20. Storm Water Design#
Course Website
Readings#
Videos#
Importance of Stormwater Management#
Stormwater management is critical in both urban and rural environments, though the challenges and strategies differ significantly between the two. In urban areas, increased impervious surfaces such as roads, buildings, and parking lots prevent natural infiltration, leading to higher runoff volumes, flash flooding, and water quality degradation. Without proper management, urban stormwater can overwhelm drainage infrastructure, erode waterways, and carry pollutants such as oil, heavy metals, and sediment into local rivers and lakes. Cities address these issues through detention ponds, green infrastructure (e.g., bioswales and permeable pavements), and municipal stormwater regulations, ensuring that development does not exacerbate flood risks or degrade water quality.
In rural settings, stormwater management is equally important but presents different challenges. While there is typically more natural infiltration due to open land, agricultural and ranching activities can contribute to nonpoint source pollution, such as sediment, fertilizers, and pesticides washing into streams. Additionally, poor drainage infrastructure in rural areas can lead to road washouts, soil erosion, and damage to farmland. Effective rural stormwater management relies on strategies such as vegetative buffers, retention basins, contour farming, and proper drainage design to reduce runoff impacts. Whether in cities or the countryside, proactive stormwater management protects property, infrastructure, and water resources, ensuring long-term environmental sustainability and resilience against extreme weather events.
Objectives of Stormwater Infrastructure Design#
The primary objective of stormwater infrastructure design is to control, convey, and treat stormwater runoff in a way that minimizes flooding, erosion, and water quality degradation while supporting sustainable land development. A well-designed system must efficiently manage peak flows during heavy rainfall, ensuring that urban and rural areas remain resilient to storm events. Key design considerations include detaining or infiltrating runoff to prevent downstream flooding, safely conveying excess water through storm drains, culverts, and channels, and integrating Best Management Practices (BMPs) to reduce pollutant loads before stormwater reaches natural water bodies.
Beyond immediate flood control, modern stormwater infrastructure aims to mimic natural hydrologic processes, promoting groundwater recharge, ecological balance, and climate resilience. Green infrastructure solutions, such as bioretention systems, permeable pavements, and constructed wetlands, enhance traditional stormwater networks by providing additional storage and filtration capacity. Additionally, stormwater designs must account for long-term maintenance and adaptability, ensuring that infrastructure remains functional under changing land-use conditions and increasing rainfall variability due to climate change. By balancing hydraulic efficiency, environmental protection, and urban development needs, stormwater infrastructure plays a critical role in safeguarding communities and water resources for the future.
Stormwater Collection#
Stormwater collection systems are designed to efficiently capture, convey, and manage runoff from various surfaces, directing it to a controlled outlet or discharge point to prevent flooding, erosion, and water quality issues. These systems play a crucial role in urban and rural environments by mitigating the impact of excess surface water and ensuring that runoff is safely transported or treated before re-entering the natural hydrologic cycle.
Sources of stormwater runoff include:
Impervious surfaces such as parking lots, roads, and rooftops, where rainfall cannot infiltrate, leading to rapid accumulation and potential flooding.
Residential areas, including driveways, sidewalks, and yards, which generate runoff that must be directed to storm drains or detention facilities.
Industrial and commercial properties, where large paved surfaces and warehouses contribute significant stormwater volumes.
Agricultural land and open spaces, which, depending on soil conditions and land use, can contribute both sheet flow and concentrated runoff into drainage channels or retention basins.
Natural landscapes that experience increased runoff due to land development, deforestation, or soil compaction.
Stormwater systems ensure that collected water is routed appropriately, either through underground storm sewers, open ditches, detention ponds, or infiltration systems.
Depending on site conditions, runoff may be temporarily stored, treated for pollutants, or directly released into streams, rivers, or recharge areas. Effective stormwater infrastructure design balances conveyance, storage, and treatment to protect both the built and natural environments.
Regulatory Guidance Documents#
Regulatory guidance documents are a principal tool in system design, along with the designers creativity, and the owners access to right-of-way. Jurisdictions that issue guidance include:
FEMA (Federal Emergency Management Agency) writes federal regulations for construction, maintenance, treatment and operation of storm water facilities;
USEPA, USACOE, USBR all have roles;
State Environmental Departments, Transportation Departments , Cities, are charged with regulating aspects of the standards and associated permitting.
Regulatory Frameworks in Texas
In Texas, local governance operates under a unique framework that historically limits county authority while granting broad regulatory powers to cities. Unlike many other states, Texas counties have traditionally lacked ordinance-making power, meaning they cannot regulate land use, construction, or other municipal-style governance issues in the same way that cities can. Additionally, special legislative districts serve as a regulatory tool, filling gaps where city and county jurisdiction may be insufficient or politically constrained.
