Technical Release 55 (TR-55) provides simplified hydrology methods for small watersheds, initially published in January 1975.
It calculates runoff volume, peak discharge, hydrographs, and storage for stormwater management, relying on manual calculations and Type II rainfall distribution.
What is TR-55?
Technical Release 55, commonly known as TR-55, is a pivotal document in the field of urban hydrology, specifically designed for analyzing small watersheds. First released in January 1975 by the Soil Conservation Service (SCS), now the Natural Resources Conservation Service (NRCS), it offers a streamlined methodology for estimating the quantity and timing of storm runoff.
At its core, TR-55 provides simplified procedures to calculate four key hydrological parameters: storm runoff volume, the peak rate of discharge, hydrographs illustrating flow over time, and the necessary storage volumes for floodwater reservoirs or stormwater management structures. This makes it an invaluable tool for engineers, planners, and anyone involved in managing water resources in developing areas.
Initially, the manual focused on entirely manual computation methods, assuming a standardized NRCS Type II rainfall distribution for all calculations. This approach aimed to provide a consistent and relatively easy-to-use framework for assessing the impact of land use changes on runoff characteristics. TR-55’s primary application lies in small watersheds, particularly those undergoing urbanization, within the United States.
Historical Context of TR-55 (1975)
The genesis of Technical Release 55 (TR-55) in 1975 arose from a growing need for a standardized, yet accessible, method for evaluating stormwater runoff in small watersheds. Prior to its release, hydrological analysis often relied on complex and time-consuming calculations, hindering effective stormwater management, especially in rapidly developing urban areas.
The Soil Conservation Service (SCS), recognizing this gap, developed TR-55 as a simplified procedure. The initial version was entirely manual, reflecting the computational limitations of the time. A key assumption was the universal application of the NRCS Type II rainfall distribution, streamlining the process but also introducing a degree of generalization.
The 1970s witnessed increasing concerns about the environmental impacts of urbanization, including increased flooding and water pollution. TR-55 provided a practical tool to address these concerns by enabling engineers to estimate runoff and design appropriate stormwater control measures. It quickly became a foundational document for stormwater management practices across the United States, offering a common language and methodology for professionals in the field.
Purpose and Applicability
TR-55’s primary purpose is to present simplified procedures for calculating key hydrological parameters – storm runoff volume, peak discharge rate, hydrographs, and required storage volumes – essential for designing floodwater reservoirs and other stormwater management structures. It’s specifically tailored for application in small watersheds, particularly those undergoing urbanization, within the United States.
The manual’s applicability extends to a wide range of projects, from small-scale developments to larger watershed-scale analyses. It assists in determining the impact of land use changes on runoff patterns and helps engineers design effective control measures to mitigate flooding and erosion. However, it’s crucial to understand its limitations.
TR-55 is most effective when applied to watersheds where its underlying assumptions hold true. When these assumptions are significantly violated, more sophisticated tools like TR-20 are recommended. Despite its age, TR-55 remains a valuable reference, particularly for its comprehensive curve number tables and rainfall data, even as more advanced methodologies gain prominence.
Key Concepts in TR-55
TR-55 centers around calculating runoff volume and peak discharge, generating hydrographs, and determining storage volumes. These calculations rely heavily on curve numbers and rainfall data.
Runoff Volume Calculation
TR-55 employs a relatively straightforward method for estimating runoff volume, a crucial step in stormwater management design. The core of this calculation revolves around the Curve Number (CN) method, which represents the land’s runoff potential. Initially, the total rainfall depth for a specific storm event must be determined, often utilizing regional rainfall frequency data.
This rainfall depth is then adjusted for initial abstraction – the amount of rainfall lost to infiltration and depression storage before runoff begins. The CN value, representing a combination of land cover and hydrological condition, is then used in a formula to calculate the runoff depth. This formula essentially subtracts the initial abstraction from the rainfall, weighted by the CN value.
