AS/NZS 1547:2012 and DIR ADJUSTMENT for SIZING ON-SITE WASTEWATER DRIP IRRIGATION AREAS on SLOPING SITES

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1 AS/NZS 1547:2012 and DIR ADJUSTMENT for SIZING ON-SITE WASTEWATER DRIP IRRIGATION AREAS on SLOPING SITES 1.0 AS/NZS 1547 Table M2, DIR slope adjustment factors On-Site NewZ has received queries related to the origin of Table M2 [Ref.1, p.163] in the Standard. Questions asked include: Where did the Table M2 DIR slope adjustment factors come from? What actual and real risk(s) are being addressed here? Are they based on real field experience/research? If so can the reference be provided? It has been pointed out that the factors used in Table M2 are severe and that in some locations around the country they may well be impossible to comply with. 2.0 Dealing with Slope The Standard provides guidance in Appendix K on selecting and installing on-site wastewater systems on sloping sites. Table K2 [Ref.1, p.136] sets out guidelines for indicating when slope may be a constraint for individual land application methods, and provides information on possible modifications to mitigate slope constraints as well as information on installation practice and system management where installed on slopes. The relevant section of Table K2 is reproduced as Attachment 1 below. The slope value limits beyond which it is considered that design adjustment may have to be made are: Surface irrigation (spray and drip) up to 10% slope; ETA/ETS beds and trenches - up to 10% slope; Mounds, conventional trenches, LPED up to 15% slope; and Subsurface drip irrigation up to 30% slope It is noted in the table that use of land application systems on greater slopes should not be precluded where a careful assessment of the site and soil conditions and contaminant pathways is completed, as soil depth and soil category are often more important factors than slope. Hence no limit is placed on overall slope values above which on-site wastewater management is not feasible. However, for drip irrigation systems an adjustment for reducing design irrigation rate (DIR) at various slopes is provided in Table M2 [Ref.1, p.163] as follows: TABLE M2 RECOMMENDED REDUCTIONS IN DIR ACCORDING TO SLOPE Flat up to 10% Slope 10% to 20% 20% 20% to 30% 50% No reduction Reduction in DIR > 30% Advice required from a suitably qualified and experience person NOTE: See Table 1.1 for conversion of slope per cent grade into slope angle and slope ratio. This table applies only to sub-surface drip irrigation systems. For covered surface drip (and spray irrigation) the commentary to Table M2 states: CM9.3 Covered surface drip and spray irrigation pose risks from effluent run-off in wet weather for slopes > 10%. This 10% slope constraint for covered surface and spray irrigation is consistent with Table K2 (see Attachment 1 below). On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 1

2 3.0 The Origin of the Reduction in DIR % in Table M2 The percentage value reductions are purely arbitrary, and based on taking a precautionary approach to installing systems on sloping land. The Standards Committee had no specific data on which to set the above reduction criteria, and thus used an adhoc approach which is likely to be conservative. Hence In situations where steep slope drip irrigation has been practiced successfully over time and information from such experience has been accumulated, then the DIR reduction, if any, can be based on local knowledge. The advice from a suitably qualified and experienced person can thus be applied to setting the level of reduction of DIR (if judged necessary) for any slope and for both surface and subsurface irrigation. 4.0 Case Study on Slope Issues in the Marlborough Region The application of Table M2 in Marlborough has become a recent issue related to on-site wastewater designs in the Sounds area where many lots are situated on steeply sloping bush covered sites. Much of the shoreline is bordered by subdivisions created decades ago in expectation that when holiday baches/homes or permanently occupied dwellings were erected they would be serviced by on-site systems. The availability of modern treatment units to replace septic tanks and drip irrigation systems to replace soakage trenches has enabled greatly improved on-site wastewater practice over the past two or more decades. However, due to the density of bush cover on most sites subsurface irrigation is impractical. The alternative has been surface laid drip lines threaded through the bush along the contour, with leaf/bush litter allowed to cover the drip lines over time. Many sites have slopes much greater than the 30% value, and with the 50% DIR reduction doubling the irrigation field size the resulting design area cannot be accommodated on many lots. The application of Table M2 for design control in such situations has created a quandary for many designers in the Marlborough region who claim that past experience in use of drip irrigation on steep slopes over many years sized at normal DIR values for given soil conditions have not resulted in any problems. Council is currently seeking confirmation from some consultants as to their verification of this view. Hence, use of table M2 as the sole criteria for judging suitability of design sizing for drip irrigation fields is thus not appropriate, particularly if its strict application will result in many properties being unable to accommodate a suitable system. The adhoc DIR percentage reduction values in Table M2 need to be considered in the light of both constraining factors and mitigating factors associated with use of drip irrigation on steep slopes. To assist this balancing between constraints and mitigation, references from the Standard related to steep and sloping land and drip irrigation have been extracted and compiled into Attachment 2 below. Examination of Attachment 2 enables identification of potential constraints associated with installation of systems, together with mitigating factors that can be applied. The following tables set these out. POTENTIAL CONSTRAINTS Attachment 2 Item No. Wet weather runoff potential for surface irrigation on steep slopes (particularly for high 3 & 5 rainfall areas) Recommended maximum slope for subsurface irrigation is 30% 2, 4 & 10 Recommended maximum slope for surface irrigation is 10% 3, 4, 6, 7 & 10 Avoid ground disturbance during construction 4 Increasing slope gradient adds to risk of off-site impacts 9 POTENTIAL MITIGATION MEASURES Attachment 2 Item No. Increase monitoring inspection levels and frequency 1 & 4 Undertake erosion monitoring 4 Decrease scale of occupancy/water use 1 Provide high quality effluent treatment 1 Specific design for subsurface irrigation on>30% slopes 2 Ensure application rate for surface irrigation is less than natural infiltration rate 3 On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 2

