Stormwater Detention, Kestrel Flats Phase 2

Basin areas, flow control pond sizing, wet pool water balance, and the outfall culvert under SR 291 for the Kestrel Flats Phase 2 preliminary plat, analyzed per the Stormwater Management Manual for Eastern Washington and the WSDOT Hydraulics Manual. The pond on the preliminary plat needs to be longer, and the SR 291 culvert needs a larger pipe set deeper.

Plat area 16.0 ac, Climate Region 3, discharge to the SR 291 roadside ditchSBUH, 24-hour SCS Type IA storm, NOAA Atlas 2 depthsElevations NAVD 88Prepared Nov 5, 2026

Summary

Flow control · Core Element #6

The 2-year release limit, not the 25-year storm, sizes the pond

Release is capped at 50% of the existing 2-year peak (0.08 cfs) and at the existing 25-year peak (0.70 cfs). The 170-ft pond on the preliminary plat holds the 25-year storm below the riser crest at 0.67 cfs, but the 2-year storm rises into the upper orifice and releases 0.11 cfs.

Lengthen the permanent pool to at least 181 ft; 190 ft leaves margin and gives 26,100 CF of live storage below the upper orifice.

Water balance · wet pool

Evaporation alone dries the shallow bench by day 48, even with a liner

July lake evaporation averages 0.25 in./day (WRCC pan data × 0.70). Over a 60-day dry season the lined permanent pool drops 1.26 ft, and the 1.0-ft shallow bench is dry by day 48. Unlined, seepage through the silt loam (0.02 in./hr, geotechnical report p. 14) adds 2.4 ft.

Line the pond and plan for makeup water or a deeper bench; a liner alone does not keep the bench wet.

Culvert · WSDOT Hydraulics Manual

WSDOT minimums, not flow, set the culvert under SR 291

The 12-in. HDPE on the 30% plans carries 3.2 cfs against a 1.0-cfs design flow and has 2.10 ft of cover, but a culvert under a main roadway must be at least 18 in. (Section 3-3.1.7). An 18-in. HDPE leaves 1.56 ft of cover against the 2.0 ft required (Section 8-12.1).

Lower the culvert 0.44 ft, or use 18-in. concrete pipe in the shallow-cover class with supporting calculations and Region Hydraulics review.

Calculations step by step

Four linked calculations on the same inputs. Change the storm, pond length, dry season, or culvert design and watch the results and checks update. Values in the summary reflect the default settings.

16.0 acPlat area

7.2 acImpervious area (45%)

0.16 cfsExisting 2-year peak (CN 79, Tc 45 min)

0.70 cfsExisting 25-year peak

Subbasins from the preliminary plat

Schematic layout, about 1,050 × 660 ft. Click an area or a row to see the source.

Land cover and curve numbers

Land cover	Area, ac	CN	Share
Total	16.0	–	100%

Design storm, 24-hour

Permanent pool length (width 85 ft)

170 ft

–Peak inflow, developed

–Peak release

–Max water surface above permanent pool

–Live storage used

Inflow and release hydrographs

SBUH, 10-minute time step, routed through the pond and riser (level pool).

Inflow, developed Release from pond Allowable release

Pond section and outlet control

Outlet control structure: 1.5-in. orifice at the permanent pool, 7-in. orifice at +1.50 ft, 48-in. riser crest at +2.25 ft, berm at +3.25 ft. Runoff: Q = (P − 0.2S)² / (P + 0.8S), S = 1000/CN − 10, impervious and pervious areas computed separately. SBUH routing: Q₂ = Q₁ + w(I₁ + I₂ − 2Q₁), w = Δt/(2Tc + Δt). Pond: rectangular permanent pool, 3H:1V side slopes.

Dry season with no runoff

60 days

Month

Pond bottom

–Drop in permanent pool

–From evaporation

–From seepage

–Shallow bench (1.0 ft) dry

Permanent pool drawdown over the dry season

No inflow. The pool is assumed full at the start of the period.

Evaporation + seepage Evaporation only Bottom of shallow bench

Inputs

Month	Pan evap., in.	Lake, in./day	Precip., in.

