Subdivision Stormwater Design: From Concept to Construction

Subdivision stormwater design balances developer economics, municipal requirements, environmental protection, and long-term maintenance. This guide covers the complete process from initial site assessment through construction.

Planning Phase

Site Assessment

Before any design work, thoroughly assess the existing conditions:

Hydrologic Assessment:

• Existing drainage patterns and watersheds
• Off-site contributing areas
• Downstream receiving waters and constraints
• Floodplain boundaries (if any)
• Groundwater levels

Physical Characteristics:

• Topography and slopes
• Soil types and infiltration rates
• Existing vegetation and wetlands
• Utilities and easements
• Property boundaries

Regulatory Framework:

• Local stormwater ordinances
• MS4 permit requirements
• FEMA/floodplain regulations
• State environmental permits
• Water quality requirements

Conceptual Design Approach

Key Questions:

1. Where does water come from and where must it go?
2. What peak flow and volume controls are required?
3. What water quality treatment is needed?
4. Where can detention/BMPs be located?
5. What are the downstream constraints?

Development Impact

Subdivision development dramatically changes site hydrology:

Drainage System Components

Minor Drainage System

Handles frequent storms (typically 2 to 10-year):

• Street gutters and curbs
• Storm sewer pipes
• Inlets and catch basins
• Swales and small channels

Design Objective: Convey frequent storms with minimal property flooding and traffic disruption.

Major Drainage System

Handles extreme storms (typically 100-year):

• Street overflow capacity
• Overland flow paths
• Large channels/creeks
• Emergency spillways

Design Objective: Safely convey extreme storms to prevent life-safety hazards and major property damage.

Lot-Level Drainage

Individual lot drainage to prevent foundation and yard problems:

• Finished floor elevation (minimum 1 ft above street)
• Positive lot grading (6” fall in 10 ft from foundation)
• Side yard swales between lots
• Proper driveway drainage

Regional Detention

Centralized stormwater management for the entire subdivision:

• Detention ponds
• Retention basins
• Underground storage
• Water quality facilities

Street Drainage Design

Street Cross-Sections

Typical Residential Street:

• Pavement width: 24-32 feet
• Crown height: 1.5-2.5%
• Curb height: 6 inches
• Gutter width: 1.5-2 feet
• Back slope: 2-4%

Gutter Flow Calculations

Using Manning’s equation for gutter flow:

Where:

• Q = Flow (cfs)
• n = Manning’s n (0.016 for concrete)
• Sx = Cross slope (ft/ft)
• S = Longitudinal slope (ft/ft)
• T = Spread (ft)

Spread Criteria

Limit water spread to maintain traffic safety:

Inlet Spacing

Inlets must capture flow before spread becomes excessive:

1. Calculate contributing area to first inlet
2. Determine flow using Rational Method
3. Calculate spread at that flow
4. If spread exceeds criteria, add inlet
5. Calculate inlet interception efficiency
6. Add bypass to next inlet downstream

Rule of Thumb: Start with inlets at 300-foot spacing, then adjust.

Storm Sewer Design

Pipe Network Layout

Design Principles:

• Follow street alignments
• Minimize length and turns
• Outlet to detention or receiving water
• Maintain adequate cover (2-3 ft minimum)
• Avoid conflicts with other utilities

Pipe Sizing

Use Manning’s equation for gravity pipe flow:

Design Criteria:

System Sizing Example

Subdivision Layout:

• 50 lots at 1/4 acre each
• Street total: 2,000 linear feet
• Total impervious: ~8 acres
• Design storm: 10-year

Step 1: Divide into subareas Segment the development into tributary areas for each inlet.

Step 2: Calculate peak flows Use Rational Method with appropriate Tc for each point.

Step 3: Size pipes Accumulate flows downstream, sizing each pipe segment.

Step 4: Check velocities Verify self-cleaning and non-erosive velocities.

Calculate pipe capacity →

Lot Grading Standards

Finished Floor Elevation

Minimum elevations prevent structure flooding:

Lot Grading Requirements

Front Yards:

• Slope toward street
• Minimum 2% slope
• Maximum 10% for mowing
• Swale along property line if needed

Rear Yards:

• Slope away from house
• 2% minimum within 10 ft of foundation
• Outlet to drainage easement or front
• Avoid directing to neighbor’s yard

Side Yards:

• Minimum 5 ft wide with slopes to front or rear
• Swale if needed for high side drainage
• Drainage easement if carrying off-site flows

Detention/Retention Facilities

Sizing Requirements

Most jurisdictions require controlling post-development peaks to pre-development rates:

For multiple storms (e.g., 2, 10, 100-year).

