TerminalGreen

Introduction to Storm Water Management

Storm water runoff is generated when precipitation from rain flows over impervious surfaces such as pavement areas and into adjacent waterways or storm water facilities. Runoff traveling over streets, parking lots or similar impervious surfaces can accumulate a variety of debris, chemicals, sediments, and pollutants. These materials can be carried into natural watercourses and without treatment can adversely affect water quality and plant and animal life. Best Management Practices (BMPs) represent the primary methods to manage storm water discharges. Proper storm water management reduces pollutants in runoff, decreases the volume of discharged storm water, and protects the water quality of streams, rivers and other bodies of water where direct or indirect discharge occurs.

Storm water management has always played a significant role in TransDevelopment’s projects as our development initiatives often incorporate large expanses of paved terminal areas and a wide variety of industrial cargoes. The large, open-air nature of terminal development necessitates unique approaches to managing storm water runoff. For instance, a typical parking lot is engineered not to allow water to accumulate on its surface, as this inhibits foot travel. In contrast, during a critical storm event, an industrial terminal might be designed to store several inches of water for a short duration. In the same vein, rail yards, which are integral to most TransDevelopment projects, can be designed as open or paved-in facilities with the ability to manage storm water runoff through natural infiltration or structure-based drainage systems. In many cases, large industrial cargo facilities can be master planned to incorporate green buffers and transitional areas, which make excellent storm water management best practices.

Storm Water: Structural Systems

There are a variety of manufactured structures that capture storm water and provide primary and secondary treatment. Some of these structures are simplistic and are intended only to remove debris and sediment, while other more advanced systems can remove high amounts of oil and grease as well as soluble metals. Utilizing these systems either by themselves or in combination with other best management practices can greatly increase the quality of water leaving a site, positively impacting nearby bodies of water.

Catch Basins & Catch Basin Inserts

Catch basins provide an inlet to the storm drain system that can serve as a pretreatment mechanism for the storm water. Runoff enters the catch basin through a curb inlet or grate. The primary treatment function of a standard catch basin structure is to allow the water to accumulate for a time period long enough that sediment and other particulates can settle out before discharging or entering the next stage of the storm water system.

Standard Catch Basin

Also referred to as a “Lynch Style Catch Basin” is designed to capture sediment, but does not effectively remove pollutants and Total Suspended Solids (TSS). There are several factors that determine the effectiveness of a standard catch basin. The depth of the structure can be a factor if the catch basin fills too quickly and sediments cannot settle out. Rapid filling and overflow can also cause re-suspension of already captured sediments. In order for catch basins to function properly regular maintenance practices are necessary. If collected sediment is not frequently removed from the catch basin, sediment will not settle out of the incoming storm water, defeating the purpose of the catch basin. However, there are more advanced methods that can be utilized to filter out sediment, pollutants and TSS prior to entering the catch basin (See Catch Basin Inserts).

Open Bottom Catch Basin

This style of catch basin allows for storm water to infiltrate into the underlying soils through the bottom of the catch basin. The advantage of this system is that water that accumulates in the basin for a long enough period of time can infiltrate and never has to be released into a secondary storm water system. An open bottom catch basin can only be used in areas where the underlying soils will allow for infiltration. A disadvantage of this system is that without regular cleaning, accumulated sediments will clog the underlying soil and diminish natural infiltration. The cleaning and maintenance for these types of basins is complicated by need to remove sediments and other materials from rock and other drainage medium versus the smooth concrete base of a standard catch basin. However the use of a catch basin insert could reduce this problem.

Catch Basin Inserts

There are typically two types of catch basin inserts installed at the inlet to reduce sediment and pollutants entering the catch basin. The first type is referred to as a “witch hat.” It is made of geotextile fabric to retain sediment and a “sock” that will absorb the larger droplets of oil. This insert can hold up to 40 pounds of sediment and 1.4 gallons of hydrocarbons. Maintenance is straightforward - the geotextile fabric is emptied and power washed and the oil absorbent material is replaced. This option reduces the frequency of sediment removal from the catch basin floor.

