Stormwater improvements for a senior living community in Frisco, Texas

What the project was

An existing senior living community in Frisco, in the northern Dallas–Fort Worth metroplex, had a drainage system that was no longer behaving the way its original design intended. RSP Engineers was engaged to find out why, and to produce a set of stormwater improvement drawings the City of Frisco could review.

This was not a greenfield site. The campus was built, occupied and landscaped, with residents in place, parking in daily use and a network of water, sewer and storm utilities already in the ground. Everything proposed had to work around that. The scope ran from investigation through to a complete plan set: an assessment of existing drainage conditions, research into the permit record, drainage calculations, hydrologic and hydraulic modelling of the stormwater system, coordination of an independent geotechnical investigation of subsurface soil conditions, and a design addressing the deficiencies that work identified.

RSP acted as the civil engineer of record for the stormwater improvements. The resulting set runs to seventeen sheets and includes existing composite utility plans, paving and grading plans, drainage plans, drainage and structure details, profiles and cross-sections, demolition plans, and the City of Frisco's own standard notes and details.

The constraint

The difficulty on this site was not the rainfall. It was that the drainage system already existed, was partly undocumented, and sat on top of other buried infrastructure that could not be moved.

Three things made it awkward. First, the site's stormwater depends on an existing retention area with a bank running from elevation 652 feet at the top down to 642 feet at the bottom — a ten-foot drop that any new grading had to respect rather than disturb, alongside an existing detention pond held under a drainage and detention pond easement. Second, the campus is crossed by an easement carrying a drainage and floodway designation, which constrains what can be built and where. Third, the existing utility network was dense and only partly recorded: the composite utility plans document existing stormwater lines in 12-inch PVC, 18-inch PVC and 42-inch reinforced concrete, threaded among water and sewer mains whose depths the drawings explicitly instruct the contractor to field verify, because the proposed storm runs pass close enough that the top of the new storm pipe and the bottom of the existing water and sewer lines are a recorded point of conflict.

That combination is what turns a routine drainage repair into an engineering problem. A new inlet cannot simply be placed where the ponding is worst; it has to be placed where there is room between an existing water main, an existing sewer and a 42-inch storm trunk, at a depth that still drains, on a slope the receiving system can accept, without cutting into a floodway easement or destabilising a ten-foot retention bank.

What was done

The investigation came first. RSP assessed the existing drainage condition on the ground, researched the permit record for what had been approved historically, and used that to build a picture of how the system was supposed to work against how it actually behaved. Drainage calculations and hydrologic and hydraulic modelling of the stormwater network followed, which is what converts "it ponds near the parking" into a located, quantified deficiency with a hydraulic cause. An independent geotechnical investigation was commissioned and coordinated, so that subsurface soil conditions informed the grading and structure work rather than being assumed.

The design that came out of it is a collection system rather than a single intervention. Fourteen new structures were laid out — six catch basins and eight curb inlets — and connected by a storm network sized run by run, with pipe diameters from 8 inches up to 64 inches in PVC and HDPE depending on the contributing area and the available cover. The storm pipe schedule works through each leg individually, including the short connections such as the 8-inch HDPE run of 29 linear feet at a 1.0 percent slope that picks up a localised low point.

Grading was handled with berms rather than wholesale regrading, because the campus is occupied and its finished surfaces largely stay. The plans set out three-foot-wide berms at specified elevations across the site — top of berm at 661.00 against a bottom of 660.00 in one location, 665.50 over 664.50 in another, 667.687 over 666.687, 668.68 over 667.68, and 670.00 over 669.00 — each one a one-foot rise placed to redirect sheet flow toward the new inlets instead of toward buildings and paving. A four-foot wall was added where a level change was needed in a position a berm could not occupy.

A substantial part of the work is repair and reconnection rather than new build. The drawings call for an existing stormwater drain manhole to be removed and replaced with a proposed inlet, an existing catch basin to be replaced and reconnected to the new network, and existing inlets to be cleaned. That last item matters more than it sounds: part of what degrades an older system is simply that structures silt up, and a deficiency assessment has to separate a system that is undersized from one that is blocked, because the fixes are not the same.

The set also carries demolition plans, which on an occupied campus are not a formality. Removing a manhole, taking out a catch basin and cutting into live paving all have to happen in an order that leaves the site drained and passable between stages, because residents, deliveries and emergency access continue throughout. Separating demolition onto its own sheets lets the sequence be read independently of the proposed work, which is what a contractor pricing phased construction actually needs.

Profiles and cross-sections were drawn along two section lines so the proposed surface can be read directly against existing grade. On a site where the design intent is a one-foot berm here and a matched elevation there, a plan view alone does not show whether the result drains; the sections are where the relationship between proposed and existing ground becomes checkable, by the engineer and by the reviewer.

Agencies and approvals

The City of Frisco is the reviewing authority. The plan set carries a dedicated sheet of City of Frisco general notes and a sheet of City of Frisco standard details, which is the practical shape municipal review takes in this part of Texas: the city's own construction standards are incorporated into the drawings rather than referenced, so a reviewer can check the proposal against the standard on the same sheet.

Because the work sits near a drainage and floodway easement and modifies an existing detention and retention system, the review is concerned with more than pipe sizing. It is concerned with whether the existing detention function is preserved, whether the changes alter flow leaving the site, and whether anything encroaches on the easements. That is why the existing conditions work and the permit records research are not preliminaries — they are the evidence the review depends on.

Work of this kind on an occupied site also carries a construction-phase dimension that the drawings address directly: the contractor is instructed to protect existing infrastructure along the retention area, to maintain natural grade elevation where equipment is placed over existing utilities, and to connect existing pipe at matching invert elevations rather than re-grading to suit.

Outcome

The stormwater improvement design was approved for construction by the City of Frisco on 24 April 2025. For an occupied campus that is the point at which the investigation work pays off: the approval covers a scheme that keeps the existing detention and retention function intact, routes new storm runs around utilities that stay in place, and reshapes surface flow with berms rather than regrading a site that residents live on.