City Ordinances vs. County Guidelines
City Ordinances: Local Law with Legal Authority
Cities in Texas, particularly home-rule cities (those with populations over 5,000 and a voter-approved charter), have broad authority to pass ordinances governing a wide range of local matters. These ordinances carry the force of law and can regulate:
Zoning and land use (e.g., residential, commercial, and industrial districts).
Building codes and construction standards.
Public safety, noise control, and nuisance abatement.
Health and sanitation regulations.
Traffic control and parking enforcement.
The ability to enact ordinances allows cities to customize regulations to address local concerns, making them the primary regulators of urban development and public services.
County Guidelines: Historically Weak but Evolving
Unlike cities, Texas counties have historically been restricted in their regulatory authority. The Texas Constitution does not grant counties ordinance-making power, meaning they cannot enforce zoning, building codes, or development restrictions outside of city limits. Instead, counties traditionally rely on:
Guidelines and recommendations rather than enforceable laws.
Health and safety regulations (e.g., septic system rules).
Road and infrastructure maintenance.
Emergency management (in coordination with state agencies).
Subdivision regulations (which are limited and often weaker than city zoning codes).
However, there has been some evolution in county power. In certain cases, the Texas Legislature has granted counties specific regulatory authority, such as floodplain management or environmental regulations in unincorporated areas. But the general rule remains: counties lack the ability to enforce zoning or comprehensive land-use plans like cities do.
Special Legislative Districts: A Flexible Regulatory Tool
To bridge the regulatory gap between cities and counties, special legislative districts have become a key governance mechanism in Texas. These districts are created by acts of the Texas Legislature to address specific issues that cities and counties may not effectively regulate. Some examples include:
Municipal Utility Districts (MUDs) – Created to finance and manage water, wastewater, drainage, and sometimes road infrastructure in unincorporated areas.
Flood Control Districts – Manage stormwater runoff, drainage infrastructure, and flood mitigation efforts.
Hospital Districts – Provide healthcare services where counties and cities lack the resources to do so.
Economic Development Districts – Established to spur investment and infrastructure in designated areas.
Special Zoning Districts – Used to implement land-use controls in areas not covered by city zoning laws.
These districts fill regulatory voids by allowing targeted governance without expanding city or county control. They are often funded by special taxes, bonds, or fees and can enforce regulations within their specific jurisdiction.
Texas’ unique regulatory framework significantly influences the development and enforcement of stormwater management policies, particularly due to the division of authority between cities, counties, and special legislative districts. Because counties lack ordinance-making power, stormwater regulations are largely driven by municipal governments, which can enact drainage requirements, impervious cover limits, and detention pond standards through city ordinances. This means that stormwater regulations are often highly localized, varying from city to city based on priorities, resources, and political will. In contrast, counties rely on state and federal guidelines, such as those issued under the Texas Commission on Environmental Quality (TCEQ) Municipal Separate Storm Sewer System (MS4) program, but often lack enforcement mechanisms beyond subdivision regulations in unincorporated areas.
To address gaps in county authority, special legislative districts, such as flood control districts and drainage districts, play a crucial role in regional stormwater management. The Harris County Flood Control District (HCFCD), for example, is a state-created entity that oversees large-scale flood mitigation projects beyond the jurisdiction of individual cities. Similarly, Municipal Utility Districts (MUDs) often implement stormwater controls in developing areas, particularly where cities have not yet annexed land. This fragmented regulatory landscape leads to uneven stormwater standards across jurisdictions, requiring collaboration between cities, counties, and districts to develop regional solutions. While the decentralized nature of Texas governance encourages local flexibility, it can also create regulatory inconsistencies that complicate watershed-scale stormwater planning and infrastructure development.
Representative Community Manuals#
Stormwater System Design#
Stormwater system design is a fundamental aspect of urban and rural infrastructure planning, aimed at effectively managing runoff to prevent flooding, erosion, and water quality degradation. While specific design standards vary across jurisdictions, stormwater infrastructure follows common hydraulic engineering principles adapted to local site conditions and regulatory requirements. These systems can take various forms, including piped conveyance, engineered open channels, detention basins, and natural drainage features. Regardless of the conveyance type, the objective remains the same: to capture, control, and safely discharge stormwater while minimizing environmental and infrastructure impacts.
Project Layout and Design Approach#
The layout of a stormwater system is determined by land use, topography, hydrology, and regulatory constraints. Design manuals often provide detailed guidance on submittal requirements, but the actual system configuration remains flexible within the available right-of-way. A typical stormwater design process includes:
Defining the Drainage Network – Establishing a stormwater collection system layout using aerial imagery, topographic maps, and GIS-based hydrologic data.
