Post Construction Stormwater Management

Post-Construction Stormwater Management
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Table of Contents
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1.0 Introduction  PAGEREF _Toc387999392 \h
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1.1 Background  PAGEREF _Toc387999393 \h
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1.2 Applicability  PAGEREF _Toc387999394 \h
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1.3 Organization  PAGEREF _Toc387999395 \h
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2.0 Best Management Practices (BMP) Selection  PAGEREF _Toc387999396 \h
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2.1 Site Considerations  PAGEREF _Toc387999397 \h
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2.1.1 Right-of-way  PAGEREF _Toc387999398 \h
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2.1.2 Soils  PAGEREF _Toc387999399 \h
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2.1.3 Hydraulic Head  PAGEREF _Toc387999400 \h
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2.1.4 Slopes  PAGEREF _Toc387999401 \h
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2.1.5 Groundwater/Hydrogeology/Geology  PAGEREF _Toc387999402 \h
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2.1.6 Anticipated Pollutant Loading  PAGEREF _Toc387999403 \h
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2.2 Off-site Run off  PAGEREF _Toc387999404 \h
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2.3 Receiving waters  PAGEREF _Toc387999405 \h
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2.4 Countrywide NPDES I MS4  PAGEREF _Toc387999406 \h
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2.5 NPDES Statewide General Construction Activity Permit  PAGEREF _Toc387999407 \h
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2.6 NPDES Phase II MS4 Permit Requirements  PAGEREF _Toc387999408 \h
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2.7 Local Requirements and Controls  PAGEREF _Toc387999409 \h
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2.7 Technical Requirements  PAGEREF _Toc387999410 \h
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2.9 Water Quality  PAGEREF _Toc387999411 \h
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2.10 Pollutants of Concern  PAGEREF _Toc387999412 \h
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2.11 Performance  PAGEREF _Toc387999413 \h
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2.12 Maintenance  PAGEREF _Toc387999414 \h
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2.13 Flow Mitigation  PAGEREF _Toc387999415 \h
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2.14 Health and Safety  PAGEREF _Toc387999416 \h
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3.0 BMP Sizing Criteria  PAGEREF _Toc387999417 \h
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3.1 Low Impact Design (LID)  PAGEREF _Toc387999418 \h
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4.0 Recommendations  PAGEREF _Toc387999419 \h
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5.0 Conclusions  PAGEREF _Toc387999420 \h
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6.0 Appendix  PAGEREF _Toc387999421 \h
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6.1 Figures  PAGEREF _Toc387999422 \h
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6.2 Tables  PAGEREF _Toc387999423 \h
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7.0 References  PAGEREF _Toc387999424 \h
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Post Construction Stormwater Management
1.0 Introduction
1.1 Background
Over the duration of past 2 decades, the rate of land development throughout the United States has been over 2 times greater than population growth rate. If not checked, the increased impermeable surface including sidewalks, parking lots, roads and rooftops linked to this development would increase the volume of stormwater and degrade quality of water, which might harm coastal areas, streams, rivers and lakes (Hoskins & Peterein, 2013). With the growing amounts of impermeable cover, there is a corresponding increase in the volumes of stormwater runoff, along with an increase in the amount of contaminants carried by stormwater runoff. As such, post construction stormwater management is essential for the protection of not only property, but also environmental quality. In essence, stormwater management has today become a matter of great significance.
As land continue to be developed, the impermeable surfaces which are created increase the volume of stormwater runoff during events of rainfall, and thereby disrupt the natural hydrologic cycle. With no stormwater controls, the increased runoff could increase contaminant loadings, erode stream channels, prevent recharge of groundwater, and lead to downstream flooding. It is noteworthy that the increased stormwater runoff could degrade quality of water in every type of water, even those which are categorized as nutrient-sensitive waters, shellfish waters, and water supply watersheds (Ackerman & Stein, 2008). It is important to protect these waters for various reasons such as the protection of wildlife and fish habitat, supplies of drinking water, recreation, as well as human health.
