The City of Highland contracted with Oates Associates to develop a city-wide master stormwater management plan. The results of this study will be incorporated into the City comprehensive plan and will be used to guide development practices in the City of Highland for years to come. The project is being completed in phases as the City’s available funding allows.
Phase 1 – Lindenthal and Laurel Branch Analysis and Proposed FEMA Mapping Review
The first phase of this project consisted of a hydraulic and hydrologic analysis of the two major streams which drain the City of Highland, Lindenthal Creek and the Laurel Branch Creek. Oates calculated flow volumes at each major crossing location, and also utilized the results from numerous drainage studies and hydraulic reports conducted for the City of Highland over the years. A field survey of culvert sizes and elevations was also conducted.
One of the primary reasons for this analysis was to evaluate draft Flood Insurance Rate Maps for Madison County, Illinois recently issued for review by the Federal Emergency Management Agency (FEMA). These new maps indicated a significant increase in the flood elevation along Lindenthal Creek, particularly in the City’s densely developed central district. Oates compared our independent flow calculations and the results of our culvert surveys to the HEC-RAS model used by FEMA to determine the proposed flood elevations. We then advised the City on the need for a formal appeal of the draft maps, and suggested possible courses of action to mitigate the impact of the new flood elevations on property owners throughout the flood zones.
Phase 2 – Regional Retention Pond Analysis
The second phase of this project involved an evaluation of possible locations for regional retention ponds. OA utilized topographic mapping and aerial photography to identify locations which appeared to meet the selection criteria. Over a dozen pond locations were considered, and two locations were identified as being the most promising. Preliminary capacity calculations were performed which were used to estimate the approximate amount of new impervious area each pond could serve.
Phase 3 – Regional Retention Lake Preliminary Design
The current phase of this project will involve topographic and boundary survey and preliminary design for a proposed 15-acre lake which will provide regional retention capacity as well as recreational opportunities. The lake will be located in the upper reaches of the Lindenthal Creek basin and may significantly reduce peak flow rates in Lindenthal Creek during peak rainfall events. This may be helpful in mitigating some of the flooding concerns identified on the proposed FEMA flood maps, and could contribute to a future application to FEMA for a change in the maps and a reduction in base flood elevations along the creek.