14.4 Stormwater Storage: Detention, Retention & Water Quality Basins

Core Focus: Stormwater storage facilities mitigate the adverse hydrological impacts of urbanization by attenuating peak flood discharges, reducing total runoff volumes, and removing waterborne pollutants. Landscape architects must master the functional typologies of dry detention, wet retention, and constructed wetlands, design multi-stage hydraulic risers, size sediment forebays, and engineer stable earthen embankments with proper freeboard, internal seepage cutoff, and safety benches.

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1. Functional Typologies of Stormwater Basins

Stormwater basins are categorized based on whether they maintain a permanent pool of water and their primary hydrologic and water quality mechanisms.

+-------------------------------------------------------------------------+
|                    STORMWATER BASIN TYPOLOGIES                          |
+-------------------------------------------------------------------------+
| 1. DRY DETENTION BASIN / EXTENDED DETENTION BASIN (EDB)                 |
|    - Drains completely between storms (dry bottom)                      |
|    - Primary Goal: Peak flood discharge rate attenuation                |
|    - EDB adds small low-flow orifice for 24-48 hr WQv extended drawdown |
+-------------------------------------------------------------------------+
| 2. WET RETENTION POND (WET POND)                                        |
|    - Maintains permanent pool (depth 4.0' to 8.0')                      |
|    - Primary Goal: Particulate settling + biological nutrient uptake    |
|    - Displaces existing water; excellent TSS and phosphorus removal     |
+-------------------------------------------------------------------------+
| 3. CONSTRUCTED STORMWATER WETLAND                                       |
|    - Shallow marsh system (depth 6" to 18" over 70% of area)            |
|    - Primary Goal: Maximum biological uptake, denitrification, filtration|
|    - High plant diversity; superior aesthetic and ecological value      |
+-------------------------------------------------------------------------+

Comprehensive Comparison Table

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2. Hydraulic Outlet Control Structures & Multi-Stage Risers

A stormwater basin regulates outflow through a multi-stage outlet control structure (typically a cast-in-place reinforced concrete riser box or large CMP structure). The riser integrates multiple hydraulic openings operating at different water surface elevations (stages) to control distinct storm recurrence intervals.

+-------------------------------------------------------------------------+
|              MULTI-STAGE OUTLET CONTROL RISER SCHEMATIC                 |
+-------------------------------------------------------------------------+
|                                 Top Grate (100-Year Peak / Anti-Vortex) |
|                     =================================                   |
|                     |                               |                   |
|                     |                               |                   |
|  Stage 3: 10-Year   |   Intermediate Weir Notch     |                   |
|  Water Surface ~~~> |   ========================    |                   |
|                     |   |                      |    |                   |
|  Stage 2: 2-Year    |   +----------------------+    |                   |
|  Water Surface ~~~> |   Intermediate Orifice        |                   |
|                     |   +---+                       |                   |
|  Stage 1: WQv       |   |   |                       |                   |
|  Water Surface ~~~> |   +---+                       |                   |
|                     |   Low-Flow WQv Orifice        |  Outlet Barrel    |
|                     |   (Perforated pipe / plate)   |  (PFS Pipe)       |
|  Basin Floor =====> +===============================+===============>   |
|                     |                               |                   |
+-------------------------------------------------------------------------+