In mitigating stormwater issues associated with land development, there are several measures that could be undertaken in reducing the impacts of increased impermeability. The optimal way of mitigating the impacts of stormwater from new developments is by using practices in treating, storing and infiltrating runoff onsite before it could affect downstream water bodies. It is noteworthy that innovative site designs that decrease imperviousness, as well as smaller-scale low impact practices of development scattered across a site are some of the best ways of realizing the objectives of decreasing flows and improving quality of water (White & Boswell, 2011).
1.2 Applicability
This paper is applicable to new land development and major redevelopment, which discharge to the storm sewer system. New land development basically entails areas that were not earlier built to urban usage including although not limited to green space, woodland, pasture, and farmland. Major redevelopment encompasses areas which are presently built to suburban and urban land uses, and they are being invigorated with rehabilitation of current structures, or pulling down of the current structures and building of new ones (Treadway, Reese & Noel, 2009).
1.3 Organization
This paper covers the key elements that have to be considered during the selection of the right Best Management Practices. These elements include such things as site considerations, offsite run-off, pollutants of concern, water quality, technical requirements, local requirements and control, performance, maintenance, as well as NPDES phase I and phase II requirements. This paper also covers the BMP designing and sizing criteria. Conclusions are made at the ending of this paper along with recommendations.
2.0 Best Management Practices (BMP) Selection
There are a lot of BMPs that can be utilized during the post construction phase of development. Stormwater Best Management Practices comprise various techniques which are practical and simple in design, nevertheless they offer effective management of stormwater and aesthetic enrichments for rural, suburban and urban landscapes. These techniques could be cost-effective to construct whilst offering lasting sustainability for the infrastructure of the city and conservation of water resources (Rosenthal, 2008; Bean, Hunt & Bidelspach, 2007). Some of the most effective post-construction Best Management Practices include (i) bio-retention: bioretention areas are basically plant-based and soil-based storm water management practices which filter the storm water runoff from developed areas by way of imitating natural vegetated systems. In essence, these control hydrology naturally by means of evapotranspiration and infiltration. Bioretention regions are small vegetated depressions where surface water is diverted into. Storm water will flow into the bioretention area, pools on the surface of the area, and will then slowly infiltrate into the soil bed (Ackerman & Stein, 2008). An archetypal application of bioretention area is infiltrating and treating surface runoff from parking lots, where the bioretention area might comprise a slotted-curb, recessed island. In terms of effectiveness, bioretention areas help to improve the quality of water. They can eliminate 65% of phosphorous, 90% of suspended solids, 80% of metals, and 50% of nitrogen from storm water (Cole, 2011). See figure 1 and figure 2 at the Appendix.
(ii) Wet Detention – Lakes and Ponds: wet detention is characteristically a constructed lake or pond. It might also be a lake or pool that is integrated into a storm water treatment system. Wet detentions are usually deemed as end-of-the-pipe best management practices. Ponds and lakes are standing water bodies which are defined in terms of effective storage, effective height, as well as capacity. Lakes are typically larger compared to ponds, usually with total storage exceeding fifty acre-feet, and the product of effective storage in acre-feet and effective height in feet exceeding 1,250 (Cole, 2011). It is of note that every development that involves the construction of a pond or lake has to be in conformance with federal, state, as well as local laws. Ponds could be altered in order to raise their storage capacity. They can also be enhanced using vegetation so as to increase their effectiveness of water-quality treatment. Wet detention is an efficient contaminant removal. Wet detention ponds and lakes could eradicate nearly 40% to 80% of soluble nutrients, 20% to 40% of biochemical compounds, 30% to 90% of total phosphorous, and 50% to 90% of suspended soils (Daeryong & Roesner, 2013). As such, it is effective. The key functions of wet funds include detaining stormwater and facilitating removal of pollutant through settling as well as biological uptake. See figure 3 and figure 4 at the Appendix.
