RFI - Appendix 5 - Water Managment Plan · MCU19/0117

1.0 Introduction and Background
Tod Consulting has been engaged by Mark Bain Constructions to provide engineering advice on a
proposed wave pool at 90 Beach Road North Shore. The purpose of this report is to outline the
strategies proposed to manage the operational water level in the proposed wave pool.

The proposed pool will cover an approximate area of 4,000m2, and will vary from 0.0 to 1.5m in
depth. The pool will be contained within walls approximately 2.0m high above normal water levels
in order to retain the design wave heights. The operational tolerance for the water level within the
pool is +/- 100mm


2.0 Maintaining water levels in the pool
As noted above the operational tolerance for the wave pool is +/- 100mm. In order to maintain the
water level in the pool within this operational range it will be necessary to top the water level up
during prolonged dry periods and dispose of excess water during prolonged wet periods.

Using Bureau of Meteorology data on rainfall and evaporation rates we have calculated the
necessary disposal volume for the 95th percentile wettest day and month during any one year and
likewise determined the required top up volume based on 5th percentile driest month.

This information is tabulated in Appendix A & B. In summary (after operational tolerances are
allowed for):

    •   The worst month for the 95th percentile wettest month (March) results in a disposal volume
        of 1,448,000 Litres.

    •   The worst case of the 95th percentile wettest day results in a disposal volume of 840,000
        Litres.

The worst case scenario for the 5th percentile lowest rainfall month (January) (95% driest) results
in a demand volume for top up of 160,000 Litres, however the cumulative effect of consecutive dry
months (Oct-Feb) comes to 329,600 litres.

Geotechnical investigations and testing have revealed that the existing ground conditions for the
site consist mainly of sands with a hydraulic infiltration rate of 1.5m per day (62.5mm/hr).

As such it is proposed to install one or more water tanks with natural ground base to dispose of any
surplus pool water via ground infiltration. The water from the pool will pass through a dosing process
to neutralise any chlorine or salt treatment used for the pool water prior to it being placed into the
water tank for infiltration into the ground. Based on our analysis we believe that a single tank 12m
in diameter and 2.5m high (capacity of 280,000 litres) will be enough to maintain the water levels at
operational levels for all but a few days of the year during the 95th percentile wettest conditions.
Should the pool operator wish to reduce the number of “down days” then a second tank could to
provided. Note the average infiltration volume for a 12m diameter tank equates to 169,000 litres/day.

To cater for the driest periods, again a single tank of a similar size (with a sealed base) will be
enough to cater for the majority of the 5th percentile driest months. However as noted above should
cumulative dry months occur consecutively then there is a risk that the water level in the pool could
drop below the operational level for an extended period (more than one month). As such a second
tank may be considered necessary.

As noted in the introduction the pool will have 2m high sides to retain the proposed wave, as such
up to 2m of rainfall can occur without the pool over topping. This represents almost an entire year
of rainfall for the 90th percentile wettest conditions. As such and based on the proposed disposal
method for excess water noted above the disposal of excess water can be easily managed without
the need for any emergency overflow weir.

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