Multi-Stage Hydraulic Components

1. Stage 1: Low-Flow Water Quality Orifice (WQv Drawdown):
   • Located at the basin invert in dry basins or at the permanent pool elevation in wet ponds.
   • Sized specifically to detain and slowly release the Water Quality Volume (WQv) (typically the first $0.5\text{ to }1.0\text{ inch}$ of runoff over the contributing impervious watershed) over an extended drawdown period of 24 to 48 hours.
   • Prolonged detention allows fine clay silts, heavy metals, and particulate nutrients to settle out through gravitational sedimentation.
   • Governed by the Orifice Equation: $Q_o = C_d \cdot A_o \sqrt{2 g H}$ Where $Q_o$ is orifice discharge (cfs), $C_d$ is the discharge coefficient (typically $0.60$ for a sharp-edged circular orifice), $A_o$ is cross-sectional area of the orifice opening ($\text{ft}^2$), $g$ is gravity ($32.2 \text{ ft/s}^2$), and $H$ is the total hydraulic head measured from the center of the orifice to the water surface elevation (ft).
2. Stage 2: Intermediate Weir Notch or Orifice Plate (2-Year and 10-Year Storms):
   • Located at an intermediate elevation above the water quality elevation.
   • Sized to attenuate the peak rate of discharge for frequent storm events (the 2-year and 10-year, 24-hour storms) so that post-development peak runoff rates do not exceed pre-development peak runoff rates.
   • Operates under the Weir Equation (when flow is unsubmerged): $Q_w = C_w \cdot L \cdot H^{3/2}$ Where $C_w$ is the weir discharge coefficient (typically $3.0\text{ to }3.3$ for sharp-crested or broad-crested concrete weirs), $L$ is effective crest length (feet), and $H$ is hydraulic head above the weir crest (feet).
3. Stage 3: Top Riser Opening or Grate (25-Year and 100-Year Control):
   • A horizontal metal grate or open concrete top on the riser box.
   • Controls major storm events, passing large volumes into the barrel culvert once the stage overtops the intermediate notches.
   • Equipped with an anti-vortex plate and a debris trash rack to prevent floating woody debris from blocking the structure.
4. Emergency Overflow Spillway:
   • A broad-crested earthen or riprap-lined weir channel cut into undisturbed native cut ground (never built across the fill portion of the embankment dam).
   • Designed to safely pass the peak discharge of the 100-year storm (or Probable Maximum Flood / PMF for state-regulated dams) in the event that the primary riser structure is $100%$ completely clogged with debris.
   • Protects the earthen embankment from being overtopped, which would cause immediate catastrophic structural breach.

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3. Stage-Storage-Discharge Relationships & Reservoir Routing

Basin routing is the mathematical procedure used to calculate the outflow hydrograph given an inflow hydrograph and the physical storage characteristics of the basin.

Stage-Storage and Stage-Discharge Curves

• Stage-Storage Curve: A plot of water surface elevation (stage, in feet) versus cumulative basin storage volume (in acre-feet or cubic feet). Volumes are calculated using the Contour Area or Average End Area methods.
• Stage-Discharge Rating Curve: A combined hydraulic plot showing stage versus total outflow discharge ($Q$), summing the concurrent discharges of all active outlet openings (low-flow orifice + intermediate weir + top grate + emergency spillway) at each successive elevation.
• Storage Routing Concept (Modified Puls Method): Based on the fundamental continuity equation of fluid dynamics over a time increment $\Delta t$: $\frac{I_1 + I_2}{2} - \frac{O_1 + O_2}{2} = \frac{S_2 - S_1}{\Delta t}$ Where $I$ is inflow, $O$ is outflow, and $S$ is storage volume. The routing hydrograph demonstrates how the basin attenuates (flattens and delays) the peak post-development runoff wave.

+-------------------------------------------------------------------------+
|                    STORMWATER HYDROGRAPH ATTENUATION                    |
+-------------------------------------------------------------------------+
|  Discharge (cfs)                                                        |
|       |             Pre-Development Hydrograph                          |
|       |             - - - - - - -                                       |
|       |            /             \                                      |
|       |           /               \                                     |
|       |   Post-Development Inflow  \                                    |
|       |   ======================    \                                   |
|       |  /          |           \    \                                  |
|       | /       Peak Attenuation \    \                                 |
|       |/            v             \    \                                |
|       |             +              \    \                               |
|       |            / \              \    \                              |
|       |           /   \   Routed Basin Outflow                          |
|       |          /     \  --------------------                          |
|       |         /       \                     \                         |
|       +--------+---------+---------------------+-----------------> Time |
|                         <=== Peak Delay ====>                           |
+-------------------------------------------------------------------------+

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4. Forebays: Design, Sizing & Maintenance

A sediment forebay is a distinct, compartmentalized pre-treatment pool situated at the primary inflow points of a detention basin, retention pond, or wetland.

Engineering Objectives

1. Kinetic Energy Dissipation: Reduces the high velocity of stormwater jetting from incoming pipe culverts or riprap channels.
2. Coarse Sediment Trapping: Traps $50%\text{ to }70%$ of total suspended solids (gravel, coarse-to-medium sand, and asphalt grit) in a small, accessible cell before runoff enters the main basin body.
3. Preserving Main Basin Volume: Prevents sediment plumes from filling the permanent pool or choking emergent wetland vegetation, dramatically extending the design lifespan of the primary facility.