Importantly, TR-55 accounts for the influence of impervious areas, assigning them a CN of 98, signifying minimal infiltration. Composite CN values are calculated when a watershed contains a mix of land cover types, weighted by their respective areas. The resulting runoff volume is then determined by multiplying the runoff depth by the watershed area, providing an estimate of the total water volume generated by the storm event. Accurate CN determination is paramount for reliable runoff volume estimates.
Peak Discharge Rate Estimation
Following runoff volume calculation, TR-55 provides methods to estimate the peak discharge rate – the maximum flow of water during a storm event. This estimation relies heavily on the Time of Concentration (Tc), which represents the time it takes for water to travel from the most remote point in the watershed to the outlet.
TR-55 offers several methods for estimating Tc, considering factors like overland flow length, slope, and surface roughness. Once Tc is determined, it’s used in conjunction with the runoff volume and watershed characteristics to calculate the peak discharge rate. The Rational Method, a simplified approach, is often employed for smaller watersheds.
For more complex scenarios, TR-55 introduces dimensionless peak flow factors that adjust the discharge rate based on watershed shape, slope, and antecedent moisture conditions. These factors account for the dynamic nature of runoff processes. The calculated peak discharge rate is critical for designing appropriately sized stormwater conveyance systems, such as channels and culverts, to prevent flooding and ensure safe drainage. Accurate Tc estimation is vital for reliable peak discharge predictions.
Hydrograph Generation
TR-55 extends beyond peak discharge estimation by providing procedures for generating hydrographs – graphical representations of flow rate over time. These hydrographs are essential for understanding the timing and duration of runoff events, crucial for designing effective stormwater management facilities.
The hydrograph generation process in TR-55 involves dividing the rainfall event into several time increments and calculating the runoff volume for each increment. These incremental runoff volumes are then combined to create the overall hydrograph. The Time of Concentration (Tc) plays a significant role, defining the shape and timing of the hydrograph’s rising limb.
TR-55 utilizes unit hydrograph concepts, adjusting the shape based on watershed characteristics. Storage effects within the watershed, such as depressions and vegetation, are also considered to refine the hydrograph. The resulting hydrograph provides valuable insights into the watershed’s response to rainfall, aiding in the design of detention basins and other control measures to mitigate flood risks and manage water resources effectively.
Storage Volume Determination
TR-55 provides methodologies for determining the required storage volume in stormwater management facilities, such as detention basins or ponds. This calculation is vital for controlling peak discharge rates and mitigating downstream flooding. The process relies heavily on the generated hydrograph and the desired level of flood protection.
The storage volume is calculated based on the difference between the inflow hydrograph (runoff from the watershed) and the outflow hydrograph (controlled release from the facility). TR-55 offers simplified methods, including tables and graphs, to estimate the required storage based on the peak discharge rate, time of concentration, and the chosen outlet structure.
Several factors influence the storage volume, including the watershed area, curve number, rainfall distribution, and the desired outflow rate. Engineers utilize TR-55’s procedures to balance the need for flood control with the practical constraints of land availability and construction costs, ensuring effective and sustainable stormwater management practices.
Understanding Curve Numbers (CN)
Curve Numbers (CN) represent the runoff potential of a watershed, crucial for TR-55 calculations. They reflect soil type, land cover, and antecedent moisture conditions, impacting runoff volume estimates.
Definition and Significance of CN
Curve Numbers (CN), central to the TR-55 methodology, are dimensionless parameters representing the runoff potential of a land area. They abstractly reflect various hydrological characteristics, primarily the infiltration capacity of soils and the amount of overland flow generated during rainfall events. A higher CN value indicates less infiltration and greater runoff potential, typically associated with impervious surfaces or soils with poor drainage.
The significance of CN lies in its ability to quantify the impact of land cover and treatment practices on watershed hydrology. By assigning appropriate CN values to different land uses – such as forests, grasslands, cultivated lands, and urban areas – engineers can estimate the amount of rainfall that will become runoff. This estimation is fundamental for designing stormwater management facilities, assessing flood risks, and evaluating the effectiveness of conservation measures.
