The proposals initially lay partly within SEPA‘s surface water and small watercourse flood mapping and required the diversion of an existing drainage ditch – a combination that had attracted a holding objection from SEPA prior to Aegaea’s involvement.

Through detailed flood modelling of baseline conditions, the proposed realigned channel, and the resulting flood levels, Aegaea demonstrated that the scheme would not increase flood risk elsewhere and could be made safe and operational throughout its design life, both of which are critical requirements of NPF4 Policy 22, and North Lanarkshire’s Supplementary Planning Guidance documentation SPH09 – which states that new development should not be at risk of flooding, materially increase the probability of flooding elsewhere, or increase areas of land which require protection by flood prevention measures.

Policies DSP2 (B) and DSP4 of the North Lanarkshire Local Plan (2022) were also adhered to – which state that new developments must protect and enhance the physical characteristics of watercourses throughout the region

SEPA subsequently removed its holding objection, and the watercourse diversion was consented under a EASR licence, with North Lanarkshire Council granting full planning permission.

To resolve SEPA’s objection, Aegaea developed a site-specific 2D hydraulic flood model representing the drainage ditch, its structures and the surrounding ground, and used it to inform the design at three key stages.

1. Establishing baseline conditions. Aegaea built a baseline flood model of the existing, pre-development site. This characterised how surface water currently behaves across the site — confirming that the published flood extents are driven by runoff from the higher ground to the east interacting with the existing ditch and its culverted outfall. 
2. Testing the proposed realigned channel. The proposed diversion, designed by Woolgar Hunter Engineers, replaces the existing ditch with a new 155m open channel — retaining a comparable length of open watercourse — with a 1.5m base, graded side slopes, and a new headwall replicating the existing culvert inlet dimensions, controlled by an orifice plate at a new manhole. Aegaea modelled this proposed configuration for the critical 1 in 200 year plus climate change event across a range of storm durations. To meet SEPA’s requirements, culvert blockage scenarios of 25% and 50% were also tested and compared against equivalent blockage scenarios for the existing culvert. Comparing the design and baseline models confirmed betterment downstream — a general reduction in peak flood depths — with no increase in risk elsewhere.
3. Informing finished floor levels. The modelling defined a peak water level of 132.13m AOD for the 1 in 200 year plus climate change event including a 25% blockage. On this basis, Aegaea recommended raising the finished floor level of the eastern extension by 0.15m so that it sits safely above the design flood level. Recognising the operational realities of an Essential Infrastructure site — where heavy plant and generators require level, accessible enclosures that would not be entered during a severe weather warning — Aegaea presented the modelled evidence to justify a proportionate 150mm freeboard for the plant enclosures, while keeping the development safe and operational in line with Policy 22.

The findings were documented in a hydraulic modelling report (including completion of the required SEPA’s FRA Checklist) and a supporting Flood Risk Assessment, setting out the methodology, topographic inputs, blockage assumptions and the pre- versus post-development comparison.