13.5 Retaining Walls, Outlet Protection, and Wet Ponds

3. Retaining Walls (12.5.7)

Retaining Walls (12.5.7) are structural earth-retention systems constructed to hold back steep or vertical soil masses where narrow right-of-way, property lines, utilities, or environmental buffers prevent grading a stable natural slope.

Common Retaining Wall Categories

• Gravity Walls: Rely on their own massive self-weight to resist lateral soil thrust (includes unreinforced concrete walls, boulder walls, and gabion walls).
• Cantilever Walls: Cast-in-place reinforced concrete walls with an L-shaped or inverted T-shaped stem and base slab (heel and toe), where the weight of the soil sitting on the heel slab helps stabilize the wall against overturning.
• Crib Walls: Interlocking open rectangular boxes of precast reinforced concrete or pressure-treated timber stretchers and headers filled with free-draining crushed stone.
• Mechanically Stabilized Earth (MSE) / Segmental Modular Block Walls: Dry-stacked precast concrete facing units locked into horizontal layers of high-tensile polymeric geogrid reinforcement extending deep into the compacted structural backfill.
• Sheet-Pile and Soldier-Pile Lagging Walls: Driven steel sheet piling or H-piles placed in drilled shafts with horizontal timber or precast concrete lagging boards (often braced with tieback anchors for deep excavations).

#1 Cause of Retaining Wall Failure — Subsurface Hydrostatic Water Pressure: Retaining walls are engineered to resist the lateral earth pressure of drained soil—NOT a standing column of trapped groundwater! Water weighs $62.4\text{ lb/ft}^3$ and exerts full horizontal hydrostatic pressure against the back of a wall, which can double or triple the overturning and sliding thrust while simultaneously saturating and weakening the foundation bearing soil at the wall toe.

Consequently, every CESSWI inspecting a retaining wall (12.5.7) must verify four mandatory drainage and stability features:

1. Geotextile-Wrapped Crushed Stone Drainage Chimney: A continuous vertical blanket ($12\text{ to }24\text{ inches}$ wide) of clean, free-draining washed crushed stone/gravel placed directly behind the wall, completely wrapped in non-woven geotextile filter fabric so native silt and clay cannot migrate into and plug the gravel voids;
2. Perforated Foundation Footing Drain Pipe (Underdrain): Installed at the base (heel) of the gravel drainage chimney on a positive longitudinal grade and connected to a free-draining storm sewer or daylighted outlet;
3. Open, Unobstructed Weep Holes: Spaced regularly through the lower face of concrete, masonry, or sheet-pile walls to relieve pore-water pressure—inspect to ensure weep holes are not plugged by mortar, soil, or mineral encrustation;
4. Top-of-Wall Surface Diversion Swale: A paved or vegetated diversion swale (over a $12\text{-inch}$ low-permeability clay cap at the top of the backfill) constructed behind the top crest of the wall to intercept overland surface runoff so it cannot infiltrate into the structural backfill or cascade over the wall face.

Field Failure Indicators: Immediately document and escalate to a PE if a retaining wall exhibits forward tilting / rotation, horizontal sliding at the toe, cracked or bulging face blocks, plugged weep holes with water weeping through structural cracks, or tension cracks in the soil terrace directly behind the top of the wall.

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4. Outlet Velocity Protection (12.5.5)

When stormwater travels through a smooth storm drain pipe, box culvert, pipe slope drain, or paved chute, reduced friction allows the flow to accelerate to high, supercritical velocities ($V = \frac{Q}{A}$). If that concentrated high-velocity jet discharges directly onto a natural streambed or vegetated swale, it immediately scours a deep plunge pool that undermines the pipe end section and triggers upstream headcutting. Outlet Velocity Protection (12.5.5) structures dissipate this kinetic energy and expand the concentrated jet into wide, shallow, tranquil flow before it enters the receiving channel.

Common Outlet Velocity Protection Devices

1. Flared Riprap Outlet Apron: A flat, trapezoidal blanket of well-graded angular rip-rap placed over non-woven geotextile immediately downstream of a pipe end section or headwall, flaring outward (typically expanding from $1\times$ pipe diameter $D_o$ at the outlet to $3\times D_o$ at the downstream end) to allow the water jet to spread laterally and lose velocity over the rough stone surface.
2. Preformed Riprap Scour Hole (Stilling Basin): An intentionally excavated depression ($0.5\times$ to $1.0\times$ pipe diameter deep) lined with heavy rip-rap over geotextile that holds a shallow pool of water to cushion the impact of high-velocity jets where tailwater is low.
3. Baffled Concrete Impact Basins: Engineered reinforced-concrete energy dissipators—such as the USBR Type VI Impact Basin (which hangs a vertical concrete baffle wall directly in front of the pipe jet) or the Saint Anthony Falls (SAF) Stilling Basin (which uses chute blocks, floor baffle piers, and an end sill to force a controlled hydraulic jump)—used where exit velocities exceed the capacity of loose rip-rap ($> 15\text{ to }20\text{ ft/s}$).