(iii) Dry detention basin: also referred to as dry ponds, dry detention basins are basically storm water basins which are designed to intercept a volume of storm water run-off and momentarily hold the water for slow release into the storm water system or receiving stream. Dry detention basins are usually designed to totally empty out between events of stormwater run-off, characteristically in 48 hours, and thus largely provide runoff control and not water-quality control. Detention basins could restrict downstream scour as well as loss of water habitat by decreasing energy of stormwater discharges and peak flow rate. Depending on the stormwater quantity held within the basin and the length of time that it stays there, detention basins could eradicate from 10% to 90% of suspended solids (Daeryong & Roesner, 2013). The removal of contaminants is, however, less efficient. Dry detention basin designed to detain storm water runoff temporarily during storm events. See figure 5 at the Appendix.
(iv) Infiltration trench: this is an excavation that is lined with filter fabric plus backfilled with aggregate. Rainwater will enter the trench during run-off events wherein it is firstly stored and then filtered into the adjacent soil. As rainwater goes through the filter fabric and the aggregate, and into the soil, contaminants are filtered out. This BMP could treat and hold stormwater runoff for regions at the scale of governmental and individual commercial parcels or residential blocks (Urbona, 2011). An infiltration trench is effective in the reduction of the quantity of stormwater runoff and for filtering dregs. Nonetheless, the removal of contaminants is moderately low. See infiltration trenches on figures 6, 7 and 8 at the Appendix.
(v) Infiltration basin: Infiltration basins are understood as impoundments which are designed to capture runoff and allow it to permeate into the soil over period of a few days. They do not hold a permanent pond of water. An infiltration basin is characteristically end-of-pipe, off-line BMP. The basins could differ in both shape and size and usually treat stormwater from bigger areas, from several parking lots to big parking lots, and to extensive areas like neighborhoods. An infiltration basin uses the existing vegetation and soil in facilitating water infiltration into the soil and evapotranspiration of water via vegetation into the atmosphere (Kaspersen, 2009). Plant life is essential to the effectiveness of the infiltration basin. It is notable that deep-rooting plants would improve water percolation whilst also remaining properly anchored against disturbance from water. Having an adequate area for maintaining a shallow pool that would permeate in at most 48 hours is the other vital element. Depending on the storm water quantity captured within the basin, as well as the length of time it stays there, an infiltration basin could eliminate from 10% to 90% of suspended solids. The removal of contaminants depends on the soil media and the capability to decompose or adsorb contaminant compounds (Kaspersen, 2009). See figure 9 at the Appendix.
(vi) Filter strips: these are essentially grassed, densely-vegetated practices which accept sheet flow storm water run-off from nearby surfaces. A filter strip will filter out sediment and other contaminants; slow stormwater run-off; and improve percolation of surface water runoff. Filter strips are used in treating shallow sheet flows and to uniformly distribute storm flows over short contributing distance areas. They are appropriate to areas which are close to parking lots and other impermeable surfaces wherein stormwater run-off could be conveyed and then filtered before it is released into surface water bodies, stormwater systems, or swales. They are also well suited to developing land and construction sites to filter dregs from overland sheet flow (Alminagorta et al., 2013). The effectiveness of filter strips varies depending on the kind of plants as well as the filter strip size. See figures 10 and 11 at the Appendix.
(vii) Grassed swale: this is a low-cost alternative to the usual hard-engineered conveyance in commercial and residential environs. As storm water runoff moves down the swale, contaminants and suspended solids settle out; they do not enter the stream systems. Grassed swales are very much efficient in scattering the stormwater flow across a greater distance and area. However, they are very effectual in removing contaminants or sediments that are suspended (Ackerman & Stein, 2008). Selecting the most excellent post-construction stormwater BMP for a given site calls for understanding of the performance criteria and BMP technology. See figures 12 and 13 at the Appendix.
