Report To: Western Plains Drainage District Committee From: Drainage Manager Date: 14 February 2011 File reference: Document: 694443 Portfolio holder: Councillor Challis Meeting date: 22 February 2011 Subject: WPDD Drainage Manager Report February 2011 Recommendation: THAT the report be received. 1.0 OPERATIONS 1.1 Chemical spraying A small length of drains were sprayed, but holidays and unsuitable weather limited the activity in the period. 1.2 Machine cleaning A section of the Waikaka stream, and Patersons Outlet wee machine cleaned in the month. Drain blockages were removed from Coopers and Ngatea Town drains. 1.3 Maintenance/Fencing/Culverts No minor maintenance works were undertaken in the month. 1.4 Pump operation Regular inspection runs were undertaken in this period. Pump hours for January were 67 The total to the end of January is 13,443 hours, which is 137% of the annual average hours run. 57 man-hours were spent on weed screen cleaning. The total weed screen cleaning hours is 2.0% of the pump hours run, compared to the last four years average of 4.1%. In addition to the floodpump running, 126 hours of mobile pumping time were run. 1.5 Floodgates Floodgates cleared were Paul Leonard and Ngatea Town. Page 1 FRED_n694443_v1_WPDD_Drainage_Manager_Report_Feb_2011.doc 1.6 Stopbanks No stopbank maintenance was undertaken in the month. 2.0 BYLAW APPROVALS By law approvals were granted in the period as follows: 2.1 Culverts: Nil 2.2 Other: Nil 3.0 SUBDIVISIONS COMMENTED ON Subdivisions commented on during the period as follows: Nil 4.0 RAINFALL A chart of the Ngatea Monthly Rainfall depths since 1993 is attached. The rainfall total for January was 270 mm. The total from July to January is 843 mm which is 199 mm above the average for the period. Some extreme events were recorded in the western Hills over the night of 28-29 January. Six and 12 hour falls at Mangakawa and Mangatangi were between 50 and 80 years return periods. The six hour total of 99.5mm at Maukoro Landing is over 100 years ARI. 5.0 CAPITAL WORKS 5.1 Ngatea capital works The two capital works projects approved at the November meeting will be undertaken this summer. They are the piping of an open drain off Dent Street between Pinnock Place and Arapito Street, and the reach of the Paul Leonard Drain downstream of Pipiroa Road alongside the Catholic Church property. 5.2 Stopbank capital works Stopbank capital works programmed for the 2010-2011 year include the Waitakaruru RB downstream of the SH 25 bridge to the Indian floodgate, and part of the Maukoro Canal LB. Works on the Waitakaruru Stream can commence at the bridge and progress downstream. The reach of the Maukoro SB immediately upstream of the SH 25 bridge, opposite the school is the lowest point of the stopbank. Further investigation of this reach is required to determine the best solution. It has been commented that some excavation of the Waitakaruru Stream channel, to improve flood flows and provide material for stopbank maintenance may be an option to consider. Page 2 FRED_n694443_v1_WPDD_Drainage_Manager_Report_Feb_2011.doc 5.0 GENERAL 6.1 Kaiaua area Although WPDD has no responsibilities in the Kaiaua area, this report is included for members’ information. The area has experienced two storm/tide events in the last two weekends of January 2011. 1: 22-23 January Environment Waikato’s Tararua tide recorder peaked at RL 2.446 at 09.35 (NZST) on Sunday 23 January. The two following tides were also over 2.0m RL. In context, the highest recorded tide was estimated at RL 3.0m in May 1938. This is accepted as the 100 yr tide level and is the basis for the foreshore stopbank design levels for the Piako and Waihou river schemes stopbanks. In Jul 1995 a tide of 2.48m was recorded at the Tararu gauge. This tide reached as high as 2.9m in the south west Firth, and at points up river. The level of 2.5 was accepted as a 50 year tide level. The last tide overtopped the East Coast Road at Kaiaua and other points north. No regular rainfall readers have been ascertained in the area yet, but the EW gauge at Mangatangi recorded 165mm over the night of 22-23 January. No major stream problems were recorded in this event. A more comprehensive survey of damage, houses inundated etc is currently being undertaken. 