3 POTENTIAL MITIGATION MEASURES (ct d) Attachment 2 Item No. Use conservative DIR values (particularly in high rainfall areas) 1 & 5 Provide reserve area 1 Undertake careful site and soil evaluation 4 Reduce surface infiltration from rainfall (use diversion trenches) 4 Protect system from access by casual pedestrian traffic 4 Reduce DIR in accordance with Table M2 7 Ensure drip emitter flow rates are uniform on sloping ground 8 All permeability soils are suitable for surface drip provided DIR values promote unsaturated subsoil conditions 9 Additional design and implementation measures that could be considered in adapting designs to the steeply bushed lots in the Sounds include: Using special drip lines with textile wicking strip to spread effluent evenly into the soil along the line between emitters; Lowering the daily discharge per emitter by increasing the number of emitters per given drip line length (i.e. by reducing emitter spacing). Allowing natural leaf/bush litter to cover drip lines over time. Monitoring drip line performance after peak load use such as immediately following high occupancy holiday periods. Locating irrigation areas upslope of dwellings so that seepages become obvious to occupants. Issuing a design loading certificate which emphasizes the limitation of the system relevant to installation on a steep slope. Installing downslope surface flow seepage collection channels with sand filter treatment and outflow to on-property soakage. The Marlborough District Council has subsequently held a workshop with designers to discuss this slope issue along with several other matters related to use of AS/NZS 1547:2012. There was unanimous agreement amongst designers that the Table M2 values are overly onerous with respect to reduction in DIR, and that the provisions of the 2000 version of the Standard (no reduction) were more appropriate. However, the 2000 version does not incorporate a risk management approach, and so where circumstances arise where DIR reduction for steep slopes result in making it impractical to fit a system on a specific site the DIR reduction requirement could be waived where an appropriate risk reduction plan is provided. The Council Design Guidelines could outline what type of risk reduction measures might be utilised singly or in combination to enable such a waiver. The designer could then use these along with other site factors to support the risk reduction plan. REFERENCE: 1. AS/NZS 1547:2012 On-site domestic wastewater management, Standards Australia/Standards NZ, February 2012 ATTACHMENTS: Attachment 1: Attachment 2: Table K2, Selecting the Land Application System to Fit the Site and Soil [Ref.2, p.136] AS/NZS 1547:2012 References Relevant to Use Of Steep and/or Sloping Sites for Land Application via Drip Irrigation On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 3

4 Attachment 1: Table K2, Selecting the Land Application System to Fit the Site and Soil [Ref.2, p.136] Site or soil constraint System selection Possible modifications to mitigate constraint Installation practice System management Slope The following values are guidelines for the point at which the effects of slope are becoming a factor in the systems performance. Use of land application systems on greater slopes should not be precluded where a careful assessment of the site and soil conditions and contaminant pathways is completed, as soil depth and soil category are often more important factors than slope. As an example conventional trenches on absorptive soil may work effectively on slopes greater than 15 %, although factors such as build-ability become more of an issue Benching may be feasible if soil is stable Pay special attention to reducing surface infiltration (for example, interception drains uphill and rainwater-shedding designs) If a mound is used, design toe to encourage infiltration Trenches more difficult to dig, particularly for slopes above 25% Pay extra attention to the risk of erosion during construction Needs monitoring of erosion and seepage Access is best discouraged Surface irrigation (spray and drip)- up to 10% slope ETA/ETS beds and trenches - up to 10% slope Mounds, conventional trenches, LPED up to 15% slope Subsurface drip irrigation up to 30% slope The primary criterion for selection may be cost (for example, mounds will need much soil on downhill side) On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 4