The second incorporates a stainless steel insert with a basket made of a similar geotextile fabric and also includes an oil filter mechanism. In addition to capturing debris and sediment, this insert can remove large amounts of oil and grease as well as TSS and phosphorus. These inserts are significantly more expensive than the “witch hat” model, but are more effective at removing oils and other pollutants from storm water. Maintenance for this insert is the same as the other model.

Oil Water Separators

Oil water separators are commonly used in areas where the spill of automotive oils and hydraulic fluids are expected, such as trucking areas and parking lots with older vehicles. Similar to standard catch basins oil water separators will remove some debris and sediment, but they are specifically designed to remove large amounts of oil and grease.

The traditional style of oil water separator seen above relies solely on gravity to move sediment to the bottom of the chamber and oil and grease to the top. This method can be effective as long as the water is able to sit in the chamber for an adequate period of time with minimal disruption. However, there are steps that can be taken to increase the speed of the gravitational process. As seen below the use of coalescing plates in an oil water separator allows smaller molecules of oil to attach to the plates where they are able to bind with other oil molecules. This creates larger globs of oil resulting in increased buoyancy, allowing the oil to rise to the surface at a faster rate. Due to the increased capability to capture smaller oil molecules there are significant improvements in the removal rate.

Hydrodynamic Separators

There are many companies that manufacture hydrodynamic systems, each of which has its own unique design or mechanism. The fundamental idea is that the water moves in a swirling motion to push sediment to the bottom of the structure and uses a wall or chamber to capture oil before the water is released. Similar to standard catch basins, these structures must be periodically maintained or they will lose their effectiveness. The structure above utilizes the swirling motion in the first chamber to move debris and sediment into the grit chamber, the structure then relies on gravity to push the oil to the surface while the water makes its way underneath the wall and eventually to the outlet pipe.

The structure above uses a completely different design, but still incorporates the same swirl motion. The swirling motion pushes the sediment into the bottom chamber and moves the oil and grease to the edges where it is kept in the outer chamber. The clean water is then able to move through the outlet pipe, and the internal hood prevents sediment from resuspending. The dark blue arrow demonstrates where the maintenance truck would access the structure.

Storm Water: Natural Systems

Natural storm water systems provide an alternative to traditional storm water infrastructure systems. Such features not only function as a part of a storm water management system, but also contribute to the site’s aesthetics and provide open space. The use of natural storm water systems can be limited by the amount of available space, but the multiple benefits associated often make it worth it.

Retention Pond

A retention pond is a storm water best management practice that is used to manage storm water runoff by promoting natural infiltration and evaporation. A retention pond is designed to hold a specific amount of water indefinitely. The water level may go up and down, however it is unusual for a retention pond to be dry. The pond typically has vegetation around the perimeter and is designed to have drainage leading to another location or watercourse when the water level exceeds the designed pond capacity.

Storm water can be channeled to a retention pond by sheet drainage or through a system of storm drains and underground piping. Retention ponds are designed to allow relatively large flows of water to enter, but discharge to receiving waters is restricted to flooding events through the use of weirs or other similar overflow structures.

Retention ponds are often landscaped with a variety of grasses, shrubs and/or wetland plants to provide bank stability and aesthetic benefits. Vegetation also provides water quality benefits by removing soluble nutrients.

Detention Pond

A detention pond receives and reduces storm water discharge velocity to a downstream drainage system or watercourse. When water falls on an impervious area such as pavement it will drain much faster than in a natural environment. The total volume of water remains the same however the velocity of runoff and discharge is accelerated. Detention ponds are designed to temporarily detain the water and manage discharge to a desired flow rate. As the storm event ends, the detention pond will empty shortly afterwards.

