85 Molesworth Street PO Box 3942, Wellington 6140, New Zealand T: +64 4 473 7551 // F: +64 4 473 7911 E: [email protected]// www.ch2mbeca.co.nz Our Ref: 6519300 NZ1-9725249-7 0.7 Kapiti Coast District Council Private Bag 60601 Paraparaumu 5254 New Zealand Attention: Darryl Lew 19 March 2015 Dear Darryl KCDC Proposed District Plan - Stormwater Submission Review Number 86 - M Klimenko This letter is in response to your request for a site-specific assessment of the submission 86 to the Proposed District Plan (PDP) by M Klimenko. This submission covers the whole Otaki Beach area, although the addresses provided in the supporting information are at 7 and 11 Health Camp Road, Otaki Beach. This assessment addresses only the aspects of the submission related to stormwater hazards. Submission The aspects of the submission related to stormwater hazards are as follows: 1. Desk Top modelling is by itself insufficient to give an accurate classification of the Otaki Beach area with regard to “Ponding” 2. That the basis for establishing areas of “Ponding” appears to be directly attributable to flooding of both the Waitohu Stream and Otaki River without regard to buffers and natural barriers which are present near to these waterways. 3. That no consideration has been given to the unintended consequences on existing properties’ insurance cover of classifying an area solely on the basis of Desk Top modelling without any attempt having been made to confirm assumptions made in the model.” Note that no assessment has been made in this review as to the effect of the modelling on insurance cover. Site description Otaki Beach is located in an area of coastal sand dunes, which has been established as an urban / residential area for many years. The location, topography and drainage system are show in Figure 1. All levels (mRL) in this report are in terms of Wellington Vertical Datum 1953.
Transcript
85 Molesworth Street PO Box 3942, Wellington 6140, New Zealand T: +64 4 473 7551 // F: +64 4 473 7911 E: [email protected] // www.ch2mbeca.co.nz
Our Ref: 6519300 NZ1-9725249-7 0.7
Kapiti Coast District Council Private Bag 60601 Paraparaumu 5254 New Zealand
Attention: Darryl Lew
19 March 2015
Dear Darryl
KCDC Proposed District Plan - Stormwater Submission Review Number 86 - M Klimenko
This letter is in response to your request for a site-specific assessment of the submission 86 to the Proposed District Plan (PDP) by M Klimenko. This submission covers the whole Otaki Beach area, although the addresses provided in the supporting information are at 7 and 11 Health Camp Road, Otaki Beach.
This assessment addresses only the aspects of the submission related to stormwater hazards.
Submission
The aspects of the submission related to stormwater hazards are as follows:
1. Desk Top modelling is by itself insufficient to give an accurate classification of the Otaki Beach area with regard to “Ponding”
2. That the basis for establishing areas of “Ponding” appears to be directly attributable to flooding of both the Waitohu Stream and Otaki River without regard to buffers and natural barriers which are present near to these waterways.
3. That no consideration has been given to the unintended consequences on existing properties’ insurance cover of classifying an area solely on the basis of Desk Top modelling without any attempt having been made to confirm assumptions made in the model.”
Note that no assessment has been made in this review as to the effect of the modelling on insurance cover.
Site description
Otaki Beach is located in an area of coastal sand dunes, which has been established as an urban / residential area for many years.
The location, topography and drainage system are show in Figure 1. All levels (mRL) in this report are in terms of Wellington Vertical Datum 1953.
The whole area sits in a basin bordered by the frontal dune to the northwest and other dunes. It would seem logical that if there was sufficient rainfall that exceeded the capacity of the primary stormwater drainage system and of the infiltration rate of the soil (or groundwater level rose), the area would start to fill up and flood. This area would appear to discharge to the Waitohu Stream to the north via a restricted opening (about 7 m at its narrowest point) in the dunes. There is a stormwater pump station adjacent to the property at 4 Rupini Street. This pump station discharges to an ocean outfall.
Piped reticulation conveys water from the surrounding areas via pipes down Health Camp Road, Tasman Road, Moana Street, Manuka Street and Toi Street. The total catchment may be about 50 ha. The piped system has a gravity discharge to the Waitohu Stream.
The sites at 7 and 11 Health Camp Road are approximately at 3.0 mRL, and sit below the road level. The topography indicates the properties are situated in a localised low point with no overland flow path.
