Model
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Project
Project info
NOAA Atlas 14 rainfall
Unit hydrograph (Section 4)
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Network layers
Click Subcatchment to draw…
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Terrain
GeoTIFF, ESRI ASCII grid, x,y,z points, CAD drawing (DXF) or lidar (LAS). Declare what the file cannot tell us:
Contours
Seam check
Channels
Click the outlet. The boundary, area, overland reach and watercourse come from the terrain, and anything you then change is kept.
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Public datasets
▶Elevation (USGS 3DEP)
DEM Hillshade
▶FEMA (NFHL)
Flood Hazard Zones
Base Flood Elevations
▶NRCS Soils (SSURGO)
HSG Group A
HSG Group B
HSG Group C
HSG Group D
▶USGS Hydrography (NHD)
Streams / Flowlines
Water bodies
▶USFWS Wetlands (NWI)
Wetlands
▶Land cover (NLCD)
NLCD 2021 Land Cover
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User layers
Shapefiles as a .zip holding the shp, dbf and prj. State Plane is reprojected automatically. You can also drop a file on the map.
Checks run as you work. Click one to go to the element.
Editing boundaries and nodes. Drag a white square to reshape a subarea, or drag a node to move it.
Tool: Select
Move cursor over map
Zoom 14
Map: Esri Topo
Storm 100-yr
P6 none
Ready
Not run
Element
Map
Project
Which conditions should the report cover?
The choice is remembered for this project.
Select an element to see its properties
SUse the select tool and click a subarea, node or link on the map.
P J B O LDraw a subarea, junction, basin, outfall or link (pipe, gutter, swale, channel).
DDelineate a watershed from the terrain.
Ctrl EnterRun the analysis.
Ctrl ZUndo, Ctrl Y to redo.
Getting started
1
Set up the project
Name, engineer, site and design storm on the Project tab
2
Rainfall
NOAA Atlas 14 for the site, fetched or loaded from the CSV
3
Draw the drainage network
Subareas, nodes and links, then Run Analysis
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Unit hydrograph
Section 4, NRCS method. Single basin, as the SDHydroTools SD Unit Hydrograph module.
Curve number
Lag and time to peak
Rainfall at any duration
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Your projects
Info
Rainfall
Options
Project
Engineer of record
Site coordinates
Drag the site pin to refine the location. NOAA rainfall follows the site.
Study parameters
One pair per line: duration in minutes, intensity in in/hr, as in an older study's input. Interpolated linearly between pairs, as older rational method programs do. The depths below are filled from the pairs for the hydrograph. For 2026 submittals use NOAA Atlas 14 (Section 3.1.3).
NOAA Atlas 14 depths (in)
5-min
10-min
15-min
30-min
60-min
2-hr
3-hr
6-hr (P360)
12-hr
24-hr
12 and 24 hour depths feed the Section 4 unit hydrograph only. The rational method uses 5 minutes to 6 hours.
No depths entered yet
Three ways in: load the NOAA CSV, drag the CSV onto the boxes above, or paste the NOAA table into any box. The ARI column matching the design frequency is used, and latitude, longitude and location name are read from the file. NOAA blocks direct fetching from a browser, so the file is the fastest path.
Open NOAA PFDS manually
Manual guidance on rainfall data (Appendix B)
Analysis options
Ti under 5 minutes is carried downstream; the intensity is read at 5 minutes (Section 3.3, step 2).
Pipes set to be sized for the flow take the smallest standard size at or above the minimum, and not smaller than the pipe upstream, that carries the flow full at the factor times the pipe slope. The factor also sets the friction slope for pipe velocity; 1.00 uses the pipe slope.
No proxy, automatic fetching is off
NOAA and the USDA soils service refuse browser requests from another origin. Deploying the included proxy removes that limit. Troubleshooting
Backs the intensity out of the combined Q and reads the matching Tc, so downstream flows do not fluctuate. How the app reads Note 3
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Subarea A
Subcatchment
100.0
Properties
Tc / Flow
Hydrograph
Identity
With both set, Tt is summed along the flow paths between them.
Geometry and land use
Computed C - Table 3-1 [Section 3.1.2]
Runoff coeff C
0.51
Weighted CA
3.11
C = 0.90 × (% impervious) + Cp × (1 - % impervious). The 0.90 is the coefficient for impervious surface. Cp is the pervious coefficient by soil group: A 0.20, B 0.25, C 0.30, D 0.35.
Travel time: Tt
Velocity (fps)
5.40
Tt (min)
1.1
Peak flow: Q = CIA [Section 3.1.1]
Tc = Ti + Tt (min)
8.5
I at Tc (in/hr)
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C value
0.51
Qp (cfs)
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I from NOAA Atlas 14 log-log interpolation at Tc [Section 3.1.3]
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Section 6: Rational method hydrograph
Run Analysis to generate the 6-hour nested storm hydrograph for this subcatchment
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Node
Junction
Node
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Elevation
No elevation yet
Uses a loaded DEM if there is one, otherwise queries USGS 3DEP.
Flow from another study (code 7)
Peak flow and Tc carried in from an offsite or earlier study. It joins this node as a stream of its own.
Flow paths at this node
Streams arriving here
MRM junction result [Section 3.4]
Controlling Tc (min)
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Qp out (cfs)
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→
Flow path
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Route
This project uses the earlier fixed hydraulic radius for velocity. The section below takes effect once the links are updated to Section 3.1.4.2. Results will change.
Profile
Section
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Detention Basin
Detention basin, Hydraulic Design Manual Chapter 6
Node
Outlet
Stage-Storage
Routing
Low flow orifice
Eq 6-12, Q = Co A sqrt(2 g Ho), continuous from part full through submerged
More openings in the riser (multi-stage)
Openings sum (6.4.6). A riser crest runs as a weir over its rim, then as an orifice through its top (Eq 6-13, 6-14).
Outlet barrel
Caps the openings: inlet control as an orifice, outlet control as full pipe flow. Chapter 4 covers a full culvert analysis.
Emergency spillway
Broad crested, Eq 6-10, Cw 3.0 usually. It passes the undetained 100 year inflow with 1 ft of freeboard to the embankment (6.2.3, 6.2.4.3).
Storage from
Stage-storage table
| Elev (ft) | Volume (ac-ft) |
|---|
Floods the depression at this node from its low point. Step is the level increment, the second box is how deep to go.
Cumulative storage at each elevation, interpolated linearly between rows. Editing a row makes the table yours, and the builder stops changing it.
Allowable release (6.2.1)
Modified Puls routing (Eq 6-4)
Set the outlet and stage-storage, make sure flow reaches this basin, then route.