Hydrology Stormwater

Services: Hydrology & Stormwater Management

diagram of surface water flows before and after constructionHydrology is the study of the movement, distribution, and quality of water throughout the earth. Our staff of civil and site engineers focus on the hydrology of a property from the very beginning of a project. In doing so, this allows us to plan a development project which makes stormwater management and stormwater quality provisions from the initial planning stages of a development. Here at Arthur H. Howland & Associates, P.C., we feel that a solid stormwater management plan should be a priority (rather than an afterthought) for any development project.

Planning & Design Stages

During the planning and design stages of a development project, it is extremely important to analyze the overland movement of stormwater runoff throughout a property prior to development. By analyzing the pre-development flow patterns of a site, the hydrologist is able to predict how a proposed development may potentially affect the existing overland flow patterns of a watershed as well as any other downgradient properties. Once the pre-development condition of a property is analyzed, the hydrologist then performs a similar hypothetical post-development analysis for the improvements proposed (not yet constructed) on the site.

Since most proposed developments introduce rather than reduce areas of impervious surface of a site, the amount of runoff from an area of new development is expected to increase from the pre-development condition. In order to attenuate the effects a proposed development could have on a watershed, hydrologists and engineers must develop a stormwater management plan that focuses both on stormwater quantity and stormwater quality.

hydrograph imageStormwater Quantity

In order to regulate the amount of runoff generated from a site, many municipalities institute a policy of “Zero Increase in Peak Runoff Rates.” What this means is that the peak runoff rates from a developed site must be less than or equal to the various design storm peak runoff rates prior to construction. Typical design storms are the 2, 5, 10, 25, 50, and 100-year and represent the typical rainfall intensity and duration for the worst storm in whatever the design storm year is. For example, the 100-year storm represents the intensity and duration of the worst storm in 100 years. For each year, there is a 1% chance that a 100-year storm could occur. The 25-year storm represents the intensity and duration of the worst storm every 25 years. For each year there is a 4% chance that a 25-year storm could occur. Since these design storms are based on probability, there is always a chance that there could be more than one 100-year storms in 1 year or 100 years.

To develop the stormwater quantity aspect of a stormwater management plan, it is necessary to determine the impact that the proposed impervious surfaces of a development will have on the existing hydrology of the site. In order to determine the potential impact a development may have on a property, it is necessary to model both the pre-development and hypothetical unattenuated post-development condition for the site. Stormwater modeling of the pre and post-development conditions of a site is typically done in the State of Connecticut using the TR-55, NRCS methodology for a Type III, 24 hour storm duration. This soil-based methodology involves the input of times of concentration, runoff coefficients, and drainage areas to generate stormwater hydrographs for various design storms. A hydrograph displays the rate of flow from a watershed as a function of time. As the amount of impervious surface for a watershed increases, peak flow rates and runoff volumes of a watershed tend to increase.

image of dtention pond designIn order to provide “Zero Increase in Peak Runoff Rates,” it is necessary to compare the peak runoff rates for the pre-development condition with the unattenuated post-development condition. Once this comparison is made and assuming that the unattenuated post-development peak runoff rates exceed the pre-development peak runoff rates, it is then necessary to determine means of on-site stormwater storage in order to attenuate the peak flow rate increase associated with development. Typical means of on-site stormwater storage include facilities such as detention basins, subsurface detention systems, and other means of temporary stormwater storage. Between storms, these facilities are typically designed to either drain completely or to a certain level (for water quality purposes) in order to provide storage for the next storm. Selection of stormwater storage methods depends on factors such as amount of storage needed, site conditions, land availability, maintenance, and aesthetics.

Once the type of stormwater storage is selected, it is then necessary to implement the storage facility into the post-development model to develop the attenuated post-development condition of a proposed development. During this phase of the stormwater management plan, the stormwater storage system is sized and designed so that the peak runoff rates from each watershed are reduced so that they are less than or equal to those of the pre-development condition. Once the attenuated post-development peak flow rates are less than or equal to the pre-development condition, the stormwater quantity aspect of the stormwater management is then considered acceptable.

Stormwater Quality

In recent years, local, state, and federal land use agencies have been focusing more and more attention on the maintenance and provision of stormwater measures that enhance the quality of stormwater runoff from a site. During the initial stages of a storm, rainwater falling on an improved site tends to transport sediment and surface pollutants downgradient toward existing watercourses and waterbodies. In order to minimize the passage of sediment and pollutants to earth’s vital surface water resources, it is necessary to provide water quality measures as part of site development plans to ensure that runoff has been properly treated prior to leaving a site.

Water quality measures typically include some means of prolonged surface water storage, infiltration, filtration, or combination thereof that promotes the removal and settlement of pollutants and sediment from the runoff being directed to them. The 2004 Connecticut Stormwater Quality Manual as published by the Connecticut Department of Environmental Protection separates stormwater quality measures into primary and secondary treatment practices. The primary treatment practices are “stand-alone” devices that promote high levels of water quality treatment. These primary treatment measures are considered capable of:

  1. Capturing and treating the design Water Quality Volume (WQV) or Water Quality Flow (WQF) as defined in the CTDEP Stormwater Quality Manual;
  2. Providing a minimum 80% removal of average annual total suspended solids (TSS);
  3. Providing a minimum of 80% removal of floatable debris, including oil and petroleum based products, for all flows up to the design Water Quality Flow
  4. Acceptable performance or operational longevity in the field.

