2.3 Site Hydrology, Drainage & Stormwater Management

Site Hydrology, Drainage & Stormwater Management

Land development alters natural site hydrology by replacing absorbent vegetated landscapes with impervious surfaces such as rooftops, asphalt roads, and concrete plazas. This transformation accelerates runoff velocity, reduces groundwater infiltration, amplifies peak discharge rates, and exacerbates downstream flooding and environmental contamination. Architects and civil engineers must integrate site drainage networks and stormwater Best Management Practices (BMPs) to manage runoff quality and quantity at the source.

---

Site Hydrology & The Rational Method

Impervious development generally reduces infiltration and accelerates runoff. The actual partition among runoff, infiltration and evapotranspiration depends on soil, rainfall, vegetation, slope and groundwater; generalized watershed percentages are not project design inputs.

The Rational Method Formula

For small watershed catchments (under 200 acres), civil engineers calculate peak stormwater runoff using the Rational Method:

$Q = C \times I \times A$

Where:

• $Q$ = Peak runoff discharge rate in cubic feet per second (cfs, $\text{ft}^3/\text{s}$).

• $C$ = Dimensionless runoff coefficient, representing the fraction of rainfall converted to direct surface runoff based on surface permeability, slope, and soil type.

• $I$ = Rainfall intensity in inches per hour (in/hr), determined from regional Intensity-Duration-Frequency (IDF) curves for a specified design storm recurrence frequency (such as a 10-year storm) for a storm duration equal to the catchment's Time of Concentration ($t_c$).

• $A$ = Drainage basin catchment area in acres (ac).

(Note: 1.0 acre-inch per hour equals 1.00833 cubic feet per second, making the conversion factor in the Rational Method effectively 1.0).

Runoff Coefficients ($C$) for Common Surfaces

Composite Runoff Coefficient ($C_{\text{comp}}$)

When a site comprises multiple surface covers, a weighted composite runoff coefficient is calculated:

$C_{\text{comp}} = \frac{\sum (C_i \times A_i)}{\sum A_i} = \frac{C_1 A_1 + C_2 A_2 + \dots + C_n A_n}{A_{\text{total}}}$

Time of Concentration ($t_c$) & Pre- vs. Post-Development Discharge

The Time of Concentration ($t_c$) is the time required for runoff to travel from the hydraulically most remote point of the watershed to the design discharge outlet. Land development shortens $t_c$ by replacing overland sheet flow through tall grass with fast conduit flow in concrete gutters and pipes. Shorter $t_c$ corresponds to higher rainfall intensity ($I$) on IDF curves, which drastically inflates peak flow ($Q$).

For the worked example, assume the local ordinance requires $Q_{post}\le Q_{pre}$ for the specified design storm. Other jurisdictions may require additional volume retention, water-quality treatment, different design storms or downstream analysis. Peak-flow reduction alone does not prove compliance with those requirements.

Worked Calculation: Pre- vs. Post-Development Peak Runoff

A 5.0-acre parcel is being developed into an office building. The 10-year storm event is analyzed:

• Pre-Development: 5.0 acres of natural meadow ($C = 0.15$). Site $t_c$ is 30 minutes, corresponding to a 10-year rainfall intensity $I = 2.4\text{ in/hr}$. $Q_{\text{pre}} = 0.15 \times 2.4\text{ in/hr} \times 5.0\text{ ac} = 1.80\text{ cfs}$

• Post-Development: The developed parcel comprises:

  • 1.5 acres building roof ($C_1 = 0.90$)
  • 2.0 acres asphalt parking and drives ($C_2 = 0.85$)
  • 1.5 acres landscaped turf lawn on clay ($C_3 = 0.20$)
  • Site $t_c$ decreases to 15 minutes, increasing 10-year rainfall intensity to $I = 3.6\text{ in/hr}$.

1. Calculate composite runoff coefficient ($C_{\text{comp}}$): $C_{\text{comp}} = \frac{(1.5 \times 0.90) + (2.0 \times 0.85) + (1.5 \times 0.20)}{5.0} = \frac{1.35 + 1.70 + 0.30}{5.0} = \frac{3.35}{5.0} = 0.67$

2. Calculate post-development peak runoff ($Q_{\text{post}}$): $Q_{\text{post}} = 0.67 \times 3.6\text{ in/hr} \times 5.0\text{ ac} = 12.06\text{ cfs}$

3. Required Peak Flow Attenuation: The post-development peak flow is nearly seven times the pre-development rate ($12.06\text{ cfs}$ vs. $1.80\text{ cfs}$). The on-site stormwater management system must store the surge volume and restrict peak outflow to $\le 1.80\text{ cfs}$.

---

Surface & Subsurface Drainage Conveyance

Surface Conveyance Systems

• Vegetated Swales: Open, shallow linear drainage channels with gently sloping side banks (maximum 3:1) and a parabolic cross-section. Designed with a longitudinal slope of 1.0% to 4.0%. Slopes under 1.0% cause ponding and mosquito breeding; slopes exceeding 4.0% generate velocities ($>4-5\text{ ft/s}$) that cause erosive scouring, requiring check dams or rock rip-rap.

• Sheet Flow: Dispersed overland runoff across broad planar surfaces before it concentrates into rills or channels. Sheet flow typically transitions into concentrated flow within 100 to 300 feet.