Peak Discharge Estimation – Conducting hydrologic analysis to determine expected runoff rates at different locations for various storm events.
Selection of Conveyance Methods – Evaluating whether runoff should be conveyed using piped systems, open channels, swales, or detention basins.
Routing of Discharges – Ensuring runoff is efficiently directed to appropriate outfalls, retention/infiltration areas, or natural waterways.
Future Capacity Planning – Designing for current drainage needs while allowing for future expansion as land development progresses.
Managing Ponding Depths – Ensuring that low-lying areas, intersections, and residential zones do not experience excessive water pooling during storms.
Stormwater conveyance systems are not limited to underground pipes. Many designs incorporate engineered open channels, grassed swales, detention basins, and floodplain modifications to manage flow efficiently. While piped systems are common in urban environments due to space constraints, open channel systems offer cost-effective and environmentally friendly alternatives, particularly in suburban and rural settings.
Hydraulic Modeling and Conveyance Considerations#
Hydraulic modeling plays a crucial role in stormwater design, regardless of whether the system uses pipes, culverts, or open channels. Modern stormwater infrastructure design relies on computational modeling tools to assess flow rates, water surface elevations, and system performance under various conditions.
Piped Conveyance Systems – Typically designed using Manning’s equation for pressurized or non-pressurized flow. The goal is to size pipes to:
Accommodate peak runoff without surcharge.
Maintain self-cleaning velocities to prevent sediment buildup.
Optimize pipe slope and depth for cost-effective installation.
Open Channel Systems – Designed based on gradual or rapidly varied flow conditions using equations such as Manning’s equation for open-channel flow. These systems include:
Grass swales and bioswales, which slow and filter runoff.
Engineered channels, such as concrete-lined ditches or rock-lined conveyances.
Detention and retention basins, which store water to reduce peak discharge.
Hybrid Systems – Many stormwater designs incorporate both piped and open-channel elements, with pipes handling high-flow urban runoff and channels providing detention and filtration functions.
Regardless of the conveyance method, hydraulic models calculate flow depths, velocities, and potential energy losses, ensuring that the system performs effectively across various storm conditions.
Key Design Considerations#
Existing Drainage Infrastructure and Capacity A stormwater system must be integrated with existing drainage networks while ensuring adequate capacity for anticipated runoff. Reliable land use plans, zoning maps, and hydrologic models help establish drainage needs, with drainage maps guiding the placement of collection points, culverts, and outfalls.
Pipe and Channel Sizing Both pipe diameter selection and open channel dimensions directly affect stormwater conveyance efficiency. While trenching costs often drive pipe size selection, open channel designs require careful consideration of:
Side slopes for stability and maintenance.
Vegetative or structural lining to minimize erosion.
Freeboard and overflow provisions to manage extreme storm events.
Junction Location and Elevation Junction points, or collection nodes, play a critical role in both piped and open-channel networks. These locations serve as calculation points in hydraulic models, ensuring proper:
Water depth and velocity control to prevent excessive erosion or backflow.
Pipe and channel connectivity, considering gradients and system efficiency.
Integration with natural waterways, balancing engineered solutions with environmental impact.
Material Selection The choice of materials affects the long-term durability and functionality of stormwater systems. Common materials include:
Reinforced Concrete Pipe (RCP) – Often used for high-load applications in urban drainage systems.
PVC and HDPE – Lightweight, corrosion-resistant alternatives for piped systems.
Vegetated swales and riprap-lined channels – Common in open-channel drainage for erosion control.
Designers must ensure compliance with local material specifications, as regulatory documents often mandate preferred pipe types, channel linings, and allowable flow velocities.
Stormwater system design requires a comprehensive approach that integrates piped and open-channel infrastructure to provide effective flood mitigation, erosion control, and water quality management. Whether using closed conduits, engineered channels, or naturalized drainage features, hydraulic engineers must apply consistent design principles while tailoring calculations to the specific conveyance method. A well-designed stormwater system not only protects property and infrastructure but also enhances sustainability and resilience against changing climate and land-use conditions.
Warning
Stormwater system design is inherently a three-dimensional challenge, requiring careful attention to both horizontal alignments and vertical elevations. Neglecting elevation changes can result in hydraulically incoherent designs, leading to ineffective flow conveyance, unintended ponding, or system failures. Proper grading, slope considerations, and elevation coordination are essential to ensure stormwater is efficiently collected, conveyed, and discharged without adverse hydraulic consequences.
Plan and profile drawings are always required, even in fairly simple contexts.