5 Attachment 2: AS/NZS 1547:2012 References Relevant to Use Of Steep and/or Sloping Sites for Land Application via Drip Irrigation Compiled By: Ian Gunn Environmental Engineering Consultant Date: 2 March 2013 ITEM 1: APPENDIX A Risk management process guidelines (informative) TABLE A1 [page 78] EXAMPLES OF DESIGN RISK REDUCTION MEASURES (continued) Cause Site constraints Factors that increase likelihood Small lot size Steep slopes Examples of design risk reduction measures - Decrease/limit scale of development/occupancy capacity permitted - Use water conservation measures, maintaining 100% design land application area sizing established on basis of full flow design allowances without any reduction for low water use fixtures - Incorporate factors of safety through conservative design criteria (including pumped timer dose loading for optimum use of land application area), or additional wastewater treatment capacity/ level of treatment, or both - Reticulate - Increase performance monitoring inspection level and frequency - Require subdivision into larger lot areas where soil conditions are poor - For subdivisions, lots are to be sufficient size to ensure adequate capacity for the on-site wastewater system appropriate to site-and-soil conditions (reserve areas) along with suitable space for dwelling building platform and all onsite amenity areas) ITEM 2: APPENDIX K Land application systems Guidance on selection (informative) TABLE K1 [page 133] LAND APPLICATION SYSTEMS LIMITATIONS DUE TO SITE, SOIL, AND CLIMATIC FACTORS (continued) Land application system Subsurface irrigation systems Slope gradient System: Maximum slope without specific design 30 % Environment: More soil disturbance and erosion during construction on steep slopes On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 5

6 Attachment 2: AS/NZS 1547:2012 References Relevant to Use Of Steep and/or Sloping Sites for Land Application via Drip Irrigation (ct d) ITEM 3: APPENDIX K Land application systems Guidance on selection (informative) TABLE K1 [page 133] LAND APPLICATION SYSTEMS LIMITATIONS DUE TO SITE, SOIL, AND CLIMATIC FACTORS (continued) Land application system Surface irrigation systems (spray and drip) Slope gradient System: Steep slopes can cause greater run-off during wet weather; max. recommended slope 10% Environment: Very little disturbance during construction; slope angle not very important if application rate is less than natural infiltration rate; even distribution is very important On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 6

7 Attachment 2: AS/NZS 1547:2012 References Relevant to Use Of Steep and/or Sloping Sites for Land Application via Drip Irrigation (ct d) ITEM 4: APPENDIX K Land application systems Guidance on selection (informative). Site or soil constraint System selection TABLE K1 [page 136] SELECTING THE LAND APPLICATION SYSTEM TO FIT THE SITE AND THE SOIL Possible modifications to mitigate constraint Installation practice System management Slope The following values are guidelines for the point at which the effects of slope are becoming a factor in the systems performance. Use of land application systems on greater slopes should not be precluded where a careful assessment of the site and soil conditions and contaminant pathways is completed, as soil depth and soil category are often more important factors than slope. As an example conventional trenches on absorptive soil may work effectively on slopes greater than 15 %, although factors such as build-ability become more of an issue. - Benching may be feasible if soil is stable. - Pay special attention to reducing surface infiltration (for example, interception drains uphill and rainwatershedding designs) - If a mound is used, design toe to encourage infiltration - Trenches more difficult to dig, particularly for slopes above 25 % - Pay extra attention to the risk of erosion during construction - Needs monitoring of erosion and seepage - Access is best discouraged - Surface irrigation (spray and drip)-up to 10 % slope - ETA/ETS Beds and Trenches - up to 10 % slope - Mounds, conventional trenches, - LPED - up to 15 % slope - Subsurface drip irrigation - up to 30 % slope The primary criterion for selection may be cost (for example, mounds will need much soil on downhill side) On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 7

