Where the Water Goes: Drainage, Flow & Wetness

Water runs downhill, and the shape of your land decides where. From your bare-earth elevation model we trace where runoff gathers, map the channels, and flag ground where water tends to linger.

First, we fill the sinks

Elevation data has tiny pits, some real, some noise. We fill them so modeled water runs on downhill instead of stopping in a one-cell hole.

Flow accumulation (where runoff concentrates)

What it is: for every spot, flow accumulation counts how much upslope land drains through it. It's near zero on a ridge and huge in a draw where slopes funnel together.

When to use it: finding problem spots before a storm. It shows where sheet flow becomes a channel, where a crossing meets concentrated flow, and where runoff scours a road.

The Strahler drainage network (the channels, ordered)

What it is: where flow passes a threshold it becomes a channel, and the channels form a connected drainage network.

The Strahler order ranks each branch: headwater trickles are order 1, and where two channels of equal order meet, the channel below steps up one. So you see the minor fingers and the main arteries.

When to use it: siting, permitting and land planning: where the natural watercourses run, where to cross, and where to keep structures clear.

A modeled network is in the downloadable sample set as embry-sample-drainage.kmz, which opens in map apps that read KMZ.

The Topographic Wetness Index (TWI)

What it is: TWI combines how much land drains into a spot with how flat it is into one "wetness" score.

Lots of upslope water on flat ground pools and scores high; a little water on a steep face runs off and scores low.

When to use it: high-TWI ground is likely wet, soft for equipment and slow to dry. It flags where a pond could hold and where to keep a road out of the mud.

Reading them together

Flow accumulation shows the paths, the Strahler network the channels, and TWI the wet ground: together, a working model of your property's water. The Land & Hydrology Analysis page describes the full service.

The honest limit

Drainage, flow, terrain, and floodplain layers are derived analysis computed from our elevation data to support planning and design. They are not a certified survey, a stamped engineering study, or a FEMA flood-zone determination. This models how water behaves, for planning and early design. It does not replace a hydrology and hydraulics study from a licensed engineer.

It is not an official floodplain or flood-insurance determination; for that, work from FEMA's mapping and a licensed professional. Boundary and title work needs a licensed Texas surveyor; we don't do that work.