Horticultural Project : Holsworthy FRA Flood Risk Assessment Revision – Horticultural Project : Greenhouses, Packaging Building, Access Roads & Parking Areas, Holsworthy Bio-Gas Plant, Holsworthy, Devon 1 The site is not within a designated Flood Zone. However, the site is within a Critical Drainage Area. 2 Flowers are grown in the Greenhouse 365 days per day. As a consequence, there is a constant demand for water. Rainwater is harvested the from the glasshouse roofs into underground storage tanks. It will be stored until needed and then pumped back and filtered for use in irrigating the growing plants. This will also require the construction of three rainwater tanks. These will be of a size to cater for flood events as well as day-to-day needs. In the case of a shortfall a borehole will be installed to augment the supply. 3 The annual demand for water is approx. 58,200 m3 for these Greenhouses. This significantly exceeds the Annual Average Rainfall. Based on AAR of 1,250mm, this equates to an annual rainfall harvesting of 24,880 m3. Therefore the borehole will have to provide some 33,320 m3 per annum. Clearly the holding tanks will require ‘spare’ capacity for a storm event. Also the ‘Outflow’ figure can only take account of the minimum water demand. 4 Assessment of Storage Capacity Return Period : 100 years + 30% allowances for peak rainfall intensity. Ratio of 60 minute to 2 day rainfalls of 5 year return period (BRE 365, Fig. 1), r = 0.25 Impermeable Area A = 24,311 m2 ‘Outflow’ = Min. Water Demand1 + Greenfield Run-off Rate2 O = 0.00184 m3/s + 0.0524 m3/s = 0.0542 m3/s M5 rainfalls are calculated from table 1, BRE 365, Factor Z1 Duration 5 mins 10 mins 30 mins 1 hour 2 hours 4 hours 6 hours 10 hours 24 hours 1 2 M5 rainfalls mm 6.2 9.2 15.0 20.0 25.8 33.8 39.2 47.0 65.4 Growth factor Z2 1.80 1.91 1.99 2.03 2.01 1.94 1.89 1.83 1.71 100 year rainfall x 30% 14.5 22.8 38.8 52.8 67.4 85.2 96.3 111.8 145.4 Inflow m3 352.7 555.4 943.4 1283.1 1638.9 2072.4 2341.5 2718.3 3534.4 Outflow m3 16.3 32.5 97.6 195.1 390.2 780.5 1170.7 1951.2 4682.9 Storage required 336.4 522.8 845.8 1088.0 1248.7 1291.9 1170.8 767.1 # Min. Water Demand, based on 8l/m2/day = 1.84 l/s Greenfield Run-off Rate : HR Wallingford calculation attached. Sherry Consultants May 2014 Horticultural Project : Holsworthy FRA 5 Therefore the maximum required storage capacity for a storm event is 1,292 m3 6 If it assumed that there is no water demand from the plants, using just the greenfield run-off rate of 0.0524 m3/s then :Duration 5 mins 10 mins 30 mins 1 hour 2 hours 4 hours 6 hours 10 hours 24 hours M5 Growth factor rainfalls mm Z2 6.2 1.80 9.2 1.91 15.0 1.99 20.0 2.03 25.8 2.01 33.8 1.94 39.2 1.89 47.0 1.83 65.4 1.71 100 year rainfall x 30% 14.5 22.8 38.8 52.8 67.4 85.2 96.3 111.8 145.4 Inflow m3 352.7 555.4 943.4 1283.1 1638.9 2072.4 2341.5 2718.3 3534.4 Outflow m3 15.7 31.4 94.3 188.6 377.3 754.6 1131.8 1886.4 4527.4 Storage required 337.0 523.9 849.1 1094.5 1261.7 1317.8 1209.7 831.9 # Then the maximum required storage capacity for a storm event is 1,318 m3. (This demonstrates that the crop water demand is not significant.) 7 Risk Assessment 7.1 Andigestion Ltd. owns all of the land between the Greenhouse and the river on the northern boundary. There are no properties adjacent to the proposed development. The tributary runs to west where it joins the River Deer. The nearest property that is adjacent to the River Deer is at Manworthy Mill. This is some 1,500m from the proposed surface water discharge point. The next properties on the River Deer are around Rydon Bridge, A3072. This is a further 1,200m downstream. Sherry Consultants May 2014 Horticultural Project : Holsworthy 7.2 FRA In the event of a major failure, there will be localised flooding around the proposed discharge point. There will be no immediate effect on property, buildings or third parties. There may be some small increase in water levels around Manworthy Mill and possibly Rydon Bridge. 