There are a number of catchpits in the area. If the catchpits were blocked, or there was a storm event exceeding the capacity of the catchpit and pipe system, water would build up in the street.
Photograph 1 – 7 Health Camp Road looking south
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Photograph 2 – Driveways of 7, 9 and 11 Health Camp Road looking north
Site Visit and Meeting
A meeting was held between Darryl Lew (KCDC), Mike Yarrall (Beca), the submitters John and Margaret (Peggy) Klimenko and Ron Couchman was held on the 16th December 2014.
Key points of the meeting included:
n P & J Klimenko have lived at 8 Health Camp Road for 11 years. R Couchman has lived at 11 Health Camp Road for 25 years. All three had not witnessed any surface ponding on their properties.
n The road has been known to locally flood. When this is blocked, it can spill on to the properties which are located below the road level.
n The submitters believed the maps showed that their property and the Otaki Beach area were being flooded as a result of inundation from the Waitohu Stream. Darryl Lew confirmed the ponding was not due to the Otaki River or Waitohu Stream. The submitters seemed to accept this, and acknowledged this addressed a significant concern of theirs.
n J Klimenko was concerned about soil variability and how the model reflected this. n J Klimenko said there had been 3-4 events exceeding the 100 year ARI, and no evidence of
ponding. n A brief inspection of the site was carried out at the time of the meeting.
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Data available
For this assessment we have relied on the following data sources:
• A meeting with the submitters and site visit to the properties.
• Kāpiti Coast District Council on-line GIS for data on stormwater systems, 0.5 m contours, property boundaries, natural hazard mapping and aerial photographs.
• Site-specific flood modelling results for the property supplied on request by Jacobs/SKM, who undertake the flood modelling for KCDC. Their input has also included specific responses to questions we have asked to enable us to properly understand the reliability and applicability of the modelling in this area.
• The District Plan.
• Review of rainfall records for the nearest rain gauge with records at Otaki Depot.
Appropriate standards
In assessing flood vulnerability, it is important to use appropriate standards, and to understand how stormwater systems are designed and managed.
Primary flow is typically contained within a formal drainage system such as pipes or drains (and does not “cause nuisance”), while secondary flow will often flow along streets or overland through properties, or pond in natural flood plains.
It is common practice across New Zealand for primary stormwater systems to be designed to accommodate the 5 year or 10 year storm runoff (20% or 10% Annual Exceedance Probability - AEP), and for consideration to be given to the effects of secondary flow in the 50 or 100 year storm (2% or 1% AEP). KCDC typically designs its primary system for the 10% AEP storm, although in older urban areas historical standards may be lower. Therefore inevitably there will be insufficient capacity in the primary system for larger storms, resulting in temporary ponding or flow in the street or secondary flow onto adjacent land.
KCDC has plotted the flood extent for the 1% AEP storm (including freeboard to allow for uncertainty in the modelling), which aligns with their design standard for overland flow
We note that there is a trend across New Zealand towards the adoption of 1% AEP as the assessment standard for secondary flow and flood protection, and in some major flood protection schemes for urban areas even higher standards are adopted.
In its District Plan KCDC requires building floor levels to be above the 2% AEP flood levels (or where there is more than one building, above the 1% AEP flood level), and recommends that all new or relocated building floors be above the 1% AEP flood level.
The KCDC Subdivision and Development Principles and Requirements (2012) requires secondary flow paths to be designed to adequately cater for the full 1% AEP (100 year) flow less an appropriate contribution from the primary drainage system.
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We have undertaken this assessment against the 1% AEP flood modelling.
Flood modelling Flood model reliability and interpretation
The flood analysis tools now available use “2D” models that can operate catchment-wide, and will pick up localised areas where there is isolated flooding. They also use more accurate and detailed LiDAR survey data to delineate catchment, flow paths and ponding areas, and can interface between surface flow paths and underground pipe systems to provide a reasonably reliable prediction of flood levels. The results of updated modelling carried out by Jacobs/SKM for KCDC is presented as “District Plan Natural Hazards”, “Latest Natural Hazards” and “Latest Flood Extent” data on KCDC’s on-line GIS system, at http://apps.geocirrus.co.nz/?Viewer=KCDC. An extract showing the flood risk identified on this mapping is attached as Figure 1. From the point of view of data reliability, it is appropriate that this latest flood data is used for assessing flood vulnerability, rather than the District Plan maps.
