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Water level forecasts / flooding indices

In February 2015, CDIP updated the 'Flooding Index' forecast plots. In accordance with current reseach on wave runup, the new plots are not based only on the tide and wave height, but also take the wave period into account. The resulting water level predictions are generally lower than those from the old model, but they should be significantly more representative of the actual water levels observed at the coast. For more information, please refer the documentation below.

For Cardiff, the forecast plots include specific 'mild' and 'moderate' flooding thresholds.
View documentation of the Cardiff flooding thresholds.

View the CDIP flooding indices.

Shoreline Water Levels
High tides and large waves erode beaches, damage property, and flood coastal highways. Tides in California can be predicted accurately for many years in the future. Wave forecasts are usually poor more than a few days in advance. Furthermore, even given specific wave conditions, the amount of flooding depends on poorly understood site details. For example, for unknown reasons, some locations along Highway 101 can overwash (lower left) while other nearby locations do not. Modern studies of wave overwash use high power computers to simulate a few waves over limited areas with recent surveys of sand levels. Such surveys are usually lacking, and computations are in any case not feasible for large reaches of coastline. The objective of the CDIP water level forecasts (Figure 3) is to estimate the vertical elevation to be reached a given coastal location, using a simple, computationally fast model that requires little information beyond wave conditions.
Figure 1: Photographs of wave overwash in California.

Figure 2: Schematic of a breaking wave, and wave runup, on a sandy beach with slope β. SWL is the still water line (water level without waves) and is determined primarily by the astronomical tide. R is the vertical elevation of the runup above SWL.

Figure 3: CDIP water level forecast (3 day) at Point Sal.

Coastal engineers worldwide have studied wave runup for more than 60 years, including sea and swell waves running up steep seawalls, low slope beaches, and everything in between. Wave runup on fine sand differs from runup on cobbles, and coastal structures are often protected with boulders (Figure 1). Runup is important, complicated, and an active area of research. Movies of wave runup around the world have been carefully ortho-rectified and used to develop empirical (e.g. based on observations) formula for wave runup (Stockdon et al, 2006). Most of those, and other, formula include the beach slope β. Unfortunately, β changes as beaches erode, and can be steep in some depths and low slope in others. Fortunately, Stockdon et al (2006) provide an estimate for storm wave runup that does not depend on β. The vertical level reached by 2% of waves (R2%) depends on incident wave significant height Ho, and period T (Stockdon et al, 2006):

R2% = 0.043 (Ho Lo)1/2

where the deep water wavelength Lo = (g/2pi) T2. Long period swell causes higher runup than short period seas of the same height. Higher incident waves of course cause higher runup. Note that Stockdon uses the spectral peak Tpeak, whereas the mean period Tmean is used here to avoid jumps in R2% in bimodal waves with approximately equal sea and swell energy.

On figure 3, the maximum water level is equal to the Tide Level + R2%. Ho and Lo are estimated, in 10m depth, from the CDIP regional wave model. Estimates are averaged over a 500m alongshore length of beach. Tide predictions from the nearest long-term NOAA tide station are used, and the predicted tides are adjusted by adding in the offset between the last observed monthly mean MSL value and the MSL datum (1983-2001) for the tide station.

Reference: Stockdon, H.F., Holman, R.A., Howd, P.A., Sallenger, A.H., 2006. Empirical parameterization of setup, swash, and runup. Coast. Eng. 53 (7), 573-588.
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