In many areas, the Council plans are based on a rapid flood hazard assessment approach and are generally conservative in nature. Flood prone areas may also over-estimate or under-estimate the extent of ponding depending on the situation. We have expertise at assessing all of these problems, as demonstrated by the following projects:
- Waitemata Drive, Ranui: Owner wanted to add minor unit to rear of existing dwelling in area subject to overland flowpath from 1.5ha catchment. We assessed the existing overland flowpath and made some changes to it to allow overland flow to pass the new unit without changing its entry point from the road and its exit point to the adjacent lot. We also specified the required finished floor level for the minor unit.
- Oxford St, Manurewa: Owner wanted to subdivide the existing lot into three new lots and construct additional houses on-site. The site is subject to an overland flowpath and partially located in a flood prone and floodplain area. We undertook a site investigation which determined ponding was caused by water backing up behind Gloucester Road. We then developed a HEC-HMS model which assessed the 100 year ponding water depth, which was used to establish the required finished floor level for the new dwellings.
- Misty Place, Papatoetoe: Owner wanted to undertake a 3 lot subdivision of the site, which is located adjacent to a watercourse within the Kenderdine catchment where Fraser Thomas has a lot of experience, having prepared a Catchment Management Plan (CMP) for this area some years ago. We had to determine appropriate peak flood flows and water levels, allowing for partial blockage of the downstream culvert under the motorway and use culvert and open channel design software, HY-8 and HEC-RAS to determine corresponding flood water levels adjacent to the property.