2.1 Site Considerations
A scrupulous evaluation of the site-specific conditions will assist in identifying the Best Management Practices that are most appropriate for the given location. It is notable that not every BMP would fit site-specific conditions. For instance, a Best Management Practice might require too much maintenance or fail altogether after only a short time period if it is unsuitable to the conditions of the site. As illustrated in figure 14, the roadway planner has to take into consideration various site-specific conditions which could impact BMP selections and integration into a project (Treadway, Reese & Noel, 2009). See figure 14 on the Appendix.
2.1.1 Right-of-way
It is imperative to estimate the drainage area so as to establish and/or design suitable Best Management Practice storage and/or treatment volumes. In areas wherein there is limited space and/or in urban settings, it might be essential to retrofit post-construction Best Management Practices into structures that already exist. To contain the retrofit, it might be vital for the Best Management Practice to be able to be broken up into smaller units, or be able to be installed some distance away, or be compact (Houser & Pruess, 2009). For instance, detention basins could either be split up into small, several pocket basins in order to contain the necessary volume of runoff. On the contrary, in rural and more open areas, just a single Best Management Practice with a larger footprint might be more cost effective and nevertheless still attain the level of treatment needed. Additionally, except for stack interchanges, roadway footprints are primarily linear, and thus Best Management Practices which could contain linear footprints are deemed advantageous for roadway transportation systems (Barnard, 2012).
2.1.2 Soils
The applicability as well as envisaged functionality of many Best Management Practices depends on subsurface and surface geology. For instance, subsurface geology and soils have to be appropriately characterized for permeation practices. The chemistry of surface soils also has to be taken into consideration. For instance, soils that have high organic content might have high absorptive capacity for highway contaminants like petroleum hydrocarbon compounds (Cole, 2011). Terrain and soil conditions differ greatly across the country, and it is essential that designers explicitly take into consideration these regional factors in selecting the right BMPs. The selected best management practices for the site have to go with the terrain and soil. Particular question that the designer has to ask herself is: is the site having shallow soils or exposed bedrock? Some parts of the country have very shallow soils or exposed rock which might prevent the usage of some best management practices. For instance, infiltration practices might not be practical in soils which are shallow because of the restricted soil separation distance between the bedrock and the bottom of the BMP. Other practices like wetlands and ponds might be practical, although they might be expensive or hard to design and build, for instance they might necessitate liners for preventing rapid drawdown (Dallman & Spongberg, 2012).
2.1.3 Hydraulic Head
Hydraulic head is understood to mean the depth of water such as for storage or infiltration devices, or the overall drop in the level of water for flow-through designs, like for swales. It is notable that the majority of structural treatment Best Management Practices always operate under gravity flow conditions. The least hydraulic head has to be taken into consideration in selecting a BMP (Daeryong & Roesner, 2013).
2.1.4 Slopes
Both the slope within a site and the upstream slope are crucial in assessing the applicability of a Best Management Practice. In essence, the slopes would determine the vulnerability of a given site to surface erosion and the need of implementing post-construction controls so as to avert elevated sediment loading. In general, steep slopes could result in breakthrough or short-circuiting of treatment Best Management Practices if there are insufficient velocity dissipation controls (Dallman & Spongberg, 2012).
2.1.5 Groundwater/Hydrogeology/Geology
Some subsurface geologic features including subsidence, fissures and sinkholes could create gaps and enlarged joints within the soil. These voids could greatly weaken the sifting capacity of vadose zone soil before recharged stormwater gets to the groundwater. In addition, the depth to groundwater could be another vital criterion in the selection of the appropriate Best Management Practice. The recharging of stormwater using permeation practices might aggravate pollution plumes that exist within a shallow aquifer underneath the area of permeation (Kaspersen, 2009). A shallow depth to the groundwater alone could decrease substantially the infiltration rate of stormwater runoff from percolation practices. Bio-retention has to be separated from the water-table in order to ensure that the groundwater does not intersect with the bottom of the bio-retention area that serves to prevent potential groundwater pollution and practice failure.