• Catch Basins & Curb Inlets: Paved areas shed runoff toward curb openings and grated catch basins. Grates placed within accessible pedestrian paths must comply with ADA Section 302.3: openings must not exceed 0.5 inches (13 mm) in the dominant direction of travel, with elongated slots oriented perpendicular to the primary walking path to prevent wheelchair caster entrapment. Catch basins feature a below-invert sump (2 to 3 feet deep) to capture settling road grit and heavy sediment.

• Culverts: Enclosed conduits (reinforced concrete pipe RCP, corrugated high-density polyethylene HDPE) conveying stormwater beneath roadways, driveways, and pedestrian paths. Sized using Manning's equation for open-channel gravity flow.

Subsurface Drainage & Foundation Protection

• Perimeter Foundation Drain (Footing Drain): A continuous 4-to-6-inch perforated PVC or corrugated HDPE pipe positioned along the exterior perimeter of building footings. Invert elevations must be maintained at or below the top of the footing. The pipe is bedded in washed crushed stone (ASTM No. 57 aggregate) wrapped in non-woven geotextile filter fabric to intercept perched groundwater and reduce lateral hydrostatic head against basement walls.

• French Drains: Gravel-filled sub-surface trenches incorporating a perforated pipe at the base, used across site lawns or behind retaining structures to intercept shallow groundwater tables and convey subgrade seepage away from athletic fields and paved areas.

---

Stormwater Best Management Practices (BMPs) & Low Impact Development

Modern stormwater management balances peak rate control (flood prevention) with volume reduction (groundwater recharge) and water quality treatment (pollutant removal).

Detention Basins vs. Retention Ponds

• Detention Basins ("Dry Ponds"): Impoundments designed to temporarily hold stormwater during a storm and release it gradually through a calibrated multi-stage outlet structure (orifice plates, v-notch weirs, and overflow risers). The design drawdown period is established by the applicable criteria; 24–48 hours is an illustrative range, not a universal rule. Primary function: attenuating post-development peak discharge rates ($Q_{\text{post}} \le Q_{\text{pre}}$). They provide minimal total volume reduction and limited water quality polishing.

• Wet retention ponds maintain a permanent pool and temporary storage above it. Settling and biological processes improve water quality, and a controlled outlet can reduce peaks. A permanent pool alone does not prove runoff-volume reduction; that requires a water balance including infiltration, evaporation and reuse.

Low Impact Development (LID) & Green Infrastructure

LID prioritizes decentralized, small-scale micro-catchments that mimic natural hydrologic function directly at the source:

1. Bioretention uses engineered soil, vegetation and surface storage to filter runoff, with infiltration and/or an underdrain according to site conditions. Specify media composition and ponding depth under local guidance and tested performance. Check separation from groundwater and foundations, overflow routing, pretreatment and maintenance; a single sand/soil recipe is not valid everywhere.

2. Bioswales: Vegetated swales enhanced with bioretention filter media, mulch, and small rock check dams that slow runoff velocity, promote infiltration, and filter suspended particulates along linear parking lot medians and roadway right-of-ways.

3. Permeable Pavements: Porous asphalt, pervious concrete, or Permeable Interlocking Concrete Pavers (PICP) installed over an open-graded aggregate reservoir (ASTM No. 57 and No. 2 stone) without sand fines. Stormwater rapidly drains through the paver surface and is stored in the 30% to 40% void space of the crushed stone base, either infiltrating into native subgrade or draining slowly through an underdrain. Requires annual vacuum sweeping to prevent silt and organic debris from clogging void spaces.

4. Green roofs store some rainfall and return water through evapotranspiration. Performance depends on substrate depth, antecedent moisture, vegetation and the storm. A saturated roof may produce substantial runoff. Coordinate saturated structural load, waterproofing, root resistance, drains, overflow, irrigation and maintenance access. Deeper planted systems generally weigh more and support a broader plant palette.

5. Rainwater Harvesting Cisterns: Underground precast concrete tanks or indoor mechanical room cisterns that collect rooftop runoff. Stored non-potable water is filtered and used for building toilet flushing, cooling tower evaporative makeup, and exterior landscape irrigation, reducing municipal potable water demand.

---

Construction Erosion & Sediment Control (ESC)

Construction stormwater permitting generally covers disturbance of one acre or more and smaller sites within a common plan of development meeting that threshold. The applicable EPA or state permit establishes authorization, SWPPP, inspection, stabilization and discharge-control requirements. Check the actual permit and local erosion-control rules.

Key temporary construction BMPs include:

• Silt Fences: Geotextile filter fabric fastened to wooden stakes or steel posts, installed along contour lines at the toe of cleared slopes to filter sheet runoff. The bottom 6 inches of fabric must be trenched and backfilled with compacted earth. Silt fences are engineered exclusively for sheet flow and fail catastrophically if placed across swales or concentrated channels.

• Construction controls must be sized and maintained for the site and applicable permit: stabilize entrances, capture sediment before discharge, divert clean runoff, and stabilize exposed soil on the required timetable. Entrance length, basin volume and inactivity deadlines vary; do not treat 50 feet, 3,600 cubic feet per acre or 14 days as universal requirements.