Secondary treatment practices are typically viewed as stormwater treatment practices that may not be suitable as stand-alone treatment measures because they are either not capable of meeting the water quality treatment performance criteria described above or have not yet received the thorough evaluation needed to demonstrate the capabilities for meeting the performance criteria. Secondary treatment practices may be classified as primary practices at the discretion of the local review authority and/or DEP. In order to be considered a primary stormwater treatment practice, a water quality measure must demonstrate the ability to treat the design water quality volume or an equivalent design water quality flow, meet the 80 percent TSS and floatables criteria, and have proven operational longevity.

Primary Treatment Measures:

The primary treatment measures are categorized and described in the 2004 Stormwater Quality Manual as follows:

Stormwater Ponds usually maintain a permanent pool of water or combination of a permanent pool of with extended stormwater detention. By providing a permanent pool, these stormwater systems enhance pollutant removal by promoting sedimentation, biological uptake, microbial breakdown, gas exchange, volatilization, and decomposition. Design of stormwater ponds typically involves collaboration with landscape architects, ecologists, wetland specialists, and other land use professionals who analyze a design and develop planting plans that will promote nutrient and pollutant removal with minimal impact to valuable natural resources and wildlife. Examples of Stormwater Ponds include: wet ponds, micropool extended detention ponds, wet extended detention ponds, and multiple pond systems.

Stormwater Wetlands are constructed wetland systems designed to treat polluted stormwater runoff by promoting sedimentation, adsorption, biological uptake, photodegradation, and microbial breakdown. Stormwater wetlands usually include sediment forebays, shallow and deep pool areas, meandering flow paths, and vegetative measures to enhance pollutant removal. Design of stormwater wetlands also usually involves collaboration with landscape architects, ecologists, wetland specialists, and other land use professionals who analyze a design and develop planting plans that will promote nutrient and pollutant removal with minimal impact to valuable natural resources and wildlife. Examples of stormwater wetland practices include shallow wetlands, extended detention wetlands, and pond/wetland systems.

Infiltration measures are designed to capture, temporarily store, and infiltrate stormwater into porous soils. Pollutant removal
occurs through the adsorption of pollutants onto soil particles and subsequent biological and chemical processes in the soil. Infiltration practices promote groundwater recharge but must be carefully designed and maintained to prevent clogging and system failure. Examples of infiltration practices include infiltration trenches and infiltration basins.

Filtering practices treat stormwater runoff by capturing, temporarily storing, and filtering stormwater through sand, soil, organic material, or other porous media. As the stormwater passes through a media filter, sediment particles and some soluble pollutants are removed through physical straining and adsorption. Pretreatment practices are typically required to remove debris and floatables and prolong the life of a filter. Filtering practices include bioretention as well as surface, underground, and perimeter sand filters.

Water quality swales reduce stormwater velocities, provide temporary stormwater runoff storage, and promote infiltration. Pollutant removal processes in water quality swales are similar to those of constructed wetlands which include sedimentation,
adsorption, biological uptake, and microbial breakdown. Water quality swales differ from conventional grass channels and ditches, which are designed for conveyance, in that they provide higher levels of pollutant removal. Practices in this category include dry swales and wet swales.

Secondary Treatment Measures:

Secondary treatment practices are categorized and described in the 2004 Stormwater Quality Manual as follows:

Conventional Practices are practices that have traditionally been used to provide some water quality benefits but do not provide the same level of treatment or broad water quality functions as primary stormwater treatment practices. Consequently, their application is limited to use as pretreatment or supplemental treatment practices in conjunction with primary practices or to achieve other objectives such as groundwater recharge, channel protection, and peak runoff attenuation. Conventional secondary treatment practices include dry detention ponds, underground detention facilities, deep sump catch basins, conventional oil/particle separators, dry wells, permeable pavement, vegetated filter strips, level spreaders, and grass drainage channels.

Innovative/Emerging Technologies are measures in which preliminary performance data indicates that they may provide valuable stormwater treatment, however, they have not been evaluated in sufficient detail to demonstrate proven capabilities for meeting established performance standards which include pollutant removal and field longevity. Many of these technologies are proprietary in nature. Emerging technologies are generally suitable for pre-treatment and are also good candidates for stormwater retrofits and in situations where land is unavailable for larger systems. Their use as primary treatment devices should be evaluated using consistent and technically rigorous protocols. Examples of innovative/emerging technologies include catch basin inserts, hydrodynamic separators, media filters, underground infiltration systems, alum injection, and advanced treatment.

“Treatment Train” Approach:

Stormwater treatment practices can be combined in series to enhance pollutant removal or achieve multiple stormwater objectives. The use of a series of treatment practices, site planning techniques, and source controls is referred to as a “stormwater treatment train”. The use of a treatment train approach can improve levels of pollutant removal, accomplish multiple stormwater objectives, increase treatment measure life span, and reduce potential for resuspension of sediment. A “treatment train” approach may consist of multiple primary treatment measures, a combination of primary and secondary treatment measures, or multiple secondary treatment measures (at the discretion of the reviewing agency).

What We Do

Here at Arthur H. Howland & Associates, P.C. we offer a great deal of expertise and knowledge with regard to the field of hydrology and stormwater management. We pride ourselves on our ability to tailor stormwater management plans based on your needs while still adhering to the various pertinent local, state, and federal regulations and guidelines. We make stormwater management a priority from the initial planning stages of a development through to design. We provide clear, concise, high quality stormwater management reports and construction plans so that all entities involved with the approval and construction phases of the project have a full understanding of the stormwater management plan. Whether you are looking to design a development project from scratch or enhance an existing project, we are here to help you along the way. Contact us at (860) 354-9346 to discuss how we can help you develop a stormwater management plan that is tailored to your project’s needs.

Reference: Connecticut Department of Environmental Protection (CTDEP). 2004. Connecticut Stormwater Quality Manual. Hartford, CT.

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