drainage study Help & FAQ

Get a drainage study for any project.

Parcels, roads and culverts, detention ponds: draw the area and get peak flows, pond sizing and a report you can put your stamp on.

Click the point where water matters — a culvert, an outfall, a parcel's discharge — and the watershed draining to it becomes the study. Or draw an area by hand.

Pick a study point to begin. Click Study point, choose what it is (parcel outfall, culvert…), then click the map. The watershed draining to that point is delineated from USGS elevation data and the study runs on it. Prefer to outline the area yourself? Use Draw area.
Delineating the watershed from the 10 m DEM… (large watersheds can take a minute)
Still working — large sites can take a few minutes.

Road crossings FHWA HDS-5

Outlet control is sized on these. Blank barrel length assumes a shallow fill (travelway + 10 ft). Tailwater 0 means the receiving channel is assumed NOT to submerge the outlet — raise it for a crossing that discharges into a pond, a backwater or a larger stream in flood.
Delineating each crossing's watershed…
#StationArea Q CulvertHW/D Control

Proposed surface LandXML

Now click the study point — the outfall or crossing the graded site drains to. The study runs on the existing terrain AND the proposed grade, and differences them.
Leave both covers equal for a grading-only study — the terrain change is then the only variable.
Delineating on both terrains… (two full delineations)
Existing Proposed
StormPeak pre Peak postΔ peak Δ vol

Site

–acres
–centroid

Jurisdiction

Criteria drive the design — required storms, freeboard, side slopes and the rainfall distribution — and the study is then checked against them.

Rainfall

Querying NOAA Atlas 14…
Storm24-hr depth

Enter this site's 24-hour rainfall depths in inches. The study will say they were entered by you, not taken from NOAA Atlas 14.

Soils / HSG SSURGO

Querying Soil Data Access…
Map units

    Floodplain FEMA NFHL

    Querying FEMA NFHL…

    Curve numbers

    –CN pre
    –CN post

    Parcel program TR-55 districts

    Running the density ladder…
    District~Lots CNΔ peak ~Storage
    Click a row to set it as the post-development cover and re-run the study.

    Private storm drainage uploaded

    The DEM cannot see pipes. Areas added here are run as their own sub-area hydrographs and summed at the study point — they are not folded into the watershed's curve number.
    Querying SSURGO and NLCD for each area…

    Time of concentration

    –pre (min)
    –post (min)
    Flow path segments (editable)

    Drainage impact

    Computing…
    StormPQ preQ post Peak prePeak postΔ peak

    Sub-basins

    BasinArea acCNTc min Q ownQ outTo

    Mitigation — detention pond

    Placing the pond on the terrain…
    –
    –catches (ac)
    –pond (ac)
    –deep (ft)
    Drag the pond to move it, drag a corner to reshape it. Every edit is re-checked on the terrain.
    StormAllowedNo pond With pondStage
    Outlet stageElement
    Stage–storage–discharge
    Stage ftStorage ac-ft Q cfs
    Sizes the pond at several depths, compares development densities, and suggests fixes — takes ~30 s.

    Design options screening level

    Sizing alternatives…
    Depth ↔ land trade-off
    DepthPond land % of site
    Development density
    CoverCN ~Storage~Land

    Criteria findings

      Hydrographs

      Assumptions

      Report

      Worksheet-style report with substituted equations, data sources, and limitations. Preliminary — not a sealed document.
      NRCS/TR-55 methods · US customary
      Legend