<ul><li><p>SoakawaysDesign Example for BRE Digest 365If the area of your development being drained by a soakaway exceeds 100m2 it mustbe designed in accordance with BRE Digest 365 to ensure that it will functioncorrectly.</p><p>Carrying out a soakaway test</p><p>In order to carry out the test you need to excavate a trial pit to an adequate depthbeneath the estimated invert level of the inlet pipe. This is to ensure that the soilbeing tested is representative of what will actually be constructed. The trial pit needsto be filled with water by a bowser to ensure it is filled as rapidly as possible. Using ahose is not acceptable as water may infiltrate prior to the pit being full renderinginaccurate results. The height of the water below ground level must be recorded atregular intervals in order for the calculation of the infiltration rate of the soil to bemade.</p><p>The BRE Digest 365 guidance document contains a worked example on page 5explaining how to calculate the soil infiltration rate.</p><p>Designing a rectangular soakaway</p><p>Example: Calculating the size of a soakway to receive storm water from 125m2</p><p>impermeable surface for a site in Shropshire. (Refer also to BRE Digest 365)</p><p>The following rainfall amounts have been calculated for a range of storm durations inShropshire using r= 0.39. These figures should be used when calculating the size ofyour soakaway.</p><p>Table 1: Rainfall results for a range of storm durations in Shropshire</p><p>Storm duration Dmin</p><p>M5-Dmin</p><p>=20mm x Z1</p><p>Z2 M10-D min=R mm</p><p>10 10.4 1.22 12.7</p><p>15 12.6 1.23 15.5</p><p>30 16.0 1.24 19.8</p><p>60 20.0 1.24 24.8</p><p>120 24.2 1.24 30.0</p></li><li><p>240 29.2 1.22 35.6</p><p>360 32.4 1.21 39.2</p><p>600 36.4 1.19 43.3</p><p>Assuming the results from a soakage trial pit giving a soil infiltration rate off = 3.33 x 10-5 m/s were obtained at the site, this can be used to design a soakawaywhich will be filled with granular material having 30% free volume. The percentagevoid space of any granular fill material must be pre-determined for use in the design.</p><p>Assuming the trial pit dimensions were 2.4m wide x 2.5m long x 1.5m (effectivestorage depth below incoming invert), this can then form part of the full scalesoakaway. The required width of the soakaway therefore needs to be calculateddepending on the rainfall amount and infiltration rate of the soil. The base area of thesoakaway is not included in the calculations as it is assumed that this will becomeless permeable over time due to siltation.</p><p>Calculate the design width of the soakaway: Volume equation I - O = S</p><p>Inflow to soakaway I</p><p>I = A x R</p><p>= Impermeable surface area of development x M10-D min rainfall</p><p>e.g. for 10 min storm duration, M10-10min = 12.7mm = 0.0127m</p><p>I = 125 x 0.0127</p><p>= 1.588m3</p><p>Outflow from soakaway O</p><p>O = a s50 x f x D</p><p>= Internal surface area of soakaway pit to 50% storage depth (excluding basearea) x soil percolation rate x storm duration</p><p>For rectangular pit 2.4m long x 1.5m effective depth x W m wide:</p><p>as50 = 2 x (2.4 + W) x (1.5 2)</p><p>= 3.6 + 1.5 W m 2</p><p>f = 3.3 x 10 -5 m/s from soakage trial</p><p>O = (3.6 + 1.5 W) x (3.3 x 10-5) x (D x 60) m3</p></li><li><p>Soakaway storage volume S</p><p>S = effective volume of soakaway with 30% free volume</p><p>= 2.4 x 1.5 x W x 0.3</p><p>= 1.08 W m3</p><p>For satisfactory storage of the M1010min runoff, therefore D = 10</p><p>I O = S</p><p>1.588 ((3.6 + 1.5 W) x (3.3 x 10 -5) x (10 x 60)) = 1.08 W</p><p>1.588 ((3.6 + 1.5 W) x 0.000033 x 600) = 1.08 W</p><p>1.588 ((3.6 + 1.5 W) x 0.0198) = 1.08 W</p><p>1.588 0.07128 0.0297W = 1.08W</p><p>1.5167 0.0297W = 1.08 W</p><p>1.5167 = 1.11W</p><p>1.366 = W</p><p>Required soakaway width, W = 1.366 m</p><p>Repeat the calculation for a range of M10-Dmin storms and determine the maximumwidth required for the soakaway, this is the size of soakaway you will need toconstruct. Results are summarised in Table 2.</p><p>Table 2: Rainfall results for a range of M10-D min storms</p><p>Storm duration D- min Required soakaway width W -m</p><p>10 1.37</p><p>15 1.63</p><p>30 1.93</p><p>60 2.12</p><p>120 2.02</p><p>240 1.53</p></li><li><p>As shown in Table 2 your results should show an increase in width to a certain pointand then decrease. The largest width calculated is that required for construction.</p><p>A soakaway 2.4m long x 1.5m effective depth x 2.12m wide would be suitable for thecritical storm duration of around 1 hour for a 10 year event.</p><p>To check the time for half emptying of the storage volume, ts50.</p><p>ts50 = S x 0.5 = (1.08 x 2.12)x 0.5______</p><p>as50 x f (3.6 + [1.5 x 2.12]) x (3.3 x 10-5) seconds</p><p>ts50 = 1.6 hours</p><p>This design is satisfactory with the soakaway half empty within 24 hours.</p><p>You should note that if your soakaway is designed to cater for flows from a 1 in 10year event, as above, you will still need to manage exceedance flows from a 1 in 100year (+ appropriate allowance for climate change) within your site boundary.</p><p>The following rainfall amounts have been calculated for a 100 year return periodstorm in Shropshire and can be used to calculate the size of soakaway required tocater for such an event. You should note, however, that an appropriate allowancefor climate change should be added (20% for non residential development, 30% forresidential development) when considering how flows generated by a 100 yearreturn event will be managed on site.</p><p>Table 3: Rainfall results for a range of storm durations</p><p>Storm duration Dmin</p><p>M5-Dmin</p><p>=20mm x Z1</p><p>Z2</p><p>(M100)</p><p>M10-D min=R mm</p><p>10 10.4 1.91 19.864</p><p>30 16.0 1.99 31.84</p><p>60 20.0 2.03 40.6</p><p>120 24.2 2.01 48.6</p><p>240 29.2 1.97 57.5</p><p>600 36.4 1.89 68.8</p></li></ul>