Detention ponds make for excellent best management practices and are designed to manage the surges of water associated with critical storm events that are often responsible for high erosion rates. A detention facility allows unrestricted flows of water to enter but restricts the outflow by incorporating specialized outfall structures. The size of the outfall structure is determined by the capacity and velocity requirements of the discharge water body. Offset concrete blocks at the entrance spillways are often used to slow down entering water. Near the entrance, debris drop vaults can be incorporated to collect larger debris or sediment.

Bioswale

Bioswales are natural features that slow the flow of water, allow the water to cool, remove sediment and pollutants, and assist in the recharge of ground water. Good locations to install bioswales are in areas where porous paving cannot be utilized. This includes areas with heavy truck traffic that porous paving is too weak to support and where oil and hydraulic fluid spills are inevitable. Additionally, employee parking lots where cars are likely to leak oil and possibly other polluting substances is a good location to implement a bioswale. In the case of a critical storm event when the bioswale is overwhelmed with water, secondary outfall structures can be incorporated to direct overflow from the bioswale into a storm water sewer system.

The preceding image is an example of a bioswale that could be used in an employee parking lot or other areas where the runoff will pick up small amounts of oil and sediment. Likewise, bioswales can be constructed on a larger scale as demonstrated in the following image. These larger bioswales could be put to use in an area adjacent to a truck loading facility with increased incidents of oil and hydraulic fluid spills. The bioswale can also be used as a mechanism to cool the water before entering an adjacent watercourse. Allowing the water to cool will prevent unwanted disruption of shallow water habitats.

Buffer Zone

A riparian buffer zone is a strip of land intended to protect an adjacent river or stream. The buffer presented in the preceding image is planted with native grasses, shrubs, and trees that slow down high speed storm water flows allowing for sediment to settle out while also removing pollutants such as oil. The slowed flow of water helps to reduce erosion rates and increases riverbank stability. Riparian buffer zones can also provide shade allowing the storm water to cool before entering the watercourse, and improved habitat for both in-stream and local wildlife.

Shallow Water Habitats

Shallow water habitats are often located in close proximity to marine terminal facilities. Though not widely discussed, shallow water habitats form a crucial and sensitive ecosystem. These habitats are important because they keep sediment in place, recycle nutrients, provide migratory paths for fish and create a healthy environment for various species of fish that dry land species rely on as a food source. Consequently, destruction of these ecosystems can be damaging to a broad population. Impacts to shallow water habitats can be minimized both in the design and construction phase as well as when the facility is fully operational.

Design and Construction

Creation of shaded areas: Many of the plants that are the pillars of shallow water ecosystems rely on sunlight to grow and remain healthy. Incorporating physical structures such as vessel berths that cast large and deep shadows can potentially ruin the immediately adjacent shallow water habitat. Designs must be sensitive to the ecosystem which could suggest the use of smaller floating berths structures positioned in deeper water as opposed to fixed wharf systems, and especially those which require bulkheads, significant back fills or other shade creating components.

Disruption of Currents: The construction of certain barriers, in-fill areas or berth structures can block or alter currents that shallow water habitats rely on to receive nutrients and carry away waste products. These structures also destroy the shallow water habitat during the construction process. Again, the use of floating berths can be an environmentally friendly alternative to traditional berths

Daily Terminal Operations

Pollution and Contaminants: Allowing pollutants and contaminants to enter shallow water habitats untreated can potentially wipe out an entire ecosystem. These pollutants can range from oil or other materials picked up by storm water runoff, to an actual material release or chemical spill. To minimize these effects it is important to have storm water treatment systems in place as well as spill containment policies and action plans.

Noise: Loud noises can cause wildlife to abandon their home and relocate; movement of wildlife can negatively impact the rest of the ecosystem. Shallow water wildlife are the most vulnerable during spawning and mating seasons, therefore daily terminal operations should keep this in mind and attempt to minimize disruption over that time period.

Disruptions From Ships: Wakes from ships can be damaging to shallow water plants that form the pillars of the entire ecosystem. Implementing speed regulations near these habitats can reduce the negative impacts associated with ship traffic.

