Tomball staff outline drainage master plan, propose CIP projects and stricter stormwater rules

City of Tomball consultants and staff presented a drainage master plan at a workshop that maps flood-prone areas and proposes capital improvement projects, including storm sewers and subregional detention basins to reduce structure inundation in Old Town Tomball and several other basins.

The plan uses 2D hydrologic and hydraulic modeling integrated with Harris County Flood Control District stream models to refine inundation maps beyond FEMA flood data, project consultant Kurt Killian said. “HECRAS is a board of engineers program that's used worldwide specifically for stream modeling capabilities,” Killian said, adding that darker map colors indicate deeper ponding.

The city and its consultants identified near-term and ultimate improvements by basin. Examples shown to the council included phased channel deepening and culvert upgrades in the M118 basin near Lizzie Lane; enclosure and storm-sewer upgrades on the J‑13103 tributary and along Carroll, Lovett and Lubbock streets (a project tied to a separate Harris County roadway improvement); a subregional detention basin and storm-sewer lateral to serve North Cherry and Old Town; and a package of laterals and channels to relieve southern Old Town and areas near the medical complex in the M121 basin.

The consultant team said the plan is intended to eliminate structure inundation for modeled events by providing, in most street corridors, storm-sewer capacity sized to roughly one pipe diameter larger than existing systems and to target a 25‑year level of service for new or reconstructed roadways. The consultants said they also propose raising minimum detention requirements for development from 0.55 acre‑feet to 0.75 acre‑feet to align with Harris County practice.

Project packets with maps and order‑of‑magnitude cost estimates will be prepared for each CIP item to help the city, council and the public understand implementation timing and responsibilities, the consultant said. The presentation repeated a key implementation point: drainage work should proceed downstream to upstream to avoid creating downstream impacts.

Funding paths discussed include traditional city funding, state and federal grants (Texas Water Development Board, FEMA, GLO, RAISE), joint partnerships with TxDOT and Harris County precincts, and drainage impact fees; the consultants said impact‑fee reevaluation is underway for four basins. The team said a congressional community project appropriation routed through HUD has funded design and right‑of‑way work for the J‑13103 trunk project; construction funding for that project remains pending.

Council members and workshop attendees asked about the plan’s assumptions, including which future‑land‑use map the study used; the consultants said they used the city’s 2019 future land‑use map combined with as‑built plans provided by the city and that the plan includes a modeled “no‑build” future to test where growth could increase flooding. Bob Bertrand of CSC told the workshop, “Generally, you look at these master plans, you look 10 years out, okay, and then you come back and you develop a 5 and 10 year CFP.”

The workshop also covered material and technical clarifications: the draft design criteria would allow HDPE and polypropylene pipe in city right‑of‑way and would expand the design storm used for roadways from a 2‑year event to a 25‑year event to improve emergency‑vehicle access during larger storms. Several council members asked whether detention basins could double as park or athletic fields; staff said basins are intended as dry detention but will pond after storms and may remain wet for a time, and that deeper basins (a staff estimate ranged from “6 to 8” feet; another participant mentioned 12–14 feet in parts of the site) would limit recreational use.

Next steps the consultants described include preparing the CIP packets and cost estimates, coordinating required work with Harris County Flood Control District, TxDOT and county precincts where improvements are in their right of way, updating the city’s stormwater design criteria, and returning to council with prioritization for 5‑ and 10‑year capital forecasts. The presentation materials and project packets will be used to pursue grant funding and potential joint‑agency partnerships; individual project construction would require separate funding approvals.

The plan does not adopt changes tonight; the workshop was presented as a planning‑level briefing and discussion. The city’s timeline for formal adoption of the design criteria or for budgeted construction of specific CIP projects was not specified during the session.