ERDC/CHL CHETN-II-47
March 2004
As a corollary to the hot spot, a "cold spot" may form some distance up- or down-coast, where there
is more accretion than the adjacent beach. This signal may be present in the seaward movement of
the shoreline, or an increase in volume of sand on the profile above adjacent areas, or a gain in sand
above the amount placed as fill. The presence of hot and cold spots may indicate three-dimensional
(3-D) redistribution of sediment along the coast, where sand is removed from the hot spots and is
deposited in the cold spots. This differential erosion and accretion along the coast may result in
increased vulnerability to erosion during storms at the areas of hot spots and in some cases may
result in problems of too much sand in the areas of cold spots.
Federal beach nourishment project design includes a planned renourishment interval when additional
sand is placed on the beach to maintain the design profile and provide the level of protection
required. In cases where hot spots are present, the level of protection might be compromised. The hot
spot area is presumed to require additional fill or other remedial action to protect the upland property
in a shorter time frame than the planned renourishment.
SUSPECTED CAUSES OF HOT SPOTS: In order to understand how to anticipate and mitigate
for hot spots, an understanding of the causes and types of erosional hot spots is needed. Dean et al.
(1999) first identified 12 types of hot spots and their causes. These are identified in Table 1. Kraus
and Galgano (2001) modified these 12 types and added six additional hot spot types as seen in
Table 1. Bodge et al. (1999) presented another list of eight basic hot spot types and causes (Table 2).
These 26 types of hot spots cover all types of coastal erosion and have various causes.
Table 1
Hot Spot Type Identification I
No.
Type
Cause
1-12 from Dean et al. (1999) modified by Kraus and Galgano (2001)
1
Dredge selectivity
Variable grain size alongshore placement
2
Pre-existing structure induced slope
i.e., groin, jetty
3
Wave transformation over borrow pit
Wave refraction, focusing
4
Gap in bar
Wave focusing
5
Differential volume of placed fill
Insufficient volume placed to develop equilibrium profile
6
Profile lowering in front of seawall
Wave reflection induced scour
7
Headlands and embayments
Shoreline orientation change
8
Residual fill bathymetry
Uneven fill profile influence on wave refraction
9
Permanent offshore loss
Fill loss through gap in reef or submarine canyon
10
Offshore translation of fill profile
Wave focusing due to irregular translation of fill in offshore direction creating
irregular nearshore bathymetric contours
11
Nearshore bathymetry variation
Wave focusing due to irregular nearshore bed
12
Borrow pit located in active profile
Sand moves to fill pit, wave refraction
13-18 from Kraus and Galgano (2001)
13
Updrift barrier
Blockage of longshore transport
14
Relict inlet offset
Relict (former) ebb and flood shoal bathymetry
15
Transitory longshore sand wave
Excess slug of sand moving alongshore
16
Standing or random sand wave
Seasonal or random sediment supply
17
Isolation of beach from sand source
Change in sediment budget
18
Rip currents on open beach or
Move sand seaward
adjacent to groins/jetties
2
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