Stormwater drainage design is an integral component of both site and overall stormwater management design. Good drainage design must strive to maintain compatibility and minimize interference with existing drainage patterns; control flooding of property, structures and roadways for design flood events; and minimize potential environmental impacts on stormwater runoff.
Stormwater collection systems must be designed to provide adequate surface drainage while at the same time meeting other stormwater management goals such as water quality, streambank channel protection, habitat protection and groundwater recharge.
Storm Drainage System Components
In every location there are two stormwater drainage systems, the minor system and the major system. Three considerations largely shape the design of these systems: flooding, public safety and water quality.
The minor drainage system is designed to remove stormwater from areas such as streets and sidewalks for public safety reasons. The minor drainage system consists of inlets, street and roadway gutters, roadside ditches, small channels and swales, and small underground pipe systems which collect stormwater runoff and transport it to structural control facilities, pervious areas and/or the major drainage system (i.e., natural waterways, large man-made conduits, and large water impoundments). Paths taken by runoff from very large storms are called major systems.
The major system (designed for the less frequent storm up to the 100-yr level) consists of natural waterways, large man-made conduits, and large water impoundments. In addition, the major system includes some less obvious drainageways such as overload relief swales and infrequent temporary ponding areas. The major system includes not only the trunk line system that receives the water from the minor system, but also the natural backup system which functions in case of overflow from or failure of the minor system.
Storm Drainage Planning and Design
The following is a general procedure for drainage system design on a development site.
Analyse topography-
a) Check off-site drainage pattern. Where is water coming onto the site? Where is water leaving the site?
b) Check on-site topography for surface runoff and storage, and infiltration
Determine runoff pattern; high points, ridges, valleys, streams, and swales. Where is the water going?
Overlay the grading plan and indicate watershed areas; calculate square footage (acreage), points of concentration, low points, etc.
a) Land use and physical obstructions such as walks, drives, parking, patios, landscape edging, fencing, grassed area, landscaped area, tree roots, etc.
b) Soil type determines the amount of water that can be absorbed by the soil.
c) Vegetative cover will determine the amount of slope possible without erosion.
Analyse areas for probable location of drainage structures and facilities.
Consideration Factors During Stormwater Drainage Design
Introduction
The traditional design of stormwater drainage systems has been to collect and convey stormwater runoff as rapidly as possible to a suitable location where it can be discharged. This means that:
Stormwater conveyance systems are to remove water efficiently enough to meet flood protection criteria and level of service requirements, and
These systems are to complement the ability of the site design and structural stormwater controls to mitigate the major impacts of urban development.
General Drainage Design Considerations
Stormwater systems should be planned and designed so as to generally conform to natural drainage patterns and discharge to natural drainage paths within a drainage basin. These natural drainage paths should be modified as necessary to contain and safely convey the peak flows generated by the development.
Runoff must be discharged in a manner that will not cause adverse impacts on downstream properties or stormwater systems. In general, runoff from development sites within a drainage basin should be discharged at the existing natural drainage outlet or outlets. If the developer wishes to change discharge points, he or she must demonstrate that the change will not have any adverse impacts on downstream properties or stormwater systems.
It is important to ensure that the combined minor and major system can handle blockages and flows in excess of the design capacity to minimize the likelihood of nuisance flooding or damage to private properties. If failure of minor systems and/or major structures occurs during these periods, the risk to life and property could be significantly increased.
In establishing the layout of stormwater networks, it is essential to ensure that flows will not discharge onto private property during flows up to the major system design capacity.
Gutters are efficient flow conveyance structures. This is not always an advantage if removal of pollutants and reduction of runoff is an objective. Therefore, impervious surfaces should be disconnected hydrologically where possible and runoff should be allowed to flow across pervious surfaces or through grass channels. Gutters should be used only after other options have been investigated and only after runoff has had as much chance as possible to infiltrate and filter through vegetated areas.
It may be possible not to use gutters at all, or to modify them to channel runoff to off-road pervious areas or open channels. For example, curb opening type designs take roadway runoff to smaller feeder grass channels. Care should be taken not to create erosion problems in off-road areas. Protection during construction, establishment of strong stands of grass, and active maintenance may be necessary in some areas.
Use road cross sections that include grass channels or swales instead of gutters to provide for pollution reduction and reduce the impervious area required.
To protect the edge of pavement, ribbons of concrete can be used along the outer edges of asphalt roads.
