5 July 2023
Media: Local Democracy Reporter
Topic: High lake levels at Rotoma and Rotoehu
Enquiry
I am writing a bigger piece on the situation with the lake levels, I was wondering if I could please get a bit of info?
Wider qs:
What examples can be given of the impact lake levels have had in the district?
How many jetties and boat ramps are in the district and how many are under water?
What RLC roads are underwater, which are threatened? Closed?
What damage has been done to council infrastructure/facilities/spaces, and where?
Hopes, concerns and expectations going forward? is El Nino expected to have any impact?
Any other comment?
Response
From RLC Infrastructure and Environmental Solutions DCE Stavros Michael:
The Rotomā/Rotoehu Response Team (RRRT) acknowledges the impact and stress this situation is causing affected residents. Their safety is our priority and we are here to provide whatever support we can. Anyone who feels unsafe should call us on (07) 348 4199.
Council has received 15 calls for a range of reasons including seeking rates relief and requests for sand bags and road closure information. Currently, Manawahe Road and Pongakawa Valley Road between Kennedy Bay and Soda Springs have been closed to all traffic. We understand there may be frustration around road closures but ask that everyone respect the barriers that are in place to protect our communities and not attempt to move them, which we are aware has happened.
We are watching lake levels closely and assisting, where we can, to continue to ensure safe access and offer assistance to landowners where possible.
The challenges created by elevated water levels are due to record rainfall and there has been inundation of homes, roads and networks infrastructure, as well as significant erosion and inundation of land. Inundation from a lake is not like inundation from a river where properties are flooded and then the flows recede quickly. With lakes levels rising over time and not receding, property inundation may happen over time and even when the waters eventually recede, the ground may be compromised.
Persistent high lake levels are generating a range of issues including:
- Erosion at the lake margins;
- Raised groundwater levels;
- Undermining of embankments and road foundations;
- Increased infiltration of water into the wastewater network;
- Increased flood risk due to higher tail water (at river mouths and upstream of elevated lake levels).
- Erosion at the lake margins;
- Raised groundwater levels;
- Undermining of embankments and road foundations;
- Increased infiltration of water into the wastewater network;
- Increased flood risk due to higher tail water (at river mouths and upstream of elevated lake levels).
Groundwater
High lake levels raise the groundwater table in areas near the periphery of the lake. This has the following implications:
- Increased water infiltration into the wastewater network, creating challenges with effective treatment and final disposal costs;
- Reduction in the rainfall infiltration capacity of the land where the groundwater table is elevated, resulting in higher volumes of rainfall runoff at higher concentration velocities during storm events and reduction in infiltration rates for any soakage systems;
- Groundwater coming into contact with wastewater disposal fields for septic tanks which can cause contamination;
- Can cause challenging geotechnical issues such as rate of settlement or dampness under buildings and foundations;
- Warm water has been coming to the surface in some areas as Lake Rotomā has risen. There has been so much rainfall, that the saturation of the groundwater is forcing warm water to the surface. There is naturally geothermal activity in this area at depth and the high rainfall and high lake levels have recharged groundwater in the area to saturation, which forces the warm water to the surface. The warm water is also bringing other minerals up from deeper areas, evident by orange coloration that is typical of water containing high levels of iron.
Persistent high lake levels are generating a range of issues including:
Implications on flooding and stream processes:
High lake levels reduce the velocity of the stream flows entering the lake. This encourages sediment to be deposited at the stream mouth, further reducing the hydraulic capacity and potentially worsening flooding.
Shoreline erosion:
Sustained high lake levels increase the risk of shoreline erosion due to wind waves. Uncertainty in the range of lake levels is a factor in land owners (including RLC) opting to construct hard erosion protection structures such as rock revetments. If the lake level limit could be more reliably controlled, land owners may be able to justify a lower level of protection using soft options such as planting or beach nourishment to cater for shoreline erosion.
Note to reporter: In regards to SH30, you will need to contact Waka Kotahi to see how the lake levels have affected it.