Sizing and Physical Detailing Standards

• Volumetric Sizing: Standard civil engineering criteria mandate that the forebay must hold $10%\text{ to }15%$ of the total design basin storage volume, or provide a dedicated storage volume equivalent to $0.10\text{ to }0.20\text{ inches}$ per contributing impervious watershed acre.
• Divider Wall: Separated from the main basin by an internal gabion wall, riprap berm, or reinforced concrete weir. The crest of the divider wall is set at the permanent pool elevation (or $0.5\text{ to }1.0\text{ foot}$ below normal pool) to allow clean water to sheet gently into the main pool without re-suspending settled sludge.
• Hardened Bottom & Depth Gauge: The forebay floor must be armored with a hardened concrete slab or grouted riprap to provide a stable, solid footing for maintenance equipment (skid-steers and backhoes) during dredging. A vertical cast-iron or fiberglass cleanout marker post graduated in feet must be permanently anchored in the center of the cell to indicate when sediment has reached $50%$ capacity, triggering mandatory cleanout.
• Heavy Maintenance Access: Must include a heavy-duty crushed stone or concrete access ramp measuring at least 12 feet wide with a maximum slope of $10%\text{ to }15%$ ($6.7:1\text{ to }10:1$) capable of supporting 20-ton tandem dump trucks and excavators.

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5. Embankment Design, Seepage Control & Dam Safety

When a stormwater basin is created by constructing an earthen dam across a draw or fill perimeter, the structural integrity of the embankment is governed by state and federal Dam Safety Regulations.

+-------------------------------------------------------------------------+
|                    EARTHEN EMBANKMENT DAM CROSS-SECTION                 |
+-------------------------------------------------------------------------+
|                                Embankment Crest (min 10' to 14' wide)   |
|                                ==============================           |
|   100-Year Peak Pool Stage     |                            |           |
|   ~~~~~~~~~~~~~~~~~~~~~~~      |  Freeboard (min 1.0'-2.0') |           |
|                         \      +----------------------------+           |
|   Interior Slope         \     |                            |  Exterior |
|   (3:1 to 4:1)            \    |   Impervious Clay Core     |   Slope   |
|                            \   |   Trench (Keyway)          |   (3:1)   |
|   Safety Bench              \  |   ====================     |  /        |
|   (10' wide, <= 10:1)        \ |   |                  |     | /         |
|   ==================+         \|   |                  |     |/          |
|   Normal Pool        \         |   |                  |     |           |
|   ~~~~~~~~~~~~        \        |   |                  |     |           |
|   Aquatic Bench        \       |   |                  |     |           |
|   (10' wide, 1'-2' d)   \      |   |                  |     |           |
|   ================+      \     |   |                  |     |           |
|                   |       \    |   |                  |     |           |
|   Basin Floor ====+        \   |   |                  |     |           |
|                             \  |   |                  |     |           |
|                              \ |   |                  |     |           |
|   Pervious Native Subsoil     \|   |                  |     |           |
|   -----------------------------+---+                  +-----+           |
|   Impermeable Bedrock / Clay Layer ==================================   |
+-------------------------------------------------------------------------+

