In 2008, the City of Sugar Land created an Integrated Stormwater Management Model (ISWMM). This model is a comprehensive geo-database of all the City of Sugar Land’s drainage infrastructure. The geographic information system (GIS) model consists of hydraulic models, shape files, attribute tables, and links to applicable information for all of the city’s drainage facilities and internal watersheds.
The ISWMM model incorporated existing hydraulic models, created city-wide ponding maps, integrated existing storm sewer facilities and riverine models for the Brazos River and Oyster Creek into the GIS-database.
As a result of this modeling effort, the City was able to have immediate access to floodplain delineations for all channels and streams, a clear understanding and identification of critical ponded areas by storm frequency and areas at risk of flooding. ISWMM is also helpful in identifying and developing drainage Capital Improvement Program (CIP) projects to address flooding concerns and assist with planning of flood emergency response activities. This information can also be used during storm events by the Office of Emergency Management, Police, Fire, Public Works and Engineering Departments.
In 2017, City Council approved Phase I of the ISWMM model update. Phase I incorporated the most recent flood and drainage study models created after Hurricane Harvey and updated the available storm sewer infrastructure based on the best available GIS data. Phase I also included elements such as data collection, incorporating flood studies, integration of FEMA flood maps, developing terrain model, updating storm sewer GIS data, updating ponding analysis, and identifying gaps in the City needing updated ponding maps.
Phase II of the ISWMM Update will complete the ISWMM update of the geodatabase by incorporating updated flood and drainage studies provided by the Levee Improvement Districts in 2018 (LID 2, LID7 and LID 11) to cover New Territory and Greatwood. Phase II will also produce ponding maps using the Atlas 14 rainfall for 2-, 5-, 10-, 25-, 100- and 500-yr events (issued on September 2018), and update existing hydrologic and hydraulic models using Atlas 14 rainfalls to produce new and updated model outputs. Areas that have not been improved since the latest existing condition was simulated will be merged in with the existing map where appropriate.
Additionally, Halff Associates will develop levee freeboard information for each Levee Improvement District, based on river event water surface elevations at Richmond, Texas, and 2014 LiDAR Data. An updated set of ponding maps that will be used for flood warning during storm events will be created. These ponding maps will be divided into 30 Zones (corresponding to the proposed location of the rain gauges). Through this update the City will receive an updated city-wide ponding Web Map. The developed ISWMM Web Map will utilize the City's organizational ArcGIS Online Portal to provide City staff access on a regular basis and during emergencies, with the possibility to publish the information if necessary.
Funding for the project is included in the Engineering Department professional services budget.
The Engineering Department recommends that the City Council authorize the execution of a professional services contract with Halff Associates, Inc., for the Integrated Stormwater Management Model (ISWMM) Update - Phase II, in the amount of $91,780.00.