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Chapter 2 (Sections 2.1 and 2.2) provides some examples of qualitative matrix-type approaches for the comparative evaluation of differing SuDS options based on a subjective analysis of their functional characteristics and performance. Table 2.6 utilises a matrix of SuDS pollutant removal and flow attenuation capabilities to provide a comparative evaluation, whilst Tables 2.9 and 2.10 provide more generalised indicator approaches based on effectiveness potential for the triad of water quantity, water quality and amenity functions. As indicated by Table 2.10, such approaches can be made semi-quantitative and all can be fairly readily and cheaply developed and applied. However, these examples are essentially technical performance applications and do not consider the full range of social or sustainability criteria or life time costing criteria. In addition, site-specific conditions and/or local operating experience can strongly influence the value and weighting placed against any particular category. Good working knowledge of reservoir porous paving structures for example, might well result in this SuDS being favoured over other source control options.

Table 7.4a SuDS Technology Evaluation Matrix

Criterion

Infiltration Systems Porous Paving (with reservoir structure) Grass Swales Grass Filter Strip Wet Retention Basins Constructed Wetlands Planning cost

(Pre-planning and design)

+ o + o +

-Construction cost

(Capital investment)

+ o + o - o

O & M cost

(Including personnel, plant replacement and sediment disposal)

+ + o + o o

Technical implementation effort (excavation, lifetime

O & M, decommissioning)

+ o + + - o

Water re-use

(not including groundwater recharge)

- - - - + +

Whole-life cost

(Duration, affordability, flexibility for retrofitting etc)

+ - + + + o

Reliability against Failure (Forced and planned outage during lifetime)

- o + + + o

Planning and Practical Experience

(System performance knowledge)

o + o - +

-KEY:

+ more advantageous as compared to other technologies

o neither advantageous nor disadvantageous as compared to other technologies - less advantageous as compared to other technologies

Sometimes provided (but with careful design) 9 Groundwater recharge Normally provided

?

A further supporting comparative matrix approach which includes sustainability referencing for various types of SuDS stormwater treatment systems is given in Tables 7.4a to c which can be used in conjunction with Tables 2.6, 2.9 and 2.10. This type of multi-matrix approach is primarily intended for general planning support in the pre-selection of integrated urban BMP systems and cannot be used for detailed design. Although wetlands may have the possibility of water re-use, there may be an overall water loss as a result of plant evapotranspiration during the summer period than occurs from an unvegetated open water system such as a wet retention pond.

Table 7.4b SuDS Stormwater Control Evaluation Matrix Peak Discharge

Control SuDS

RI 1:2 RI

1:10 RI 1:10

0

Volume Control Groundwater Recharge Potential Direct Water Re-use Downstream Erosion and Flood Control

Extended detention

basin

?

Wet retention

basin 99

Constructed

wetland 99

Infiltration

basin 9

Porous paving

(with reservoir structure)

9

Grass swale

9 Grass filter

strip 9

KEY: RI Return Interval (years)

Seldom or never provided 99 Direct re-use potential

Inspection of Table 7.4a would suggest that wetlands generally seem to be neutral in terms of advantages and disadvantages over other SuDS systems. However, they gain in terms of overall operational efficiency of pollutant removal (Table 2.6) and have more advantage in terms of environmental and community amenity provision (Table 7.4c). Whilst the technical evaluation as identified in Table 7.4a would seem to place constructed wetlands at some disadvantage compared to other SuDS systems, the gains in performance and environmental capacity as well as in potential community benefits more than compensates for any technical shortfalls. Much of the neutrality recorded in technical requirement, effort and cost is due to the relatively limited experience of constructed wetlands for stormwater treatment as well as to the widespread (and generally unfounded) concerns associated with open water bodies in urban areas. Given an extensive plant cover and restricted access to deep water and contaminated sediment areas that can be safeguarded by barrier planting, such concerns are much less appropriate in the case of constructed wetlands.

Table 7.4c SuDS Environmental and Urban Community Amenities Evaluation Matrix

SuDS

Receiving Water Low Flow Status Aquatic Habitat Creation Wildlife Habitat Creation Landscape Enhancement Recreational Benefits Hazard and Safety Reduction Aesthetics Community Acceptance

Extended detention basin Wet retention basin

Constructed wetland

Infiltration basin

Porous paving

Grass swale

Grass filter strip

KEY Seldom or never provided

Sometimes provided (but with careful design modification) Normally provided

Can be overcome with careful site design Generally not a restriction

Constructed wetlands are also generally neutral in terms of comparative advantage/disadvantage to other SuDS technologies when considering storage volume, flow attenuation and groundwater recharge (Tables 2.6, 2.9 and 7.4b).

However, whilst many other systems provide the latter benefit, they can also at the same time raise the possibility of groundwater contamination (see Table 2.10). The potential advantages offered by constructed wetlands become much clearer in Table 7.4c where they score highly as a result of a combination of integrated environmental and urban community benefits. However, it must be noted that these benefits will only accrue if they are considered early on in the design and planning process as they are frequently difficult and costly to retrofit into existing structures. Their success and long term community benefit is essentially dependent on adoption agreements and continued, positive management either by public or private agencies. In these terms

"bigger is better", as large wetland facilities with wide surrounding buffer zones offer the greatest development opportunities for amenity/recreational provision and associated income streams. In addition, both constructed wetlands and wet retention basins can offer possibilities for water re-use such as park or garden irrigation.

Table 7.5 SuDS Restrictions Evaluation Matrix SuDS

Gradient High Water Table Proximity to Bedrock Proximity to Building Foundations Land Take Maximum Depth Multifunctional Uses High Sediment Input Management and Liability

Extended detention basin

Wet retention basin

Constructed wetland Infiltration basin Porous paving Grass swale

Grass filter strip

KEY

May preclude the SuDS use

The matrix criteria identified in Table 7.4c essentially relate to stormwater wetland systems that are intended for introduction within residential and /or commercial/industrial areas. The full range of criteria would not necessarily be applicable to wetlands intended for the control and treatment of highway runoff where recreational, amenity, aesthetics and community acceptance are generally not applicable considerations. The major difficulties indicated in the Table 7.4c matrix, are associated with health and safety and with downstream receiving water protection of low flow and thermal regimes where rapid changes in temperature due to incoming stormwater may present difficulties to fish and other aquatic species (see also Table 2.10). Nevertheless, with careful design, constructed wetlands can avoid (or at the very least minimise) all potential hazards.

Table 7.5 provides a general guidance matrix on a range of factors which can preclude or restrict the use of particular SuDS options. Wetlands together with other wet storage facilities such as retention and extended detention basins, have fewer overall restrictions although they can score badly against important factors such as space consumption and adoption/management liability.