TR-55 utilizes CN values in conjunction with rainfall data to calculate runoff volume. The CN is directly incorporated into equations that determine the abstract retention parameter, which represents the average amount of rainfall stored in the watershed before runoff begins. Accurate CN selection is therefore paramount for obtaining reliable runoff estimates and ensuring the proper functioning of stormwater infrastructure.
Composite Curve Numbers
When a watershed contains a mixture of different land cover types, a composite Curve Number (CN) must be calculated to represent the overall runoff potential. TR-55 provides a weighted average method for determining this composite CN, considering the area and CN value of each land use component.
The calculation involves multiplying the area of each land use type by its corresponding CN, summing these products, and then dividing by the total watershed area. This weighted average effectively accounts for the relative contribution of each land cover type to the overall runoff response. For example, a watershed with 70% forest (CN=40) and 30% urban area (CN=98) would have a composite CN calculated as: [(0.70 * 40) + (0.30 * 98)] / 1.0 = 67.4.
The average percent impervious area is crucial in developing these composite CNs. TR-55 emphasizes that accurate determination of land use areas and their corresponding CN values is essential for reliable runoff estimations. Figures 2-3 and 2-4 within the manual provide guidance on selecting appropriate CN values based on specific land cover and hydrological conditions.
Impervious Area Considerations (CN = 98)
TR-55 consistently assigns a Curve Number (CN) of 98 to impervious areas, recognizing their complete inability to absorb rainfall. This high CN value reflects the direct conversion of precipitation into runoff on surfaces like rooftops, pavements, and parking lots. The manual explicitly states this assumption, simplifying calculations for urban watersheds.
However, it’s crucial to understand the underlying assumption: these impervious areas are considered directly connected to the drainage system. This means rainfall falling on these surfaces immediately flows into the nearest conveyance channel without any significant detention or infiltration. If some portion of impervious runoff experiences even minimal storage (e.g., sheet flow over a landscaped area before reaching a drain), this assumption is violated.
Accurate delineation of impervious areas is therefore paramount. Overestimation can lead to inflated runoff predictions, while underestimation can underestimate flood risks. TR-55 relies on careful assessment of land use maps and field observations to determine the extent of impervious surfaces within a watershed. Remember, the CN=98 value is a simplification, and its accuracy depends on the validity of the direct connection assumption.
Using Figures 2-3 and 2-4 for CN Calculation
TR-55 provides Figures 2-3 and 2-4 as essential tools for determining composite Curve Numbers (CNs) when a watershed contains a mix of land cover types and antecedent moisture conditions. These graphical representations allow users to move beyond simple CN assignments and account for varying hydrologic characteristics.
Figure 2-3 is specifically designed for calculating CNs based on the percentage of impervious area within a watershed. By inputting the percentage of impervious surface, a corresponding composite CN can be directly read from the graph. Figure 2-4, however, addresses scenarios with varying percentages of different pervious land cover types (e.g., forests, grasslands, cultivated lands).
Using these figures requires careful consideration of the watershed’s composition. Accurately determining the area percentages for each land cover type is crucial for obtaining a representative composite CN. The manual emphasizes that these figures are based on specific assumptions about hydrologic condition; therefore, understanding these limitations is vital for reliable runoff estimation. These figures are a cornerstone of applying TR-55 effectively.
Limitations and Alternatives to TR-55
TR-55 relies on assumptions that may not always hold true; more precise tools like TR-20 are recommended when these assumptions are invalid.
Its use is declining.
Assumptions of TR-55
TR-55 operates under several key assumptions that significantly influence the accuracy of its results. A primary assumption is that impervious areas are directly connected to the drainage system, meaning runoff from these surfaces immediately enters the flow path without significant overland flow delay. This is a simplification, as some impervious areas may have limited connectivity.
Furthermore, TR-55 consistently assigns a Curve Number (CN) of 98 to all impervious areas, representing maximum runoff potential. While generally reasonable, this doesn’t account for variations in impervious surface type or potential for on-site detention. Pervious areas are treated as equivalent to open space in good hydrologic condition, neglecting factors like vegetation density, soil type, and antecedent moisture conditions.