Porous Pavement Overview

Porous asphalt pavement provides an alternative to traditional storm water management systems. Porous paving is constructed to allow water infiltration, while maintaining most of the structural characteristics of asphalt pavement. A porous system eliminates the need for extensive piping, catch basins, and other infrastructure typically required for traditional storm water drainage. Porous systems accomplish several important environmental goals, including reductions of run-off temperature, velocity, volume, and pollutants. Surface accumulations of storm water during critical events are almost entirely mitigated. In considering whether or not a porous paving system should be pursued, the condition of the sub grade and underlying soils must be evaluated for drainage purposes. For example, a primarily sand sub grade will allow for high infiltration, whereas a denser clay sub grade may prove to be unsuitable for porous systems.

Porous pavements offer a green approach to storm water treatment that reduces the need for storm water control structures. By mimicking natural processes, the resulting infiltration is of a much higher quality than that of conventionally managed run-off. Moreover, porous pavement reduces contaminants such as total suspended solids, metals, and oil and also improves the treatment of nitrates, phosphates, and other anthropogenic compounds. Porous pavement systems can also assist in recharging local aquifers through the natural infiltration process.

Central to any feasibility analysis is cost. On its face, porous pavement is slightly more expensive than traditional asphalt. Recent studies indicate that porous asphalt costs between ten to twenty percent more than standard asphalt, on a unit basis. In addition to the increased pavement cost, the underlying stone bed, which is generally deeper than a conventional aggregate sub-base and wrapped in geotextile fabric, is also more expensive. Two major contributors to the higher initial costs of porous pavement are a limited number of experienced vendors to install the pavement, and the requirement that the asphalt batch be produced separately. The engineer/contractor must know the specifications to send to the producer who then manufactures a special load of porous asphalt. However, when other factors such as storm water fees and physical infrastructure are included in the analysis, the cost gap between conventional asphalt and porous pavement diminishes. Also, there generally is less earthwork involved because porous systems can be incorporated into the site’s natural topography. When all factors are considered, porous paving systems with infiltration have proven to be competitive with, and often less expensive than conventional impervious surfaces with traditional storm water management infrastructure.

Traditional asphalt requires resealing every few years. In contrast, porous pavement requires no resealing but does demand vacuuming to remove material that can clog the void space. Porous pavement, due to its reduced fines, is susceptible to crushing by heavy loadings. The very underlying soils that make porous pavement an attractive option also have a downside: high permeability sub grades have the potential to develop anaerobic conditions. Finally, in the event of a spill, both surface types will require removal, excavation and site remediation, though porous pavement can make the cleanup more difficult due to absorption in the pores. If installed properly and maintained to the specifications of the engineer, porous pavement has a long lifespan; existing systems dating back several decades are still functional today.

Maintenance & Repairs

The maintenance required for porous pavement systems is not necessarily intensive but it remains important nonetheless. The major intent of system upkeep is to prevent the pavement surface and infiltration bed from becoming clogged with fines (sediments). Superficial dirt will not necessarily clog the pavement voids, but if the dirt is ground-in repeatedly by tires, this can have adverse affects on the system. Thus, trucks and other heavy equipment should be prevented from tracking or spilling dirt onto the porous pavement and their travel across the surface should be limited.

Keeping the paving clean throughout the year will prolong its life span. Using a commercial cleaning unit to vacuum the surface two to three times per year is highly recommended. Pavement washing systems, compressed air units, and pressure washers should not be used because they will degrade the structural integrity of the surface. Any inlet structures in the infiltration beds should also be cleaned annually or biannually, if possible. Porous pavement surfaces should never be seal coated.

If there are swales or landscaped areas adjacent to porous pavement, these should be well maintained in order to prevent an overflow or washout, which can deposit sediments on the porous surface. If any bare spots or eroded areas are observed, steps should be taken to stabilize the region so that overflow dangers are mitigated.