Monthly pan evaporation and precipitation from Site_Climate_Data_NOAA_Atlas2_WRCC.xlsx, lake evaporation = pan × 0.70. Seepage, unlined: 0.02 in./hr measured in the pond footprint = 0.48 in./day. Lined (40-mil LLDPE): negligible. Permanent pool depth 3.0 ft; shallow bench (emergent planting zone) 1.0 ft below the permanent pool.

Sources and method

Sources

Kestrel_Flats_Ph2_Preliminary_Plat_Sheet_C2.pdfLot layout, land cover areas, and pond location and footprint1 sheet, 24 × 36
Kestrel_Flats_Ph2_30pct_Storm_Drainage_C4-C6.pdfStorm drain, pond outlet control structure, and SR 291 culvert with inverts3 sheets
WSDOT_Developer_Services_Comment_Letter_SR291_2026-09-14.pdfDischarge to the SR 291 ditch at or below existing rates; culvert per the WSDOT Hydraulics Manual2 pages
City_of_Kestrel_Flats_Stormwater_Design_Standards_2024.pdfAdopts the 2019 Eastern Washington manual (SWMMEW); SBUH with the 24-hour SCS Type IA storm and existing-condition basis22 pages
Geotechnical_Report_Kestrel_Flats_Ph2.pdfSilt loam, HSG C; infiltration test 0.02 in./hr in the pond footprint (p. 14)31 pages
Site_Climate_Data_NOAA_Atlas2_WRCC.xlsx24-hour depths from NOAA Atlas 2, Vol. IX isopluvials; monthly pan evaporation and precipitation2 tabs
Stormwater Management Manual for Eastern Washington (2019); WSDOT Hydraulics Manual M 23-03.12 (April 2026)Flow control, hydrologic methods, and design storms; culvert size and pipe coverStandards

Method

Land cover areas were taken from the preliminary plat and assigned SCS curve numbers for HSG C soils: impervious 98, gravel 89, native open space 79, lawn and landscaping 74. Existing condition: dryland pasture, CN 79.
Runoff hydrographs were computed with the SBUH method on a 10-minute step for the 24-hour SCS Type IA storm, using 1.20, 2.00, and 2.50 in. for the 2-, 25-, and 100-year events (NOAA Atlas 2 isopluvials).
Allowable release follows Core Element #6 for discharge to a nonexempt stream: 50% of the existing 2-year peak and the existing 25-year peak, per the 2019 Eastern Washington manual that the City’s 2024 standards adopt. The 2024 edition (Ecology publication 24-10-014) keeps these limits and adds a requirement to retain the 10-year, 24-hour event on site without discharge to an MS4. That requirement is not evaluated here.
Developed hydrographs were routed through the pond and the two-orifice riser by level pool routing; storage comes from the pool length, the 85-ft width, and 3H:1V side slopes.
The wet pool water balance uses daily lake evaporation (pan × 0.70) and the measured infiltration rate.
The culvert was checked against WSDOT Hydraulics Manual Sections 3-3.1.7 (minimum diameter) and 8-12.1 (pipe cover) and for full-flow capacity with Manning's equation, n = 0.012.

Assumptions and limitations

The whole plat drains to the pond. Existing conditions are used as the predeveloped basis, as the city standards allow.
The SR 291 ditch drains to an unnamed tributary, treated here as a nonexempt stream; confirm exemption status with the city.
Orifice coefficient 0.62; riser crest as a sharp-crested weir, C = 3.1. Tailwater in the SR 291 ditch is not modeled.
The dry season assumes no runoff and no groundwater inflow. Seepage is applied over the full bottom area.
The culvert design flow is the 100-year pond release, rounded up to 1.0 cfs. Runoff from SR 291 itself is not included, and headwater under inlet and outlet control is not evaluated. The capacity margin is therefore not fully verified; a licensed PE should check inlet and outlet control (HDS-5 method) before the culvert size is final.
Cover is measured from the outside of the pipe to the bottom of pavement at the upstream edge of shoulder, where it is least.
The 2024 Eastern Washington manual may require the 10-year, 24-hour event to be retained on site if the SR 291 ditch counts as discharge to WSDOT’s MS4. That could change the pond size. A licensed PE should confirm with WSDOT, as the MS4 operator, and the city which edition and requirement apply.
The 24-hour depths and evaporation values come from the site climate workbook and have not been checked independently for this location. The design engineer should confirm them against NOAA Atlas 2 and local pan evaporation data.