How can you design a grading and drainage plan on a floodplain?

2 Assess the hydrologic and hydraulic conditions

The next step is to assess the hydrologic and hydraulic conditions of the site and its surroundings. This involves estimating the amount and frequency of rainfall, the runoff coefficient, the time of concentration, and the peak discharge that will affect the site during a design storm event. You also need to evaluate the existing drainage patterns, the direction and velocity of flow, the hydraulic capacity of the drainage network, and the potential sources of flooding and inundation. You can use tools such as rainfall intensity-duration-frequency curves, runoff equations, hydrographs, and hydraulic models to perform these calculations.

3 Design the grading plan

The grading plan is the part of the grading and drainage plan that shows the proposed changes in the elevation and slope of the land surface. The main objectives of the grading plan are to create a suitable building pad, to provide adequate drainage, to minimize earthwork, and to preserve or enhance the natural features of the site. The grading plan should follow the principles of cut and fill balance, slope stability, erosion control, and floodplain management. You can use software such as AutoCAD Civil 3D, ArcGIS, or QGIS to create and edit the grading plan.

4 Design the drainage plan

The drainage plan is the part of the grading and drainage plan that shows the proposed elements and structures that will collect, convey, store, treat, and discharge stormwater runoff from the site. The main objectives of the drainage plan are to reduce flood risk, to protect water quality, to prevent property damage, and to comply with stormwater regulations. The drainage plan should include components such as gutters, downspouts, pipes, culverts, swales, ditches, catch basins, inlets, outlets, detention basins, retention basins, wetlands, bioswales, rain gardens, and permeable pavements. You can use software such as HEC-RAS, SWMM, or EPA Storm Water Management Model to design and analyze the drainage plan.

5 Prepare the grading and drainage drawings

The final step in designing a grading and drainage plan on a floodplain is to prepare the grading and drainage drawings that will document and communicate the design to the stakeholders, contractors, and authorities. The grading and drainage drawings should include the following information: the site plan, the grading plan, the drainage plan, the cross sections, the profiles, the details, the notes, the specifications, the legends, the symbols, the scales, the coordinates, the elevations, the dimensions, and the labels. You can use software such as AutoCAD, MicroStation, or Revit to prepare and print the grading and drainage drawings.