2: 28-29 January (Auckland Anniversary weekend, cyclone Wilma) Steady overnight rain over much of Hauraki District resulted in flooding and ponding predominantly in the west and North West of the district, including the streams in the Kaiaua area. The overnight rainfalls averaged 140mm in Waihi, 75mm in Paeroa, 120mm in the Plains, over 200 in the Thames hills, and at Mangatangi 165mm were recorded. A preliminary analysis by EW of their recorder sites indicates some extreme events within the rainfall period, especially the 6 and 12 hour periods. Return period falls of 50 to 80 years were recorded at Mangatangi and Mangakawa, and at Maukoro Landing a 6 hour total in excess of 100 year ARI was noted. The main river systems in the Valley coped well. The four major catchments of the Kaiaua area experienced high flood flows, producing lateral erosion and in the case of Kaiaua village, significant flooding of property and buildings including the beach front store. A summary of preliminary investigation follows: Page 3 FRED_n694443_v1_WPDD_Drainage_Manager_Report_Feb_2011.doc Waharua Stream (400ha). Lateral erosion downstream of East Cost Road (ECR). Willow and other growth in channel up and downstream of ECR will need attention. Bridge structure integrity to be checked. Waihopuhopu Stream (450 ha). Trees in channel up and downstream of ECR. A weir structure immediately downstream of the road bridge will need attention. Whakatiwai Stream (1675 ha). Significant lateral erosion in the stopbanked reach upstream of ECR, and downstream adjacent to the ‘EcoQuest’ camp. Erosion of berms in the vicinity of the road bridge. This area apparently has had a drainage area established in the past. Stopbanks and channel straightening works were undertaken, but no ongoing rating was in place for maintenance of theses works. Land containing the stopbanks is in Council ownership. Some more recent channel protection works in the form of gabion baskets has been placed, but is being outflanked by channel movement. This stream has significance to the local iwi. There are urupa and waahi tapu areas near the stream mouth. Hauarahi Stream (1420 ha). This stream flows behind and through Kaiaua village. It overflowed its left bank and flooded dwellings, the Kaiaua store and the fire station. It cut a new channel through the bund in the beachfront reserve between ECR and the beach. Mangrove growth has contributed to the realignment of the channel in the vicinity of County Bridge, just upstream of the boat marina area. Most of the floodgated outlets outside of the drainage area appeared to cope well. There was considerable surface ponding in the Rangipo Stream area, but not for extended periods. The Taramaire Drainage district (TDD) suffered ponding but no tidal inundation. Several other piped and open drain channels to the sea experienced local ponding and had basement and garage areas flooded. Roles of EW and HDC EW has established catchment zones covering most of the Waikato Region. HDC lies within the Waihou-Piako Zone which covers the areas of the river schemes, and the Waitakaruru-Miranda-Kaiaua area to the catchment divide to the west. It is assumed that this zone will be extended to incorporate that part of Kaiaua now included in the Waikato Region. Within the zone there is an area which has a catchment rate applied. In the west the northern extent is the Waitakaruru Stream, Mahuta North Road, then south between SH 27and Mahuta South Road. The direct benefit (Drainage, River and Tidal flooding) areas of the PRS do not extend west of the Maukoro Canal. Thus the area north and west of the Waitakaruru Stream, including Miranda and Kaiaua, is not rated by EW for any sort of catchment works. It is the only part of the zone not included in this coverage. EW general, Lake Taupo, and biosecurity rates are levied in the area. HDC constructs and maintains tidal and river flooding assets in the area between the Waitakaruru Stream and Miranda, within the WPDD area. Apart from the TDD area of 700ha, HDC has no formal drainage responsibilities in the Kaiaua area. Stormwater reticulation assets are located in the villages of Kaiaua and Whakatiwai. Page 4 FRED_n694443_v1_WPDD_Drainage_Manager_Report_Feb_2011.doc There are numerous floodgated outlets, beachfront outfalls, and minor bunds and protection works in the area that do not appear to have proper provision made for maintenance. Council owns esplanade and minor reserves in the Kaiaua area, including the lower reach of the Whakatiwai Stream. Council has agreed that a scoping study of the issues will need to be undertaken, and recommendations made in consultation with the ratepayers for the future management of this area. 6.2 Waitakaruru area The rainfall event of 28-29 January has been noted above. Significant medium duration events (6 to 12 hours) were calculated to between 10 and 100year ARIs. The rainfall was generally very steady, overnight, with no high winds and consequent damage. The detailed rainfall analysis is attached. The Western Hills catchments appear to have received higher rainfall than the east. The Waitakaruru Stream and catchments to the south experienced flooding and ponding over much of the weekend. A few sandbags were placed at the SH 25 bridges to contain a small overflow downstream of the bridges.