Note that the current natural hazards map in the Proposed District Plan was published in October 2012 and appears to be consistent with the information online.
Modelling
The modelling KCDC is using for the catchment represent current “best practice” in terms of tools and methods, although there is still a need for appropriate application of data, linking of model components, and calibration. Flood hydrology for the catchment has been calibrated, and takes account of different runoff characteristics for the different soil types and land uses, including relatively low runoff rates for sand areas.
We have obtained specific information from Jacobs SKM, who undertook the modelling. Specific matters we have addressed with them, including their responses, are listed below.
1. The rainfall intensity used in the modelling allows for climate change, in accordance with accepted practice and KCDC guidelines.
2. The recommended building level at 7 and 11 Health Camp road is 3.5m RL.
3. Tide level assumed includes 0.8 m of sea level rise for climate change, plus a 20 year ARI tide. Although the primary discharge from the reticulated stormwater network is to the Waitohu Stream, the sea level would be expected to affect the groundwater level and therefore infiltration rates and capacity.
4. The flood hazard modelling that informed the flood hazard mapping of the local streams and stormwater networks was based on the 100 year average recurrence interval rainfall event. Following sensitivity testing this event was modelled to coincide with the peak flows anticipated from a ten year average recurrence interval flood event in the Waitohu Stream. However it should be noted that the flood hazard maps proposed for the district plan are a combination of the local flood hazards plus Greater Wellington Regional Council flood hazards maps of the Waitohu Stream and Otaki River.
5. The hydraulic modelling has assumed water levels in the Otaki River mouth (either because of high flows in the Otaki River or high sea levels) are at a level where the flood gates on the Rangiuru Stream under the Otaki stopbanks are closed. It should be noted that the flood hazard maps proposed for the district plan are a combination of the hazard from the modelling of the local streams and networks combined with Greater Wellington Regional Councils hazard maps of the Waitohu and Otaki River. In the low lying area surrounding the lower Rangiuru Stream the flood hazard maps are dominated by the GWRC flood hazard modelling.
6. The standard hydrological modelling method prescribed by the Kapiti Coast District Council is based on the Clark Unit Hydrograph using SCS Curve Numbers. The infiltration to the soil is accounted for in two ways, through an initial abstraction, that is the depth of rainfall before runoff will occur, and through ongoing losses that depend on the underlying soil type.
7. The hydrological modelling takes into account the soils in this area. Runoff from impervious areas (roads, driveways, roofs) is at a much higher rate than for the pervious areas (lawns, gardens).
8. Locally the infiltration rates used in the modelling have been selected with consideration of the local ground conditions and infiltration rates. It should be noted that a number of infiltration tests have been carried out in the Otaki area by council and others by developers. While infiltration on the dunes is generally consistent, in low lying areas and on the alluvial (river gravels) material on which much of the Otaki township is built there are wide ranges in the recorded infiltration rates even in sites close to each other. In the Otaki area the hydraulic modelling results of the streams and pipe networks were verified against flood records and showed a close match to the available information.
9. Based on investigations and research undertaken in 2001 by Connell Wagner for KCDC initial abstraction losses of 3 mm have been have been applied to the area surrounding 7 and 11 Health Camp Road.
10. GNS maps (Begg, J.G.; Johnston, M.R. Geology of the Wellington Area) were used to determine the underlying soil type. This information identifies 7 and 11 Health Camp Road as being on “stable inland dunes”. Based on this hydrological parameters were assigned (CN of 65) representative of Group B soils which have moderate infiltration rates when thoroughly wetted (4-8 mm/h). Inland compacted sands, less well drained gravel loams and valley based gravel loams would typically fall within this soil group.
11. The modelling has assumed that the soils in this area will have moderate infiltration rates but has not factored in the elevated ground water levels that could further restrict infiltration from this site. Much of Otaki Beach, including the area surrounding 7 and 11 Health Camp Road, is at a level of approximately 3.0 m aMSL. Modelling for the development of the flood hazard maps includes a peak sea level of 3.3 m aMSL. This suggests that during the peak of the tide infiltration losses could be overestimated. Note that this sea level is a future level and includes an allowance for climate change. The level of the frontal dune and flood gates on the stream would help to prevent inundation from the sea in this situation.
