8. Email queries (2 off) from Graham Tayler to Stephen Bos dated 17 March 2009.
9. Response from Stephen Bos, MTEC, dated 18 March 2009.
10 Another question from Graham Tayler to Stephen Bos dated 24 March 2009.
11. Response from Stephen Bos, MTEC, dated 24 March 2009.
12. Yet another couple of questions from Graham Tayler to Stephen Bos dated 25 March 2009, and
13. Response from Stephen Bos, MTEC, dated 25 March 2009.
14. Request for clarification from Graham Tayler to Stephen Bos dated 27 March 2009
15. Response from Stephen Bos, MTEC, dated 31 March 2009.