| 1) Run-off Area: (a) Roofs (b) Hard externals e.g. roads, car parking, service yards, driveways, footpaths etc. (these can be grouped into an overall total area). The reasons why we differentiate these areas are (i) we usually assign a 95% run-off rate to roofs and 90% to externals, but of more importance (ii) roof run-off does not require the same level of treatment as external run-off, which may require several phases of treatment. |
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| 2) Storm Event(s) e.g. 1 in 10 years, 1 in 30 years, 1 in 100 years NB Typically we design for roofs to be attenuated/infiltrated at 1 in 30 and externals at 1 in 10 with 1 in 100 year events either temporarily stored above ground (if not allowed to leave site) or routed overland -off site (if allowed to leave site)
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| 3) Discharge Consent e.g. 5 l/s/ha (typical Greenfield run-off rate) or often expressed as a total figure for the development e.g. 20 l/s |
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4) Development plan showing buildings and external areas with proposed finished ground levels and proposed discharge location(s) (preferably in an electronic .dwg file to aid take-off accuracy and to facilitate overlaying of alternative proposals)
NB As an alternative to 2, 3 and 4 we may simply use a given attenuation volume set by another consultant, which we will typically re-distribute according to catchment sizes.
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| 5) Information to determine infiltration suitability - (a) soil permeability - ideally specific value(s) e.g. 1 x 10-5 m/s or soil descriptions e.g. 'sands', 'gravels', 'clays' etc and this should ideally be in the form of trial pit or borehole records (with a location plan if available) (b) water table level -(EA usually require minimum of 1m 'free-board' between water table and invert of soakaways) (c) Presence of contamination - (often infiltration is not acceptable over/through contaminated soils). |
| (a) soil permeability |
| (b) water table level |
| (c) Presence of contamination |
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| 6) Abnormals (regardless of infiltration considerations) e.g. high water table, contamination, access restrictions, shallow rockhead, very low soil CBR's, pumping due to outfall discharge levels AOD etc |
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| 7) Traffic loadings e.g. non-trafficked, cars only, HGV |
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| Obviously, the more information we are provided with, the greater the accuracy and ability to provide an optimised cost-effective design; however, it is possible to offer stated assumptions (or range of assumptions) where information is not available. |