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12. The modelling does not include any allowance for infiltration to ground of water that runs off impervious areas and is ponded in the low area. This is likely to be slightly conservative, although not overly so as the land is low-lying, and given the rainfall depths associated with the 1% AEP storms, and given the modelled sea level there will be some limitation as to how quickly that water can drain. An example of this is the ponding in the low lying areas near the centre of Paekakariki which were flooded for several days after the 2003 floods.
13. At a district level the hydrological approach, including the parameters that account for infiltration loses, used on the Kapiti Coast was independently peer reviewed. The review covered the methodology and parameter assumptions and calibration of the Wharemauku Stream against the weir record. The general conclusion of the review was that the methodology being used was appropriate but would benefit from further calibration where calibration information is available which was subsequently carried out.
Additional Review of Data Flood Level
The 1% AEP ponding level identified in the modelling is 3.5 mRL.
This compares with a level of approximately 3m at the properties, and is approximately the same as that of the northwest side of the road. As the properties are below the road level and appear to sit in a localised low point, there is the potential for any flow in the road above kerb level to spill into the properties. The owners have reported the water level has previously almost reached the top of the footpath.
Note that this level includes an allowance of 300mm for freeboard in addition to the modelled level. However, this is considered appropriate to allow for blockage in the piped network and other unaccounted-for obstructions in the overland flow path.
Rainfall
The rainfall records for the Otaki Depot rain gauge were obtained from Greater Wellington Regional Council. This is the closest rain gauge to the site for which records were available. The records were compared to the Annual Return Intervals (ARI) from the KCDC Isohyet Maps and the NIWA HIRDS database. This showed there were only two instances (23/04/95 and 21/10/98) since 1984 where the 24 hour rainfall exceeded the 5 year ARI rainfall, and both of these were less than the 10 year ARI. With the climate change scenario adopted by KCDC, the events were less than a 5 year ARI.
There was one event (07/02/96) since the start of hourly records in 1992 where the hourly rainfall exceeded the 60 year ARI, but this was less than the 80 year ARI. With the climate change scenario adopted by KCDC this would be between a 30 to 40 year ARI. The remainder of the rain events were a 10 year ARI or less.
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Infiltration
Jacobs confirmed that GNS maps (Begg, J.G.; Johnston, M.R. Geology of the Wellington Area) were used to determine the underlying soil type. This information identifies 7 and 11 Health Camp Road as being on “stable inland dunes”. Based on this they have assigned hydrological parameters (CN of 65) representative of Group B soils which have moderate infiltration rates when thoroughly wetted (4-8 mm/h). Inland compacted sands, less well drained gravel loams and valley based gravel loams would typically fall within this soil group.
The hydrological model does not factor in the effect of groundwater level on infiltration. As the peak sea level is expected to be about 0.3 m higher than the ground level at the property, this is likely to overestimate infiltration into the soils in extreme events.
Conclusion
In conclusion,
• Hydrological / flood hazard hydraulic models are not precise, but allow large areas to be assessed for the risk of flooding.
• The latest flood modelling has both elements of conservatism and underestimation of risks, but overall appears to provide an appropriate representation of the likely flood level in this area.
• The model indicates flooding in the Otaki Beach area is primarily due to rainfall runoff that is unable to be conveyed by the reticulated network.
• The model does not factor in the effect of groundwater level on infiltration. As the peak sea level is expected to be higher than the ground level at low lying properties in Otaki Beach (including 7 and 11 Health Camp Road) this is likely to overestimate infiltration into the soils in the model.
• Based in LiDAR information, the topography around 7 and 11 Health Camp Road indicates the properties are situated in a localised low point with no overland flow path, and with potential for any flow in the road above kerb level to spill into the properties. The owners noted that the water level has previously almost reached the top of the footpath. The combination of greater rainfall and the allowance of freeboard would increase the level above the footpath.
• Although the current owners have not experienced any ponding on the site, rainfall records show that a 100 year event has not been experienced at the property.
• The flood level shown on the maps includes an allowance for freeboard above the modelled flood level. This is considered reasonable to account for factors such as parked cars and blocked drains which can affect the overland flow, and due to uncertainties in the modelling.
In summary, the flood maps are considered appropriate in this area.
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Yours sincerely Mike Yarrall Senior Water Engineer on behalf of
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