Tasman District Council manages various stopbanks along the Waimea, Lower Motueka and Riwaka Rivers for flood protection purposes. Maintenance of the Council owned banks is funded though collection of X river rates.
Under the 1941 Soil Conservation and River Control Act the Council is responsible for managing the stopbanks for the benefit of the whole community. There are also stopbanks that are privately maintained.
Keeping our stopbanks in peak condition is vital – a stopbank is only as good as its weakest part. No one wants to be liable for a section of bank in poor condition that fails in a flood. While it has been some decades since the stopbanked areas have received a major flood, we must not be complacent.
We require good access to the banks so that we can undertake regular inspections and maintenance. There are places where fences inhibit activities like mowing, gates have been blocked off or the Council has been locked out. There are also lengths of bank that are too narrow for safe access.
We do not encourage stock on Council-maintained banks. Unfortunately most of the Council’s stopbanks have been constructed with steep side slopes and most types of grazing will compromise them. This is particularly so in wet conditions or with larger animals such as cattle and horses. Where we own the land, we are progressively removing grazing rights to prevent further damage.
Large logs, debris or farm waste can direct flows into the stopbanks in flood conditions, they can mobilise and cause blockages, or cause damage directly to the stopbanks.
Regular crossing by vehicles or animals wear the stopbank crest down, creating a low point and weakness where the bank can potentially breach or fail in flood conditions. Track locations should be rationalised and surfaced using gravel or similar material to guard against this.
We need to approve any new or modification of existing structures on Council-owned stopbanks. Irrigation pipes, tracks, fences, etc., that can weaken the bank or hinder access need to be discussed with us.
Trees and shrubs can shade out the grass growing on the banks, hinder access and reduce bank performance in flood conditions. If they are planted too close, tree roots create pathways for water to enter the stopbank. They can also blow over in a storm creating a breach as the root ball up-ends and tears a hole in the bank. Large trees should be planted a minimum of 15m from the base of any stopbank.
The Council’s contractor needs to spray and control weeds on or adjacent to our stopbanks to ensure a healthy grass sward can be maintained. Grass is considered to be the best cover on stopbanks to resist scour in flood events.