Purpose. This study establishes how surface water is collected and discharged from the SAFEBOX Pattaya site. It sets the design storm, the peak runoff, the U-ditch sizes and grades, and the outfall connection, so the project architect and licensed civil engineer can review, stamp, and detail the works. All ground levels here are digitized from the project instrument topographic survey (80 spot levels, local benchmark datum “ELE 0.000” on the east road); final invert levels must be tied to MSL and the east-road drain invert before construction.
What gets built — do we need any pipe / ditch / culverts at all? / ต้องมีท่อ / รางระบายน้ำหรือไม่
YES — a simple gravity drainage system is required. Because the site is nearly flat and paved, sheet flow must be caught and led to one controlled outfall. Specifically:
NO pumps. The whole site falls to the SE corner by gravity (1.57 m of natural fall to the low point), so discharge is entirely gravity-driven — no lift station, no power, no mechanical parts.
⭐ ENGINEER’S SCHEME — received 13 Jul 2026, ADOPTED as the working layout / แบบที่วิศวกรเสนอ (ใช้เป็นแบบหลัก)
The working drainage layout (shown one colour in Fig. 1 and the 3D model): precast concrete U-channels with steel grating along the NORTH and SOUTH boundaries, linked by an N–S connector channel inside the yard just west of the entrance forecourt, with the pad crowned so each half sheds to its nearest channel. Discharge is via buried crossings into TWO square UNDERGROUND WATER BOXES (junction / silt chambers) sitting on the east-road municipal drain — box #1 north (ties into road catch basin DRAIN 02), box #2 south. All runs fall west→east with the natural grade (NW high +1.15 → SE low −0.42; road 0.45–0.67 m below the parcel), so the scheme stays fully gravity-driven.
| This study (§2) | Engineer proposal (13 Jul) | |
|---|---|---|
| Collectors | South main + East | North + South + N–S connector (west of forecourt) |
| Channel type | Open U-ditch, grated at crossings | Grated precast U-channel throughout |
| Pad grading | Cross-fall to south/east | Crowned ridge — halves shed N and S |
| Outfalls | 1 (SE corner) | 2 underground water boxes (N + S) on the city drain — shorter, shallower runs |
Assessment: consistent with the instrument survey and with this study’s hydrology (§3–§5 apply unchanged); the two-outfall split halves each run length so channels stay shallower. Approved in principle, conditional on:
Shown in Fig. 1 (amber) and in the 3D model 💧 layer (amber channels, two red outfalls). Budget: the north-channel + second-outfall delta over §8 is roughly ฿150,000–260,000 (~100 m grated channel @ ฿1,400–1,800/m + outfall structure ฿40,000–80,000) — verify against the master budget’s drainage allowance.
The Phase 1 parcel is a five-sided lot of 4,911 m² fronting Motorway 7 to the north and a public entrance road along the east boundary (edge P3–P4). The east road already carries municipal drainage. After construction the surface is ~85% impervious (asphalt paving over the full polygon plus module roofs), so a runoff coefficient C = 0.90 is used.
The site is nearly flat: total fall across the parcel is only 1.57 m (survey range +1.154 to −0.418 m, mean +0.40 m, local datum). Crucially, the natural low point sits at the south-east corner (≈−0.42 m local) — exactly where the east road’s drainage is installed, and the road itself is lower still (−0.45 to −0.67 m) — while the high ground is the north-west corner (≈+1.15 m). The site already wants to drain to the SE corner. The design works with that grade; nothing is directed uphill toward the Motorway 7 frontage.
| Boundary corner | Elevation (m, local datum) | Position |
|---|---|---|
| NW | +1.15 | NW — HIGH point (survey max) |
| N apex | +0.21 | N (Motorway 7 frontage) |
| NE | +0.18 | NE (top of east road edge) |
| SE | −0.42 | SE — LOW / outfall (survey min) |
| SW | +0.74 | SW |
Phase-2 parcel corners (future, south): survey ~+0.45 to -0.41 m local; the southern parcel sits lower than Phase 1 and also falls toward the same SE corner.
Two perimeter U-ditch collectors run in the setback / fence strips (clear of all module footprints, the office, and the gates) and converge at a single outfall at the SE corner:
Asphalt grading. The paved yard is finished with shallow cross-falls of 1–2% toward the two perimeter ditches, so sheet flow never ponds against module bases. Module roofs shed to the aisles, which fall to the same perimeter lines.