2.1.6 Anticipated Pollutant Loading
Forecasting the expected type of pollutant and concentration is vital in the selection of the suitable Best Management Practice. Water quality requirements might as well limit the usage of certain Best Management Practices. Stormwater from high density highways typically have high concentrations of total suspended and dissolved solids, metals, petroleum hydrocarbons, and miscellaneous debris and litter in relation to less frequently used, smaller roads. Essentially, comprehending the target contaminants and the removal efficiencies needed is the initial step in correctly describing and planning the required post-construction water quality Best Management Practice (Jia et al., 2013).
2.2 Off-site Run off
The selection of the appropriate Best Management Practices for anticipated rain events and succeeding runoff rates cannot be over accentuated in the effectiveness and efficiency of BMP. Whenever stormwater causes sediment and pollutant loaded waters to start migrating off-site, effective BMPs would assist in keeping the pollutants on-site, thereby preventing them from going in the stormwater system and getting into local lakes and/streams (Stephenson, 2007). As such, in selecting the right BMPs, an entity has to take into consideration off-site run off and should select those that would prevent off-site runoff.
2.3 Receiving waters
The selection of Post-Construction BMPs should take into account the quality and habitat of the receiving waters. In essence, BMPs have to be incorporated into redevelopments and new developments so as to reduce the impacts on receiving water habitat and quality. The BMPs that greatly help to reduce the pollution of water bodies should be considered (Keller, 2008). Designers have to understand the regulatory status and nature of the waters which would receive stormwater runoff from the developing site. It is of note that the kind of receiving water greatly influences the chosen best management practice to utilize, and in some instances, might trigger increased treatment requirements (Dallman & Spongberg, 2012). There are various types of Special Waters and other sensitive waters in different states across the United States that have to be taken into consideration. Receiving waters are basically classified into 5 classes: drinking waters, trout resources, impaired waters, lakes and wetlands. The complete range of best management practices could be applied to sites which drain to receiving waters not designated as sensitive or special in a given state. For instance, only best management practices which offer a higher level of phosphorous removal might be encouraged for sensitive lakes.
Using ponds might be discouraged in trout streams primarily because of concerns over stream warming. (1) Is the site draining to a trout stream protection? It is of note that trout streams are worth special protection, and this greatly influences the selection of best management practices. Some best management practices are favored since they attain high sediment removal rates, protect channels from erosion, and promote baseflow. Ponds might be discouraged since they result in stream warming (Goldstein, 2012). (2) Is the site draining to a sensitive lake? Best management practices vary in their capacity to eliminate phosphorous, the main stormwater contaminant managed to protecting sensitive lakes – including surface water drinking supplies and trout lakes. Communities might be requiring greater water quality treatment, a particular rate of phosphorous removal, or even load decrease at the site of development in order to protect their most sensitive lakes (Dallman & Spongberg, 2012). Generally, higher phosphorous elimination requirements lead to shorter list of acceptable best management practice designs that could be employed at the site.
(3) Is the site situated in a groundwater drinking water source region? In essence, sites which are situated within aquifers utilized for drinking water supply necessitate best management practices which could recharge aquifers and simultaneously they stop groundwater pollution from contaminated storm water, especially when it is produced from possible storm water hotspots. Is the site draining to a wetland? Measham, Kelly and Smith (2011) stated that wetlands could be impacted in some way by upland development sites. As such, it is important that designers select best management practices which could maintain wetland hydroperiods and restrict loads of phosphorous. A number of best management practices offer extended detention storage and infiltration which protect natural wetlands from increased runoff and nutrient loads from upland development. (4) Is the site draining to impaired water? The selection of best management practice becomes imperative whenever a site is draining to receiving water which is not attaining standards of water quality. The designer might have to select best management practices which attain a more strict level of removal for the pollutants of concern. These pollutants include metals, hydrocarbons, phosphorous, bacteria, total suspended solids, as wel...