Effective surface water drainage is critical for protecting infrastructure, preventing flooding, and maintaining environmental quality. At Allon Consulting, we design and construct comprehensive drainage systems—from storm sewers to culverts and open channels—that manage runoff safely and sustainably. Our solutions are tailored to site conditions, regulatory requirements, and long‑term performance.
1. Hydrological Analysis
We begin by quantifying runoff:
- Rainfall Data
Use of historical records and IDF (intensity‑duration‑frequency) curves for design storms (e.g., 5‑year, 25‑year, 100‑year events). - Rational Method
For small catchments (≤ 50 ha), we compute peak flow using runoff coefficients based on land use and soil type. - Hydrologic Modeling
For larger watersheds, we use models (HEC‑HMS, SWMM) to simulate runoff, accounting for time of concentration, storage, and routing.
2. Storm Sewer Systems
We design pipe networks to convey stormwater:
- Pipe Materials
- Reinforced Concrete Pipe (RCP) – for large diameters and heavy loads.
- HDPE – flexible, corrosion‑resistant, suitable for trenchless installation.
- PVC – for smaller diameters and non‑traffic areas.
- Corrugated Metal Pipe (CMP) – with coatings for durability.
- Hydraulic Design
Pipes sized for design flow with minimum slope (0.5‑1.0%) to achieve self‑cleaning velocities (> 0.9 m/s). We use Manning’s equation with appropriate roughness coefficients. - Catch Basins and Inlets
Spacing based on gutter flow capacity and drainage area. Inlet types include curb inlets, grate inlets, and combination structures. - Manholes
Precast concrete structures at changes in direction, grade, or pipe size. We provide drop structures where needed. - Outfalls
Energy dissipaters (riprap, stilling basins) to prevent erosion at discharge points.
3. Culvert Design and Installation
Culverts convey water under roads, driveways, and embankments:
- Hydraulic Capacity
Design for design storm with freeboard. We evaluate inlet control and outlet control conditions to determine headwater elevation. - Culvert Types
- Pipe Culverts – RCP, CMP, or HDPE in circular, elliptical, or arch shapes.
- Box Culverts – reinforced concrete boxes for larger spans or where limited cover exists.
- Bridge Culverts – for wide watercourses or environmentally sensitive crossings.
- Installation
- Bedding – granular material to provide uniform support.
- Backfill – select material placed in lifts and compacted.
- End Treatments – headwalls, wingwalls, or flared ends to improve hydraulics and prevent scour.
- Fish Passage
Where required, we design culverts with natural bottom or baffles to allow aquatic organism passage.
4. Open Channels and Ditches
For larger flows or where pipes are not feasible, we construct open channels:
- Channel Types
- Grass‑Lined Swales – low‑velocity conveyance with water quality benefits.
- Lined Channels – concrete, riprap, or gabion‑lined for high velocities.
- Naturalized Channels – mimicking natural stream geometry with rock and vegetation.
- Hydraulic Design
Manning’s equation with appropriate roughness; freeboard; erosion protection. - Stabilization
Riprap aprons, gabion mats, or turf reinforcement mats (TRMs) at outfalls and bends.
5. Stormwater Management Facilities
We design facilities to control runoff quantity and quality:
- Detention Ponds
Temporary storage of stormwater to reduce peak flows. Outlet structures (orifices, weirs) control release rates. - Retention Ponds
Permanent pools with extended detention for water quality treatment and flood control. - Underground Detention
Large‑diameter pipes, vaults, or modular plastic chambers for space‑constrained sites. - Low Impact Development (LID)
- Permeable Pavements – porous asphalt, pervious concrete, or interlocking pavers that allow infiltration.
- Bioretention Cells – landscaped depressions with engineered soil and underdrains.
- Rain Gardens – smaller bioretention for residential or commercial sites.
- Infiltration Trenches – gravel‑filled trenches with perforated pipe to infiltrate runoff.
6. Erosion and Sediment Control During Construction
We implement temporary measures to protect receiving waters:
- Silt Fences – around the perimeter of disturbed areas.
- Sediment Basins – with skimmers or risers.
- Stabilized Construction Entrances – stone pads to prevent tracking.
- Temporary Swales – convey runoff to sediment traps.
7. Materials and Construction
- Pipe Installation
Trench excavation, bedding, jointing, and backfill per ASTM or AWWA standards. We perform deflection tests for flexible pipe. - Concrete Work
For catch basins, manholes, and headwalls, we use high‑strength concrete with waterproofing admixtures. - Riprap Placement
Graded stone placed over filter fabric; thickness based on velocity and duration of flow.
8. Quality Assurance and Testing
- Pipe Testing
Low‑pressure air or water testing for leakage; mandrel testing for flexible pipe. - Compaction Testing
Density of backfill around structures. - Survey Verification
As‑built surveys to confirm grades and invert elevations.
9. Typical Projects
- Highway Drainage System
Designed and installed 15 km of storm sewer, 200 catch basins, and 40 culverts for a new expressway. Managed 100‑year flood flows with minimal disruption. - Residential Subdivision Drainage
Constructed a regional detention pond with 10,000 m³ storage, underground pipe network, and swales. Reduced downstream flooding by 80%. - Culvert Replacement
Replaced a failing 2.5 m diameter CMP culvert with a 3 m × 2 m precast concrete box culvert, including fish passage features. Completed under live traffic with temporary bypass. - LID Retrofit
Converted an urban parking lot to permeable pavers and bioretention cells, reducing stormwater runoff by 90% and eliminating sewer surcharge.
Our drainage and culvert works are designed to manage water safely, protect infrastructure, and enhance environmental performance. We deliver systems that stand up to extreme weather while maintaining long‑term functionality.
