Designing for a small holiday community

Riversdale is a small coastal holiday community on the Wairarapa coast. The community is to be provided with a new sewerage scheme. Designing the scheme presented particular challenges due to a very low permanent population, extreme population increases during summer holiday period and a high groundwater table in sandy soils. Land application of the treated wastewater to a limited area required design flows to be developed with a high degree of confidence.

A daily population model was developed for Riversdale Beach by undertaking an occupancy survey over the summer holiday period and combining this with census data.

Infiltration inflow rates were assessed by first reviewing operational data from several Wastewater Treatment Plants (WWTPs) around New Zealand and typical design allowances for inflow and infiltration (I&I). This information was then assessed in the context of a significant portion of a possible gravity collection system being permanently submerged below groundwater levels.

The design flows analysis shows that groundwater inflow rates and volumes are critical factors in the design of the proposed sewerage scheme. As a result of the analysis, a pressurised wastewater collection system that effective eliminates infiltration inflow was recommended for consideration for the proposed scheme.

The design flows established were then put through an in-depth water balance modelling exercise as part of the land application design, which is the subject of another paper being presented by Hamish Lowe at this conference.