While potholes in the porous pavement are unlikely, soft spots in the sub grade can cause minor areas to sink several inches. These small potholes are called declivities. TransDevelopment has observed that the soft spots did not suffer from any surface buckling due to the flexibility of the wearing surface. For damaged areas of less than fifty square feet, a declivity can be patched with standard impervious paving. Since the area is small, any increased runoff from the conventional surface patch will infiltrate in the adjacent porous area. If there is a major event that causes a declivity of more than fifty square feet, both porous and traditional asphalt can be considered. At this time, the site should undergo an inspection by the engineer to determine why a relatively large area suffered from a soft spot issue.

Winter Conditions

Porous pavements perform well in colder, northern climates. Because water drains through the surface and into the subsurface bed, freeze-thaw cycles usually do not adversely affect the paving. The systems actually are less prone to forming black ice. Winter maintenance for a porous pavement area may be necessary but is less intensive than what is required for a traditional impervious surface. A porous pavement system with a subsurface aggregate bed has superior snow melting characteristics because the stone bed will absorb and retain heat. Therefore, freezing rain and snow melt faster on porous pavement accumulation of ice and light snow is generally not problematic.

It is important to consider heavier storm events where larger volumes of snow will be deposited on the porous surface. During and after these storms, snow plowing is acceptable as long as the operator is careful and sets the blade slightly higher than usual. Abrasive materials for traction and melting such as sand or cinders should not be applied on or near the porous paving. Salt can be used to de-ice the porous pavement; however, alternative deicers are recommended for environmental reasons. Lastly, during these storms, porous pavement provides better traction for humans and automobile tires in rain and snow.

Controlling Critical Storm Events

Most porous pavement systems that rely on infiltration are designed to handle critical storm events that occur between once every two years and once every ten years. Designers should also study a rare once-in-a-century storm event’s potential effects on the porous paving system. In order to handle the extreme volumes of water and a possible overwhelming of infiltration capacity, the stone bed can be designed with an overflow control structure. During large storm events, peak rates are controlled, so at no time can the water rise to the pavement level.

Perforated pipes or French drains along the bottom aggregate bed may be used to evenly distribute runoff over the entire bed bottom. Continuously perforated pipes can also be used to connect any underground infrastructure to provide outflow. Depending on size, the pipes themselves may provide additional water storage. Bypass systems can be designed but these structures should always maintain positive outflow.

Design & Construction Notes

In areas with poorly draining soils, infiltration beds below the surface can be designed to assist with slow discharge. In areas such as truck loading zones, where the threat of spills and groundwater contamination is heightened, porous pavement is not as suitable as conventional impervious paving. Due to the frequency of spills and the heavier-duty load requirements of haul away loading areas, these surfaces are better of being paved traditionally. Runoff from the truck and heavy equipment area can be treated with oil water separators and other filters before infiltrating into an adjacent bioswale.

Due to the activities inherent to construction, porous pavement should be installed as close to the end of other construction as possible. It is during this construction phase that porous pavement is most vulnerable to failure; special care must be taken to reduce the compaction of underlying soils, the contamination of infiltration base with sediment and fines, and the tracking of sediment by equipment. Also, any onsite drainage containing sediments should be diverted from the porous surface or constructed bed.

Immediately after the sub grade is prepared by raking and ripping, the geotextile fabric should be placed. The mat should be placed to the specifications and recommendations of the manufacturer and secured several feet outside of the stone bed to prevent runoff and sediment from infiltrating. After the site has stabilized and the installation is completed, excess geotextile can be cut back to the edge of the wearing surface. Base aggregate should be laid next after the geotextile fabric is spread, immediately followed by the choker coarse and the wearing surface (asphalt, open graded Portland cement, etc.). Porous pavement should not be installed on wet surfaces or when the air temperature is below fifty degrees Fahrenheit.

Following the completion of the porous paving installation, the permeability of the pavement surface should be tested. This can be accomplished by applying water at a rate of at least 5 gallons per minute on the surface with the use of a hose. Engineers should verify that the water infiltrates the surface directly without forming a puddle or runoff. In addition, it is recommended that the owner retain a third party to test the integrity and functionality of the system under the required state and federal safety regulations.