Anatomy of an Engineered Embankment

1. Core Trench (Keyway Cutoff Trench):
   • A vertical trench excavated along the centerline axis of the dam foundation, penetrating through permeable surficial sands, silts, or gravels, and keyed into a solid, impermeable clay or bedrock layer.
   • Backfilled with highly compacted, impermeable cohesive clay soil (Unified Soil Classification: CL or CH) compacted to $\ge 95%$ Modified Proctor Density.
   • Standard dimensions: Minimum bottom width of 4.0 to 8.0 feet with $1:1$ side slopes, extending at least 2.0 to 3.0 feet deep into the impermeable stratum.
   • Purpose: Blocks horizontal subsurface water seepage underneath the dam foundation, preventing foundation blowout.
2. Freeboard Requirements:
   • Freeboard is the vertical clearance between the maximum routed water surface elevation of the 100-year design storm and the lowest point on the settled top crest of the earthen embankment.
   • Standard engineering criteria mandate a minimum freeboard of 1.0 foot (many municipal and state dam safety criteria mandate 1.5 to 2.0 feet) to protect the dam against wave slosh, wind setup, and post-construction crest settlement.
3. Internal Piping Prevention Along the Outlet Barrel:
   • Water stored behind a dam naturally seeks paths of least resistance. Seepage water tends to travel along the smooth outside surface of the structural concrete outlet pipe barrel penetrating the embankment.
   • Piping Failure: High-velocity seepage erodes fine soil particles along the conduit, creating an internal hollow cavity ("pipe") that progressively widens until the dam collapses catastrophically within minutes.
   • Historic Mitigation: Anti-Seep Collars (large vertical concrete or steel cutoff plates projecting $2.0\text{ to }3.0\text{ feet}$ perpendicularly from the pipe barrel to increase the seepage path length by at least $15%$).
   • Modern NRCS Standard: Filter Diaphragms (Sand Filter Collars). The USDA NRCS and US Army Corps of Engineers now strongly recommend or mandate filter diaphragms over anti-seep collars. A filter diaphragm is a zone of engineered, washed silica sand and fine gravel surrounding the conduit. Instead of attempting to block water, it safely collects seepage, relieves internal hydrostatic pore pressure, and physically blocks soil particles from migrating, preventing piping.
4. Embankment Slopes and Crest Width:
   • Interior (Upstream) Slopes: Sized at $3:1\text{ to }4:1$ to maintain structural slope stability under sudden water level drawdown.
   • Exterior (Downstream) Slopes: Sized at $3:1$ (maximum allowable is $2:1$, but $3:1$ is required for stable maintenance access and turf mowing).
   • Crest Width: Minimum top width of $10\text{ to }14\text{ feet}$ to accommodate maintenance vehicles.

Safety Benches in Wet Ponds

To protect human life from drowning and satisfy municipal site design regulations, wet retention ponds with permanent pools deeper than $3.0\text{ to }4.0\text{ feet}$ must incorporate dual perimeter safety benches:

• Safety Bench (Upper / Exterior Bench): A horizontal terrace measuring at least 10 feet wide, located just above the normal pool elevation (or starting at the permanent pool waterline and extending landward). Sloped gently at $10:1$ (10%) or flatter to allow a person who slips down the embankment to regain their balance before tumbling into deep water.
• Aquatic Bench (Lower / Interior Bench): A horizontal shallow submerged terrace measuring at least 10 feet wide, located immediately inside the pool perimeter extending outward from the waterline at a constant water depth of 12 to 18 inches (1.0 to 1.5 feet). Planted with dense emergent wetland vegetation (e.g., Typha latifolia, Iris versicolor, Scirpus acutus) that serves as an impassable physical barrier to children, provides biological nutrient uptake, and allows anyone who falls into the pond to easily stand up.

Dam Hazard Classifications

State Dam Safety Offices classify earthen dams into three hazard categories based on potential downstream devastation in the event of an embankment breach:

• Low Hazard: Failure causes no probable loss of human life and only minor economic or structural damage confined to agricultural land or local roads.
• Significant Hazard: Failure causes no probable loss of human life, but causes severe economic loss, environmental damage, disruption of lifeline utilities, or damage to main highways.
• High Hazard: Failure causes probable loss of one or more human lives, flooding of residential homes, commercial buildings, or major transportation corridors. High hazard dams must be engineered to pass the Probable Maximum Flood (PMF) without failure.

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6. Real-World Case Scenario: Multi-Stage Riser & Forebay Design for a Suburban Mixed-Use Center

Scenario: A landscape architecture firm is designing an Extended Detention Basin (EDB) to serve a 20-acre mixed-use lifestyle center with an overall impervious cover of $60%$. The civil engineering stormwater report establishes the following design parameters:

• Contributing impervious area: $A_{imp} = 20\text{ acres} \times 0.60 = 12.0\text{ acres}$.
• Water Quality Volume (WQv) requirement: First $1.0\text{ inch}$ of runoff over the contributing impervious surface.
• Permanent forebay volume requirement: $15%$ of total basin storage.
• Pre-development 10-year peak discharge: $Q_{10_pre} = 22.0\text{ cfs}$.
• Unrouted post-development 10-year peak discharge: $Q_{10_post} = 54.0\text{ cfs}$.