Some also require volume control:

Detention Pond Layout

Location Considerations:

• Downstream end of development
• Accessible for maintenance
• Adequate buffer from lots
• Visible for safety
• Compatible with open space use

Key Components:

• Forebay for sediment
• Main pool for storage
• Outlet structure (multi-stage)
• Emergency spillway
• Maintenance access

Outlet Structure Design

Multi-stage outlets control different storms:

Stage 1: Water Quality (small storms)

• Extended detention for first flush
• Orifice sized for 24-48 hour drain time

Stage 2: Channel Protection (1-2 year)

• Control bankfull discharge
• Extended detention 12-24 hours

Stage 3: Flood Control (10-100 year)

• Match pre-development peaks
• Larger orifices or weirs

Stage 4: Emergency Spillway (>100 year)

• Handles extreme events
• Non-erosive overflow path

Calculate detention requirements →

Wet vs. Dry Ponds

Wet Ponds (Permanent Pool):

• Better water quality treatment
• Aesthetic value
• Wildlife habitat
• Requires larger footprint
• Mosquito management needed

Dry Ponds (Extended Detention):

• Smaller footprint
• Dual use as open space
• Less maintenance
• Less water quality benefit
• May have nuisance growth

Compare detention vs retention →

Open Channel Design

Swales and Channels

For conveying flows through common areas:

Design Criteria:

Channel Protection

Prevent erosion at critical locations:

• Velocity transitions
• Pipe outlets
• Bends and confluences
• Drop structures
• Weirs and spillways

Protection Methods:

• Riprap
• Concrete aprons
• Turf reinforcement mats
• Drop structures
• Energy dissipators

Water Quality Requirements

Common Standards

BMP Selection

Structural BMPs:

• Wet ponds
• Constructed wetlands
• Bioretention/rain gardens
• Sand filters
• Infiltration practices

Non-Structural BMPs:

• Buffer preservation
• Minimized impervious area
• Disconnected impervious surfaces
• Native vegetation

Low Impact Development

LID approaches distribute stormwater management:

• Rain gardens at individual lots
• Permeable pavement
• Green roofs
• Cisterns
• Reduced lot sizes with common open space

Regulatory Coordination

Typical Permit Requirements

Submittal Requirements

Typical Plan Set:

• Site plan with drainage areas
• Grading plan with spot elevations
• Storm sewer plan and profile
• Detention pond details
• BMP details
• Erosion control plan
• Stormwater calculations

Typical Calculations:

• Pre/post hydrology
• Peak flow analysis
• Pipe capacity analysis
• Detention routing
• Floodplain analysis (if applicable)
• BMP sizing

Construction Phase

Erosion and Sediment Control

Critical during construction:

Before Grading:

• Perimeter controls (silt fence, inlet protection)
• Sediment basin for large sites
• Stabilized construction entrance

During Grading:

• Minimize exposed area
• Divert clean water around site
• Temporary seeding of idle areas
• Maintain perimeter controls

After Grading:

• Permanent stabilization
• Storm sewer connections
• BMP installation
• Final grading verification

Construction Sequence

Typical sequence for stormwater facilities:

1. Sediment basin - Before major grading
2. Storm sewers - During road construction
3. Rough grading - Pond, channels, lots
4. Fine grading - Final elevations
5. Permanent BMPs - After stabilization
6. Outlet structure - After pond grading
7. Final stabilization - Seeding, planting

Inspection and Testing

During Construction:

• Daily erosion control inspection (during active work)
• Compaction testing of embankments
• Pipe installation inspection
• As-built surveys

Prior to Acceptance:

• Flow testing of pipes
• Pond stage-storage verification
• Outlet structure testing
• Vegetation establishment

Maintenance Considerations

Design for Maintenance

Access:

• 12-foot access roads to ponds
• Maintenance easements
• Turnaround areas

Maintainability:

• Grass-lined where possible (mowing)
• Accessible outlet structures
• Cleanout access on pipes
• Sediment forebays for dredging

Long-Term Maintenance Responsibility

Summary

Successful subdivision stormwater design requires:

1. Thorough site assessment before design begins
2. Understanding regulations and coordinating with agencies
3. Minor AND major system design
4. Lot grading that protects individual homes
5. Detention/retention for peak and volume control
6. Water quality treatment as required
7. Erosion control during construction
8. Long-term maintenance planning

Related Calculators

• Rational Method Calculator →
• SCS Curve Number Calculator →
• Detention Pond Calculator →
• Manning’s Open Channel Calculator →

References

1. American Society of Civil Engineers. (2017). Design and construction of urban stormwater management systems (ASCE Manual of Practice No. 77). ASCE Press.

2. Federal Highway Administration. (2013). Urban drainage design manual (3rd ed., HEC-22). U.S. Department of Transportation.

3. Natural Resources Conservation Service. (2004). National Engineering Handbook, Part 630: Hydrology. USDA.

4. Environmental Protection Agency. (2004). Stormwater best management practice design guide. EPA.

5. Urban Drainage and Flood Control District. (2016). Urban storm drainage criteria manual. Denver, CO.

6. Prince George’s County, MD. (1999). Low-impact development design strategies. Department of Environmental Resources.