The original TR-55 manual also assumed the NRCS Type II rainfall distribution for all calculations, which may not be representative of rainfall patterns in all regions of the United States. Composite CNs are developed using average percent impervious area, potentially masking localized variations in land cover. These assumptions, while simplifying the process, introduce potential errors and limit the applicability of TR-55 in complex watershed scenarios.
When to Use More Precise Tools (e.g., TR-20)
TR-55, while valuable, has limitations. When its underlying assumptions are significantly violated, more sophisticated hydrological models become necessary. Specifically, if the direct connection assumption for impervious areas doesn’t hold – for instance, with extensive on-site detention or disconnected impervious surfaces – TR-55’s accuracy diminishes.
Complex watershed characteristics, such as varying soil types, land uses, or irregular topography, also warrant the use of more detailed tools. Situations requiring precise hydrograph shaping, detailed routing analysis, or consideration of multiple storm events are better suited for models like TR-20.
TR-20 allows for a more nuanced representation of watershed processes, incorporating detailed channel geometry, reservoir routing, and variable rainfall distributions. It’s crucial to transition to these advanced tools when dealing with large watersheds, critical infrastructure, or projects demanding a higher level of hydrological accuracy. The TR-55 manual itself recommends considering TR-20 when its assumptions are not met, prioritizing reliable results over simplified calculations.
The Declining Use of the TR-55 Manual
Despite remaining a useful reference – particularly for its comprehensive curve number tables and rainfall maps – the direct application of the TR-55 manual has decreased over time. This decline stems from the emergence of more advanced and accurate hydrological modeling software and techniques.
Modern engineers increasingly favor tools offering greater precision, flexibility, and the ability to simulate complex watershed behaviors. Software packages now routinely incorporate continuous simulation capabilities, detailed rainfall-runoff modeling, and advanced routing algorithms, exceeding TR-55’s scope.
The availability of these alternatives has diminished the necessity of relying solely on the TR-55 methodology. While the fundamental principles within TR-55 remain relevant, its simplified approach often proves insufficient for addressing contemporary stormwater management challenges and regulatory requirements. However, its historical significance and foundational concepts continue to inform modern hydrological practices, serving as a valuable learning resource for professionals.
Resources and Further Information
The USDA affirms its commitment as an equal opportunity provider and employer. TR-55 documentation remains accessible, offering valuable insights into small watershed hydrology.
USDA and Equal Opportunity
The United States Department of Agriculture (USDA) plays a crucial role in the development and dissemination of resources like Technical Release 55 (TR-55). It’s fundamentally important to acknowledge the USDA’s steadfast commitment to equal opportunity for all, both as a provider of services and as an employer.
This commitment extends to ensuring that all individuals have access to the information and tools necessary for effective stormwater management and watershed planning. TR-55, while a technical document, is intended to be utilized by a broad range of professionals and stakeholders, and equitable access is paramount.
The USDA’s dedication to equal opportunity means that programs and resources are available without discrimination based on race, color, national origin, sex, religion, age, disability, political beliefs, or sexual orientation. This principle guides the agency’s efforts to promote sustainable land use practices and protect valuable water resources. Understanding this foundational principle is key when utilizing TR-55 and related hydrological tools.
Availability of TR-55 Documentation
While originally a cornerstone resource, the widespread availability of more advanced hydrological modeling tools has somewhat diminished the direct need for the physical TR-55 manual. However, the document remains a valuable reference, particularly for its comprehensive curve number (CN) tables and detailed rainfall maps.
Historically, the TR-55 manual was obtainable through USDA Natural Resources Conservation Service (NRCS) offices. Today, accessing the complete documentation often involves searching online repositories and governmental websites. Digital versions, including PDFs, are frequently available for download, offering convenient access to the methodologies and data presented within.
Despite the rise of software like TR-20, understanding the foundational principles outlined in TR-55 is still beneficial for engineers and hydrologists. The manual provides a clear understanding of the underlying assumptions and limitations of simplified hydrological analysis. Resources and archived versions can be found through various online searches, ensuring continued access to this historical document.