Specifications Overview

The specifications outlined below represent the guidelines used by several states including Florida and Pennsylvania for porous pavement installations. Many of the specifications have been refined by Cahill Associates and were implemented in the Port of Portland’s Terminal 6 expansion. These are not all-encompassing specifications and should not be treated as such. The engineer and contractor must adapt materials and specifications to the specific needs of each project. Additionally, the specific standards for each material were intentionally omitted from this document.

1. Base aggregate
   • The infiltration bed should be between 12-36 inches deep and comprised of clean, uniformly graded aggregate.
   • The depth of the bed is a function of storm water storage requirements, frost depth considerations, site grading, and predicted loading.
   • Stone for infiltration beds should be uniformly graded coarse aggregate (AASHTO No. 3) with one to two inches of gradation.
   • Aggregate should not have wash loss of more than 0.5% and should have about 40% void space, depending on the application.
   • Choker base coarse aggregate should be installed uniformly over the base aggregate in order to provide an even surface for paving.
   • Height should 3/8 inch to 3/4 inch of uniformly graded AASHTO No. 57.
2. Geotextile fabric
   • Geotextile cover should consist of needled, non woven polypropylene fibers
   • Examples of acceptable products: Mirafi 140N, Amoco 4547, and Geotex 451
3. Piping
   • Pipe shall be continuously perforated with a smooth interior.
   • The minimum inside diameter should be about six inches.
   • High-density polyethylene pipe should meet AASHTO standards.
4. Stormwater Structures
   • Standard inlet boxes should be modified to provide a minimum of 12" sump storage.
   • These inlets should have bottom-leaching basins and open to gravel sumps when situated in the infiltration bed.
   • Any PVC catch basins, clean outs, and drains should have H-rated grating.
   • Steel reinforcing bars installed over the outlet structure should conform to ASTM standards.
   • Pre-cast concrete inlets may be substituted for cast-in-place structures.
5. Wearing Surface
   • For porous paving, bituminous surface course or Portland cement pervious pavement mixtures can be used.
   • Consult with engineer, contractor, and manufacturer for material selection, production, and installation.
   • This document makes no attempt to capture all of the necessary specifications for the wearing surface.

Floating Berths

Floating vessel berths for Ro-Ro and Pure Car Carrier (PCC) ship discharge is an approach currently employed within the Columbia River basin, at the Ports of Portland Oregon and Vancouver Washington. These floating platform docks have provided a combination of economic, operational, and environmental benefits which qualify them as true innovations in the field of automotive ports-of-entry. As newly constructed ports-of-entry increase in permitting complexity as well as capital investment, the development of floating berths may offer an important alternative to the port industry.

Traditional approaches to vessel berthing include fixed cellular bulkheads or pile supported structures, however floating platforms can provide important advantages, especially for PCC and Ro-Ro cargoes. Through the use of trestles and hinged transfer ramps extending from the shore to the floating berth, preservation of shallow water habitats in and around the vessel discharge area can be maximized, providing important environmental mitigations related to impacts on migrating fish and other plant and animal life. The floating berths offer advantages over fixed level docks as the platforms can raise and lower with tidal or seasonal water level fluctuations, providing greater operating efficiencies by maintaining constant elevation between the dock and vessel for the ramp-based loading and unloading operations. Likewise, the permitting and capital construction costs of floating berths can be significantly lower than traditional methods of pile driving, construction of cellular bulkheads, and associated backfilling and dredging work.

The floating portion of the berth system can be flexibly constructed from refurbished barge, ship or dry-dock components or fabricated as a new floating structure. Two of the existing floating berths on the Columbia River were fashioned from World War II era Liberty Ships and have remained in service for over two decades. In addition to the trestle and floating structure, a combination of mooring and breasting dolphins are typically incorporated into the design, which allow vessels to shift positions alongside the berth depending on the requirements for discharging from the mid-ships, stern or simultaneously from both ramps.