Three Inspection Principles for Riprap Outlet Aprons

• Rule 1 — Flat ($0.0\%$ Longitudinal Grade) Flush with the Receiving Streambed: The rip-rap outlet apron floor must be constructed on a completely level ($0.0\%$ longitudinal slope) profile with its top rock surface flush with the invert elevation of the receiving channel bed. Crucial Exam Trap: If a rip-rap apron is built on a steep downward slope or terminates above the receiving streambed as a drop-off ("perched outlet"), water accelerates off the end of the apron like a waterfall, scours a deep plunge pool at the toe, rolls the end stones into the hole, and progressively unravels the entire apron and pipe headwall from the bottom up!
• Rule 2 — Non-Woven Geotextile Underlayment & Toe Cutoff Trench: The entire apron must rest on non-woven geotextile filter fabric (or a granular filter blanket) keyed into the pipe headwall cutoff and a downstream toe trench.
• Rule 3 — Straight Downstream Channel Reach: The outlet apron must be oriented to discharge into a straight downstream channel reach—never aimed straight into an opposite erodible streambank or installed at a sharp channel bend.

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5. Wet Ponds (12.5.10)

Wet Ponds (12.5.10) (also known as wet retention basins or stormwater management ponds) are permanent or multi-purpose stormwater impoundments engineered with two distinct vertical storage zones:

1. Permanent Pool ("Dead Storage"): A continuous, year-round pool of standing water maintained below the lowest outlet invert. During a storm, incoming runoff pushes out the clarified water that has been sitting and settling in the permanent pool since the previous storm (plug-flow displacement), while dissolved nutrients (nitrogen and phosphorus) and metals are removed through biological uptake by aquatic plants, algae, and microorganisms.
2. Live Detention Storage: Temporary flood storage volume between the normal pool elevation and the emergency spillway crest, metered out through a multi-stage principal spillway structure to attenuate post-construction peak discharge rates ($Q_p$) and prevent downstream channel erosion.

Five Wet-Pond Inspection Checkpoints for Wet Ponds

1. Upstream Sediment Forebay ($10\%\text{ to }20\%$ of Pool Volume): A dedicated pre-treatment settling cell at the pond inflows separated from the main pool by a submerged rock berm, gabion weir, or earthen dike. The forebay traps coarse sand and gravel in a compact, accessible basin (with a hardened equipment access ramp and fixed depth staff gauge) that can be dredged at $50\%$ capacity without draining the entire wet pond.
2. Vegetated Aquatic Bench (Safety Shelf): A shallow ($6\text{ to }18\text{ inches}$ deep, $10\text{ to }15\text{ feet}$ wide), gently graded bench around the inside perimeter of the permanent pool planted with emergent wetland vegetation. It serves a dual purpose: protecting the shoreline from wave erosion and providing a life-saving shallow shelf if a person or child slips down the embankment.
3. Multi-Stage Principal Spillway Outlet: Inspect the concrete riser or weir box for a functioning reverse-slope pipe or hooded low-flow orifice (which draws water from below the surface scum layer so floating trash, oil, and grease cannot escape), clean trash racks, anti-vortex hood, and watertight barrel joints.
4. Earthen Dam Embankment Maintenance (No Woody Trees or Burrowing Animals!):
   • The compacted earthen dam embankment must be maintained in mowed turfgrass and kept strictly free of woody trees, brush, and burrowing animals (such as muskrats, beavers, nutria, or groundhogs).
   • Why This Is Heavily Tested: Woody tree roots penetrate deep into the compacted clay dam core; when a tree dies or is blown over in a storm, the rotting root channels leave hollow pipes through the dam (root piping). Similarly, aquatic rodents burrow into the submerged upstream face and dry downstream face of earthen dams, creating direct hydraulic tunnels that trigger catastrophic dam breaches.
5. Stabilized Emergency Spillway & Outfall Apron: Verify that the emergency spillway cut in native soil around the abutment has intact turf/TRM/riprap armor, zero woody obstruction, at least $1.0\text{ ft}$ of embankment freeboard, and a stable downstream outlet velocity protection apron.

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Summary Comparison of Structural Slope, Wall, Velocity, and Pond Controls