8 Proposed Flood Storage Measures 8.1 It is proposed to construct a lagoon (or grass swale) with a capacity of 1,318 m3 + 10%, say 1,500 m3. There will be a hydrobrake (or other agreed restriction) to restrict the outflow to the greenfield run-off rate of 0.0524 m3/s. S J Sherry Sherry Consultants BSc, CEng, MICE, MCIWEM, MCIWM. May 2014 Greenfield runoff estimation for sites Site name: Bio-Gas Plant Site coordinates Site location: EX22 7HH Latitude: 50.82484° N Longitude: 4.3508° W This is an estimation of the greenfield runoff rate limits that are needed to meet normal best practice criteria in line with Environment Agency guidance “Preliminary rainfall runoff management for developments”, W5-074/A/TR1/1 rev. E (2012) and the CIRIA SUDS Manual (2007). It is not to be used for detailed design of drainage systems. It is recommended that every drainage scheme uses hydraulic modelling software to finalise volume requirements and design details before drawings are produced. Site characteristics Reference: gchchuru5hbk / 3.69 Date: 28 May 2014 Hydrological characteristics Default Edited 3.69 ha SAAR 1178 1178 mm Significant public open space 1.26 ha M5-60 Rainfall Depth 20 20 mm Area positively drained 2.43 ha ‘r’ Ratio M5-60/M5-2 day 0.3 0.3 FEH/FSR conversion factor 0.88 0.88 Hydrological region 8 8 Growth curve factor: 1 year 0.78 0.78 Growth curve factor: 10 year 1.49 1.49 Growth curve factor: 30 year 1.95 1.95 Growth curve factor: 100 year 2.43 2.43 Total site area Methodology Greenfield runoff method IH124 Qbar estimation method Calculate from SPR and SAAR SPR estimation method Calculate from SOIL type SOIL type 4 HOST class N/A SPR 0.47 Greenfield runoff rates Default Edited Qbar 21.57 21.57 l/s 1 in 1 year 16.82 16.82 l/s 1 in 30 years 42.06 42.06 l/s 1 in 100 years 52.41 52.41 l/s Please note that a minimum flow of 5 l/s applies to any site HR Wallingford Ltd, the Environment Agency and any local authority are not liable for the performance of a drainage scheme which is based upon the output of this report. www.hrwallingford.com Greenfield runoff estimation for sites Site name: Bio-Gas Plant Site coordinates Site location: EX22 7HH Latitude: 50.82484° N Longitude: 4.3508° W This is an estimation of the greenfield runoff rate limits that are needed to meet normal best practice criteria in line with Environment Agency guidance “Preliminary rainfall runoff management for developments”, W5-074/A/TR1/1 rev. E (2012) and the CIRIA SUDS Manual (2007). It is not to be used for detailed design of drainage systems. It is recommended that every drainage scheme uses hydraulic modelling software to finalise volume requirements and design details before drawings are produced. Site characteristics Reference: gchchuru5hbk / 3.69 Date: 28 May 2014 Hydrological characteristics Default Edited 3.69 ha SAAR 1178 1178 mm Significant public open space 1.26 ha M5-60 Rainfall Depth 20 20 mm Area positively drained 2.43 ha ‘r’ Ratio M5-60/M5-2 day 0.3 0.3 FEH/FSR conversion factor 0.88 0.88 Hydrological region 8 8 Growth curve factor: 1 year 0.78 0.78 Growth curve factor: 10 year 1.49 1.49 Growth curve factor: 30 year 1.95 1.95 Growth curve factor: 100 year 2.43 2.43 Total site area Methodology Greenfield runoff method FEH Qmed estimation method Calculate from BFI and SAAR BFI and SPR estimation method Calculate from dominant HOST HOST class N/A BFI / BFIHOST 0.00 Qmed N/A Qbar / Qmed Conversion Factor N/A Greenfield runoff rates l/s Default Edited Qbar --- --- l/s 1 in 1 year --- --- l/s 1 in 30 years --- --- l/s 1 in 100 years --- --- l/s Please note that a minimum flow of 5 l/s applies to any site HR Wallingford Ltd, the Environment Agency and any local authority are not liable for the performance of a drainage scheme which is based upon the output of this report. www.hrwallingford.com