Engineering Solutions:

1. Calculate Water Quality Volume (WQv): $WQv = 12.0\text{ acres} \times \left( \frac{1.0\text{ in}}{12\text{ in/ft}} \right) = 1.0\text{ acre-foot} = 43,560\text{ cubic feet}$
2. Size the Sediment Forebay: $V_{forebay} = 0.15 \times WQv = 0.15 \times 43,560\text{ cu ft} = \mathbf{6,534\text{ cubic feet}}$
   • At an average depth of $4.0\text{ feet}$, the required surface area of the forebay is: $6,534 / 4.0 = 1,634\text{ sq ft}$ (e.g., a $35' \times 47'$ cell).
   • Specify a 12-foot-wide concrete access ramp sloped at $10%$ and a concrete equipment floor slab.
3. Size the Low-Flow Drawdown Orifice (40-Hour Drawdown Target):
   • Average target outflow: $Q_{avg} = 43,560\text{ cu ft} / (40\text{ hours} \times 3,600\text{ s/hr}) = 43,560 / 144,000 = 0.3025\text{ cfs}$.
   • Using the orifice equation at mean water head ($H_{avg} = 2.0\text{ ft}$): $Q = C_d A_o \sqrt{2 g H} \implies A_o = \frac{Q}{C_d \sqrt{2 g H}} = \frac{0.3025}{0.60 \times \sqrt{64.4 \times 2.0}} = \frac{0.3025}{0.60 \times 11.35} = \frac{0.3025}{6.81} = 0.0444\text{ ft}^2$
   • Solving for circular orifice diameter ($A = \pi D^2 / 4$): $D = \sqrt{\frac{4 \times 0.0444}{\pi}} = \sqrt{0.0565} = 0.238\text{ ft} = \mathbf{2.85\text{ inches}}$
   • The landscape architect specifies a 3-inch circular steel orifice plate protected by a gravel-jacketed perforated riser screen to prevent trash clogging.
4. Size the Intermediate Weir for the 10-Year Storm:
   • At Stage $4.0\text{ feet}$, a rectangular concrete sharp-crested weir notch ($C_w = 3.2$) with a crest length of $L = 4.5\text{ feet}$ restricts outflow to $\le 22.0\text{ cfs}$, ensuring post-development discharge matches pre-development rates.
5. Establish Embankment Elevations and Freeboard:
   • Invert of basin = 100.00'.
   • WQv water surface = 102.50'.
   • Routed 100-year water surface elevation = 106.20'.
   • Minimum freeboard = 1.00 foot.
   • Top of settled embankment dam crest = 106.20' + 1.00' = 107.20'.
   • Emergency spillway crest cut into native rock at elevation 106.20'.

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7. Exam Traps & Pitfalls

1. Detention vs. Retention Confusion: Confusing dry detention (temporary capture with zero permanent water; dry bottom between storms) with wet retention (maintains a permanent, standing pool of water at all times). Detention ponds attenuate peak flood rates; retention ponds provide permanent settling pools with superior nutrient removal.
2. Freeboard Measurement Benchmark: Measuring freeboard from the normal pool or permanent pool level rather than the peak routed 100-year water surface elevation. Earthen dams fail because unrouted 100-year stages overtop the crest, causing catastrophic erosion.
3. Anti-Seep Collars vs. Modern Filter Diaphragms: Assuming anti-seep collars are the only acceptable seepage defense. Modern USDA NRCS criteria strongly discourage anti-seep collars (which often crack concrete barrels or experience poor compaction under the haunches) in favor of sand filter diaphragms.
4. Omitting the Sediment Forebay Hardened Floor: Designing a sediment forebay without a concrete bottom slab or depth marker. When an excavator dredges an unarmored earth forebay, the operator inevitably digs into native subgrade, breaching the seal and destroying the cell.
5. Emergency Spillways on Fill Embankments: Placing the emergency spillway across the engineered fill embankment. Emergency spillways must always be excavated into virgin, undisturbed native ground on the dam abutment to prevent breach during emergency overflow.