8 Attachment 2: AS/NZS 1547:2012 References Relevant to Use Of Steep and/or Sloping Sites for Land Application via Drip Irrigation (ct d) ITEM 5: APPENDIX M Land application methods Irrigation systems (normative) M3.2 Covered surface drip irrigation [page 157] CM3.2 On sloping ground in areas of high rainfall, or subject to periodic high intensity rainfall events, conservative DIR values should be adopted for sizing the covered surface drip irrigation area. ITEM 6: APPENDIX M Land application methods Irrigation systems (normative) M7.1 General [page 161] The irrigation area shall have an adequate depth of natural good quality topsoil (or imported topsoil if necessary) to store the applied effluent and to support the growth of evergreen vegetation to maximise evapotranspiration. CM7.1 The natural soil into which irrigation lines are installed should consist of a minimum depth of 150 mm (for subsoil categories 3 to 5) and 250 mm (for subsoil categories 1, 2, and 6) of good quality topsoil (see CM1). The maximum effective soakage area that should be used for design purposes is 1 m width along the irrigation lines. This applies to where the lines are 1 m or more apart, and in the case of drip systems, is proportionately less where drip lines are closer together. For flat land this effective area is taken as half this width either side of the drip or LPED line and for sloping land it is taken as the full width downslope of each line along the contour. Design irrigation rates (Table M1) apply to systems on flat to moderate slopes up to 10 %. ITEM 7: APPENDIX M Land application methods Irrigation systems (normative) M9.3 Sloping sites [page 162] On sloping ground, all irrigation lines shall be installed along the contour unless the line has non-leakage emitters, when lines may run down slope within the manufacturer s specifications. Shallow and narrow trenches for LPED systems shall be carefully constructed along the contour. The ingress of surface and seepage water into the land application area shall be controlled or prevented. A cut-off trench or diversion drain shall be constructed up-slope to divert surface and groundwater away from the irrigated area. For sloping ground the DIR for subsurface drip irrigation shall be decreased to ensure that effluent migration down slope is taken up adequately within the top soil and plant root system. See TABLE M for recommended reductions according to slope. TABLE M2 RECOMMENDED REDUCTIONS IN DIR ACCORDING TO SLOPE Slope Flat up to 10 % 10 % to 20 % 20 % 20 % to 30 % 50 % No reduction Reduction in DIR > 30 % Advice required from a suitably qualified and experience person NOTE: See Table 1.1 for conversion of slope per cent grade into slope angle and slope ratio. On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 8

9 Attachment 2: AS/NZS 1547:2012 References Relevant to Use Of Steep and/or Sloping Sites for Land Application via Drip Irrigation (ct d) CM9.3 LPED systems are not appropriate on slopes greater than 27 % due to uncertainty with even distribution being achieved on steep land. Covered surface drip and spray irrigation pose risks from effluent run-off in wet weather for slopes > 10 %. ITEM 8: APPENDIX M Land application methods Irrigation systems (normative) M10.1 Pump system [page 164] The pump system shall: (c) For drip irrigation, ensure that drip emitter flow rates do not vary by more than 10 % from the design rate over the whole of the system when installed on sloping ground. ITEM 9: APPENDIX Q Water balance and land application systems (informative) Q5.1 Covered surface drip irrigation and spray irrigation [page 183] The main factors that apply to effluent disposal are: (a) In both forms of irrigation, the effluent is applied to the soil surface. This tends to assist the process of evapotranspiration, so that Et may be greater than in any of the trenches or beds or combination evapotranspiration/absorption/seepage systems. However, for design purposes this effect is ignored; (b) Effluent applied to the soil surface may become part of the run-off component (RO) when it rains. However, pathogen removal is unlikely to be adequate, even with disinfection. Further nutrient removal takes place during slow, shallow surface run-off flow. The risk from any run-off should be assessed; (c) Irrigation systems must maintain a net deep infiltration (DI) component to remove salts, and to ensure CR = 0; (d) The magnitude of interflow (IF) depends on the permeabilities and gradient of the layers of soil in the profile; and (e) The application area may be terraced to increase Et and DI and decrease RO. Surface drip irrigation and spray irrigation are especially suited to soils of almost any permeability, as the effluent delivery rate and the area of application can be controlled by the design and operation of the system in order to maximise further in-soil treatment. Thus in highly permeable soils, the effluent can be spread thinly over a large area so as to promote unsaturated flow and achieve good additional treatment. In soils of very low permeability, the spreading out of effluent enables one to utilise the low infiltration rate of a large area to prevent surface run-off. However in that case, the risk of producing off-site impacts increases with slope gradient. On-Site NewZ Blog Item Drip Irrigation and Steep Slopes [30 October 2013] Page 9