# Validating an Operational Flood Forecast Model Using Citizen Science in Hampton Roads, VA, USA
## 4 Discussion
In a similar fashion, comparisons between the high water marks collected by citizen scientists during _Catch the King_ 2018 observed a slightly less favorable overall MHDD of 6.2 m (24.6 ft), likely attributed to the winds from the mild nor'easter that occurred in the hours leading up to the event. This MHDD was calculated from 30,920 of the 33,847 total high water marks collected after 8.6% of the citizen scientists' measurements were filtered out of the surveyed data. In the interest of improving future forecasts, it was found that less than 1% of the filtered GPS high water marks were still not within 50 m of the Tidewatch Map's predicted inundation raster. Further investigation into these sites identified two reasons for the discrepancy, both related to errors in hydrologic correction of the model's DEM calculated water depth assumptions. Figure 11 outlines a series of above-ground drainage ditches in Hampton VA, that occasionally become inundated when the water table rises with extra high tidal waters. Connection through these narrow drainage ditches can be obscured by thick canopied trees adjacent to the narrow tidal creeks and mostly non-tidal ditches that feed those creeks (Figure 12). The model's elevations are attributed to averaged digital elevations from aerial lidar surveys to source the DEM that the model uses to represent reality. Thus, the depths of the bottoms of these fine scale ditches (\(<\)1 m wide) were not likely to be correct unless the point spacing is extremely high. Naturally, this is acceptable, since the model was scaled to (at best) 1 m spatial resolution, and cannot accurately represent the slopes of such detailed drainage features without scaling to a 0.33 m resolution. Yet, these ditches were found to become tidal conduits for fluid movement capable of causing inundation far from the shoreline during king tides [46]. In other places, bridges over typically non-tidal creeks were not removed from the aerial survey data used to build the DEM, and removal of the occluding feature aided hydro-correction to correct the model's incorrect volume displacement in areas where entire creeks were shown to be dry due to the artificial dam imposed by a bridge, constricting proper fluid flow (Figure 13). Thus, one of the most important and immediately noticeable achievements that _Catch the King_ accomplished for the hydrodynamic model's validation was the aid of hydro-correction for several small streams that were obscured in the aerial lidar surveys informing the Tidewatch Maps. In the case of several ephemeral creeks that temporarily became tidal during the king tide, the citizen scientists' survey identified locations where these ditches needed to be corrected (Figure 14) [47]. Figure 12: Ditches extracted and represented at the sub-grid DEM pixel level for effective representation of drainage ditches leading to (**A**) the west edge of Tabbs Creek and (**B**) the south edge of the creek draining the fairways of Eaglewood Golf Course, both identified for hydro-correction in Hampton, VA, USA. For example, a typically non-tidal Creek feeding Wolfsnare Creek in Virginia Beach was inundated during the king tide in 2017. _Catch the King_ volunteers mapped the king tide approximately an hour before the king tide's peak, and the large initial mean horizontal distance difference from this cluster of points drew researchers' attention to investigate the hydrodynamic model's under-prediction of inundation. The error was traced back to a faulty elevation assumption attributed to obstructed flow underneath a bridge. VIMS researchers hydro-corrected the landscape to open flow using neighboring elevations from the DEM through the box culvert underneath the bridge and corrected ground elevations impacted by thick tree canopies surrounding a Creek bed with low aerial lidar-point-spacing.