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When a developer begins evaluating a piece of land, the first steps usually include a high‑level feasibility review. At this stage, both the developer and their design team look at the big‑ticket items: transportation access, roadway impacts, utility availability, and whether the site can physically support the intended project. They may even look into phase one environmental assessments and perform a title search. These are essential — but they’re not the whole picture.

There’s a third water utility beyond water and sewer that consistently gets overlooked, and ignoring it can create major liability issues down the road: this utility is stormwater. Often, stormwater isn’t even evaluated as a utility but it should be. You need to know that receiving drainage structures are within reach, what is their capacity, and what drainage improvements will be needed.

To understand this, you have to look not only at the site itself, but also beyond the property line.

Public Data vs. Survey Data: Two Different Tools, Two Different Purposes

Before a developer invests in a full boundary and topographic survey, the design team typically relies on publicly available, open‑source data to perform early screening. These tools are incredibly valuable — but they’re not precise enough to replace field verification.

Common Open‑Source / Public Data sources include:

– The National Map (USGS)  

– State GIS portals  

– LiDAR‑based elevation datasets  

– FEMA flood maps

– Soils Maps from NRCS

– County parcel viewers and GIS  

These help the team:

– Understand general terrain  

– Identify watershed boundaries  

– Spot potential outfall locations  

– Evaluate access to water, sewer, and telecom  

– Review surrounding roadway networks  

But these datasets are screening tools, not design tools. They’re somewhat adequate for early feasibility but not reliable enough to understand real‑world drainage behavior.

Survey Data

A formal topographic survey — usually completed after feasibility and sometimes after property is already purchased — provides:

– Accurate spot elevations  

– Located utilities  

– Surface features  

– Drainage structures  

– Easements and right‑of‑way limits  

Surveys are far more precise and reveal information that isn’t always available publicly, but even survey data has its limitations. And one of the biggest sources of misunderstanding comes from how survey surfaces are modeled.

What a TIN Surface Really Is — And Why It Can Mislead You

A TIN surface (Triangulated Irregular Network) is a digital terrain model created by connecting surveyed points with triangles. It’s a powerful tool, but it has inherent limitations:

– It only represents the points the surveyor collected.  

– If a gully, ditch, or erosional feature wasn’t shot, the TIN won’t show it.  

– Vegetation or debris can cause the TIN to “smooth over” the real ground.  

– Small but important drainage paths can disappear entirely.  

In other words:  

A TIN surface is a mathematical approximation of the land — not the land itself.

This is why both developers and design teams must treat TIN surfaces as helpful tools, not absolute truth.

Why a Site Visit Is Non‑Negotiable

In a previous blog, the focus was on what you can discover on the site that the survey missed. This time, the emphasis shifts to what’s happening beyond the property line.

A site visit reveals:

– Unmapped gullies  

– Blocked or damaged culverts  

– Informal drainage paths created by years of runoff  

– Downstream erosion  

– Utility conflicts  

– Real‑world flow patterns that contradict the TIN  

No dataset — public or surveyed — can replace physically walking the land and seeing where the water actually goes.

Look Beyond the Boundary: Where Does Your Water Go?

A proper feasibility study must include a downstream assessment. It’s not enough to know where stormwater leaves your site; you must understand what happens after it does. I always tell my team to start at the outfalls and determine what is downstream and how the changes to the site or land upstream will affect the drainage. Look for risks at the outfalls is my main message to team members when discussing stormwater drainage patterns.

Key questions for developers and their design teams include:

– Where are the outfalls located?  

– Are you discharging onto someone else’s property?  

– What condition are the downstream channels in?  

– How far does runoff travel before reaching a regulated waterbody?  

– Can the downstream system handle additional peak flow?  

– Even if peak rates are controlled, will added volume cause problems?  

Most states regulate peak discharge rates, but volume control is often not required. That gap can create real‑world impacts even when a design is technically compliant with regulations. Every engineer is taught in university that meeting the regulatory standards is not enough to protect themselves from professional liability. This is especially true with stormwater.

Why This Matters for Developers and Their Design Team

Stormwater isn’t just a design detail — it’s a risk factor. If downstream systems fail because of increased runoff from your project, the liability can land squarely on the developer or project engineer, or both.

By incorporating downstream analysis into the earliest stages of feasibility, developers and their design teams can:

– Avoid purchasing land with hidden drainage liabilities  

– Prevent costly redesigns  

– Reduce conflicts with neighbors  

– Improve permitting outcomes  

– Build more resilient, responsible projects  

The Bottom Line

A site feasibility study isn’t complete unless it looks both inward and outward. Understanding the downstream impacts of your project is just as important as evaluating the site itself.

With the right combination of:

– Open‑source data  

– Survey data  

– Utility research  

– Transportation considerations  

– And most importantly, a thorough site walk  

…developers and their design teams can make informed decisions and avoid surprises that surface too late in the process.

Image of John Poole, PE