Daily rainfall extremes were taken from Open-Meteo ERA5 reanalysis for the site cell, 1995–2025, and fitted with a Gumbel (EV-I) distribution. The largest 24-h total on record is 91.5 mm (8 Sep 2021). September–October is the monsoon peak (~250 / 220 mm per month).
| Return period | 2-yr | 5-yr | 10-yr | 25-yr | 50-yr | 100-yr |
|---|---|---|---|---|---|---|
| 24-h rainfall (mm), Gumbel | ≈52 | ≈66 | ≈74 | 80 | 89 | ≈98 |
Convective safety margin. ERA5 is a gridded reanalysis that smooths short convective peaks — the flash thunderstorm cells that actually govern a small paved catchment. We therefore design the ditches to a 24-h depth of ≥120 mm/day, and for the rational method adopt a short-burst design intensity of i = 90 mm/hr. This sits above the 100-yr ERA5 daily figure and is a defensible urban-drainage value for Pattaya; the civil engineer should confirm against the local IDF curve (DPT / Royal Irrigation Dept.) at detailed design.
Q = C · i · A, with the catchment split ≈60% to the south collector (larger tributary area) and ≈40% to the east collector.
Precast reinforced-concrete U-ditch, Manning n = 0.013, checked flowing 75% full (leaving ≥25% freeboard). Q = (1/n) A R⁵ᵣᵤ Sᵢᵣᵤ.
| Collector | Section W×D | Grade | Capacity @75% | Demand | Velocity | Check |
|---|---|---|---|---|---|---|
| South main | 0.40 × 0.40 m | 0.3% | 123 L/s | 66 L/s | 1.03 m/s | OK — 1.9× spare |
| East | 0.30 × 0.30 m | 0.3% | 57 L/s | 44 L/s | 0.85 m/s | OK — 1.3× spare |
Both velocities fall in the self-cleansing band (0.6–2.5 m/s): fast enough to keep silt moving, slow enough to avoid concrete scour. Minimum grade 0.2%; 0.3% preferred. Because the natural fall over the 92 m south run is ≈1.9 m (≈2%), grade is not a constraint — the ditch invert can be laid flatter than the ground to stay shallow while still exceeding minimum self-cleansing slope.
The west (rear) boundary is a 3 m solid wall; the north, east and south are see-through fence. Because the east side — toward the SE outfall — is see-through, provide a continuous kerb / edge upstand along the east (and south) fence line so surface water cannot sheet-flow through the fence onto the road; it is steered into the perimeter U-ditch and forced to the single SE outfall. The east U-ditch runs in the setback strip just inside the fence line.
| Item | Qty | Unit rate (฿) | Amount (฿) |
|---|---|---|---|
| South main U-ditch, 0.40 m, incl. bed & grating at crossings | 92 m | 1,400–1,800 | 129,000–166,000 |
| East U-ditch, 0.30 m, incl. bed & grating at crossings | 52 m | 1,000–1,400 | 52,000–73,000 |
| Catch basins / sumps with sediment trap | 4 no. | 8,000–15,000 | 32,000–60,000 |
| Outfall structure + silt trap / interceptor + road tie-in | 1 no. | 40,000–80,000 | 40,000–80,000 |
| Asphalt cross-fall grading (allowance) | item | — | in paving pkg |
| Indicative drainage sub-total | ฿253,000–379,000 |
Ditch-only line (~145 m at ฿1,000–1,800/m) = ฿181,000–275,000. Detention tank, if required, is additional. Rates are Thai site-works reference figures; obtain contractor quotes at tender.
Engineer-proposal delta (13 Jul 2026): the north grated U-channel (~100 m @ ฿1,400–1,800/m ≈ ฿140,000–180,000) + second outfall structure at NE (฿40,000–80,000) adds roughly ฿150,000–260,000 over the table above → indicative drainage total ฿400,000–640,000 if the full perimeter-grated scheme is adopted. Offset: the study’s “grating at crossings only” assumption becomes grated-throughout — keep truck-rated grating at aprons/crossings only and lighter covers elsewhere to pull cost back down.