Inlets and Drains
Inlets should be located to maximize overland flow path, take advantage of pervious areas, and seek to maximize vegetative filtering and infiltration. For example, it might be possible to design a parking lot so that water flows into vegetated areas prior to entering the nearest inlet.
Inlet location should not compromise safety or aesthetics. It should not allow for standing water in areas of vehicular or pedestrian traffic, but should take advantage of natural depression storage where possible.
Inlets should be located to serve as overflows for structural stormwater controls. For example, a bioretention device in a commercial area could be designed to overflow to a catch basin for larger storm events.
The choice of inlet type should match its intended use. A sumped inlet may be more effective supporting water quality objectives.
Use several smaller inlets instead of one large inlet in order to:
(1) Prevent erosion on steep landscapes by intercepting water before it accumulates too much volume and velocity.
(2) Provide a safety factor. If a drain inlet clogs, the other surface drains may pick up the water.
(3) Improve aesthetics. Several smaller drains will be less obvious than one large drain.
(4) Spacing smaller drain inlets will give surface runoff a better chance of reaching the drain. Water will have farther to travel to reach one large drain inlet.
Storm Drain Pipe Systems (Storm Sewers)
The use of better site design practices (and corresponding site design credits) should be considered to reduce the overall length of a piped stormwater conveyance system.
Shorter and smaller conveyances can be designed to carry runoff to nearby holding areas, natural conservation areas, or filter strips (with spreaders at the end of the pipe).
Ensure that storms in excess of pipe design flows can be safely conveyed through development without damaging structures or flooding major roadways. This is often done through design of both a major and minor drainage system. The minor (piped) system carries the mid-frequency design flows while larger runoff events may flow across lots and along streets as long as it will not cause property damage or impact public safety.
Culverts
Culverts can serve double duty as flow retarding structures in grass channel design. Care should be taken to design them as storage control structures if depths exceed several feet, and to ensure safety during flows.
Improved inlet designs can absorb considerable slope and energy for steeper sloped designs, thus helping to protect channels.
Open Channels
Open channels provide opportunities for reduction of flow peaks and pollution loads. They may be designed as wet or dry enhanced swales or grass channels.
Channels can be designed with natural meanders improving both aesthetics and pollution removal through increase of contact time.
Grass channels generally provide better habitat than hardened channel sections, though studies have shown that riprap interstices provide significant habitat as well. Velocities should be carefully checked at design flows and the outer banks at bends should be specifically designed for increased shear stress.
Compound sections can be developed that carry the annual flow in the lower section and higher flows above them. Below Figure illustrates a compound section that carries the 2-year and 10-year flows within banks. This reduces channel erosion at lower flows, and meandering, self-forming low flow channels that attack banks. The shelf in the compound section should have a minimum 1:12 slope to ensure drainage.
Flow control structures can be placed in the channels to increase residence time. Higher flows should be calculated using a channel slope that goes from the top of the cross piece to the next one if it is significantly different from the channel bottom for normal depth calculations. Channel slope stability can also be ensured through the use of grade control structures that can serve as pollution reduction enhancements if they are set above the channel bottom. Regular maintenance is necessary to remove sediment and keep the channels from aggrading and losing capacity for larger flows.
Design Storm Recommendations
Listed below are the design storm recommendations for various stormwater drainage system components to be designed and constructed in accordance with the minimum stormwater management standards. Some jurisdictions may require the design of both a minor and major stormwater conveyance system, sized for two different storm frequencies. Please consult your local review authority to determine the local requirements. It is recommended that the full build-out conditions be used to calculate flows for the design storm frequencies below.
Storm Drainage Systems
Includes storm drainage systems and pipes that do not convey runoff under public roadways, sometimes called lateral closed systems.
10- to 25-year design storm (for pipe and culvert design)
Cross drainage facilities that transport storm runoff under roadways.
25- to 100-year design storm, whichever is more stringent. (Criteria to be taken into consideration when selecting design flow include roadway type, depth of flow over road, structures and property subject to flooding, emergency access, and road replacement costs)
Open Channel Design
Open channels include all channels, swales, etc.
25-year design storm Channels may be designed with multiple stages (e.g., a low flow channel section containing the 2-year to 5-year flows, and a high flow section that contains the design discharge) to improve stability and better mimic natural channel dimensions. Where flow easements can be obtained and structures kept clear, overbank areas may also be designed as part of a conveyance system wherein floodplain areas are designed for storage and/or conveyance of larger storms.
